* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download A multi interface total .. internal reflection based electro .. optic
Survey
Document related concepts
Photomultiplier wikipedia , lookup
Photon scanning microscopy wikipedia , lookup
Retroreflector wikipedia , lookup
Birefringence wikipedia , lookup
Night vision device wikipedia , lookup
Laser beam profiler wikipedia , lookup
Ultraviolet–visible spectroscopy wikipedia , lookup
Optical tweezers wikipedia , lookup
Surface plasmon resonance microscopy wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Gaseous detection device wikipedia , lookup
Anti-reflective coating wikipedia , lookup
Transcript
A multi . . interface total .. internal ...reflection based electro ..optic switch Cibby Pulikkaseril Department of Electrical and Computer Engineering McGiII University Montréal, Québec, Canada December 2003 A thesis submiUed to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements of the degree of Master of Engineering © Cibby Pulikkaseril, 2003 1+1 Library and Archives Canada Bibliothèque et Archives Canada Published Heritage Branch Direction du Patrimoine de l'édition 395 Wellington Street Ottawa ON K1A ON4 Canada 395, rue Wellington Ottawa ON K1A ON4 Canada Your file Votre référence ISBN: 0-612-98559-8 Our file Notre référence ISBN: 0-612-98559-8 NOTICE: The author has granted a nonexclusive license allowing Library and Archives Canada to reproduce, publish, archive, preserve, conserve, communicate to the public by telecommunication or on the Internet, loan, distribute and sell th es es worldwide, for commercial or noncommercial purposes, in microform, paper, electronic and/or any other formats. AVIS: L'auteur a accordé une licence non exclusive permettant à la Bibliothèque et Archives Canada de reproduire, publier, archiver, sauvegarder, conserver, transmettre au public par télécommunication ou par l'Internet, prêter, distribuer et vendre des thèses partout dans le monde, à des fins commerciales ou autres, sur support microforme, papier, électronique et/ou autres formats. The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. L'auteur conserve la propriété du droit d'auteur et des droits moraux qui protège cette thèse. Ni la thèse ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation. ln compliance with the Canadian Privacy Act some supporting forms may have been removed from this thesis. Conformément à la loi canadienne sur la protection de la vie privée, quelques formulaires secondaires ont été enlevés de cette thèse. While these forms may be included in the document page count, their removal does not represent any loss of content from the thesis. Bien que ces formulaires aient inclus dans la pagination, il n'y aura aucun contenu manquant. ••• Canada Abstract A novel method for implementing a lx3 electro-optic switch in lithium tantalate is presented. Using multiple total-intemal-reflection interfaces, an input beam is deflected to switch ports by reflection at a grazing angle; these interfaces are tumed on by application ofhigh voltage to a surface electrode. The expected performance of the device was simulated using a combination of in-house and commercial software, and the device was fabricated using a combination of university facilities and service vendors. Testing of the device showed that the concept ofusing multiple electro-optic interfaces is feasible and has potential applications in the creation of a package optical switch. • Sommaire Cette dissertation présente une méthode originale pour mettre en application un commutateur électrooptique lx3e dans le tantalate de lithium. En utilisant des interfaces multiples basées sur la réflexion interne totale, un faisceau d'entrée est guidé pour commuter des ports par réflexion à un angle de écorcher; ces interfaces sont allumées par l'application d'un haut voltage à une électrode extérieure. L'exécution prévue du dispositif a été simulée en utilisant une combinaison de simulations internes et commerciales, et le dispositif a été fabriqué en utilisant une combinaison des équipements d'université et des services industriels. L'essai du dispositif a prouvé que le concept d'employer les interfaces électrooptiques multiples est non seulement faisable, mais a aussi de nombreuses applications potentielles dans la création d'un commutateur optique de paquet. • Acknowledgements I would like to express my gratitude to my supervisor David Plant, for his unconditional support for my Master's program. He has never flinched or wavered in supporting any of the unsubstantiated, exploratory projects I have tried to implement, no matter how bizarre or ludicrous. I would like to thank Vincent Aimez and Pierre Langlois for their aid in processing our wafers and their indefatigable desire to try new projects. In addition, thanks to professors Andrew Kirk and Lawrence Chen, for always opening their do ors to me, despite my insistent questioning. Sincere acknowledgments go out to the electro-optic switch group: Eric Tremblay, Yiying Zuo, and Madeleine Mony have pushed the switch beyond • expectations, and have never turned down a good buffet. Thanks to Babak Bahamin and Ehab Shoukry, Les Éléphants Bébés, for diving right into the project, complete with high voltages and electro-optic arcs. A number of shout-outs go to the Photonic Systems Group: Nikolaos Gryspolakis for overseas aid; Chris Rolston for getting my equipment here on time; Dominic Pudo for his splicing; Colin Alleyne for his Gaussian beam analysis and grating simulations; Andre Isaac for laughing when 1 tried to align the Newfocus detector; Wei Tang for handling the Chinese suppliers; Eric Bisaillon for looking in the microscope; Roberto Rotili for sushi parties; James Prentice for weekend island-getaways; Fred TD for producing my script; Ted Zambelis for energy-saving hockey strategies; Ali Soleymani for Iranian cookies and Carlos, for recycling my discarded papers. 1 would like to thank my family; my mother, Anne, for her quiet support in my strange pursuits, my father, Babu, for his direct intelligence and my little brother, Danny, who 1 can no longer intimidate with force. This thesis is dedicated to my dear friend Puja, a girl that has an unusual sense of timing, an uncanny luck with salmon, an unerring shot from the three-point line, an unbelievable energy on the mountain and an incredible vitality that makes a thesis seem pale in comparison ... This work was supported by Agile AU-Photonic Networks (AAPN), a Research Network funded by NSERC and industrial partners. Table of Contents ! INTRODUCTION 1 1.1 MOTIVATION 1 1.2 SWITCHING TECHNOLOGIES 2 1.3 REFERENCES 6 ~ ELECTRO-OPTIC CRYSTALS 2.1 OPTICAL CRYSTALS 2.1.1 2.2 UNIAXIAL CRYSTALS ELECTRO-OPTIC EFFECT 2.2.1 2.3 MATRIX REPRESENTATION OF RUK LITHIUM-BASED CRYSTALS 10 10 10 12 13 14 2.3.1 ELECTRO-OPTIC COEFFICIENTS OF LITHIUM-BASED CRYSTALS 14 2.3.2 LITHIUM NIOBATE 16 2.3.3 LITHIUM TANTALATE 17 2.4 RESPONSE TIME OF LITHIUM TANTALATE 18 2.5 REFERENCES 20 J. DOMAIN INVERSION 3.1 BACKGROUND 21 21 3.1.1 FERROELECTRIC MATERIALS 22 3.1.2 FERROELECTRIC PROPERTIES OF LITHIUM-BASED CRYSTALS 26 3.2 MOTIVATION FOR DOMAIN INVERSION 27 3.2.1 !MPROVING INDEX DIFFERENCE 27 3.2.2 SHARP INDEX INTERFACES 28 3.3 POLING METHODS 29 3.3.1 METHODS IN LITERATURE 29 3.3.2 POLING WITH EXTERNAL ELECTRIC FIELD 29 3.4 REFERENCES 30 ~ DESIGN 4.1 CONCEPT 4.1.1 MOTIVATION 4.1.1.1 32 33 33 4.1.2 SINGLE INTERFACE SWITCH 34 4.1.3 MULTIPLE INTERFACE SWITCH 36 4.2 SIMULATION 37 4.2.1 RAYTRACE 38 4.2.2 GAUSSIAN SIMULATION 39 4.2.2.1 Theory 39 4.2.2.2 Simulation results 44 4.2.2.3 Interface width 47 4.2.3 • Problems with refraction-based switches 32 BEAM PROPAGATION 48 4.2.3.1 Code V Setup 48 4.2.3.2 Simulation Results 50 REFERENCES 51 FABRICATION 52 4.3 2 5.1 LITHIUM TANTALATE 52 5.1.1 LITHIUM TANTALATE PREFERENCE 53 5.1.2 W AFER CHARACTERISTICS 53 5.1.3 WAFERORIENTATION 54 5.2 LITHOGRAPHY 5.2.1 MASKDESIGN 55 56 5.2.1.1 Design considerations 57 5.2.1.2 Mask alignment 58 5.2.2 ELECTRODE PATTERNING 59 5.2.2.1 Cleaning 59 5.2.2.2 Photolithography 60 5.2.2.3 Metal evaporation and liftoff 60 5.3 5.3.1 POLING EXPERIMENTAL SETUP 61 62 5.3.1.1 63 5.3.2 POLING PROCEDURE 64 5.3.3 DETERMINING QUALI1Y OF POLED DEVICES 65 5.3.3.1 Charge transfer 65 5.3.3.2 Visualization 67 5.4 DICING, POLISHING AND AR COATING 69 5.4.1 DrCING 69 5.4.2 POLISHING 70 5.4.3 ARCOATING 72 5.5 ANNEALING 73 5.6 REFERENCES 76 §. • High Voltage supplies DEVICE TESTING 77 6.1 EXPERIMENTAL SETUP 78 6.2 SWITCHING BEHAVIOR 79 6.2.1 PERFORMANCE 79 6.2.2 CAMERA SCANS 81 6.2.3 VOLTAGE DEPENDANT BEHAvrOR 83 6.2.4 POLARIZATION 87 6.3 REFERENCES 89 Z CONCLUSION 90 7.1 FUTURE DEVELOPMENTS 91 7.2 REFERENCES 93 ~ 8.1 APPENDIX PAPER SUBMITTED TO CLEO 2004 94 94 List of Figures FIGURE 1 - TIR SWITCH ................................................................................................................................... 5 FIGURE 2- INDEX ELLIPSOID .......................................................................................................................... 12 FIGURE 3- TRIGONAL STRUCTURE ................................................................................................................. 15 FIGURE 4 - SCHEMA TIC REPRESENTATION OF PYROELECTRIC AND FERROELECTRIC CRYSTALS ..................... 23 FIGURE 5 - UNDER- AND OVERPOLING DIAGRAM ........................................................................................... 24 FIGURE 6 - HYSTERESIS LOOP ........................................................................................................................ 25 FIGURE 7 - INDEX DIFFERENCE OF (A) NORMAL FERROELECTRIC MATERIAL AND (B) FERROELECTRIC MA TERIAL WITH A REVERSED DOMAIN ................................................................................................. 27 FIGURE 8 - INDEX VARIATION OF AN UNPOLED FERROELECTRIC .................................................................... 29 FIGURE 9 - BASIC TIR CONCEPT .................................................................................................................... 32 FIGURE 10 - PRISM-BASED SWITCH ................................................................................................................ 33 FIGURE Il - FIRST INTERFACE RAYTRACE ..................................................................................................... 35 FIGURE 12 - Ix3 SWITCH SCHEMATIC ............................................................................................................ 36 FIGURE 13 - DESIGN OF 3-PORT SWITCH ........................................................................................................ 37 FIGURE 14 -RAYTRACE OF (A) 1 8T INTERFACEREFLECTION AND (B)BOTH INTERFACE REFLECTION .............. 38 FIGURE 15 - INPUT GAUSSIAN BEAM .............................................................................................................. 40 • FIGURE 16 - BEAM WAIST THROUGH DEVICE ................................................................................................. 41 FIGURE 17 - CURVED WAVEFRONT INCIDENT ON AN INTERFACE ................................................................... 42 FIGURE 18 - SIMULATION OF REFLECTION OF MULTIPLE INTERFACES-OPERATED AT 1046 V ...................... 44 FIGURE 19 - SIMULATION RUN AT 1075 V ..................................................................................................... 45 FIGURE 20 - EFFECT OF VOLTAGE ON INSERTION LOSS .................................................................................. 45 FIGURE 21 - EFFECT OF ANGLE ON SWrrCH INSERTION LOSS .......................................................................... 46 FIGURE 22 - BEAM WIDTH AT REFLECTING INTERFACE .................................................................................. 47 FIGURE 23- LENS DATA MANAGER FOR PORT 1.............................................................................................. 48 FIGURE 24 - 2D PLOT OF CODE V SySTEM ..................................................................................................... 49 FIGURE 25 -LENS DATA MANAGER FOR PORT 3 ............................................................................................. 49 FIGURE 26 - BEAM PROPAGATION RESULTS FOR PORT (A) TWO AND (B) THREE ............................................. 50 FIGURE 27 - SETUP TO DETERMINE WAFER ORIENTATION .............................................................................. 55 FIGURE 28 - (A) POLING AND (B) DEVICE ELECTRODES .................................................................................. 56 FIGURE 29 - PHOTOLITHOGRAPHY MASKS (A) METAL lAND (B) METAL 2 ................................................ 57 FIGURE 30 -ALIGNMENTMARKS (A) FROM METAL 1, (B) FROM METAL 2 AND (C) ASSEMBLED ................ 59 FIGURE 31 - POLING SETUP ............................................................................................................................ 63 FIGURE 32 - ELECTRICAL CONNECTION TO POLING SETUP ............................................................................. 64 FIGURE 33 -POLING CURRENT TRACES: RAW AND FILTERED DATA ............................................................... 66 FIGURE 34 - POLED DOMAINS UNDER CROSS-POLARIZERS ............................................................................. 68 FIGURE 35 - DIClNG CUT LINES ...................................................................................................................... 70 FIGURE 36 - POLISHING LINES ....................................................................................................................... 71 FIGURE 37 - ANTI-REFLECTION COATING ...................................................................................................... 72 FIGURE 38 - CAMERA SCANS OF PORT 1 (A) BEFORE AND (B) AFTERANNEALING .......................................... 74 FIGURE 39 -PORT 3 (A) BEFOREAND (B) AFTERANNEALING ......................................................................... 74 FIGURE 40 - DEVICE CHIP .............................................................................................................................. 77 FIGURE 41 - TEST SETUP ................................................................................................................................ 78 FIGURE 42 - CAMERA SCANS OF ALL SWITCHING PORTS ................................................................................ 81 FIGURE 43 - DETAILED EXAMINATION OF (A) PORT 2 AND (B) PORT 3 ........................................................... 82 FIGURE 44 - MISMATCH LOSS OF PORT 3 ....................................................................................................... FIGURE 45 - PORT 2 INSERTION LOSS - VOLTAGE DEPENDENCE ..................................................................... 84 FIGURE 46 - PORT 3 INSERTION LOSS - VOLTAGE DEPENDENCE ..................................................................... FIGURE 47 - PORT FIGURE 48 - SCAN OF (A) PORT 2, AND THE CROSSTALK IN PORT 2 WHEN (A) THE FIRST INTERFACE IS 87 - CAMERA SCANS OF THREE SWITCHING STATES FOR P-POLARIZATION ........................................ 