A simple bulk optic structure is described that is suitable for multiplexing or demultiplexing many closely spaced wavelengths of light. Aside from input/output coupling hardware, the structure is comprised of a single glass, high finesse, Fabry-Perot etalon with a low efficiency diffraction grating etched into one face. The device concept provides a capability for multiplexing ratios in excess of 10, channel frequency separations smaller than 1 GHz, and can be designed to support optical networks having a total communication capacity in excess of 100 GHz. A laboratory model of the multiplexer was constructed and evaluated to demonstrate concept feasibility. The demonstration model, adjusted to demultiplex nine channels separated by 2.2 GHz for a total capacity of 20 GHz, exhibited a throughput optical coupling of -7 dB and a signal-to-crosstalk ratio greater than 15 dB.