Smart lens: tunable liquid lens for laser tracking

Transcription

1 Smart lens: tunable liquid lens for laser tracking Fan-Yi Lin a, Li-Yu Chu b, Yu-Shan Juan a, Sih-Ting Pan b, Shih-Kang Fan* b a Institute of Photonics Technologies, Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan; b Institute of Nanotechnology, National Chiao Tung University, Hsinchu 300, Taiwan; ABSTRACT A tracking system utilizing tunable liquid lens is proposed and demonstrated. Adapting the concept of EWOD (electrowetting-on-dielectric), the curvature of a droplet on a dielectric film can be controlled by varying the applied voltage. When utilizing the droplet as an optical lens, the focal length of this adaptive liquid lens can be adjusted as desired. Moreover, the light that passes through it can therefore be focused to different positions in space. In this paper, the tuning range of the curvature and focal length of the tunable liquid lens is investigated. Droplet transformation is observed and analyzed under a CCD camera. A tracking system combining the tunable liquid lens with a laser detection system is also proposed. With a feedback circuit that maximizing the returned signal by controlling the tunable lens, the laser beam can keep tracked on a distant reflected target while it is moving. Keywords: Electrowetting, Liquid lens, Tunable, Laser tracking 1. INTRODUCTION Tunable focal length is useful for applications that need auto-focusing, auto alignment, or object tracking. For conventional optical system that has components with fixed focal lengths and positions, it is very difficult to adjust the effective focal length of the system rapidly. Although the focal length of an optical system can be adjusted with some parts that can be moved mechanically, the response time is often slow. Moreover, the weight, size, and power consumption of the moving parts limit its portability. For the refraction of light at the surface of liquid in a medium with mismatched refractive index, a droplet can be regarded as a liquid lens, which may be lightweight and smooth at surface. Researchers have demonstrated the possibilities to alter the curvature of a liquid lens by external pressure 1 or the liquidsolid interfacial tension. 2-4 The modified curvature of the liquid lens can therefore alter the focal length instead of changing the relative positions of multiple lenses. EWOD (electrowetting-on-dielectric) 5 is one of the electrical methods to change the liquid-solid interfacial tension. By controlling the droplet electronically, the focal length can be changed rapidly (~ms). 6 Moreover, without mechanical moving parts, an integrated optical system can be compact, lightweight, and with little power consumption, which is desired for portable devices. In this paper, we study the optical properties of the liquid lens based on EWOD. The effective focal length of the liquid lens in a set of multiple lenses is investigated, which is applied to a laser tracking system. 2. PRINCIPLE 2.1 EWOD Electrowetting is an electric means to alter the interface tension between liquid and an electrode. 5,7 Depending on the configuration and materials used in experiments, electrowetting has been mostly studied in the forms of electrocapillary, continuous electrowetting, and EWOD. Over a hundred years ago, Lippmann monitored the position of the interface between electrolyte and liquid metal (mercury) in a fine capillary and discovered that the interfacial tension would be influenced by applying an external electric potential across the interface. 8,9 This so-called electrocapillary effect was then modified to move a mercury slug in an electrolyte-filled capillary by applying a voltage at the two ends of the capillary. 10 The driving mechanism was named continuous electrowetting and applied in optical switches and microfluidics 11,12 In *skfan@mail.nctu.edu.tw; phone ext Adaptive Optics for Laser Systems and Other Applications, edited by Gilles Cheriaux, Chris J. Hooker, Michal Stupka, Proc. of SPIE Vol. 6584, 65840D, (2007) X/07/$18 doi: / Proc. of SPIE Vol D-1

