Abstract: An interferometer that is used with a frequency shifting phase conjugate mirror (PCM), to produce a 90.degree. polarization rotation, employs only two adjustable elements: a specially shaped prism, and a beam deflector that is preferably a porro prism. The primary prism is shaped and oriented so that a linearally polarized input beam is divided into ordinary (o) and extraordinary (e) subbeams. One of the subbeams is refracted out of the prism along a controlled length path, and then returned back into the prism by the deflector. The second subbeam is retained within the prism by total internal reflection, and directed onto a exit path that coincides with the path of the first subbeam after its reentry into the prism. The recombined output beam is directed into the PCM, from which it is reflected back into the prism for a reverse pass. The difference in path lengths between the o and e subbeams is selected to produce a net 90.degree. polarization rotation after both passes.