Evaluation of KTiOPO4 crystals for high-power laser

The gray-tracking of KTiOPO4 nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The susceptibilities of gray-tracking were evaluated by measuring the reduction of transmittance of the crystal with the second harmonics pulse of Nd:YAG laser, and were compared with the optical absorption coefficients and contaminants in each crystal. As result, the susceptibility indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it was prevented with increasing hydroxyl concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals annealing process under control if the humidity.