Reduction of the spectral width and beam divergence of a BBO-OPO by using collinear type-II phase matching and back reflection of the pump beam

We report on the reduction of the divergence and the spectral width of the output of a 355 nm pumped pulsed optical parametric oscillator (OPO) of beta -barium borate (BBO). Detailed theoretical investigations indicated that type-II phase matching in combination with double-passing the pump beam should simultaneously reduce the spectral width and the divergence of the OPO output beam. These predictions are confirmed in the experiments reported in this paper. In fact the bandwidth is reduced by more than a factor of 20 to less than 0.1 nm. Simultaneously the divergence of the OPO waves is reduced in the phase matching plane by more than a factor of 5 to 1.0 mrad, The small divergence and the reduced bandwidth allows efficient frequency doubling of the 5-ns-long visible OPO signal pulses into the UV. Doubling in a 7-mm-long BBO crystal provided conversion efficiencies of up to 35%.