A study was made of the spectral dependence of the laser-induced absor
ption in a crystal of potassium titanyl phosphate KTiOPO4 (KTP), The n
onlinear absorption coefficients were determined for KTP crystals diff
ering in respect of quality. It was found that the formation of colour
centres in the course of generation of the second harmonic of neodymi
um laser radiation was mainly due to two-photon absorption.