Damage properties of the frequency conversion crystal KDP limits the output fluence of high power laser facilities. Damage behavior of KDP was investigated for many years, and results show that intrinsic or extrinsic defects are responsible for highly localized absorption in the bulk of KDP materials may cause the damage. Irradiated by laser fluence under damage threslond would improve the damage performance of KDP crystal. The mechanisms of laser-induced damage and laser conditioning are still not well understood. Lots of research concerntrated on optimizing the laser parameters to improve the conditioning effect, since no visible damage pionts in the KDP ctrystal formed under subdamage fluence irradiation, few attention focused on changes of the intrinsic crytal.

We investigated the characterisitic of KDP crystal before and after laser irradiation. The crystal were irradiated by different fluences below damage threshold,  and absorption coefficient, fluorescence spectrum, x-ray diffraction of the irradiated area were investigated, and obviously variation obtained.

Bulk absorption vs laser fluence

Our results show that subdamage laser fluence irradiation modified performance of KDP crystal, such as absorption coefficient of the material, that is, as the damage threshold improve, the absorption of the material increase, too, considering the low threshold defect precursors have been eliminated, we wonder whether the damage occurred after laser conditioning has something to do with this.

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