Coupling length adjustment for an optical directional coupler by loading a metal film

Abstract
A simple method for realizing the desired light coupling for an optical directional coupler is presented. This method consists of loading a metal film on one of the waveguides in the undesired coupling region. The effective coupling length, fabricated in c‐plate LiNbO3, could be changed from 0 to 3L0 (L0: 0‐dB coupling length) by controlling the length of the metal film.