Optimum exposure intervals for light-induced optical waveguides in lithium niobate crystals by laser micromachining

The optimal exposure intervals for optical waveguides induced by laser micromachining in LiNbO₃ crystals are theoretically and experimentally investigated. When the focused laser beams scan along different directions, the optimal intervals for waveguide fabrications are numerically specified by solving the photorefractive dynamic equations. The simulations show that the index distributions of the fabricated waveguides are strongly dependent on the optimal exposure interval, and the optimal exposure interval is not dependent on the scanning directions of the writing beam. When the writing beam scans the crystal along the c axis for sandwich illumination, optical waveguides cannot be fabricated efficiently. However, in this case, symmetric refractive index changes can be obtained. Experimental demonstrations are performed by scanning LiNbO₂ :Fe crystal with a focused green laser beam. The light-induced index changes are measured by digital holography. The experimental results conform to the theoretical analyses. Additionally, a Y-branch waveguide is experimentally formed and the results of the guiding tests are showed in this paper.