UBC Faculty Research and Publications

Y-branch optical modulator. Jaeger, Nicolas A. F.; Lai, Winnie C. 1991

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Y-Branch optical modulatorNicolas A.F. Jaeger and Winnie C. LaiUniversity of British Columbia, Department of Electrical EngineeringVancouver, British Columbia, Canada V6T 1Z4ABSTRACTTo characterize z-cut Ti:LiNbO3 Y-branch optical modulators, numerical simulations wereperformed showing that high on/off ratios are attainable without special Y-junction asymmetries orlong electrodes. The effective index and 2-D finite difference beam propagation methods were usedfor the simulations. A modulator with a 2° Y-branch was fabricated. The measured on/off ratioswere 5:1 at 25V,12: 1 at 50V,and 60: 1 at 75V,corresponding to 4: 1 ,12:1 ,and62: 1 for thesimulations for =632.8nm. 1. iNTRODUCTIONIncreasing research is being done in the area of electro-optic modulators in part due to theirpotential use in time division multiplexers' as well as in digital switches2 for high bit rate opticalcommunications. Y-branch optical modulators are more suitable for such use than Mach-Zehndertype modulators because they are easier to fabricate and to control. Y-branch optical modulators arenon-interferometric by nature and are tolerant of variations in parameters such as applied voltage,branch angle, and wavelength.The Y-branch optical modulator operates by the electro-optic effect. In its neutral state, withoutany voltage application, each branch arm guides an equal amount of light. Application of voltageto its electrodes channels light into one of the arms of the Y-branch while at the same timechannelling light out of the other arm. The arm to which the light is channelled or not channelleddepends on the polarity of the applied voltage.We examine, both numerically and experimentally, a Y-branch optical modulator fabricated inz-cut lithium niobate (LiMbO3), where the waveguides are formed by titanium (Ti) indiffusion.Previous devices with high on/off ratios have had very small branch angles3 (i.e., less than 0.2°)and/or asymmetric branch arms4. Small branch angles give rise to long devices and long electrodesresulting in wasted real estate on the LiNbO3 substrate and increased capacitance. Furthermore,asymetric branches are more difficult to fabricate. Here, we show that devices with high on/offratios can be obtained without long horn lengths or any special Y-junction asymmetries.202/ SPIE Vol. 1583 Integrated Optical Circuits (1991) O-8194-0714-3/91/$4.OODownloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms2. NUMERICALSIMULATIONSAlthough there are 3-D numerical methods which can simulate a propagating optical field withina waveguide, a 2-D beam propagation method (BPM) is chosen due to its increased speed andcomparable accuracy5. The effective index method6 (ElM) is first applied to reduce the refractiveindex profile from 3-D to 2-D. The refractive index n(x,z) for metal indiffused strip waveguides(here, we use z instead of y to indicate the z-axis of the LiNbO3) is defined as follows6:2 22 2 Z2x7z (x,z)= D W (la)where = exp(-z2/D2) (ib)andg(4) =ii{eit[(i+4)}+e,t[(i_.)]} (ic)where D is the diffusion depth, W is the width of the Ti strip prior to diffusion, nb is the bulkrefractive index for z-cut LiNbO3 at ) =632.8nm, and n is the maximum refractive index at thesurface due to the Ti indiffusion. The device configuration and fabrication parameters of the Y-branch optical modulator are given in Figure 1 and Table I respectively.After using the ElM to reduce the refractive index profile, giving the effective index ne(x) fromn(x,z), the finite difference beam propagation method7 (FDBPM) is used to calculate the optical fielddistribution. The FDBPM is based on the Fresnel approximation to the scalar Helmholtz equation.The evolution of the electric field for a single polarization of a monochromatic optical wave in ourcase is7: -n)dy'= Deb e 'ohbY'(xy) +O(Ly)3 (2a)where 1÷D- 8kOnb aXX -iiya2 (2b)8kOnb aX2SPIEVol. 1583 Integrated Optical Circuits (1991) / 203Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsI IdiffusedLINbO3 Ti diffusionat 1050°Cfor6hrFigure 10100-I-)a4-4-0050Dimensions and fabrication parameters of the Y-branchoptical modulator.Figure 2On/off ratios and percent guided power vs. applied voltagefor the 2° V-branch optical modulator for =632.8nm.204 I SPIE Vol. 1583 Integrated Optical Cfrcuits (1991)228Un (2 horn lengths)I-- - —11 4UflAA ,n5-2.2041 A'1 50125 60on/off ratio— — % guided power75 // a)00///25 -aa):31020 30 40 50 60applied voltage (V)Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/termsThe FDBPM is chosen over the BPM because it remains accurate for larger longitudinal step lengths8and therefore it is