89 FIGURE 50 - USING ELECTRO-OPTIC LENSES IN THE TIR SWITCH ................................................................... • 85 1 CROSSTALK - VOLTAGE DEPENDENCE ............................................................................ 86 ACTIV ATED OR (B) BOTH INTERFACES ARE ACTIVATED ........................................................................ FIGURE 49 83 93 List of Tables TABLE 1- CONVERSION TO CONTRACTED MATRIX REPRESENTATION ............................................................ 13 TABLE 2 - OPTICAL PROPERTIES OF LITHIUM NIOBATE .................................................................................. 17 TABLE 3 - OPTICAL PROPERTIES OF LITHIUM TANTALATE ............................................................................. 18 TABLE 4 - FERROELECTRIC PROPERTIES OF LITHIUM NIOBATE AND TANTALA TE ........................................... 27 TABLE 5 - EXPECTED BEAM SEPARATION AT SWITCH STATES ........................................................................ 39 TABLE 6 - CODE V SIMULATION RESULTS ...................................................................................................... 51 TABLE 7 - LITHIUM TANTALATE WAFERCHARACTERISTICS .......................................................................... 54 TABLE 8 - CHARGE TRANSFER ....................................................................................................................... 67 TABLE 9 - IMPROVEMENT IN INSERTION LOSS SURE TO ANNEALING .............................................................. 75 TABLE 10 - SWITCH INSERTION LOSS AND SEPARATION ................................................................................. 80 TABLE Il - CROSSTALK BETWEEN PORTS ...................................................................................................... 80 TABLE 12 -PERFORMANCEOFP-POLARIZATION ............................................................................................ 88 • 1 Introduction 1. 1 Motivation Optical communication has received considerable focus in recent years as the next step in high-speed, high-volume communication. This interest is due to the sizable increase in transmission bandwidth and speed of an optical fiber compared to the physically larger electrical wires. One optical fiber, using wavelength-division multiplexing, can carry much more capacity than a single 4" coaxial cable. As long distance networks slowly convert electrical components to optical, in se arch of the alloptical network, attention has been drawn to the concept of an optical switch, avoiding • the electrical bottleneck in an optical-electrical-optical switch [1]. There are a wide variety of technologies under study for use in optical switching, and it is not presently clear if one specific method will prove to be the clear choice for next-generation networks. Every method has its benefits and drawbacks, and different switching applications can apply a particular technology to meet particular demands. This thesis documents the design, simulation, fabrication and testing of a multiport optical switch using a novel geometry in a bulk lithium tantalate crystal. Using the electro-optic effect, multiple total internaI reflection interfaces are used to direct a beam to one of three output ports. The electro-optic effect is an incredibly fast effect, and bulk electro-optic switches can change state as fast as the electric field can change. 1 1.2 Switching Technologies The first commercially available optical switch was based on sorne optomecharncal technology, either using prisms or mirrors [3]. Interest has waned in these types of switches, as they are difficult to scale, have extremely slow switching speeds, and questionable long-term reliability. Microelectromechanical systems (MEMS) are, essentially, a type of optomechanical switch, but are in their own category due to the considerable differences. MEMS have attracted considerable interest for switching and cross-connect applications, since they intrinsicaUy provide polarization and wavelength insensitivity, low crosstalk and ease of scalability [4]. While the majority of the MEMS designs involve using a mirror to reflect a free-space beam [5]-[7], sorne research is being done into using MEMS • to move an input waveguide to one oftwo output waveguides [8]. Large port count switches can use 3D MEMS technology, where mirrors can tilt on two axes, in an analog fashion, to direct the beam towards one of several output ports [9]. However, this scheme is exceedingly complex and requires fine position control and feedback to prevent drift. Switches based on MEMS technology generally exhibit high losses for high-port switches; commercial MEMS products have a loss of3.7 dB for an 8x8 switch [3]. MEMS switches have, historically, demonstrated slow switching speeds, roughly 7 ms, due to the mechanical motion used to deflect light [4]. However, in the past few years, MEMS have been built with micro second switching speeds[lO]. Thermo-optic switches use the change of refractive index of a material when heated; typically, switches are created by inducing a phase change in a Mach-Zehnder 2 interferometer [11], [12], but switches have been fabricated by using thermo-capillary action [l3]. The main disadvantage ofthermo-optic switches is their slow switching speed (3 ms) and high power requirements, but recent developments have shown that switches can be operated as low as 45 m W [14]. Liquid crystals have been the focus of switching interests for some years. An electric field is used to change the optical properties of a liquid that maintains a mean relative orientation, similar to a crystal [15], [16]. Polarization effects are generally used to drive the switching behavior, usually using some type of polarization filter to screen one switch state from another. Switches designed in the late eighties carried slow switching times, 4 ms, and excellent crosstalk, around -30 dB, but generally exhibited poor insertion losses, due to the polarization losses [17]. Recently, liquid crystal • holographic switches have been illustrated, enabling a single-stage switch to act as a 1 x N switch, with N potentially as high as 1000 [18]. In fact, Fracasso et. al. c1aim that their scheme is inherently more accurate than 3D MEMS; they report a switch with an 8 dB insertion 10ss, and a switching time of a few hundred micro seconds. Acousto-optic switches use the interaction of light and sound in a waveguide to build a switching effect. Through the acousto-optic effect, a surface acoustic wave is created in the same direction as the traveling light, and subsequently, creates an effective grating which directs the light to one oftwo ports [3]. These switches can carry multiple wavelengths, and demonstrate good isolation (-30 dB) and insertion losses (3 dB) [19], [20]. Historically, acousto-optic switches have demonstrated slow switching times, but recent designs have shown rise times of 40 ~s [20] and 300 ns [21]. 3 A Semiconductor Optical Amplifier (SOA) switch uses on-off voltages to amplify an incoming signal. An 'on' voltage creates a population inversion which amplifies the signal, but an 'off signal actually attenuates the beam, creating excellent extinction ratios [22], [23]. This effect can also be applied to multiple-quantum weIl switches [24]-[27], and semiconductor waveguides [28]. The switching times ofthese devices can be extremely fast, especially as the active regions shrink in size; rise times have been reduced recently from 2 ns to 200 ps [29]. Other switching technologies include the spatial collisions of solitons [30], thin film ferroelectrics [31], directional coupler switches [32], and A WG-blocking switches [33]. This proj ect uses the electro-optic effect to create an index change in a • ferroelectric material (see Section 2 and 3.1). The electro-optic effect is one of the fastest effects used in switching technology today [3], but has been identified as a method to deflect laser beams since the late 60's [34]. This effect is often used in waveguide technology to create switches, or used for quasi-phase matching [35], since the electrooptic effect is a waveguide requires low voltages, but we are interested in exploring switches fabricated in bulk electro-optic materials. A research group at Carnegie-Mellon, lead by T.E. Schlesinger and D.D. Stancil, have produced most of the comerstone work on bulk lithium niobate switches, using a series of electro-optic prisms [36]-[38]. Prism-based switches use a negligible amount of refraction imparted by each prism that will have a net cumulative effect on the final output of the beam. 4 This thesis describes an optical switch made in bulk lithium tantalate using total internal reflection (TIR) to deflect a beam, as shown in Figure 1. A laser beam is directed at a glancing angle at an index interface, created in an electro-optic material by the application of an electric field, and reflects off the interface to an output port. The novel aspect ofthis project is that this concept is expanded to create a multiple-port switch. electro-optic interface • Figure 1 - TIR switch The concept and design of our switch will be discussed in detail in Section 4, but the idea is based on papers written by Boyland et. al., describing the first published bulk TIR switch [39], [40]. They reported nearly 100% reflection off the TIR interface and a crosstalk of less than -20 dB, suggesting that this concept is ideal for our switching application. Optical switches using TIR have been explored for sorne time; waveguide switches using PbwLaxZryTiz (PLZT) thin films were fabricated in 1984 [41] and nonlinear films were used to switch a laser beam in 1989 [42]. More recently, an optical switch using TIR in a liquid crystal cell has been proposed [43]. 5 1.3 References [1] Bregni, S., Guerra, G., Pattavina, A., "State of the Art of Optical Switching Technology for All-Optical Networks", Communications World, WSES Press, 200l. [2] Yasui, T., Goto, H., IEE Communications Magazine, vol. 25, no. 5, 1987. [3] Papadimitriou, G.I., Papazoglou, C., Pomportsis, A.S., J. Light. Tech., vol. 21, no. 2,2003. [4] Ma, Xiaohua, Kuo, G.-S., IEEE Opt. Comm., pp S16-S23, 2003. [5] Aksyuk, V., Barber, B., Giles, C.R., Ruel, R, Stulz, L., Bishop, D., Elec. Lett. vol. 34, no. 14, 1998. • [6] Shubin, 1., Wa, P.L.K, Elec. Lett., vol. 37, no. 7,2001. [7] Babic, R, TELSIKS 2003, pp 503-505. [8] Ollier, E., IEEE J. Select. Top. Quant. Elec., vol. 8, no. l, 2002. [9] Kaman, V., Zheng, X., Helkey, RJ., Pusarla, C., Bowers, lE., Phot. Tech. Lett., vol. 15, no. 6,2003. [10] Ford, J.E.; Aksyuk, V.A.; Bishop, DJ.; Walker, J.A., l Light. Tech., vol. 17, no. 5, 1999. [Il] Hida, Y., Onose, H., Imamura, S., Phot. Tech. Lett., vol. 5, no. 7, 1993. [12] Espinola, R,L., Tsai, M.-C., Yardley, J.T., Osgood Jr., RM., Phot. Tech. Lett., vol. 15, no. 10,2003. [l3] Sato, M., Makihara, M., Shimokawa, F., Nishida, Y., ECOC 1997, pp73-76, 1997. [14] Sohma, S., Goh, T, Okazaki, H., Okuna, M., Sugita, A., Elec. Lett., vol. 38, no. 3, 2002. 6 [15] Crossland, W.A., Manolis, I.G., Redmond, M.M., Tan, K.L., Wilkinson, T.D., Chu, A., Handerek, V.A., Hoimes, M.l, Parker, T., Bonas, I.G., Robertson, B., Warr, S.T., Stace, C., White, H.J., Wooley, R.A., Henshall, G., IEE Seminar on Microdisplay and Smart Pixel Technology, 2000. [16] Chu, H.-H., Holmes, M.l, IEE Seminar on Microdisplay and Smart Pixel Technology, 2000. [17] Skinner, J., Lane, C.H.R., J. Selected Areas in Comm., vol 6., no. 7, 1988. [18] Fracasso, B., de Bougrenet de la Tocnaye, Razzak, M., Uche, c., J. Light. Tech., vol. 21, no. 10, 2003. [19] Smith, D.A., d'Alessandro, A., Baran, J.E., Fritz, D.J., Jackel, lL., Chakravarthy, R.S., J. Light. Tech., vol. 14, no. 9, 1996. [20] Park, H.S., Song, K.Y., Yun, S.H., Kim, B.Y., l Light. Tech., vol. 20, no. 10, 2002. [21] Enguang, D., Deming, W., Anshi, X., OFC 2000, pp 7-10. [22] Morito, K., Leuthold, l, Melchior, H., ECOC 1997, pp 81-84, 1997. [23] Kalman, R.F., Kazovsky, L.G., Goodman, lW., Phot. Tech. Lett., vol. 4, no. 9, 1992. [24] Nakamura, H., Kanamoto, Y., Nakamura, Y., Ohkouchi, S., Ikeda, N., Tanaka, Y., Sugimoto, Ishikawa, Asakawa, K., LEOS 2002, pp 764-765, 2002. [25] Yao, J., Walker, N., Sherlock, G., IEE Colloquium on Optical Amplifiers for Communications, pp 5/1-5/6, 1992. [26] Fisher, M.A., Davies, D.A.O., Adams, M.J., Kennedy, G.T., Grant, R.S., Sibbett, W., IEE Colloquium on Ultra-Short Optical Pulses, pp 711-7/4, 1993. 7 [27] Kim, C., May-Arrioja, D.A., LiKamWa, P., Newman, P., Pamulapati, J., Elec. Lett. Vol. 36, no. 23, 2000. [28] Penty, RV., IEE Colloquium on Ultra-Short Optical Pulses, pp 6/1-6/4, 1993. [29] Gallep, C.M., Conforti, E., Phot. Tech. Lett., vol. 14, no. 7,2002. [30] Andrade-Lucio, J.A., Alvarado-Mendez, B., Rojas-Laguna, R, Ibarra-Manzano, O.G., Torres-Cisneros, M., Jaime-Rivas, R, Kuzin, B.A., Elec. Lett., vol. 36, no. 16,2000. [31] Lee, M.G., Aoki, S., Yokouchi, K., LEOS 2002, pp 734-735, 2002 [32] Hunter, D.K., Smith, D.G., IEE Colloquium on Optical Multiple Access Networks, pp 1211-12/4, 1991. [33] Cheyns, l, Van Breusegem, E., Ackaert, A., Pickavet, M., Demeester, P., Elec. Lett., vol. 39, no. 6, 2003. [34] Lotspeich, J.F., "Electrooptic light-beam deflection", IEEE Spectrurn, pp 45-52, 1968. [35] Matsurnoto, S., Lim, E.l, Hertz, H.M., Fejer, M.M., Elec. Lett., vol. 27, no. 22, 1991. [36] Li., l, Cheng, H.C., Kawas, M.J., Lambeth, D.V., Schlesigner, T.B., Stancil, D.D., Phot. Tech. Lett., vol. 8, no. 11, 1996. [37] Chiu, Y., Gopalan, V., Kawa, M.J., Schlesinger, T.E., Stancil, D.D., Risk, W.P., J. Light. Tech., vol. 17, no. 3, 1999. [38] Chen, Q., Chiu, Y., Lambeth, D.N., Schlesinger, T.E., Stancil, D.D., l Light. Tech., vol. 12, no. 8, 1994. 8 [39] Boyland, A.J., Sakellaris, M., Hendricks, J.M., Smith, P.G.R., Eason, R.W., Opt. Comm., vol. 197, pp 193-200,2001. [40] Boyland, A.J., Ross, G.W., Mailis, S., Smith, P.G.R., Eason, R.W., Elec. Lett., vol. 37, no. 9, 2001. [41] Wasa, K., Yamazaki, O., Adachi, H., Kawaguchi, T., Setsune, K., J. Light. Tech., vol. LT-2, no. 5, 1984. [42] Khoo, I.e., Zhou, P., J. Opt. Soc. Am. B., vol. 6, no. 5., 1989. [43] Yang, D.-K., J. Opt. A: Pure Appl. Opt., vol. 5, pp 402-408,2003. 9 2 Electro . .optic crystals 2. 1 Dpt/cal crystals The crystals we are interested in using, for our electro-optic switch, are lithium based, specifically, lithium niobate (LiNb03) and lithium tantalate (LiTa03)' Both lithium compounds are uniaxial crystals, which are defined by the symmetry of the crystallattice. 