Comparison of calculated and measured refractive index profiles of continuous wave ultravoilet written waveguides in LiNbO3 and its analysis by effective index based matrix method

Abstract

The refractive index profiles of the continuous wave (CW) UV-written waveguides in congruent LiNbO3 crystal are theoretically computed and compared with the evaluated profiles from the measured mode intensity distributions. It has been observed that change in activation energy of Li-indiffusion affects the computed refractive index profiles to a large extent, and the computed and measured profiles match fairly well for 305 nm and 275 nm written waveguides with an activation energy equal to ∼1.3 eV. The calculated refractive index profiles are used in effective index based matrix method (EIMM) to compute the guided mode propagation constants of these waveguides at 632.8 nm transmitting wavelength. For both the writing wavelengths, the waveguides are single mode in nature, which tally with experimental results. The bending losses of the bent waveguides for different bending radii are also computed using a conformal mapping technique along with EIMM. Finally, the behavior of CW UV-written waveguides under an applied electric field is physically analyzed. Accumulated space charge for 305 nm written waveguides is identified for its comparatively low electro-optic coefficient than the waveguide written with 275 nm UV laser.