By combining a polarization beam splitter cube (PBS) and a quartz quarter waveplate, an isolator is done. Through the PBS, incident lights become linearly polarized, which then become circularly polarized by the quarter waveplate. When the lights are reflected back into the isolator itself, they will be converted into linearly polarized ones perpendicular to the input lights, which can be cut off by the PBS.
Chenter supplies isolators with the following specifications.