Three-dimensional Numerical Simulation on the Directly Written Waveguide Formation with 320 nm UV Laser in Lithium Niobate

Three-dimensional numerical simulation on the waveguide formation written directly by 320[Formula: see text]nm ultraviolet laser in congruent lithium niobate (LN) crystal is presented. With the recent development of praseodymium (Pr)-doped laser crystal, the 320 nm UV laser has become easily available today. Though the absorption coefficient of this wavelength laser in LN is relatively low at room temperature, which might deter the direct waveguide writing fabrication, the situation will be much different when temperature rises. The absorption coefficient of this laser in LN will increase significantly under elevated temperature. When the temperature-dependent absorption coefficient is taken into consideration, the coupling of temperature and absorbed laser power will bring about interesting absorption characteristics. That is, the penetration depth of the laser will be affected by its power and the focus condition. In this work, we investigate the waveguide formation process in heated LN crystal via the lithium ion thermal-diffusion model. Cases where the laser is focused onto the surface and into the crystal are studied separately. The results show that when extra heated, the LN crystal can strongly absorb the 320[Formula: see text]nm laser focused on the surface and then form surface waveguides. Besides, when focused into the crystal, the laser will penetrate into the crystal and create internal high-temperature area with appropriated power and focusing parameter. However, in this case, only a relatively small refractive index change is obtained, which is not large enough to support efficient waveguide mode.