Femtosecond laser writing of low-loss three-dimensional waveguide coupler in LiNbO3 crystal

With different interactions between material and femtosecond lasers, two-dimensional (2D) and three-dimensional (3D) waveguide couplers, whose separation distances are fabricated in z- cut lithium niobate crystal by femtosecond laser writing, are reported. Experimentally and numerically, it is shown from results that the guidance is only propagating along TM polarization due to the Type I modification and holds equal splitting ratios, which are the same as power splitters at 632.8 nm. The propagation losses of 2D and 3D waveguide couplers exhibit better transmission properties than those of the previously reported Type I Y-junction waveguide splitters.