Distributed fluctuations in the effective index of a waveguide can explain all the nonideal optical characteristics (including optical cross talk) observed in Ti-diffused LiNbO3 directional-coupler switches. We find that these spatial-index fluctuations are not a result of the photolithographic processes used to define the channel waveguides. Measurements of undiffused Ti films have an envelope of concentration variations approximately seven times that of the diffused Ti channel waveguides. The magnitude of these inhomogeneities is consistent with that required to produce the optical cross talk in current devices.
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McCaughan et al. (1987) studied this question.