Abstract

Short pulse excimer lasers are an ideal tool for material processing. The poor beam quality results in large focus diameters and prevents a cutting directly with the focused laser beam. An improvement in beam quality can be obtained by replacing the plane-parallel resonator by an unstable resonator with an output coupler of super-Gaussian reflectivity profile. For such type of lasers, a low number of round trips (n=2) and geometrical limitations in cavity magnification M (max. M=5) caused by the discharge chamber and electrode dimensions are significant. For this conditions, beam parameters and output energy for different resonator types have been calculated using the beam propagation method. The beam quality was characterised by the M²-factor on the basis of the second moment of the energy density distribution. For the plane-parallel cavity, the variation of parameters like cavity length and diameter of intra-cavity apertures showed a remarkable correspondence between simulation and experimental behaviour for the M²-factor and output energy in the vertical beam direction. In horizontal beam direction, an additional introduction of diffuse scattering at the electrodes was necessary to complete the model. The temporal gain behaviour has the strongest influence on the correlation between theory and experiment and its exact measurement was quite difficult in our case.

© 1998 IEEE