An all-optically induced spatial shift in the interference between the two lowest-order modes of a lithium niobate channel waveguide was demonstrated based on the power-dependent nonlinear phase shift induced via quadratic cascading

Assanto, Gaetano

Baek, Yongsoon

Schiek, Roland

Sohler, W.

Stegeman, George

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University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1998 All-optical mode mixer spatial switch based on cascading in lithium niobate Yongsoon Baek University of Central Florida Roland Schiek University of Central Florida George Stegeman University of Central Florida Gaetano Assanto W. Sohler Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Baek, Yongsoon; Schiek, Roland; Stegeman, George; Assanto, Gaetano; and Sohler, W., "All-optical mode mixer spatial switch based on cascading in lithium niobate" (1998). Faculty Bibliography 1990s. 2171. https://stars.library.ucf.edu/facultybib1990/2171 Appl. Phys. Lett. 72, 3405 (1998); https://doi.org/10.1063/1.121647 72, 3405© 1998 American Institute of Physics.All-optical mode mixer spatial switch basedon cascading in lithium niobateCite as: Appl. Phys. Lett. 72, 3405 (1998); https://doi.org/10.1063/1.121647Submitted: 19 March 1998 . Accepted: 27 April 1998 . Published Online: 23 June 1998Yongsoon Baek, Roland Schiek, George Stegeman, Gaetano Assanto, and W. SohlerARTICLES YOU MAY BE INTERESTED INOptical waveguides in lithium niobate: Recent developments and applicationsApplied Physics Reviews 2, 040603 (2015); https://doi.org/10.1063/1.4931601All-optical mode mixer spatial switch based on cascadingin lithium niobateYongsoon Baek,a) Roland Schiek, and George StegemanCREOL, University of Central Florida, Orlando, Florida 32816-2700Gaetano AssantoDepartment of Electronic Engineering, Terza University of Rome, 00146 Roma, ItalyW. SohlerAngewandte Physik, Universitaet-GH Paderborn, D-33098 Paderborn, Germany~Received 19 March 1998; accepted for publication 27 April 1998!An all-optically induced spatial shift in the interference between the two lowest-order modes of alithium niobate channel waveguide was demonstrated based on the power-dependent nonlinearphase shift induced via quadratic cascading. ©1998 American Institute of Physics.@S0003-6951~98!00926-7#It was pointed out that in the early days of nonlinearoptics by Ostrovskii that a nonlinear phase shift can be in-duced on a fundamental beam when it participates in a near-phase-matched second-harmonic-generation~SHG! process.1It took almost 30 years before this effect, now called cascad-ing, was proposed for all-optical, guided wave, switchingdevices.2–4 One of the drawbacks was that the phase shiftrequired strong coupling to, and significant generation of, asecond-harmonic wave which acts like a loss for the funda-mental beam, reducing the fundamental beam throughput.However, a variation in the SHG phase-matching conditionwith propagation distance turned out to be almost ideal forminimizing the loss to the harmonic commensurate with ob-taining a large phase shift.5 This led to the successful experi-mental demonstration of both a nonlinear Mach–Zehnder in-terferometer and a nonlinear directional coupler inbirefringently phase-matched LiNbO3 channel waveguides.6A nonlinear mode mixer consists of a single-channelwaveguide fabricated to support the two lowest-order guidedmodes, for example the TM00(v) and TM01(v) modes. Itsoperational principles are sketched in Fig. 1. The key is thatthe TM01(v) mode has a two-lobed field distribution withthe lobesp out of phase with one another. When both theTM00(v) and TM01(v) are launched, and with the TM00(v)wave in phase with one of the lobes of the TM01(v) mode,the total field at the input is localized to one side of thechannel because of interference between the two modes. Fur-thermore, because the two modes propagate with differentwave vectors, b00 and b01, then the field maximum‘‘switches’’ periodically back and forth across the wave-guide with distance with periodLc52p/@b002b01#, seeFig. 