The frequency-selective optical multiplexer comprises, an LC polarization controller, a spectral demux/mux and input/output optics including two ports, a first optical path spatially separated from a second optical path and polarization-dispersive optics. The polarization dispersive optics are disposed between the ports and the optical paths; generate from an optical signal a pair of polarization components, composed of a first polarization component and a second polarization component having orthogonal polarizations; and output the first and second polarization components via the first and second optical paths, respectively. The first and second polarization components have first and second polarizations when the optical signal is received at one port, and have the second and the first polarizations, respectively, when the optical signal is received at the other port. The spectral demux/mux generates first pairs of spatially-separated spectral components from a first pair of orthogonal polarization components received from the input/output optics, outputs the spectral components to the polarization controller, receives respective second pairs of spectral components from the polarization controller, and spatially overlaps the second pairs of spectral components to generate a second pair of polarization components. Either the first pair or the second pair of polarization components passes between the spectral demux/mux and the input/output optics via the optical paths. The polarization controller receives the first pairs of spectral components at a zero angle of incidence, and rotates the polarizations of each of them individually and selectively through 0.degree. or 90.degree. to generate one of the second pairs of spectral components.