Three-dimensional femtosecond laser fabrication of waveguide beam splitters in LiNbO_3 crystal

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We report on the fabrication of waveguide beam splitters in z-cut LiNbO3 crystal by direct femtosecond laser writing. The guidance is valid only for TM polarization due to the Type I modification of extraordinary refractive index (ne) induced by femtosecond laser pulses. With single scan of femtosecond laser beams over the crystal bulk, the structures with channel geometry have been produced. In this work, such waveguide configurations were created as one-dimensional (1D) straight waveguide, two-dimensional (2D) 1 × 2 and three-dimensional (3D) 1 × 4 waveguide beam splitters. The waveguide beam splitters are characterized at the wavelength of 632.8 nm and 1064 nm both experimentally and numerically. This work opens the way for laser-written 3D LiNbO3 waveguide beam splitters as novel 3D nonlinear photonic devices.