Soft-Proton-Exchange Tapers for Low Insertion-Loss LiNbO3 Devices

We present the results of an experimental study of waveguide tapers realized in LiNbO₃ using the soft-proton-exchange process and the waveguide segmentation technique. Measurements show that they can be favorably introduced between the nonlinear or electrooptic part of the components, which require strong mode confinement to increase efficiency, and the coupling section with a standard optical fiber, which requires low mode confinement in order to have low coupling losses. These tapers allow almost lossless mode-shape transformation. Preliminary results show that they allow reducing the coupling losses by up to 0.7 dB, thus proving the practical interest of the approach.