Wavelength-flattened directional couplers for mirror-symmetric interferometers

In the context of guided optics, the authors derive, analytically and geometrically, a rigorous general criterion to design wavelength-insensitive interferometers with mirror symmetry, which are needed for wavelength multiplexing/demultiplexing. The criterion is applied to a practical case, resulting in an interferometer that works on a band wider than 70 nm.