Investigation on the characteristics of cross type optical isolator system with a double pass amplifier and two symmetric 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.
The reflectivity of SBS PCM using FC-75 index matching oil was over 90% in single mode pumping. But the transmittance of SBS PCM using FC-75 index matching oil was 6% in single mode pumping. This energy loss was generated by surface reflection of optical component, especially polarization beam splitter. The reflectivity of SBS PCM in multi mode pumping was 65%.
Also, polarization distortion by thermal birefringence was measured in the case of single Pass amplifier and double pass amplifier with SBS PCM using Faraday Rotator and Quarter Wave Plate. Depolarization ratio in single pass amplifier was 4%. But depolarization ratio in double pass amplifier using Faraday Rotator was 0.6%. So, the polarization distortion was considerably compensated using a Faraday Rotator In double pass amplifier. On the contrary, using Quarter Wave Plate, the polarization distortion was not compensated.
The leaked back reflection from the post multi-pass amplifier stage can be coupled with an oscillator and cause severe optical damage even if an optical isolator is employed. A new method to cut off the back reflection by using SBS PCM was performed. In this experimental work, the back reflection could be removed completely if the optical path length between the two SBS PCMs was over 6.5 m. The double pass amplifier combined with this isolator had low gain because of low optical pumping energy, break down in SBS PCM and inappropriate delay time. But the leaked back reflection was completely cut off and laser beam quality was improved.
