Fiber Bragg Gratings in Small-Core Ge-Doped Photonic Crystal Fibers

Transcription

1 T T Manuscript TF FT JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER Fiber Bragg Gratings in Small-Core Ge-Doped hotonic Crystal Fibers Yiping Wang, Hartmut Bartelt, Wolfgang Ecke, Reinhardt Willsch, Jens Kobelke, Michael Kautz, Sven Brueckner, and Manfred Rothhardt Abstract Th paper reports fiber Bragg gratings (s) inscribed in a small-core Ge-doped photonic crystal fibers with a UV laser and a Talbot interferometer. The responses of such s to temperature, strain, bending, and transverse-loading were systematically investigated. The Bragg wavelength of the s shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. The bending and transverse- loading properties of the s are sensitive to the fiber orientations. Index Terms Fiber Bragg gratings, photonic crystal fibers, optical fiber sensors. 1. Introduction hotonic crystal fibers (CFs) have been undergoing rapid development over the past decade. A large number of optical fiber gratings have been demonstrated in various [1],[2] CFs with different inscription techniques. For example, the first long-period fiber grating in an air-core photonic bandgap fiber has been reported. Such a grating showed special optical charactertics due to their unique [2] microstructures and dpersive properties. Recently, small-core CFs attracted a great deal of attention because of their potential sensing and communications applica- [3] tions. In case of small core, reasonable field overlap with the surrounding holey region can be expected. Therefore, the microstructure can strongly affect the light transmsion properties. Ge-doping in the CF core of great interest received September 18, 28; reved October 2, 28. Th work was supported by the Alexander von Humboldt Foundation, the National Science Foundation of China under Grant No , and the Thuringian Mintry of Education and Cultural Affairs. Y. Wang with Institute of hotonic Technology, Albert- Einstein-Str. 9, 7745 Jena, Germany and State Key Lab of Advanced Optical Communication Systems and Networks, Shanghai Jiaotong University, Shanghai, 224, China (ypwang@china.com). H. Bartelt, W. Ecke, R. Willsch, J. Kobelke, M. Kautz, S. Brueckner, and M. Rothhardt are with Institute of hotonic Technology, Albert-Einstein-Str. 9, 7745 Jena, Germany. when one tries to achieve photosensitivity for the grating inscription. In th paper, we report s in a small-core Ge-doped CF and systematical investigations into the responses of such s to temperature, strain, bending and transverse-loading. 2. Fabrications A small-core Ge-doped CF with an air hole pitch of 4.2 µm employed in our experiments to inscribe a, as shown in Fig. 1. The diameters of fiber core, cladding, and air holes are 4.1 µm, 82.7 µm, and 3.5 µm, respectively. The central region of the fiber core with a diameter of about 1 µm doped with a high concentration of germanium. We inscribed high-quality s in the CF by use of a 248 nm [4] UV laser and a Talbot interferometer. As shown in Fig. 2, a phase mask with a period of 161 nm was employed as a beam splitter. The orientation of the mirrors in the Talbot interferometer defined the spatial frequency of the interference pattern and, therefore, the Bragg wavelength. Fig. 1. Cross section images of CF (IHT-252b5) from the Institute of hotonic Technology (IHT), Hhttp:// In order to observe the reflection spectra development during grating fabrication, the CF employed was spliced to a conventional single mode fiber by a splicing technique [5] reported earlier by the authors. Such a splice joint does not result in light reflection, due to the complete fusion of the fiber ends. As shown in Fig. 3, an, i.e., B1B, with a reflectivity of 35.2% at the Bragg wavelength of 83.5 nm and a 3 db bandwidth of.148 nm was inscribed in the CF. Since the was inscribed by exposure to about 6 UV laser pulses with an energy of 16 mj/pulse, it obvious that hydrogen loading not required to inscribe a high-quality in the CF.

