Polarization dependence in Ti-diffused LiNbO3 directional couplers

The effect of design and fabrication parameters on the dependence on polarization of the splitting ratio in directional couplers produced by Ti diffusion in LiNbO3 has been investigated experimentally at a wavelength of 1.54 mum. The parametric study was carried out at a diffusion temperature of 1025 degreesC, which was found to suppress outdiffusion effects. The directional couplers were produced by use of various combinations of waveguide separation, Ti film thickness, and diffusion time. Of particular interest was the identification of parameter sets for which the sum of TE and TM splitting ratios equals unity, as required for electro-optic and acousto-optic tunable filter designs with relaxed beam-sputter requirements. Directional couplers that closely match this criterion were obtained by diffusion of 3.5-mm-long, 7-mum-wide Ti strips separated by 11 mum for 11 h. It was found that a bending angle of 0.6 degrees for input and output waveguides produces lower transmission loss for both polarizations than a 1.0 degrees bend angle (>1-dB loss reduction). (C) 2002 Optical Society of America.