Coupling-length adjustment for an optical directional coupler as a 2×2 switch

Abstract
A novel method for realizing the desired light coupling of an optical directional coupler for 2×2 switching is presented. This method consists of loading a high‐index film on one of the waveguides and reducing the light coupling to zero in the undesired coupling region. The operation principle was experimentally examined using Ti‐diffused LiNbO3waveguides and Se‐S–based chalcogenide glass film. The effective coupling length for the TE mode could be changed from 0 to 3L 0 (L 0 : 0‐dB coupling length) by trimming the high‐index strip.