The problem of brightness enhancement during laser frequency doubling

The characteristics of the transformation of the spatial structure of powerful laser system beams when their frequencies are doubled in nonlinear crystals are investigated qualitatively. The aim is to derive a condition on the wavefront of the initial beam which must be satisfied in order to increase the brightness of second-harmonic radiation. The analysis is based on wave equations written in parabolic approximation for slowly fluctuating amplitudes of the interacting waves propagating in the direction of synchronism in the crystal. It is found that the change in brightness is determined by the ratio of the geometric component (characteristic angles) of the beam divergence to the diffraction component at the crystal input. An unchanged radiation brightness, and a 3.5-fold brightness enhancement are demonstrated experimentally as a function of the curvature of the wavefront for 88% power conversion in a KDP crystal with harmonic laser radiation of B = 5x10 to the 17th power w/square centimeters. Brightness enhancement is also observed in a KDP crystal cut for type-II synchronism (L= 2.5 cm). It is found that depolarization of no more than 0.3% is required in order to obtain near maximum brightness. B = 5X10 to the 17th power w/square centimeters.