2.1.1 Uniaxial crystals To describe a uniaxial crystal, we have to look at the dielectric tensor of the material [44]. The dielectric tensor can be described as ( 1) where Xij{W) is the dielectric susceptibility, a tensor that relates the e1ectrical polarization, P, and the electrical excitation, E, as shown by ( 2) The indices i andj represent two orthogonal axis of the crystal which indicate the plane of excitation. These indices will be defined in Section 2.2.1, to translate them to customary X,Y, and Z axes. The dielectric tensor can be inverted, (a-1)jj, to create an index tensor, which is represented by (l/n2)jj. The dielectric energy density, W, is given by ( 3) 10 where E is the electric field intensity and D is the electric flux density. From (3), we can extract the following ellipsoid: ( 4) This is called the index ellipsoid, which has principal axes intersections of .rsx, .rSy and .rsz, as shown in Figure 2. Light traveling along these axes will see a refractive index equal to the values ofthe dielectric tensor in the other two axes. For example, light traveling along the X-axis, will see nl = -Ji:,n2 = Ji: , (5) whereas light traveling in an isotropic material will have Sx = Sy = Sz = S = n2 . In an uniaxial crystal, like lithium tantalate or niobate, we have symmetry in the • index ellipsoid, such that lix = liy = li perp = no2 li z = ( 6) and li par = 2 ne , ( 7) where sperp and spar are the perpenrucular and parallel dielectric constants, respectively. The existence of a difference between the ordinary index, 110, and the extraordinary index, ne, is called birefringence. 11 z y Figure 2- Index ellipsoid 2.2 Electro-optic effect The electro-optic effect is the change in the optical properties of a material based on an extemal electric field [44]. Yariv shows how to evaluate the index ellipsoid under the influence of an electric field. We expect this effect to be quite small; therefore, if the change in index, (l/n2)ij, is expressed in terms of the electric field, E, only the first- and second-order term are of interest: ( 8) where rijk is designated as a linear electro-optic coefficient tensor and Sijkl is the quadratic electro-optic coefficient tensor. The linear effect is called the Pockels effect, 12 whereas the quadratic effect is known as the Kerr effect - both ofwruch are influenced by the light frequency and the modulation frequency. For the purpose ofthis document, the quadratic effect will be ignored, and we treat our devices as ifthey exist only with a linear electro-optic effect. 2.2.1 Matrix representation of rijk For ease ofuse, we represent the linear electro-optic coefficient tensor, l"ijk, as a two dimensional matrix, rmk, based on the symmetry (l/n2)jj = Cl/n2h. The index m is used to represent ij with the following designation: Table 1- Conversion to contraded matrix representation m Axes l XX 2 yy 3 ZZ 4 YZ=ZY 5 XZ=ZX 6 XY=YX Using trus notation, we can express the change in index, ~(l/n2)i, as 13 L1( n12 )1 'il r21 'i2 r22 1î3 r32 r33 L1C2 )4 r31 r41 r42 r43 L1CJ s rS1 rS2 rS3 L1C2 )6 r61 r62 r63 L1(n\ )2 L1( n12 )3 r23 El E2 ( 9) E3 2.3 Lithium-based crystals The electro-optic crystals ofprimary interest for this project are lithium niobate and lithium tantalate, which are considered ferroelectric crystals. Ferroelectric materials have a spontaneous polarization, P s, which can be reversed in polarity by an external electric field larger than the intrinsic coercive field of the material. This phenomenon will be discussed in greater detail in Section 3. 2.3.1 Electro-optic coefficients of lithium-based crystals Lithium niobate and tantalate are both trigonal crystals, which means, as shown in Figure 3, that a single molecular group has threefold symmetry about the cation row. Symmetry ofthis type for LiNb03 or LiTa03 is often designated as 3m symmetry. 14 Nb or Ta Figure 3- Trigonal structure The symmetry of the basic molecule results in a simplification of the electro-optic • tensor, reducing the complexity of an analytic expression for the effect. In our case, 3m materials have an electro-optic tensor of the following form: r Ij.. 0 -r22 li3 0 r22 li3 0 0 r33 0 r51 0 r 51 0 0 -r22 0 0 (10) This simplified form of the electro-optic tensor clarifies the effect of an electric field on the crystal. For example, the row with the term r33 has zero elements in the other two columns, which indicates that this crystal only experiences an index change in the zaxis when exposed to an electric field in the same axis. 15 Even though we have a selection of coefficients to use in an electro-optic device, only the r33 component will be used. This coefficient is the strongest, and for such a negligible effect, we would like to have the largest coefficient possible. Knowing that r33 will be used, we plan to apply an electric field only in the Zaxis. Ifthis is true, then equation (10) will simplify to [54] ( 11) 2.3.2 Lithium Niobate Lithium niobate is a popular material for electro-optic applications, due to its relatively high electro-optic and non-linear coefficients. The popularity oflithium • tantalate has increased the number of companies offering optical-quality wafers at low prices, which in turn, boosts the popularity of the material for frequency conversion, modulators and holographie storage [47]. The ease of finding quality LiNb0 3 wafers has lead to intense developmental research work on processing lithium niobate, such as diffusing waveguides [48], or, lately, periodically poled lithium niobate [49]. One of the disadvantages of using lithium niobate is that is readily susceptible to photorefractive damage, making it difficuIt to pass high laser power through the crystal. However, this problem can be greatly diminished by doping the LiNb03 crystal with compounds such as MgO or ZnO [50]. Table 2 supplies sorne basic optical properties oflithium niobate [53]. 16 Table 2 - Optical properties of lithium niobate Symbol Units Value Ordinary index I10 - 2.23 Extraordinary index lle - 2.16 Electro-optic coefficient r33 pmN 30.9 Dielectric constant B33 relative to Bo 30 Property 2.3.3 Lithium Tantalate Lithium tantalate is, as seen in Section 2.3.1, structurally similar to lithium niobate, and since it has the same form of the electro-optic tensor, can be used to • fabricate identical devices . The demand for lithium tantalate is much lower than lithium niobate, so it becomes much more difficult to find optical grade wafers of the right orientation. In fact, we had to deal with suppliers in China, resulting in various difficulties from the extreme time difference and language barrier. However, lithium tantalate is much less vulnerable to photorefractive damage and has a lower piezoelectric constant, making it suitable for surface-acoustic wave devices and quasi-phase matching, even though the nonlinear effect in lithium niobate is twice as large [51]. Table 3 lists sorne basic optical properties of lithium tantalate [53]. 17 Table 3 - Optical properties of lithium tantalate Symbol Units Value Ordinary index no - 2.176 Extraordinary index ne - 2.180 Electro-optic coefficient r33 pm/V 35.8 Dielectric constant 833 Property relative to 80 43 2.4 Response time of lithium tantalate Our interest in using electro-optic materials for switching is based primarily on speed. CUITent switching technologies can only switch in milliseconds, and next• generation networks anticipate faster switching rates. The response time of an electro-optic material, 't, is used to evaluate the switching speed. Agullo-Lopez states that the speed of the direct electronic mechanism in an electro-optic material is roughly 't ~ 1O-14S, depending on the material [44]. However, in the laboratory, we understand that the response time is more accurately characterized by our electrical setup, and the intrinsic RC constant derived from the circuit. Still, we can determine a theoretical RC constant for our electro-optic device, and use this time constant as the maximum possible ideal we can achieve. The response time, 't, is given by (12) The capacitance, C, for a parallel-plate capacitor is given by 18 C=&lI d' (13) where E is the dielectric constant ofthe material, S is the surface area of the electrode, and dis the material thickness separating the electrodes [45]. In order to approximate the resistance of the electrode, we use the expression for a parallel-plate electrode: ( 14) where 1 and w are the length and width of the electrode, respectively; /le is the permeability of the material used, and cre is the conductivity. From Cheng, we find that, for gold, the values of /le and cre are 1.257 x 10-6 Hlm and 4.10 x 107 SIm. From equation (14), we find the resistance ofthe go Id electrode to be • 1.25 x 10-30 . For the capacitance, we use an area of 523.8 mm2, the largest electrode in our designs; the relative dielectric constant of lithium tantalate is given as 44.5 in the z-axis of the crystal. With these values, we evaluate the smallest capacitance of our devices to be 412 pF. Injecting these values into equation (12) allows us to suggest a time constant of 0.515 ps for our devices. This value is so small that it seems safe to assume that the limit to our fast switching speeds will come from the drive electronics. 19 2.5 References [44] Agullo-Lopez, F., Electrooptics, Academic Press,1994. [45] Cheng, D.K., Field and Wave Electromagnetics, Addison-Wesley Publishing Company, 1992. [46] Yariv, A., Optical Electronics, Saunders College Publishing, 1991. [47] Xue, D., Betzler, K., Hesse, H., Solid State Comm., vol. 115, pp 581-585, 2000. [48] Alfemess, R.c., Ramaswamy, V.R., Korotky, M.D.D., Buhl, L.L., IEEE Trans. Microwave Theory and Tech., vol. MTT-30, no. 10, 1982. [49] Grilli, S., Ferraro, P., Nicola, S.D., Finizio, A., Pierattini, G., Natale, P.D., Chiarini, M., Optics Express, vol. Il, no. 4, 2003. • [50] Xue, D., Kitamura, K., J. Crystal Growth, vol. 249, pp 507-513,2003. [51] Ahlfeldt, H., Webjom, J., Arvidsson, G., IEEE Phot. Tech. Lett., vol. 3, no. 7, 1991. [52] Shuto, Y., Amano, M., l Appl. Phys., vol. 77, no. 9, 1995. [53] Luther-Davies, B., Davies, P.H., Cound, V.M., Hulme, K.G., J. Phys. C: Solid State Phys., vol. 3, pp L106-L107, 1970. [54] Fang, J.c., Kawas, M.l, Zou, J., Gopalan, V., Schlesinger, T.B., Stancil, D.D., IEEE Phot. Lett., vol. 11., no. 1, 1999. 20 3 Domain inversion 3.1 Background The symmetry of a crystal can be deterrnined by looking at its unit cell and categorizing the crystal into one of32 point groups, or classifications [56]. Eleven of these 32 groups are centrosymmetric, wmch means that the crystal has no polarity. AU except one ofthe remaining 21 point groups, which are not centrosyrnrnetric, have a piezoelectric effect about sorne axis; the piezoelectric effect is the development of an electrical polarity when subjected to physical stress. Arnong these crystals, 10 of the 20 piezoelectric groups have an asymmetric atomic arrangement, which gives them a • polarity, or further, a spontaneous polarization . Spontaneous polarization is a measure of the value of the dipole moment per unit volume and is given by: Ps = [HJudv]1 volume, (15) where Il is the dipole moment per unit volume. Materials exhibiting a spontaneous polarization have positive and negative ions at sorne minimum free energy state at a certain temperature, called the Curie temperature. As the temperature drops below this Curie temperature, the ions are slightly displaced, creating electric dipoles; the spontaneous polarization is, as previously mentioned, the resulting sum of these dipole moments. The value of the spontaneous polarization is temperature dependant, which, from a different perspective, is to say that a change oftemperature cause a flow of current to the surface of the crystal. This is called the pyroelectric effect and can be expressed as (16) 21 where L\Ps is the change in spontaneous polarization, !lT is the change in temperature and p is the pyroelectric coefficient in C m-2 K- 1 • 3.1.1 Ferroelectric materials From the family of pyroelectric crystals, certain materials, when exposed to an electric field, will actually reverse the orientation oftheir spontaneous polarization [55]. This reversaI remains permanent even after the electric field is removed. These types of materials are considered to be ferroelectric. The condition for ferroelectricity has a requirement for an experimental procedure, which suggests that there is no crystal classification to determine ferroelectricity. In fact, after the spontaneous polarization has been reversed, it is not • always clear if the change is permanent or will revert after some time . Figure 4 summarizes the ideas behind the differences between ferroelectric and pyroelectric materials. Both materials have a polarity, but only the ferroelectric substance will change that polarity under an electric field; for comparison, an antipolar crystal is shown, which has no polarity in any condition. 22 No electric field 8 '" F erroelectric "8 8j 8 ",. Antipolar @ ,,8 e e 8j @ ,w"8 ,8 e @' :8j 8 e negative ion Ee positive ion ,~è ""We e ~ e e, "",,8 8 8,"" wB, ",8 S3 Pyroelectric 'B Electric field 8 EB e,~ 8 8j: ,·,·,···8 8 ,'u @ 8, B' é 8 ",8, 8 '8j e @ 8· 8 Electric field direction Figure 4 - Schematic representation of pyroelectric and ferroelectric crystals Ferroelectric materials, like lithium niobate and tantalate, have regions where the polarization of the electric dipoles are an arranged in the same direction, called ferroelectric domains. It is not clear why domains exist in ferroelectrics, but it suggested that the arrangement of domains results in a state of minimum free energy. The entire domain can have its spontaneous polarization reversed by an extemal electric field, in a process called domain reversaI, or as we caU the procedure, poling. Poling requires an electric field to reverse the domains, but the magnitude of this field is critical to successfully pole devices. If the field is too strong, we can potentially overpole the device, a phenomena where the domain reversaI extends outside the 23 electrodes. This effect, as weIl as underpoling, is displayed in Figure 5; this emphasizes the need to understand how the poling process in order to fabricate devices. electrode +v -v Underpoled Well-poled Overpoled Figure 5 - Under- and overpoling diagram From the piezoelectric equations given by Xu [56], the electric displacement in a • crystal is given by: (17) where E is the electric field vector, Ps is the spontaneous polarization, sis the dielectric constant, d is the piezoelectric coefficient and T is the stress on the crystal. While this relation gives us an analytic expression for the polarization in a crystal, the experimental results are strikingly different. In fact, a D-E plot taken from experiments, specifically, a Sawyer-Tower circuit, the following hysteresis shape occurs. 24 D Ps -------- ------------ E Figure 6 - Hysteresis loop In the tirst branch, when the electric field drops to zero, there is sorne polarization • smaller than the actual spontaneous polarization; this is called the remnant polarization and is a measure ofhow many of the domains will remain aligned to the positive direction. In the second branch, the polarization reaches zero at a specific electric field value, which is called the coercive field. It is this coercive field that we need to overcome in order to make domain reversaI occur. The value ofthis field can be analytically determined by E c = -+_2_ Iaf 3.J3 V-;:; , (18) where al and a2 are given by: _1_ 2833' a 2 (19) 25 and 633 is the component value of the dielectric tensor in the Z-axis given an Ez (see Section 2.2) [58]. From this relation, we get values of the coercive field to be 2750 kY/cm and 5420 kY/cm for lithium tantalate and lithium niobate, respectively. Unfortunately, experimental results from the literature actually report substantially lower values: 17 kY/cm and 40 kV/cm for lithium tantalate arid niobate. These values are orders of magnitude lower than the predicted values, but has been suggested the discrepancy lies in the existence of defect sites where domain reversaI occurs at a faster rate. Gopalan and Gruverman offer a modified equation, but albeit one without enough information to provide a numerical value. 