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Towards noncritical phasematching in thin-film lithium niobate waveguides

We theoretically investigate noncritical phasematching in thin-film, periodically poled lithium niobate (LN) waveguides. Noncritical phasematching relaxes fabrication tolerances and is needed for long devices or when ideal tuning curves are required. Geometries exist for noncritical phasematching with respect to waveguide width, but we could not identify geometries noncritically phasematched with respect to LN thickness (the least well-control geometrical parameter). Our calculations showed that thicker waveguides had less sensitivity to thickness variation. We present a model of how geometrical variations affect the nonlinear tuning curves. We estimate limits on the acceptable thickness variation and discuss the limits scale with device length.