UNIVERSITY OF CINCINNATI

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1 UNIVERSITY OF CINCINNATI Date: July 5, 005 I, Weizhuo Li, hereby submit this work as part of the requirements for the degree of: Doctorate of Phyilosphy in: Electrical & Computer Engineering and Computer Science It is entitled: Wavelength Multiplexing of MEMS Pressure and Temperature Sensors Using Fiber Bragg Gratings and Arrayed Waveguide Gratings This work and its defense approved by: Chair: Joseph T. Boyd Peter Kosel Altan M. Ferendeci Howard E. Jackson Chong H. Ahn 1

2 Wavelength Multiplexing of MEMS Pressure and Temperature Sensors Using Fiber Bragg Gratings and Arrayed Waveguide Gratings A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirement for the degree of DOCTORATE OF PHILOSOPHY (Ph.D.) in the Department of Electrical and Computer Engineering and Computer Science of the College of Engineering 005 by Weizhuo Li B.S., Electrical Engineering, Northern Jiaotong University, Beijing, China, 1994 Committee Chair: Professor Joseph. T. Boyd

3 Abstract This thesis presents the design, fabrication and testing of wavelength multiplexing of optically interrogated MEMS pressure and temperature sensors using both fiber Bragg gratings (FBGs) and arrayed waveguide gratings (AWGs). The pressure sensors and temperature sensors have been designed and fabricated using MEMS techniques. For the pressure sensor, fabrication is initiated with a standard fused-silica wafer (Pyrex 7740) polished on both sides that is patterned to form a series of cavities for the Fabry-Perot interferometer. Positive photoresist is patterned and serves as a mask for etching the cavities in the glass wafer with buffered HF. A silicon wafer polished on both sides is then electrostatically bonded to the patterned glass wafer. Bulk etching techniques are used to thin the silicon wafer down to the desired diaphragm thickness, while the other side of the Si/glass assembly is protected. The configuration of a Fabry-Perot temperature sensor involves a thin layer of silicon bonded to a glass wafer. The fabrication process is similar to that of a pressure sensor, but with no air cavity. Pressure measurements were made over the 0 to 30 psi range while temperature measurements were made over the 4 to 100 C range. Pressure sensor sensitivities of 0.0mv/psi for the multiplexing system using AWGs, and of 0.007mv/psi for the multiplexing system using FBGs were obtained. The pressure sensor were designed with cavity diameter R 0 = 300 μm, cavity depth d 0 = 0.64 μm for the sensor operating at 850 nm, and d 0 = 1.1 μm for the sensor operating at 1550 nm. Diaphragm thickness for the two sensors were 14 μm and 15.5 μm. The temperature sensor was fabricated by bonding a silicon wafer with different thickness on the glass wafer. An anti-reflective coating of SiO was deposited on the top surface of the pressure sensor by evaporation. The purpose

4 of this anti-reflective coating was to reduce sensor s temperature sensitivity. An oxidantresistant encapsulation scheme for the temperature sensor was proposed, fabricated and tested, namely aluminum coated silicon nitride (Al/Si3N4). The multiplexed sensor system using FBGs has the potential of multiplexing about seventy sensors depending on FBGs bandwidth and the one using AWGs has the potential of multiplexing about eight sensors depending on the number of AWGs channels and the bandwidth of each channel. A dual- wavelength method incorporating a tunable laser was used to interrogate either the applied pressure or temperature experienced by the sensor, while a three-wavelength method was used to simultaneously interrogate pressure and temperature. Experimental results, including response as a function of pressure or temperature, were characterized by good agreement between experimental and theoretical results. There is no observable cross-talk between the multiplexed sensors. 3

5 Acknowledgements First and foremost, I would like to thank my advisor, Dr. Joseph T. Boyd. His guidance and instruction throughout this project greatly added to my research and study experience, and his overall believe in my ability to finish this project was also invaluable for me. Second, I would like to thank the committee members, Dr. Chong H. Ahn, Dr. Altan M. Ferendeci, Dr. Peter Kosel and Dr. Howard Jackson for their review of this work. Thirdly, I would also like to acknowledge the help and friendship of the optoelectronics lab mates who have helped and encouraged me throughout my endeavor at my doctoral program. They are Anish Saran, Jie Zhou, Pete L. Vassy and Nan Zhang. A special acknowledgement is extended to Dr. Don Abeysinghe, a research scientist at Taitech Inc for his involvement and guidance in my thesis project. Finally, I would like to thank my family for their support, especially to my husband, Jianhui Zhao. He comes along with me to the US, sacrifices his career in China for supporting me to pursue and complete this Ph D degree. Without his support and love, I would not succeed. So, I would like to dedicate this thesis to my beloved husband. 4

6 Table of Content Table of Contents 1 List of Figures List of Tables INTRODUCTION Review of Fabry-Perot Cavity-Based Pressure and Temperature Sensor 1. Review of Multiplexed Sensors 1.3 Introduction to Optical Fiber Telecommunications and WDM System 1.4 Summary 1.5 References.0 DESIGN OF SENSORS.1 Introduction. Plane Wave Propagation in Homogeneous Media..1 Reflection and Transmission at an Interface Between Two Materials.. Reflection from Isotropic Layered Media.3 Design of Ideal Fabry-Perot Cavity-Based Pressure sensor.3.1 The Dynamic Properties of the Fabry-Perot Pressure Sensor.3. Calculation of the Light Loss Inside the Sensor.3.3 Initial Diaphragm Deflection.3.4 Anti-reflection Layer.4 Design of Ideal Fabry-Perot Temperature Sensor.4.1 Selection of Temperature Sensitive Material.4. Determination of Fabry-Perot Temperature Sensor s Thickness.4.3 Design of Encapsulant for Temperature Sensor.5 Summary.6 References 5

7 3.0 MULTIPLEXED SENSORS 3.1 Introduction 3. Multiplexed Pressure Sensors Using Fiber Bragg Gratings (FBGs) 3.3 Multiplexed Pressure Sensors Using Arrayed Waveguide Gratings (AWGs) 3.4 Ripples Due to the Long Coherence Length of Tunable Laser 3.5 Dual-Wavelength Interrogation Technique for Multiplexed Pressure Sensors 3.6 Multiplexed Temperature Sensors and Interrogation Technique 3.7 Summary 3.8 References 4.0 FABRICATION AND CALIBRATION OF SENSORS 4.1 Introduction 4. Sensor Fabrication 4..1 Photolithographic Patterning and Wet Etching for Cavity Formation on Glass wafer 4.. Anodic Bonding of Silicon-to-Glass 4..3 Silicon Wafer Thinning to Form Diaphragm 4..4 Polishing Silicon Diaphragm 4..5 Sensor Package 4.3 Sensor Characterization Measuring Initial Diaphragm Deflection and Cavity Depth 4.3. Measuring Diaphragm Thickness 4.4 Pressure Sensor Calibration Calibration of Multiplexed Pressure Sensor System 4.4. Comparison of the Two Multiplexed Pressure Sensor Systems Temperature Sensor Calibration Calibration of Multiplexed Temperature Sensor System 4.5 Summary 4.6 References 6

8 5.0 ANTI-RELECTION LAYER FOR PRESSURE SENSOR AND ENCAPSULATING STRUCTURE FOR TEMPERATURE SENSOR 5.1 Introduction 5. SiO Evaporation Process 5..1 Calibration of Pressure Sensor with Anti-reflective Coating 5.3 Si 3 N 4 and Aluminum Deposition Process Calibration of Temperature Sensor with Encapsulation Structure 5.4 Summary 5.5 References 6.0 MULTIPLEXED SENSOR SYSTEM FOR SIMULTANEOUS MEASUREMENT OF PRESSURE AND TEMPERATURE 6.1 Introduction 6. Prerequisite for Simultaneous Measurement of Pressure and Temperature 6.3 Multiplexed Sensor System for Simultaneous Measurement of Pressure and 6.4 Summary 6.5 References 7.0 CONCLUSIONS 7

9 List of Figures 1.1 Schematic diagram illustrating an optical-interrogated Fabry-Perot pressure sensor interconnected via an optical fiber. 1. Cross-sectional view of the sensing element. 1.3 Illustration of two configurations of fiber optically interrogated MEMS pressure sensors. Figure 1(a) shows the usual configuration, which consists of a glass plate with a shallow cylindrical cavity etched into one surface with the cavity covered by a thin silicon diaphragm that has been anodically bonded to the patterned glass wafer. Figure 1(b) shows the configuration where the cavity is formed on the end of the optical fiber and a silicon diaphragm is bonded anodically. 1.4 Measurement system for a Fabry-Perot cavity-based pressure sensor. 1.5 Frequency multiplexing scheme. 1.6 Wavelength Division Multiplexing Bragg grating-based laser sensors. 1.7 Time-Division-Multiplexed fiber Bragg grating sensor array..1 Configuration of optically interrogated MEMS pressure sensors. (a) gauge pressure sensor; (b) absolute pressure sensor; (c) differential pressure sensor. Reflection and refraction of plane wave at a boundary between two dielectric media..3 Reflection and refraction of S wave (TE)..4 Reflection and refraction of P wave (TM)..5 Transmission and reflection from one layer Fabry-Perot interferometer..6 Transmission and reflection from two layer Fabry-Perot interferometer..7 A multiplayer dielectric medium..8 Reflectance from the Fabry-Perot cavity-based sensor as a function of cavity depth when operating at 850nm (n 0 = n glass =1.474, n 1 = n air =1.0, n = n si =3.46)..9 Reflectance from the Fabry-Perot cavity based sensor as a function of cavity depth when operating at 1550 nm with the diaphragm thickness at t = um (n 0 = n glass =1.473, n 1 = n air =1.0, n = n si =3.478, n 3 = n air =1.0)..10 Reflectance from the Fabry-Perot cavity based sensor as a function of silicon thickness when operating at 1550 nm with the cavity depth at d 0 = 1.1 μm. 8

10 .11 Reflectance from the Fabry-Perot cavity based sensor as a function of cavity depth (0- μm) and silicon thickness ( μm) when operating at 1550nm..1 Computer simulation of diaphragm deflection of the Fabry-Perot cavity-based sensor under the pressure using Mathematic Calculation of light loss in sensor..14 Calculation of change of pressure inside the cavity under diaphragm bending..15 Fabry-Perot cavity-based pressure sensor with a thin layer of antireflection coating (SiO) on the top surface of silicon diaphragm..16 Illustration of how an antireflection coating reduces the reflected light intensity..17 Illustration of how the condition for antireflection coating is wavelength dependent (The thickness of the antireflection coating is designed for operating at 1550 nm). The antireflection condition is satisfied at the wavelengths of the cross points of the two lines. Red line: Reflectivity from the three layer Fabry-Perot interferometer (pressure sensor without antireflection layer); Blue line: Reflectivity from the one layer Fabry-Perot interferometer (pressure sensor with antireflection layer)..18 Configuration of the Fabry-Perot temperature sensor..19 Reflectance spectra shift with temperature increasing..0 Temperature range as a function of silicon thickness at λ = 1550 nm for a Fabry- Perot temperature sensor..1 Configuration of the Fabry-Perot temperature sensor with encapsulating layers.. Comparison of reflectance spectra shift for the sensor with encapsulant and the sensor without encapsulant under several values of temperature. 3.1 Principle of fiber Bragg gratings (FBGs). 3. Schematic diagram of the multiplexed pressure sensor system using FBGs. 3.3 Simulation of the results from multiplexed pressure sensor sytem using FBGs. 3.4 Schematic diagram of the arrayed waveguide gratings (AWGs). 3.5 Schematic diagram of the multiplexed pressure sensor system using AWGs. 3.6 A cosinusoidal wave train modulated by a Gaussian envelope along with its transform, which is also Gaussian. 3.7 Unwanted interferometric signal between the connector and FBG. 3.8 Structure of a basic ferrule connector (FC/PC and FC/APC). 9

11 R( λ1 ) I( R, λ) = from the Fabry-Perot cavity-based pressure R( λ ) + R( λ ) 3.9 Ratio 1 sensor as a function of cavity depth when operating at λ = nm and λ = nm with the silicon diaphragm thickness at t = μm (n 0 = n glass =1.473, n 1 = n air =1.0, n = n si =3.478, n 3 = n air =1.0) Reflectivity from the Fabry-Perot temperature sensor as a function of silicon refractive index with silicon thickness at t = 0 μm (n 0 = n glass =1.473, n 1 = n si, n = n air =1.0). R( λ1 ) 3.11 Ratio I( R, λ) = from the Fabry-Perot temperature sensor as a R( λ ) + R( λ ) 1 function of silicon refractive index when operating at λ = nm and λ = nm and with silicon thickness at t = 0 μm. 3.1 Multiplexed temperature sensors using broad band mirror fiber gratings Reflection spectrum of a broad band mirror fiber grating Simulation of the results from multiplexed temperature sensors using broad band mirror fiber gratings (Bandwidth 10 nm) Simulation of temperature range (a) and temperature sensitivity; (b) as a function of silicon thickness for multiplexed temperature sensors using broadband fiber gratings (Bandwidth 10 nm). 4.1 MEMS fabrication processing steps. 4. Mask for Fabry-Perot cavity-based pressure sensors. 4.3 Cavity depth measured by Profilometer The sensor above has the cavity depth around μm The sensor below has the cavity depth around μm. 4.4 Glass-to-silicon anodic bonding machine in our lab. 4.5 Glass-to-silicon anodic bonding setup. 4.6 Typical current flow during anodic bonding. 4.7 A Teflon beaker for silicon diaphragm etching. 4.8 Wafer protector for etching bonded Si substrate. 10

12 4.9 A Fabry-Perot cavity observed from the silicon side after thinning down the silicon diaphragm The profile of a Fabry-Perot cavity after cleaving through the glass-diaphragm structure Surface of the silicon diaphragm after etching: (a) before polishing; (b) after polishing. 4.1 Package configurations for housing the optical fiber and MEMS pressure sensor: (a) differential pressure sensor; (b) absolute and gauge pressure sensor (a) Nikon optical microscope in our lab; (b) Place a sensor under microscope for observation of Newton s rings Illustrating the formation of Newton s rings Change of Newton s fringes with pressure increasing for a gauge pressure sensor Change of Newton s fringes with pressure increasing for an absolute pressure sensor Change of Newton s fringes with pressure increasing for a differential pressure sensor Microscope focusing can be used to determine the silicon diaphragm thickness by cleaving through the glass-diaphragm structure and measuring its thickness on edge Silicon diaphragm thickness can be determined by the measured reflectance spectrum from the sensor. 4.0 The relationship between interferometer cavity depth and cavity reflectance is shown for LED emission at 80, 850 and 880 nm. Note the reflectance is wavelength dependent at each cavity depth. 4.1 The static pressure response results from the pressure sensor operating at 850 nm. 4. Computer simulation of pressure fluctuation in wind tunnel. 4.3 Measured pressure fluctuation in wind tunnel. 4.4 Schematic diagram of single pressure sensor measurement. 4.5 Measured reflectance spectra at several values of pressures for single pressure sensor measurement (cavity depth = 0.95 μm; diaphragm thickness =.95 μm). 4.6 Measured reflectance spectra at several values of pressures for single pressure sensor measurement when using FC/PC connectors in the system instead of FC/APC connectors. 11

13 4.7 Air pressure control system in our lab. 4.8 Experimental set-up for multiplexed pressure sensor system. 4.9 Results of multiplexed pressure sensor system using fiber Bragg gratings (FBGs) Results of multiplexed pressure sensor system using arrayed waveguide gratings (AWG) Measured reflectance as a function of pressure (0 30 psi) extracted from the multiplexed signal for sensor # at two wavelengths: λ 1 = nm and λ = nm. 4.3 The ratio R( λ ) I( R, λ) = (λ 1 = nm and λ = nm) for sensor # R( λ ) + R( λ ) 1 as a function of pressure. Dotted lines: theoretical curve; Solid lines: measured response Schematic diagram of single Fabry-Perot temperature sensor measurement Experimental set-up for Fabry-Perot temperature sensor measurement Configuration of a Fabry-Perot temperature sensor # Measured reflectance spectrum shift under several values of temperature for sensor # Configuration of a Fabry-Perot temperature sensor # Measured reflectance spectrum shift under several values of temperature for sensor # Results of multiplexed temperature sensor system using fiber Bragg gratings (FBGs) Measured reflectance as a function of temperature (4 C 35 C) extracted from multiplexed signal for sensor #1 at two wavelengths: λ 1 = nm and λ = nm Ratio of the reflectance R( λ ) I( R, λ) = measured at two wavelengths (λ 1 = R( λ ) + R( λ ) nm and λ = nm) for sensor # Mathematics simulation of reflectance spectrum shift for sensor #1 using the multiplexed temperature sensor system. (The reflected light from a broadband fiber grating with 10 nm bandwidth was used to interrogate sensor #1 using wavelengthencoded measurement). 1

14 5.1 Vacuum evaporation oven. 5. A Fabry-Perot cavity observed from the silicon side after thinning down the silicon diaphragm (a) without anti-reflective coating; (b) with anti-reflective coating. 5.3 (a) Theoretical reflectance spectrum for a Fabry-Perot cavity-based pressure sensor with anti-reflective coating; (b) Measured reflectance spectrum from a Fabry-Perot cavity-based pressure sensor with anti-reflective coating. 5.4 Measured reflectance spectra at several values of pressure for a Fabry-Perot cavitybased pressure sensor with anti-reflective coating. 5.5 Measured reflectance as a function of pressure at two wavelengths: λ 1 = 1548 nm and λ = 1550 nm for a Fabry-Perot cavity-based pressure sensor with anti-reflective coating. 5.6 Measured reflectance spectra at several values of temperature from C. 5.7 Sputtering machine. 5.8 Electron-beam evaporation machine. 5.9 Thickness and refractive index measurements of silicon nitride on silicon using spectral ellipsometer (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor without encapsulating layers (silicon thickness = 13 μm); (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor without encapsulating layers (silicon thickness = 13 μm) (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor with 00 nm Si 3 N 4 ; (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor with 00 nm Si 3 N (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor with 00 nm Si 3 N μm Aluminum; (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor with 00 nm Si 3 N μm Aluminum Measured reflectance as a function of temperature ( C) for a temperature sensor with encapsulating layers at two wavelengths: λ 1 = 155 nm and λ = 1557 nm. 13

15 5.14 The ratio R( λ ) I( R, λ) = (λ 1 = 155 nm and λ = 1557 nm) for a temperature R( λ ) + R( λ ) 1 sensor with encapsulating layers as a function of temperature. Dotted lines: theoretical curve; Solid lines: measure response. 6.1 Newton s rings change under temperature for a Fabry-Perot cavity-based pressure sensor without pressure applied: (a) gauge pressure sensor; (b) absolute pressure sensor; (c) differential pressure sensor. 6. The absorption coefficient (α) vs. wavelength (λ) for various semiconductors. The penetration depth is equal to 1/α. 6.3 Reflectance from the Fabry-Perot cavity-based sensor as a function of cavity depth when operating at 850 nm. 6.4 The ratio R( λ1 ) I( R, λ) = from the Fabry-Perot cavity-based pressure R( λ ) + R( λ ) 1 sensor as a function of cavity depth when operating at nm with (a) λ λ 10 nm; (b) λ λ 0nm; (c) λ λ 30 nm. 1 = 6.5 The ratio 1 = 1 = R( λ1 ) I( R, λ) = from the Fabry-Perot cavity-based pressure R( λ ) + R( λ ) 1 sensor as a function of cavity depth within the maximum cavity depth change at d 0 = 4.9 μm and λ - λ 1 = 0 nm. 6.6 The ratio R( λ1 ) I( R, λ) = for the relative reflected intensities at two R( λ ) + R( λ ) 1 wavelengths (λ 1 = 860 nm; λ = 840 nm) from the Fabry-Perot sensor as a function of applied pressures 0 30 psi. 6.7 The ratio R( λ1 ) I( R, λ) = for the relative reflected intensities at two R( λ ) + R( λ ) 1 wavelengths (λ 1 = 860 nm; λ = 840 nm) from the Fabry-Perot sensor a function of applied pressures 0 30 psi at 5 C and 100 C respectively. 6.8 Theoretical reflectance R(λ) from the Fabry-Perot cavity-based pressure sensor at several values of pressure and temperature. 6.9 Schematic diagram of the multiplexed Fabry-Perot cavity-based sensors for simultaneous measurement of pressure and temperature. 14

16 6.10 Simulation of the multiplexed Fabry-Perot cavity-based sensors for simultaneous measurement of pressure and temperature Temperature range of a multiplexed Faby-Perot sensor as a function of silicon thickness when using a broadband FBGs (Bandwidth nm) as light source in 1550 nm wavelength region. (The temperature range of a Fabry-Perot sensor is defined as Δ T = T T1, where T 1 is room temperature). 6.1 Simulation of the reflectance spectra shift with temperature increasing (4 90 C) for a multiplexed Fabry-Perot sensor using a broadband chirped FBGs ( nm) as light source in 1550 nm wavelength region Cross-sectional view of a fused-fiber coupler having a coupling region W and two tapered regions of length L. The total span L+W is the coupler draw length An optical isolator transmits light in only one direction Responsivity from a New Focus 1554-A photodetector. 15

17 List of Tables.1 The natural frequencies of circular plates.. Properties of candidate Fabry-Perot materials. 4.1 <100> silicon etch rates in [μm/h] for various KOH concentrations and etch temperature as calculated from Eq. () by setting Ea = ev and k 0 = 480 μm/h (mol/liter) Data for pressure sensor measurement (sensor operating at 850 nm). 4.3 Data for temperature sensor measurement (sensor #1). 4.4 Data for temperature sensor measurement (sensor #). 4.5 Data for temperature sensor measurement using multiplexed system (sensor #1). 16

18 1 Introduction This dissertation describes on investigation of pressure and temperature sensors integrated by optical techniques. The pressure and temperature sensor are based on the principle of Fabry-Perot interferometer and fabricated using MEMS-technology. This thesis research primarily concentrates on multiplexing pressure and temperature sensors based on WDM principles and existing WDM technology using tunable laser. Optical sensors utilizing a Fabry-Perot cavity have been demonstrated to be attractive for the measurement of temperature, strain, pressure and displacement, due to their high sensitivity. The major advantages of optical sensors over conventional electrical sensors include immunity to electromagnetic interference, resistance to harsh environment and potential for multiplexing. Microelectro-mechanical systems (MEMS) techniques make Fabry-Perot sensors more attractive by reducing the size and the cost of the sensing element. Miniaturization of sensors (< 1mm) will lead to realization of dense sensor arrays that will change the state-of-the-art of pressure mapping. Optical sensors are of great interest for application in aerospace because their immunity to EMI can provide a significant weight savings through the elimination of cable shielding and surgeprotection electronics. Optical sensors also show proven success in biomedical field resulting from their reliability and biocompatibility and the simplicity of the sensorphysician interface. Low-coherence light sources, such as light-emitting diodes (LEDs), can be used as light source for these sensors, since the optical length of a low-finesse Fabry-Perot cavity is shorter than the coherence length of typical LEDs. However, for situations involving multiple sensors, wavelength division multiplexing (WDM) is advantageous. To implement WDM, laser sources are required. Use of lasers in the 1550 nm wavelength range will allow use of commercial WDM products. So, we choose lasers instead of LEDs as the light source for Fabry-perot cavity-based sensors in this project. 1.1 Review of Fabry-Perot Cavity-based Pressure and Temperature Sensor For Fabry-Perot cavity-based sensors, pressure sensing is currently the most promising market. Some of the common applications include biomedical blood pressure 17

19 sensing, industrial process monitoring and automotive engine control. Fabry-Perot cavitybased pressure sensors have been reported in a variety of configurations, such as those using modulation of the air gap of a Fabry-Perot interferometer by deflection of a diaphragm [1-5], and interferometric detection of the stress-dependent resonant frequency of an optically excited vibrating beam [6-7]. A simple Fabry-perot cavity-based pressure sensor consists of a diaphragm, which deflects in response to a change in pressure. The diaphragm deflection changes the depth of the cavity and thus the conditions for optical interference when light is reflected from successive reflections of the initial beam. As the diaphragm moves in response to pressure, the Fabry-Perot reflectance changes accordingly. There are different designed structures for those Fabry-Perot cavity-based pressure sensors. Youngmin Kim, in 1995, developed a surface micromachined Fabry-Perot cavity based pressure sensor. Dielectric film stacks consisting of layers of silicon dioxide and silicon nitride were used as mirrors, shown in Fig 1.1. Polysilicon was used as a sacrificial layer that was then removed to form an air gap cavity. The Fabry-Perot sensor was optically interrogated using a multimode optical fiber [1]. In 1996, O. Tohysma developed a new type of fiber-optic pressure micro-sensor (sensing-element size: 350 μm 350 μm 350 μm) suitable for medical instruments such as catheters and endoscopes. The sensing element consists of a diaphragm structure, glass plate and alignment structure, a multimode fiber 50/15 μm (core/clad) in diameter has been used, shown in Fig. 1.. Pressure measurements of the sensor with sufficient sensitivity ( 0.19 μw MPa -1 on the average) and accuracy have been obtained []. Don C. Abeysinghe, in 000, described a novel optically interrogated pressure sensor in which the entire MEMS structure is fabricated directly on an optical fiber. A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity, and anodic bonding of a silicon diaphragm is utilized. The structure of the sensor is shown in Fig Both 00-μmdiameter and 400-μm-diameter multimode optical fibers have been demonstrated. The pressure range is 0-80 psi, with the sensitivity of about 0.11 mv/psi with ±0.01mV/psi departure from linearity [3]. 18

20 The interrogation methods for Fabry-Perot sensors include employing optical spectrum modulation for Fabry-Perot sensors illuminated with LEDs [8], generation of quadrature phase-shifted interferometric signals through the utilization of dualwavelength illumination [9-10], dual-length coupled cavities [11-1] and dualwavelength technique [1]. We will choose a dual-wavelength interrogation technique for the Fabry-Perot sensor illuminated with a tunable laser in this project. Fiber Bragg gratings (FBGs) and Arrayed waveguide gratings (AWGs) are utilized as spectral discriminators with properties suitable to the interrogation of Fabry-Perot cavity based pressure sensor using our dual-wavelength technique. For Fabry-Perot sensors illuminated with LEDs, an interrogation method employing optical spectrum modulation was demonstrated on biomedical pressure and temperature sensors by Roger A. Wolthuis in 1991 [8]. This method uses a LED centered at 850 nm for interrogating the Fabry-Perot cavity-based pressure sensor. Since the LED has an emission bandwidth of about 60 nm and the reflectance of each emission wavelength varies uniquely with optical cavity depth, the reflected light from the sensor appears to be spectrally shifted by external pressure. The sensor input and output are measured by the ratio of the output of the photodetector with the bandpass filter and the output of the second photodetector with no filter, as shown in Fig. 1.4 [4]. Thus, the sensor signal is the ratio of measured input and output, which is used to detect the spectral shift. The benefit of this method is that the sensor signal is relatively immune to changes in LED output intensity or to the efficiency of light-handling components within the optical system. One of the interrogation methods for Fabry-Perot sensors illuminated with laserdiode source is a dual-wavelength technique, in which relative reflected intensities at two different wavelengths are separately measured and the ratio is calculated and used to trace back to pressures for Fabry-Perot sensor. This technique eliminates errors resulting from wavelength independent changes in the fiber interconnect to the sensor. The response curve is a periodic function with respect to the cavity depth [1]. A Fabry-Perot temperature sensor usually consists of a thin layer of a material that has a temperature-dependent refractive index. The temperature changes the optical path of successive reflected beams, so the reflectance from the sensor changes accordingly. 19

