Abstract

Short-pulse lasers deliver pulses that have made only a few round trips inside the cavity before leaving it definitively, before the equilibrium state of the beam parameters is reached. Hence, a good knowledge of the beam parameters dynamics inside the laser cavity is necessary to develop improvements of the overall output brilliance. In fact, the beam quality depends on the spatial coherence of the beam, which grows with the number of transits made inside the cavity and affects the beam divergence. We have developed a theoretical model¹,² giving the main beam parameters (spot size, radius of curvature and beam quality factor M²) of the intracavity radiation at each round trip, starting from the ASE radiation of low coherence and high divergence. The numerical results from this model have been compared with experimental measurements made on the laser pulse from a XeCl excimer laser, collecting data on the spot size and divergence in correspondence to each round trip¹,². The laser cavity structure has been changed for three different groups of measurements. For each group, mirrors with a Gaussian, a super-Gaussian and a phase-unifying reflectivity profile, respectively, have been used as output coupler of a plane-parallel cavity containing the XeCl laser medium. Results of the theoretical analysis and of the experimental measurements result in good accordance, indicating that the beam parameters dynamics can be understood and then controlled.

© 1996 IEEE