An optical multiplexer/demultiplexer provides for arbitrarily closely spaced wavelength channels, the spacing and locations thereof being determined by the thickness of birefringent crystals such as, for example, quartz. Such a device can also be used for duplexing bidirectional signals. Methods for...http://www.google.com/patents/US4566761?utm_source=gb-gplus-sharePatent US4566761 - Birefringent optical wavelength multiplexer/demultiplexer
