Nonlinear optical properties of different types of proton-exchanged LiNbO 3 and LiTaO 3 waveguides

Abstract

Reflection second-harmonic generation (SHG) from the polished waveguide end face is used to investigate the second-order nonlinear optical properties of different types of PE LiNbO 3 and LiTaO 3 waveguides in different H x Li 1-x NbO 3 and H x Li 1-x TaO 3 phases: as-exchanged and annealed proton-exchanged (APE) waveguides, high-temperature proton-exchanged (HTPE) waveguides, as well as reverse proton exchanged (RPE) waveguides. A detailed correlation is done between the nonlinear properties, the processing conditions, crystal structure parameters, the refractive index changes and the optical losses of the waveguides. The intensity of reflected SH signal, strongly reduced after the initial proton exchange in both LiNbO 3 and LiTaO 3 , is found to be restored and even increased after annealing. However, this apparent increase of the nonlinearity is accompanied by high scattering and a strong degradation of the quality of the SHG reflected beam in the region of initial step-like PE waveguides. The recently proposed HTPE technique has been shown to produce high-quality alpha-phase PE LiNbO 3 or PE LiTaO 3 waveguides with essentially undegraded nonlinear optical properties. It has been also shown that the nonlinear properties of annealed proton exchanged LiNbO 3 and LiTaO 3 waveguides can be effectively recovered by the reverse proton exchange technique.