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High-speed optical commutator switch for digital optical pulses. Jaeger, Nicolas A. F.; Chen, Mingche; Lai, Winnie C.; Dower, Roger G. 1993

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A high speedopticalcommutator switch for digital optical pulsesNicolas A. F. Jaeger, Mingche Chen, Winnie C. Lai,and Roger G. DowerUniversity of British Columbia, Department of Electrical EngineeringVancouver, British Columbia, Canada V6T 1Z4ABSTRACTA new optical commutator switch, capable of very high speed pulse generation and pulsemultiplexing/demultiplexing, is proposed. It consists of an integrated optical Y-branch modulator anda "cul-de-sac" microstrip resonator. A possible compound digital optical modulator, using these opticalcommutators, is described. The results of measurements made on a cul-de-sac resonator, fabricatedon an alumina substrate, and on optical Y-branch modulators, fabricated on z-cut lithium niobatesubstrates, are presented. For the resonator the unloaded quality factor was measured. For the opticalY-branch modulators the on/off ratios and percent guided powers were measured as functions of theapplied voltage for several branch angles. The results indicate that optical commutators, of the typeproposed, could be made. 1. INTRODUCTIONA novel optical commutator switch that is based on the integrated optical Y-branch modulator1and the "cul-de-sac" resonator is proposed; by an optical commutator we mean an optical switch thatalternately directs light from one input waveguide into one of two output waveguides. Our switch usesan optical Y-branch, fabricated in z-cut lithium niobate by the controlled in-diffusion of titanium, inconjunction with a microstrip resonator. The resonator has the shape of a "cul-de-sac" traffic sign,i.e. ,ithas two substantially parallel legs which are unconnected at one end and connected via an openring at the other end. The resonator is designed to be a half-wave (or an odd multiple of a half-wave)resonator. It is oriented so that its legs induce electric fields in the optical waveguides of the Y-branchand, via the electrooptic effect, direct light from the input waveguide into each of the output brancheson alternate half-cycles of the centre frequency. Figure 1 is an illustration of an optical commutator;here we are assuming that the microwave is fed into the resonator by a matching quarter-wavetransformer.One application of these commutator switches is the generation, from a continuous wave input,of high speed optical pulses in the output branches. A second application is the directing of previouslygenerated pulses into the two output waveguides on alternate half-cycles of the resonant frequency,e.g. ,aspart of a synchronized demultiplexer; naturally, the reverse operation of combining previouslygenerated pulses may be accomplished, e.g. ,aspart of a synchronized multiplexer. Also, compounddigital optical modulators are possible.Figure 2 illustrates a compound digital optical modulator that uses optical commutators. Herenarrow optical pulses are first generated and then distributed to a series of slower acting optical "killer"switches using two tiers of optical commutators, one operating at a centre frequency f and twooperating at f/2. The killer switches selectively destroy the pulses. The surviving pulses are then770ISPIE Vol. 1801 High-Speed Photography and Photonics (1992) 0819409995/93/$4.00Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. Terms of Use:  http://spiedl.org/termsmultiplexed onto a single output waveguide using two more tiers of commutators in the reverse mode.The final pulse train has a repetition rate of 2f.Naturally, strict phase relationships must bemaintained among all of the commutators. While the modulator illustrated in Figure 2 has only twotiers of commutators on both the input and the output, modulators having larger numbers of tiersshould be possible.In this paper we provide some of the results of our work on both the resonators, here fabricatedon alumina substrates, and the optical Y-branch modulators, fabricated on z-cut lithium niobatesubstrates. These results indicate that optical commutators, of the type proposed, fabricated on z-cutlithium niobate substrates, could be made.2. THE RESONATORThe resonator is intended to act as a voltage transformer, i.e. ,developinga relatively largevoltage between the ends of its legs while dissipating relatively little power. For our application, thevoltage that develops between the ends of the legs should be 50Vfor about 100 mW dissipated powerat 15 GHz on a z-cut lithium niobate substrate.While, as part of the optical commutator, the resonator would be fabricated on a z-cut lithiumniobate substrate, in order to verify our concept, we fabricated resonators designed to operate at 7 GHzon alumina substrates. The model, as well as various other considerations, used in the design of theresonators is discussed in Reference 2. Also given in Reference 2 is the relationship between theoutput voltage V0 and the source voltage V, (a lossless line between the source and the resonator isassumed); V02 {4B/(1 +13)2}X {8QuZr/(nZs)}XV2, where i3 is the coupling coefficient between thesource and the resonator, Zr is the characteristic impedance of the microstrip constituting the ringportion of the resonator, Z is the characteristic impedance of the source, and Qisthe unloadedquality factor of the resonator.The resonators were fabricated on 0. 