The birefringent filter was first developed by astronomers at the turn of the century so that they could make celestial observations of the Sun through a narrow-band spectral window. The classic papers on these filters were written by Loyt [1] Evans [2] and Solc [3]. (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) In 1965 Harris et al [4] published a method of filter synthesis for the birefringent filter. In the late 1980s, Buhrer [5] brought the birefringent filter into the optical communications world with his demonstration of such a filter designed for optical wavelength multiplexing and demultiplexing.
The filters built by Lyot, Evans, and Solc, FIG. 1a used polarizers placed before and after the birefringent filter proper to isolate one polarization state at the input and then discriminate the state of polarization at the output of the filter. This was sufficient for their application because the Sun is an intense source and the polarization of light from the Sun is completely random.
In the late 1980s, Buhrer recognized that the use of input and output polarizers created excessive polarization-dependent loss and was therefore unsuitable for telecommunication applications. He instead created a method for polarization diversity. New input and output stages were proposed to allow the birefringent filter to handle simultaneously two orthogonal polarizations and to allow the output of the filter to couple to two complimentary ports. In his scheme, there is no loss of light, regardless of polarization. Buhrer U.S. Pat. No. 4,987,567 [6] further describes the use of a birefringent filter as an optical signal multiplexer/demultiplexer.
This invention makes material improvements to the Buhrer scheme for polarization diversity at the input and output of a birefringent filter. This invention increases the isolation between signals and decreases the chromatic dependence.