21 The reflectance from the sensor is a function of wavelength. So, a wavelength-encoded temperature measurement can be provided by a Fabry-Perot temperature sensor. At resonance, the reflectivity of the sensor is a minimum, and the wavelength of one of these reflectivity minima can serve as a signal-level-insensitive indicator of temperature. The first temperature measurements using a fiber-linked Fabry-Perot sensor were described by Christensen in He tested two sensors: a fused-silica window, 3.-mm thick, and a glass window, mm thick. Multi-layer dielectric mirrors were deposited on both surfaces of each window. The high reflectivities of these mirrors provided a high finesse, which is a measure of the optical resonator s quality factor. The temperature ranges were 11 C and 150 C for the fused-silica and glass interferometers respectively [13-14]. In 1986, using a SiC Fabry-Perot interferometer, Beheim demonstrated a temperature resolution of 1 C over the 0 C to 1000 C range. The temperaturesensitive material was cubic SiC, with a thickness of 18 μm, that was deposited on a silicon wafer [15]. C. E. Lee, in 1988, described a Fabry-Perot temperature sensor which was characterized over a 150 C temperature range. A thin TiO coating was evaporated onto the end of the sensing fiber prior to fusion splicing it to a transmission fiber. Lee s group found that the reflectivity of the splice could be controlled via the fusion parameters; by the continued application of heat the titanium was made to diffuse further into the surrounding SiO, thereby reducing the splice s reflectivity [16]. Silicon is becoming increasingly important in the optoelectronic device field due to its well developed and inexpensive technology allowing an effective integration between optical and electronic devices. Besides, silicon has a high thermo-optic coefficient. In 1988, L. Schultheis described a Fabry-Perot temperature sensor that used silicon as the temperature-sensitive material. Amorphous silicon films can be deposited directly onto the tip of the fiber by means of evaporation, sputtering, or chemical vapor deposition without having problems of stoichiometry. After annealing, the previously amorphous silicon films become polycrystalline. Temperatures in excess of 400 C can be measured without deteriorating the sensor performance. An absolute temperature limit is solely 0

22 determined by the thermal stability of the silicon-glass interface and is expected to be in the range of C [17]. In most structure or process control applications, it is often desirable that both the pressure and temperature be known at a given point. In keeping with these needs many researchers have suggested configurations to measure both the pressure and temperature simultaneously. Gregory N. De Brabander, in 1998, demonstrated an optical pressure sensor that uses an unbalanced integrated optical Mach-Zehnder interferometer that is mechanically coupled to a micromachined silicon diaphragm. The sensor s TE and TM output were separated and measured respectively. TM mode was found to be substantially more sensitive to pressure than the TE mode, whereas the thermal responses of the two modes were found to be similar. Based on this, pressure and temperature can be measured simultaneously [18]. 1. Review of Multiplexed Sensors: The use of optical sensors for measuring temperature, strain and pressure in engineering structures often requires many sensors. This makes multiplexing a very important issue for optical sensors. Moreover, multiplexing optical sensors is simpler than multiplexing electrical signals from multiple sensors. By sharing components, the cost per sensor is greatly reduced, improving the competitiveness of optical sensors against conventional electro-mechanical sensors. Optical multiplexing techniques also substantially reduce the overall system weight which is very critical for applications in aerospace. Since the mid-1980s there has been growing interest in the development of multiplexing techniques for optical sensors. Several multiplexing techniques which have been reported: frequency [19], wavelength [0], time [1], spatial [], and coherence domain multiplexing [3]. The multiplexing capacities of these techniques have been limited to approximately 10 sensors due to various factors, including speed, cross-talk, signal-to-noise ratio, and wavelength bandwidth. Frequency multiplexing is the most highly developed topology for multiplexed sensors demonstrated to-date. The lasers are modulated at different frequencies, the 1

23 outputs carried by each output fiber will consist of signals at each modulation frequency, and the sensor outputs can be separated by frequency selection using appropriate band filtering or synchronous detection. This type of array has been shown to be capable of providing good phase detection sensitivity and low crosstalk for systems. And it has the potential to consist of large number of sensors. A frequency multiplexing scheme is shown in Fig. 1.5 [19]. In 1995, K. P. Koo described a wavelength division multiplexing technique for multiplexing Bragg grating-based laser sensors. Bragg grating-based laser sensor consists of two Bragg fiber gratings of matched Bragg wavelength to create an in-fiber cavity. The lasing wavelengths of the two fiber laser sensors were μm and μm respectively. The wavelength encoded signals from each of these isolated sensor signals were interrogated by two separate MZI s, as shown in Fig This wavelength division multiplexing/demultiplexing scheme should be expandable to arrays of 8 or more sensors in the Er bandwidth [0]. In 000, Chi Chiu Chan developed a time-division-multiplexed fiber Bragg grating sensor array by using a tunable laser source. Fig. 1.7 shows a schematic diagram of the multiplexed FBG sensor system. This system uses TDM and a tree array topology. The pulses from different FBG channels were separated in the time domain when they arrive at the photodetector and could be separated by electronic switching after the photodetector. Computer simulation shows that an array of 0 FBG sensors with 3 με resolution can be realized [1]. In this project, we choose a wavelength-division-multiplexing technique for multiplexing Fabry-Perot pressure and temperature sensors. Because current optical techniques can handle an extremely a large number of wavelengths, there exists the opportunity for multiplexing large number of sensors. This technique also makes the dual-wavelength interrogation method for a single MEMS Fabry-Perot sensor possible. 1.3 Introduction to Optical Fiber Telecommunications and WDM System Optical fiber communications has grown rapidly during the past decade. Present trends are to use single-mode fibers for most telecommunication applications due to the low loss (reduction of Rayleigh scattering losses because of lower doping density in the

24 fiber core [4]) and low dispersion. Currently, the fourth-generation optical fiber communication systems utilize the lowest-loss 1550 nm wavelength region ( third window ). The demands for global broadband communications are driving fiber optical-based telecommunications towards wavelength division multiplexing (WDM). WDM systems provide multiple wavelengths to carry optical signals simultaneously through a fiber. WDM is a key element in photonics technology that has opened a new era in the optical telecommunication industry. But it takes one laser to produce each of those wavelengths, and as the power of WDM has grown to 160 wavelengths per fiber, the collection of lasers needed has become unwieldy. This has lead to intense research and development of tunable lasers for over ten years. Tunable lasers offer many compelling advantages over fixed wavelength solutions in WDM in that they provide extremely good resolution, monotonic, and continuous wavelength scans, and cover the major wavelength range of interest [5]. External-Cavity diode laser (ECDL) technology is used to build tunable lasers in order to provide wider tuning ranges (~100 nm) with higher output powers. The utility and reliability of ECDLs have been proven in test-and-measurement applications. We choose a 638 tunable laser based on ECDL technology from Newfocus Inc.; this laser offer continuous tuning over wide wavelength ranges, fast linear sweeps, excellent tuning resolution, and narrow, stable linewidths. Tunable lasers will dominate many applications areas within the next few years. This, along with their convenience for laboratory demonstration, is why we choose a tunable laser as the light source for multiplexed sensors. 1.4 Summary This chapter has reviewed a number of most significant developments in Fabry- Perot type sensor technology, and has highlighted the importance of multiplexed sensors. The purposes of this project include: 1. MEMS fabrication of Fabry-perot cavity based pressure sensors. Using single-mode fiber in system design and operating in 1550nm region for the purpose of compatibility with optical fiber communications. 3

25 3. Realizing multiplexing pressure and temperature sensors system based on WDM principles and existing WDM technology using tunable laser, and expanding the number of sensors which can be multiplexed to more than 10 sensors. 4. Simultaneous measurement of temperature and pressure Multiplexing sensors systems using both FBGs and AWGs were set up in our lab. Experimental results have been compared with theoretical analysis using Mathematica

26 1.5 References: [1] Youngmin Kim and Dean P. Neikirk, Design of Manufacture of Micromachined Fabry-Perot Cavity-based Sensors, Sensors and Actuators, Vol. 50, pp , [] O.Tohyama, M. Kohashi, K. Yamamoto, H. Itoh, A Fiber-optic Silicon Pressure Sensor for Ultra-thin Catheters, Sensors and Actuators, Vol. 54, pp.6-65, [3] Don C. Abeysinghe, Samhita Dasgupta, Joseph T. Boyd and Howard E. Jackson, A Novel MEMS Pressure Sensor Fabricated on an Optical Fiber, IEEE Photonics Technology Letters, Vol. 13, No. 9, 001. [4] Jie Zhou, Samhita Dasgupta, Howard E. Jackson and Joseph T. Boyd, Optical Interrogated MEMS Pressure Sensors for Propulsion Applications, Optical Engineering, Vol. 40, No. 4, pp , 001. [5] Z. Xiao, O. Engstrom and N. Vidovic, Diaphragm Deflection of Silicon Interferometer Structures Used as Pressure Sensors, Sensors and Actuators, Vol. 58, pp , [6] D. Angelidis and P. Parsons, Optical Micromachined Pressure Sensor for Aerospace Applications, Optical Engineering, Vol. 31, No. 8, pp , 199. [7] H. Unzeitig and H. Bartelt, All-Optical Pressure Sensor with Temperature Compensation on Resonant PECVD Silicon Nitride Microstructures, Electronics Letter, Vol. 8, pp , 199. [8] Roger A. Wolthuis, Gordon L. Mitchell, Elric Saaski, James C. Hartl and Martin 5

27 A. Afromowitz, Development of Medical Pressure and Temperature Sensors Employing Optical Spectrum Modulation, IEEE Transctions on Biomedical Engineering, Vol. 38, No. 10, [9] M. Dahlem, J. L. Santos, L. A. Ferreira and F. M. Araujo, Passive Interrogation of Low-Finesse Fabry-Perot Cavity Using Fiber Bragg Gratings, IEEE Photonics Technology Letters, Vol. 13, No. 9, 001. [10] A. Ezbiri and R. P. Tatam, Passive Signal Processing for a Miniature Fabry- Perot Interferometric Sensor with a Multimode Laser-diode Source, Optical Letters, Vol. 0, pp , [11] K. A. Murphy, M. F. Gunther, A. M. Vengsarkar, And R. O. Claus, Quadrature Phase-shifted, Extrinsic Fabry-Perot Optical Fiber Sensors, Optical Letters, Vol. 16, pp , [1] J. L. Santos and D. A. Jackson, Passive Demodulation of Miniature Fiber-opticbased Interferometric Sensors Using a Time-Multiplexing Technique, Optical Letters, Vol. 16, pp , [13] D. A. Christensen, Temperature Measurement Using Optical Etalons, Annual Meeting of the Optical Society of America, Houston, Texas, October 15-18, [14] D. A. Chresitensen, An Optical Etalon Temperature Probe for Biomedical Applications, 8 th ACEMSB, Paper D4a.1, Fairmont Hotel, New Orleans, Louisiana, pp. 0-4, [15] G. Beheim, Fiber-Optic Thermometer Using Semiconductor-Etalon Sensor, Electronics Letters, Vol., pp. 38,

28 [16] C. E. Lee, R. A. Atkins, and H. F. Taylor, Performance of a Fiber-Optic Temperature Sensor from 00 to 1050 C, Optical Letters, Vol. 13, No. 11, pp , [17] L. Schultheis, H. Amstutz, and M. Kaufmann, Fiber-Optic Temperature Sensing with Ultrathin Silicon Etalons, Optical Letters, Vol. 13, No. 9, pp , [18] Gregory N. De Brabander, Joseph T. Boyd, and Glenn Beheim, Integrated Optical Ring Resonator With Miromechanical Diaphragm for Pressure Sensing, IEEE Photonics technology letters, Vol. 6, No. 5, [19] A. Dandridge et al., Frequency Division Multiplexing of Interferometric Sensor Using Phase Generated Carrier, NRL memorandum Report, No. 6457, May [0] K. P. Koo and A. D. Kersey, Bragg Grating-Based Laser Sensors Systems with Interferometric Interrogation and Wavelength Division Multiplexing, Journal of Lightwave Technology, Vol. 13, No. 7, pp , [1] Chi Chiu Chan, Wei Jin and H.L.Ho, Performance Analysis of a Time- Division-Multiplexed Fiber Bragg Grating Sensor Array by Use of a Tunable Laser Source, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 6, pp , 000. [] Y. J. Rao, A. B. Lobo Ribeiro, D. A. Jackson, L. Zhang and I. Bennion, Simultaneous Spatial, Time and Wavelength Division Multiplexed In-fiber Grating Sensing Network, Optics Communications, Vol. 15, pp , [3] Vonbieren Karlhein and Frederick Donald, Coherence Multiplexed 7

29 Interferometric Signal Processing System and Method, Process Control and Quality, Vol. 9, pp. N48-N49, [4] Stewart E. Miller and Ivan P. Kaminow, Optical Fiber Telecommunications, pp , San Diego, CA, Academic Press, [5] B. Mason, G. A. Fish, S. Denbaars and L. A. Coldren, Widely tunable sampled grating DBR laser with integrated electo-absorption modulator, IEEE Photonics Technology Letters, Vol. 11, No. 6, [6] P. Zorabedian and W. R. Trutna, Tunable External-Cavity Semiconductor Lasers, Optical Letters, Vol. 13, pp.86-88,

30 Fig. 1.1 Schematic diagram illustrating an optical-interrogated Fabry-Perot pressure sensor interconnected via an optical fiber [1]. 9

31 Fig. 1. Cross-sectional view of the sensing element []. 30

32 Glass Optical fiber Fig. 1.3 Illustration of two configurations of fiber optically interrogated MEMS pressure sensors. Figure 1(a) shows the usual configuration, which consists of a glass plate with a shallow cylindrical cavity etched into one surface with the cavity covered by a thin silicon diaphragm that has been anodically bonded to the patterned glass wafer [4]. Figure 1(b) shows the configuration where the cavity is formed on the end of the optical fiber and a silicon diaphragm is bonded anodically [3]. 31

33 Fig. 1.4 Pressure sensor measurement system [4]. 3

34 Fig. 1.5 Frequency multiplexing scheme [19]. 33

35 Fig. 1.6 Wavelength Division Multiplexing Bragg grating-based laser sensors [0]. 34

36 Fig. 1.7 Time-Division-Multiplexed fiber Bragg grating sensor array [1]. 35

37 Design of Sensors.1 Introduction The optical Fabry-Perot cavity-based pressure sensors are fabricated using MEMS techniques. There are three different types of pressure sensors fabricated in our lab, as shown in Fig..1, gauge pressure sensors, absolute pressure sensors and differential pressure sensors. Gauge pressure is measured relative to ambient pressure. Absolute pressure is measured relative to a perfect vacuum. And differential pressure is the difference in pressure between two points of measurement. Applied pressure deflects the upper diaphragm and changes the optical path length of the cavity, which is sensed by a change in the reflectivity from the sensor. A broadband light emitting diode (LED) centered at 850 nm is used as the illumination source for the sensor designed for Air Force Research Lab with the pressure range 0-1 psi. For the present demonstration of a multiplexed pressure sensor system, a nearinfrared, tunable diode laser (center wavelength is at 1550 nm) is used to illuminate the sensor with the pressure range 0-30 psi. Since silicon is transparent in this region, the reflectivity from the sensor operating at 1550 nm is quite different than that from the sensor operating at 850 nm. A detailed analysis will be provided for these two types of sensors in this chapter. The Fabry-Perot temperature sensor consists of a thin layer of a material that has a temperature-dependent refractive index. A tunable laser (center wavelength at 1550 nm) is used as light source for the temperature sensor and a wavelength-encoded temperature measurement is used to interrogate the temperature sensor.. Plane Wave Propagation in Homogeneous Media Since light is an electromagnetic phenomenon, its properties and behavior must satisfy Maxwell equations [1] D = ρ (.1) B E = (.) t 36

38 B = 0 (.3) D H = J + (.4) t In these equations, E and H are the electrical field vector (in volts per meter) and magnetic field vector (in amperes per meter), respectively. The quantities D and B are called the electric displacement (in coulombs per square meter) and the magnetic induction (in webers per square meter), respectively. For the case of linear isotropic media with constant properties, the Maxwell equations become H E = μ (.5) t E H = σ E + ε (.6) t These equations indicate a cross-coupling between the electric and magnetic field intensities. Rearranging above equations, we can get the wave equation for charge-free, non-conductive homogeneous media ( σ = ρ = 0 ): V V με = 0 (.7) t Here the scalar function V ( r, t ) represents each of the six field components of E and H, taken in turn. The simplest solutions to wave equation in a homogeneous medium are time-harmonic plane waves which can be described by the complex electric field representation: j ( ω t k r ) E = E o e (.8) Where E o = ( E ( r ), E ( r ), E ( r )) is an amplitude vector having a position ox oy oz dependence. k is called the propagation vector and the magnitude is given by ω π k = = (.9) v λ where v is the phase velocity of the time-harmonic wave at the frequency f = ω / π which depends on the media properties through 1 v = (.10) με For free space the phase velocity takes on the characteristic value 37

39 1 8 1 c = = 3 10 ms μ o ε o The refractive index is defined by o (.11) c ε n = = = ε (.1) r v ε The phase velocity and consequently the refractive index cannot be constants of the medium. In fact, they depend on frequency. The variation of the refractive index with frequency constitutes the phenomenon of dispersion. The dependence of the refractive index on frequency is expressed as [] n Ne 1 = 1 + ( ) m ε ω + jγω ω o o (.13) where γ = damping constant, N = the electron density, m = electron mass, e = electronic charge, ω o = natural frequency. The magnitude and direction of the wave s energy flux are given by the time-average Poynting vector S 1 Re[ * av = E H ], (.14) and the wave s intensity, I, is simply the magnitude of S av...1 Reflection and Transmission at an Interface Between Two Materials The reflection and refraction of a plane wave at a plane interface between two media with different dielectric properties are familiar phenomena. A plane wave incident on the interface will, in general, be split into two waves: a transmitted wave and a reflected wave. The relationship between the three waves is governed by the boundary conditions. Suppose the xy plane is the plane of incidence and let the z-axis be directed from medium 1 with refractive index n 1 into medium with refractive index n, shown in Fig... The incident wave in the E-field representation is therefore [3] E i o ω j [ ω t ( x sin θ + z cos θ )] j ( ω t k r ) v 1 = E o e = E e (.15) The transmitted wave is ω j [ ω t ( x sin θ cos φ + y sin θ sin φ + z cos θ )] j ( ω t k r ) v = E o e E = E e (.16) t o 38

40 The reflected wave is ω j [ ω t ( x sin θ cos φ + y sin θ sin φ z cos θ )] j ( ω t k r ) v 1 = E o e E = E e (.17) r o Satisfaction of Maxwell s equations requires continuity of the tangential components of Ē and Ħ across the interface. Now, applying tangential boundary condition ˆ 1 = z ( E E ) 0 (.18) where E = E in E E =, and 1 r t E In order to satisfy the tangential boundary condition, three conclusions must be reached: 1. No frequency change is produced by reflection and transmission at an interface. ω = ω = ω (.19). Transmitted, reflected and incident waves are coplanar. φ = φ = 0 (.0) 3. Snell s law: n 1 sin θ = n sin θ (.1) and Law of reflection θ = θ (.) Continuity of the tangential E and H components also determines the amplitude reflection and transmission coefficients r = E r /E i and t = E t /E i. However, the amplitudes coefficients depend on the direction of polarization of the incident wave as well as the angle of incidenceθ. Since E lies in a plane transverse to its direction of propagation, it is conveniently resolved into: a component E s perpendicular to the plane-of-incidence shown in Fig..3, and a component E p parallel to the plane-of-incidence shown in Fig..4. Applying tangential boundary conditions in each case, we can get the Fresnel s equation for non-magnetic media [1] r p E p n cosθ + n1 cosθ = =, E n cosθ + n cosθ p 1 (.3) Es n1 cosθ n rs = = E n cosθ + n t p s 1 E p = = E n p cosθ, cosθ n1 cosθ, cosθ + n cosθ 1 (.4) (.5) 39

41 Es n1 cosθ t = = (.6) s E n cosθ + n cosθ s 1.. Reflection from Isotropic Layered Media The expressions for transmission and reflection coefficients from isotropic layered media can be derived by summing the amplitudes of successive reflections and refractions. Such derivation was first carried by G.B.Airy in His method is described as follows. Consider a beam incident from the left (see Fig..5). The incident beam is partially reflected and partially transmitted at the first interface. The transmitted part is subsequently reflected back and forth between the two interfaces as shown. The reflection and transmission coefficients can be obtained by adding the amplitudes of the successive reflected and transmitted rays. In doing this, it is important to include the phase factor that accounts for the geometric path difference between any two successive reflected or transmitted rays [4]. First, consider the reflection of light from a Fabry-Perot interferometer, which, as shown in Fig..5, is comprised of a single film having a thickness of d and refractive index of n 1. The phase difference is given by -φ, with φ given by: φ = π (ndcosθ ) / λ (.7) For normal incidence φ 4πn d λ = (.8) Let the amplitude of the incident wave be 1. Taking the phase difference into account as a factor and adding the amplitudes of the reflected rays, we obtain the reflection and transmission coefficients for normal incidence from Eqs. (.4) and (.6). At 1- interface n n = (.9) n0 + n1 r r n n = (.30) = r1 n0 + n1 40

42 n t = + r (.31) 0 1 = 1 n0 + n1 1 1 = 1 n0 + n1 1 n t = + r (.3) At -3 interface r n + n = (.33) n1 n r n 1 3 = (.34) n1 + n We obtain the reflected electric vector by adding the amplitudes of the reflected rays: E r = r = r t + t t r 3 ( r 3 e e jφ t1 jφ + t 1 )(1 + r j We set α = r r e ), φ 1 ( 3 ( r 1 3 ( r e 3 jφ ) r1t1 e jφ ) + r 1 + t ( r 1 3 e ( r 3 jφ ) e jφ 3 + r ) ( r ( r ) 3 e t 1 jφ 3 + L ) + L) (.35) Sum = 1+ α + α + α +L So, reflected electric vector can be expressed as E r Since jφ ( 3 = r + t t r e ) Sum 3 4 α = α + α + α + α L 3 (.36) Sum (.37) Combining Eqs. (.34) and (.35) ( 1 α ) Sum = 1 (.38) and Sum = = = (.39) jφ 1 α 1 r + r r e jφ 1( r3e ) 1 The reflected electric vector is

43 E r r 1 = r + r where t t t 1 1 ( r r3e + 1+ r r 3 e jφ jφ jφ r3e jφ 1 3e r ) Sum = (1 r r 1 r3e 1+ r ) r1 + r3e = 1+ r r e 1t1 1+ r1)(1 + r1) = (1 + r1)(1 r1) = jφ + t1t1r3 jφ 1r3e jφ jφ 1 e jφ = (.40) = ( r (.41) The corresponding reflectance R(λ) is found from the square modulus R jφ * r1 + r3e ( λ) ErEr = jφ 1 + r1r3e E r E r = (.4) Now, consider the reflection of light from a Fabry-Perot interferometer, which, as shown in Fig..6, is comprised of two films having a thickness of d, refractive index n 1 and thickness of t, refractive index of n respectively. First considering the region, 3 and 4 as a simple Fabry-Perot interferometer with one layer, the electrical field vector from this one layer Fabry-Perot interferometer is jθ r3 + r34e E r = (.43) jθ 1+ r3r34e with r 3 given by Eq. (.31) r 34 n n 3 = (.44) n + n 3 θ 4πn t λ = (.45) 0 Second, considering the region 1, and 3 as a simple one layer Fabry-Perot interferometer with interferometer is φ 4πn d E r substituting r 3. The phase difference for this Fabry-Perot 1 = (.46) λ0 From Fig..6, we can see that the reflected electric vector from the Fabry-Perot interferometer with two layers is 4

44 E r jφ jφ jφ 3 = r + t t E e + t t r ( E e ) + t t r ( E e ) + L r1 + Ere = 1+ r E e = r e 1+ r 1 1 r jφ 1 jφ jφ r r3 + r e ( 1+ r r r3 + r ( 1+ r r jθ 34 jθ 3 34e jθ 34e ) jθ 3 34e ) r r (.47) with r 1 given by Eq. (.9) Using * ( λ ) E E r r R = interferometer R( λ), we can get the reflectance from the two layer Fabry-Perot jθ jφ r3 + r34e r1 + e ( ) jθ 1 + r3r34e = (.48) jθ jφ r3 + r34e 1 + r1e ( ) jθ 1 + r3r34e The reiterative reflectance formula can be used reiteratively to calculate the reflectance of any multiplayer Fabry-Perot interferometer. For example, the reflectance for l-layer Fabry-Perot cavity is [5] R( with E jφ l r1 + e E l 1 λ ) l = (.49) jφl 1 + r 1e El 1 l 1 r = jφl e El jφl e El E l = L M (.50) This procedure is continued, again and again, until the final reflected electric vector is that of the one layer Fabry-Perot interferometer. The method described above can be used to solve for the amplitudes of electric and magnetic fields of any isotropic layered media. However, when the number of layers becomes too large, the analysis becomes very complicated because of the large number of 43

45 equations involved. The matrix method is especially useful when a computer is available that can handle the matrix algebra. Referring to Fig..7, we now consider the case of multiplayer structures. The dielectric structure is described by [6] n0 x < x0, n1 x0 < x < x1, n x1 < x < x, n( x) = { M M (.51) nn xn 1 < x < xn, n x < x, s N where n l is the refractive index of the l th layer, ns is the position of the interface between the l th layer and the ( l +1)th layer, ns is the substrate index of refraction, and n0 is that of the incident medium. The layer thickness d l are related to the x l s by: d d 1 = x1 x0, = x x M 1, d (.5) N = xn xn 1. The electric field of a general plane wave solution of the wave equation can be written as: E i( ωt βz ) = E( x) e (.53) The electric field E(x) consists of a right-traveling wave and a left-traveling wave and can be written as ik x ik xx E = Re x + Le A( x) + B( x) (.54) So, the electric field distribution E (x) for multiplayer structure can be written as A0e E( x) = { A e where klx A e ik 0 x ( x x ) iklx ( x xl ) l ikns ( x xn ) s 0 + B 0 e + B e l + B e s ik ik 0 x ( x x ) 0 ik ( x x ) lx Nx l ( x x is the x component of the wave vectors N, ),, x l 1 x < x x 0, < x < x N < x, l, (.55) 44

46 ω 1/ klx = [( nl ) β ], l = 0, 1,,, N, s, (.56) c and is related to the ray angle θ l by ω klx = nl cos θl (.57) c According to Eqs. (.54) and (.55), Al and B l represent the amplitude of plane waves at interface x = x l. and expressed as A B 0 0 = D 1 0 Al = Pl Dl Bl A D1 B 1 D 1 1 l+ 1, Al Bl where N+1 represents s, , l = 0, 1,,, N (.58) A = A B +1 = B and the matrices can be written as N +1 s, N s l l Dl = nl cosθl nl cosθl for s wave (.59) and cosθ cosθl n l Dl = nl respectively, and P j e 0 φl 0 l = jφ with l lx l e l l for p wave, (.60) (.61) φ = k d (.6) A B The relation between A 0, B0 and 0 0 M = M 11 1 M M 1 with the matrix given by M M M As Bs N Do [ Dl Pl Dl 1 M = l = 1 A s, B s can thus be written as, (.63) ] D s (.64) 45

47 Here we recall that N is the number of layers, A0 and B 0 are the amplitudes of the plane waves in medium 0 at x = x0, and A s, B s are the amplitudes of the plane waves in medium s at x =. xn The reflection and transmission coefficients of multiplayer structure are defined as: M M 1 r = (.65) 11 and 1 t = M (.66) 11 Reflectance is given by: M 1 = r (.67) R = M 11 For the simple one layer Fabry-Perot interferometer M11 M1 1 1 = D1 DP D D3 M 1 M For a two layer Fabry-Perot interferometer (.68) M11 M = D1 ( DP D D3P3 D3 ) D4 M 1 M (.69) Substitute M 1, M 11 into Eq. (.67), the results are the same as Eqs. (.4) and (.48)..3 Design of Ideal Fabry-Perot Cavity-based Pressure Sensor The Fabry-Perot cavity-based pressure sensor (shown in Figure.1) is a Faby-Perot interferometer consisting of two parallel, partially reflecting surfaces separated by a gap. Light is introduced into the Fabry-Perot cavity through the optical fiber. Multiple reflections within the cavity are carried back through the fiber. One of the reflecting surfaces is sensor s diaphragm, which deflects in response to a change in pressure. The diaphragm deflection changes the depth of the cavity and thus the conditions for optical interference when light is reflected from successive reflections of the initial beam. As the 46

