75%-Efficiency blue generation from an intracavity PPKTP frequency doubler

We report on a high-efficiency 461 nm blue light conversion from an external cavity-enhanced second-harmonic generation of a 922 nm diode laser with a quasi-phase-matched KTP crystal (PPKTP). By choosing a long crystal (L_C = 20 mm) and twice looser focusing (w₀ = 43 μm) than the "optimal" one, thermal lensing effects due to the blue power absorption are minimized while still maintaining near-optimal conversion efficiency. A stable blue power of 234 mW with a net conversion efficiency of η = 75% at an input mode-matched power of 310 mW is obtained. The intra-cavity measurements of the conversion efficiency and temperature tuning bandwidth yield an accurate value d₃₃(461 nm) = 15 (±5%) pm/V for KTP and provide a stringent validation of some recently published linear and thermo-optic dispersion data of KTP.