Effect of Optical Inhomogeneities on Phase Matching in Nonlinear Crystals

Experimentally, we have observed variations in the phase‐matching temperature (Tpm) for second harmonic generation (SHG) along the pull axis of lithium niobate (LiNbO3) crystals. We have related these to variations in the extraordinary index of refraction caused by changes in crystal stoichiometry occurring in the growth process. We have established a technique, for filtering pathological crystals, based upon a point‐by‐point mapping of the birefringence variations along the direction of interest. This provides information about the details of the large and small‐scale spatial variations. Empirically we can get toleration limits on the birefringence variations for application to parametric oscillator design. We have studied several physical models (single step, multiple step, and linear ramp) for the manner in which monotonic changes in index, appropriate for LiNbO3, are introduced as a function of position. We have found that the usual quality criterion based upon the width of the central peak in an SHG ...