48 diaphragm moves in response to pressure, the Fabry-Perot reflectance changes accordingly. We chose the size of the diaphragm as R 0 = 300 μm which is compatible with testing and MEMS fabrication. A single-mode 9/15 μm (core diameter/cladding diameter) step-index fiber with NA = 0.13 is used for multiplexed pressure sensor, a multimode 100/140 μm step-index fiber with NA = 0.14 is used for the pressure sensor operating at 850nm. Since the size of optical fiber is much smaller than the diaphragm, the diaphragm deflection can be considered as uniform throughout the fiber endface; this simplifies sensor s calculation. Moreover, a little misalignment of the optical fiber to the center of the diaphragm during sensor packaging will not deteriorate the performance of the sensor. When the light source for the sensor is an LED operating at λ = 850 nm, the pressure sensor can be considered as a simple Fabry-Perot interferometer with one resonator layer. Refractive indices of glass, air, and silicon are denoted as n 0 = n glass (=1.474), n 1 = n air (=1.0), and n = n si (=3.46) respectively and the cavity depth is denoted as d. Here, we have assumed normal incidence of light. And the reflectance from the Fabry-Perot pressure sensor is given by Eq. (.4). This results in a periodic variation of the reflectance as a function of the cavity depth d (Figure.8). The depth of the etched cavity in the glass wafer must be greater than λ / 4, but to avoid ambiguities, must be small enough to yield a cavity where free spectral range (the wavelength range corresponding to a phase change 0 ΔΦ = π is termed the free spectral range, Δ λfsr ) [] somewhat greater than the LED linewidth of 100 nm. That is, the spectral width of LED imposes an upper bound for the cavity depth. It is assumed here that the source spectrum is a subset of the analyzer s range. For moderately large fringe order m, the free spectral range is FSR m Δ λ λ / Λ (.70) OPD The above requirement can then be written as m Δλ λ / Λ. (.71) LED OPD A free spectral range of 100 nm corresponds to a cavity depth of 3.6 μm. We chose a cavity depth of 0.64 μm to lie within these two limiting values. As the applied pressure 47

49 increases, cavity depth decreases, the sensor reflectance decreases in a monotonic fashion, shown in Fig..8. When the light source for the sensor is a tunable laser operating at center wavelength λ = 1550 nm, silicon is transparent. Therefore, the silicon diaphragm constitutes a second Fabry-Perot resonator. The reflectance from the Fabry-Perot pressure sensor is given by Eq. (.45) with n 0 = n glass (=1.473), n 1 = n air (=1.0), n = n si (=3.478) and n 3 = n air (=1.0) respectively. The reflectance as a function of the cavity depth d is shown in Fig..9 with silicon diaphragm thickness at t = 0 um. The reflectance from the sensor is also a periodic variation as a function of the silicon diaphragm thickness, as shown in Fig..10 with cavity depth at d 0 = 1.1 um. Fig..11 shows the reflectance as a function of cavity depth and silicon thickness. From Fig..11, we can see that in order to decide the initial cavity depth of the sensor, we need to decide the silicon diaphragm thickness first. The silicon diaphragm is modeled as a circular membrane. The deflection of the diaphragm due to the application of pressure P is given by [6, 7] 4 3PR 0 (1 ν ) r w( r) = 3 1 (.7) 16Et R0 where r is the distance from the center of the plate; w (r) is the deflection at r ; P is the normal pressure; R 0 is the radius of the diaphragm; ν is the Poisson s ratio for silicon, E is the Young s modulus, and t is the diaphragm thickness. The diaphragm deflection under the pressure can be simulated using Mathematic 4.0, which is shown in Fig..1. To optically interrogate the pressure sensor shown in Fig..1, an optical fiber is aligned in the center of the diaphragm. Since the size of the optical fiber is much smaller than diaphragm, the diaphragm deflection can be considered as uniform throughout the fiber endface. So, cavity depth d of the sensor is a function of the maximum diaphragm deflection w defined as d = d 0 w (.73) where d 0 is the initial etched cavity depth. The maximum diaphragm deflection is obtained from Eq. (.70) when r = 0 4 PR0 w = (.74) 64 D 48

50 where D is the flexural rigidity expressed by 3 Et D = (.75) 1(1 ν ) The maximum cavity depth change is designed for a deflection of λ 0 / 4 where λ 0 is the operating wavelength. Thus, the pressure that causes a deflection of λ / 4 is given by 4Et λ0 P =. (.76) 3 R (1 ν ) The pressure sensor operating at 850 nm is designed to respond over the pressure range 1-1 psi. Calculations indicate that at λ 0 = 850 nm, the diaphragm thickness required for making a diaphragm-center-deflection of λ / 4 at 1 psi is 14 μm. The intention of this design where we use a thinner silicon diaphragm is to obtain a greater response. The multiplexed pressure sensor operating at 1550 nm is designed to respond over the pressure range 1-30 psi. Calculations indicate that at λ 0 = 1550 nm, the required diaphragm thickness for making a center deflection of λ / 4 at 30 psi is μm. The depth of the etched cavity in the glass wafer must be greater than λ / 4. From Fig..8, we choose a cavity depth at d 0 = 1.1 μm. As the applied pressure increases, the reflectance from the sensor decreases in a monotonic fashion. It is important to investigate if ultra-thin silicon can withstand the strains resulting from the above pressure ranges. Diaphragm theory [8] can be used in predicting the maximum tolerable pressure P m at maximum tensile stress (σ m ) at the center of diaphragm, and the maximum pressure P o at which strain versus pressure relation remains linear. By modeling silicon as a clamped circular diaphragm, P m and P o can be related to sensor parameters by the following equations: P m and 8 t = σ m 3(1 ), (.77) + ν R

51 P o 8E = 3(1 ν ) t R 0 4, (.78) where E is Young s modulus of silicon, ν is Poisson s ration of silicon, R 0 is the radius of the diaphragm (or the cavity), and t is the diaphragm thickness. Though the calculated thickness of silicon diaphragm in one of the sensors is very small (14 μm), its maximum stress is about three orders less than the yield strength of silicon ( dynes/cm ). The maximum pressure Po at which strain versus pressure relation remains linear is about 400 psi. This ensures that these membranes can perform very well for many pressure sensors application..3.1 The Dynamic Properties of the Fabry-Perot Pressure Sensor Static pressure is measured under steady-state or equilibrium conditions, but most real-life applications deal with dynamic or changing pressure. The most important parameter for the dynamic measurement of the sensor is the natural (resonant) frequency of the sensing element and response time. The natural frequency of a sensor determines the dynamic capabilities that a sensor can measure. As we know, when a pressure sensor is working near the resonance frequency, the membrane deflection is much higher so that it is easily damaged. Generally, the operating frequency range for the sensor should be selected well below (at least 60%) or above the natural frequency. The natural frequency f of the circular plate is a function of the dimensionless parameter κ [9] 0 1/ 3 κ ij Et f n = (.79) πr 1ρ(1 ν ) where R 0 is the radius of the sensor diaphragm, t is the diaphragm thickness, E is the Young s modulus, ν is the Poisson s ratio and ρ is the mass per unit area for silicon. κ = 10. for the fundamental mode of the circular plates with clamped edge, which is shown in Table.1. Calculation indicates that the natural frequency of the fundamental mode for Fabry-Perot pressure sensor (radius of the diaphragm: R 0 = 300 μm, thickness of silicon diaphragm: t = 5 μm) is 19.8 khz. 50

52 To measure changing pressure, the frequency response of the sensor must be considered. Frequency response is an important dynamic characteristic of a sensor as it specifies how fast the sensor can react to a change in the input stimulus. As a rough approximation, the sensor frequency response should be 5-10 the highest frequency component in the pressure signal. The frequency response is defined as the highest frequency that the sensor will measure without distortion or attenuation [10]. Sometimes the response time is given instead. For a first-order system, they are related as follows f = 1/ πτ (.80) B where f B = frequency where the response is reduced by 50% τ = time constant where the output rises to 63% of its final value following a step input change This value is usually obtained by experiment. From above calculation, we can see that the dynamic property of the pressure sensor can be improved by reducing the size of the sensor and increasing the diaphragm thickness..3. Calculation of the Light Loss Inside the Sensor Light is supplied into the sensor and the reflected light from the sensor is coupled back by using an optical fiber. Part of the light will be lost inside the sensor due to the numerical aperture of the optical fiber and the light divergence. The single-mode fiber used in the multiplexed pressure sensor system has the size: 9/15 μm (core diameter/cladding diameter) and NA = The multimode fiber used for the sensor operating at 850 nm has the size 100/140 μm and NA = 0.3. The numerical aperture, NA, is a measure of how much light can be collected by an optical system and is defined as [11] NA = Sin( θ c ) (.81) where θ c is the critical angle. Light divergence in the sensor cavity is w air = dtan( θ c ) (.8) 51

53 At the interface of the sensor cavity and silicon diaphragm, Snell s law can be expressed as n Sin ) n Sin( θ ) si ( si air c θ = (.83) So, light divergence in the silicon diaphragm is w si = ttan( θ si ) (.84) At the interface of the silicon diaphragm and air, Snell s law can be expressed as n glass Sin θ ) = n Sin( θ ) (.85) ( glass air c So, light divergence in glass wafer is w = θ ) (.86) glass ltan( glass where w air is the light divergence in the sensor cavity, w si is the light divergence in the silicon diaphragm, w glass is the light divergence in the glass wafer, measured by the angles θ c, θ si and θ glass in Fig.6 and where d is the sensor cavity depth, t is the thickness of the silicon diaphragm and l is the thickness of glass wafer. From geometrical considerations in Fig..13, the maximum power that can be coupled back into the single-mode fiber at designed cavity depth d = 1.1 μm and silicon diaphragm thickness t = μm, glass wafer thickness l = 50 μm and radius of the single-mode fiber core region a = 4.5 μm is P P out in = π (w air πa + w + w si glass + a) =.6% (.87) The calculation is based on the assumption that light is uniformly distributed in the core region of the optical fiber. So that the power which can be coupled back into fiber is determined by the area of input light and output light from the sensor. For a single-mode fiber with Gaussian power distribution, the irradiance is symmetric about the beam and varies radially outward from this axis with the form [11] 0 r / r1 I ( r) = I e (.88) 5

54 r 1, by definition, is the radius of the beam where the irradiance is 1/e of the value on the beam axis I 0. So, the maximum amount of optical power that can be coupled back into the fiber is larger than the value from the above calculation. For multimode fiber, the designed cavity depth d = 0.64 μm and silicon diaphragm thickness t = 14 μm, glass wafer thickness l = 50 μm and radius of the multimode fiber core region a = 50 μm. The maximum power that can be coupled back into the multimode fiber is P P out in = π (w air πa + w + w si glass + a) =.75% (.89) We can see from the above calculation that the maximum optical power that can be coupled back into single-mode fiber is significantly lower than that for a multimode fiber and the loss inside the sensor can be reduced greatly if we reduce the thickness of the glass wafer..3.3 Initial Diaphragm Deflection For a gauge pressure sensor, atmospheric pressure is hermetically sealed inside the cavity during silicon-glass anodic bonding. During the process of anodic bonding, a 7740 Pyrex glass wafer with cavities is placed on top of a silicon wafer, heating this sandwich up to 400 C and applying a large mechanical pressure to bring them into intimate contact. This may be done by applying a large voltage ( V) on the glass and the silicon wafer for a time period long enough to allow the current to settle at a steady state minimized level. The bond is formed by silicon bonds to oxygen in the glass. The residual gas generated during the anodic bonding process and that desorbed from the silicon and glass surface increase the pressure in a sealed cavity [1]. So, after bonding process is finished, the pressure inside the cavity P is no longer atmospheric pressure P 1 = 1 atm. The difference between P 1 and P will cause the deflection of sensor diaphragm until the pressure inside the cavity is equal to the atmosphere P 1 = P = 1 atm. We will observe the Newton s ringes from the sensor under the microscope to determine the initial diaphragm deflection. For an absolute pressure sensor, anodic bonding of glass-silicon in vacuum is used to obtain vacuum sealed cavity. However, it is not possible to make a high vacuum cavity 53

55 by this method. Two residual gas sources which pose a problem for vacuum sealing have to be considered. One is gas generation during the anodic bonding process [13]. The other is gas desorption from the inner surface of the sealed cavity. A starting pressure of typically 10-5 mbar results in a final internal cavity pressure of 1 mbar in the best case [14]. This problem does not exist for differential pressure sensors. Because the residual gas generated during anodic bonding is evacuated through an opening to the atmosphere. Eq. (.74) shows a linear dependence between the center diaphragm deflection and the applied pressure, P. This simple expression is practical for estimation, but must be handled with care. It is valid only for a case where the change of pressure inside the cavity can be neglected when the diaphragm bends, i.e., for cavities open to the air and for small diaphragm bending. For thicker diaphragms, i.e., for higher pressure ranges, the change of pressure inside the cavity is not significant compared to the applied pressure, so this approximation may be used. But for gauge pressure sensors, if the diaphragm bending is under pressure close to atmospheric pressure with hermetically sealed cavities, one has to take into consideration the increasing pressure inside the cavity due to the decreasing cavity volume. This means that the pressure P in the treatment above is to be considered as the difference, P out -P in, between the pressures outside and inside the cavity respectively [15]. The gauge pressure sensors fabricated in our lab are designed to respond over the pressure range 1-30 psi. At 30 psi, the center diaphragm deflection is λ / 4. The diaphragm radius is R 0 = 300 μm, the cavity depth is d 0 = 1.1 μm and center wavelength of the light source is λ 0 = 1.55 μm. Assuming the temperature is constant: T 1 = T and atmosphere pressure inside cavity without pressure applied, P 1 = 1atm. The volume inside the cavity before diaphragm bending is 1 0 d0 V πr = (.90) The volume inside the cavity under diaphragm bending is 1 V = πr0 d0 π ( λ0 / 4) (3R λ0 / 4) (.91) 3 where 0 54

56 R0 + ( λ0 / 4) R = (.9) ( λ / 4) 0 According to Boyle s law, we can get the pressure inside the cavity under 30 psi, which is shown in Fig..14 P =.P 1. atm 1 1 = The change of pressure inside the cavity under 30 psi is Δ P = 0.atm =. 94 psi Since ΔP<<30 psi, the change of pressure inside the cavity due to the diaphragm bending can be neglected. This problem does not exist for absolute pressure sensors and differential pressure sensors. Finally, we must be aware that Eq. (.74) is valid only for small pressures such that the diaphragm is the only part of the device which is influenced. For higher pressures, the whole device body may be deformed depending on the mounting method..3.4 Anti-reflection Layer For multiplexed pressure sensors operating at 1.55 μm, part of the light will be propagating in silicon. In applications, the sensor will be exposed to variable temperature as well as pressure. Since the refractive index of silicon has significant temperature dependence, this will cause a problem of cross sensitivity to pressure and temperature for the pressure sensor. One solution for reducing and eliminating the sensor s temperature dependence is to utilize a proper coating as an anti-reflection layer on the top surface of silicon diaphragm, shown in Fig..15. It is known that when an electromagnetic radiation reaches a dielectric boundary between two media of different refractive indices, n1 and n, respectively, Fresnel reflection occurs. The reflection is a result of the dielectric discontinuity and can be reduced or even eliminated by the proper coatings of a few layers of homogeneous material to eliminate reflections. The intensity of reflected electromagnetic radiation at a boundary between two dielectric media is given by the reflectance 55

57 R ( n n1 ) = (.93) ( n + n1 ) The single homogeneous layer antireflection coating is the simplest dielectric structure to achieve this goal. For example, on the top surface of the silicon diaphragm, we can coat a thin layer of a dielectric material, such as SiO (silicon monoxide), that has an intermediate refractive index. Fig..16 illustrates how the thin dielectric coating reduces the reflected light intensity. In this case, n 1 (Si) = 3.478, n (SiO) = 1.86, and n 3 (air) = 1 when the light source is at 1.55 μm. Light is first incident on the Si/SiO surface and some of it becomes reflected and this reflected wave is shown as A in Fig..16. Wave A has experienced a 180 phase change on reflection as this is an external reflection. The wave that enters and travels in the coating (SiO) then becomes reflected at the SiO/air surface. This wave, which is shown as B in Fig..16, also suffers a 180 phase change since n 3 > n. When wave B reaches A, it has suffered a total delay of traversing the thickness d of the coating twice. The phase difference between A and B is φ = 4πn d λ Δ (.94) where λ is the free-space wavelength, d is the thickness of the coating. To reduce the reflected light, A and B must interfere destructively and this requires the phase difference to be π or odd-multiples ofπ, mπ in which m = 1, 3, 5, is an oddinteger. Thus [16] πn λ d = mπ or λ d = m 4n Thus, the thickness of the coating must be multiples of the quarter wavelength in the coating and depends on the wavelength. When sensor is operating at 1.55 μm, the (.95) thickness of the antireflection coating (SiO) is obtained from Eq. (.95) when m = 1, because the thickness of the coating is limited when a sputtering or evaporation process is used. So, d = λ 10nm 4n (.96) 56

58 To obtain a good degree of destructive interference between wave A and B, the two amplitudes must be comparable. It turns out that we need [] n = (.97) n1n3 n = then the reflection coefficient between the silicon and coating is n n When 1 3 equal to that between the coating and the air. In this case we would need an index of or In practice, materials with such an index of refraction may not exist. Nevertheless, using available materials with an index of refraction close to that given in Eq. (.97), a great reduction in reflectance is obtained. Thus, SiO is a good choice as an antireflection coating material on a silicon diaphragm. After applying a thin antireflection coating on the top surface of the pressure sensor, the reflectance from the sensor is obtained from Eq. (.49) for a three-layer Fabry-Perot interferometer. When the condition for antireflection coating is met, the reflected wave A from Si/SiO interface is canceled out by the reflected wave B from SiO/air interface. So, the reflectance from the sensor can be considered to be from a simple one layer Fabry- Perot interferometer, which is obtained from Eq. (.40). But, the condition for antireflection coating is wavelength dependent, as shown in Fig Design of Ideal Fabry-Perot Temperature Sensor A model of the thin-film Fabry-Perot temperature sensor will be developed in this section. This model will provide a basis for a preliminary sensor design, which consists of the selection of a material and the determination of the optimum thickness range. A Fabry-Perot temperature sensor usually consists of a thin layer of a material that has a temperature-dependent refractive index. The temperature changes the optical path of successive reflected beams, so the reflectance from the sensor changes accordingly. The reflectance from the sensor is a function of wavelength. A wavelength-encoded temperature measurement can be provided by the Fabry-Perot temperature sensor. At resonance, the reflectivity of the sensor is a minimum, and the wavelength of one of these reflectivity minima can serve as a signal-level-insensitive indicator of temperature. 57

59 The Fabry-Perot temperature sensor has a layer of film with thickness of d and a refractive index of n 1, which is temperature-dependent. It is surrounded by a material of index of n 0 and n. The reflectance from this sensor is given by Eq. (.4) R jφ r1 + r3e ( λ) jφ 1 + r1r3e = (.98) where: 4πn φ 1 d = (.99) λ The reflectance is minimized at resonance, or φ = mπ, where m is an integer. In terms of wavelength, the resonance condition is φ = 4πn1d λ m1 = mπ where λ m1 is the resonance wavelength at room temperature. When the temperature (.100) increases, the refractive index of n 1 changes to n 1. According to Eq. (.98), the resonance condition doesn t change, or φ = mπ, but the resonance wavelength will be shift to λ m. 4πn d λ m1 = πn1 d λ m = m π Assuming the thermal expansion coefficient of the thin film is neglected, d 1 = d, the above equation can be simplified as n λ n (.101) 1 1 = (.10) m1 λ m By measuring resonance wavelengths at room temperature and elevated temperature respectively, refractive index n 1 at an elevated temperature can be obtained from Eq. (.10), since the refractive index n 1 at room temperature can be obtained from literature. This wavelength-based measurement method, because it is signal-level insensitive, has a high degree of immunity to the effects of changes in the transmissivities of the optical fibers and connectors. 58

60 .41 Selection of Temperature Sensitive Material As the first step in the selection of a thin-film interferometer material, the temperature sensitivities of various candidate materials will be compared. This requires a sensitivity figure-of-merit which is independent of the film thickness. Assume that the temperature is determined as a function of one of the resonant wavelength λ. [17] The temperature sensitivity of the Fabry-Perot temperature sensor is determined by k φ = k n + k L (.103) where, k n is the thermo-optic coefficient, 1 dn = (.104) n dt 1 k n 1 and k L is the thermal-expansion coefficient, 1 dl k L = (.105) L dt Table. ranks the candidate materials in descending order of k φ. This table also m provides n, k and k, together with the wavelengths at which the optical properties were n L measured. For all the optical materials listed in Table., k >> k, so the temperatureinduced resonance wavelength change is almost entirely caused by the change in refractive index and the thermal expansion effect can be neglected. The temperature sensitivity k n is greatest for the high-index semiconductors, which are, in descending order of sensitivity, GaAs, Ge and Si. For these semiconductors, dependent. With decreasing λ,. for silicon. n L k n is highly wavelength k n generally increases, as shown by the entries in Table Of the three materials with the largest values of k φ, silicon is best suited for this application. The material with the highest temperature sensitivity GaAs, is not sufficiently stable at the temperature exceeding 50 C. Evaporation of arsenic from GaAs has been detected after only 10min at 450 C [31]. Germanium, which has the second highest k φ, is readily sputter-deposited, however, it has very small bandgap (0.67ev) and is highly absorbing at the wavelength range of the tunable laser in our lab (1510 nm 59

61 1580 nm). At 1550 nm, the absorption coefficient of germanium is cm -1 [7]. For this reason, silicon is preferable for the sensor application at 1550 nm region, because light is transparent to silicon in this wavelength range. In addition to a high k φ, the Fabry-Perot material should have a high refractive index. A higher value of n provides a larger reflectance, which increases the maximum reflectivity and the maximum phase sensitivity. Based on the above considerations and the fact that silicon can be bonded to a Pyrex glass wafer, silicon is determined to be the best material for the fabrication of a thin-film Fabry-Perot temperature sensor. Moreover, silicon is becoming increasingly important in the optoelectronic device field due to its well developed and inexpensive technology allowing an effective integration between optical and electronic devices. Using silicon as the material for a Fabry-Perot temperature sensor will reduce the cost for each sensor and provide the potential for compatibility with other devices. The configuration of the Fabry-Perot temperature sensor using silicon as a thin-film interferometer is shown in Fig..18. For temperature interrogation, the effect of temperature on the refractive index of the silicon should be estimated. It was found that the refractive index of silicon is a linear function of temperature at all wavelengths, but the coefficient of the linear term is a function of wavelength. The equation for representing thermo-optic coefficients of silicon is expressed as [3] dn n = GR + HR (.106) dt with the normalized dispersive wavelength R defined as: λ R = (.107) λ λ ig In the above equations, n and dn / dt are the room-temperature refractive index and its derivative with respect to temperature and λig is related to the E ig, the isentropic band gap. Optical constants G and H are given in [3]. The sensor s sensitivity to temperature is 1 5 n dn / dt = / C [3]. 60

62 From the above analysis, we can see that the silicon refractive index can be traced back to the temperature experienced by the Fabry-Perot temperature sensor. At room temperature, the silicon refractive index is equal to at 1550 nm [7]. The following subsection will develop the criteria used to determine the thickness of the temperaturesensitive silicon film..4 Determination of the Fabry-Perot Temperature Sensor s Thickness The design principle of Fabry-Perot temperature sensor is similar to that of the pressure sensor because both sensors are based on the Fabry-Perot interferometer. However, since we have used a different measurement system for temperature sensor, some design criteria in tracking of spectral resonance is outlined below. For temperature sensors, we are using wavelength-encoded temperature measurement, explained in section.41. From Eq. (10), we can see that with temperature increasing, silicon refractive index increases, causing the reflectance spectral shift to longer wavelength, as shown in Fig..19. But in order to avoid ambiguity, the wavelength shift under the temperature must be less than the distance between two neighboring resonance wavelengths for the Fabry-Perot temperature sensor. We assume λ m1, λ m1+ 1 which satisfy the resonant condition are the two neighboring resonance wavelengths at room temperature 4πn1d λ m1 = m π (.108) and 4πn1 d = ( m + λ m1+ 1 1) π (.109) With temperature increasing, the resonance wavelength λ m1 shifts to λ m the resonance condition at elevated temperature, which satisfies 4πn λ 1 m d = mπ (.110) where n1 is the silicon refractive index at elevated temperature. In order to avoid ambiguity, the condition for wavelength shift must be satisfied, as shown in Fig..19: 61

63 λ λ > λ λ m1 1 m1 m m1 + (.111) Combining Eq. (.108), (.109), (.110), and substitute λ m1+ 1, λ m into Eq. (.111), we get λ m Δ n = n1 n1 < (.11) 4d The resonance wavelength at room temperature we are choosing is around 1550 nm. So, from Eq. (.11), we can get the conclusion that the temperature range of the Fabry-Perot temperature sensor is only determined by the silicon thickness, as shown in Fig.0. Since the refractive index of silicon is a linear function of temperature at all wavelengths, the temperature range of the Fabry-Perot sensor is an inverse function of silicon thickness. For example, if the thickness of silicon is 15 μm, from Eq. (.11), we can get 1550 nm Δn < 4 15 μm = From Eq. (.106) and (.107), this is corresponding to the temperature range: 4 C -35 C. If the thickness of silicon is 15 μm 1550 nm Δn < 4 15 μm = This corresponds to the temperature range: 4 C -115 C. The sensitivity of the wavelength-encoded temperature measurement is determined by the resolution of the tunable sensor. The tunable external-cavity diode laser (New Focus 638 Velocity Laser) used in our lab has the resolution of 0.01 nm. This corresponds to the temperature resolution of 0.07 C for the Fabry-Perot temperature sensor..4.3 Design of Encapsulant for Temperature Sensor A critical function of the Fabry-Perot sensor s encapsulating layers is the protection of the silicon surface from oxidation, dirt and other contaminants. The encapsulant should therefore be opaque, to prevent the guided light from interacting with contaminants and also to block any external light. Aluminum has proven to be a superior coating for the environment protection at temperatures as high as 400 C. It was determined to use aluminum as the thin-film 6

64 interferometer s outermost encapsulating layer, since it can provide the required lightblocking and reflectance-enhancing functions. Since the oxide of aluminum, Al O 3, is strongly adherent and highly impermeable, aluminum oxidizes very slowly [33]. Therefore, a dense aluminum coating ( 1 μm) could protect the silicon layer from oxidation. However, aluminum deposited on a low-temperature substrate by evaporation or sputtering typically has a columnar structure [34]. A high reliance, therefore, was not placed on the oxidant-blocking abilities of this type of film. In any event, an intermediate diffusion barrier is required to prevent dissolution of the silicon in the aluminum, as the solubility of silicon in aluminum, at 300 C, is 0.1% by weight [35]. Silicon nitride oxidizes very slowly and is largely impermeable to oxygen and water [36]. For these reasons it is widely used as a mask for the selective oxidization of silicon [37]. Dense stoichometric Si 3 N 4 films can be deposited at temperatures well below 300 C using reactive sputtering [38]. For Fabry-Perot temperature sensors, a layer of silicon nitride will be used as a diffusion barrier between the silicon and aluminum. The optical properties of the three-film sensor structure, Si/Si 3 N 4 /Al, will be considered in the design of the encapsulation structure. The configuration of the encapsulation structure for a Fabry-Perot temperature sensor is shown in Fig..1. To be an effective diffusion barrier, the Si 3 N 4 layer need not be thick, d > 10 nm is adequate. The Si 3 N 4 layer constitutes a second Fabry-Perot resonator, so the Fabry-Perot temperature sensor can be considered as two layer Fabry-Perot interferometer. The reflectance from the sensor with encapsulation layer is given by Eq. (.48) R jφ r1 + r13e ( λ) jφ 1 + r1r13e = (.113) with n 0 = n glass = , n 1 = n air = , n = n Si N = and n = nal = respectively. The phase shift of the Si 3 N 4 resonator is 3 j 4πn Si N d 3 4 φ = (.114) λ where d is the thickness of Si 3 N 4 layer. The amplitude reflectance of the nitride-coated aluminum is

65 r 13 jϕ r3 + r34e = (.115) jϕ 1+ r r e r 13 The reflectivity of the nitride-coated aluminum, R =, is maximized at the antiresonance condition, which is [17] ϕ + φ + φ = π (m 1) (.116) whereφ 1 and φ3 are the phase shifts associated with r 1 and r 3 and m is an integer. Since n 1 > n, φ1 π. If the aluminum is modeled as an ideal metal, thenφ 3 π. The reflectivity R 13 is then a maximum for ϕ = π ( p +1/ ). The zeroth-order reflectance maximum, ϕ = π /, is in this case provided by a quarter-wave layer, i.e. d = 00 nm for λ = 1550 nm. The effect of encapsulating films on the temperature sensitivity was shown to be negligible in a previous study [17]. So, the wavelength-encoded temperature measurement discussed in section.4 can be used for Fabry-Perot temperature sensor with encapsulating films. Fig.. compares the reflectance spectra shift for the sensor with encapsulating films and the sensor without encapsulating films under several values of temperature. The resonance wavelength shift under the same temperature is the same for the two types of sensors..5 Summary The expressions for reflectivity from a single layer and two layers that form Fabry- Perot interferometers were derived using Maxwell s equations and matrix techniques. A pressure sensor based on a single layer Fabry-Perot interferometer was designed for operating in the pressure range 1-1 psi. The design was done for use of the pressure sensor in a 850nm LED based reflectometry measurement system. A pressure sensor based on a two-layer Fabry-Perot interferometer was designed for operating in the pressure range 1-30 psi. The design was done for use of multiplexed pressure sensors using a tunable diode laser operating in the nm wavelength range. The configurations of three types of Fabry-Perot cavity-based pressure sensors (gauge, 64

