A vertically coupled InGaAsP/InP directional coupler filter of ultranarrow bandwidth

A report is presented on the design, analysis, and experiment of an InGaAsP/InP ultranarrow-bandwidth wavelength filter. The device consists of two vertically coupled waveguides of dissimilar cross sections and different material parameters. Whereas the upper thin waveguide uses air cladding in the lateral direction, the lower thick waveguide is weakly confined laterally. The difference in the optical confinements of the two waveguides is designed to give rise to a rapid decrease in the coupling strength with the wavelength deviating from the central wavelength, thereby achieving a narrow bandwidth. The measured -3-dB bandwidth (full width at half maximum) is >