1. If, in addition, there is a power-dependent(P-dependent! relative phase shift between the modes,fNL(L,P), then the total relative phase between the modesis L @b002b01#1fNL(L,P). All-optical switching is ob-tained whenfNL(L,Ps)2fNL(L,O)5p so that the fieldmaximum moves from one side of the waveguide to theother when the power is increased toPs . This type of devicehas been demonstrated before in semiconductors using third-order nonlinearities.7 Here, we demonstrate this operation ina LiNbO3 channel waveguide using quadratic cascading togenerate the additional relative phase shift.8The 15mm wide, titanium in-diffused, LiNbO3 channelwaveguide used can support the TM00(v) and TM01(v)modes at the fundamental wavelength~l51.32 mm!. Sinceb00Þb01, the wave-vector mismatch for SHG is differentfor each mode at a specific phase-matching temperature. Fig-ure 2 shows the calculated effective indices as a function oftemperature for several different fundamental and SH modesover the temperature range of interest here. Over a 15 Ktemperature range there are many possible phase-matchingresonances involving the TM00(v) and TM01(v) modeswhose strength depends on the respective overlap integrals.At a fixed temperature the wave-vector mismatch is verydifferent for the various SHG processes and differentamounts of nonlinear phase shift can be obtained in the fun-damental modes for each interaction. This is the origin of thedifferential power-dependent phase shift between the modes.To launch each mode separately, the experimental setupshown in Fig. 3 was employed. AQ-switched, mode-lockedNd:YAG laser with a pulse slicer produced 90 ps pulses at a500 Hz repetition rate. The waveguides were placed in anoven, which produced a spatial dispersion in the wave-vector-matching condition along the waveguide. This led toa!Current address: Dept. of Physics, Washington State University, Pullman,WA 99164-2814.FIG. 1. Principle of a nonlinear mode mixer. When the input powerP isincreased, the field maximum switches from one side of the waveguide tothe other due to the additional power-inducedp phase shift between themodes.APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 26 29 JUNE 199834050003-6951/98/72(26)/3405/3/$15.00 © 1998 American Institute of Physicslarge nonlinear phase shifts at fundamental depletions of lessthan 10%.5 The TM00(v) mode was excited via arm 1. Byadjusting mirror M1 in arm 2, the higher-order mode wasexcited by launching an incident beam with a small angle tothe waveguide axis. The launching of each mode was veri-fied by imaging the output from the waveguide into a cameraand using the half-wave plate before the polarizing beamsplitter, the power fraction between the two arms was ad-justed carefully to make the output power equal for eachmode.Different SHG tuning curves with launching TM00(v)and TM01(v) separately as well as together were measuredto verify the location of the resonance temperatures in thevicinity of the dominant 2TM00(v)→TE00(2v) interaction.Figure 4 shows the SHG tuning curve when both TM00 andTM01 modes were launched. There are two major contribu-tions, the dominant one from 2TM00(v)→TE00(2v) at336.7 °C and a secondary one at 337.8 °C corresponding tothe TM00(v)1TM01(v)→TE01(2v) resonance. The reso-nance from 2TM01(v)→TE02(2v) occurs at 338.8 °C.Other resonances involving the TM01(v) mode only lie ateven higher or much lower temperatures. Note that due toinaccuracies in the Sellmeier equations used,9 the calculatedphase-matching temperatures differ by;18 K with respectto the experimental values~ ee Fig. 2! 