66 absolute and differential pressure sensors) were introduced and the different properties were investigated. The Fabry-Perot temperature sensor was designed for operating in different temperature ranges. Aluminum-coated silicon nitride (Si 3 N 4 /Al) was proposed as a structure, which should act as an oxidant-impermeable encapsulant providing long-term stability. The optimum thickness of Si 3 N 4 layer was determined to be 00 nm. 65

67 .6 References: [1] C. R. Paul, and S. R. Nasaar, Introduction to Electromagnetic Fields, McGraw- Hill, New York, nd ed., [] Max Born and Emil Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, New York, 7 nd ed., [3] Pochi Yeh, Optical Waves in Layered Media, New York, Wiley, c1998. [4] J. M. Vaughan, The Fabry-Perot Interferometer: History, Theory, Practice and Applications, pp , Philiadelphia, A. Hilger, c1989. [5] Johan M. Muller, Multimirror Fabry-Perot Interferometers, Journal of Optical Society of America. A, Vol., No. 8, [6] L.Landeau and L. Lifschitz, Theory of Elasticity, London, Pergamon Press, Reading, Mass., Addison-Wesley Pub. Co., [7] J. R. Vinson, Structural Mechanics: The Behavior of Plates and Shells, New York, John Wiley and Sons, [8] Mario Di Giovanni, Flat and Corrugated Diaphragm Design Handbook, Mercel Dekker, NewYork, 199. [9] Robert D. Blevins, Formulas for Natural Frequency and Mode Shape, New York, Van Nostrand Reinhold Co., [10] Jacob Fraden, AIP Handbook of Modern Sensors: Physics, Designs, and Appicaitions, pp.8-31, New York, American Institute of Physics, c

68 [11] Jeff Hecht, Understanding Fiber Optics, Upper Saddle River, NJ: Prentice Hall, c00. [1] H. Henmi, S. Shoj, K. Yoshimi, and M. Esashi, Vacuum Packaging for Microsensors by Glass-silicon Anodic Bonding, Sensors and Actuators A, Vol. 43, pp , [13] K. Ichimura, M. Matsuyama and K. Watanabe, Alloying Effect on the Activation Process of Zr-alloy Getters, The Journal of Vacuum Science and Technology, Vol. 5, pp. 0-5, [14] G. Stemme, Resonant Silicon Sensors, Journal of Micromechanics and Microengineering, Vol. 1, pp , [15] Z. Xiao, O.Engstrom and N. Vidovic, Diaphragm Deflection of Silicon Interferometer Structures used as Pressure Sensors, Sensors and Actuators, Vol. 58, pp , [16] S. O. Kasap, Optoelectronics and Photonics: Principles and Practices, Upper Saddle River, NJ: Prentice Hall, c001. [17] Glenn Beheim, Fiber-Optic Temperature Sensor Using a Thin-Film Fabry-Perot Interferometer, NASA Technical Memorandum , Lewis Research Center, [18] D. T. E. Marple, Refractive Index of GaAs, Journal of Applied Physics, Vol. 35, pp. 141, [19] S. S. Ballard, K. A. McCarthy, W. L. Wolfe, Optical Materials for Instrumentation, University of Michigan Report No S,

69 [0] H. W. Icenogle, B. C. Platt, W. L. Wolfe, Refractive Indexes and Temperature Coefficients of Germanium and Silicon, Applied Optics, Vol. 15, pp. 348, [1] W. G. Driscoll, ed., Handbook of Optics, McGraw-Hill, New York, [] S. M. Sze, Physics of Semiconductor Devices, nd Edition, pp. 851, John Wiley & Sons, New York, [3] G.Cocorullo and I. Rendina, Thermo-Optical Modulation at 1.5 μm in Silicon Etalon, Electronics Letters, Vol. 8, No. 83, 199. [4] H. H. Li, Refractive Index of Silicon and Germanium and its Wavelength and Temperature Derivatives, Journal of Physical and Chemical Reference Data, Vol. 9, pp. 561, [5] G. E. Jellison, Jr. and F. A. Modine, Optical Functions of Silicon at Elevated Temperature, Journal of Applied Physics, Vol. 76, pp. 3758, [6] R. J. Harris, G. T. Johnston, G. A. Kepple, P. C. Krok, And H. Mukai, Infrared Thermooptic Coefficient Measurement of Polycrystalline ZnSe, ZnS, CdTe, CaF, and BaF, Single Crystal KCl, and TI-0 Glass, Applied Optics, Vol.16, pp. 436, [7] E. D. Palik, ed., Handbook of Optical Constants of Solids, Academic Press, Orlando, [8] W. J. Lu, A. J. Steckl, T. P. Chow, and W. Katz, Thermal Oxidation of Sputtered Silicon Carbide Thin Films, Journal of the Electrochemical Society, Vol. 131, pp.1907,

70 [9] G. Beheim, unpublished data. [30] G. Beheim, K. Fritsch, and D. J. Anthan, Fiber-Optic Temperature Sensor Using a Spectrum-Modulating Semiconductor Etalon, NASA TM , [31] J. M. Molarius, E. Kolawa, K. morishita, M-A. Nicolet, J. L. Tandon, J. A. Leavitt, and L. C. McIntyre, Jr., Tantalum-Based Encapsulants For Thermal Annealing of GaAs, Journal of the Electrochemical Society, Vol. 138, pp. 834, [3] Gorachand Ghosh, Temperature Dispersion of Refractive Indices in Crystalline and Amorphous Silicon, Applied Physics Letter, Vol. 66, No. 6, pp , June [33] J. Grimblot and J. M. Eldridge, Oxidation of Al Films, Journal of the electrochemical Society, Vol. 19, pp. 369, 198. [34] J. A. Thornton, Influence of Apparatus Geometry and Deposition Conditions on the Structure and Topography of Thick Sputtered Coatings, The Journal of Vacuum Science and Technology, Vol. 11, pp. 666, 197. [35] D. B. Fraser, Metallization in VLSI Technology, S. M. Sze, ed., pp.367, McGraw-Hill, New York, [36] C. A. Goodwin, The Use of Silicon Nitride in Semiconductor Devices, Ceramic Engineering & Science Proceedings, Vol. 5, pp. 109, [37] J. A. Appels, E. Kooi, M.. Paffen, J. J. Schatorje, and W. H. C. G. Verkuylen, Local Oxidation of Silicon and its Application in Semiconductor Device 69

71 Technology, Philips Research Reports, Vol. 5, pp. 118, [38] S. M. Hu and L. V. Gregor, Silicon Nitride Films by Reactive Sputtering, Journal of the Electrochemical Society, Vol. 114, pp. 86,

72 R 0 silicon air t glass d Optical fiber Light output Light input Gauge pressure sensor silicon R 0 vacuum t glass d Optical fiber Light output Light input Absolute pressure sensor 71

73 P 1 R 0 silicon t P glass d Optical fiber Light output Light input Differential pressure sensor Fig.1 Configuration of optically interrogated MEMS pressure sensors 7

74 z kˆ y n n 1 θ θ θ φ φ kˆ kˆ x Fig. Reflection and refraction of plane wave at a boundary between two dielectric media z kˆ H t E st n n 1 H i θ θ kˆ θ kˆ x H r E si E sr Fig..3 Reflection and refraction of S wave (TE) 73

75 z kˆ H t n θ E pt n 1 θ θ kˆ kˆ x H i E pi E pr. H r Fig..4 Reflection and refraction of P wave (TM) 74

76 1 d θ r1 t1t1 t1r3t1 t1r3r1t1 t1r3t1(r3r1) t1(r3r1) t1 n0 n1 n 1 3 Fig.5 Transmission and reflection from one layer Fabry-Perot interferometer 75

77 1 d t θ r1 t1 E'r t1 t1 E'r t1 E'r r1 E'r t1 E'r t1 (r1 E'r) n0 n1 n n3 Fig.6 Transmission and reflection from two layer Fabry-Perot interferometer 76

78 Fig..7 A multi-layer dielectric medium 77

79 1 0.8 Reflectivity Cavity depth(µm) Fig..8 Reflectance from the Fabry-Perot cavity based sensor as a function of cavity depth when operating at 850nm (n 0 = n glass =1.474, n 1 = n air =1.0, n = n si =3.46) 78

80 1 0.8 Reflectivity Cavity depth(µm) Fig..9 Reflectance from the Fabry-Perot cavity based sensor as a function of cavity depth when operating at 1550 nm with the diaphragm thickness at t = µm (n 0 = n glass =1.473, n 1 = n air =1.0, n = n si =3.478, n 3 = n air =1.0) 79

81 1 0.8 Reflectivity Silicon Cavity thickness(µm) depthhmml Fig..10 Reflectance from the Fabry-Perot cavity based sensor as a function of silicon thickness when operating at 1550 nm with the cavity depth at d 0 = 1.1 μm 80

82 Reflectance Cavity depth (µm) Si thickness (µm) Fig..10 Reflectance from the Fabry-Perot cavity based sensor as a function of cavity depth (0- μm) and silicon thickness ( μm) when operating at 1550nm. 81

83 The deflection of the membrane (µm) µm µm µm 300µm Fig..1 Computer simulation of diaphragm deflection of the Fabry-Perot cavity-based sensor under the pressure using Mathematic

84 W si W air θ θ 1 W glass 9 μm 56.8μm Fig..13 Calculation of light loss in sensor 83

85 R pressure λ 0 /4 cavity V 1 = π ( λ 0 / 4) (3R λ0 / 4) 3 Fig..14 Calculation of change of pressure inside the cavity under diaphragm bending. 84

86 SiO silicon air glass Fig..15 Fabry-Perot cavity-based pressure sensor with a thin layer of antireflection coating (SiO) on the top surface of silicon diaphragm. Reflections out of phase Fig..16 Illustration of how an antireflection coating reduces the reflected light intensity 85

87 0.46 Reflectivity WavelengthHmmL (µm) Fig..17 Illustration of how the condition for antireflection coating is wavelength dependent (the thickness of the antireflection coating is designed for operating at 1550 nm). The antireflection condition is satisfied at the wavelengths of the cross points of the two lines. Red line: Reflectivity from the three layer Fabry-Perot interferometer (pressure sensor without antireflection layer) Blue line: Reflectivity from the one layer Fabry-Perot interferometer (pressure sensor with antireflection layer) 86

88 d silicon glass Fig..18 Configuration of the Fabry-Perot temperature sensor λ m1 λ m λ m1+1 Temp increase Reflectance WavelengthHmmL (µm) 1.58 Fig..19 Reflectance spectra shift with temperature increasing 87

89 300 ( ºC ) Temp R ang e C º Si thicknesshmml (µm) Fig..0 Temperature range as a function of silicon thickness at λ = 1550 nm for Fabry- Perot temperature sensor 88

90 Al Si 3 N 4 silicon glass Fig..1 Configuration of the Fabry-Perot temperature sensor with encapsulating films. sensor with encapsulant 1 λ m1 λ m λ m3 0.8 Reflectance sensor without encapsulant WavelengthHmmL (µm) Fig.. Comparison of reflectance spectra shift for the sensor with encapsulant and the sensor without encapsulant under several values of temperature 89

91 Table.1 The natural frequencies of circular plates Notation: R 0 = radius of plate; t = thickness of plate; i = number of nodal circles; j = number of nodal circles, not counting the boundary; r = a radius Natural Frequency (hertz), Description f n κ = π κ ij ij R0 3 Et 1 ρ (1 ν ) 1 / ; i = 0, 1, ; j = 0,1, Mode Shape 1. Free Edge. Simply Supported Edge i j * * i j Radii of Nodal Circles, r/r 0: i j Mode Shape: κ r J i ( κ ) κ r [ J i ( ) I ( )] cos( iθ ) ; i R I ( κ ) R 0 r i = 0, 1,, Radii of Nodal Circles, r/r 0: j i

92 3. Clamped Edge 4. Clamped at Center, Free Along Edge i j Polar Symmetric Modes (i = 0) j κ Mode Shape: κ r J i ( κ ) κ r [ J i ( ) I ( )] cos( iθ ) ; i R I ( κ ) R 0 r i = 0, 1,, Radii of Nodal Circles, r/r 0: i j In higher modes values of κ are separated by π. 0 91

93 Table. Properties of candidate Fabry-Perot materials. Units of k n, k 6 / C. L and k φ are 10 - Material λ (μm) n k n k L k Φ GaAs [18] 10 [18] 5.7 [18] 16 Ge [0] 100 [0] 5.7 [1] 106 Si [0].6 [] [3] [4] 59 [4] [5] 76 [5] 79 CdTe [6] 53 [6] 5.0 [6] 58 TiO , n o [1] 16 [19] 7.1 [19] 3 (Rutile).83, n e [1] 35 [19] 9. [19] 44 ZnS [6] 7 [6] 6.9 [6] 34 Al MgO [19] 8.1 [19] 14 [19] Si 3 N [7] NA.8 [8] 19 [9] SiC [30] NA [30] SiO [1] 15 [1] 0.5 [1] 15 Al O [1] 10 [1] 5 [1] 15 9

94 3 Multiplexed Sensors 3.1 Introduction The use of optical sensors for measuring temperature and pressure in engineering structures often requires many sensors. This makes multiplexing a very important issue for optical sensors. Moreover, multiplexing optical sensors is simpler than multiplexing electrical signals from multiple sensors. By sharing components, the cost per sensor is greatly reduced, improving the competitiveness of optical sensors against conventional electro-mechanical sensors. Optical multiplexing techniques also substantially reduce the overall system weight which is very critical for applications in aerospace. The purpose of this chapter is to design multiplexed pressure and temperature sensor systems based on WDM principles and existing WDM technology using a tunable laser, and expanding the number of sensors which can be multiplexed to more than 10 sensors. An experimental demonstration of the operation of these systems is included in a later chapter. 3. Multiplexed Pressure Sensors Using Fiber Bragg Gratings (FBGs) Fiber Bragg gratings (FBGs) represent a key element in the established and emerging field of optical communications. Their unique filtering properties and versatility as in-fiber devices have led to a variety of lightwave applications including wavelength stabilized lasers, fiber lasers, remote pump amplifiers, Raman amplifiers, phase conjugators, wavelength converters, passive optical networks, wavelength division multiplexers and demultiplexers, add/drop multiplexers, dispersion compensators, and gain equalizers [1]. Fiber Bragg gratings are set to revolutionize telecommunications, and will also have a critical impact on the optical fiber sensor field. FBGs are produced by exposing a photosensitive optical fiber to a spatially varying pattern of ultraviolet intensity, which results in a periodic perturbation to the effective refractive index of the guided modes. The index of refractive profile can be expressed as [1] n( z) = n0 πz + ΔnCos( ) Λ (3.1) 93

95 where Δn is the amplitude of the induced refractive index perturbation (typical values 10-5 to 10-3 ), and z is the distance along the fiber longitudinal axis. Several types of FBGs have been reported: uniform Bragg gratings, blazed Bragg gratings, and chirped Bragg gratings. The most common FBG is the Bragg reflector, which has a constant pitch. The Bragg reflector functions as a narrowband filter. Light propagating along the core of an optical fiber, will be scattered by each grating plane. If the Bragg condition is not satisfied, the reflected light from each of the subsequent planes becomes progressively out of phase and will eventually cancel out. Additionally, light that is not coincident with the Bragg wavelength resonance will experience very weak reflection at each of the grating planes because of the index mismatch, this reflection accumulates over the length of the grating. As an example, a 1-mm grating at 1.5 μm with a strong Δ n of 10-3 will reflect 0.05% of the off-resonance incident light. Where the Bragg condition is satisfied, the contributions of reflected light from each grating plane add constructively in the backward direction to form a back-reflected peak with a center wavelength defined by the grating parameters. This will occur for only one wavelength for a FBG with constant pitch. The Bragg grating condition is the requirement that both energy and momentum conservation are satisfied. Energy conservation ( h ω = hω ) requires that the frequency of the incident radiation and the reflected radiation is the same. Momentum conservation requires that the incident wavevector, k i, plus the grating wavevector, K, equal the wavevector of the scattered radiation i k f f k f. This is simply stated as [] k + K = (3.) where the grating wavevector, K, has a direction normal to the grating planes with a magnitude π / Λ ( Λ is the grating period shown in Fig 3.1 [] ). The diffracted wavevector is equal in magnitude, but opposite in direction, to the incident wavevector. Hence, the momentum conservation condition becomes πn ) = π eff ( λ Λ B which simplifies to the first-order Bragg condition i (3.3) 94

96 λ n Λ (3.4) B = eff where the Bragg grating wavelength, λ B, is the free space center wavelength of the input light that will be back-reflected from the Bragg grating, and neff is the effective refractive index of the fiber core at the free space center wavelength. The FBGs used in the present experiments are the Bragg Reflector type with 99% reflectivity and 0.nm bandwidth, the resonant wavelengths of the four FBGs are: λ = nm λ = 155nm λ = nm λ = nm A tunable external-cavity diode laser is used to illuminate the Fabry-Perot cavity, as shown in Fig 3.. The reflection light from the two FBGs are routed into one sensor through the x (50:50) couplers at the resonant wavelengths of the two gratings. The reflection light from the FBG will be separated in the time domain by sweeping the wavelength of a tunable laser. The isolators between the FBG are chosen to separate the sensors. Therefore, two wavelengths are assigned to each sensor, and for demonstration purposes, the four wavelengths for the two sensors are separated in the time domain. Two photodetectors are used to measure the input and output from the sensors, respectively. The ratio of the signals from the two photodetectors at two wavelengths is used as the measure of the sensed parameter. Fig 3. shows the schematic diaphragm of the multiplexed pressure sensors using fiber Bragg gratings, and Fig 3.3 shows the simulation of the results anticipated from the multiplexed pressure sensor system. Single-mode 9/15um step index fibers with cut-off wavelength at 150nm are used for the whole system. In order to avoid spurious reflections, angle-polished FC/APC connectors and isolators are used. The maximum sensor number that can be multiplexed using this system is determined by the span of wavelength over which the tunable laser can operate. The tunable laser in our lab operates in the nm wavelength range. Therefore, this system is capable of multiplexing about 70 sensors. 95

97 3.3 Multiplexed Pressure Sensors Using Arrayed Waveguide Gratings (AWGs) The AWG, which is also called a phased array grating (PHASAR) or waveguide grating router (WGR), plays a key role in the optical multiplexing/demultiplexing technology of telecommunication links and networks. Fig 3.4 shows the schematic layout of a PHASAR-demultiplexer [3]. The operation is understood as follows. When the beam propagating through the transmitter waveguide enters the free propagating region (FPR) it is no longer laterally confined and becomes divergent. On arriving at the input aperture the beam is nearly uniformly coupled into the waveguide array and propagates through the individual array waveguides to the output aperture. The length of the array waveguides is chosen such that the optical path length difference n co Δ L between adjacent waveguides equals an integer multiple of the central wavelength λ o of the demultiplexer π λ o n co Δ L = mπ (3.5) Here nco is the effective index of the arrayed channel waveguide mode at the central wavelength, m is called the grating order. For this wavelength the fields in the individual waveguides will arrive at the output aperture with equal phase (apart from an integer multiple of π), and the field distribution at the input aperture will be reproduced at the output aperture. The divergent beam at the input aperture at λ o is thus transformed into a convergent one with equal amplitude and phase distribution, and an image of the input field at the object plane will be formed at the center of the image plane. The dispersion of the PHASAR is due to the linearly increasing length of the array waveguides, which will cause the phase change induced by a change in the wavelength to vary linearly along the output aperture. As a consequence, the outgoing beam will be tilted and the focal point will shift along the image plane. Interference pattern dictates that maxima will occur when total path length differences are integer multiples of the wavelength of the light [4] π π necδ L + nesd asinθ = mπ (3.6) λ λ The total path length differences include the length difference between adjacent waveguides and the length difference in the output FPR region, as shown in Fig 3.4. Here 96

98 nes is the slab mode effective index of the output FPR region, d a is the spacing between adjacent arrayed waveguides, and θ is the angle at which the interference maximum will occur relative to the normal at the center of the image focal line of the output FPR region. ΔL is a fixed distance as defined in Eq. (3.5), and nec is a function of wavelength, or n ec (λ). By placing receiver waveguides at proper positions along the image plane, spatial separation of the different wavelength channels is obtained. A schematic diaphragm of the multiplexed pressure sensor system using AWG is shown in Fig 3.5. The AWG used in present experiments is a JDS Uniphase AWG demultiplexer with 8 channels. The center wavelengths of 8 channels are: (The 3dB bandwidth of each channel is 0.7nm) λ 1 = nm ; λ = nm ; λ 3 = nm ; λ 4 = nm λ 5 = 155.5nm ; λ 6 = nm ; λ 7 = nm ; λ 8 = nm A tunable external-cavity diode laser is used as light source for this system. The output from two channels of AWGs are routed into one sensor through a X (50:50) coupler. The sensors will be separated in the time domain by sweeping the wavelength of the tunable laser. Isolators with FC/APC connectors are placed just after the AWG to minimize the effect of the reflected light from the sensors to the AWG. Low crosstalk between channels is a key requirement for the performance of this system. The schematic diagram of the multiplexed pressure sensors using AWG is shown in Fig 3.5. The loss of the AWGs is very low, so that the output power from each channel is high enough to support about 5 sensors through a X5 coupler. Therefore, the maximum number of sensors that can be multiplexed using this system is about Ripples Due to the Long Coherence Length of Tunable Laser The output of many lasers is highly monochromatic. Ideally, the light from the laser is a single color. But, it actually consists of light within a small band of wavelength, Δ λ. There are a number of ways of describing the degree of monochromaticity in addition to the wavelength bandwidth, Δ λ. It can be characterized as a frequency bandwidth, Δ ν. The two are related by 97

99 Δλc Δ ν = (3.7) λ where c is the light velocity in free space. The smaller the bandwidth, the more monochromatic the light source. Another way of describing the monochromaticity of the source is through its coherence length. If a source were totally monochromatic, the output would consist of an electromagnetic field of constant amplitude that oscillates for an infinitely long time with a single output frequency and wavelength. Any deviation in the amplitude or shortening of the length of oscillation results in an increase in the bandwidth of the source or, alternatively, a decrease in the coherence length of the radiation. One may think of this description as trains of waves whose lengths represent the monochromaticity of the source. The relation between the coherence length and the bandwidth is [5] c λ l c = = (3.8) Δν Δλ As will become evident presently, the coherence length is the extent in space over which the wave is nicely sinusoidal so that its phase can be predicted reliably. Coherence time is defined as Δlc Δ tc = (3.9) c Fig 3.6 shows a single wave train in time and frequency domain [6]. The frequency bandwidth of the tunable laser in our lab is 5 MHz, and the center wavelength is 1550 nm. So, the coherence length of the tunable laser is l c 60m. If the light source has a short coherence length, the optical path difference between two beams from the same light source increases. Then, identically paired wave groups will no longer be able to arrive at the same place exactly together. There will be an increasing amount of overlap in portions of uncorrelated wave groups, and the contrast of the fringes due to interference will degrade. When the difference of optical path is of the order of, or much greater than, the coherence length, instead of two correlated portions of the same wave groups overlapping, the fringes will vanish. In this case, interference is no longer appreciable. Since the coherence length of the tunable laser in our lab is relatively long, the light interference will occur in wide optical path difference range. Therefore, the 98

100 parasitic reflected waves occurring in the system may interfere with the signal wave and result in unwanted interferometric signals. The parasitic reflections may occur at various fiber terminations such as fiber joints, unused fiber ports, etc. This will lead to the problem of ripples in the reflectance spectra due to the unwanted interferometric signals. The existence of ripples in the system would affect the measurement accuracy of the sensor. One of the sources of unwanted interferometric signal in the multiplexed sensors system is caused by the refractive index difference between the glass (wafer and fiber) and air and connectors. This is due to the existence of an air gap between the sensor and the fiber endface. Index matching oil is filled in the gap to reduce the reflection due to the refractive index difference. Another source of unwanted interference is caused by the back reflection from FBGs and connectors or other fiber terminations, show in Fig 3.7. The structure of a basic ferrule connector is shown in Fig 3.8 [7]; FC/PC connector has back reflection of -30 to -40 db and FC/APC connector has back reflection smaller than - 60 db. Therefore, FC/APC connectors are used to couple light into sensors. The angled finish will result in substantially reducing reflection at fiber endfaces. To avoid spurious reflections at other fiber terminations, refractive index matching fluid is used at the unused fiber ports and fiber isolators are placed at suitable locations along the fiber path. These two methods will reduce the ripples in the system by eliminating unwanted interferometric signals. 3.5 Dual-Wavelength Interrogation Technique for Multiplexed Pressure Sensors In chapter, we have derived the equation for the sensor s reflectivity as a function of cavity depth. The response curve is shown in Fig..9. Unfortunately, for Fabry-Perot cavity based pressure sensors, it is usually not possible to measure the absolute value of reflectance, because of the existence of optical losses in the system, such as in fiber couplers and connectors. An alternative is the dual wavelength technique, in which relative reflected intensities at two different wavelengths λ1 and λ are separately measured, and then a ratio I( R, λ) is calculated using [8] a( λ1 ) R( λ1 ) I( R, λ) = (3.10) a( λ ) R( λ ) + a( λ ) R( λ )

101 where R ( λ 1 ), R ( λ ) are the reflectance at two wavelengths λ 1 and λ, defined in Eq. (.45). a(λ) accounts for any losses induced by the optical system. a λ ) R( ) ( 1 λ1 a( λ ) R( λ ) are the measured reflectance at two wavelengths respectively. Assuming these losses are wavelength independent over the wavelength range, the ratio becomes equal to the corresponding ratio of absolute reflectance: R( λ1 ) I( R, λ) = (3.11) R( λ ) + R( λ ) 1 This technique eliminates errors resulting from wavelength-independent changes in the fiber interconnect to the sensor. The response curve is still a periodic function with respect to cavity depth t, as shown in Fig 3.9. This dual-wavelength interrogation technique requires the signal at two wavelengths for each sensor. In multiplexed pressure sensors system introduced in section 3.1 and 3., two wavelengths from FBGs or AWGs will be provided to each sensor for pressure interrogation. Because the wavelength of the tunable laser source is scanned, the two wavelength signals will be separated in the time domain upon arrival at the same photodetector. and 3.6 Multiplexed Temperature Sensors and Interrogation Technique In chapter, we have derived the Eq. (.98) for the reflectivity from a Fabry-Perot temperature sensor. The response curve is a periodic function with respect to silicon refractive index, as shown in Fig Since it s usually not possible to measure the absolute value of reflectance, because of the existence of optical losses in the system, we need to use the dual-wavelength interrogation technique introduced in Section 3.4 or the wavelength-encoded measurement described in Section.4. The two different interrogation techniques will lead to different multiplexed systems for Fabry-Perot temperature sensors. Dual-wavelength interrogation technique described in Section 3.4 can be used to interrogate temperature sensors. Reflectance at two different wavelengths λ1 and λ are separately measured, and then a ratio I ( R, λ) is calculated using Eq. (3.11) assuming the losses are wavelength independent over the wavelength range. In Eq. (3.11), R λ ), ( 1 100

