Maximization of second-harmonic power using normal-cut nonlinear crystals in a high-enhancement external cavity

When an antireflection-coated normal-cut nonlinear crystal is used in an external cavity for the generation of high second-harmonic power, a small residual reflection at the crystal facets causes a round-trip loss and prevents the realization of a large fundamental enhancement. This problem is eliminated when the reflected beams at the crystal facets are subject to constructive interference. We demonstrate that the temperature tuning of a beta-BaB(2)O(4) crystal of at most 3 K is sufficient to realize constructive interference at any wavelength. We achieve an enhancement factor of 125, and a second-harmonic power of 125 mW is generated at 398 nm from a fundamental power of 390 mW.