A WDM demultiplexer/multiplexer comprising a plurality of narrow band reflective filters linearly disposed along an optical axis, each narrow band reflective filter reflecting a single channel or group of channels and transmitting the remaining channels, is described. In a demultiplexing mode, an optical signal initially carrying channels at &lgr;1&lgr;2 . . . &lgr;N travels along the optical axis. Each narrow band reflective filter reflects a distinct channel and is tilted with respect to the optical axis such that it directs the reflected beam away from the optical axis to an output. Each narrow band reflective filter is substantially transparent to the remaining channels of the optical signal, such that the remainder of the optical signal proceeds along the optical axis substantially undisturbed. Advantageously, the device is highly robust against tilt variations or other mechanical variations in the narrow band reflective filters, because such variations are not compounded as the optical signal travels through the device. When many channels “N” require multiplexing/demultiplexing, the incoming beam may be split into “m” separate beams and sent to “m” separate narrowband reflective filter arrays, each comprising about N/m narrowband reflective filters. An optimal number of beams “m” for best energy efficiency may be computed based on specified system parameters.