89 mm thick alumina substrates.The microstripconstituting the resonator was made of gold, electroplated to a thickness of 6 m. The ring portionof the resonator had a mean radius of 0. 86 mm and a width of 0. 14 mm, Zr was 95ftThe legs were1 .3 mm long, 0. 14 mm wide, and were separated by a 12 tm gap. The predicted value for Qwas180 for a centre frequency f of 7 GHz. The assumed values for Z and 13were50ci and1,respectively. Using these values we predicted that 50Vwould develop between the legs for an inputpower of 24 mW.The measured normalized reflected power, for one such resonator, is shown in Figure 3 .Sincewe used a reflection technique on a scalar network analyzer to take the measurements, the value of Qwasnot given by simply dividing the centre frequency by the 3dBbandwidth but had to be adjustedto account for the different reflection coefficients at the various measurement points. The measuredvalue for Qwas123 and forwas 7. 12 GHz. Using these values we calculated that, to develop50Vbetween the legs of this resonator at 7. 12 GHz, an input power of 35mWwould be needed.Using the same model, as that used above to design the resonators fabricated on the aluminasubstrates, we designed resonators for z-cut lithium niobate substrates that would operate at 15 GHz.We assumed that Z would be 50c2, that1wouldbe 1, and that the microstrip constituting theSPIEVol. 1801 High-Speed Photography and Photonics (1992) / 771Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. Terms of Use:  http://spiedl.org/termsresonator would be made of 6 jm thick gold. In this case the legs would be 0.35mmlong, 0.03 mmwide, and the gap between them would be 4 m. The ring would have a mean radius of 0.79 mm anda width of 0.02 mm. Our predicted Qwas71 and Zr was 62 c2;giving59Vbetween the ends of thelegs for 100 mW input power. If we assume a 30% decrease in the Qofan actual resonator, as inthe case above, this resonator should develop 49 V for 100 mW input power.3. THE OPTICAL MODULATORA study of the optical Y-branch modulator was carried out to determine its usefulness in anoptical commutator. Y-branches were both simulated and fabricated. Our study was used to determinethe on/off ratios and percent guided powers, for various applied voltages and branch angles, fordevices fabricated on z-cut lithium niobate.In the simulations, the fundamental TM mode (= 0.6328j.m), having unit power, isassumed to be input to the Y-branch. The branch angles studied were 1 ,Øø,1.5°,and2.0° and theelectrodes were twice the length of the horn of the Y-branch, i.e. ,0.46mm, 0.30 mm, and 0.23 mmlong, respectively; the horn is that portion of the Y-branch in which the two output waveguides beginto diverge from the input waveguide but are not yet separate. The electrodes run parallel from thenarrow end of the horn, extending from the edges of the input waveguide, to the point at which theoutput waveguides begin to separate, they then run down the centres of the output waveguides for onemore horn length. Simulations were performed for one, two, and three horn length electrodes fordevices with both 1 .5°and2.0° branch angles, the two horn length electrodes gave the best results.In the simulations the effective index method3 (ElM) and the finite difference beam propagationmethod4 (FDBPM) were used. First the ElM was used to reduce the refractive index distribution ofthe Y-branch from three dimensions to two dimensions by calculating an effective index for each pointon the surface of the substrate, i.e. ,removingthe depth dimension, leaving only a planar structure.The three dimensional refractive index profile, for the Y-branch, was calculated as described inReference 3 assuming that titanium strips 4 m wide (increasing to 8 m in the horn) and 500 Athickwere in-diffused for 6 hr at 1050° C. The effect of applying a voltage to the electrodes was calculatedusing conformal mapping5. These voltages may be large enough to induce electric fields in the Y-branch that reduce the effective index, in specific locations, to values less than the refractive index ofthe undiffused substrate. To calculate effective indices for such locations we used the methoddescribed in Reference 6.Optical Y-branch modulators were fabricated in lithium niobate. The fabrication parameterswere those used in the simulations. The Y-branch pattern was formed by thermally evaporating auniform film of titanium onto a z-cut lithium niobate substrate and then by patterning the titaniumusing photolithography and plasma-etching. The diffusion was done in wet oxygen to suppress lithiumoxide out-diffusion7. A silicon dioxide layer was sputtered onto the surface to act as an optical bufferbetween the titanium in-diffused