a passband - flattened phasar including two free space regions coupled by a plurality of waveguides having predetermined differences between their lengths so as to act as an arrayed waveguide grating . the phasar is particularly useful in a wavelength - division multiplexed optical communication system transmitting multiple wavelength systems arranged in a wavelength comb with a constant wavelength channel spacing . the input waveguide is coupled to the first free space region through a mach - zehnder interferometer having two waveguide arms of differing lengths receiving approximately equal amounts of the input signal . the arms differ in lengths so as to produce a phase difference between them . in a wdm network , the waveguide arm produce a phase difference such that the free spectral range of the mzi equals the wavelength channel spacing , such that the wavelength response of the mzi is the same for each of the wdm wavelengths . the two outputs of the mzi are coupled into the input end of a multi - mode interferometer with a lateral separation which provides a lateral spatial dispersion in the mmi equaling the lateral spatial dispersion of the conventional phasar . thereby , a larger portion of the passband is equally passed through the phasar .