3.1.2 Ferroelectric properties of lithium-based crystals The properties of lithium niobate and lithium tantalate are quite similar, since they have nearly identical crystal structures (see Section 2.3), but poling domains in these materials is quite different. Poled domains in lithium niobate are triangular, whereas lithium tantalate tends to form hexagonal shapes. Straight domain wans are harder to fabricate with lithium tantalate, but the hexagonal domains have much less tendency to merge, making it easier to pole small features [57]. Table 1 compiles the ferroelectric properties oflithium niobate and tantalate [58]. Lithium tantalate has a substantially smaller coercive field, making it easier to pole. 26 Table 4 - Ferroelectric properties of lithium niobate and tanfalate Spontaneous Polarization (C/mL) Ec (kV/cm) LiNb03 0.75 40 LiTa0 3 0.55 17 3.2 Motivation for domain inversion The electro-optic effect is typically very small, which induces a change in index that is almost negligible, on the order of 10-5 . To make functional devices with such a weak effect, we need to use either high voltages or use long interaction lengths. 3.2.1 Improving index difference In our devices, we do not need a large absolute change in index; rather, we are concemed with the index difference from one region to another. Domain inversion allows us to use the electro-optic effect to extract twice the index difference from a ferroelectric material. +~n -~n +~n Figure 7 - Index difference of (a) normal ferroelectric material and (b) ferroelectric mate rial with a reversed domain 27 As shown in Figure 7, reversing the polarization of a ferroelectric domain, gives us an index change of 2.6.n - this means we can reduce the voltage required by a factor of one half, a substantial improvement for our high speed devices. 3.2.2 Sharp index interfaces Without poling the ferroelectric domains, we encounter another problem with designing electro-optic devices. The electric field lines from a pattemed electrode are shown in Figure 8, suggesting that there will be an index gradient around the edges of any electrodes. An the models for our devices rely on the fact that the index interfaces are sharp and fiat, and obviously this does not fit the case. However, poling the device before applying the electric field creates a sharp interface, called a domain wall. The domain wall will create abrupt changes in index, thus avoiding the fringing effects outlined above. The Gaussian shape of the laser beam will be minimally disturbed if the domain walls are smooth and even. Top Electrode Electric Field Lines 28 Figure 8 - Index variation of an unpoled ferroelectric 3.3 Poling methods 3.3.1 Methods in literature Poling has been reported in ferroelectrics since the 70's, and interest was sparked in lithium niobate and tantalate from the obvious electro-optic and piezoelectric properties. Since then, several methods to induce domain inversion have been investigated, including electron beam poling [59], chemical patterning [60] and thermal poling [61], [62]. 3.3.2 Poling with external electric field Recently, aU major advances in poling have developed from using an external electric field to pole LiNb03 and LiTa03 [63], [64], [65], [66]. Using patterned electrodes on one face, and a ground electrode on the backside, it is quite easy to create inverted domains with sharp boundary walls. The method that is used in our project is to apply a high-voltage pulse to the sample, monitoring the current that passes through. Since the crystal acts like a capacitor, current will only be measured when domain reversaI occurs, as the shift in polarity is due to moving charges. The poling procedure can be evaluated by measuring this charge transfer and comparing it to a theoretical estimate, where the charge, Q, is given by ( 20) 29 where A is the area ofthe pattemed electrode and P s is the spontaneous polarization, as defined before. Q is an estimate, because this relation hold true for a pure, stoichiometric crystal; in reality, every crystal has defects and impurities. Occasionally, equation (20) is modified by multiplying an empirical factor, EF, to factor in difference between wafer batches. Pulse poling is the particular method we used to invert our device domains. By raising the DC voltage across the wafer to generate an electric field almost equal to the coercive field, a small voltage pulse can be applied to nudge the field higher. In this manner, we achieve fine control over the quantity of poled domains. This is ideal for poling interfaces, as overpoling willlead to lateral spreading of the domains, destroying the sharpness of the domain walls. 3.4 References [55] Lines, M.E., Glass, A.M., Principles and Applications ofFerroelectrics and Related Materials, Clarendon Press, 1977. [56] Xi, Y., Ferroelectric Materials and Their Applications, Elsevier Science Publishers B.V., 1991. [57] Meyn, J.-P., Laue, c., Knappe, R., Wallenstein, R., Fejer, M.M., Appl. Phys. B, vol. 73, pp 111-114,2001. [58] Kim, S., Gopalan, V., Gruverman, A., Appl. Phys. Lett., vol. 80, no. 15,2002. [59] Haycock, P.W., Townsend, P.D., Appl. Phys. Lett., vol. 48, no. 11, 1986. [60] Risk, W.P., Lau, S.D., Appl. Phys. Lett., vol. 69, no. 26, 1996. 30 [61] Brooks, R., Townsend, P.D., Hole, D.B., Callejo, D., Bermudez, V., Dieguez, E., J. Phys. D: Appl. Phys., vol. 36, pp 969-974, 2003. [62] Houé, M., Townsend, P.D., Appl. Phys. Lett., vol. 66, no. 20, 1995. [63] Hsing, J.L., Cheng, C., Kawas, M.J., Lambeth, D.N., Schlesinger, T.B., Stancil, D. D., IEEE Phot. Tech. Lett., vol. 8, no. 11, 1996. [64] Busacca, A.C., Sones, c.L., Apostolopoulos, V., Bason, R.W., Maillis, S., Appl. Phys. Lett., vol. 81, no. 26, 2002. [65] Yamada, M., Saitoh, M., J. Appl. Phys., vol. 84, no. 4, 1998. [66] Grisard, A., Lallier, B., Polgar, K., Péter, A., Elec. Lett., vol. 36, no. 12,2000. • 31 4 Design 4.1 Concept Electro-optic TIR switches have been documented for use as a 1x2 switch extensively. By using multiple interfaces, we aim to expand this concept to build a multiple-port switch. In this thesis, a lx3 switch is reported, but this is an intermediate step to building a lx4 switch, which has immediate applications in an all-optical network. The TIR switch is based on one idea: a collimated laser beam is incident at a grazing angle onto an electro-optic interface. When no voltage is applied, the beam passes straight through, but is reflected when the electric field is active, as displayed in • Figure 9 . on off 0c Figure 9 - Basic TIR concept 32 4.1.1 Motivation As mentioned before, electro-optic devices have great advantages in switching applications, due to the fast response times. Two principle styles are used to design these devices: 1) Deviees based on light refraction 2) Devices based on light reflection The Photonic Systems Group at McGill aiso designed and built refraction-based electro-optic devices, using a series of electro-optic prisms to gradually bend the beam, as shown in Figure 10. • Figure 10 - Prism-based switch The prism-based devices work extremely weIl [67], with insertion losses of3.6 dB and crosstalk of -37 dB. However, the device concept has sorne inherent problems that we will try to solve using reflection-based electro-optic switches. 4.1.1.1 Problems with refraction-based switches The prism-based switches use a series of prisms to deflect the beam; because the index change associated with the lithium tantalate electro-optic effect is so small, the effect of a single prism is quite small. 33 For example, from equation (11), the change of index from a 0.5 mm thick lithium tantalate with a voltage of 1000 V is 3 X 10-4 • Using Snell's law, the deflection from one 300 prism is given by (8 + 30°) = arctan(n-An n+An sin 30°) • ( 21) Substituting the calculated value of ~n, the angular deflection provided from one interface is only 9 x 10-3 degrees; since each prism has two interfaces, the total deflection provided by one prism is 1.8 x 10-2 . In order to deflect the beam one degree, 55 prisms are required in series! The problem is that the beam passes through each interface and is distorted by any defects in the domain wall. Increasing numbers of interfaces that are passed through by the beam, result in more distortion of the wavefront. While this has minimal effect on the lx2 switch built by McGill University, a multi-port switch will have this distortion amplified. Another problem: the 1.22 0 deflection reported by the prism switch is still a tiny deflection, too small for collimators to be placed side-by-side. The on-state has to be deflected by a prism and captured by a collimator mounted on the side of the package. This addition increases insertion 10ss, cost and complexity. 4.1.2 Single interface switch The problems mentioned in the previous section can be addressed by the design of an electro-optic TIR switch. One immediate benefit is gained by using reflection; since the beam does not have to pass through any poled domains, the quality of the domains does not degrade our beam quality to the same extent. 34 Equation 21 can be reduced to calculate the critical angle, 8 c , as shown in Figure 9, by substituting sin(300) with sin(900). Then the equation reduces to e C = 90° - arcsin(n+&n) n-Iln , (22) where Lln is -3.34 X 10-4 and n is 2.1807, giving us a critical angle of 1.418°, which me ans that the beam will experience an angular deflection of28c , or 2.836°. Using a raytrace, we can calculate the final displacement of the beam at the output of the device, designed to be 68 mm long, shown in Figure Il with only one interface. The length of the device was selected based on the wafer diameter; 68mm was the longest device that could fit on the wafer. 1 - hRnnn Figure Il - First interface raytrace The distance ofthis beam separation is calculated to be 2.83 mm, which is a substantial improvement over the prism-based switch. In fact, the separation is large enough to place fiber collimators side-by-side, as the collimators have a diameter of 1.3 mm. 35 Using only one interface to pro duce a significant deflection is quite a remarkable improvement, since the beam can only be distorted by the quality of that one interface. 4.1.3 Multiple interface switch In order to make a multi-port switch, another interface is required to further deflect the beam to a third switch state. This interface must be electrically isolated from the first interface, and has to be designed so that the reflected beam from the first interface reaches this electrode at a critical angle. This is illustrated in Figure 12, which indicates how each switching state is achieved. Port 3 Port 2 Port 1 Figure 12 - 1x3 switcn scnematic The first design of this device is quite simple, since we are exploring the feasibility ofusing multiple TIR interfaces. If the initial beam angle is 0°, then our first interface should have an angle of 1.418°, The next interface is trivial to design as well - since the beam will be deflected a total of 28c , the next interface must be at 38c , or 4.254°, relative to the input beam. 36 Figure 13 shows our final device, on the mask, used to fabricate the devices, a process covered in detail in Section 5. [ • 1 Figure 13 - Design of 3-port switch 4.2 Simulation In order to simulate the free-space electro-optic switch, three separate methods were used to extract different performance characteristics. Initially, a simple raytrace was used to design the device; since the phenomenon at work is reflection, the raytrace is a useful first design step. Next, a Gaussian simulation was written in MatLAB to determine the effect of voltage and input angle on the performance ofthe device. 37 Finally, Code V was used to perform a beam propagation analysis of the switching states, reinforcing the initial raytrace. 4.2.1 Raytrace As a first step, the double-interface TIR switch was modeled in MatLAB using a raytrace, as outlined in the previous sections. Figure 14 shows the graphical output of the simulation, where the solid lines are the electro-optic interfaces, and the dotted hne represents the light path. / / / / / (a) (b) Figure 14 - Raytrace of (a) I st interface reflection and (b) both interface reflection The raytrace allows us to visualize how much separation we expect to obtain from this design, as surnrnarized in Table 5. In addition, the dimensions from the raytrace were used irnrnediately for the Gaussian simulation, as detailed in Section 4.2.2, and then imported into Autocad to create the mask for the wafer. 38 Table 5 - Expected. beam separation ai switch states Separation (mm) Port 1 0.00 2 2.82 3 4.51 4.2.2 Gaussian simulation Although the raytrace simulation was a useful design step, we require a more comprehensive examination of the behavior of our devices. Using MatLAB, a Gaussian beam simulation was written to estimate the amount of power that could be reflected at each interface. 4.2.2.1 TbeOl'Y An the equations used for the Gaussian beam simulation are taken from either Hecht [69] or Saleh and Teich [68]. Waves that travel at small angles with respect to the propagation axis are called paraxial waves and must satisfy the Helmholtz equation [68]. One solution to the equation is a Gaussian beam, which has most of its power concentrated in the center of the beam, and is describes by E = ~ 2P exp(_K) ai, JUDo o ( 23) 39 where P is the total power in the beam and (00 is the beam waist, and has a transverse power profile as shown in Figure 15. ~ , , ,,, , ___ ~ __ ~ _ _ _ _ _ _ _ _ ~ _ _ _ _ _ _ _ _ L __ , ,,, -------r--------t------ , ,,, " --------T--------T " l , ------T--------y------1 , 1 1 1 1 1 1 1 1 l , 1 1 1 1 l 1 , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 J -------t--------r---- --r--------T--------T-- ----T--------T------1 J 1 -------r--------r- -----r--------T--------T----- --T--------T------1 1 1 1 1 1 1 , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l 1 , 1 1 1 l 1 , 1 r 1 1 1 1 1 1 1 1 1 1 Figure 15 - Input gaussian beam A beam waist of 220 !lm was used, since the beam diameter of our collimators is roughly 440 !lm, at a wavelength of 1310 nm. The Rayleigh range of a Gaussian beam is a parameter that indicates the distance over which a beam will remain focused, and is given by zo = ( 24) where Ais the wavelength and n! is the index ofthe material. Given the Rayleigh range, the beam waist of a Gaussian beam changes according to ( 25) 40 where ID is the beam waist, z is the distance traveled by the beam and Zo is the Rayleigh range. The radius of curvature of the beamfront, R, is expressed as ( 26) From equation 25, we can plot the beam waist as a function of distance in free space, as depicted in Figure 16, and see that our beam waist will be larger than 250 Ilm from 0-25 mm. The thickness of our wafer is only 500 Ilm; hence, a 250llm beam waist would correspond to a 500llm beam diameter (lIe"2) and would experience dramatic leakage from the wafer. Using these collimators willlead to sorne 10ss from this effect, wmch we can reduce by moving the collimators back from the front facet of the device. From Figure 16, it is suggested that the collimators should be moved back 2.5 cm, but we leave this to experimental verification. ~ ~ · . _ • • _ • .1 . . . . . . __ . . . . . . .1. . . . . . . __ . . . . . . . l. .......... __ . . . .t ...... __ ...... __ 1. . . . . __ .......... " ___ .......... 