waveguides and the electrodes, preventing losses due to the opticalwaves interacting with the electrodes8. Then a 4000 Athicklayer of aluminum was thermallydeposited and patterned into ordinary electrodes (not resonators) using photolithography and chemicalwet-etching. The sample containing the modulators was then cut and the ends were polished to allowlight to be coupled into the devices.772ISPIE Vol. 1801 High-Speed Photography and Photonics (1992)Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. Terms of Use:  http://spiedl.org/termsPolarized light, at ) =0.6328jim, was launched into the TM-like mode of the devices froma polarization-preserving, single-mode, optical fibre. The outputs of the devices were focused, usinga microscope objective, onto a pinhole in front of a detector. A slowly varying, 1 Hz, triangle wavewas applied to the electrodes and the output was measured. In order to reduce the photorefractiveeffect, the power in the waveguides was kept to a few micro-watts. The on/off ratios from both thesimulations and the measurements are shown in Figure 4. The percent guided powers for both thesimulations and the measurements are shown in Figure 5. As can be seen, the agreement between themeasurements and the simulations is quite good, showing close agreement in the predicted (theoretical)and measured (experimental) values as well as in the trends.4. CONCLUSIONSWe have fabricated, on alumina substrates, resonators capable of developing relatively largevoltages between the ends of their legs while dissipating small amounts of power. We predict that suchresonators, fabricated on z-cut lithium niobate substrates, would be capable of developing 50 V forabout 100 mW dissipated power at 15 GHz. The legs of such a resonator could be used as theelectrodes of an integrated optical Y-branch modulator to form an optical commutator. We havesimulated such Y-branch modulators, as well as having fabricated some on z-cut lithium niobatesubstrates. We have measured on/off ratios of 10, 10, and 1 1 dB and percent guided powers of 69%,64%, and 39% for 50 V applied to the electrodes of modulators having 1.0°, 1.5°, and 2.00branchangles, respectively. By combining the two structures it should be possible to fabricate opticalcommutators of the type proposed.5 .ACKNOWLEDGEMENTThe authors gratefully acknowledge the financial support of the Natural Sciences andEngineering Research Council of Canada.6. REFERENCES1. N.A.F. Jaeger and W.C. Lai, "Y-Branch Optical Modulator," SPJE Vol. 1583 Integrated OpticalCircuits, pp. 202-209, 1991.2. N.A.F. Jaeger and M. Chen, "Cul-de-sac' Microstrip Resonators for High Speed OpticalCommutator Switches," SPIE Vol. 1 794 Integrated Optical Circuits II, to appear, 1992.3. G.B. Hocker and W.K. Burns, "Mode Dispersion in Diffused Channel Waveguides by the EffectiveIndex Method, "Appl.Opt. ,Vol.16, pp. 1 13-1 18, 1977.4. D. Yevick and B. Hermansson, "Split-Step Finite Difference Analysis of Rib Waveguides,"Electron. Let,'. ,Vol.25, pp. 461-462, 1989.5. N.A.F. Jaeger and L. Young, "Voltage-Induced Optical Waveguide Modulator in LithiumNiobate, "IEEEJ. Quantum. Electron.,Vol.25, pp. 720-728, 1989.6. N.A.F. Jaeger and W.C. Lai, "Calculation of Effective Index in Non-Guiding Regions," Appl.Opt., accepted for publication July 1992.7. J.L Jackel, "Suppression of Outdiffusion in Titanium Diffused LiNbO3: A Review," J. Opt.Commun., Vol. 3, pp. 82-85, 1982.8. M. Masuda and J. Koyama, "Effects of a Buffer Layer on TM Modes in a Metal-Clad OpticalWaveguide Using Ti-Diffused LiNbO3 C-Plate," Appl. Opt., Vol. 16, pp. 2994-3000, 1977.SPIEVol. 1801 High-Speed Photography and Photonics (1992) / 773Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. Terms of Use:  http://spiedl.org/termsnicrowave inputcwI nput+resonator electrodeFigure1. An integrated optical commutator.output lightJLftILoutputf774 ISPIEVol.1801 High-Speed Photography and Photonics (1992)input ughY-branch waveguidek L [er swI tchesdQtQ tokHerswitchesFigure2. A compound digital optical modulator.Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. Terms of Use:  http://spiedl.org/terms6)0011)06)6)-oa)M0E0C90!ao.—0 706)-o60C6)0a)ciSP!E Vol. 1801 High-Speed Photography and Photonics (1992)! 7753dB BW=55.4 MHz0.0—2.0—4.0—6.0—8.0—10.0—12.0—14.05.5 6.0 6.5 7.0 7.5 5.0frequency in GHzFigure3. The normalized reflected powervs. frequency for a cul-de-sac resonator.f0=7.12 0Hz '8580-70-o 60--4-J0L 50-30-20-10-Legend:1 .0° — theoretical1 .0° — experimental— — 1 .5 — theoretical0 1 .5 — experimental. -7— — 2.0 — theoretical0 2.0 — experimental 1/I 01/1/////Legend:1 .0 — theoretical1.0 — experimental— — 1.5W _ theoretical0 1.5 — experimental— — - 2.0 — theoretical -.--0- 05040-300 10 20 3'O 40 5b 6b 70 80applied voltage (V)Figure4. The on/off ratios vs. appliedvoltage for the optical Y-branch modulators. 0 10 20 3b 40 50 60 70 80applied voltage (V)Figure5.Thepercent guided powers vs.applied voltage for the optical Y-branchmodulators.Downloaded from SPIE Digital Library on 07 Jun 2011 to 137.82.117.28. 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