``Noncritical'' phase matching in optical waveguides

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Abstract
Phase matching of nonlinear three‐wave optical processes in optical waveguides is shown to be possible with the use of only zero‐order TE and TM modes, such that the phase‐match condition is made insensitive to waveguide thickness over a limited region. This is accomplished by adjusting the phase shifts on total internal reflection for lowest‐order modes, near cutoff, in a thin nearly symmetric waveguide. This ``noncritical'' situation eases the requirements on waveguide fabrication. Limitations due to bulk dispersion, waveguide asymmetry, and surface scattering losses are discussed.