Generation of 740??mW of blue light by intracavity frequency doubling with a first-order quasi-phase-matched KTiOPO_4 crystal

We report on efficient intracavity frequency doubling of a cw diode-pumped Nd:YAG laser on the ${}^4{F}_{3/2}\to {}^4{I}_{9/2}$ laser transition at 946 nm. The nonlinear crystal used in the experiments was a first-order quasi-phase-matched flux-grown ${\mathrm{KTiOPO}}_4$ crystal (period, $6.09 \mu \mathrm{m}$; thickness, 1 mm; length, 9 mm). The fluctuations in the generated second-harmonic wave were lower than 3% at output powers of as much as 500 mW. The overall optical-to-optical efficiency was 5.7%. A maximum output power of 740 mW of blue light was generated, which was stable for only 0.5 min. The decrease the output power at this power level was attributed to heating and thermal lensing in the periodically poled ${\mathrm{KTiOPO}}_4$ crystal. The short-term behavior of the second-harmonic wave exhibited switching between a cw mode and chaotic intensity fluctuations.