The polarization matching mixer (U.S. Patent No. 4,723,315) is a device that uses polarization beamsplitters to first combine local oscillator and signal light beams into an orthogonal-polarization mixed beam and then split this beam into two matched-polarization mixed beams ideally suited for differential detection. It has potential advantages for coherent communications systems since it offers improvements in optical efficiency through sensing and controlling the polarization state of the incoming signal beam in the receiver. In multiple-wavelength laser diode systems, it may make it possible to increase the number of channels used within a given wavelength band by allowing fine pointing, polarization, and wavelength sensing through diffraction grating dispersers. This paper discusses the polarization matching mixer, how it works, how it compares to the use of amplitude beamsplitters, and how it can be used to reduce signal loss due to degradation in signal polarization. Its potential application to multiple-wavelength laser diode systems is briefly discussed. The paper closes with a discussion of high efficiency polarization beamsplitters.