The invention relates to a double-grating subtractive-dispersion optical
channel multiplexer/demultiplexer for combining or separating optical
channels and providing the output channels with a flat-top response. The
FSR of the first grating is substantially equal to the channel spacing of
the optical channels, such that the emission from the first grating
achieves a cyclic offset of incidence angle into the second grating of
the system.