Phase Tuning in Optical Directional Coupler

A new phase tuning method for an optical directional coupler fabricated in a c-plate LiNbO3 by TiO2 diffusion is presented. The method consists of applying a bias voltage to an additional electrode placed closely outside one of the modulation electrodes. The propagation constants of the two waveguides could be made equal by a 5 V bias voltage without affecting the 8 V modulation voltage.