Second-harmonic generation by Nd3+:YAG/Cr4+:YAG-laser pulses with changing state of polarization

We describe the passive Q-switching regime of a neodymium laser that contains a Cr4+:YAG saturable absorber inside the cavity. Two cofigurations of the laser cavity are modeled: the cavity containing a partial polarizer as an additional unit and the microchip laser cavity in which the functions of an active element and saturable absorber are combined in a single piece of a Nd3+:YAG/Cr4+:YAG crystal with reflecting facets. It is shown that both lasers are able to generate a giant pulse with a changing state of polarization. Both the kinetics of the state of polarization of the pulse and its impact on second-harmonic generation are treated numerically. We find that the up-conversion efficiency can be enhanced and that harmonic-pulse compression can be obtained by means of the proper orientations of the intracavity polarizing elements, for the first configuration, or by choosing the polarization of the longitudinal pump and angular orientation of the doubling crystal, for the second one. The ability of the doubling crystal to analyze the state of polarization of pulses with nonlinearly changing polarization is discussed. (C) 2000 Optical Society of America.