Relationship between propagation loss and crystallinity in proton-exchanged and annealed LiNbO₃ optical waveguides

The propagation loss of proton-exchanged and annealed LiNbO₃ optical waveguides was studied in relation to the change in the crystal phase. With annealing at relatively low temperatures (≤325°C) or for a short time (at 375 or 400°C, ≤15 min), the propagation loss increases. Annealing at relatively high temperatures (375 to 400°C) for a longer time (30 to 45 min) greatly reduces the propagation loss. The reasons for the increase in propagation loss with insufficient annealing is the deterioration of crystallinity due to changes in the crystal phase and the coexistence of two different crystal phases.