High-index proton-exchanged MgO:LiNbO₃ optical waveguides using adipic acid

We report the fabrication and characterization of proton-exchanged optical waveguides in z-cut MgO:LiNbO₃ using adipic acid as a new proton source. These waveguides exhibit propagation losses of less than 1 dB/cm, a linear-step index profile, and a surface index increase of 0.149 measured at 0.633 μm wavelength. This is the largest surface index change reported for proton-exchanged MgO:LiNbO₃ waveguides. The diffusion parameters were characterized optically, and the diffusion constant D₀ and the activation energy Q were found to be 5.40×10¹⁰ μm ²/h and 103.27 kJ/mol, respectively. The diffusion rate is slower than that of the popular phosphoric acid.