1. Introduction . Increasing the capacity of existing fiber networks using wavelength division multiplexing (WDM) is driving the demand for high performance integrated photonic filters. Low-loss, flat-top, and low crosstalk filters are needed for (de)multiplexing closely spaced wavelength channels. Different structures have been employed to implement such filters, including echelle diffraction gratings [ 1 ], arrayed-waveguide gratings [ 2 , 3 ], Mach-Zehnder Interferometers (MZIs) [ ...