Beam Quality of a Fast Axial Flow CO2-Laser with RF-Excitation

Abstract

The beam quality of a laser system is defined as a figure of merit for the optimum focus geometry (minimum spot size and maximum focus length), obtained with an ideal focusing optic. Optimization of this beam quality is one objective of laser development, especially in the field of lasers for precision machining. We have investigated the beam quality of a fast axial flow CO2-laser with transverse rf-excitation. Nearfield and farfield intensity distribution have been measured as a function of resonator aperture radius in order to calculate the beam quality. The goal was to optimize the aperture radius for the gauss mode with maximum beam quality. The experiments show the existence of optimum apertures for the observed gauss and donat modes where the beam quality exceeds a local maximum. With these optimized apertures, the beam quality is close to the theoretical limit. In the transition region between gauss and donat mode, the beam quality decreases sharply and the near-field intensity distribution is superimposed by strong diffraction fringes. It has been demonstrated, that the transition region between two modes can be shifted either to higher or lower aperture radii by varying the spatial gain distribution.