This paper reviews the growth and properties of KTiOPO4 (KTP) and its analogue crystals including its isomorphous crystals and doped crystals. Based on the calculation of the refractive indices and phase matching of KTP crystals, it is shown that non-critical phase matching can be achieved theoretically. In a 4%Nb-doped KTP crystal, non-critical phase matching was obtained with a pulsed Nd:YAG laser in an efficiency of ca. 4%. Most of the isomorphous and doped KTP crystals possess similar physical, optical and nonlinear optical properties as those of KTE RbTiOPO 4, KTiOAsO 4 and RbTiOAsO4 crystals have better electrooptical figure of merit and RbTiOPO4 crystals have a fast ion conductor character. Some doped KTP crystals have shown high second harmonic generation efficiencies and damage thresholds.
But the homogeneity of KTP analogue crystals is poorer than that of KTR On the base of damage threshold tests using a CW Argon laser, it is concluded that the best second harmonic generation crystal among all the KTP analogue crystals is still KTP itself. Special care must be taken to eliminate impurity centers to avoid the formation of gray track and photorefractive centers.