OPTICAL WAVELENGTH MULTIPLEXERS/DEMULTIPLEXERS AND DEMULTIPLEXER/REMULTIPLEXER A multichannel wavelength demultiplexer has two or more cascaded birefringent filter stages for dividing an input signal consisting of a plurality of adjacent channels of optical signals among a number of outputs, each of said filter stages having a transmission function with even symmetry about optical frequencies at the centers of both the longest and the shortest wavelength channels. This symmetry, common to all transmission functions, permits all birefringent filter stages to be constructed from sequences of the same one size crystal element, thereby greatly lowering the cost of construction because solely one size crystal element is fabricated, tested and assembled. A multichannel multiplexer, identical to the above defined demultiplexer, but operated in reverse, is set forth. A combination multichannel wavelength demultiplexer/ remultiplexer consists of a single birefringent filter stage for use in conjunction with multichannel multiplexer/demultiplexer to divide a common set of adjacent wavelength channels on its primary input between its primary and secondary outputs in any combination, and simultaneously to accept at its secondary input different optical signals within the same wavelength channels as those exiting its secondary output and recombine them to exit at its primary output, wherein said single filter stage has a transmission function with even symmetry about optical frequencies at the centers of both the longest and the shortest wavelength channels. This symmetry, common to its transmission function and to those of said multichannel multiplexer/demultiplexer, permits all birefringent filter stages to be constructed also from sequences of the same one size crystal element.
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