Optical multimode interference couplers of Ti:LiNbO3 waveguides and electrical tuning of power splitting ratio

Abstract We analyzed the optical properties of optical couplers based on multimode interference (MMI) in titanium-diffused LiNbO 3 (Ti:LN) waveguides using the beam propagation method (BPM) with refractive index distributions by Ti diffusion. From these results, we designed and fabricated 2 × 2 MMI couplers operating at 1 . 55 μ m wavelength using Ti:LN waveguides. An optical power splitting ratio of 0.53:0.47 (0.52 dB power deviation from equal split) and an insertion loss of less than 1.5 dB were observed with a coupler consisting of an MMI waveguide 50 μ m wide and 2.12 mm long. Electrical tuning of the power splitting ratio by the electro-optic effect of LiNbO 3 was also considered. The electrodes were formed directly on the MMI waveguide section of the coupler and the power splitting ratios were controlled almost in proportion to the voltage applied to the electrode and a deviation of about 1 dB (0.56:0.44) of the power splitting ratio from an equal split could be adjusted to zero by applying a voltage of −6.1 V.