Low-loss waveguides in a single-crystal lithium niobate thin film.

We report low-loss channel waveguides in a single-crystal LiNbO(3) thin film achieved using the annealed proton exchange process. The simulation indicated that the mode size of the α phase channel waveguide could be as small as 1.2  μm(2). Waveguides with several different widths were fabricated, and the 4 μm-wide channel waveguide exhibited a mode size of 4.6  μm(2). Its propagation loss was accurately evaluated to be as low as 0.6 dB/cm at 1.55 μm. The single-crystal lattice structure in the LiNbO(3) thin film was preserved by a moderate annealed proton exchange process (5 min of proton exchange at 200°C, followed by 3 h annealing at 350°C), as revealed by measuring the extraordinary refractive index change and x ray rocking curve. A longer proton exchange time followed by stronger annealing would destroy the crystal structure and induce a high loss in the channel waveguides.