Evanescent coupling in buried planar waveguide arrays written by a femtosecond laser in neodymium-doped yttrium lithium fluoride (Nd : YLiF4)

We report on the fabrication of buried planar waveguide arrays in neodymium-doped yttrium lithium fluoride (Nd : YLiF4) by femtosecond laser writing. With optimized fabrication parameters, the individual waveguide modes are coupled to each other through the evanescent tails of adjacent guided fields, while the coupling constant is calculated to be 1.18 cm-1 at 633 nm based on coupled-mode theory, which shows good agreement with the experiment results. Micro-luminescence spectra reveal that the Nd3+ fluorescence properties are not significantly affected by the waveguide formation processing, which indicates a fairly good potential for further laser actions in a compact device. (paper)