2 R = 3 for for 43 Mirror UV laser st +1 α Lens hase st 1 mask Revolvable mirror Fig. 2. Scheme of experimental setup for inscribing s with two-beam interference and a UV laser. Reflection (db) Fig. 3. Reflection spectra of B1B. 3. Sensor roperties of s We systematically investigated the responses of such s inscribed in the CF to temperature, strain, bending, and transverse-loading by employing a specific fiber Bragg [6] grating interrogation system. The grating section of the CF was stripped of its coating before inscribing a in order to avoid the influence of the coatings on the investigated sensing properties. The s used in the following measurements were all made to have such properties as exemplified by Fig. 3. Thus the reflection spectra of B2B, B3B, and B4B are similar to that of B1B as illustrated in Fig Temperature roperties We investigated first the wavelength shift of B1B while the temperature of the gratings was adjusted by a semiconductor cooler and a temperature controller with a resolution of.1 C (THORLABS TED2). As shown in Fig. 4, the Bragg wavelengths of the gratings shifted linearly toward the longer wavelength with increasing temperature KBT =5 51pm/ C Temperature ( C) Fig. 4. Temperature properties of B1B. β JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER 28 Splice SMF CF (83.5nm, 1.12dB) 35.2% Light OSA The temperature-induced shift of the Bragg wavelength, λ B, due to the fiber expansion and the thermo-optic effect and may be written as : ( ) Δ λ = λ α + α Δ (1) B B Λ n T where ΔT the temperature change, αbλb the thermal 6 expansion coefficient for the fiber (about.55 1 silica), Λ the grating period, and αbnb the thermo-optic 6 coefficient, which approximately equal to the Ge-doped silica-core fiber. Clearly, the index change by far the dominant effect. From (1), the expected temperature sensitivity for an 83 nm Bragg grating inscribed in a conventional Ge-doped silica fiber about 7.6 pm/ C. As shown in Fig. 4, the temperature sensitivities of B1B 5.51 pm/ C. 3.2 Strain roperties Next we investigated the strain properties of another, i.e., B2B, inscribed in the CF by employing an experimental setup as illustrated in Fig. 5. B2B was bonded to the surface of a metal cantilever beam with a length of L=33 mm and a thickness of D=5.8 mm by means of a heat-curing epoxy (EO-TEK 353ND). The gratings were stretched along the fiber ax while the free end of the cantilever beam was moved upwards, whereas they were compressed along the fiber ax while the free end was moved downwards. The tensile and compressed strains can be calculated by 3 DW ( L x) ε ( W ) = 3 2L where x the dtance between the grating and the fixed point of the cantilever beam, and W the positive (upward) or negative (downward) dplacement of the free end of the beam. The maximum dplacement applied in our experiments was W = ± mm. As shown in Fig. 6, the Bragg wavelength of the gratings shifted linearly toward the longer or shorter wavelength with increasing tensile or compressed strain, respectively. D x L + : tensile strain W - : compressed strain L=33 mm D=5.8 mm x=18 mm Fig. 5. Schematic diagram of the experimental setup used to measure the strain properties of 2. The strain-induced shift of the Bragg wavelength, λbbb, due to the photo-elastic effect and the change of grating [1] pitch, and may be written as : (2)

3 WANG et al.: Fiber Bragg Gratings in Small-Core Ge-Doped hotonic Crystal Fibers Fig. 6. Strain properties of B2B. ( 1 p ) Δ λ = λ ε (3) B B e where εbzb the tensile or compressed strain applied to the s, and pbeb an effective strain-optic constant defined as 2 neff pe = p12 v( p11 + p12 2 ) (4) where and p are components of the strain-optic p11 12 tensor, and v the oson s ratio. For a typical germanosilicate optical fiber, the parameters are =.113, p =.252, v=.16, and B=1.482 nbeff. From (3) p11 12 λbbb=829.82nm KBsB=.67pm/μs Tensile strain Compressed strain Tensile or compressed strain (με) and (4), the expected strain sensitivity for an 83 nm Bragg grating inscribed in a conventional Ge-doped silica fiber approxi- mately.65 pm/με. As shown in Fig. 6, the strain sensiti- vities of B2B.67 pm/με. 3.3 Bending roperties We then investigated the bending properties of another, i.e. B3B,B Binscribed in the CF by employing an experimental setup as illustrated in Fig. 7 (a). The left end of a CF with a was fixed, and the right end was moved gradually toward the left side in order to bend the symmetrically. Th bending procedure was repeated after the fiber was turned around its ax by angles of 9, 18, and 27. The curvature of the bent due to the shift of the fiber end can be approximated by the relation: 2 2 ( ) z C = 2H H + L 4. (5) As shown in Fig. 8, the Bragg wavelength of the grating shifted approximately linearly toward longer or shorter