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 (NAg ~ 10-2 - 10-3 molar fractions where NAg = CAg /(CAg + CNa) and Ci 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 NAg [4] saturating at ~ 0.1 for pure Ag NO3 melts in most soda-lime glasses. Since for fiber compatible single-mode guides, NAg ~ 10-4 MF, this strong dependence of Δn on NAg calls for a precise control of NAg 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 NAg > 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 NAg will permit the solution of the diffusion equation with concentration-dependent interdiffusion coefficient [2].
© 1987 Optical Society of America
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