2 both electrocapillary and continuous electrowetting cases, the applied voltage accumulates charges in the electrical double layer (EDL) 13 at the interfaces of electrolyte and mercury and changes the interfacial energy. Therefore, EDL can be regarded as a capacitor, and the electric property of the capacitor plays an important role in the electrowetting phenomenon. However, EDL is established by the surface charges of the liquid metal and the attracted counterions (ions of opposite charge) from the electrolyte, the electric property is easily affected by the surface condition and the ion concentration. Researchers found that the EDL was fragile and easily broken down by an excess voltage. In addition, the interface is prone to be contaminated by particulates in the system, reducing lifetime of such devices. 14 EWOD is another approach to realize electrowetting on a dielectric layer coated electrode. Different from using EDL as a capacitor, a well-defined and robust dielectric layer is prepared to be a capacitor in EWOD. Figure 1 shows a sessile drop experiment which has been widely used to study EWOD. A droplet is placed on an electrode covered by a dielectric layer and a hydrophobic coating. A hydrophobic-material-coated probe is immersed into the droplet. During the experiment, a voltage V is applied between the electrode and the probe. Similar as accumulating charges in the capacitor of EDL in the previous cases, when applying V between the electrode and the probe, charges will be distributed in the dielectric layer and change the interfacial tension of liquid and solid. Droplet: no voltage Droplet: voltage applied V θ 0 θ(v) Hydrophobic coating Dielectric layer Electrode Substrate (a) 1 mm Probe Reflected image (b) (c) Fig. 1. EWOD principle and experimental study by sessile drop experiment. (a) Set up of the sessile drop experiment. (b) Front view of a 1.2 µl DI water droplet when no voltage was applied. The contact angle is ~113 o. (c) Front view when 60 V is applied. The contact angle is measured to be ~88 o. Since a glass substrate is used, a reflected droplet was observed. The interfacial tension change is observed and measured by the change of contact angle of the droplet in the sessile drop experiment. The voltage-controlled contact angle can be expressed by Lippmann-Young equation: ε ε cosθ ( V ) cos + V 0 2 = θ0, (1) 2 γ LGt Proc. of SPIE Vol D-2

3 where θ 0 denotes the equilibrium contact angle at V = 0 V, θ(v) is the contact angle at V applied, ε o is the permittivity of vacuum ( F/m), ε and t are the dielectric constant and the thickness of the dielectric layer, and γ LG is the interfacial tension of liquid and gas. AC or DC voltages can be applied to generate EWOD. 1 khz AC signals are used in the study. The conformation of the droplet in the sessile drop experiment is monitored and recorded by a CCD through zoom lenses. Figure 1(b) is the front view of a 1.2 µl de-ionized (DI) water droplet dispensed on a Teflon (from DuPont) and SU-8 coated ITO (Indium-Tin-Oxide) electrode. ITO was first deposited on a glass substrate and then covered by a 1-µm-thick SU-8 which is a photosensitive polymer and widely used to construct microstructures. SU-8 was spin-coated, UV exposed, and baked to enhance its mechanical and electrical properties. 180-nm-thick Teflon was also coated by spinning to provide a hydrophobic surface. As can be seen from Fig. 1(b), with no voltage applied, the original contact angle of the droplet is greater than 90 o (hydrophobic). The accurate contact angle, analyzed and measured by software from the front view image, is ~113 o. Figure 1(c) shows the side view of the droplet when 60 V was applied. Obviously, the contact angle becomes smaller than 90 o and measured to be ~88 o. With sufficient voltage, the surface can be changed from hydrophobic to hydrophilic. Moreover, the surface wettability change is reversible and repeatable. For its controllability, reversibility and ease of fabrication, EWOD has been applied to microfluidics. 17 The applied voltage is increased from 0 V to 80 V continuously. The contact angles under varied voltages are measured and analyzed from the recorded front view images. The measured contact angles as well as the calculated ones from Lippmann-Young equation (Eq. (1)) are plotted in Fig. 2. As can be noticed, before 65 V, the experimental and theoretical contact angles agree with each other. However, when more than 65 V was applied, the contact angle stops to change and becomes saturated. This contact angle saturation occurring at high voltages has been noticed and reported. 2, Contact angle (deg) Experiment Theory Focal length Focal Length (mm) Saturation region Applied voltage (V) Fig. 2. Voltage dependent experimental contact angle, theoretical contact angle, and focal length in the sessile drop experiment. For the surface tension and the cohesion force of water molecules, when no other forces exert on the droplet, the shape of the droplet tends to be a sphere. In this study, since the volume of the tested droplets are small (~1.2 µl), the gravity force can be neglected. On the scale of our interest, surface tension and cohesion forces are dominant. Therefore, the droplets are considered to be spherical at the liquid-gas interface. The spherical droplet can be used as a liquid lens when its refractive index is different form the medium. For a droplet in the sessile drop experiment, the curvature of the droplet depends on the contact angle. When performing as a plano-convex liquid lens, the focal length f is determined by the radius of the curvature or the contact angle θ and expressed by: f 3V droplet = 2 π (1 cosθ )(2 cos θ cosθ ) 1 3 n L n G (2) Proc. of SPIE Vol D-3

