Optical Isolator
    Optical isolator is a combination of a polarization Beamsplitter cube (PBS) and a quarter waveplate made of crystal quartz. Incident light is linearly polarized by PBS and converted to circular polarization by the quarter waveplate. If any portion of the emerging beam is reflected back into the isolator, the quarter waveplate will convert the reflected beam into a linearly polarized beam which is perpendicular to the input beam. This beam is then blocked by PBS and it will not return to the input side of the system.