Efficient resonant frequency doubling of a cw Nd:YAG laser in bulk periodically poled KTiOPO_4

A cw Nd:YAG laser was quasi-phase-matched frequency doubled by a bulk periodically poled flux-grown ${\mathrm{KTiOPO}}_4$ crystal in an external resonant cavity. The conversion efficiency and the second-harmonic power with a pump of 225 mW were 55% and 123.5 mW, respectively. The loss of the 1-cm-long crystal, which operated near room temperature, was 1.3% and was dominated by scatter from the cleaved facets of the crystal. No damage, photorefractive or other, was observed at intensites of $370 {\mathrm{Wmm}}^{-2}$, but beam size had to be optimized to eliminate thermal-induced instabilities in the doubling cavity.