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

Abstract 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 LiNbO 3 , the index profile of APE waveguides in MgO:LiNbO 3 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:LiNbO 3 . The lattice distortion in the guiding layer of an as-exchanged waveguide in Y-cut MgO:LiNbO 3 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:LiNbO 3 than that in pure LiNbO 3 in our study. It was also found that HNbO 3 phase of high hydrogen concentration can be formed more easily in Y-cut MgO:LiNbO 3 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:LiNbO 3 waveguides.