Optimization of second-harmonic generation efficiency in potassium niobate

Abstract

KNbO3 has been shown to be a promising crystal for second-harmonic generation (SHG) of tunable blue light by doubling of near-infrared emission of laser diodes and Ti:sapphire lasers. Although low power resonant cavity SHG has been previously described, there has been no reported systematic experimental determination of the maximum cw or quasi-cw single-pass SHG efficiency. Particularly, limitations to the SHG efficiency imposed by incomplete phase matching due to the nonuniform temperature distribution in the crystal have not been reported. Here we describe, we believe for the first time, cw and quasi-cw measurements of KNbO3 frequency doubling efficiencies as a function of the fundamental beam diameter, crystal length, and incident fundamental power. In optimum conditions, a maximum of 8%/(Watt-cm) was achieved using a diffraction-limited pump beam from a Ti: Al2O3 laser operating at 860 nm. Over 300 mW of 430 nm was measured in single pass SHG with 3.7 W of incident fundamental power. For high input powers, where the frequency doubled power was above a few hundred milliwatts, a large decrease in the SHG efficiency was observed for long pulse lengths and cw operation, because of heating of the active volume of the crystal causing incomplete phase matching.