US Patent 7149387 Double diffraction grating with flat passband output

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.