High-power UV light generation in picosecond pulse trains

This paper focuses on the investigation of detrimental UV induced effects during fourth harmonic generation (FHG) in BBO crystals. An accumulation of partly recoverable UV two-photon induced optical defects has been observed in the 140 s train generated by frequency quadrupled 1.5 GHz Nd:YLF laser with about 300 W UV power per train, even for a relatively low peak pulse intensity of about 100 MW/cm². BBO crystals with length of 4.2, 8.5 and 12 mm have been tested. In a shorter crystal less distortion have been observed for the same UV output power. Although the growth of optical defects is attributed to the UV two-photon absorption, the already accumulated defects lead to a linear absorption of green and UV co-propagating pulses which in turn results in train envelope degradation due to deterioration of a phase-matching and thermal beam distortion. The UV beam exhibited focusing behavior on a short distance (crystal length) which cannot be explained by temperature dependence of BBO refractive indexes. The UV beam focusing is addressed as a strong photo-elastic effect, caused by the stress induced by essentially non-uniform temperature distribution inside the beam propagation volume.