4 where V droplet is the volume of the droplet, n L and n G are the refractive indices of the liquid and the gas respectively. As can be known from the equation, the focal length can be altered by the contact angle using EWOD. For example, the theoretical focal length of the droplet tested in the sessile drop experiment is calculated from Eq. (2), and the f-v curve is plotted in Fig. 2. By applying voltage on the 1.2 µl DI water droplet, the focal length can be easily changed from ~2.1 mm to ~2.7 mm. Besides altering the applied voltage, the refractive indices and the volume of the droplet can also vary the focal length. 2.2 EWOD on coplanar electrodes Since the immersed probe may interfere with the light passing through the liquid lens, we further develop a liquid lens actuated by the embedded electrode shown in Fig. 3. When applying a voltage between the pair of embedded coplanar electrodes, voltage is distributed evenly in the dielectric layer on both the electrodes. The charges are accumulated in the capacitor, and the surface becomes hydrophilic by EWOD. However, it requires more voltage to achieve a certain contact angle change in the coplanar electrode experiment than that in a sessile drop experiment. Droplet: no voltage Droplet: voltage applied θ 0 θ(v) Hydrophobic coating Dielectric layer Electrode Substrate V (a) 1 mm θ 0 Baseline θ(v) Reflected image (b) (c) Fig. 3. EWOD study by coplanar electrode experiment. (a) Set up of the coplanar electrode experiment. (b) Front view of a 1.2 µl DI water droplet when no voltage was applied. The contact angle is ~107 o. (c) Front view when 100 V is applied. The contact angle is measured to be ~88 o. In the sessile drop experiment, the applied voltage V is consumed mostly in the dielectric layer above the bottom electrode on the substrate. Nevertheless, in the coplanar electrode experiment, the applied voltage is distributed evenly in the dielectric above both electrodes. Only V/2 is applied to the dielectric on each electrode, reducing the contact angle change. As a result the contact angle change on each electrode should be modified as: Proc. of SPIE Vol D-4

5 1 ε ε V cosθ ( V ) = cosθ + (3) ( ) 2 γ LG t 2 To obtain the same contact angle change achieved by the sessile drop experiment, now in the coplanar electrode experiment, twice as much voltage is necessary. Figure 3(b) and (c) are the front views of the droplet without and with voltage applied. A 1.2 µl DI water droplet is placed on a pair of patterned ITO electrodes covered by 1-µm-thick SU-8 and 180-nm-thick Teflon layers. Since EWOD is generated by powering the embedded electrodes, the above probe is not necessary. When there is no voltage applied, the contact angle is ~107 o analyzed from Fig. 3(b). The measured initial contact angle is about 6 o smaller than that of the sessile drop experiment. The initial contact angle reflects the surface wettability of Teflon. The initial contact angle difference implies there is a degree of variation of Teflon coating between experiments. We also suspect the difference is caused by the contact angle hysteresis and the influence of the top immersed probe. When applying 100 V, the contact angle is changed to ~88 o as shown in Fig. 3(c). It is interesting to compare Fig. 2(c) and Fig. 3(c). To obtain a ~88 o contact angle, 60 V and 100 V are required in sessile drop and coplanar electrode experiments. If the initial contact angles are the same in both experiments, the required voltage in coplanar electrode experiment should follow Eq. (3) and be twice as much as that in sessile drop experiment. Similarly, the contact angles under various applied voltages are measured from CCD-recorded fort view images. The measured and the theoretical (Eq. (3)) contact angles are plotted in Fig. 4. Between 20 V and 105 V, the values of theoretical and measured contact angles almost agree with each other. Before 20 V, we suspect the initial contact angle has influence on the results. After 105 V, the contact angle starts to saturate. The calculated theoretical contact angles are substituted into Eq. (2) to obtain the corresponding focal lengths. As can be seen from Fig. 4, focal length of the 1.2 µl droplet can be changed from ~2.2 mm to ~2.5 mm by applying voltage on the embedded coplanar electrodes Contact angle (deg) Experiment Theroy Focal length Focal Length (mm) Applied voltage (V) Saturation region 2.2 Fig. 4. Voltage dependent experimental contact angle, theoretical contact angle, and focal length in the coplanar electrode experiment. 3. EXPERIMENTS The setup of the proposed laser tracking unit utilizing the tunable liquid lens is shown in Fig. 5. The laser used is a continuous wave frequency doubled Nd:YAG laser at 532 nm. The laser beam diameter is 2 mm with an output power of 4 mw. Due to the limited volume of the liquid lens that has a diameter of about 1 mm, the laser beam must be narrowed before sending through the droplet. A pair of iris and pinhole is used to narrow down the beam diameter and prevent the diffraction. The tunable liquid lens is placed horizontally on the substrate to avoid the influence of gravity force. In our experiment, the laser beam passes through the liquid lens from the bottom to the top. While the change of the focal Proc. of SPIE Vol D-5

