Two-port optical wavelength circuits composed of cascaded Mach-Zehnder interferometers with point-symmetrical configurations

Wavelength circuit characteristics are investigated systematically for a family of two-port optical wavelength circuits composed of cascaded Mach-Zehnder interferometers (MZIs) with point-symmetrical configurations. A novel design method is presented for flattening their cross-port passbands (through-port stopbands). The design principle is based on a consideration of the general relations between wavelength circuit characteristics and circuit configuration symmetries. It is shown that the role of this novel design method is complementary to that of the conventional flattening method which is used for simple wavelength-insensitive couplers (WINCs) with a single-stage MZI configuration. Various optical wavelength circuits are designed by using both the novel and conventional design principles, including fixed and tunable WINCs, a wavelength-insensitive switch (WINS), and a wavelength multi/demultiplexer with a flattened passband and stopband. Measurement results are provided for some of the designed optical circuits which were fabricated using silica-on-silicon planar lightwave circuits (PLCs)