Laser damage investigation in KTiOPO4 (KTP) and RbTiOPO4 (RTP) crystals: threshold anisotropy and the influence of SHG

The laser damage probability of nonlinear optical crystals depends on many different factors. In addition to the fabrication process (crystal growth, cutting, polishing, coating) the damage threshold of non-linear crystals is influenced by the usage of the crystal. The anisotropy of material properties like the complex refractive index, the mechanical yield strength and the polarizability may cause anisotropy of the Laser Induced Damage Threshold (LIDT). The LIDT may depend on the propagation direction or the polarization of the light. Anisotropy in the LIDT has in fact been observed in different crystals. The dependency of the KDP-LIDT on the propagation direction and its independence on the polarization direction reported by Burnham et al. is an example that is not yet fully understood. For KTP it has been evidenced by Hu et al. that the grey-tracking threshold is polarization dependent. In this contribution we discuss the bulk laser damage resistance of two isomorphous non-linear crystals: KTiOPO4 (KTP) and RbTiOPO4 (RTP). All tests are performed using a nanosecond laser at 1064nm wavelength. For both crystals all polarization and propagation directions parallel to the principal axes have been tested. In addition we investigated two typical devices: two types of Pockels cells in RTP and a SHG-cut crystal in KTP. The results are analyzed on the basis of an anisotropic sensitivity of the crystal to the electric field and the efficiency of second harmonic generation.