A method and apparatus for multiplexing/de-multiplexing optical signals is disclosed. The method and apparatus are applicable to a range of optical multiplexing techniques including, but not limited to: wavelength division multiplexing (WDM), dense wavelength division multiplexing (DWDM) and frequency division multiple accessing (FDMA). The disclosed devices do not require active components. The disclosed devices may be implemented with fiber or fiberless optical communication systems including telecommunications systems. The devices may be used on their own or as part of a larger system such as a multi-stage mux/demux, an optical switch, or router. In an embodiment of the invention, the devices are passively thermally stabilized with the result that their tuning is substantially invariant across a wide temperature range. The optical device includes a linear polarizer, wave plates and a beam displacer/combiner. The wave plates optically couple to the linear polarizer. The wave plates rotate both odd and even channel components of an optical beam between a linear and an orthogonal relationship depending on the propagation direction. Each wave plate is of a selected length and index of refraction which together determine a free spectral range which correspond to a spacing between adjacent gridlines of the selected wavelength grid. Each of the wave plates is tuned to even symmetry with the selected wavelength grid. The beam displacer/combiner displaces and combines orthogonally polarized odd and even channel components of an optical beam depending on a propagation direction.