Efficient cw high-power frequency doubling in periodically poled KTP

We report on efficient second harmonic generation in periodically poled KTiOPO4 (ppKTP) at 423 nm pumped by a Ti:Sapphire solid state laser system. An overall optical conversion efficiency of over 60% was achieved at a moderate input power of 400 mW. The nonlinear crystal used in the experiments was a first-order quasi-phase-matched flux-grown KTP crystal (period 4 μm, length 10 mm). The crystal has a single-pass conversion efficiency of 1.38±0.08%/W cm and was placed in a ring cavity. We have demonstrated stable operation with a generated cw power at 423 nm of up to 225 mW. With a sweeping cavity we generated 425 mW peak power with an incident pump power of 680 mW. At this power level it was not possible to lock the cavity due to a bistability caused by localised heating of the crystal.