Abstract: 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 (WDM) 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 (MZI) 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.

Abstract: An optical filter, such as a wavelength-division multiplexer, demultiplexer, or optical router, in which several single-mode waveguides are coupled to the sides of an optical interaction region containing a wavelength dispersive element that collects light from one or more input waveguides inputting light to the interaction region and disperses it according to wavelength to one or more output waveguides outputting wavelength-separated light. According to the invention, a multi-moded waveguide is interposed between one or more of the single-moded waveguides and the optical interaction region. It has a predetermined length to create at one end a multiply peaked image of a singly peaked profile presented to it at the other end, thus being a multi-mode interference (MMI) filter that presents a flatter filter profile at the interface between the MMI and the optical interaction region and affords reduced filter sensitivity to wavelength drift of an optical signal.