Critically index-matched second-harmonic generation in multiple crystals

A new method is presented for reducing the conversion-limiting effects of crystal anisotropy in critically index-matched second-harmonic generation (SHG). A number of crystals are put in series; adjacent crystals are oriented in such a way that their walkoff effects are opposite one another. The harmonic field and the harmonic power of a compensated two-crystal generator are calculated. Numerically computed curves show the dependence of the SH power on the mismatching of the crystals and on the focusing of a TEM00laser beam. Theoretically predicted results are confirmed by experiment. As a typical result, the conversion efficiency of a compensated ADP generator (2 crystals, 1 cm each, in series) at optimum focusing is about twice the efficiency obtained with a single crystal of the same total length (2 cm, single crystal).