All-optical switching in lithium niobate directional couplers with cascaded nonlinearity

Abstract

We report on intensity-dependent switching in lithium niobate directional couplers. Large nonlinear phase shifts that are due to cascading detune the coupling between the coupler branches, which makes all-optical switching possible. Depending on the input intensity, the output could be switched between the cross and the bar coupler branches with a switching ratio of 1:5 and a throughput of 80%. © 1996 Optical Society of AmericaIn a non-phase-matched second-order nonlinear interaction between a fundamental light wave and its second harmonic, the difference in the phase velocities leads to a periodic energy exchange with propagation distance between the interacting waves. Because the interaction is coherent, the phases are also affected, and a nonlinear phase shift accumulates with a propagation distance whose magnitude depends on the input fundamental intensity (cascaded nonlinearity). This idea is particularly intriguing because there are many semiconductor and organic materials with second-order nonlinearities .100 pm͞V. Therefore, under conditions of near phase matching, the nonlinear phase shift that is due to cascading can be orders of magnitude larger than phase shifts produced by the thirdorder nonlinearity (nonlinear refractive index) for the same input power. This should dramatically reduce the power levels required for a given application. Although both all-optical switching and solitonlike propagation based on the nonlinear refractive index have been successfully demonstrated during the past decade, 4,5 the first experiments to conf irm the predictions that typical all-optical x ͑3͒ phenomena can be mimicked with the cascaded second-order nonlinearity were performed only a few years ago. Specifically, in LiNbO 3 channel and slab waveguides the nonlinear phase shifts have been measured, 6 one-dimensional spatial solitons have been observed in planar waveguides, 7 and the nonlinear phase shift was used for what was to our knowledge the f irst demonstration of all-optical switching in a hybrid Mach-Zehnder interferometer. 8 However, large cascading nonlinear phase shifts in the fundamental beam would be of limited value if they were inseparably accompanied by strong depletion of the fundamental. And, in general, this would seem to be the case, because the largest phase shifts occur near phase matching, where conversion of the fundamental into its second harmonic is strongest. However, our previous experiments and calculations showed that varying the wave-vector mismatch in a specific way along the propagation direction, for example, by changes in the waveguide dimensions, the quasi-phase-matching period, or the temperature distribution (our case), leads to large nonlinear phase shifts commensurate with weak (,10%) net conversion to second-harmonic generation (SHG). 6 -8 This has led us to test the feasibility of designing and fabricating waveguide-directional couplers in LiNbO 3 for a f irst demonstration of all-optical switching owing to cascading in a fully integrated optics device. The results are ...

Tunable Pulse Compression by Nonlinear Cross-Phase Modulation using Cascaded Electro-Optic Effects

“…The third-order nonlinearities exist in medium with any symmetry and have already been utilized for pulse compression [1], all-optical switching [2], self-focusing and self-defocusing [3] with an intense light. Hence it is significant to enhance the intrinsic weak third-order nonlinearity.…”

PT-symmetric coupler withχ(2)nonlinearity

“…In the latter context, switching in two parallel waveguides was studied in [31]. The intensitydependent switching in lithium niobate directional couplers was subsequently first observed in [32]. The particular case of a dimer with one nonlinear and one linear waveguide was considered in [33].…”

Electro-optic modulation on nonlinear phase shift and fundamental transmission in quasi-phase-matched second-harmonic generation

“…Therefore, the cost of obtaining the required nonlinear phase shift in the fundamental beam is nearly complete depletion of the fundamental beam. There have been many works to solve this problem by varying the wave-vector mismatch in a specific way along the propagation direction, for example, by changes in the waveguide dimensions, the quasi-phase-matching period, or the temperature distribution which leads to required nonlinear phase shifts commensurate with weak conversion to SH [7][8][9][10][11], and large nonlinear phase shifts (>2p) accompanied with low fundamental depletion (<10%) by adjusting temperature distribution have been studied experimentally [7][8][9][10].…”