1 1 1 1 l , 1 1 1 1 1 1 1 1 l , 1 1 • 1 1 1 1 1 l l l , 1 1 1 , 1 l • 1 1 " ' 1 ' 1 .............. .. -:- .................. :- .............. .... ................ .. -:- .................. .............. .. ~ - - - . . . . . - - - , . . . MM -- - : - - - - - - .. 1 1 1 , .. -- .......... 1 1 ~_ 1 1 ................ -:-...... 1 l 1 ,, , - - - ~- , , ·· • 1 1 1 l , 1 l , 1 l 1 , 1 1 1 1 l 1 1 , l 1 1 , .. .. Mi ........ -- .... -; .................. -;- ............ M_" , 1 1 1 .. --, .......... -- .... -,_"" 1 "" ", , , , , , , , , l 1 1 ' 1 M_ ...... - - ~_ ............ .. r .............. .. 1 , , , . . . .. . .. - - - , - - - - - - - - - .,- - - - -- - - - -r - - - - - - - - - , - - 1 - 1 . • 1 1 1 • 1 ................ ,- ...... -- ...... -,- ...... -- .... --r""" 1 ~ , 1 , 1 ---r------ . --,-- ----- . --:- ---- .. _- --i- ----- .. -1 1 1 J f i . , 1 . 1 1 l, 1 1 l , l 1 1 1 , , 1 - - -,- - - - - - - - - - r - - - - - - - l , _ _ _ _ _ _ _ "' __ • _ _ _ _ _ - _, ___ - _ _ _ _ _ _ .. _ _ _ _ - _ _ _ _ ..f _ _ _ _ _ _ _ _ _ _ , _ _ _ _ _ · • 1 1 1 . • • 1 1 , , l l l 1 , l , , , , , , , Figure 16 - Bearn waist through device 41 The Gaussian bearn simulation begins by calculating the propagation distance to the first interface and calculating the beam waist and radius of curvature at the interface. The radius of curvature is important, since different parts of the wavefront will graze the interface at a different angle than the center, as illustrated in Figure 17. Transverse axis, x interface Figure 17 - Curved wavefront incident on an interface For every element of the wavefront, the angle of incidence must be calculated to see if it meets the requirement for total internal reflection. The angle, 'P, of a differential element of the wavefront from the central ray is given by '1' = arctan( ~) , ( 27) where x is the location along the transverse axis, as depicted in Figure 15, and R is the radius of curvature. The angle of incidence of this differential element is just the sum of'P and 0, the angle of incidence of the central ray. 42 At every point along the Gaussian beam, the angle of incidence is evaluated to determine if it is lower than the required critical angle. Angles that exceed the critic al angle are lost from the beam and are considered for 10ss. Furthermore, if less than 2000 meets the critical angle condition, the beam is no longer considered Gaussian and the simulation will transmit no power to the next interface. This description of 10ss is, indeed, a simplification and only serves to evaluate the amount of light 10st through failure to meet the critical angle. Elements that successfully meet the total internaI reftection condition are reftected from the interface. A reftection coefficient, r, is calculated from Fresnel equations [68], and is calculated for S-polarized light by r e -j2rp ( 28) where t ( m=arcan 't' ~Sin2 e-sin2 ee ) e s"', co ( 29) where e is the angle of incidence of an arbitrary point on the wavefront. Modifying the Fresnel equations [69], Boyland et. al. suggest the reftection coefficient as r = ([cos(s~n ~ ~ [nit s~n e Înc -cos e incnit ])] ) [cos(sm [nit sm e inc +cos e incni( ])] 2 , ( 30) The power of the reftected beam is calculated, considering losses for the TIR condition, and a Gaussian beam is reformed with the reftected power. This Gaussian beam is then projected to the next interface. 43 4.2.2.2 Simulation re~mlts Figure 18 shows the results of the simulation ron at 1046 V, the design voltage. The 10ss of power is expected, and confirms the accuracy of the simulation, as at the design voltage, sorne parts of the beam will not meet the TIR condition, and will be 10st. The relative peak power of the reflected beam from the first interface 1S -3 dB, compared to the initial source, and the second interface has a peak power of -6 dB. , ,, ,, l , 1 1 , , ,, ,, , ,,, , 1 1 1 1 1 1 1 , , , --_.----~--------~--------~-------- ----r--------r--------f--------f--------f---1 l 1 1 1 1 1 --------r--------r--------,--------T--------,------1 1 1 l , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 --~--------~--------~-------_&_------_&_-------~--------~-1 1 1 1 1 1 1 1 l 1 1 ' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l , 1 1 1 1 1 I 1 -----;--------r--------}--------f--------f--------t--------f----1 i i 1 1 1 1 1 1 1 1 1 1 1 1 1 l 1 , 1 1 1 1 1 1 1 1 1 1 1 l 1 1 1 , 1 1 1 1 1 l 1 1 1 , 1 1 1 1 1 1 1 1 1 1 -------r--------r--------r--------T--------,--------T·-------T------ Figure 18 - Simulation of reflection of multiple interfaces - operated at 1046 V This is a significant loss for a theoretical model, especially one that neglects material properties and poling aberrations. It becomes apparent that the device will have to be operated at a higher voltage than the design voltage, as shown in Figure 19. Using a voltage of 1075, we get theoretical insertion losses close to zero. 44 ~ - , ~ - - --- r - - - - - - -- - - - --- - - - - -. - --- ---- -- -- , T - - -- - - - - - - - -- , , ---r-, -- -----,.------ - ---.-.-. -------- --- ---- -or-------T, , I 1 l 1 1 1 , , ,,, ,,, , ---- ..., -------- .., --- .---- -- ------ ___ w. ___________ +, ________ , +1 ____ _ Figure 19 - Simulation mn at 1075 V The relation between the insertion 10ss and the voltage was investigated, to • detennine a working voltage at which to operate the device. Figure 20 shows the effect of voltage on the effectiveness of the switch; it would suggest that the device should not operate below 1060 V, since the reflected power drops off quickly. ....-_. 15t interface ---- 2nd interface Figure 20 - Effed ofvoUage on insertion loss 45 The Gaussian simulation also predicts the tolerance of the system to angular alignment. Ideally, the laser source will be inserted at 0° relative to the wafer flat, or the device die boundary - this will ensure that the beam encounters the interface at exactly the required critical angle. Alignment is always a difficult procedure, and aligning angles can pose an impressive obstacle. Our initial intuition suggested that the laser could be injected at a shallower angle than the critical angle, to guarantee total internaI reflection. Unfortunately, as Figure 21 shows, the device is sensitive to changes in angle in both directions. The explanation lies in simple geometry: a decrease of the incident angle at the first interface will correspond to a higher angle at the second interface. This limits us to • be able to align to 0.02 degrees in the yaw direction. 1- 1st interface 1 1- 2nd interface 1 Figure 21 - Effect of angle on switcb. insertion loss 46 4.2.2.3 Interface width During Gaussian beam simulations, an important concem began to stress its importance: the question of electrode length. The interface responsible for reflecting the beam must be long enough to capture the entire wavefront. Due to the small grazing angle, the beam will be spread out over a large distance, and a short electrode will let sorne part of the beam leak through. beam Figure 22 - Bearn width at reflecting interface From Figure 22, it is clear that the required interface length, d, is simply d = sinE>c 2OJ o ; ( 31) for a critical angle of 1.418° and a beam waist of 200 !lm, an electrode length of 16.2 mm is required. As previously mentioned, the slight grazing angle requires a huge, relatively speaking, interface length to fully capture the beam. The immediate effect of this is that our 70 mm usable wafer length can only support three electro-optic interfaces. 47 4.2.3 Bearn Propagation In order to augment the Gaussian bearn simulations, a bearn propagation simulation was created in Code V by Yiying Zuo. Bearn propagation was used to evaluate the switching separation of the device; the Gaussian beam simulation do es not provide this information and only a simple raytrace was used to calculate the locations of the switching states. 4.2.3.1 Code V Setup To model the Ix3 electro-optic switch, a system consisting of9 surfaces was implemented in Code V. Two materials were used for LiTa03: with no electric field, and with electric field. By selecting which surfaces have the appropriate material, we can essentially 'tum on and off the interfaces for out device. Figure 23 is the screen capture of the Lens data manager in Code V, showing the 9 surfaces defined for the off state, or port 1, of the switch. ,-SUlf..,,~t1OS 1.0000 -C""",a!Id_ Infinit? Infinit? ~\_Spl- Pt"'''"' Cotait..;! o.t!! """.t. c"..,., 0aI:0 '"'ll5lklgs u.t tho (ont..,\> 01 tl 'Î')~W, ~eaamTY&:è dbby.pc<tLtxt ta.OOQO io.oOco - $phere , "Ïnf iïiù; y' ~. :i3'pilé~ë ~Sp'here ':.. In1~'1ni't1 ;. . lntinit"V' ~~,' 10.0000 -, LT.I.O' ;$J)he:re ~p'.h::t;a Q<ô:kIDPIot Figure 23- Lens data manager for port 1 48 ~" r - -_________ ---------------'-- _:::::::=- ~- - 8.06 New lens from CVMACRO:cvnewlens.seq scale: 3.lU MM 29-Nov-03 Figure 24 - 2D plot of Code V system Figure 24 is the quick 2D plot of the system defined by the Lens manager, • showing the two interfaces, in this case, not tumed on. Switching states is simply a matter of changing the index to that of LiTa03 under an electric field, and setting the refract mode to Only TIR for the appropriate surfaces, as indicated in Figure 25. :. Lem I)OUO Mo....g..- Zoom5\'''''' Syst... o.t. s..f_Pt_ ~w...,.. 2_Sj!<_ ."'I«ot. Cot<oIog (lot• .. ""''''' Cot<oIog [)oI:.: 8li~ . wt the CorIW>t. of t :.;;""""'" Wlod<w.s ,·G~aeaMTl'..:e ,Ibby-"",",u..t QWd<20PIot Figure 25 - Lens data manager for port 3 49 4.2.3.2 Simulation ResuIts The beam intensity images from Code V at the image surface are shown in Figure 22 for both port 2 and port 3. Due to the scaling of Code V, the spot sizes are not relative . • (a) (b) Figure 26 - Bearn propagation results for port (a) two and (b) three However, we can see that the beam propagation predicts Gaussian spots for the output states, an important result for a successful switch. Table 6 summarizes the findings from the Code V simulation, including separation at output facet, the radius of the spot at the output, and the calculated insertion 10ss. The separation of port 2 is identical from that calculated by the raytrace simulation, which is an optimistic sign for our device. Port 3, however, is predicted to have a larger separation than the raytrace predicts - an error of 0.24 mm. This discrepancy is unaccounted for and inexplicable at this time. 50 Table 6 - Code V simulation results Port 2 Port 3 Switching separation (mm) 2.82 4.75 Bearn radius (um) 172 171 Insertion 10ss (dB) 0.26 0.26 4.3 References [67] Tremblay, E., Pulikkaseril, C., Shoukry, E., Baharnin, B., Zuo, Y., Mony, M., Langlois, P., Aimez, V., Plant, D.V., "A fast lx2 fiber-optic switch based on electro-optic bearn scanning", submitted to CLEO 2004. [68] Saleh, B.E.A., Teich, M.C., Fundamentals ofPhotonics, John Wiley & Sons, Inc., 1991. [69] Hecht, E., Optics 3rd edition, Addison-Wesley, 1998. 51 5 Fabrication At the writing ofthis thesis, electro-optic switches were only created after a successful integration of services and skills from different companies and university groups. As a result, making a batch of switches takes a dedicated amount oftime, organization and planning, mainly due to aIl the delays in shipping and scheduling. Fabrication of the switches requires five major steps: 1) Purchasing lithium tantalate wafers 2) Photolithography of poling electrodes 3) Poling 4) Photolithography of device electrodes 5) Dicing and polishing 6) Annealing From start to finish, this process can take between 3 and 6 months, based on the availability of materials and equipment. 5. 1 Lithium tanta/ate Lithium tantalate wafers were purchased from Foctek Photonics, Inc (www.foctek.com). a company located in China. We purchased three inch wafers, since the processing facility was better equipped to handle three inch wafers. As mentioned in Section 2.3.1, the electric field will be applied to the Z-axis. The simplest method to accomplish this is to procure wafers with the Z-axis perpendicular to the face of the wafer - then the device electrodes can be pattemed directly on these faces. 52 Wafers like this are termed z-cut wafers and were, fortunately, available from Foctek in 70 mm diameter and 500 !lm thick wafers. 5.1.1 Lithium tantalate preference Lithium tantalate was selected as the material of choice for the electro-optic switch. This decision was based on the following reasons: 1) Higher electro-optic coefficient - Compared to lithium niobate, a lower voltage would need to be applied to the device or, with the same voltage, a larger index change would be induced. 2) Increased durabHity - According to suppliers, lithium tantalate is easier to handle, an important factor considering our collective ignorance in handling these crystals. 5.1.2 Wafer characteristics The wafers from Foctek Photonics, Inc. were received and had the following characteristics, as quoted by the company, shown in Table 7. 53 Table 7 - Lithium tantalate waCer characteristics Variable Unit Value Extraordinary index ne - 2.180 Ordinary index 110 - 2.176 Electro-optic coefficient r33 pmN 30.4 One problem with the supply ofwafers is that Foctek neglected to mark the orientation of the z-axis. Since the wafers were z-cut, they have a positive and negative face, and important factor when processing the wafers. If the wafers are upside-down, poling will not occur, since the poling field is in the same direction as the internaI coercive field . • 5.1.3 Wafer orientation In order to determine the orientation of the lithium tantalate wafers, Foctek recommended that we etch a section of the wafer with hydrofluoric acid, or HF, watching to see which face etched faster. This is an extremely dangerous acid, and an invasive procedure, so we elected to use an easier, non-invasive method. Since lithium tantalate is a pyroelectric material, changes in temperature will cause an electric dipole moment [70]. The pyroelectric coefficient,p, as used in equation 16, is given as 19.0 nC/cm2 oK. Using the setup illustrated in Figure 27, we measure the voltage across a lithium tantalate wafer as the temperature is increased. The brass plate is used as an access point to the voltage on the backside of the wafer. 54 C wafer~ brass plate ---. '. . . _ _ _ _ _ _-' hot plate Figure 27 - Setup to determine wafer orientation Using this voltmeter polarity, when the wafer is heated, a negative voltage will be measured if the +z face of the wafer is facing up. This method is so simple and fast, that even if the supplier indicates the polarity of the wafers, they are double checked with this procedure. 