102 R ( λ ) are the reflectance from Fabry-Perot temperature sensor at two wavelengths λ 1 and λ, defined in Eq. (.98). The response curve is still a periodic function with respect to silicon refractive index, as shown in Fig The temperature range of the Fabry- Perot temperature sensor using dual-wavelength interrogation technique is determined by silicon thickness and the two wavelengths being chosen. A temperature sensor must operate in less than ½ period defined by the response curve I ( R, λ) as a function of silicon refractive index in order to avoid ambiguity. The multiplexed temperature sensor system using dual-wavelength interrogation technique is similar to the multiplexed pressure sensor system described in Section 3.1 and Section 3. and shown in Fig (3.) and Fig (3.5), except replacing pressure sensors with temperature sensors. Wavelength-encoded measurement described in Section.4 can be used to interrogate a temperature sensor by tracking spectral resonance. At resonance, the reflectivity of the sensor is a minimum, and the wavelength of one of these reflectivity minima can serve as a signal-level-insensitive indicator of temperature. The multiplexed temperature sensors system using wavelength-encoded measurement is shown in Fig 3.1. In this system, we are using reflected light from broadband fiber gratings to interrogate the multiplexed temperature sensors. The broadband fiber gratings used in the present experiments are chirped fiber Bragg gratings with bandwidth 10 nm. Chirped fiber Bragg gratings are devices in which the period of the modulation of the refractive index changes along the grating. By progressively increasing the spacing between the grating planes, different wavelengths are reflected back as light travels through the grating. The result is a spectral response that will be characterized by a wide bandwidth as shown in Fig 3.13 [1]. The wavelengths for the two broadband fiber gratings in the system are nm and nm, respectively. A tunable external-cavity diode laser is used to illuminate the Fabry-Perot cavity. The reflection light from one of the broadband fiber gratings is routed into one sensor through the x (50:50) couplers. The reflected light from the two broadband fiber gratings will be separated in the time domain by sweeping the wavelength of a tunable laser. Therefore, one wavelength region is assigned to each sensor, and for demonstration purposes, the two wavelength regions for the two sensors are separated in the time 101

103 domain. Two photodetectors are used to measure the reflectance spectra from the sensors. Temperature experienced by the sensor is determined by tracking the wavelength shift of one of the reflectivity minima within the bandwidth of the broadband fiber grating. Fig 3.14 shows the simulation of the results from a multiplexed temperature sensor system. The temperature range of the multiplexed Fabry-Perot temperature sensor using broadband fiber gratings is determined by silicon thickness and bandwidth of the broadband fiber gratings. Fig 3.15 shows the simulation of temperature range and temperature sensitivity as a function of silicon thickness for multiplexed temperature sensors using broadband fiber gratings. The maximum sensor number that can be multiplexed using this system is determined by the span of wavelength over which the tunable laser can operate. The tunable laser in our lab operates in the nm wavelength range. Therefore, this system is capable of multiplexing about seven sensors using this technique. 3.8 Summary Wavelength multiplexing of optically interrogated MEMS pressure and temperature sensors using both fiber Bragg gratings (FBGs) and arrayed waveguide gratings (AWGs) has been designed. The multiplexed pressure sensor system using FBGs has the potential of multiplexing about seventy sensors depending on FBG bandwidth and the one using AWG has the potential of multiplexing about eight sensors depending on the number of AWG channels and the bandwidth of each channel. There are two different interrogation techniques for temperature sensor which lead to different multiplexed systems. The multiplexed temperature sensor system using FBGs has the potential of multiplexing about seventy sensors and the one using broadband fiber gratings has the potential of multiplexing about seven sensors. 10

104 3.7 References: [1] Andreas Othonos and Kyriacos Kalli, Fiber Bragg gratings: Fundamentals and Applications in Telecommunications and Sensing, Boston: Artech House, [] Raman Kashyap, Fiber Bragg gratings, San Diego, CA: Academic Press, [3] Meint K. Smit and C. Van Dam, PHASAR-based WDM devices: Principles, Design and Applications, Journal of Selected Topics in Quantum Electronics, Vol., No., [4] Masaki Kohtoku, Semiconductor Arrayed Waveguide Gratings for Integrated Photonics Devices, Electronics and Communications in Japan, Part, Vol. 83, No. 8, pp , [5] Eugene Hecht, Optics, Reading, Mass.: Addison-Wesley, 4 nd ed., c00. [6] Max Born and Emil Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, New York, 7 th ed., [7] John Senior, Optical Fiber Communictions, Englewood cliffs, NJ : Prentice-Hall International, [8] Youngmin Kim and Dean P. Neikirk, Design of Manufacture of Micromachined Fabry-Perot Cavity-based Sensors, Sensors and Actuators, Vol. 50, pp ,

105 λ B = neff Λ Reflections in phase Λ Cladding Core Jacket FBGs-specific wavelengths are reflected by small mirrors in fiber that are periodic changes in the refractive index of the core n( z) = n0 πz + ΔnCos( ) Λ Fig 3.1 Principle of fiber Bragg gratings (FBGs) [1] 104

106 Tunable laser λ1 FBG λ λ3 4 3dB coupler λ FBG 3dB coupler Isolator FC/APC connector union 3dB coupler detector detector Sensor #1 Sensor # Fig 3. Schematic diagram of the multiplexed pressure sensor system using FBGs 105

107 Power (mw) Wavelength (nm) Laser output Power (μw) Sensor #1 Sensor # Reflectance Sensor Input Sensor #1 Sensor # Wavelength (nm) Reflectance from the Sensor 1580 Wavelength (nm) Fig 3.3 Simulation of the results from multiplexed pressure sensor system using FBGs 106

108 Fig 3.4 Schematic diagram of the arrayed waveguide gratings (AWGs) [3] 107

109 Tunable laser AWG FC/APC connector union 3dB coupler 3dB coupler Isolator 3dB coupler detector Sensor input 3dB coupler detector Sensor output Sensor # Sensor #1 Fig 3.5 Schematic diagram of the multiplexed pressure sensor system using AWGs 108

110 Fig 3.6 A cosinusoidal wave train modulated by a Gaussian envelope along with its transform, which is also Gaussian [6] Tunable laser 3d FBG Unwanted interferometric signal Signal Fig 3.7 Unwanted interferometric signal between the connector and FBG 109

111 FC/PC (Back reflection: -30 to -40dB) 8 FC/APC (Back reflection: >-60dB) Fig 3.8 Structure of a basic ferrule connector (FC/PC and FC/APC) [7] 110

112 R atio I R,l Cavity depth (µm) R( λ1 ) I( R, λ) = from the Fabry-Perot cavity-based pressure R( λ ) + R( λ ) Fig. 3.9 Ratio 1 sensor as a function of cavity depth when operating at λ = nm and λ = nm with the silicon diaphragm thickness at t = μm (n 0 = n glass =1.473, n 1 = n air =1.0, n = n si =3.478, n 3 = n air =1.0) 111

113 Reflectivity Si refractive index Fig Reflectivity from the Fabry-Perot temperature sensor as a function of silicon refractive index with silicon thickness at t = 0 μm (n 0 = n glass =1.473, n 1 = n si, n = n air =1.0) 11

114 Ratio I R,l Si refractive index R( λ1 ) I( R, λ) = from the Fabry-Perot temperature sensor as R( λ ) + R( λ ) Fig Ratio 1 a function of silicon refractive index when operating at λ = nm and with silicon thickness at t = 0 μm λ = nm and 113

115 Tunable laser 3dB coupler λ1 Broad band mirror fiber gratings 3dB coupler λ Broad band mirror fiber gratings Isolator 3dB coupler FC/APC connector union 3dB coupler detector detector Sensor #1 Sensor # Fig. 3.1 Multiplexed temperature sensors using broad band mirror fiber gratings Fig Reflection spectrum of a broad band mirror fiber grating [1] 114

116 Power (mw) Wavelength (nm) Laser output Power (μw) Sensor #1 Sensor # Sensor Input Wavelength (nm) Reflectance Sensor #1 Sensor # Reflectance from the Sensor 1580 Wavelength (nm) Fig 3.14 Simulation of the results from multiplexed temperature sensors using broad band mirror fiber gratings (Bandwidth 10 nm) 115

117 ( ºC ) Temp R ang e C Si thicknesshmml (µm) (a) 116

118 Temp s ensitivit y ( n nm/ºc m C ) Si thicknesshmml (µm) (b) Fig 3.15 Simulation of temperature range (a) and temperature sensitivity (b) as a function of silicon thickness for multiplexed temperature sensors using broadband fiber gratings (Bandwidth 10 nm) 117

119 4 Fabrication and Calibration of Sensors 4.1 Introduction The optical Fabry-Perot cavity-based pressure sensors are fabricated using MEMS techniques. Fig 4.1 shows the processing steps used during the fabrication process. The process is initiated with a standard fused-silica wafer (Pyrex 7740) polished on both sides that is patterned to form a series of cavities for the Fabry-Perot interferometer. Standard photolithographic and etching techniques are used for this process. A silicon wafer polished on both sides is then electrostatically bonded to the patterned glass wafer. Bulk etching techniques are used to thin the silicon wafer down to the desired diaphragm thickness, while the other side of the Si/glass assembly is protected. The configuration of a Fabry-Perot temperature sensor is shown in Fig.18. It consists of a thin layer of silicon bonded to a glass wafer. The fabrication process is similar to that of a pressure sensor, but with no air cavity (t = 0). Pressure sensors designed to respond over the pressure range 0 1 psi and operating at 850 nm were sent to Air Force Research Lab for testing. Pressure sensors designed to respond over the pressure range 0 30 psi and operating at 1550 nm were tested in our lab for static pressure calibration. Temperature sensors with different temperature ranges were tested in our lab. The designed multiplexed system for two Fabry-Perot cavity-based pressure sensors and temperature sensors has been demonstrated. The results given show good agreement between experimental and theoretical results. 4. Sensor Fabrication We have devised a fabrication plan where the key design parameters for a pressure sensor such as silicon diaphragm thickness, initial cavity depth, and cavity diameter can be achieved for the calculated values to provide linear response over various pressure ranges. For temperature sensors, the key design parameter is the thickness of the silicon layer which determines the temperature range of the sensor. The main processing steps are summarized in this section (Fig 4.1). 118

120 4..1 Photolithographic Patterning and Wet Etching for Cavity Formation on Glass Wafer Prior to application of photoresist, the glass wafer is cleaned and put into an oven for 1 hr at 10 degrees. This operation is for dehydration and it promotes adhesion of the photoresist. An adhesion promoter, HMDS, must be applied on the glass wafer before application of photoresist. Then photoresist is spun on the wafer at 4000 rpm to yield a thickness of approximated μm. The wafer is then patterned using a mask, as shown in Fig 4.. The alignment mark 上 is used to distinguish which side of the glass wafer is patterned. Wet etching was chosen for forming cavities because it is simple, and yet it can be done in a controlled manner. Hydrofluoric (HF)-based etchants are typically used for etching of silicon dioxide. Several similar reactions for the HF-based etching of silicon dioxide are given in literature. For pure HF etching, the overall reaction is [1], [], SiO + 6HF H SiF 6(aq) + H O. The reaction in Buffered HF solutions is SiO + 4HF + NH 4 F (NH 4 )SiF 6 + H O. And the reaction involving the HF ion (discussed below) is SiO + 3HF - + H + SiF H O. HF is a weak acid; except when present in very small concentrations, it does not completely dissociate into H + and F - ions in water. The etch rate of silicon dioxide - increases linearly with the concentrations of both HF and HF for concentrations lower than 10M, while being independent of the concentration of F - - ions alone. The HF complex attacks oxide about 4.5 times faster than HF. Higher-order complexes, such as H F - 3, appear to occur at higher HF concentrations (e.g., in 49% HF) and attack oxide - even faster than HF [3]. Thus, etch rate increases faster than linearly with HF concentration. As HF and HF - are consumed, the etch rate decreases. Buffering with NHF therefore, helps to keep the ph and thus the concentrations of HF and HF - constant, stabilizing the etch rate [4]. Since the cavity depth (0.64 μm for sensors operating at 850 nm and 1.1 μm for sensors operating at 1550 nm) of a Fabry-Perot cavity-based pressure sensor must be 119

121 etched very accurately, the etching process requires high control. For this reason, we used buffered HF in etching of patterned fiber end faces. 5:1 refers to five parts by weight of 40-weight-percent ammonium fluoride (the buffer) to one part by weight 49- weight-percent hydrofluoric acid. It has been found that the etch rate for borosilicate glass is too high (780 nm/min), and as a result, the total etch time for etching 0.64 μm and 1.1 μm deep cavities is less than one minute. This low etching time causes significant inaccuracies in etch depth; therefore, NH4F: HF 1:1 (BHF) has been used. This resulted in a measured etch rate of 10 nm/min, thus making the total etching time to be 5-6 min for 0.64 μm deep cavity and min for 1.1 μm deep cavity. The etching was done at room temperature. Because this etchant is buffered, its etch rate does not vary much with temperature. Moreover, this etchant can be masked with photoresist (the adhesion is much better than in higher concentrated HF). Etch rate was determined by measuring the depth of sensor cavity etched at different time intervals. Cavity depths were determined using a Profilometer, as shown in Fig 4.3. Wet etching of cavities described above was followed by removal of photoresist from the glass wafer. Dipping in Acetone at room temperature was done for this purpose, but at certain instances where photoresist was not removed easily, Acetone cleaning was done at elevated temperatures (70 C) by keeping the Acetone beaker on a hot plate for up to 5 10 minutes. 4.. Anodic Bonding of Silicon-to-Glass A 15 μm silicon wafer polished on both sides was then electrostaticly bonded to the patterned glass wafer. The most common glass used for anodic bonding is Pyrex (corning #7740, Hoya SD- (Hoya) and Tempax (Schott #8330). These inorganic borosilicate glasses all contain around 4% sodium; other metal ions in the glass are lithium, calcium, and aluminum. Most alkali-containing glasses behave at elevated temperatures like solid electrolytes. This property of glass is a key mechanism for anodic bonding. When fabricating devices extremely sensitive to residual strain such as anodically bonded optical pressure sensors, the thermal expansion match of glass to silicon is of great importance [5]. The thermal expansion coefficient of this Pyrex 7740 borosilicate glass best matches the thermal expansion coefficient of the silicon over a 10

122 wide temperature range [6]. This enables a low-stress bonding of the glass and silicon wafers. Anodic bonding was first discovered and performed in Since then it has been established as a standard process in MEMS technology. Before anodic bonding, base cleaning of silicon and glass wafer must be done. Mixing 1 part of Ammonium Hydroxide with 5 parts of DI H O and heating this mixture up to 70 C on a hot plate and then by adding another part of Hydrogen Peroxide (H O ) made the base cleaning solution. Silicon and glass wafers were dipped in this solution for 15 minutes and rinsed in running DI H O, and dried with nitrogen blow gun. Anodic bonding was performed by an automated bonding machine, as shown in Fig 4.4. The most common reference is a standard atmosphere for gauge pressure sensors and differential pressure sensors or a perfect vacuum for absolute pressure sensors. Bonding was obtained by placing a Pyrex glass wafer on top of a polished silicon wafer, heating this sandwich up to about 400 o C and applying a large mechanical pressure to bring them into intimate contact. This may be done by applying a large voltage ( V) on the glass and the silicon wafer. As the bond progresses the interface becomes dark and homogeneous in color when the bond is formed. When the glass-silicon sandwich was heated, the positive ions Na + become mobile which causes the glass to behave as conductor. Hence, in the very first moment, most of the applied voltage applied to the glass-silicon sandwich is dropped across the small gap of a few microns between the two surfaces. The high electric field in this area created strong electrostatic force, acting on the two surfaces and effectively pulling them together, thus forming an intimate contact. This facilitated the chemical reaction between the two surfaces and leads to formation of a bond. At the same time, the Na + ions started drifting to the cathode, neutralizing the cathode while creating a depletion zone adjacent to the silicon anode (see Figure 4.5). This depletion zone, which had a thickness of less than 1 μm in the beginning, can be compared with a capacitor that is being charged. The existing electric field at the interface was high enough for oxygen cations to drift to the interface. These cations once at the interface reacted with silicon and formed Si-O links. Because of the movement of the Na + ions the bonding current peaks at the beginning of the process as shown in Figure 4.6. When the glass and silicon were bonded, the current stabilizes, indicating that the bond was complete [7]. 11

123 Anodic bonding parameters have to be selected carefully in order to guarantee a satisfactory and reproducible seal. Incorrect choices result in unwanted effects such as oxidized silicon tracks, non-hermetic seals [8], warped wafer stack [9], plastic deformation of glass [10], sealed cavities [5], and void formation, which will hamper the quality of the performance of a device. Anodic bonding process parameters such as voltage, temperature, time, electrode configuration, pre-treatment of wafers, bonding atmosphere, surface morphology and heating source have to be pondered upon before application Silicon Wafer Thinning to Form Diaphragm After the bonding process was done, bulk etching techniques (KOH etchant) was used to thin the silicon wafer down to the desired diaphragm thickness. The thickness of the silicon diaphragm is decided by the pressure range of sensors and the operating wavelength. As discussed in chapter, the thickness of silicon diaphragm is t = 14.0 μm for the pressure sensor operating at 850 nm and designed to respond over the pressure range 0 1 psi and t = 15.5 μm for the pressure sensor operating at 1550 nm and designed to respond over the pressure range 0 30 psi. Because during anodic bonding the silicon wafer was exposed to air at high temperature, a very thin silicon oxide film would be generated on the silicon surface. In KOH etching, approximately etching selectivity in <100> silicon/silicon oxide will be achieved. Thus, if a small amount of silicon oxide was left on the wafer surface, the etched window would show very rough surface after KOH etching due to the residual oxide mask. This would result in failure to achieve a uniform diaphragm. To remove the residual silicon oxide film and contamination, the wafer was etched in % HF solution for 30 seconds before dipping into the silicon etching solution. The etching solution used in this experiment is PSE-00, a commercially prepared solution of isopropyl alcohol, KOH, and water purchased from Transene Co. An etching protector was used to protect the glass wafer from the etching solutions. This protector, made in three pieces from polypropylene, seals with two O-rings, the smaller one to the front of the silicon wafer, the bigger one at the cap, as shown in Fig 4.8. The etching protector was placed into an etching solution, which was maintained at 60 C by an etch control 1

124 system (Fig 4.7). This control system consists of a hot plate (Cole-Parmer 04644) with provision for closed loop control using an RTD temperature probe constructed within an enclosure from stainless steel and enclosed by a Teflon sleeve closed at one end. The etchant was contained in a Teflon beaker that rested directly on the hotplate surface. The temperature probe passed through an opening in the center of the condenser-cap and its tip was immersed to a depth of.5 cm in the etchant. KOH is crystal-orientation-dependent etchant of silicon. The basic reaction is [11] Silicon + Water + Hydroxide Ions Silicates + Hydrogen The reaction sequence appears to be the following. Silicon atoms at surface react with hydroxyl ions. The silicon is oxidized and four electrons are injected from each silicon atom into the conduction band, Si + OH - Si(OH) + + 4e - Simultaneously, water is reduced, leading to the evolution of hydrogen, 4H O + 4e - 4OH - + H The complexed silicon, Si(OH) +, further reacts with hydroxyl ions to from a soluble silicon complex and water, Si(OH) + + 4OH - SiO (OH) - + H O Thus the overall reaction is, Si + OH - + H O SiO (OH) - + H For KOH solutions within a concentration ranging from 10-60%, the etch rate is determined by the KOH concentration and etch temperature. The following empirical formula for the calculation of the silicon etch rate was found to be close agreement with the experimental data [1] Etch Rate = k 0 [H O] 4 [KOH] 1/4 e -Ea/KT (4.1) The values for the fitting parameters were found to be E a = ev and k 0 = 480 μm/h (mol/liter) -4.5 for a <100> surface and E a = 0.6 ev and k 0 = 4500 μm/h (mol/liter) -4.5 for a <110> surface. Etch rates calculated by this formula for different KOH concentrations and etch temperatures for the <100> surface are listed in Table 4.1 [1]. The silicon wafer used for sensor fabrication is n type (100) orientation with inch diameter and 15 μm thickness. An etch rate of 45% KOH solution is about 17.1 μm/h at 60 C. To obtain a t = 14.0 μm 13

125 diaphragm, the time needed to etch is about 6.5 hr; to obtain a t = 15.5 μm diaphragm, the time needed to etch is about 6.4 hr. Upon completion of the etching, wafers are rinsed in running DI water for 3 minutes and dried with a nitrogen blow gun. The silicon diaphragm thickness could be estimated from sensor s reflectance spectra using a tunable laser. If the diaphragm thickness was greater than the desired value, we would repeat the etching process. The main drawback of this method historically has been the difficulty in obtaining consistent thickness and uniformity of the pressure diaphragm across the wafer. Fig 4.9 shows a Fabry-Perot cavity observed from the silicon side after thinning down the silicon diaphragm. And Fig 4.10 shows the profile of a Fabry-Perot cavity after cleaving through the glass-diaphragm structure Polishing Silicon Diaphragm After etching, the surface of silicon diaphragm was very rough, as shown in Fig 4.11 (a). Light would be scattered or total internally reflected in the interface of siliconair. This might make the measurement inaccurate. Polishing the diaphragm surface was necessary for the accuracy of sensor s measurement. Polishing papers of three different grit-size-papers were used; they are (in the order polishing was carried out) 3 μm, 1μm, and 0.3 μm. Each time, the polishing paper was changed from higher to lower grid size paper, the surface of silicon diaphragm was cleaned with isopropyl alcohol and dried immediately afterwards by blowing dry nitrogen [13]. In order to obtain a smoother surface polishing, Cerium Oxide polishing liquid (COPL) by Bueler Inc. was used. COPL is supposed to act not only as a good mechanical polishing agent, but also as a chemical agent that enhances the surface smoothness further. Again, polishing was carried out by diluting COPL with water in the following order: undiluted COPL, :1 diluted in water, 4:1 diluted in water. The glass-diaphragm structure was mounted onto the polishing disc for uniform polishing. Fig 4.11 (b) shows the diaphragm surface after polishing. The diaphragm surface became very smooth after the polishing process. 14

126 4..5 Sensor Package For initial evaluation the sensors were separated using a standard dicing saw. The glass-diaphragm structure and the fiber were mounted in a sturdy Lexan package which was able to maintain their proximity to etch other in the presence of harsh external environments. Fig 4.1 shows the package configuration. The package sensors will then be placed onto a mounting assembly suitable for static and dynamic testing. Pressure sensors and temperature sensors designed for multiplexing experiments were not packaged when tested in our lab. 4.3 Sensor Characterization To interrogate Fabry-Perot cavity-based sensors, we need to measure the ratio of the reflectance at two wavelengths for each sensor, and using Eq. (.45) and Eq. (3.11) to extract pressure. In order to do that, some of the sensor s parameters in Eq. (.45) and Eq. (3.11) have to be accurately measured, including initial diaphragm deflection and cavity depth, diaphragm thickness Measuring Initial Diaphragm Deflection and Cavity Depth As discussed in Section.3.3, the pressure inside the sensor cavities is higher than the atmosphere pressure for gauge pressure sensors because of the residual gas generated during anodic bonding and gas desorption (oxygen) from the surfaces inside the cavity. The sensor diaphragm would be deflected after etching process was completed due to the pressure difference. Newton s rings can be observed from the sensor under the optical microscope. Newton s rings were formed due to optical path difference in the film between an optical flat and curve surface. For Fabry-Perot cavity-based pressure sensors, the flat surface is the Pyrex glass wafer and the curved surface is the deflected silicon diaphragm. Since the cavity depth is only a few half-wavelengths, the orders of interference in the monochromatic pattern are very low, and fringes are visible with a white light source [14]. For this reason, Newton s rings can be observed under a Nikon optical microscope (Fig 4.13 (a)) with white light source in our lab. Fig 4.13 (b) shows how to place a sensor under microscope for observation of Newton s rings. 15

127 The fringes are circles about the center point of the sensor diaphragm. The cavity depth that satisfies the condition for a bright fringe is m λ d =, = 0, 1,... m (4.) The cavity depth difference for two neighboring bright fringes is λ d 1 d = (4.3) If the cavity depth decreases, the points of the film with given d move inwards and the fringes collapse towards the center, where one disappears each time the cavity depth decreases by λ /. If the cavity depth increases, the points of the film with given d move outwards with one disappearing each time the cavity depth increases by λ /. Initial diaphragm deflection can be determined by counting number of fringes observed from the sensor as shown in Fig For example, at 0 psi, we can observe one and half bright fringes. According to Eq. (4.3), diaphragm deflection at 0 psi is 0.75 λ; At 5 psi, the number of bright fringes we can observe is two and half. So, diaphragm deflection at 5 psi is 1.5 λ. The sensor s diaphragm will deflect in response to a change in pressure. The diaphragm deflection changes the depth of the cavity. Therefore, we can observe the movement of Newton s fringes with pressure increasing. Fig 4.15, Fig 4.16, and Fig 4.17 show the change of Newton s fringes with pressure increasing for a gauge pressure sensor, absolute pressure sensor and differential pressure sensor, respectively. For a differential pressure sensor, cavity was not sealed during bonding process. The residual gas generated during anodic bonding was evacuated through an opening to the atmosphere. So, the sensor diaphragm would not deflect after silicon etching. For this reason, no fringes were observed without pressure applied. Initial cavity depth is equal to the value measured by a Profilometer after etching cavity on glass wafer plus the initial diaphragm deflection measured by Newton s fringes Measuring Diaphragm Thickness The final diaphragm thickness for all diaphragms fabricated for this work was determined by the length of time the wafer remained in the silicon etchant. Batch production having one thickness is highly desirable, but in application, it s very difficult 16

128 to maintain uniform thickness on one wafer. For this work, however, a method is desired to accurately achieve a desired diaphragm thickness. Diaphragm thickness was initially determined by subtracting glass wafer thickness, measured with a micrometer before and after etching. The digital micrometer (Mitutoyo Digimatic) only resolved to one micron, giving an uncertainty of ±.5 μm in diaphragm thickness [15]. Besides, the etched diaphragm was not uniform, which made the measurement inaccurate. Microscope focusing can be used to determine the silicon diaphragm thickness by cleaving through the glass-diaphragm structure and measuring its thickness on edge, as shown in Fig The nonuniformity of the etched diaphragm would limit the accuracy of this method. Because the diaphragm thickness was different at the cleaved position from that of the sensor. An optical method of measuring diaphragm thickness using a tunable laser (New Focus 638 Velocity Laser) and locating the wavelengths at two neighboring reflectivity minima was developed. The experimental set up for measuring diaphragm thickness in this way is shown in Fig 4.4. Two photodetectors are used to measure the input and output from the sensors, respectively. The ratio of the input and output from the sensor was called reflectivity. We can measure the reflectance spectrum by sweeping the wavelength ( nm) of the tunable laser, as shown in Fig The reflectance from the pressure sensor is given by Eq. (.48), with n 0 = n glass (=1.473), n 1 = n air (=1.0), n = n si (=3.478) and n 3 = n air (=1.0) respectively R( λ) jθ jφ r3 + r34e r1 + e ( ) jθ 1 + r3r34e = (4.4) jθ jφ r3 + r34e 1 + r1e ( ) jθ 1 + r3r34e where 4πn θ t = λ (4.5) 4πn φ 1 d = λ (4.6) 17

129 d is the cavity depth of the sensor and t is the diaphragm thickness. The reflectivity is minimized at the resonance condition [16] φ + θ mπ (4.7) The phase difference between two neighboring reflectivity minima λ m1, λ m is 4π 4π Δφ + Δθ = ( nt + n1d ) ( nt + n1d ) = π λ λ m1 m (4.8) Since n 1 t >> n d, n 1 d can be neglected. So, silicon diaphragm thickness is calculated using the following equation 4π n λ m1 si 4π t n λ m si t π t (4.9) This optical method provides great advantages over the previous two methods. One of the important advantages is that the measurement accuracy is not affected by the nonuniformity of the silicon diaphragm. Therefore, we chose this method to measure diaphragm thickness in this project. 4.4 Pressure Sensor Calibration Pressure sensors designed to respond over the pressure range 0 1 psi and operating at 850 nm (LED) were sent to Air Force Research Lab for testing. The optical detection scheme was based on the fact that the reflected light from the sensor was spectrally shifted [17]. This measurement scheme is shown in Fig 1.4. A broadband light emitting diode (LED) centered at 850 nm was routed onto the sensor through a coupler. The reflected light was collected back through the same fiber and was routed to a second coupler that split the reflected light into two equal intensity signals. One of the signals was routed to a photodetector and the other to a high band pass optical filter and photodiode combination. This detection scheme effectively measured the change in reflectivity that results from shifting of the spectrum of the Fabry-Perot cavity-based pressure sensor (Fig 4.0). The ratio of the output of the two photodetectors was taken to be a measure for the sensed parameter [18]. A pressure sensor with the design values of cavity radius r = 300 μm, cavity depth d = 0.64 μm, and silicon diaphragm thickness t = 14.0 μm was tested showing an 18

