Investigation on the characteristics of degenerate 4-pass amplifier system with a stimulated Brillouin scattering phase conjugation mirrors(SBS PCM) was performed in this study. $Nd^{3+}$:YAG was used as gain medium for the double pass amplifier and FC-75 index matching oil was used as phase conjugation mirror medium.
We have tested the 4 pass amplifier using SBS-PCM as a rear mirror. The input beam energy is less than 1mJ. In the case using SBS-PCM as a rear mirror the output energy is 108mJ and in the case using mirror the output energy is 135mJ at the 35J electrical pumping energy. It's reason that reflectivity of SBS-PCM is lower than that of mirror in the lower input energy.
In the 4 pass amplifier system, the distorted beam profile and its imperfect polarization states are originated from the thermally induced distortion effect. In the case of using a conventional mirror as a rear mirror, its beam profile is distorted very much and its polarization does not rotate 45° exactly by the Faraday rotator due to the thermally induced birefringence effect. In order to compensate this effect, we employ SBS-Phase conjugation mirror as a rear mirror. In the case of using a SBS-PCM, the beam profile shows that the distortions are compensated and its profile is much better than the former case of using a conventional mirror.
And polarization distortion by thermal birefringence was measured in the case of four pass amplifier with SBS-PCM using Faraday Rotator and Quarter Wave Plate. Using a Faraday rotator between amplifier and mirror, the polarization was considerably compensated. On the contrary, using Quarter Wave Plate, the polarization distortion was not compensated. It is verified experimentally that the depolarization of a laser pulse in the four-pass amplifier with a Faraday rotator was only 1/10 of that with a quarter-wave plate.
To summarise, using SBS-PCM as a rear mirror in the four pass amplifier we can improve beam quality and compensate thermal distortion in the four pass amplifier.
