US Patent 7101431 Periodically poled potassium titanyl phosphate crystal

A thermal treatment process for improving the resistance of a flux grown, periodically poled KTiOPO4 crystal to photorefractive or photochromic damage comprising the steps of:i) heating said crystal from ambient temperature up to an annealing temperature in the range of from about 200° C. to about 400° C.;ii) maintaining said crystal at said annealing temperature in an oxygen containing atmosphere;iii) allowing said crystal to slowly cool down from said annealing temperature to ambient temperature.