An experimental study of phase conjugation via stimulated Brillouin scattering applied to a high power Nd:YAG oscillator/amplifier laser system
In the thesis we investigate the use of phase conjugation techniques for the correction of amplifier aberrations in a Nd:YAG oscillator/ amplifier system. The phase conjugation process examined was stimulated Brillouin scattering in methane gas. In the first part of the thesis a TEMoo, single longitudinal mode, pre-lase Q-switched oscillator is described, which gave a high quality output beam. We discuss the properties of Nd:YAG laser amplifiers of the type used to amplify this output, and the ensuing beam distortions. The processes of phase conjugation and stimulated Brillouin scattering are then explained. In the latter part of the thesis experimental investigations of these processes are described. These include the effects of depolarisation due to thermally induced birefringence in the laser amplifier and the extent to which the phase conjugation process can tolerate this depolarisation. The effect of the process upon the laser linewidth, and vice versa, are also discussed. When a single amplifier stage was used in a two-pass arrangement incorporating either a conventional mirror or a phase conjugate mirror, the latter gave a factor of four improvement in beam brightness over the former. This factor was increased to sixteen when the phase conjugate mirror was used in a four-pass configuration.