Stimulated Brillouin scattering(SBS) phase conjugate mirrors are used for coupling of parallel gain media(beam combination) in a repetition-type Nd:YAG laser system. The characteristics of SBS reflection and the beam combination were investigated with experiments and simulations.
The beam combination means the coupling of parallel amplifiers to form a single coherent output beam. The technique can be used for achieving high power and high repetition solid state laser beyond traditional limits imposed by the maximum available volume of a single active medium. These limits mainly arise from crystal growth technology and thermal effects. In this experiment, the wave front of the input beam was divided using square type wedges. Then each divided beam was amplified separately, reflected by SBS mirrors and recombined. The stimulated Brillouin scattering results from parametric coupling between light and acoustic waves. The SBS process has properties of phase conjugation, pulse compression, beam smoothing, and so on. Phase conjugation of backward scattered beam by these SBS mirrors compensated aberrations in the laser gain media.
The laser system used in the experiments had MOPA(master oscillator and power amplifier) configuration. A SBS mirror($CCl_4$) was inserted in the middle of the amplifier chain. The reflectivity of the mirror depended on the input energy, and had maximum 35% in the experiment. This reflectivity characteristic was used for optical isolation, that is, blocking of the back-reflected beam from the next SBS mirrors.
Simulations for the beam combination showed how the output beam characteristics change with different phase distribution between the separated beams. The changes of the spatial beam profiles observed in the beam combination procedure were similar to the simulation results. The aberration compensation effect was confirmed, but the gap between squares appeared in the combined pattern. This gap is thought to be due to the relative phase difference between them.