Three-dimensional beam-splitting transitions and numerical modelling of direct-laser-written near-infrared LiNbO3 cladding waveguides

We report novel results on the fabrication of near-infrared waveguides inside lithium niobate (LiNbO3) crystals with different three-dimensional beam-splitting architectures, comparing the effects that each type of architecture has on the propagation losses and mode evolutions. Optimized waveguides are then studied in detail to obtain the refractive index profiles within the femtosecond-laser-written claddings with sub-micron resolution. This knowledge is currently impossible to obtain with experimental techniques and allows for the proper understanding of the laser-writing process, as well as to design novel waveguides and photonic circuits with optimized properties.