5.2 Lithography In order to place electrodes on the wafers, we designed photolithography masks to perform metallift-off. Our process requires two masks: one for poling, and one for device activation. The poling mask is a pattern in the shape of the device we want to construct. This way, once the poling electrodes are in place, and we pole the wafer, we will have inverted domains in the shape of the electrode. The device electrode is simply a blanket electrode over the poled and unpoled regions of the device. When a voltage is applied, a 2~n index difference will be induced, as described in Section 3.2.1. This device electrode is usually made to be larger than the 55 expected useable area of the device, to avoid any fringing effects at the edges of the electrodes. Figure 28 shows one style of device, with its poling and device electrode. ---] 1 ~--- r~ -- ----- ...----.. . (a) l (b) Figure 28 - (a) Poling and (b) device electrodes • 5.2.1 Mask Design Our masks were 4-inch dark field masks made from soda lime glass with chrome patteming, fabricated by Adtek Photomask (www.adtekphotomask.com). The orientation ofthe mask is right reading chrome up, indicating that looking at the chrome side, the text will be legible from left to right. The poling electrode mask was labeled METAL 1 and the device electrode mask was labeled METAL 2, as indicated in Figure 29. 56 [J (a) (b) Figure 29 - Photolithography masks (a) METAL 1 and (b) METAL 2 This mask has a total of 6 TIR devices - three are double interface devices, but with both interfaces connected. This is meant to be a 1x2 switch, but using a second • interface to obtain a larger deflection. The remaining three are triple interface devices, to be used as a lx4 switch, but were only tested as ifthey were lx3 switches. METAL 1 has a minimum feature size of 2 !-lm, whereas MET AL 2 has a 100 !-lm minimum; this is due to the gratings that were placed on METAL 1. Having small features on the poling mask has one direct advantage: if the small features are resolved properly on the wafer, we can be certain that the rest of the larger devices should be defined weIl. 5.2.1.1 Design considerations Since we plan to use high voltage to pole the electro-optic devices, some precautions must be made to ensure that there will be no arcing. Arcing will be further 57 discussed in Section 5.3, but certain guidelines have been developed from past expenence. First, aIl electrodes must be separated from each other by at least 1000 /lm. This distance is considered the minimum distance that will not arc between devices on the wafer, given our poling voltage. Ideally, a separation of 3000 /lm is desired, but due to space constraints on the lithium tantalate wafers, we have compromised with 1 mm. Second, due to arcing at the edges of the wafer, devices on the edges are typically going to be difficult to pole, or will not pole at aIl. With tms in mind, devices that have a design that has been through more iterations are placed in the center of the wafer, in an attempt to produce consistent working devices. Last, devices should be oriented on the wafer so that dicing cuts can be made • without having to remount the wafer. That is, aIl the dicing cuts should go all the way through the wafer, and should be straight lines. This design rule is mainly to keep the dicing and polismng costs low, as well as minimizing the time needed to dice and polish. 5.2.1.2 Mask alignment In order to guarantee that the device electrodes are placed exactly on top of the poling electrodes, we require sorne feature on the mask that can be used for alignment. The alignment marks shown in Figure 30 were placed in several spaces on the masks for this purpose. 58 CJ CJ CJ CJ CJ CJ CJ CJ CJ CJ CJ CJ (a) ç 9 9 b:d b:d b:d (b) (c) Figure 30 - Alignment marks (a) from METAL 1, (b) from METAL 2 and (c) assembled The minimum dimension of the alignment marks is 100 J.!m; consequently, we aim to be able to align the second mask to a tolerance of roughly 10 J.!m. 5.2.2 Electrode patterning An of the devices used in this thesis were fabricated at the microfabrication facility at the Université de Sherbrooke, in Sherbrooke, Québec, by Pierre Langlois, under the supervision of Vincent Aimez. The photolithography-liftoffprocess was developed by Pierre Langlois to the point where we could resolve 2 J.!m features on our lithium tantalate wafers. His procedure for photolithography is summarized in the following three sections. 5.2.2.1 Cleaning Wafer cleaning is an imperative step before any photolithography takes place, as a dirty wafer will have problems with photoresist adhesion. A standard procedure is used to clean the lithium tantalate wafers. 59 First, the wafer is immersed with OptiClear, a commercial optical cleaner that removes the organic compounds, such as grease, from the wafer. Next, the wafer is rinsed with acetone, and then isopropanol, which is then dried with nitrogen gas. After cleaning, the wafer is immediately prepared for lithography, before any contaminants have a chance to settle on the wafer surface. 5.2.2.2 Photolithography As indicated in Section 5.2, a metallift-offprocedure is used to define metal electrodes on the wafer surface. The wafer is first baked in the oyen and a special resist called Lift-offResist (LOR) 1S deposited. LOR is a special resist used to improve the definition of the pattern; in effect, it is a layer that is easily removed by the lift-off • deve1oper, but is fairly resistant to the other procedures . After the LOR is applied, the wafer is hard baked in the oyen before adhesion promoter is applied, followed by the photoresist. The adhesion promoter 1S a chemical that makes the photoresist adhere better to the LOR layer. The wafer is soft baked again and then exposed using the contact mask aligner, and a UV lamp. A commercial deve10per is used to remove the unwanted resist, and the remaining pattern on the wafer is ready for metal evaporation after a de-ionized water rinse, nitrogen drying and plasma de-scum. 5.2.2.3 Metal evaporation and liftoff The metal evaporation is done in a thermal evaporator, and deposits a 20 nm layer of chrome, followed by an 80 nm layer of gold. Gold is preferred because of its excellent conductivity and resistance to oxidation, but a chrome layer is required for adhesion. 60 After evaporation, the unwanted metal is removed by soaking the wafer in a commercial LOR remover. After a rinse with acetone, isopropanol and nitrogen drying, the wafer is inspected to evaluate the photolithographie definition. 5.3 Poling Poling is, theoretically, a fairly simple process when using an external electric field. In reality, however, it is quite an art, where intuition becomes useful. The high voltages required to pole a 0.5 mm thick lithium tantalate wafer could easily exceed the breakdown voltage of air and cause areing in sorne areas. Arcing is avoided at an costs, since the electricity will bum and vaporize parts of the electrode, and damage the crystal. Wafer that have experienced severe arcing tend to shatter at a further point in the fabrication process, especially during polishing. Arcing may also have an unusual effect, where the arc actually poles the device - but this usually results in serious overpoling. It has also been suggested that perhaps the crystal wafers that are likely to arc contain defects that would destroy them during polishing. However, the arcing that has been observed in the lab is, in our opinion, forceful enough to shatter the wafer. Arcing between devices occurred in the early developmental stages of the project, and has been remedied by leaving at least a 3 mm gap between aH poling electrodes. The device electrodes can be plaeed close to each other, sinee the operational voltage is nearly lOtîmes less than the poling voltage. The most common problem is the arcing between the top electrodes and the wafer mounting puck. Any devices close to the edge of the wafer are potentially liable to arc once the voltage is ramped up. This problem can be minimized by designing the mask 61 such that there is a gap between the edge and the closest device; aIl our masks include a gap of 3 mm, to reduce this problem as much as possible. Space on the wafer is at a premium, though, and sometimes this gap is squeezed, to maximize the number of devices on a wafer, or to align the die boundaries. As a result, arcing may render the outer devices useless. 5.3.1 Experimental Setup The basic setup used to pole lithium tantalate is shown in Figure 31. The probe used to bring the +1000 V pulse to the wafer is a BECU probe from IDI Industries. This probe has a spring-tip, allowing the probe to contact the wafer and exerting minimal stress. The mounting puck is a solid brass wafer holder, polished to maintain electrical • contact with the wafer. A small amount of conductive grease is aiso applied to improve this contact. An electrical connection is made in the bottom of the mounting puck to connect to the high voltage supplies, detai!ed in Section 5.3.1.1. 62 Pulse generator Function generator Oscilloscope Probe Lithium tantalate wafer High voltage supplies Mounting puck Figure 31 - Poling setup • 5.3.1.1 High Voltage supplies The fast, high voltage supplies used for poling and for device testing were bought from Glassman High Voltage, Inc. The power supplies have a fast rise time, steady operation and current limiting protection, and can be reversed in polarity to provide either positive or negative voltages. Two high voltage supplies were used, a 1.5 kV and aiS kV supply, for the pulse and the DC supply, respectively. These supplies are connected as in Figure 32, with the 15 kV supply used in reverse, and the 1.5 kV supply used as normal. With a pulse voltage of 1000 V and a DC voltage of 10 000 V, a total voltage of Il 000 V is created across the wafer. 63 The pulse generator is a device that takes a high voltage and will output a pulse at that voltage when the generator receives a 5 V pulse. A function generator is used to provide the gate voltage. function generator pulse generator r- probe + 1.5 kV supply l..- I gnd 1 c:: :::::::> -15 kV supply mounting puck • Figure 32 - Electrical connection to poling setup 5.3.2 Poling procedure A standard procedure for poling lithium tantalate wafers was created to apply to any 0.5 mm thick wafer, regardless of device design. Using this procedure, devices are weil poled, with straight domain walls and little- to no-overpoling. 1. Conductive grease is applied to the backside of wafer and spread evenly in a thin coat. 2. The wafer is placed on the mounting puck. 3. Coffee filter paper is placed on the wafer, with a smail hole over the electrode to be poled. 64 4. Baby oil is applied to the filter paper, to minimize arcing. 5. The probe is brought into contact with the wafer. 6. The pulse voltage is set at 1000 V. 7. The DC voltage is raised to 3000 and the device is pulsed. 8. The DC voltage is raised in 2000 V Increments, and pulsed, unti19000 V is reached. 9. The DC voltage is raised in 100 V increments, and pulsed, until a response is observed on the oscilloscope. 10. The device is pulsed repeatedly until there is no further response on the oscilloscope. Il. An extra two pulses are sent, for any residual poling. • 5.3.3 Determining quality of poled devices Once a device has been poled, it is desired to estimate the quality of the poling; badly poled devices can then have no further processing wasted on them. Invasive methods cannot be used, obviously, therefore, acid etching [71] is not a possibility. We have two methods at our disposaI, charge transfer and visualization. Charge transfer is calculated for every device, as the CUITent curves for each pulse are saved regardless, but visualization is rarely used, unless the wafer shows sorne strange behavior whenpoled. 5.3.3.1 Charge transfer Using extemal electric field poling, as detailed in Section 3.3.2, domain inversion can be monitored by measuring the CUITent through the device at the moment of reverse 65 polarization. The total charge transferred by poling can be calculated by integrating the cUITent, as indicated by equation 32. q = fldt ( 32) The CUITent traces from the poling process are captured in LabVIEW, and then filtered in MatLAB with a low-pass filter, to remove excess noise [72]. The resulting data is plotted in Figure 33 . • Figure 33 - Poling current traces: raw and filtered data The measured area of the inverted domain is taken from AutoCAD, from the original mask design; the are as of the 1st and 2nd interfaces are 0.4495 cm2 and 0.6773 cm2 respectively. 66 Table 8 - Charge transfer Calculated (flC) Measured (flC) % 1st interface 44.95 7.27 16.2 2nd interface 67.73 11.11 16.4 Table 8 shows a summary of our analysis of the charge transfer during poling. Unfortunately, the measured charge is substantially lower than the expected charge, which would suggest that the devices are only partially poled. However, experience has shown that, regardless of the measured charge, the domains are usually of excellent quality when using the pulse-poling procedure [73]. At this moment in time, it appears that this quantification of poling method is not • applicable to our project, and further investigation will have to be made into the project. 5.3.3.2 Visualization Once the devices on the wafer have been poled, the domains can be visually inspected to determine the quality of the devices. For the TIR switches, the reflecting interface can be evaluated, to observe any domain spreading that may have occurred. Poled domains become visible when observed under a microscope with crosspolarizers. The polarizers are set to block the light passing through the wafer, making the unpoled regions dark. Since the poled regions have a slightly different index, due to temperature, the light through the regions pass through the polarizers. The immediate effect is that the domains appear illuminated, and their shape becomes distinct. 67 Unfortunately, this procedure is invasive to the device; in order to see the light passing through the domains, the metal electrodes must be removed first. As a result, visualization is not used often, especially since the poling procedure has matured to the point where excellent quality devices are fairly easy to fahricate . • Figure 34 - Poled domains under cross-polarizers As shown in Figure 34, the inverted domains are quite sharp, with good definition; this particular screen capture is from a prism-hased switch, as none of the TIR devices were examined with this method. Defects in the device are immediately visible as weH, for example, the mispoled prisms in Figure 34. ldeally, the examination ofthe poled regions can he used to simulate a more realistic model of the device, reducing the gap between simulated and experimental results. 