130 approximately linear response to static pressure 0 1 psi. The slope of the best fit to the data gives a sensitivity of about 0.1 mv/psi. The static pressure response results are shown in Table 4. and plotted in Fig 4.1. Each pressure point was the average of 100 readings acquired. The pressure was also monitored with a Druck pressure calibrator ant the output was acquired with a computer based data acquisition system. The LED and photodiodes were maintained at constant temperature with a thermoelectric cooler assembly that can be monitored during experiment to insure constant LED output. The individual pressure sensor was then tested for its dynamic response in wind tunnel. Fig 4. shows computer simulation of pressure fluctuation in wind tunnel and Fig 4.3 shows the measured pressure fluctuation in wind tunnel by a Fabry-Perot cavity-based pressure sensor. The agreement between the experimental results and computer simulation confirms the applicability of the Fabry-Perot cavity-based pressure sensor for high speed application, such as gas turbines [19]. Pressure sensors designed to respond over the pressure range 0-30 psi and operating at 1550 nm (tunable laser) was testing in our lab. Air pressure applied to the silicon diaphragm was controlled using a pressure regulator and monitored with a gauge, as shown in Fig 4.7 and Fig 4.8. The reflectance from this pressure sensor is given by Eq. (.45). The reflectance is a function of the cavity depth d (see Fig..9) and wavelength when the designed diaphragm thickness is obtained after the etching process. We measured the reflectance spectra R(λ) at several values of pressure for a Fabry-Perot cavity-based pressure sensor using the schematic diaphragm shown in Fig 4.4. The experimental results are shown in Fig 4.5. Sensor s cavity depth was 0.95 μm measured with a profilometer, and diaphragm thickness was.95 μm measured with the optical method mentioned in Section From Fig 4.5, we can see that the reflectance spectra R(λ) from the pressure sensor will move upwards with pressure increasing. If we measured the reflectance spectra R(λ) using FC/PC connectors instead of FC/APC connectors, unwanted interferometric signals in the system would cause ripples, as discussed in Chapter 3. The results are shown in Fig 4.6. Comparing Fig 4.5 with Fig 4.6, we can see that the existence of ripples in the reflectance spectra R(λ) would affect the measurement accuracy of the sensor. 19

131 4.4.1 Calibration of Multiplexed Pressure Sensor System Wavelength multiplexing of MEMS pressure sensors using FBGs or AWGs have been fabricated and tested in our lab as a part of this project. Schematic diagrams of the systems are shown in Fig 3. and Fig 3.4 in Chapter 3. The experimental set-up is shown in Fig 4.8. Air pressure applied to the silicon diaphragm was controlled using a pressure regulator and monitored with a gauge for each sensor. The air pressure control system in our lab is shown in Fig 4.7. Scanning speed of the tunable laser was set at 0. nm/sec, and scanning wavelength was set from nm. Results of multiplexing pressure sensors using FBGs are shown in Fig 4.9. The pressure range was 0psi 30psi. sensor #1 was operating at: λ 1 = nm and λ = nm, sensor # was operating at: λ = 155 nm, λ = nm From Fig 4.9, we can see there was no observable cross-talk between the two sensors. And the outputs of the individual sensor were easily extracted from the multiplexed signal. Results of multiplexing pressure sensors using an AWG are shown in Fig Outputs at channel #1 and channel #8 of AWG were directed into sensor #1, while outputs at channel #4 and channel #6 were directed into sensor #. The center wavelengths of the four channels at room temperature are: λ 1 =1547 nm, λ 4 = nm, λ = nm, λ = nm There was no significant cross-talk between the two sensors. The outputs of the individual sensor were extracted from the multiplexed signal in Fig 4.9 for investigation. Fig 4.31 shows the measured reflectance as a function of pressure (0psi 30psi) for sensor #. The data at two wavelengths were slightly different, reflectance at nm was larger than the reflectance at 155.1nm at the same pressure, but they all showed a steady, nearly linear variation with pressure. From this, a maximum sensitivity of 0.0 mv/psi for the multiplexing system using an AWG, and of

132 mv/psi for the multiplexing system using FBGs were obtained. Fig 4.3 shows the ratio of the reflectance R( λ ) I( R, λ) = measured at two wavelengths (λ 1 = nm R( λ ) + R( λ ) 1 and λ = nm) for sensor #. The theoretical curve was obtained from combining Eq. (.45) and Eq. (3.11). Without pressure applied, the cavity depths for the two sensors were 1.19 μm and 0.95 μm measured with a Profilometer, and the effective reflectance were.5% and 1.7% respectively. The diaphragm thickness for the two sensors were μm and.95 μm, measured with an optical method mentioned in Section The good agreement between the experimental and theoretical curve confirms the applicability of the two wavelength interrogation technique for the multiplexed pressure sensor system. Therefore, we can say that two Fabry-Perot cavity-based pressure sensors multiplexed using the designed systems have been successfully demonstrated Comparison of the Two Multiplexed Pressure Sensor Systems FBG is a comparatively simple device. Multiple Bragg Gratings can be inscribed into one single-mode fiber. They have important applications because of their integration with fibers and the large number of device functions that they can facilitate, thereby increasing flexibility [0]. Their very low insertion loss and narrowband wavelength reflection offer the potential of multiplexing large number of pressure sensors using a single-mode optical fiber. However, the sensor s pressure sensitivity associated with multiplexing system using FBGs is low, mainly because of the loss in the couplers and connectors. This can be improved by using one single-mode fiber with multiple Bragg gratings. AWGs are relatively expensive devices. But AWGs are monolithic devices that can separate many optical channels at once [1]. This makes them much simpler for systems with high channel counts, which means a large number of sensors can be multiplexed. Therefore, the cost per channel or per sensor can be much lower. The multiplexing system using an AWG is simper than using FBG, which leads to the lower loss in the system because of reducing the number of fiber couplers and connectors. The sensor s sensitivity using an AWG is thus much higher. For multiplexing pressure sensors using 131

133 AWG, the great concern for system design is crosstalk between two neighboring channels and the bandwidth of the channels Temperature Sensor Calibration Two Fabry-Perot temperature sensors consisting of a layer of silicon that has a temperature-dependent refractive index have been fabricated and tested in our lab. The configuration of sensor #1 is shown in Fig It consists of a layer of silicon with the thickness 15 μm and the reflectance from this sensor is given by Eq. (.4), with n 0 (SiO ) = 1.473, n 1(Si) = 3.478, and n (air) = 1. The configuration of sensor # is shown in Fig It consists of a thin layer of silicon with the thickness 5 μm, bonded on a Pyrex glass wafer. The reason of glass-to-silicon bonding was due to the difficulty of handling thin silicon wafers. The reflectance from this sensor is given by Eq. (.4), with n 0 (SiO ) = 1.473, n 1(Si) = 3.478, and n (SiO ) = As discussed in chapter, the temperature range of a Fabry-Perot temperature sensor is an inverse function of silicon thickness. For sensor #1, silicon thickness is 15 μm, we can get maximum refractive index change for silicon without causing ambiguity from Eq. (.11) Δn < 1550 nm 4 15 μm = Combining Eq. (.106) and (.107), this corresponds to the temperature range: 4 C 35 C. For sensor #, silicon thickness is 15 μm, the maximum refractive index change for silicon without causing ambiguity is Δn < 1550 nm 4 15 μm = This corresponds to the temperature range: 4 C 115 C. In essence, a Fabry-Perot temperature sensor provides a temperature-sensitive reflectance spectrum []. Accordingly, a wavelength-encoded temperature measurement was developed by tracking the shift of one of the resonance wavelengths. We used the schematic diaphragm as shown in Fig 4.33 to measure the reflectance spectrum R(λ) from a Fabry-Perot temperature sensor. In order to accurately determine λ m, the 13

134 reflectivity from the sensor was measured using a sampling interval of 0.1 nm. The experimental set up is shown in Fig Temperature was applied to the sensor by a digital hot plate (Cole-Parmer 04644). A thermo couple was glued on the sensor for temperature calibration. The output from the thermo couple was monitored by a Multimeter. The two sensors were calibrated by tracking λ m (T) over the temperature range. Fig 4.36 and Fig 4.38 show the reflectance spectrum shifting under several values of temperature for the two sensors, respectively. By measuring resonance wavelength shift at elevated temperature, silicon refractive index at elevated temperature was obtained from Eq. (.10). Therefore, the temperature experienced by the Fabry-Perot temperature sensor can be determined from the silicon refractive index at elevated temperature by combining Eq. (.106) and (.107) [3]. A comparison of the temperature measured by the Fabry-Perot temperature sensor with the temperature measured by thermo couple for the two sensors is shown in Table 4.3 and Table Calibration of Multiplexed Temperature Sensor System As discussed in chapter, there are two different interrogation techniques for temperature sensors that lead to different multiplexed systems. The multiplexed temperature sensors system using dual-wavelength interrogation technique is similar to the multiplexed pressure sensors system described in Section 3.1 and Section 3. and shown in Fig (3.) and Fig (3.5), except replacing pressure sensors with temperature sensors. The experimental set up is shown in Fig Scanning speed of the tunable laser was set at 0. nm/sec, and scanning wavelength was set from nm. Results of multiplexing temperature sensors using FBGs are shown in Fig sensor #1 was operating at: λ 1 = nm and λ = nm, sensor # was operating at: λ = 155 nm, λ = nm 133

135 From Fig 4.39, we can see there was no observable cross-talk between the two sensors. And the outputs of the individual sensor were easily extracted from the multiplexed signal. The outputs of the individual sensor were extracted from the multiplexed signal in Fig 4.39 for investigation. Fig 4.40 shows the measured reflectance as a function of temperature (4 C 35 C) for sensor #1. The data at two wavelengths were different, reflectance at nm was decreasing with temperature increasing while reflectance at nm was increasing with temperature increasing, but they all showed a steady, nearly linear variation with temperature. From this, a maximum sensitivity of 0.08 mv/ C for the multiplexing system using FBGs was obtained. Fig 4.41 shows the ratio of the reflectance R( λ ) I( R, λ) = measured at two wavelengths (λ 1 = nm and λ = R( λ ) + R( λ ) nm) for sensor #1. Dual-wavelength interrogation described in Section 3.4 was used to interrogate temperature sensors. Reflectance at two different wavelengths λ1 and λ were separately measured, and then a ratio I ( R, λ) was calculated using Eq. (3.11) assuming the losses were wavelength independent over the wavelength range. In Eq. (3.11), R λ ), R λ ) were the reflectance from Fabry-Perot temperature sensor at two wavelengths λ 1 and λ, defined in Eq. (.98). For the purpose of dual-wavelength interrogation, silicon thickness has to be accurately measured. An optical method of measuring silicon thickness using a tunable laser (New Focus 638 Velocity Laser) by locating the wavelengths at two neighboring reflectivity minima was developed. The schematic diagram for measuring silicon thickness is shown in Fig We can measure the reflectance spectrum by sweeping the wavelength ( nm) of the tunable laser, as shown in Fig 4.36 and Fig The reflectance from the temperature sensor is given by Eq. (.98), with n 0 = n glass (=1.473), n 1 = n si (=3.478) and n 3 = n air (=1.0) for sensor #1 and n 3 = n glass (=1.473) for sensor # respectively R jφ r1 + r3e ( λ) jφ 1 + r1r3e where = (4.10) ( 1 ( 134

136 4πn φ 1 d = (4.11) λ d is silicon thickness. The reflectivity is minimized at the resonance condition φ = mπ (4.1) The phase difference between two neighboring reflectivity minima λ m1, λ m is 4π 4π Δφ = n1d n1d = π λ λ m1 m Accordingly, silicon thickness can be calculated using the following equation 4π n λ m1 si 4π d n λ m si d = π (4.13) d (4.14) After the silicon thickness was determined, the silicon refractive index at elevated temperature was obtained from Eq. (3.11) using the data extracted from multiplexed signal. Therefore, the temperature experienced by the Fabry-Perot temperature sensor can be determined from silicon refractive index at elevated temperature combining Eq. (.106) and (.107). The temperature range of the Fabry-Perot temperature sensor using dualwavelength interrogation technique was determined by the silicon thickness and the two wavelengths being chosen. For sensor #1 operating at λ 1 = nm and λ = nm and with silicon thickness 15 μm, the temperature range for using multiplexed system was 4 C 36.5 C; For sensor # operating at λ = 155 nm, λ4 = nm and with silicon thickness 15 μm, the temperature range was 4 C 136 C. A comparison of the temperature measured by sensor #1 using multiplexed system with temperature measured by the thermocouple is shown in Table. The good agreement of the data from the temperature sensors and the data from the thermocouple shows the excellent performance of the novel temperature sensors for accurate temperature measurement. The multiplexed temperature sensor system using wavelength-encoded measurement is described in Section 3.6 and shown in Fig 3.1. In this system, we are using reflected light from broadband fiber gratings to interrogate the multiplexed temperature sensors. Due to the limitation of funding, we cannot buy broadband fiber gratings to demonstrate this system. Instead, we used Mathematica 4.0 for the simulation and demonstration. Fig 4.4 shows the reflectance spectrum shift for sensor #1 using the 135

137 multiplexed temperature sensor system. The reflected light from a broadband fiber grating with 10 nm bandwidth was used to interrogate sensor #1 using wavelengthencoded measurement. 4.5 Summary A simple micromachining process compatible with MEMS was developed in fabricating Fabry-Perot cavity-based pressure and temperature sensors. Some of the important parameters of pressure and temperature sensors were accurately measured using optical methods including Newton s fringes and reflectance spectrum. Common problems with optical sensing systems have been presented. Examples of optical pressure and temperature sensor measurement systems of various designs have been provided. Wavelength multiplexing of two Fabry-Perot pressure and two temperature sensors using both fiber Bragg gratings (FBGs) and arrayed waveguide gratings (AWGs) have been tested and demonstrated in our lab. A multiplexed temperature sensor system using broadband fiber gratings was demonstrated by simulation using Mathematica

138 4.6 Reference [1] S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford University Press, New York, [] J. Lasky, Bonding of Silicon to Silicon, Applied Physics Letters, 48(1), pp. 78, [3] S. K. Ghandi, VLSI Fabrication Principles, New York: Wiley, Chapter 9, [4] J. S. Judge, A Study of the Dissolution of SiO in Acidic Fluoride Solutions, Journal of Electrochemical Society, Vol. 118, No. 11, pp , [5] H. Engelke, and M. Harz, Curvature Changing or Flattening of Anodically Bonded Silicon and Borosilicate Glass, Sensors and Actuators, A 55, pp , [6] A. J. Moser, The practising scientist's handbook, Van Nostrand Reinhold Company Inc., New York, [7] M. Despont, H. Gross, F. Arrouy, C. Stebler, Fabrication of a Silicon-Pyrexsilicon Stack by Anodic Bonding, U. A. IBM Research Division, Zurich Research Laboratory, 8803 Rüschlikon, Switzerland. [8] D. Carlson, K. Hang, and G. Stockdale, Ion Depletion of Glass at a Blocking Anode II, Journal of the American Ceramic Society, 57(7), pp , [9] T. Corman, P. Enoksson, and G. Stemme, Low-pressure-encapsulated Resonant Structures with Intergrated Electrodes for Electrostatic Excitation and Capacitive Sensors, Sensors and Actuators, A(66), pp ,

139 [10] W. R. Runyan and K. E. Bean, Semiconductor Integrated Circuit Processing Technology, Reading, MA: Addison-Wesley, [11] Gregory T.A., Micromachined Transducers Sourcebook, Boston: WCB/McGraw- Hill, [1] H. Seidel, L. Csepregi, A. Heuberger, H. Baumgärtel, Anisotropic Etching of Crystalline Silicon in Alkaline Solutions, Journal of Electrochemical Society, Vol. 137, No. 11, pp , [13] Guide to Connectorization and Polishing Optical Fibers, ThorLabs Inc., [14] Max Born and Emil Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, New York, 7 nd ed., [15] G. N. De Brabander, Integrated Optical Interferometers with Micromachined Diaphragms for Pressure Sensing, in ECECS Department, Cincinnati: University of Cincinnati, [16] G. Beheim, K. Fritsch, and D. J. Anthan, Fiber-Optic Temperature Sensor Using a Spectrum-Modulating Semiconductor Etalon, NASA TM , [17] L. A. Johnson and S. C. Jensen, Problems and Approaches for Remote Fiber Optic Absolute Sensors, presented at Fiber Optic and Laser Sensors III, San Diego, CA, and published in Proceedings of SPIE, Vol. 566, pp , [18] Jie Zhou, Samhita Dasgupta, Hiroshi Kobayashi, Howard E. Jackson and Joseph T. Boyd, Optically Interrogated MEMS Pressure Sensors for Propulsion 138

140 Applications, Optical Engineering, 40(4), pp , 001. [19] H. Kobayashi, T. Leger, and J. M. Wolff, Experimental and Theoretical Frequency Response of Pressure Transducers for High Speed Turbomachinery, International Journal of Turbo and Jet Engines, in press. [0] Andreas Othonos and Kyriacos Kalli, Fiber Bragg gratings: Fundamentals and Applications in Telecommunications and Sensing, Boston: Artech House, [1] Meint K. Smit and C. Van Dam, PHASAR-based WDM devices: Principles, Design and Applications, Journal of Selected Topics in Quantum Electronics, Vol., No., [] G. Boreman, R. Walters, and D. Lester, Fiber Optic Thin Film Temperature Sensor, Fiber Optic and Laser Sensors III, E. L. Moore and O. G. Ramer, eds., Proceedings of SPIE, pp , [3] Gorachand Ghosh, Temperature Dispersion of Refractive Indices in Crystalline and Amorphous Silicon, Applied Physics Letter, Vol. 66, No. 6, pp ,

141 Photoresist 7740 Pyrex glass Spin Photoresist Mask Lithograph Develop Photoresist Etch in BHF and strip resist Silicon Silicon-glass Anodic bonding Protected KOH Etching to form sensor diaphragm Fig. 4.1 MEMS fabrication processing steps 140

142 Fig. 4. Mask for Fabry-Perot cavity-based pressure sensors 141

143 Fig. 4.3 Cavity depth measured by Profilometer The sensor above has the cavity depth around μm The sensor below has the cavity depth around μm. 14

144 Fig. 4.4 Glass-to-silicon anodic bonding machine in our lab. Fig. 4.5 Glass-to-silicon anodic bonding setup 143

145 Fig. 4.6 Typical current flow during anodic bonding Fig. 4.7 A Teflon beaker for silicon diaphragm etching 144

146 Fig. 4.8 Wafer protector for etching bonded Si substrate 145

147 Fig. 4.9 A Fabry-Perot cavity observed from the silicon side after thinning down the silicon diaphragm sensor cavity glass silicon diaphragm Fig The profile of a Fabry-Perot cavity after cleaving through the glass-diaphragm structure 146

148 (a) (b) Fig Surface of the silicon diaphragm after etching (a) before polishing (b) after polishing 147

149 Fig. 4.1 Package configurations for housing the optical fiber and MEMS pressure sensor: (a) differential pressure sensor (b) absolute and gauge pressure sensor 148

150 (a) 149

151 cavity glass silicon pressure (b) Fig (a) Nikon optical microscope in our lab (b) Place a sensor under microscope for observation of Newton s rings 150

152 0psi Δd 0.75λ glass d1 d ' d 1 ' d 1.5λ pressure ' Δd deflected Si membrane 5psi Fig Illustrating the formation of Newton s rings 151

153 Without etching 0psi 5psi 10psi 15psi 0psi 5psi 30psi Fig Change of Newton s fringes with pressure increasing for a gauge pressure sensor 15

154 0psi 5psi 10psi 15psi 0psi 5psi 30psi Fig Change of Newton s fringes with pressure increasing for an absolute pressure sensor 153

155 0psi 5psi 10psi 15psi 0psi 5psi 30psi Fig Change of Newton s fringes with pressure increasing for a differential pressure sensor 154

156 silicon diaphragm glass Fig Microscope focusing can be used to determine the silicon diaphragm thickness by cleaving through the glass-diaphragm structure and measuring its thickness on edge 155

157 Measured reflectance Wavelength (nm) λm1 λ m Fig Silicon diaphragm thickness can be determined by the measured reflectance spectrum from the sensor 156

158 Fig. 4.0 The relationship between interferometer cavity depth and cavity reflectance is shown for LED emission at 80, 850 and 880 nm. Note the reflectance is wavelength dependent at each cavity depth 157

159 Fig. 4.1 The static pressure response results for the pressure sensor operating at 850 nm 158

160 Fig. 4. Computer simulation of pressure fluctuation in wind tunnel 159

161 Fig. 4.3 Measured pressure fluctuation in wind tunnel 160

162 Tunable laser 3 d B detector detector sensor Fig. 4.4 Schematic diagram of single pressure sensor measurement 0.06 Pressure Measured reflectance psi 5psi 10psi 15spi 0psi 5psi 30psi W avelength (nm) Fig 4.5 Measured reflectance spectra at several values of pressures for single pressure sensor measurement (cavity depth = 0.95 μm; diaphragm thickness =.95 μm) 161

163 Measured reflectance 0.08 pressure Wavelength (nm) 30psi 5psi 0psi 15psi 10psi 5psi 0psi Fig 4.6 Measured reflectance spectra at several values of pressures for single pressure sensor measurement when using FC/PC connectors in the system instead of FC/APC connectors 16

164 Fig 4.7 Air pressure control system in our lab 163

165 Index matching fluid Connector Sensor cavity Polishing disc Silicon-glass Wafer Sensor #1 Glass tube Sensor # Pressure gauge 1 Valve 1 Valve Vent to Fine atmosphere adjustment Valve Pressure regulator To 80 psi building compressed air Pressures (0-30psi) applied to the two sensors can be controlled by two valves and two pressure gauges Fig. 4.8 Experimental set-up for multiplexed pressure sensor system 164

166 0.07 Measured Reflectance psi 5psi 10psi 15psi 0psi 5psi 30psi Wavelength (nm) Fig. 4.9 Results of multiplexed pressure sensor system using fiber Bragg gratings (FBGs) 165

167 M easured Reflectivity psi 5psi 10psi 15psi 0psi 5psi 30psi Wavelength (nm) Fig 4.30 Results of multiplexed pressure sensor system using arrayed waveguide gratings (AWGs) 166

168 λ 1 =1557.1nm Measured Reflectance λ 3 =155.1nm Pressure (psi) Fig Measured reflectance as a function of pressure (0 30 psi) extracted from the multiplexed signal for Sensor # at two wavelengths: λ 1 = nm and λ = nm. 167

169 Ratio Theoretical curve Measured curve Pressure (psi) Fig. 4.3 The ratio R( λ ) I( R, λ) = (λ 1 = nm and λ = nm) for R( λ ) + R( λ ) 1 sensor # as a function of pressure Dotted lines: theoretical curve; Solid lines: measured response. 168

170 Tunabl e laser detector detector Temperature sensor Fig Schematic diagram of single Fabry-Perot temperature sensor measurement Connector Polishing disc Silicon-glass Wafer Digital hot plate Temperature sensor Thermal couple Multimeter Fig Experimental set-up for Fabry-Perot temperature sensor measurement 169

171 13 μm silicon Fig Configuration of a Fabry-Perot temperature sensor #1 π Measured Reflectance Wavelength (nm) 3.8 C 8.8 C 3. C 35.8 C λ1 λ Fig Measured reflectance spectrum shift under several values of temperature for sensor #1 170

172 5μm silicon glass Fig Configuration of a Fabry-Perot temperature sensor # π Measured Reflectance C 46.8 C 6.3 C 8.9 C 107. C λ λ 1 Wavelength (nm) Fig Measured reflectance spectrum shift under several values of temperature for sensor # 171

173 Measured reflectance Sensor #1 Sensor # Wavelength (nm) 5.7 C 9.6 C 3.6 C 35.8 C Fig Results of multiplexed temperature sensor system using fiber Bragg gratings (FBGs) 17

174 Measured Reflectance 0.45 λ 1 = nm λ = nm Temperature ( 癈 ) Fig Measured reflectance as a function of temperature (4 C 35 C) extracted from multiplexed signal for sensor #1 at two wavelengths: λ 1 = nm and λ = nm 173

175 Ratio Temperature ( 癈 ) Fig Ratio of the reflectance R( λ ) I( R, λ) = measured at two wavelengths R( λ ) + R( λ ) (λ 1 = nm and λ = nm) for sensor #

176 Reflectivity WavelengthHnmL (nm) 4 C 90 C Fig. 4.4 Mathematica simulation of reflectance spectrum shift for sensor #1 using the multiplexed temperature sensor system. (The reflected light from a broadband fiber grating with 10 nm bandwidth was used to interrogate sensor #1 using wavelength-encoded measurement) 175

177 Table 4.1 <100> silicon etch rates in [μm/h] for various KOH concentrations and etch temperatures as calculated from Eq.() by setting E a = ev and k 0 = 480 μm/h (mol/liter) -4.5 % Temperature [ C] KOH Table 4. Data for pressure sensor measurement (sensor operating at 850 nm) Pressure (psi) Vsensor (Volts) Vkulite (Volts) Table 4.3 Data for temperature sensor measurement (sensor #1): Temp (T 1 ) on 3.8 C 8.8 C 3. C 35.8 C thermocoupler Temp (T ) on RT 9.16 C 3.7 C 35.4 C sensor ΔT = T - T C C C RT -- room temperature 176

178 Table 4.4 Data for temperature sensor measurement (sensor #): Temp (T 1 ) on thermocouple 4.3 C 46.8 C 6.3 C 8.9 C 107. C Temp (T ) on sensor RT 46. C C 83 C C ΔT = T - T C C +0.1 C -1.5 C RT -- room temperature Table 4.5 Data for temperature sensor measurement using multiplexed system (sensor #1): Temp (T 1 ) on 4.7 C 9.6 C 3.6 C 35.6 C thermocouple Temp (T ) on RT 30 C 3.4 C C sensor ΔT = T - T C -0. C C RT -- room temperature 177

179 5 Anti-reflection Layer for Pressure Sensor and Encapsulating Structure for Temperature Sensor 5.1 Introduction Sensors need an encapsulation layer for the protection of the silicon surface from dirt and other contaminants. The encapsulant should therefore be opaque, to prevent the guided light from interacting with contaminants and also to block any external light. The interface between the encapsulant and the silicon must be extremely stable, with no optically detectable reaction or diffusion of the constituents of the films, throughout the sensor s operational life. To protect the silicon from oxidation, the encapsulant should be an effective barrier to the diffusion of oxidizing species. Further, the encapsulant, itself, should be highly stable. Finally, the encapsulant should have a reflectivity which provides the thin-film interferometer with a high fringe visibility. An anti-reflection layer on silicon would improve the operation of pressure sensors. The anti-reflective coating of SiO was deposited on the top surface of Fabry-Perot pressure sensor by evaporation. The purpose of this anti-reflective coating is to reduce the sensor s temperature sensitivity. The encapsulating layers of Si 3 N 4 and aluminum were deposited on the top surface of Fabry-Perot temperature sensor for the protection of the silicon surface from oxidation, dirt and other contaminants. In this section, optimization of the Si 3 N 4 and aluminum deposition will be described, and the characteristics of the Si 3 N 4 film will be discussed. The Fabry-Perot cavity-based pressure sensor with antireflective coating and the temperature sensor with encapsulating layers will be calibrated and the data will be compared with theoretical results. 5. SiO Evaporation Process As discussed in chapter, the anti-reflective coating, which consists of a quarterwave layer of SiO, on the top surface of the silicon membrane, prevents unnecessary interference within the membrane and reduces pressure sensor s temperature dependence. Silicon monoxide, SiO, is an amorphous solid, with similar hardness with silicon. Silicon 178

180 monoxide is brown in powder or pressed form and black in granular or lump form. SiO is stable at room temperature, but at temperatures of C, SiO can be decomposed into silicon oxide and silicon. Silicon monoxide is a highly desirable coating material for optical applications in reflectors, mirrors and other products due to its high deposition rate and ease of evaporation using low cost resistance-heated coating systems. SiO is also noted for its excellent environmental stability [1]. In preparation for SiO deposition, the substrate was first cleaned in order to allow the coating material to adhere to the surface of the substrate without leaving visible stains or pinholes. SiO films were produced by evaporating at x 10-6 Torr with substrate temperatures of 100 to 1600 C. Resistance heating in baffle boats was used to generate the high substrate temperature. Fig 5.1 shows a vacuum evaporation oven for SiO deposition. Once the machine had reached proper operating conditions, SiO was deposited by vaporizing a SiO target with an electron gun, using very high voltage. As the material evaporated, it condensed on the substrate. Once the thickness of this condensate layer had reached the optimum design value 10 nm, the machine operator terminated the deposition. We prepared two samples with anti-reflective coating. The vapor-deposited SiO film shows a green color (Fig 5.) Calibration of Pressure Sensor with Anti-reflective Coating After applying a thin anti-reflective coating of SiO on the top surface of the pressure sensor, light interference within the membrane was significantly reduced to a negligible level. Therefore, the reflectance from the sensor is obtained from Eq. (.49) for a three-layer Fabry-Perot interferometer. Fig 5.3 (a) shows the theoretical reflectance spectrum at 0 psi for a Fabry-Perot cavity-based pressure sensor with anti-reflective coating using Eq. (.49). Fig 5.3 (b) shows the measured reflectance spectrum at 0 psi for a pressure sensor with anti-reflective coating. The cavity depth of the sensor was 0.9 μm, measured with a Profilometer. From a comparison, we can see that the results from pressure sensor with anti-reflective coating show good agreement between experimental and theoretical results. Fig 5.4 shows measured reflectance spectra at several values of pressure for a pressure sensor with anti-reflective coating. The response of the reflectance as a function of pressure has significant wavelength dependence. The reflectance from 179

