Optical and structural characterization of annealed proton exchange waveguides in Y-cut MgO:LiNbO3

In this paper, studies are presented on proton exchanged (PE) and annealed proton exchanged (APE) waveguides in Y-cut magnesium doped lithium niobate based on our systematic fabrication and characterizations. The analysis is focused on the optical and structural properties from the measurements of prism coupler, X-ray diffraction and infrared absorption. The evolution of the index profile with annealing is studied. Unlike the Gaussian index profile for the APE waveguides in pure LiNbO3, the index profile of APE waveguides in MgO:LiNbO3 still keeps the characteristics of step-like profile though with a long index tail. From the measurement results of X-ray diffraction, the strain induced by proton exchange depends on the exchange condition in Y-cut MgO:LiNbO3. The lattice distortion in the guiding layer of an as-exchanged waveguide in Y-cut MgO:LiNbO3 can be up to +0.73%, which can be restored to +0.20% by annealing for a short time. The proton exchange causes more serious non-uniformity of the strain in Y-cut MgO:LiNbO3 than that in pure LiNbO3 in our study. It was also found that HNbO3 phase of high hydrogen concentration can be formed more easily in Y-cut MgO:LiNbO3 by proton exchange from the analysis of 3300 cm−1 peak in the IR spectra. The intensity of the infrared absorption peak at 3500 cm−1 decreases and the broad peak at 3300 cm−1 disappears after annealing in Y-cut MgO:LiNbO3 waveguides.