Influence of KDP crystals' refractive-index non-uniformities on 3ω conversion efficiency

The spatial refractive index distribution of typical rapid-growth (RG) crystals and conventional-growth (CG) crystals were obtained with orthogonal polarization interferometry. By numerical simulation, the influence of crystal refractive-index non-uniformities on 3ω conversion efficiency was also conducted for a fixed doubler detuning angle of 0 and 220 μrad respectively. The results show that the RMS refractive index uniformities error of RG crystals is about sixfold compared with that of CG crystals, and 3ω conversion efficiency is insensitive to refractive-index non-uniformities when 1ω irradiance was low, however, it is sensitive around that inflexion point. The refractive index variation of a doubler contributes more to the degrading of 3ω conversion efficiency than that of a tripler, on condition that the 2ω beam is not overmuch when tripling. At present, the CG crystals made in China are acceptable for high power third harmonic generation (THG) experiments. (authors)