Sidemode suppression of optical add-and-drop multiplexer utilizing vertically coupled InGaAsP waveguides

A sidemode suppression for a vertically and contra-directionally coupled semiconductor waveguide-type optical add-and-drop multiplexer is investigated theoretically and experimentally, by introducing a window function to a grating width. Grating configuration dependence of device characteristics was numerically calculated based on the finite element method and the coupled-mode equations. It was found that the sidemode suppression ratio (SMSR) could be improved to be more than 30dB even by a simple linear changing of the grating width. In the device fabrication, a trimmed grating was formed by a multi-step exposure method, including an interferometric exposure and a laser beam direct drawing. SMSR of 18 dB was obtained for a fabricated device, and improvement was obvious in contrast with a uniform grating. A comparison between the calculated and experimental results is also given.