181 the sensor at two wavelengths (λ 1 = 1548 nm and λ = 1550 nm) were extracted for investigation. Fig 5.5 shows the measured reflectance as a function of pressure at two wavelengths. The data at two wavelengths were quite similar and all showed a steady, nearly linear variation with pressure, because the two wavelengths were chosen to be very close. We are expecting reduced temperature sensitivity for the pressure sensor with antireflective coating. Therefore, the response of the reflectance from a pressure sensor with anti-reflective coating as a function of temperature was investigated. Fig 5.6 shows measured reflectance spectra at several values of temperature from 50 C 10 C with temperature increment at 10 C. From Fig 5.6, we can see that the temperature response from the pressure sensor with anti-reflective coating was quite random. And the temperature sensitivity was also too high, which was not good for the pressure sensor. There exists a problem of cross sensitivity of pressure and temperature for the Fabry- Perot cavity-based pressure sensor. The reason for the poor performance of the sensor under temperature is due to the nonuniformity of the deposited SiO film. As discussed in chapter, the condition for antireflective coating is wavelength dependent [] λ d = (5.1) 4n where d is the SiO thickness and n is the refractive index of SiO. For a certain thickness d of SiO film, the condition for anti-reflective coating is met only at certain wavelengths which can satisfy Eq. (5.1). So, accurate measurement of SiO thickness is very critical for determining the wavelengths that the pressure sensor can operate with the anti-reflective coating condition met. For some reason, spectroscopic ellipsometry was unable to measure the thickness of our SiO film. Therefore, we can not tell if the condition for antireflective coating is met or not for a Fabry-Perot pressure sensor. And if the condition can not be met, the temperature sensitivity will be undesirably high. 5.3 Si 3 N 4 and Aluminum Deposition Process An encapsulated sensor has two oxidant-barrier layers deposited on one side of silicon: silicon nitride layer (00 nm design thickness) and aluminum layer (1 μm thickness). The reasons for choosing the above layer materials and the design 180

182 considerations are described in chapter. In fabricating the encapsulated temperature sensor, we first anodically bonded the silicon wafer onto the Pyrex glass wafer and then deposited the above layers by sputtering on silicon. Silicon nitride is widely used to passivate semiconductor devices because it is an excellent barrier to contaminants such as moisture and sodium [3]. A popular way to deposit silicon nitride is by plasma-enhanced chemical vapor deposition (PECVD), at temperature between 50 C and 350 C. This PECVD nitride is termed SiN or SiN x to emphasize its lack of stoichiometry. It is generally silicon rich (x = 0.8 to 1.), with a large concentration of hydrogen (0 to 5 atomic %). Both these factors cause PECVD SiN x to be thermally unstable, with a refractive index that is dependent on the film s thermal history. Silicon nitride deposited by PECVD is, therefore, ill-suited for this application, since changes in the refractive index of the silicon-nitride layer can cause the sensor s output to drift. Another disadvantage of PECVD SiN x is its lower density, compared to stoichometric and hydrogen-free Si 3 N 4. A film with the highest possible density is desired in order to best protect the underlying silicon. Another disadvantage of PECVD SiN x is the incompatibility of PECVD with Pyrex glass substrate. Hydrogen-free and stoichiometric Si 3 N 4 films are most readily deposited, at low substrate temperatures, by RF magnetron or diode sputtering [4-9]. Sputtering a Si 3 N 4 target in an inert gas, such as argon, produces a film which is nitrogen deficient. Stoichiometric Si 3 N 4 is obtained only by reactive sputtering in nitrogen (argon can be added to increase the deposition rate). Impurities are minimized by using a silicon target. Also, Si 3 N 4 contain about 1% binder, such as MgO. Fig 5.7 shows the sputtering machine used for Si 3 N 4 deposition. The source material and target substrate are connected to opposing terminals of a high-voltage power supply. The potential difference causes the source material to be bombarded with ions from a plasma gas in the chamber. The source releases molecules ions into the chamber which are deposited onto the wafer. In the process of depositing Si 3 N 4, the silicon target is bombarded with N + and N + ions which can develop energies in excess of 1 kev as they are accelerated across the cathode dark space. Bombardment by nitrogen ions produces a Si 3 N 4 coating on the surface of the target. The steady-state surface condition should therefore be developed by pre-sputtering prior to deposition. The sputtered particles (mostly atoms) typically have 181

183 several ev of kinetic energy when expelled from the target. These particles will impact the substrate in a manner which is strongly influenced by the gas pressure. With increasing pressure, the sputtered particles suffer more collisions with gas molecules, so that their energies at the substrate are reduced, and the range of their angles of incidence is broadened. Eventually, at a pressure which is inversely proportional to the target-tosubstrate spacing, the sputtered particles are thermalized to the average gas energy. Higher impact velocities densify the film by increasing the surface mobilities of the atoms. Normal incidence, as is obtained at low pressures, also promotes a greater density, since oblique incidence causes shadowing and void formation. A reduced pressure, therefore, aids the development of a dense and stoichiometric Si 3 N 4 film. Si 3 N 4 films were deposited by sputtering in pure N with pressures ranging from 1.8 to 5.3 mtorr. The target-to-substrate distance was 6 cm. Considerable care was exercised to minimize contamination of the sputtering system. If the sputtering system was previously used to deposit some other material, the first Si 3 N 4 film was generally of poor quality, as indicated by a high void fraction [10]. Following silicon nitride deposition, 1 μm thick layer of aluminum was e-beam deposited at a chamber pressure of 10-7 Torr and at an e-beam current of 55 ma (Deposition rate: 0. micron/min). Fig 5.8 shows the e-beam deposition machine. When inspected under an optical microscope, the surfaces of the deposited aluminum films appear to be fully comprised of tightly packed pyramidal hillocks, each of which has a width of several μm. Because aluminum is highly reactive, residual oxygen is readily incorporated into the film. The chemisorption of oxygen along the edges of growing grains limits the mobility of atoms between grains, thereby promoting columnar growth [11]. Although the surfaces of the deposited aluminum were diffusely reflective, when viewed through the substrate the films were specular. Therefore, these films were deemed suitable for use as the thin-film interferometer s rear reflector. Because these deposited aluminum films are comprised of columnar crystallites and voids, little confidence was placed initially in their effectiveness as barriers to oxidants. However, these aluminum films were experimentally determined to be good barriers to oxygen, despite their poor appearance [1]. 18

184 Spectroscopic ellipsometry was used in determining the thickness of silicon nitride layer. This is a nondestructive optical technique typically used to determine the optical properties of substrates and thin films. This experimental method is based on measuring the polarization ellipse of a light beam reflected off a sample at a given angle. The angle of incidence on our ellipsometer can be varied from 55 to 75 degrees in 5-degree increments. Using a 75W Xenon are lamp source, the ellipsometer has wavelength range from 50 nm to 1000 nm. A monochromatic light beam of know polarization is reflected off the sample at a given angle of incidence altering both the intensity and polarization state of the beam. A polarization analyzer and photodetector are used to measure φ and δ angles as a function of incident angle and wavelength. These measured parameters are defined as [13] R p Tan ( φ ) Exp( jδ ) = (5.) R s where R p and R s are the pseudo-fresnel reflection coefficients of the sample, with p denoting the direction of the plane of incidence and s the direction perpendicular to the incident plane. From this data, the complex index of refraction and film thickness can be determined using a computer model fit. First, an initial model of the sample is created. Then, using a nonlinear regression technique, the model parameters are varied to fit eth experimental data. Experimental data (Fig 5.9) of spectral ellipsometer is as follows: Mean Square Error = 1.53 Thickness = Real part of refractive index = Imaginary part of refractive index = After several processing runs, we obtained the film with the above set of optical properties where the thickness and refractive index was shifted minimally from the expected values (the design thickness by nearly 1 nm, and the refractive index shifted by 0.04). The above shifts can be attributed to the change in actual temperature at which the deposition was done from the specified temperature. 183

185 5.3.1 Calibration of Temperature Sensor with Encapsulation Structure We prepared one sample with encapsulating layers. The silicon thickness of the temperature sensor is 13 μm, measured with a tunable laser and using an optical method mentioned in Section There are two different interrogation techniques for the temperature sensor, wavelength-encoded temperature measurement and dual-wavelength interrogation technique. We will calibrate the temperature sensor with encapsulation structure using these two techniques. As discussed before, a Fabry-Perot temperature sensor provides a temperaturesensitive reflectance spectrum [14]. Accordingly, a wavelength-encoded temperature measurement was developed by tracking the shift of one of the resonance wavelengths. We used the schematic diaphragm as shown in Fig 4.33 to measure the reflectance spectrum R(λ) from a Fabry-Perot temperature sensor. Fig 5.10 (a) and (b) show the theoretical and experimental results of reflectance spectra shift under several values of temperature for a sensor without encapsulating layers. Fig 5.11 (a) and (b) show the theoretical and experimental results of reflectance spectra shift for a temperature sensor with 00 nm Si 3 N 4. Fig 5.1 (a) and (b) show the theoretical and experimental results of reflectance spectra shift for a temperature sensor with encapsulation structure Al/Si 3 N 4. From comparison, we can see that the results from temperature sensor with both encapsulating layers show good agreement between experimental and theoretical results when using wavelength-encoded temperature measurement. The dual-wavelength interrogation technique described in Section 3.4 can be used to interrogate temperature sensors. Reflectance at two different wavelengths λ1 and λ are separately measured, and then a ratio I ( R, λ) is calculated using Eq. (3.11), assuming the losses are wavelength independent over the wavelength range. The ratio I ( R, λ) is used to trace back to the temperature experienced by the sensor. Fig 5.13 shows measured reflectance as a function of temperature (5 C 35.5 C) for a temperature sensor with encapsulating layers at two wavelengths λ 1 = 155 nm and λ = 1557 nm. Fig 5.14 shows the ratio of the reflectance R( λ ) I( R, λ) = measured at two wavelengths (λ 1 = R( λ ) + R( λ ) nm and λ = 1557 nm) as a function of temperature for the temperature sensor with 184

186 encapsulating layers. The theoretical curve was obtained from combining Eq. (.98) and Eq. (3.11). From Fig 5.14, we can see that there exists a significant mismatch between the experimental and theoretical curve. The reason for this mismatch is due to the instability of the Si 3 N 4 /Al interface. The diffusion of aluminum in Si 3 N 4 films was studied by Ogata, et al [15]. From the shift in the KL L 3 Auger peak they determined that the diffused aluminum is not metallic but is the form of an oxide. The oxygen concentration in the film was observed to track that of the diffused aluminum, with a ratio approximately equal to that of Al O 3. The formation of Al O 3 then appears necessary for the diffusion mechanism observed by Ogata et al. Possible sources of oxygen for the formation of Al O 3 are the Si 3 N 4 layer s native oxide and the oxygen impurities in the film. Even at very low temperatures, aluminum in contact with SiO will react, yielding Al O 3 and silicon [16]. This reaction caused aluminum to be strongly adherent to SiO. For sufficient thickness of SiO, this reaction is self limiting, probably because the Al O 3 acts as a barrier to the diffusion of aluminum. In the fabrication of the Fabry-Perot temperature sensor, the aluminum was deposited 3 days after the Si 3 N 4 layer. There exists the chance of surface oxide on the Si 3 N 4 layer [17]. The oxygen impurities in the Si 3 N 4 are another source of oxygen for the formation of Al O 3 that could then diffuse through the Si 3 N 4. So, in order to improve sensor s performance, the aluminum should be deposited immediately after the Si 3 N 4 layer without breaking vacuum in the fabrication process. The thickness of the Si 3 N 4 surface oxide would be thereby minimized. 5.4 Summary The optimum conditions for deposition of SiO on the top surface of Fabry-Perot cavity-based pressure sensor and Si 3 N 4 /Al on the top surface of Fabry-Perot temperature sensor were determined. A pressure sensor with anti-reflective coating and a temperature sensor with encapsulating layers were calibrated in our lab. The experimental results were compared with theoretical results. Detailed analysis has been made for the mismatch between the experimental and theoretical curve. 185

187 5.5 Reference [1] William Vaughan, Handbook of Optics, Sponsored by the Optical Society of America, New York: McGraw-Hill, [] S. O. Kasap, Optoelectronics and Photonics: Principles and Practices, Upper Saddle River, NJ: Prentice Hall, 001. [3] A. C. Adams, Dielectric and Polysilicon Film Deposition, in VLSI Technology, S. M. Sze, ed., New York: McGraw-Hill, [4] T. Serikawa and A. Okamoto, Properties of Magnetron-Sputtered Silicon Nitride Films, Journal of the Electrochemical Society, Vol. 131, pp. 98, [5] S. M. Hu and L. V. Gregor, Silicon Nitride Films by Reactive Sputtering, Journal of the Electrochemical Society, Vol. 114, pp. 86, [6] T. Carriere, B. Agius, I. Vickridge, J. Siejka and P. Alnot, Characterization of Silicon Nitride Films Deposited on GaAs by RF Magnetron Cathodic Sputtering, Journal of the Electrochemical Society, Vol. 137, pp. 158, [7] X. Qiu and E. Gyarmati, Composition and Properties of SiN x Films Produced by Reactive R.F. Magnetron Sputtering, Thin Solid Films, Vol. 151, pp. 3, [8] C. J. Mogab, P. M. Petroff and T. T. Sheng, Effect of Reactant Nitrogen Pressure on the Microstructure and Properties of Reactively Sputtered Silicon Nitride Films, Journal of the Electrochemical Society, Vol. 1, pp. 815, [9] C. J. Mogab and E. Lugujjo, Backscattering Analysis of the Composition of Silicon-Nitride Films by RF Reactive sputtering, Journal of Applied Physics, Vol. 47, pp. 130,

188 [10] E. D. Palik, ed., Handbook of Optical Constants of Solids, Orlando: Academic, [11] K. H. Guenther, Microstructure of Vapor-Deposited Optical Coatings, Applied Optics, Vol. 3, pp. 3806, [1] M. Wittmer, Barrier Layers: Principles and Applications in Microelectronics, The Journal of Vacuum Science and Technology, Vol. A, pp. 73, [13] G. N. De Brabander, Novel MEMS Pressure and Temperature Sensors Fabricated on Optical Fibers, in ECECS Department, Cincinnati: University of Cincinnati, 001. [14] G. Boreman, R. Walters, and D. Lester, Fiber Optic Thin Film Temperature Sensor, Fiber Optic and Laser Sensors III, E. L. Moore and O. G. Ramer, eds., Proceedings of SPIE, pp , [15] H. Ogata, K. Kanayama, M. Ohtani, K. Fujiwara, H. Abe and H. Nakayama, Diffusion of Aluminum into Silicon Nitride Films, Thin Solid Films, Vol. 48, pp. 333, [16] Y. E. Strausser and K. S. Majumder, Abstract: Chemical Structure of the Al- SiO Interface, The Journal of Vacuum Science and Technology, Vol. 15, pp. 38, [17] S. I. Raider, R. Flitsch, J. A. Aboaf and W. A. Pliskin, Surface Oxidation of Silicon Nitride Films, Journal of the Electrochemical Society, Vol. 13, pp. 560,

189 Fig 5.1 Vacuum evaporation oven 188

190 (a) (b) Fig 5. A Fabry-Perot cavity observed from the silicon side after thinning down the silicon diaphragm (a) without anti-reflective coating (b) with anti-reflective coating 189

191 Reflectivity WavelengthHnmL (nm) (a) Reflectivity Wavelength (nm) (b) Fig 5.3 (a) Theoretical reflectance spectrum for a Fabry-Perot cavity-based pressure sensor with anti-reflective coating (b) Measured reflectance spectrum from a Fabry-Perot cavity-based pressure sensor with anti-reflective coating 190

192 0.07 Reflectivity psi psi 4psi 6psi 8psi 10psi Wavelength (nm) Fig 5.4 Measured reflectance spectra at several values of pressure for a Fabry-Perot cavity-based sensor with anti-reflective coating 191

193 Reflectivity Pressure (Psi) 1548n m 1550n Fig 5.5 Measured reflectance as a function of pressure at two wavelengths: λ 1 = 1548 nm and λ = 1550 nm for a Fabry-Perot cavity-based pressure sensor with anti-reflective coating 19

194 0.045 Reflectivity Wavelength (nm) Fig 5.6 Measured reflectance spectra at several values of temperature from 50 C 10 C 193

195 Fig 5.7 Sputtering machine 194

196 Fig 5.8 Electron-beam evaporation machine 195

197 Fig 5.9 Thickness and refractive index measurements of silicon nitride on silicon using spectral ellipsometer 196

198 1 0.8 Reflectivity WavelengthHnmL (nm) 1560 (a) Measured Reflectivity Wavelength (nm) 3.8(RT) 8.8(40) 3.(50) 35.8(60) (b) Fig 5.10 (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor without encapsulating layers (silicon thickness = 13 μm) (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor without encapsulating layers (silicon thickness = 13 μm) 197

199 1 0.8 Reflectivity WavelengthHnmL (nm) 1560 (a) Measured Reflectivity Wavelength (nm) (b) Fig 5.11 (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor with 00 nm Si 3 N 4 (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor with 00 nm Si 3 N 4 198

200 1 0.8 Reflectivity WavelengthHnmL (nm) 1560 (a) Measured Reflectivity Wavelength (nm) (b) Fig 5.1 (a) Theoretical reflectance spectra for a Fabry-Perot temperature sensor with 00 nm Si 3 N μm Aluminum (b) Measured reflectance spectrum from a Fabry-Perot temperature sensor with 00 nm Si 3 N 4 + 1μm Aluminum 199

201 Measured Reflectivity λ 1 =155 nm λ =1557 nm Temperature ( C) Fig 5.13 Measured reflectance as a function of temperature (5 C 35.5 C) for a temperature sensor with encapsulating layers at two wavelengths: λ 1 = 155 nm and λ = 1557 nm. 00

202 Measured curve Ratio Theoretical curve Temperature ( C) Fig 5.14 The ratio R( λ ) I( R, λ) = (λ 1 = 155 nm and λ = 1557 nm) for a R( λ ) + R( λ ) 1 temperature sensor with encapsulating layers as a function of temperature Dotted lines: theoretical curve; Solid lines: measured response. 01

203 6 Multiplexed Sensor System for Simultaneous Measurement of Pressure and Temperature 6.1 Introduction There is ongoing demand for small and reliable sensors capable of monitoring multiple environmental parameters. For example, in most structure or process control applications, it is often desirable that both the pressure and temperature be known at a given point. In keeping with these needs many researchers have suggested configurations to measure both the pressure and temperature simultaneously. Gregory N. De Brabander, in 1998, demonstrated an optical pressure sensor that uses an unbalanced integrated optical Mach-Zehnder interferometer that is mechanically coupled to a micromachined silicon diaphragm. The sensor s TE and TM output were separated and measured respectively. The TM mode was found to be substantially more sensitive to pressure than the TE mode, whereas the thermal responses of the two modes were found to be similar. Based on this, pressure and temperature can be measured simultaneously [1]. Louis C. Philippe and Ronald K. Hanson presented an application of wavelength modulation spectroscopy and second-harmonic detection to the measurement of temperature, pressure, and velocity in transient flows for an AlGaAs laser diode sensor. A least-squares fit of the experimental line shapes to theoretical second-harmonic line shapes permits simultaneous determination of the pressure and the temperature. The use of high-repetition-rate (10-kHz) linear scans of the studied wavelength region permits application of the technique to high-speed unidimensional transient flows generated in a shock tube. Velocity is derived from the Doppler shift of the absorption profiles []. In addition, the use of optical sensors for measuring pressure and temperature in engineering structures often requires many sensors. This makes multiplexing a very important issue for optical sensors. A multiplexed diode-laser sensor system, comprised of two InGaAsP diode lasers and fiber optic components, has been developed to nonintrusively measure gas pressure and temperature over a single path for a closed-loop 0

204 process control system using laser absorption spectroscopy techniques by E. R. Furlong in the department of Mechanical Engineering in Stanford University. This system was applied to measure and control the gas pressure and temperature in the post-flame gases along a 7-cm long path 6 mm above the surface of Hencken burner (multiple C 4 H-air diffusion flames). Temperature was determined from the ratio of measured peak absorbances. H O concentration was determined from the measured peak absorbance of one transition at the measured temperature. The results demonstrate the potential of multiplexed diode lasers for rapid, continuous, simultaneous measurement and control of important parameters such as gas pressure and temperature in a combustion environment [3]. The purpose of this chapter is to design a novel multiplexed sensor system for simultaneous measurement of pressure and temperature in harsh environments such as gas turbine engine or biomedical situations. It follows a different philosophy than that followed by the sensors describe above. We use two wavelength regions as the light source for each sensor and two interrogation methods for pressure and temperature measurement. The sensor is to be designed to operate over the pressure range of 0 30 psi and temperature range of C. The Fabry-Perot cavity-based pressure sensor (shown in Figure.1) offers the potential for simultaneous measurement of pressure and temperature. For a Fabry-Perot cavity-based pressure sensor, the sensing element is susceptible to cross sensitivities to pressure and temperature. If the sensing element simultaneously experiences both pressure and temperature change, the resulting interferometric spectra change will be caused by a combination of both effects. In this project, we are searching a method to separate signal from the sensor output for simultaneous measurement of pressure and temperature. 6. Prerequisite for Simultaneous Measurement of Pressure and Temperature The prerequisite for successful separation of pressure and temperature signals from the sensor output is that pressure and temperature must have different effect on the sensor s parameters. For a Fabry-Perot cavity-based pressure sensor, applied pressure deflects the upper diaphragm and changes the cavity depth, while temperature changes 03

205 the refractive index of the silicon diaphragm because the silicon refractive index has significant temperature dependence. But for a gauge pressure sensor, with the configuration shown in Fig.1 (a), the cavity depth of the sensor will also change with temperature. Because the pressure trapped inside the cavity is assumed to be P 1 that is higher than atmosphere at room temperature T 1 for a gauge pressure sensor. The reason is explained in Section.3.3. With temperature increasing to T, the pressure inside the cavity will change according to the Boyle s law assuming the volume inside sensor cavity doesn t change V 1 =V P 1V 1 PV = (6.1) T 1 T where P 1 is the pressure inside the cavity at room temperature T 1, P is the pressure inside the cavity at elevated temperature T. The difference between P 1 and P will cause deflection of the sensor diaphragm. Deflection of the sensor diaphragm will cause a change of the cavity volume V 1 < V. Since the thermal expansion of Pyrex glass wafer is very small, the change of cavity diameter is negligible. The change of sensor s cavity depth with temperature increasing is the main contribution to the change of cavity volume. For a Fabry-Perot cavity-based pressure sensor, applied pressure deflects the upper diaphragm and changes the cavity depth. So, if the sensing element of a gauge pressure sensor simultaneously experiences both pressure and temperature change, the resulting cavity depth change is caused by a combination of both effects, and neither can separately be identified. It is impossible to distinguish the pressure and temperature signal from the sensor output. For an absolute pressure sensor, with the configuration shown in Fig.1 (b), the pressure trapped inside the cavity is vacuum at room temperature T 1. A vacuum bonding machine in the UC MEMS group is used for silicon-glass bonding. However, it is not possible to make a high vacuum cavity by this method. Two residual gas sources which pose a problem for vacuum sealing have to be considered. One is gas generation during the anodic bonding process [4]. The other is gas desorption from the inner surface of the sealed cavity. A starting pressure of typically 10-5 mbar results in a final internal cavity pressure of 1 mbar in the best case [5]. Under current technology, it is impossible to accurately measure the pressure inside sensor s cavity. Therefore, we cannot predict 04

206 whether the pressure change due to the change of temperature inside the sensor s cavity is negligible or not compared to sensor s pressure range. So, for an absolute pressure sensor, if the sensing element simultaneously experiences both pressure and temperature change, the resulting cavity depth change remains unknown. For a differential pressure sensor, with the configuration shown in Fig.1 (c), the pressure inside the cavity is open to the ambient pressure. Since the cavity is not sealed, Boyle s law is not relevant in this case. A temperature change would not affect the pressure inside the cavity, nor does the cavity depth. This satisfies the prerequisite for successful separation of pressure and temperature signal from the sensor output. We have investigated the different effect of pressure and temperature on a sensor s parameters for these three different types of pressure sensors. From this analysis, we find out that only differential pressure sensors can satisfy the condition for simultaneous measurement of pressure and temperature. But, we need further proof from experiment. Fortunately, Newton s rings provide a means of determining the cavity depth changes with a very simple apparatus. Newton s rings are formed due to optical path difference in the film between an optical flat and curve surface. For a Fabry-Perot cavity-based pressure sensor, the flat surface is the Pyrex glass wafer and the curve surface is the deflected silicon diaphragm. Since the cavity depth is only a few halfwavelengths, the orders of interference in the monochromatic pattern are very low, and fringes are visible with a white light source [6]. For this reason, Newton s rings can be observed under a Nikon optical microscope (Fig 4.13 (a)) with white light source in our lab. Fig 4.13 (b) shows how to place a sensor under microscope for observation of Newton s rings. The position of each bright fringe is determined by the cavity depth. We use a hair dryer to heat the sensor and observe how the fringes change under the microscope. If the cavity depth of the sensor changes with temperature increasing, we would expect to observe fringes moving. As shown in Fig 4.14, the fringes are circles about the center point of the sensor diaphragm. The cavity depth that satisfies the condition for a bright fringe is m λ d =, = 0, 1,... m (6.) 05

207 The cavity depth difference for two neighboring bright fringes is λ d 1 d = (6.3) If the cavity depth decreases, the points of the film with given d move inwards and the fringes collapse towards the center, where one disappears each time the cavity depth decreases by λ /. If the cavity depth increases, the points of the film with given d move outwards with one disappearing each time the cavity depth increases by λ /. Fig 6.1 shows the Newton s rings change under temperature for gauge, absolute and differential pressure sensors respectively. From Fig 6.1, we can see that the position of bright fringes moves outwards with temperature increasing for gauge and absolute pressure sensors, which means the cavity depth of the sensor increases with temperature. No fringes are observed for differential pressure sensors at room temperature or elevated temperature. Since the thermal expansion of Pyrex glass is negligible, we have confidence to make conclusion that sensor s cavity depth does not change with temperature for a differential pressure sensor. From previous analysis, temperature could change the cavity depth of a gauge pressure sensor according to Boyle s law. But for an absolute pressure sensor, the theoretical analysis cannot provide a convincing result. Experimental results from observation of Newton s fringes show a similar situation to that of a gauge pressure sensor. For a differential pressure sensor, the experimental result shows good agreement with the theoretical analysis. 6.3 Multiplexed Sensor System for Simultaneous Measurement of Pressure and Temperature In the previous section we have demonstrated the different effect of pressure and temperature on the sensor s parameters for a Fabry-Perot cavity-based differential pressure sensor. In this section, we need to separate signal from the sensor output for simultaneous measurement of pressure and temperature based on the different effect of pressure and temperature on the sensor s parameters. When the light source for the sensor is operating at wavelength shorter than [7] hc 1.4 λ ( μ m ) = = μ m E E ( ev ) = (6.4) g g 06