wavelengths with increasing curvature, depending on the applied bending orientations. In other words, dtinct bending sensitivities of the Bragg wavelength were observed when the was bent toward different orientations of the fiber. Although air holes are symmetrically aligned in the ideal hexagonal symmetric CF, the microstructures are asymmetrical in the actual CF. Moreover, the single-side irradiation of the UV laser induced an asymmetrical index modulation within the cross section of the fiber core during the grating inscription. As a result, the center of the mode field not identical with the geometrical center of the cross-section of the grating, as shown in Fig. 7 (b). Thus, when the was bent toward the two opposite orientations within the diametral plane including the center of mode filed, a tensile or compression strain occurred in the center region of the mode field, resulting in a red or blue shift of the Bragg wavelength, respectively (Fig. 8). L Laser Mode field Turn center H fiber 27 9 Moved Fixe 18 (a) (b) Fig. 7. Schematic diagram of (a) the experimental setup used to measure the bending features, and (b) cross-section of the fiber and its mode field center. Bending radius (mm) λbbb=832.7nm Wavelength shift (pm) Curvature (1/m) Fig. 8. Bending properties of B3B. 3.4 Transverse-Loading roperties We finally investigated the transverse-loading properties of another, i.e. B4B,B Binscribed in the CF by employing an experimental setup (Fig. 9). A CF with a and another identical CF (in order to assure a symmetrical arrangement) were placed between two metal beams. L the length of the loaded CF/ segment, L=1 mm. A force, F, was applied to the upper metal beam via a pressure gauge and the change of the Bragg wavelength was measured. The CF/ was then unloaded. Such testing procedure was repeated after the o CF/ was rotated around its ax by angles of 45, 9 and F CF Beam Turn fiber Fig. 9. Schematic diagram of the experimental setup used to measure the transverse-loading properties of the. As shown in Fig. 1, the Bragg wavelength of the grating experiences a shift towards longer wavelengths with increasing transverse load. Again, the loading sensitivities of the Bragg wavelength show a strong dependence on fiber orientation. F L=1 mm FB

4 As dcussed in Section 3.3, an initial birefringence exts in the practical CF/. The external transverse-loading applied to a grating in the fiber having an initial birefringence will rotate the principal axes of the fiber, except when the transverse-loading directed along the initial polarization ax, i.e. fast ax or slow ax, of the [8] fiber. The application of the transverse-loading could enhance or remove the initial birefringence, depending on the angle between the loading direction and the initial polarization axes. As a result, the wavelength sensitivities of the to the transverse-loading are dependent on the fiber orientation and the wavelength shifts of the grating are [8] not necessarily linear, as shown in Fig JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY OF CHINA, VOL. 6, NO. 4, DECEMBER 28 [4] M. Rothhardt, C. Chojetzki, and H. R. Mueller, High mechanical strength single-pulse draw tower gratings, roc. SIE, vol. 5579, pp , May 25. [5] Y. Wang, H. Bartelt, S. Brueckner, J. Kobelke, M. Rothhardt, K. Mörl, W. Ecke, and R. Willsch, Splicing Ge-doped photonic crystal fibers using commercial fusion splicer with default dcharge parameters, Opt. Express, vol. 16, pp , May 28. [6] W. Ecke, A. A. Chertoriki, and V. L. Vesnin, A high-speed system for strain and temperature measurements based on fiber Bragg sensors, Instruments and Experimental Techniques, vol. 5, pp , Jun. 27. A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamental and applications in Telecommunications and Sensing, Boston, London: Artech House, 1999, ch. 8. [8] F. Bosia,. Giaccari, M. Facchini, J. Bots, H. Limberger, and R. Salathe, Characterization of embedded fibre Bragg grating sensors written in high-birefringent optical fibres subjected to transverse loading, roc. SIE, vol. 4694, pp , Feb Transverse loading (Nmm ) Fig. 1. Transverse-loading properties of B4B. 4. Conclusion Even without hydrogen loading, small-core Ge-doped CFs with high photosensitivity are well usable for inscription of high-quality s using a UV laser recording method. The Bragg wavelength of the s shifts toward longer wavelengths with increasing temperature, tensile strain and transverse-loading. The bending and transverse-loading properties of the s depend strongly on the bending and loading orientations, respectively. At the same time, the influence of asymmetries on the charactertic sensitivities and the possible dependence on the orientation of the fiber has to be considered for practical applications. References [1] B. J. Eggleton,. S. Westbrook, R. S. Windeler, S. Spalter, and T. A. Strasser, Grating resonances in air-silica microstructured optical fibers, Opt. Lett., vol. 24, pp , Nov [2] Y. Wang, W. Jin, J. Ju, H. Xuan, H.