336.5 °C. Based onthis tuning curve, the experimental temperature was set at336.5 °C. At this temperature the nonlinear phase shift forany TM01(v) mode interaction is expected to be small com-pared to that of the TM00 mode, because the resonances forthe TM01(v) are relatively far away and the SHG efficiencyis also quite small compared to that for the TM00(v) mode.This feature allows an optimized relative nonlinear phasedifference between the modes.First the relative input phase between the modes wasadjusted so that the field maximum at the output could belocated on either side of the waveguide. With proper adjust-ment of the wedge plate in arm 2~see Fig. 3!, the output canexit either the right or left side of the channel as shown inFig. 5. Different linear interference conditions occur betweenthe modes when the relative phase of the two launchedmodes changes byp. Figure 6 compares the normalized out-put intensity pattern at high~340 W in each mode! and lowinput powers keeping the input phase of the two modes un-changed. As expected, the location of the output was modi-fied due to the different amounts of nonlinear phase shiftaccumulated for each mode through cascading. This is inexcellent agreement with previous measurements in Ref. 6and calculations in which the nonlinear phase change in theTM01(v) mode was completely ignored, such that a switch-ing power of around 300 W~corresponding to an inducedpphase shift! was estimated.The output data in Fig. 6 clearly show that the high-power output beam profile is broader than that of the linearoutput, and asymmetric. This is a direct consequence of thepulsed excitation used in this experiment. Only the peakpower portion of the pulse is switched completely over, withthe switching efficiency decreasing with decreasing poweralong the pulse envelope. This results in a long tail to theFIG. 2. Calculated effective mode indices for the fundamental and secondharmonic as a function of temperature in a 15mm wide waveguide.FIG. 3. Experimental setup for all-optical switching by coherent mode mix-ing.FIG. 4. SHG tuning curve for launching both the TM00 and TM01 funda-mental modes.FIG. 5. Output intensity pattern for the low-power case and a phase differ-ence of the modes at the output of 0 andp, respectively.3406 Appl. Phys. Lett., Vol. 72, No. 26, 29 June 1998 Baek et al.output intensity pattern, stretching toward the side of thechannel which corresponds to the low-power intensity maxi-mum.In summary, we have demonstrated spatial switching bythe nonlinear mixing of the two lowest-order guided modesin a channel waveguide. The prevailing nonlinearity was cas-cading, i.e., the nonlinear phase shift accumulated by thefundamental beam near a SHG phase-matching condition.Note that if aY junction is fabricated at the end of the wave-guide to separate the outputs, an all-optical switching devicecan be built which changes its output port according to inputpower.This research was supported by DARPA and ARO.1L. A. Ostrovskii, JETP Lett.5, 272 ~1967!.2G. Assanto, G. I. Stegeman, M. Sheik-Bahae, and E. W. Van Stryland,Appl. Phys. Lett.62, 1323~1993!.3R. Schiek, Opt. Quantum Electron.26, 415 ~1994!.4C. N. Ironside, J. S. Aitchison, and J. M. Arnold, IEEE J. Quantum Elec-tron. 29, 2650~1993!.5R. Schiek, M. L. Sundheimer, D. Y. Kim, Y. Baek, G. I. Stegeman, H.Seibert, and W. Sohler, Opt. Lett.19, 1949~1994!.6Y. Baek, R. Schiek, and G. I. Stegeman, Opt. Lett.20, 2168 ~1995!; R.Schiek, Y. Baek, G. Krijnen, G. I. Stegeman, I. Baumann, and W. Sohler,ibid. 21, 940 ~1996!.7For example, P. LiKamWa, A. Miller, C. B. Park, J. S. Roberts, and P. N.Robson, Appl. Phys. Lett.57, 1846~1990!.8A. De Rossi, C. Conti, and G. Assanto, Opt. Quantum Electron.29, 53~1997!.9Y. Baek, Ph.D. thesis, University of Central Florida, 1997.FIG. 6. Normalized output modal interference pattern at 340 W input powercompared to the pattern for low power for the same initial relative phasedifference.3407Appl. Phys. Lett., Vol. 72, No. 26, 29 June 1998 Baek et al.

All-optical mode mixer spatial switch based on cascading in lithium niobate

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All-optical mode mixer spatial switch based on cascading in lithium niobate

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