Three-dimensional polarization-dependent full-wavelength beam splitter written by femtosecond laser in LiNbO₃ crystal

We report on the fabrication of Y-branched waveguide beam splitters with cladding structures in LiNbO₃ crystal by direct femtosecond laser writing. The femtosecond laser writes tracks near the surface of the crystal, constructing a square structure based on the Type II geometry. The waveguide beam splitters support the propagation of full-wavelength light from the visible to mid-infrared, which was experimentally and numerically investigated. In addition, it has been found that the guidance is only along the vertical (i.e., TM) polarization, which is due to the different refractive indices in the longitudinal and transverse directions. The femtosecond-laser writing here also represents an alternative for fabricating complex integrated light guiding in crystals.