68 5.4 Dicingg Polishing and AR coating In order to test our devices, we need to separate the individual devices from the wafer, and once this is done, we need to have an optical polish on the end facets to minimize scattering losses at the interface. The optical polish can be augmented by the application of an anti-rejlection coating or AR coating. AU dicing, polishing and AR coating services are available from Nova Phase Inc., 5.4.1 Dicing The original mask design includes boundaries on every device that defines an area called a die. These die are separated from the wafer by a pro cess called dicing, where the wafer is mounted on a chuck, and a diamond-edged blade cuts through specified lines on • the wafer. The dicing lines are totally independent of the boundary lines on the wafer; an Autocad file is usually sent to the dicing company with dicing lines drawn in, as shown in Figure 35. 69 --~::f"___._._..'''.._ ...._.._._ •.+ ___.._...____ . ~rm~ _ _. __._m"'.__·'_.__ ._.._....__......__.__.._.._. Chip 1 __....•....... __.._... __.._.._. __..._"'...._._._...__. ~. - ~ ;-.~ ·1f§g1~1§!j+-'--J Chip2 \i .........................-.........-.... ··t-·····,--_ .. ·__····•········__······· !. ~~~~~~~~~ ~ Ir Il • / Figure 35 - Dicing cut lin es As previously mentioned in Section 5.2.1.1, the dicing lines on our wafer are • typically made to run straight through the wafer. As Figure 35 shows, we made only 5 cuts through the wafer, which will deliver two chips, labeled Chip 1 and Chip 2. Chip 1 has two identical copies of a one-bit double-interface, TIR switch. Unfortunately, the curvature of the lithium tantalate wafer makes the polishing ofthis chip a little more difficult and more susceptible to chipping or cracking. Chip 2 has three copies of a three interface TIR switch. 5.4.2 Polishing The edges of a chip are typically chipped and rough after dicing, and this causes several problems with the operation ofthe electro-optic device. First, the insertion 10ss will be extremely high, since a rough surface will scatter the beam. Second, the unevenness of the facet can displace the beam, giving it an angle that was not accounted for in simulation. 70 The solution to these problems is to polish the input and output facets of the device. Polishing is done by grinding the edges of the device with sandpaper. The coarseness of the sandpaper is progressively reduced until an optical grade polish is produced. This pro cess is the much more time- and expensive than dicing, but is critical to producing low-loss devices. In order to minimize the time required to polish devices, the dicing Hnes are placed as close to the polishing lines as possible. Figure 36 shows the dicing and polishing lines on our mask, in an area where there is a large separation. In tms unfortunate case, the device we want is close to the curve of the wafer, and far from the dicing lines, making this chip much more work-intensive. • 'IIi dicing line· polismng line ~ Figure 36 - Polishing !ines 71 5.4.3 AR coating Any material that has an abrupt index change win have sorne reflection associated with this interface. In our case, light entering our lithium tantalate wafer will be partially reflected by the boundary. Similarly, light exiting the device will see the same reflectivity. Recht defines the reflectance of a material, R, to be [70] ( 33) where Eor and EOi are the reflected and incident waves at the interface, and nt and n r are the materials that are on the transmission and reflection side, respectively, as indicated in Figure 37 • Figure 37 - Anti-reflection coating Substituting the refractive index for air and lithium tantalate (2.1807), the value of Ris 13.78%. Converting this to a power 10ss, the reflection 10ss at one interface is calculated to be 0.64 dB. 72 In other terms, the total reflection loss for a device made in lithium tantalate is 1.28 dB. However, it is possible to minimize tms 10ss, by depositing an anti-rejlection coating (AR coating) on the facets of the device. This coating is usually a double-layer, quarter wavelength stack, and is designed by [74] ( 34) ( 35) where nj and n2 are the indices of the double layer stack, dis the thickness ofboth layers, and ns and no are the substrate and the initial material indices. This thin film can be applied to the end facets ofthe device after polishing, and is • designed for a small range ofwavelengths around a specified design wavelength . Novaphase offers this service, but we chose not to have the AR coating applied, since these devices were aU preliminary designs, and AR coating is an expensive treatment. 5.5 Annealing Annealing was a process done after an fabrication steps on the device had been completed. It was never part of the original process recipe, and was only done as an enhancement step. The need for annealing became obvious when the output quality of the beam was shown to be severely degraded. 73 Figure 38 - Camera scans of port 1 (a) before and (b) after annealing Figure 39 - Port 3 (a) before and (b) aCter anneaIing Annealing has been mentioned in the literature as a method to homogenize the refractive index profile, which may be irregular due to strain in the crystal or electric 74 charges at the dornain boundaries [75]. As shown in Figure 38 and Figure 39, the annealing process irnproved the bearn quality through the device; Figure 38 is the scan of the bearn with no electric field. Even in this inactive state, the reversed dornains have an effect on the refractive index profile. Table 9 shows, quantitatively, the irnprovernent in the insertion 10ss of the device. • Table 9 - Improvement in insertion loss sure to annealing Port 1 Port 2 Port 3 IL before annealing (dB) 4.4 7.9 8.5 IL after annealing (dB) 1.5 4.8 5.3 93% 105% 110% % change in pwr 75 5.6 References [70] Hossain, A., Rashid, M.H., IEEE Trans. Industry Appl., vol. 27, no. 5, 1991. [71] Shur, V.Y., Rumyantsev, E.L, Nikolaev, E.V., Shishkin, E.I., Batchko, R.G., Fejer, M.M., Byer, R.L., Ferroelectrics, vol. 257, pp 191-202,2001. [72] Proakis, J.G., Manolakis, D.G., Digital Signal Processing 3rd edition, Prentice Hall,1996. [73] Tremblay, E.J., "The design, simulation, fabrication and testing of electro-optic beam scanners in domain inverted LiTa03", Master of Engineering Thesis, 2003. [74] Hecht, E., Optics 3rd edition, Addison-Wesley, 1998. [75] Yamada, M., Saitoh, M., J. Appl. Phys., vol. 84., no. 4, 1998 . • 76 6 Deviee Testing Three wafers were processed with the mask shown in Section 5.2.1, to have a total of9 devices to test the feasibility ofusing TIR interfaces for a multi-port switch. Unfortunately, one wafer broke during poling, and another wafer shattered during cleaning. The remaining wafer was successfully diced and polished, and then aIl three devices were screened to determine if any were operational. Only the middle device, as indicated in Figure 40, provided sufficient beam deflection to be useful as a switch. After all the fabrication steps, the top coyer electrodes experienced sorne degradation, and began to spark when the test voltage was applied. A layer of silver paint • was used to reinforce the electrode to prevent this. While the paint does not affect the De testing of the switch, further studies into the speed of the device may suffer from this paint layer. Top Mlcldle Botton C~I L==J c===J 1 1 L==J 1 l 1[: J Figure 40 - Deviee chip 77 6.1 Experimental Setup The electro-optic device (EOD) was tested using a camera to image the different output states. The setup of our test bed is sketched in Figure 41. The laser source is a fiber-coupled 1310 nm semiconductor laser, fixed at an output power of lmW. 13 10 nm laser source attenuator 15 cm polarization controller lem ~ DIo EOD collimator • InGaAs camera vacuum holder Figure 41 - Test setup The polarization controller has three paddles, through which, are wound fiber, allowing stress to be placed on the fiber by moving the paddles. Using this polarization controner, we can usually obtain an extinction ratio of -40 dB, but this degrades quickly over time. Originally, the collimator was selected as a 10 cm working distance, 500 /lm beam diameter collimator. These collimators caused severe leakage through the device, which is attributed to the inadequate length of the electrodes; a replacement collimator was used, one with 2 cm working distance and a 100 /lm beam diameter. The distance the beam travels from one collimator to the image plane is roughly 23 cm, a huge distance 78 for this collimator, but inspection in the camera shows that the beam is still reasonably collimated, fitting in the 1.3 mm beam aperture. The collimator is aligned to the device with a 6-axis alignment stage, providing full control over X, Y, Z axes, as weIl as yaw, pitch, and rolL Using a camera allows us to align the device properly, observe the effects of polarization, and monitor beam quality; this is possible due to the linear InGaAs camera used in our setup. This camera is very sensitive to light, and the input power must be attenuated in order to avoid destructive saturation of the CCD array. An attenuation of 45 dB was required to observe the output states without any saturation. The sensitivity ofthe camera also necessitates the use of a baffle, a metal barrel that screws into the camera, protecting the InGaAs array from damage and from ambient • light. This baffle prevents the camera from being placed doser to the device, forcing the 15 cm gap. 6.2 Switching behavior 6.2.1 Performance The TIR switch was operated as a 1x3 switch, using s-polarized light as an input. Table 10 and Table Il summarize the performance of the switch, using insertion 10ss, crosstalk and beam separation as parameters. The insertion 10ss of the switch was calculated by measuring the power visible in the InGaAs camera through a 1.3 mm aperture. This 10ss is calculated in reference to the laser in air, which the camera perceives as a 2.98 m W beam. The reported insertion 10ss 79 is not comparable to the insertion loss measured in an output fiber, as these numbers only indicate how rnuch power is in a particular position, and ignores beam shape and quality. The separation of a switch state is measured at the camera plane with respect to Port 1. Since the InGaAs array is 15 cm away from the edge of the device, the deflection appears huge. Table 10 - Switch insertion Joss and separation Insertion Loss (dB) Separation (mm) Port 1 1.5 0.00 Port 2 4.8 14.64 Port 3 5.3 18.16 Crosstalk is measured similarly to insertion loss; once the insertion 10ss has been measured, the switch state is changed and the power remaining in the original port is collected. Table Il - Crosstalk between ports Port 1 Port 2 Port 3 Port 1 X -11.7 -11.9 Port 2 -23.6 X -16.9 Port 3 -24.0 -22.2 X Crosstalk (dB} The performance of the device is inadequate to make a packaged switch, but the results do verify the concept ofusing a TIR switch. The high insertion loss in Port 2 and 3 represents a 10ss of light due to leakage at the interfaces - this manifests as poor 80 crosstalk in Port 1. For this device to be useful in a switch fabric, this 'lost' light must be directed to the appropriate port. Further improvements to the design will be discussed in Section 7. 6.2.2 Camera Scans The InGaAs camera allows critical assessment of the switching behavior of the BOD. Figure 42 displays the camera images from all three switch states, sequentially. This figure provides an accurate sense ofhow well the switch is working: the crosstalk of the device is very evident, and the power in each port is easily determined . • Figure 42 - Camera scans of aIl switching ports The performance results reported in the previous section are relative measures of how much power is in a specifie port. In a real device, however, this power has to be 81 coupled back into a fiber by using a collimator matched to the input. If the spot shape and size of the switch state do not couple well into a collimator, then more loss will be introduced. Figure 43 is a magnified look at Port 2 and 3, showing that both states deviate from an ideal Gaussian . • Figure 43 - Detailed examination of (a) Port 2 and (b) Port 3 Port 2 appears to be a good quality spot, but upon closer examination, there is a hint of smearing on the one side. This effect is even more pronounced in Port 3, where the beam now has an elliptical shape. A beam with this shape will encounter high 10ss when coupled into an output collimator. The coupling 10ss between two Gaussian beams with different beam waists is given by [76] ( 36) where W r and Wt are the waists of the two beams. Figure 44 contrasts the difference between the anticipated collimator mode and the spot as seen by the camera. 82 The beam waist of the spot at Port 3 is estimated to be 1040 !lm, 390 !lm larger than the collimator beam waist of 650 !lm. This beam waist is measured in the x-direction, as this is the axis that experiences severe degradation. Using equation 36, the mismatch loss of this coupling is estimated to be 0.81 dB, which in addition to the power 10ss reported in the previous section. However, equation 36 does not take into account the beam ellipicity, and assumes that both beams are still Gaussians. _. Collimalor mode • Figure 44 - Mismatch Joss of Port 3 6.2.3 Voltage dependant behavior It is quite important to analyze the voltage dependence of the device, since it allows us to gauge the 10west possible operational voltage. Reducing the operational voltage suggests potentially higher repetition rates as a switch. 83 Figure 45 displays the voltage dependence of insertion 10ss as seen by Port 2; that is, when the first electrode is activated. The shape of the curve is to be expected, and is similar to the shape predicted by simulation in Section 4.2.2.2, where the transmitted power 1S high at higher voltages, but drops sharply around the design voltage of 1000 V. Contrary to simulation, however, the insertion 10ss of this switch state increases as the voltage increases past the design voltage. Visually, this is observed by the beam shifting further in the camera, moving it out of the designated aperture. -- ~ -f --1 1 , l 1 -----T -.;-- -- -- -:- .. -- .... -:-- ...... -- f ........ --1---- .... -:- .. -- .... -:- --- .... l 1 l , 1 , 1 , , 1 1 1 1 1 1 l l l 1 1 , , , 1 1 1 1 l 1 1 -r---- 1 l , 1 1 1 , 1 1 1 -----,-------~------~------T------,------,-------~-----r---1 1 1 1 1 1 1 1 1 1 l , 1 l , 1 l , ............... .lI ...... ____ 1_ ...... ___ 1 1 1 1 1 1 1 1 1 1 '"' . _ .. _ ... _ ....... _ _ _ _ 1 1 1 1 ~_ l 1 1 1 1 1 1 1 1 , 1 1 1 1 1 .. _ _ _ _ ..1 ____ .... _1.,. _ _ .. _ _ _ "' _ _ .. _ _ 1 1 1 , 1 l , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l i t 1 l 1 1 , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ...... +............ ~ .......... .. -:- .......... -:- ............ f .......... -'"' .......... -:- ............ :- ............ i- ...... .. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 .......... T ............ , ............ -,- .......... -,... - - - - - T _ . _ . - - . , - - - - - - , - - - - - - -,.. - - - - - - 1 r - - -- 1 1 • 1 1 1 1 1 1 1 1 • 1 • 1 • 1 1 1 1 1 1 1 1 1 1 1 1 l , 1 1 1 1 1 1 Figure 45 - Port 2 insertion Joss - voltage dependence The movement of the output beam is caused, hypothetically, by the refraction of the beam leaving the device. As the beam crosses the edge of the device, from high index, to air, the exit angle is given by 8 2 = arcsin(n) sin8)) ( 37) where nI is the index of the lithium tantalate crystal. As the index increases, the exit angle of the beam will increase; this may be a negligible difference, but at the camera 84 image place, located 15 cm from the edge of the device, we observed a noticeable shift. The exit angle was not measured directly due to experimentallimitations and time constraints. Figure 46 displays a similar plot, voltage dependence of switch state power, for Port 3. It is at this port that the effect mentioned above, is highly exaggerated, and the design voltage is the only feasible voltage to operate the device. Increasing the voltage, in this case, causes a shift, as before, but also a marked decrease in power. -:- --- -- -:-- .... ~ .... f .... - -_ .. -! .. 1 1 ,,1 ,1 ,,, ,, ,,, , ,,, ,,, ,,, 1 1 ,l ,, , , - ........ --,- .......... -,.. ........ --T"" -_ .. - , .... -_ .. - . , - - - .... - l , l l 1 1 l 1 , 1 , , 1 1 , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 • 1 1 1 1 1 1 1 1 l , 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 l 1 1 , 1 1 1 1 1 1 1 1 1 1 , , 1 1 --- .. -t-- .... -~ -- ........ -:-- ........ -~ .... --- .. t--- -- -~ ---- .... ~- .. --- .. -:--- .. 1 1 1 1 1 1 1 l , 1 1 1 1 1 1 1 1 1 1 l l 1 .......... , ............ -,- .......... -f" ............ T ............ , ............ "l- .......... 1 -r.......... .. ...... .. Figure 46 - Port 3 insertion loss - voltage dependence The most obvious solution is extracted from our observations of the device; since the first switching port loses power at higher voltages, due to a beam shift, we hypothesize that this shift at the first interface is bringing the beam at an incident angle greater than the required critical angle, as detailed in 4.2.2.2. Two curves are shown in Figure 47 to understand the crosstalk in port 1; the solid hne is the actual crosstalk measured at an aperture at the location ofport 1, and the 85 dashed line is a measure of the power that leaks through. The leakage power is measured by moving the aperture to the apparent peak of the erosstalk power. While very little light aetually enters port l, there is a significant amount of leakage that does not meet the TIR eriterion and passes through the deviee. However, this light is still subjeet to the refractive index difference as it passes through the poled regions that are aetivated. As a result, the leakage experienees a small shi ft, enough to move it out of the physieal location ofport 2. This is easily displayed in Figure 48, where the displacement of the leaked light is evident. • actual xtalk - - leakage power Figure 47 - Port 1 crosstalk - voltage dependence 86 Figure 48 - Scan of (a) Port 2, and the crosstalk in Port 2 when (a) the first interface is activated or (b) both interfaces are activated While the actual crosstalk numbers are quite stable, the leakage results show that there is still quite a bit of light that is not totally intemally reflected, and thus, is contributing to 10ss in the other ports. The next iteration of device should be designed to improve this problem, as the insertion 10ss of the device is a critical parameter. 6.2.4 Polarization Using TIR as a method to switch a free-space beam was used since there was an idea that the switch would be less polarization dependant than the prism-based switches. As long as both s- and p-polarizations are incident at a critical angle, they will be reflected to the identical positions. However, at our initial design conception, we faüed to recognize the amount ofloss that would occur from deviations from the design angle. 87 Since both s- and p-polarizations require different critical angles, it now appears unlikely that this device could perfonn in any polarizat~on state. The polarization behavior of our switch was investigated, but ultimately, met with failure. Our device was extremely polarization sensitive, and displayed poor perfonnance with p-polarization, as shown by the results in Table 12. The separation between states is taken at the camera plane, identical to the method used to measure the s-polarization. Table 12 - Performance of p-polarization • Insertion Loss (dB) Separation (mm) Port 1 1.5 0.00 Port 2 11.3 12.68 Port 3 12.4 16.32 From these results alone, it would be hard to suggest that any switching is occurring, given that the insertion 10ss is comparable to the crosstalk of the device. Nevertheless, as Figure 49 shows, there is a faint amount oflight into the ports 2 and 3. 88 Figure 49 - Camera scans of three switching states for p-polarization The light that is successfully switched must be, unfortunately, due to the • imperfect extinction ratio of the polarization controllers. The p-polarized light experiences no reflection at aU from the electro-optic interface - this is certainly due to the fact that the electro-optic coefficient to change no, the ordinary index, is much smaller than the coefficient to change De, the extraordinary index. This could be remedied by increasing the voltage, but the power supply used to operate the device only supplies 1500 V, which is not sufficient to induce a large change in the ordinary index. 6.3 References [76] Yuan, S., Riza, N.A., Appl. Optics, vol. 38, no. 15, 1999. 89 7 Conclusion This thesis has investigated the novel concept ofusing multiple electro-optic TIR interfaces to create a multi-port switch in bulk lithium tantalate. The concept is a first step towards the eventual goal of building a packaged switch prototype, similar to one that could potentially be used in an agile all-optical network. The TIR switch was designed and simulated using a combination of in-house and commercial simulation tools; fabrication was completed using the facilities at l'Université de Sherbrooke and using a host of services from commercial suppliers. The testing and characterization of the switch was completed in the laboratory of the Photonic Systems Group in McGill University. • The concept of the lx3 switch was demonstrated to be feasible, with insertion losses of 1.5,4.8 and 5.3 dB in the respective ports. The worst case crosstalk of the switch was found to be -11.7 dB, with a best case of -24.0 dB. One strong characteristic ofthis design is that the separation of the ports from each other is large, the smallest separation being 4 mm at an image plane 15 cm from the end of the device, facilitatmg the packaging of a prototype switch, without the need for any additional separating optics. The main benefit of the device is the inherently fast switching speed that the electro-optic effect contributes; the speed ofthe device was not tested, but from other proj ects using the same materials and processes, we anticipate similar rise and faU times, around 37 ns. Unfortunately, severallimiting factors prevent this switch design from being packaged as a functional switch. A benefit of the TIR concept is that every deflection should be polarization independent, but our switch is extremely sensitive to polarization, 90 a development caused by the low voltage we designed the device to run at. The polarization intolerance is responsible for sorne ofthe detracting characteristics of the switch, namely high 10ss in switch ports 2 and 3, and the high crosstalk in port 1. In an effort to conserve space, the TIR interfaces were designed to be exactly the required length for full re:flection to occur. With no extra electrode length, it is difficult to align the device to completely re:flect the beam; as temporary solution, we changed collimators to use shorter working distance, smaller beam waist collimators. One forecasted advantage ofusing the TIR scheme was that scalability seemed easy to implement. TIR interfaces could be serially placed to provide higher switch port counts; however, the sizeable electrode length restricted the number of interfaces that could fit on one wafer. Predictably, every sequential interface becomes increasingly more • difficult to align, and with the distorted beam effect seen in Port 3, the beam quality becomes markedly more degraded. Modifications to the original design can be made to improve the device performance, and sorne new ideas are being explored to realize a high-performance multi-port switch. These ideas will be discussed in the following section. 7. 1 Future deve/opments Several improvements have been suggested for the next iteration of the TIR switch project. One immediate improvement is to design the device to run at a voltage where both s- and p-polarized light will experience a re:flection at the interfaces. Using a thinner wafer or using a material with a higher EO coefficient may solve this issue. Once polarization independence is achieved, the performance of the device is expecting to improve dramatically. 91 The next design change is to increase the length of the interfaces, so that the entire beam is reflected. In order to accomplish this, the last, unused electrode on the original mask will be removed, and the chip will have only two interfaces. It is hoped that this will improve the beam quality in the third output port. As an experimental effort, it has been suggested to change the TIR interface from a flat interface, to an interface with sorne curve to it. This idea was inspired by a 1999 paper from Hatami-Hanza et. al., where an intersecting waveguide switch was fabricated in LiNb0 3, but using a curved electrode to increase power reflectivity and extinction ratios [77]. Similarly, a curved TIR interface may help to reduce crosstalk and improve reflectivity, enhancing the insertion losses in the upper switch states. As previously mentioned, the number of TIR interfaces available for a device is limited by the size of the wafer. Rather than use larger wafers to increase port count, we propose to reduce the beam waist at each interface by using electro-optic lenses in the wafer to focus the beam. This idea was based on papers by Chiu et. al. and Yamada et. al., detailing the process of fabricating electro-optic cylindricallenses in lithium niobate [78], [79]. Our proposed ide a is shown in Figure 50, with two interfaces. 92 electro-optic lenses TIR electro-optic interfaces Figure 50 - Using electro-optic Benses in the TIR switch Implementing these new concepts into the TIR switch design will require detailed simulations, since a simple raytrace cannot be used to guide design, as it was used in this • thesis. However, the next phases of the TIR switch project are forecasted to pro duce high performance packaged switches. 7.2 References [77] Hatami-Hanza, H., Nayyer, J., Safavi-Naini, S., J. Light. Tech., vol. 12, no. 8, 1994. [78] Chiu, Y., Gopalan, V., Kawa, M.J., Schlesinger, T.E., Standl, D.D., Risk, W.P., J. Light. Tech., vol. 17, no. 3,1999. [79] Yamada, M., Saitoh, M., Ooki, H., App!. Phys. Lett., vol. 64, no. 24, 1996. 93 8 Appendix 8.1 Paper submitted ta CLEO 2004 Electro-optic beam deflection using multiple totalinternai reflection interfaces C. Pulikkaseril, E. Shoukry, B. Bahamin, P. Langlois*, V. Aimez* and D.V. Plant Department ofElectrical &Computer Engineering, McGill University, Montréal, Québec, Canada H3A-2A7 *Department of Electrical & Information Engineering, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2Rl Contact: :David Plant tel:(514)398-2989 emai/:[email protected] Abstrad: We report a novel design of an electro-optic beam deflector using two total-internai reflection interfaces creating a three-port switch. ©2004 Optical Society of America OSICS codes: (060.1810) Couplers, switches and multiplexers; (060.2340) Fiber optie components; (230.2090) Eleetrooptical devices. Electro-optic devices have seen considerable interest in recent years, particularly for applications in optical switching. Materials such as lithium niobate (LiNb03) and lithium tantalate (LiTa03) are typically used because of their relatively high electro-optic coefficient and availability; however, the index change associated with these crystals is still srnaU in bulk devices, even at high voltages. Despite this, totalintemal-reflection (TIR) switches are attractive because of the simple fabrication, theoretical polarization independence, and possible short switching times [1]. • We have investigated prism based electro-optic switches [2,3], but the deflection ofthese switches is quite smaU, around 1.22 degrees. TIR switches offer resolvable spots with large angular deflection, roughly 4 degrees, reducing sorne difficulties in building a packaged switch. We have designed and fabricated a novel design for a three-port switch, using multiple TIR interfaces to provide deflection for each state. Figure 1 shows the basic concept behind the extended TIR design as a lx3 switch. Figure 1. Switch concept We have fabricated these devices by lithographically defining electrodes on a 500 /lm, z-cut LiTa03 wafer. Vsing an external electric field, the polarization ofthe dornains is reversed, and an electrode is deposited over the entire region. Vnder an electric field, the unpoled and poled dornains will experience an index change, but opposite in sign, giving us twice the index change at the interface of the unpoled and poled dornains. The device was then diced and polished to an optical grade by Nova Phase, Inc and then annealed for an hour at 150 oc. We tested the devices by connecting a linearly-polarized 1310 nm laser source to polarization controllers, to a longworking distance fiber collimator, which was then aligned to the first electro-optic interface. A copper probe was placed in contact with the device electrode and 1050 V was applied. At the output, we used a linear InGaAs camera to gauge alignment and to provide scans of the beam shape and quality at each of the three output ports. 94 Figure 2. Camera scans of three output ports from TIR switch (S-polarization) Figure 2 shows the three switch states as seen through the camera at an output plane located 15 cm from the end facet of the device. Table 1 gives results indicating the angular deflection of the beam, and the insertion 10ss captured through a 1.3 mm aperture at each port. With this design, our concept ofusing TIR interfaces for a three-port switch is achieved; the higher than predicted crosstalk is attributed to the electrode length being too small to fully reflect the collimated beam, causing leakage through the interfaces. Tablel. Switch Deflection and Power measurements (S-polarization) • PiJ'J't. Dc4k%!t!ûl'i ;tllOt) (deg) ('n,,,,,wl1.: in J~,Jri -biSelti;l,n 1.(M>;i (dB) -ni' '1 :t'5 L5 4.8 3 .~.$2 5.3 .. n.Ü() V.(\f!>ll:~!1C t,tiB) -16.9 -22.1 References [l]Boyland, A.J., Mailis, S., Hendricks, J.M., Smith. P.O.R., Eason, R.W., "Electro-optically controlled beam switching via total internaI reflection at a domain-engineered interface in LiNb03". Optics Comm. 197, pp 193-200 (2001). [2]Li, J., Cheng, c., Kawas, M.J., Lambeth, D.N., Schlesinger, T.E., Stancil D.D., "Electrooptic wafer beam deflector in LiTa03." IEEE Photonics TecMol. Lett. 8, 1486 (1996). [3] Tremblay, E., Pulikkaseril, C., Shoukry, E., Bahamin, B., Zuo, Y., Mony, M., Langlois, P., Aimez, V., Plant, D.V., "A fast lx2 fiber-optic switch based on e1ectro-optic beam scanning", submitted to CLEO 2004. 95