considerably faster.To simulate the effect of voltage application to the electrodes of the Y-branch optical modulator,conformal mapping9 is used to calculate the electric field components in the x and z directions in theelectro-optic substrate, E, and E. These fields are used, in turn, to determine the 3-D modulatedrefractive index profile, which is subsequently used in computing the modulated effective index foruse with the 2-D FDBPM.The optical field that is input to the modulator is the fundamental TM-like mode with unit power,here called the eigenfunction U0(x). This function can be determined by using the method describedin Reference 10. The power of the fundamental mode guided by the modulator at any distance y canbe calculated using the orthogonality principle fromP(y) = fE(x,y) u:(x) t2 (3)In our simulations, we use a 100 jm wide window with a transverse grid of 1000 points. Anabsorber is placed 15 jm from either side of the window edge to dampen the field preventing highfrequency numerical instabilities11. The Y-branch spreads out in 0. 1 m steps and the power outputis computed when the two arms are 40 m apart so that the optical fields in the two arms no longerinteract with each other.Table I Fabrication parameters for a Y-branchoptical modulatorTi thickness 500 Abulkrefractive index nb 2.2maximum refractive index n 2.204diffusion depth D 3 mdiffusion temperature 1050 °Cdiffusion time 6 hrswaveguide pattern width W 4 jmelectrode gap at input 4 jmelectrode gap at output 8 mbranch angle 0 20electrodelength 228 /LmSPIEVol. 1583 Integrated Optical Circuits (1991) / 205Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms3. SIMULATION RESULTSThegoal of this numerical simulation is to study a Y-branch modulator combining short electrodelengths with high on/off ratios at intermediate voltages, i.e. about 50V.We have found thatgenerally, on/off ratios at any given applied voltage increase with increasing branch angle'2.However, the amount of guided power also decreases with increasing branch angle. A 2° branchangle is chosen for this particular study because it exhibits high on/off ratios while retaining about50%ofthe input power as well as having short electrode lengths.We have simulated the modulator using various electrode lengths, and found that a two-horn-length electrode provides better performance than a one-horn-length or three-horn-length electrode.A one-horn-length electrode is too short to cause substantial modulation, while a three-horn-lengthelectrode shows no notable improvement in the on/off ratio but obviously increases the electrodes'capacitance.The simulations show that both the on/off ratio and the percent guided power increase withapplied voltage. For example, the on/off ratio is predicted to be 2.4: 1 at 20 V with 37% guidedpower and 85:1at 80 V with 51 % guided power, as shown in Figure 2. The on/off ratio's increasewith applied voltage is to be expected because the greater the electric field the greater the change inthe refractive index and the more light directed into one of the arms. The relationship betweenapplied voltage and guided power shows that guiding efficiency increases with voltage, this is becauselight which is originally radiated at low voltages is increasingly captured and channelled into one ofthe arms as the voltage is increased. Figures 3 and 4 show the optical field in the Y-branch with 0V and 50 V applied respectively. 4. FABRICATIONA Y-branch optical modulator, having the parameters shown in Figure 1 and Table I, wasfabricated on z-cut y-propagating LiNbO3. The Y-branch pattern was formed in the 500 AthickTiusing photo-lithography and plasma-etching. The Ti was diffused into the crystal at 1050°C for 6hours in flowing wet oxygen. The wet oxygen was used to prevent surface waveguiding due to Li02out-diffusion'3. The estimated maximum refractive index change due to the Ti-indiffusion is 0.004at A0 =632.8nm.Before putting on the electrodes, a thin Si02 optical buffer layer was sputtered onto the sample.Then a 4000A thick layer of aluminum was deposited and patterned into the electrodes using photo-lithography and chemical wet-etching. The samples were subsequently cut to the approriate sizes,and the input and output ends of the waveguides were polished.206/ SPIE Vol. 1583 Integrated Optical Circuits (1991)Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms(do,) I Figure 3 Optical field distribution of the Y-branch optical modulator with 0 V applied. Figure 4 Optical field distribution of the Y-branch optical modulator with 50 V applied. I I S S At S C S.