-L. Ho, L. Xiao, and D. Wang, Long period gratings in air-core photonic bandgap fibers, Opt. Express, vol. 16, pp , Feb. 28. [3] A. Efimov, A. J. Taylor, F. G. Omenetto, A. V. Yulin, N. Y. Joly, F. Biancalana, D. V. Skryabin, J. C. Knight, and. S. J. Russell, Time-spectrally-resolved ultrafast nonlinear dynamics in small-core photonic crystal fibers: experiment and modelling, Opt. Express, vol. 12, pp , Dec. 24. Yiping Wang was born in Chongqing, China in He received the B.S. degree in precion instrument engineering from Xi an Institute of Technology, Xi an, China in 1995 and the M.S. degree in precion instrument and mechanm and the h.d. degree in optics engineering from Chongqing University, Chongqing, China in 2 and 23, respectively. He joined the State Key Laboratory on Local Fiber-Optic Communication Networks and Advanced Optical Communication Systems, Shanghai Jiaotong University, Shanghai, China as a postdoctoral fellow in 23. He then joined the Department of Electrical Engineering, Hong Kong olytechnic University, Hong Kong, China, as a research fellow in 25. He currently a Humboldt fellow in Institute of hotonic Technology, Jena, Germany. H research interests focus on optical fiber gratings, optical fiber sensor, and microstructured fibers. Dr. Wang a senior member of IEEE, a senior member of Chinese Optical Society and a member of the Optical Society of America. Hartmut Bartelt was born in Karlsruhe (Germany) in He graduated in physics from University of Erlangen-Nuremberg (Germany) in 1976 and received h h.d. for research on wavelength multiplexed optical signal processing in 198. From 1981 to 1982 he worked as a research asstant at the University of Minnesota in Minneapol (USA) in the field of volume holographic optical elements. In 1982 he returned to the University of Erlangen-Nuremberg to continue h research activities and in 1985 he joined the corporate research laboratories of Siemens AG in Erlangen (Germany). In 1994 he was appointed professor for modern optics at the University of Jena (Germany) and became head of the optics divion of the Institute for hysical High Technology (IHT), now divion of optical fibers and fiber applications in the Institute of hotonic Technology. From 1999 to 26 he also served as director (head of the executive committee) of the IHT. Research activities cover the fields of optical speciality fibers, micro and nano-structured optics (laser and e-beam lithography) and fiber optical sensors.

5 WANG et al.: Fiber Bragg Gratings in Small-Core Ge-Doped hotonic Crystal Fibers 433 Wolfgang Ecke physict (diploma 197, Dr. rer. nat. 198) and Vice-Head of Fiber Optic Systems Department at the Institute of hotonic Technology in Jena, Germany. He works in R&D of fiber optic interferometric and Bragg grating sensor components and sensor systems, with direct application of the results at industrial partners in aerospace, transport, and energy sectors. Other activities include teaching Fiber Optics at Jena University of Applied Sciences, work as program chair of Optical Fiber Sensors and SIE Smart Structures conferences. Reinhardt Willsch was born in 1949 in Jena, Germany. He studied physics at St. etersburg and Jena universities. In 1972 and 1975, he received the Diplom-hysiker and h.d. degrees, respectively, from Friedrich-Schiller University Jena in the field of nuclear magnetic resonance (NMR) spectroscopy. Since 1982, he dealing with R&D on optical fiber components, sensors, and measurement systems. Currently, he with the Institute of hotonic Technology (IHT) in Jena as head of the Optical Fiber Systems department. In 1998, Dr. Willsch was appointed a Honorary rofessor at Jena University of Applied Sciences, where he acts as guest lecturer in sensor technology. Since 2, rof. Willsch a member of the Optical Fiber Sensors International Steering Committee. Jens Kobelke studied chemtry at TH Merseburg / Germany ( ) and received the h.d. degree in heterogenous catalys in Since th time he works at IHT Jena on development and preparation of special optical fibers based on different glass materials (e.g. chalcogenide glasses, HMO glasses and high silica). Current activities include the preparation and characterization of microstructured fibers. Michael Kautz s biography not available at the time of publication. Sven Brueckner received the Diploma (FH) in physical engineering from the University of Applied Sciences, Mittweida, Germany, in Since 1999, he laboratory engineer in the divion Optical Fibers & Fiber Applications, Institute of hotonic Technology, Jena, Germany. Manfred Rothhardt s biography not available at the time of publication.