208 where E g = 1.1 ev is the bandgap energy for silicon, the pressure sensor can be considered as a simple Fabry-Perot interferometer with one resonator layer. Refractive indices of glass, air, and silicon are denoted as n 0 = n glass, n 1 = n air, and n = n si respectively and the cavity depth is denoted as d. Here, we have assumed normal incidence of light. And the reflectance from the Fabry-Perot pressure sensor is given by Eq. (.4) R jφ * r1 + r3e ( λ) Er Er = jφ 1+ r1r3e where = (6.5) n n = (6.6) n0 + n1 r r n n 1 3 = (6.7) n1 + n φ 4πn d λ 1 = (6.8) 0 Eq. (6.5) is valid only when the thickness of the silicon diaphragm is much greater than the absorption depth of the silicon at the wavelengths used [8]. Fig 6. shows the optical absorption coefficients (α) vs. wavelength (λ) for various semiconductors [9]. The absorption depth (also called penetration depth) is equal to 1/α. For silicon, the absorption depth at λ = 850 nm is around 100 μm. The designed thickness of the silicon diaphragm for Fabry-Perot sensor is around 0 μm that is much greater than the absorption depth. Therefore, λ = 850 nm is used as light source for interrogation the pressure applied on the sensor. The reflectance from the sensor is a periodic variation as a function of the cavity depth d in the 850 nm region, as shown in Fig 6.3. For the purpose of multiplexing, a tunable external-cavity diode laser ( nm) is used to illuminate the Fabry-Perot cavity. Dual-wavelength interrogation is used to optically interrogate the pressure applied on the sensor. Relative reflected intensities at two different wavelengths λ1 and λ in the wavelength region of the tunable laser ( nm) are separately measured, and then a ratio I( R, λ) is calculated using [8] R( λ1 ) I( R, λ) = (6.9) R( λ ) + R( λ ) 1 07

209 and assuming the losses in the system are wavelength independent over the wavelength range. The response curve is still a periodic function with respect to cavity depth d, as shown in Fig 6.4 (a), (b), (c) for the different spacing between two wavelengths λ λ 1 = 10 nm, 0 nm and 30 nm respectively. The depth of the etched cavity in the glass wafer must be greater than λ / 4, and to avoid significant errors in the solution of the pressure on the sensor from Eq. (6.9) due to the small inaccuracies in the measurement, must be large enough to yield a wide reflectance change within full pressure range ( λ / 4 ), which means 0 0 I( R, λ) should be d as large as possible [10]. But the depth of the etched cavity in the Pyrex glass wafer is limited by the current etching technique [11]. So far only a few deep wet chemical etching fabrication processes of fused silica have been reported [1,13]. Most of these lack practical use, since they are complex and costly. Common glass etching processes are based on metal hard mask technologies. But usually, metal hard masks exhibit a high number of point defects and require complicated and costly process steps, such as metal deposition and etching. Therefore, soft masks (photoresist) are of great interest. They show a low defect density, weak processing expense. But photoresist cannot withstand the BHF etching solution for too long, which limits the etching depth we can get. From Fig. 6.3, we can see that as the spacing ( Δ λ = λ λ1 ) between the two wavelengths being chosen increases, the same initial cavity depth can yield wider reflectance change. But the spacing of the two wavelengths is limited by the bandwidth of the tunable laser and the fiber couplers. We need to make a compromise when choosing the sensor s cavity depth and operating wavelengths. We chose an initial cavity depth of 4.9 μm and λ λ 1 = 0 nm to lie within these limiting values. As the applied pressure increases, cavity depth decreases, the ratio I( R, λ) from the sensor decreases in a monotonic fashion, shown in Fig. 6.5 within the maximum cavity depth change of λ / 4 1 nm. 0 After the initial cavity depth of the sensor is determined, we need to decide the silicon diaphragm thickness. The silicon diaphragm is modeled as a circular membrane. The deflection of the diaphragm due to the application of pressure P is given by [14, 15] 08

210 w( r) 4 3PR 0 (1 ν ) r Et R = 0 (6.10) where r is the distance from the center of the plate; w (r) is the deflection at r ; P is the normal pressure; R 0 is the radius of the diaphragm; ν is the Poisson s ratio for silicon, E is the Young s modulus, and t is the diaphragm thickness. The cavity depth d of the sensor is a function of the maximum diaphragm deflection w defined as d = d 0 w (6.11) where d 0 is the initial etched cavity depth. The maximum diaphragm deflection is obtained from Eq. (6.10) when r = 0 4 PR0 w = (6.1) 64 D where D is the flexural rigidity expressed by 3 Et D = (6.13) 1(1 ν ) The maximum cavity depth change is designed for a deflection of λ 0 / 4 where λ 0 is the operating wavelength. Thus, the pressure that causes a deflection of λ / 4 is given by 3 4Et λ0 P =. (6.14) 3 R (1 ν ) 4 0 The pressure sensor operating at 850 nm is designed to respond over the pressure range 0 30 psi. Calculations indicate that at λ 0 = 850 nm, the diaphragm thickness required for making a diaphragm-center-deflection of λ / 4 at 30 psi is 1 μm. Fig. 6.6 shows the simulation of the ratio I( R, λ) at two wavelengths (λ 1 = 860 nm; λ = 840 nm) from the sensor as a function of applied pressure 0 30 psi. After determining sensor s parameters, we need to investigate temperature effect on the sensor operating in 850 nm wavelength region. As discussed before, for a differential pressure sensor, applied pressure changes the cavity depth, while temperature primarily changes the refractive index of the silicon diaphragm. The refractive index of silicon is a

211 linear function of temperature at all wavelengths. The equation for representing thermooptic coefficients of silicon is expressed as [16] dn n GR HR dt with the normalized dispersive wavelength R defined as: R = = + (6.15) λ λ λ ig In the above equations, n and dn / dt (6.16) are the room-temperature silicon refractive index and its derivative with respect to temperature and λ ig is the wavelength corresponding to E, the isentropic band gap lying in the UV region for silicon. The ig isentropic band gap is the band gap corresponding to the band-to-band transition that is not affected by temperature [17]. This gap lies between the excitonic band and the conduction band and determines the dispersion of n( dn / dt ) [17]. Optical constants G and H are given in [16]. The designed temperature range of the Fabry-Perot cavity-based pressure sensor is C. At 5 C, the silicon refractive index at 850 nm is equal to determined using the dispersion equation for single crystal silicon [18] 4 = A + BL + CL + Dλ Eλ (6.17) n + where A = ; B = ; C = ; D = ; 1 E = ; L = λ 0.08 At 100 C, the silicon refractive index at 850 nm is equal to determined using Eq. (6.15) and Eq. (6.16). In the 850 nm wavelength region, a dual-wavelength interrogation method in which relative reflected intensities at two different wavelengths are separately measured and the ratio is calculated using Eq. (6.9) is used to extract pressure for the Fabry-Perot sensor. As shown in Fig 6.7, we plot the ratio for the relative reflected intensities at two different wavelengths (λ 1 = 860 nm; λ = 840 nm) as a function of pressure (0 30 psi) for n si = at 5 C and n si = at 100 C, respectively, in one graph. From Fig 6.7, we can see that the two curves are nearly overlapping which means there is a negligible temperature effect on the Fabry-Perot sensor operating in the 10

212 850 nm wavelength region. Therefore, applied pressure on the sensor obtained from solving Eq. (6.9) is independent of the temperature experienced by the sensor. When the light source for the sensor is operating in the 1550 nm wavelength region, silicon is transparent. Therefore, the silicon diaphragm constitutes a second Fabry-Perot resonator. The reflectance from the Fabry-Perot pressure sensor is given by Eq. (.48) with n 0 = n glass (=1.473), n 1 = n air (=1.0), n = n si (=3.478) and n 3 = n air (=1.0) respectively. The reflectance is a function of the cavity depth d which is determined by the applied pressure and silicon refractive index n si which is determined by the temperature. As discussed in chapter, wavelength-encoded temperature measurement is used for interrogation of the temperature signal in 1550 nm wavelength region. The sensed temperature can be determined by tracking the position of one of the minima in R(λ). Fig 6.8 shows the theoretical curve of reflectance spectra R(λ) from a Fabry-Perot cavitybased pressure sensor for different pressure and temperature. From Fig. 6.8 we can see that the phase shift of R(λ) spectra is mainly caused by temperature, the magnitude of reflectance change is mainly caused by pressure. Therefore, by measuring the phase shift of R(λ) spectra in the 1550 nm wavelength region, temperature signal can be extracted and is independent of the pressure applied on the sensor. This technique is valid within 5% of error for temperatures lower than 100 C and for silicon diaphragm thickness around 0 μm. The reflectance from the pressure sensor in the 1550 nm wavelength region is given by Eq. (.48) R( λ) jθ jφ r3 + r34e r1 + e ( ) jθ 1+ r3r34e = (6.18) jθ jφ r3 + r34e 1+ r1e ( ) jθ 1+ r3r34e where 4πn θ si t = λ (6.19) 4πn d φ = air λ (6.0) 11

213 where d is the sensor s cavity depth and t is the diaphragm thickness. R(λ) spectra is a periodic function with respect to wavelength, but the period is mainly determined by θ due to the fact n t >> n1d. Therefore, the reflectance is minimized at the resonance condition [19] θ mπ, m = 1,, 3 (6.1) As shown in Fig 6.8, change of temperature will cause the phase shift of R(λ) spectra. So, the position of one of the minima λ m1 will move to λ m the R(λ) minima remains the same 4πn t λ m1. But the resonance condition at 4πn1 t = m π, m = 1,, 3 (6.) λ 1 1 m where λ m1 is the resonance wavelength and n 1 is the silicon refractive index at room temperature, λm is the resonance wavelength and n 1 is the silicon refractive index at elevated temperature. Assuming the thermal expansion coefficient of the thin film is neglected, t 1 = t, the above equation can be simplified as λ = (6.3) m n 1 n1 λ m1 By measuring resonance wavelengths at room temperature and elevated temperature, respectively, refractive index n 1 at an elevated temperature can be obtained from Eq. (6.3). At room temperature, silicon refractive index n 1 is equal to at 1550 nm [0]. The resonance wavelength shift must satisfy the equation below in order to avoid ambiguity and this equation also determines the temperature range of a Fabry-Perot sensor 4πn t λ 4πn λ 1 1 m1 m t π (6.4) For temperature interrogation, the effect of temperature on the refractive index of the silicon should be estimated. It was found that the refractive index of silicon is a linear function of temperature at all wavelengths, but the coefficient of the linear term is a function of wavelength. The equation for representing thermo-optic coefficients of silicon is expressed in Eq. (6.15) and Eq. (6.16) 1

214 Combining Eq. (6.15) and Eq. (6.16), silicon refractive index n 1 obtained from Eq. (6.3) can be traced back to the temperature experienced by the Fabry-Perot sensor. This wavelength-based measurement method, because it is signal-level insensitive, has a high degree of immunity to the effects of changes in the transmissivities of the optical fibers and connectors. For the purpose of multiplexing sensors, we use a wavelength-division multiplexing (WDM) technique in which the signals at different wavelengths from different sensors are separated in the time domain when they arrive at the photodetector by sweeping the wavelength of two tunable lasers. Fig 6.9 shows the schematic diaphragm of the multiplexed Fabry-Perot sensors for simultaneous measurement of pressure and temperature. The two tunable lasers that are external-cavity diode lasers available from New Focus are used to illuminate the Fabry-Perot cavity. One of the lasers is a New Focus 6316 Velocity Laser providing output wavelengths in the range nm. The other laser is a New Focus 638 Velocity Laser providing output wavelengths in the range of nm. A computer with Lab View installed is used to control and separate the output from the two tunable lasers in time domain, as shown in the laser output from Fig Fiber Bragg gratings (FBGs) in the wavelength range ( nm) are utilized as spectral discriminators with properties suitable to the interrogation of pressure applied on the Fabry-Perot sensor. The FBGs used in the present experiments are the Bragg Reflector type with 99% reflectivity and 0.nm bandwidth, the resonant wavelengths of the four FBGs are: λ 1 = 830 nm ; λ = 840 nm ; λ 3 = 850 nm; λ 4 = 860 nm Chirped fiber Bragg gratings (FBGs) with broadband 10 nm in the wavelength range ( nm) are used as light source for the Fabry-Perot sensor to interrogate temperature experienced by the sensor. Chirped fiber Bragg gratings are devices in which the period of the modulation of the refractive index changes along the grating. By progressively increasing the spacing between the grating planes, different wavelengths are reflected back as light travels through the grating. The result is a spectral response that will be characterized by a wide bandwidth [1]. The wavelengths for the two chirped FGBs in the system are nm and nm, respectively. 13

215 When the first tunable laser is used to illuminate the Fabry-Perot cavity by sweeping in the wavelength range nm, the reflected light from the two FBGs are routed into one sensor through a x (50:50) single mode coupler (center wavelength 850 nm), a 1x (50:50) multi-mode coupler and a x (50:50) multi-mode coupler at the resonant wavelengths of the two gratings. Therefore, two wavelengths are assigned to each sensor, and for demonstration purposes, the four wavelengths for the two sensors are separated in the time domain. Two photodetectors are used to measure the input and output from the sensors, respectively. The ratio of the signals from the two photodetectors at two wavelengths is used as the measure of the sensed parameter using dual-wavelength interrogation technique, as shown in Fig When the second tunable laser is used to illuminate the Fabry-Perot cavity by sweeping in the wavelength range nm, the reflected light from a chirped FBGs is routed into one sensor through a x (50:50) single mode coupler (center wavelength 1550 nm) and shares the same multi-mode couplers with the reflected light in the nm wavelength range. The reflected light from the two chirped FBGs will be separated in the time domain by sweeping the wavelength of the second tunable laser. Therefore, one wavelength region is assigned to each sensor, and for demonstration purposes, the two wavelength regions for the two sensors are separated in the time domain. Two photodetectors are used to measure the reflectance spectra from the sensors, as shown in Fig Temperature experienced by the sensor is determined by tracking the wavelength shift of one of the reflectivity minima within the bandwidth of the broadband chirped FBG. The temperature range ( Δ T = T T1, where T 1 is room temperature) of a multiplexed Fabry-Perot sensor is determined by the silicon diaphragm thickness and the bandwidth of the broadband chirped FBGs. Fig 6.11 shows the temperature range as a function of silicon diaphragm thickness for a multiplexed Fabry-Perot sensor using a chirped FBG ( nm) as light source. The procedure for obtaining this curve is as follows: First, we combine Eq. (6.3) and Eq. (6.4) and set one of the resonance wavelength in R(λ) spectra at λ m1 1550nm at room temperature to get the maximum silicon refractive index change before ambiguity occurs and then use Eq. (6.15) and Eq. 14

216 (6.16) to trace back to the temperature range for a Fabry-Perot sensor from the maximum silicon refractive index change. The purpose of setting one of the resonance wavelengths in the R(λ) spectra at λ m1 1550nm at room temperature is to track the resonance wavelength shift with temperature increasing over a wide range, because the bandwidth of the chirped FBG ( nm) will set the upper limit of the maximum range of the resonance wavelength shift for a Fabry-Perot sensor. In addition, the Fabry-Perot sensor is designed to measure the temperature range from room temperature to the temperature above. The corresponding silicon diaphragm thickness for getting one of the resonance wavelengths at λ m1 1550nm under room temperature is t m λ m1, m = 1,, 3 (6.5) n1 Since the refractive index of silicon is a linear function of temperature at all wavelengths, the temperature range of the Fabry-Perot sensor is a reverse function of silicon thickness. When the silicon diaphragm thickness is less than 17 μm, the temperature range is only determined by the bandwidth of chirped FBG, so that the curve becomes flat. As discussed before, the silicon diaphragm thickness required for making a diaphragmcenter-deflection of λ / 4 at 30 psi in 850 nm region is 1 μm, this corresponds to m = 94 0 in Eq. (6.5). The red line in Fig 6.11 is for the silicon diaphragm thickness at 1 μm and the cross point of the black line and the red line is the temperature range (4 90 C) for the Fabry-Perot sensor designed for simultaneous measurement of pressure and temperature. When using wavelength-encoded temperature measurement for temperature interrogation in the 1550 nm wavelength region for a multiplexed Fabry-Perot sensor, we must be aware that even though the phase shift of the R(λ) spectra is mainly caused by temperature, pressure applied on the sensor still has some contribution to the phase shift. We need to compare the temperature and pressure sensitivity of the phase shift. Temperature sensitivity (nm/ C) is obtained by combining Eq. (6.15), Eq. (6.16) and Eq. (6.3) and setting one of the resonance wavelengths at room temperature at λ m1 1550nm. In fact, temperature sensitivity of a Fabry-Perot sensor is not dependent 15

217 on the silicon diaphragm thickness. From this calculation, the temperature sensitivity of the phase shift for a Fabry-Perot sensor is around 0.5 nm/ C. Pressure sensitivity is obtained by tracking the resonance wavelength shift of R(λ) spectra in nm region with pressure increasing at room temperature. The reflectance from the pressure sensor in 1550 nm wavelength region is given by Eq. (6.18), with silicon diaphragm thickness at t = 1 μm, cavity depth from d = 4.9 μm (0psi) to d = 4.7 μm (30psi). From calculation, the pressure sensitivity of the phase shift for a Fabry- Perot sensor is around nm/psi. By comparison of pressure and temperature sensitivity of the phase shift, we can make the conclusion that the pressure effect on the phase shift can be neglected because the temperature sensitivity is more than 10 times higher than pressure sensitivity of the phase shift. Fig 6.1 shows the simulation of the reflectance spectra shift with temperature increasing (4 C 90 C) for a multiplexed Fabry-Perot sensor using a broadband chirped FBGs (1550 nm 1560 nm) as light source in 1550 nm wavelength region. In the multiplexed sensor system for simultaneous measurement of pressure and temperature, Fabry-Perot sensors are operating in two wavelength regions. This leads to a new requirement for the fiber couplers and isolators used in the system, because their performances are wavelength dependent. Fiber couplers, also known as directional couplers, constitute an essential component of lightwave technology. They are used routinely for a multitude of fiber-optic devices that require splitting of an optical field into two coherent but physically separated parts (and vice versa). A common construction of the fiber coupler is the fused-fiber coupler. This is fabricated by twisting together, melting, and pulling two fibers so they become fused together over a uniform section of length W, as shown in Fig 6.13 [7]. As the input light P 0 propagates along the taper in fiber 1 and into the coupling region W, there is a significant decrease in the V number owing to the reduction in the ratio d/λ πd 1 / πd V = ( n1 n ) = NA (6.6) λ λ where d is the center-to-center spacing between the two cores. n 1 and n are the refractive indices of the core and cladding, respectively. NA is the numerical aperture of a stepindex fiber. Consequently, as the signal enters the coupling region, an increasingly larger 16

218 portion of the input field now propagates outside the core of the fiber. By making the tapers very gradual, only a negligible fraction of the incoming optical power is reflected back into either of the input ports. Assuming that the coupler is lossless, the expression for the power P coupled from one fiber to another over an axial distance z is P = P sin ( κ ) (6.7) 0 z where κ is the coupling coefficient describing the interaction between the fields in the two fibers, defined as [] πv κ = exp[ ( c 0 + c1 d + c d )] (6.8) k n a 0 0 where V is the fiber parameters, k 0 = π λ is the magnitude of propagation vector, a is the core radius, and d d / a is the normalized center-to-center spacing between the two cores ( d > ). The constants c 0, c1 and c depend on V as = V 0. V, c c1 0 V 015 V = , and c 0 V 0009 V = Eq (6.8) is accurate to within 1% for values of V and d in the range 1.5 V.5 and d 4.5. By conservation of power, for identical-core fibers we have P = P P = P [1 sin ( κ z )] = P cos ( κ ) (6.9) z If coupler length z is chosen such that κ z = π / 4, the power is equally divided between the two output ports. Such couplers are referred to as 50:50 or 3-dB couplers. From Eq. (6.7) and Eq. (6.9), we can see that the optical power coupled from one fiber to another is determined by four parameters: the axial length z of the coupling region over which the fields from the two fibers interact; the size of the fiber core radius α; the operating wavelength λ; and d, the center-to-center spacing between the two cores. Usually, single mode fiber coupler has narrow bandwidth compared with multimode coupler. For example, a multi-mode x 3dB fiber coupler (Newport F-CPL- M851) and a multi-mode 1x 3dB fiber coupler (Newport F-CPL-M1855) with center wavelength at 850 nm have a broad bandwidth nm. These two fiber couplers are used to couple the reflected light from FBGs ( nm) and chirped FBGs ( nm) into one Fabry-Perot sensor. A single mode fiber coupler (Newport F-CPL- 17

219 S855) with center wavelength at 850 nm has a bandwidth ± 0 nm. This fiber coupler is used to couple the reflected light from FBGs ( nm) into the multi-mode fiber coupler. FBGs are produced by exposing a photosensitive single mode optical fiber to a spatially varying pattern of ultraviolet intensity, which results in a periodic perturbation to the effective refractive index of the guided modes. Use of single mode fiber couplers can minimize the loss from coupling reflected light from FBGs to the multi-mode coupler, as shown in Fig 6.9. At the second stage, light from the single mode coupler is coupled into the 1 multi-mode coupler with negligible loss, because light is coupled from the single mode fiber with very small core diameter to the multi-mode fiber with large core diameter. The loss is significant when light coupling occurs from a multi-mode fiber to a single mode fiber. Fortunately, in our system, coupling the reflected light from a sensor to the FBGs is considered as noise to the sensor and should be avoided by placing an isolator in a suitable position. The second multi-mode coupler is used to send sensor input and output to the different photodetectors. A single mode fiber coupler (Newport F-CPL-S355) with center wavelength at 1550 nm has a bandwidth ± 40 nm. This fiber coupler is used to couple the reflected light from a chirped FBGs ( nm) into the 1 multi-mode fiber coupler. Since the 1 multi-mode coupler covers a broad bandwidth ( nm), reflected light in the two wavelength regions nm and nm can share the same multimode couplers, as shown in Fig 6.9. For demonstration purposes, reflected light from the FBGs ( nm) and chirped FBGs ( nm) will be separated in the time domain by sweeping the wavelength of two tunable lasers. Optical isolators are devices that transmit light only in one direction. They play an important role in fiber-optic systems by stopping back-reflection and scattered light from reaching sensitive components. The inside workings of optical isolators depend on polarization. An optical isolator includes a pair of linear polarizers, oriented so the planes in which they polarize light are 45 apart. Between them is a device called a Faraday rotator, which rotates the plane of polarization of light by 45. As shown in Fig 6.14, what happens to the light passing through an optical isolator depends on the direction it is going. 18

220 First consider light going from left to right. The input light is unpolarized, but the first polarizer transmits only vertically polarized light. Then the Faraday rotator twists the plane of polarization 45 to the right. The second polarizer transmits light if its plane of polarization is 45 to the right of vertical, which in this case is all the light that passed through the first polarizer and the Faraday rotator. Now consider light going in the opposite direction, from right to left. The polarizer on the right transmits only light polarized at 45 to the vertical. The Faraday rotator turns it another 45 to the right, so the plane of polarization is horizontal. That horizontally polarized light is blocked by the polarizer on the left, which transmits only light polarized vertically. Optical isolators can attenuate light headed in the wrong direction by 40 db or more. By placing them in suitable positions in our system, we can separate the signals from different sensors, avoid spurious reflections at various fiber terminations, and protect sensors from noise that might affect their performance [3]. The photodetector we are using is a New Focus broadband photodetector 1554-A that covers the nm wavelength range. The responsivity curve is very flat in the nm wavelength range, as shown in Fig The ratio of the signals from the two photodetectors at two wavelengths in the nm range is used as the measure of the sensed parameter. Therefore, flat responsivity of photodetector in this wavelength range is very critical for the accuracy of pressure measurement. In the nm wavelength range, the wavelength-encoded measurement described in chapter is used to interrogate temperature by tracking spectral resonance. In this system, we are using reflected light from broadband fiber gratings with bandwidth 10 nm as the light source for the sensor. At resonance, the reflectivity of the sensor is a minimum, and the wavelength of one of these reflectivity minima can serve as a signal-level-insensitive indicator of temperature. Two photodetectors are used to measure the reflectance spectra from the sensors. The temperature experienced by the sensor is determined by tracking the wavelength shift of one of the reflectivity minima within the bandwidth of the broadband fiber grating. Thus, flat responsivity of the photodector in this wavelength range is not critical for the accuracy of temperature measurement. 19

221 6.4 Summary A multiplexed sensor system using two tunable lasers for simultaneous measurement of pressure and temperature is proposed. Computer simulation using Mathematica 4.0 has been used to demonstrate the viability of the proposed design. A maximum sensor number that can be multiplexed using this system is determined by the ratio of the laser tuning range over the width of the wavelength slot allocated to individual sensors. This multiplexed sensor system has the potential of multiplexing about seven sensors. 0

222 6.5 Reference [1] Gregory N. De Brabander, Joseph T. Boyd, and Glenn Beheim, Integrated Optical Ring Resonator With Miromechanical Diaphragm for Pressure Sensing, IEEE Photonics Technology Letters, Vol. 6, No. 5, [] L. C. Philippe and R. K. Hanson, Laser Diode Wavelength-modulation Spectroscopy for Simultaneous Measurement of Temperature, Pressure and Velocity in Shock-heated Oxygen Flows, Applied Optics, Vol. 3, No. 30, pp , [3] D. S. Baer, R. K. Hanson, M. E. Newfield, and N. K. L. M. Gopaul, Multiplexed Diode-laser Sensor System for Simultaneous HO, O, and Temperature Measurements, Optical Letters, Vol. 19, pp , [4] K. Ichimura, M. Matsuyama and K. Watanabe, Alloying Effect on the Activation Process of Zr-alloy Getters, The Journal of Vacuum Science and Technology, Vol. 5, pp. 0-5, [5] G. Stemme, Resonant Silicon Sensors, Journal of Micromechanics and Microengineering, Vol. 1, pp , [6] Max Born and Emil Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, New York, 7 nd ed., [7] Gerd Keiser, Optical Fiber Communications, New York: McBraw-Hill, 3 nd ed., 000. [8] Youngmin Kim and Dean P. Neikirk, Design of Manufacture of Micromachined 1

223 Fabry-Perot Cavity-based Sensors, Sensors and Actuators, Vol. 50, pp , [9] S. O. Kasap, Optoelectronics and Photonics: Principles and Practices, Upper Saddle River, NJ: Prentice Hall, pp., 001. [10] Ashish M. Vengsarkar, Craig Michie, Ljilja Jankovic, Brian Culshaw and Richard O. Clasus, Fiber-Optic Dual-Technique Sensor for Simultaneous Measurement of Strain and Temeperature, Journal of Lightwave Technology, Vol. 1, No. 1, pp , [11] Axel Grosse, Matthias Grewe and Henning Fouchkhardt, Deep Wet Etching of Fused Silica Glass for Hollow Capillary Optical Leaky Waveguides in Microfluidic Devices, Journal of Micromechanics and Microengineering, Vol. 11, pp.57-6, 001. [1] Matzke C. M, Arnold D. W, Kravitz S. H, Warren M. E and Bailey C. G, Quartz Channel Fabrication for Electrokinetically Driven Separations, Proc. SPIE 3515, pp , [13] Koutny L. B, Schmalzing D, Taylor T. A and Fuchs M, Microchip Electrophoresis Immunoassay for Serum Cortisol, Analytical Chemistry, Vol. 68, pp. 18-, [14] L.Landeau and L. Lifschitz, Theory of Elasticity, London, Pergamon Press, Reading, Mass., Addison-Wesley Pub. Co., [15] J. R. Vinson, Structural Mechanics: The Behavior of Plates and Shells, New York, John Wiley and Sons, [16] Gorachand Ghosh, Temperature Dispersion of Refractive Indices in Crystalline and Amorphous Silicon, Applied Physics Letters, Vol. 66, No. 6, pp.

224 , [17] Gorachand Ghosh, Thermo-optic Coefficients of LiNbO 3, LiIO 3 and LiTaO 3 Nonlinear Crystals, Optics Letters, Vol. 19, No. 18, pp , [18] Michael Bass, Handbook of Optics, sponsored by the Optical Society of America, New York: McGraw-Hill, pp. 7-10, [19] Glenn Beheim, Fiber-Optic Temperature Sensor Using a Thin-Film Fabry-Perot Interferometer, NASA Technical Memorandum , Lewis Research Center, [0] Edward D. Palik, Handbook of Optical Constants of Solids, San Diego: Academic Press, [1] Andreas Othonos and Kyriacos Kalli, Fiber Bragg gratings: Fundamentals and Applications in Telecommunications and Sensing, Boston: Artech House, [] Govind P. Agrawal, Applications of Nonlinear Fiber Optics, San Diego: Academic Press, pp. 64-8, 001. [3] Jeff Hecht, Understanding Fiber Optics, Upper Saddle River, NJ: Prentice Hall, pp , 00. 3

225 (a) Gauge pressure sensor (b) Absolute pressure sensor 4