— 4 'I 'I S At -S e C '3 S 0 (Jo,) 0 ?e So Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms5. MEASURED RESULTSPolarized light from a Helium-Neon laser (A0 =632.8nm) was coupled into a single modepolarization-maintaining monomode fibre through a microscope objective. Next the light from thefibre was endfire coupled into the TM-like mode of the input waveguide of the Y-branch opticalmodulator. The output from the modulator was projected through a microscope objective and apolarizer onto a pinhole in front of the detector which was connected to a storage oscilloscope. Theoutput power from the two branches of the modulator was equalized prior to voltage application.Using a waveform generator a slowly-varying triangular wave was applied to the electrodes of theoptical modulator. The measured on/off ratios were 5: 1 at 25 V, 12: 1 at 50 V, and 60: 1 at 75 V.Here we have accounted for the effects of the radiation modes since, in our case, the detector pickedup both the guided TM-like mode as well as a portion of the radiation modes. Using our simulationresults, the amount of radiation was calculated and used to adjust the measured data accordingly.The percentage guided power can be obtained by maximizing the power at the output of themodulator and then maximizing the power at the output of a 4 j.m straight waveguide, fabricated onthe same substrate adjacent to the modulator, and comparing the two. Since the straight waveguideis adjacent to the Y-branch modulator, we assume that both waveguides have nearly identical guidingcharacteristics. The amount of guided power measured using the above method was 36% with 0 Vand 43% with 75 V applied, again we accounted for the effects of the radiation modes. Thesemeasured on/off ratios and percent guided powers compare reasonably well with the theoreticalvalues, see Table II.Table II Performance comparison of the Y-branchoptical modulatortheoreticalexperimentalon/off ratio: 25 V50V75V 41262 51260% guided power:0 V75V 35%50% 36%43%6. CONCLUSIONA Y-branch optical modulator was modelled and its behavior simulated numerically using the 2-DFDBPM with the ElM. A device was then fabricated on z-cut LiNbO3, having a 2° branch angleand an electrode length of 228 m. On/off ratios of 5: 1 at 25 V, 12: 1 at 50 V, and 60: 1 at 75Vwere measured for light at a wavelength of 632.8 nm. The measured results are in good agreementwith the theoretical performance predictions.208/ SPIE Vol. 1583 Integrated Optical Circuits (1991)Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms7. ACKNOWLEGEMENTThis work was supported by an NSERC operating grant and an NSERC post-graduate scholarship.8. REFERENCES1.A.Djupsjöbacka, "Time Division Multiplexing Using Optical Switches, "IEEEJ.Select. AreasCommun.,vol.6, No. 7, pp. 1227- 123 1 ,August1988.2.R. L. Jungerman et al. ,"High-SpeedOptical Modulator for Application in Instrumentation,"IEEE J. Light. Tech. ,vol.8, No. 9, pp. 1363-1370, September 1990.3. P. Granestrand et al. ,"IntegratedOptics 4x4 Switch Matrix with Digital Optical Switches,"Electron. Lett. ,vol.26, No. 1 ,pp.4-5,January1990.4. Y. Silberberg, P. Perlmutter, and J. E. Baran, "Digital Optical Switch, "Appl.Phys. Lett.,vol.51, No. 16, pp.1230-1232, October 1987.5.Working Group I, COST 216, "Comparison of Different Modelling Techniques forLongitudinally Invariant Integrated Optical Waveguides," lEE Proc. ,vol.136, No. 5, pp. 273-280,October 1989.6. G. B. Hocker and W. K. Bums, "Mode Dispersion in Diffused Channel Waveguides by theEffective Index Method, "Appl.Opt. ,vol.16, No. 1 ,pp.1 13-118 January 1977.7. D. Yevick and B. Hermansson, "Split-Step Finite Difference Analysis of Rib Waveguides,"Electron. Lett. ,vol.25, No. 7, pp. 461-462, March 1989.8. D. Yevick and B. Hermansson, "Efficient Beam Propagation Techniques, "IEEEJ. QuantumElectron. ,vol.26, No. 1 ,pp.109- 1 12, January1990.9.N. A. F. Jaeger and L. Young, "Voltage-Induced Optical Waveguide Modulator in LithiumNiobate, "IEEEJ. Quantum Electron. ,vol.25 ,No.4, pp. 720-728, April 1989.10. D. Yevick and P. Danielsen, "Numerical Investigation of Mode Coupling in SinusoidallyModulated GRIN Planar Waveguides, "Appl.Opt. ,vol.21 ,No.15 ,pp.2727-2733, August 1982.1 1 .J.Saijonmaa and D. Yevick, "Beam-propagation Analaysis of Loss in Bent OpticalWaveguides and Fibers, ".1.Opt. Soc. Am. ,vol.73, No. 12, pp. 1785-1791 ,December1983.12.W. C. Lai, "The Effects of Branch Angle on a Y-branch OpticalModulator," The FirstGraduate Student Conference on Opto-Electronics Materials, Devices, and Systems, pp.30,McMasterUniversity, Hamilton, Ontario, June 24-26, 1991.13. J. L. Jackel, "Suppression of Outdiffusion inTitanium Diffused LiNbO3: A Review," J.Opt.Commun., vol. 3, pp. 82-85, 1982.SPIEVol. 1583 Integrated Optical Circuits (1991)1 209Downloaded from SPIE Digital Library on 07 Jun 2011 to Terms of Use:  http://spiedl.org/terms


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