An optical spectrum analyzer and methods for its use for multiplexing and demultiplexing. The optical spectrum analyzer includes two reflectors in a fixed spatial relationship to a volume phase grating. Input light is reflected by the first reflector to the volume phase grating, diffracted by the volume phase grating towards the second reflector along a first optical path, reflected back to the volume phase grating by the second reflector along a second optical path distinct from the first optical path, diffracted again by the volume phase grating towards the first reflector, and reflected out of the optical spectrum analyzer by the first reflector. Preferably, the reflectors are surfaces of transparent blocks that are optically coupled to the volume phase grating. The reflectors are arranged so that light of a plurality of equally spaced wavelengths, that is input to the optical spectrum analyzer at a single entry point, is demultiplexed to a corresponding plurality of equally spaced exit points. Conversely, individual equally spaced wavelengths, that are input at respective equally spaced entry points, are multiplexed to exit at a common exit point.