Diffusion characteristics and waveguiding properties of proton‐exchanged and annealed LiNbO₃channel waveguides

A discussion of the modeling of the proton exchange fabrication process is given, including a brief review of results of analytical measurements of concentration and concentration/index relation, as well as some presentation of the speculations made as to mechanisms of resulting index profiles. Discussion is then given to modeling of the exchange process itself via the ion exchange equations. The concentration profiles of protons and lithium in proton-exchanged LiNbO3 crystals measured by secondary ion mass spectroscopy are then presented. The proton concentration profiles are found to be nearly rectangular in shape. The diffusion characteristics of the ion exchange process are empirically modeled by solving the ion exchange equations with concentration dependent self-diffusion coefficients. Secondary ion mass spectroscopy (SIMS) measurements on annealed H+:LiNbO3 samples show proton and lithium concentration profiles to be Gaussian in nature. The proton and lithium concentration profiles of annealed H+:LiNbO3 samples are fit to the solutions of diffusion equations obtained from standard thermal diffusion theory. The fit has resulted in values of diffusion coefficient for protons during annealing (Dp=0.77 μm2/h). Index profiles are then calculated from these equations and are found to agree with the profiles reported elsewhere. Optical mode measurements on channel waveguides fabricated under various diffusion conditions and annealed for various times, are carried out. The mode-tailoring ability of the proton exchange process is demonstrated by fabricating low-loss single-mode channel guides (λ=0.83 μm) having channel widths as large as 6 and as small as 2.5 μm, simply by changing the diffusion conditions. Some anomalous results obtained in waveguides fabricated under extreme conditions are discussed.