Download A multi interface total .. internal reflection based electro .. optic

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
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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.
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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
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..
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V.(\f!>ll:~!1C
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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