Abstract

The binary Ag⁺ - Na⁺ exchange has been widely used for fabrication of planar and channel waveguides generally using pure Ag NO₃ melt. The single-mode waveguides thus obtained [1] have large surface index change (Δ n ~ 0.1) , show large scattering losses, are incompatible with fiber-core dimensions, and their characteristics are not reproducible to desired accuracy. To circumvent these problems, low melt concentrations of silver (N_Ag ~ 10^-2 - 10^-3 molar fractions where N_Ag = C_Ag /(C_Ag + C_Na) and C_i is the absolute concentration of the cation i) in Na NO₃ bath have been used [2-4]. However, at low concentrations, Δn is a very strong function of N_Ag [4] saturating at ~ 0.1 for pure Ag NO₃ melts in most soda-lime glasses. Since for fiber compatible single-mode guides, N_Ag ~ 10^-4 MF, this strong dependence of Δn on N_Ag calls for a precise control of N_Ag and its understanding via a detailed study of the transport phenomena and thermodynamical equilibrium between the melt and glass phases. While such a study has been reported [2,3,5] for N_Ag > 10^-2, the results are not applicable to the low concentrations involved in the fabrication of single-mode waveguides. Besides, the results are likely to depend on the processing temperature and the substrate glass. Moreover, since the concentration profile depends on the interdiffusion coefficient which is concentration independent only if the silver concentration in glass is very small, a knowledge of its dependence on N_Ag will permit the solution of the diffusion equation with concentration-dependent interdiffusion coefficient [2].

© 1987 Optical Society of America