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Applied Physics Letters

Dynamics of optical TE to TM mode conversion in LiNbO3 channel waveguides

Juan Francisco Lam, Huan W. Yen

Published Dec 1, 1984• 22 citations
Publisher

Abstract

We report an analysis of the phenomenon of TE to TM mode conversion in LiNbO3 waveguides. The process is a consequence of the photorefractivity associated with the tensor character of the photovoltaic effect. We predict the existence of a nonlinear phase shift during the energy transfer process as well as a threshold behavior in the dynamics of energy exchange.

DOI
10.1063/1.95081

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We report an analysis of the phenomenon of TE to TM mode conversion in LiNbO3 waveguides. The process is a consequence of the photorefractivity associated with the tensor character of the photovoltaic effect. We predict the existence of a nonlinear phase shift during the energy transfer process as well as a threshold behavior in the dynamics of energy exchange.
We predict and theoretically study intrinsic optical bistability (IOB) mediated by nonlinear energy transfer processes in rare-earth-doped laser-related crystals. In particular, we investigate Tm-Ho and Tm-Yb systems, in which avalanche pumping is overimposed by energy transfer up-conversion. We predict that IOB can be experimentally observed in (Tm,Yb):BaY2F8(\mathrm{T}\mathrm{m},\mathrm{Y}\mathrm{b}):{\mathrm{BaY}}_{2}{\mathrm{F}}_{8}(Tm,Yb):BaY2​F8​ crystals in a wide range of experimentally achievable parameters.
It is known that two-wave coupling in photorefractive crystal may result in an intensity-dependent phase shift as well as energy transfer between the two waves without phase crosstalk. In a sense, this kind of phase shift is similar to the intensity-dependent refractive-index effects in Kerr media. Consequently, optical bistability can be established in an optical cavity with a photorefractive crystal. We formulate the model and get the expected bistable transmission with numerical simulations. Several cases, including that of the codirectional coupling and the contradirectional coupling, are considered. The effects of the photorefractive crystal’s distribution absorption and the spatial phase shift between the index grating and the light interference pattern in the crystal on the nonlinear transmission characteristics have also been studied. It is interesting to note that the output of the pumping wave is dependent on the cavity intensity and thus shows bistable behavior with respect to the cavity input as well.
Lithium niobate is a critical material extensively used in nonlinear optics. The recent development of nanophotonic lithium niobate devices further extends their importance with enhanced nonlinearity and scalability. However, the performance of nonlinear optical processes in lithium niobate is influenced by the photorefractive effect. In this work, we study the impact of the photorefractive effect on second-order nonlinear optical processes in nanophotonic lithium niobate waveguides. We simulate the parametric down-conversion for squeezed light generation and second-harmonic generation in nanophotonic lithium niobate waveguides, considering the photorefractive effect. The squeezing generation with parametric down-conversion only experiences a constant wavelength shift as the refractive index change remains constant. On the other hand, the second-harmonic generation spectrum is distorted, and a decrease in nonlinear efficiency is observed. This is because the strength of the photorefractive effect varies along the waveguide. We further show that the ideal efficiency of second-harmonic generation can be recovered by the adapted poling technique, where the phase matching condition change caused by the photorefractive effect is compensated by adjusting the poling period of lithium niobate waveguides. Eventually, we analyze the cascaded process of second-harmonic generation and parametric down-conversion for squeezed light generation, and show that squeezing level up to 17.6 dB can be realized using 10 mW power with current performance of nanophotonic lithium niobate devices.