Proton-Exchanged Optical Waveguides in Z-cut LiNb0₃ Using Phosphoric Acid

Proton-exchanged z-cut LiNbO₃ planar waveguides formed using phosphoric acid were characterized optically. The refractive index profile and the diffusion parameters were studied systematically. These waveguides have propagation losses less than 1 dB/cm, and exhibit properties which are different from those obtained using the popular benzoic acid. The index profile is not a simple step function, and can be modelled accurately by a polynomial expression. A maximum surface index increase of 0.145 was measured at 0.633-μm wavelength. The diffusion constant D_o and the activation energy Q for the proton-exchange process using this acid were found to be 6.43 x 10⁸ μm²/h and 82.91 kj/mol, respectively. The annealing properties of these waveguides were also established, and the effects of annealing on surface index change and waveguide depth increase were found to follow a power-law relationship. © 1993 IEEE