Evanescent coupling in arrays of type II femtosecond laser-written waveguides in bulk x-cut lithium niobate

We report on the first fabrication of evanescently coupled planar arrays of type II waveguides in x-cut lithium niobate (LiNbO3) by femtosecond laser inscription. A particular choice of writing parameters allows the evanescent field of the individual waveguide modes to extend significantly beyond the damage lines into the neighboring guides, thus realizing a coupling constant of 2.2 cm−1 at a wavelength of 633 nm.