6 length of the liquid lens is limited, a double-concave lens with f = -25 mm and double-convex lens with f = 25 mm are utilized to extend the effective focal length of the system. A flat mirror is used as the target for demonstration purpose. The received power is detected by a photodetector. To form an automatic tracking system, the received signal is fed back to the applied voltage of the droplet to control its focal length. PD/CCD Feedback Signal Ream Splitter mm F25ii,ii, 4 ak7 F=2smm Fused Silica 24 mm Liquid Lens Target Iris Pinhole Mirror Fig. 5. Experimental setup of the laser tracking system. A flat mirror is used as the target. PD: photodetector. To find the relation between the focal point and the applied voltage, received powers reflected from the target at different locations for different applied voltages are measured. In Fig. 6, the relative powers for different positions of each applied voltages are plotted, where the powers are normalized to the peak power for each applied voltages. The locations of the target are identified by finding the points where the powers peak to their maxima. For lower applied voltages, the laser beam focused tightly that the power drops quickly when the target is moved away from the focal point. When the applied voltage is increased to 120 V, on the contrary, the power varies slowly due to the loose focus. I I A ov * 60V A 90V 120V OVfitting curve Vfitting curve 90V curve l2ovfittingcurve. U Distance (cm) Fig. 6. The normalized received power reflected from the target at different locations for different voltages applied to the droplet. The arrows mark the maxima of the curves, showing the corresponding focal points. 42 Proc. of SPIE Vol D-6

7 The focal points of different voltages obtained from the experiment are plotted in Fig. 7. As can be seen, the focal point shifts from 33.7 cm to 37.7 cm away from the beam splitter when the voltage varies from 0 V to 120 V, respectively. With the current setup, the focus of the beam can be tuned in a range of about 4 cm, which can be further improved by tuning the parameters of the setup. Experimental data Theoretical data Fitting of experimental data Fitting of theoretical data F Applied Voltage ( Fig. 7. Target locations obtained from the maximal power positions for different applied voltages. Using the parameters measured given in Fig. 5 and the focal lengths of the liquid lens shown in Fig. 4, the theoretical focal points of different applied voltages are also plotted for comparison. It matches well in the low voltage regime. For higher voltage, the discrepancy is mainly due to the severe distortion resulting from the non-uniform curvature at the droplet surface. Moreover, due to the tight space in the setup, it is very difficult to accurately measure the separations between each optical component, which also contributes errors in calculation. 4. CONCLUSIONS A tracking system utilizing tunable liquid lens is proposed. Detailed principles of EWOD (electrowetting-on-dielectric) are given, while the relation between the contact angle and the applied voltage is investigated. Two possible liquid lens configurations with (sessile drop experiment) or without (coplanar electrode experiment) immersed probe are examined. Although it costs twice voltage in the coplanar electrode configuration, the probe-less setup facilitates the laser tracking experiment. By changing the applied voltage, the focal length of the adaptive liquid lens is adjusted respectively. The contact angle and the focal length of the tunable liquid lens are measured. Using the tunable liquid lens in an optical system, a laser tracking system is demonstrated where, by varying the voltage from 0 V to 120 V, the focal point shifts from 33.7 cm to 37.7 cm away from the system. The tuning range of the system can be further optimized by designing the optical system with different lens set. 5. ACKNOWLEDGEMENT This work was supported by the grants from National Science Council, Taiwan, R.O.C. (NSC E MY3 and NSC M ). Proc. of SPIE Vol D-7

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