Optical parametric conversion of picosecond pulses in KTiOPO4

Abstract

KTiOPO4 (KTP) is a widely used material for frequency doubling 1,06-μm Nd:YAG lasers. Here we report the first demonstration of single-pass optical parametric generation and frequency difference mixing with picosecond pulses in hydrothermally grown KTP. The laser system utilized for this work consists of a cw mode-locked Nd:YLF laser (1.053 μm), which both synchronously pumps a single jet dye laser and seeds a Nd:YLF regenerative amplifier (RA). The dye laser emits 1-ps pulses, tunable from 0.57 to 1 μm. The output of the RA is first amplified up to 75 mj in a double-pass linear Nd:YLF amplifier, then spatially filtered in vacuum, and finally frequency doubled (20 mj in a 30-ps pulse at 526 nm) for pumping a three-stage dye laser amplifier. The dye laser amplifier typically generates picosecond pulses at the millijoule level. For the parametric generation experiments the high-power 526-nm beam was used as the pump. We have investigated two schemes: (1) the conventional configuration, in which the output of the generator is built up from noise in the nonlinear crystal, and (2) parametric generation, seeded by the dye laser. In the frequency difference mixing experiments, the amplified dye laser output is mixed with the remaining 1.053-μim beam of the amplified Nd:YLF to generate IR pulses at longer wavelengths. The KTP crystals were cut parallel to the natural (201) or (011) faces for propagation in the X-Z or Y-Z planes, respectively. For both cuts, the normal to the entrance face makes an angle of 58.8° with the Z axis. The tuning range of the parametric generator is 0.6-4.3 μim. Over this entire range, we found the experimentally observed phase matching angles to be in good agreement with the ones calculated from Sellmeier equations.1 A typical energy conversion efficiency of the (nonseeded) parametric device is 10% for a 5-mJ pump in 20-mm long crystals. Optical damage in KTP does not present a problem for these applications. We measured a damage threshold of 30 GW/cm2 for 30-ps green pulses (10-Hz repetition rate).