Planar, buried, ion-exchanged glass waveguides: Diffusion characteristics

A detailed theoretical and experimental study of Ag+-Na+ exchange in soda-lime silicate glasses in a molten bath containing a mixture of NaNO3and Ag+ is presented. With no applied field, concentration profiles (and therefore, the index profiles for low concentrations) are given by complementary error function. The estimated value for the self-diffusion coefficient of Ag+ is 0.133 μm²/min for low concentrations and it monotonically increases with the surface concentration C0until it saturates at about 0.3 μm²/min for MF. However, square root dependence of diffusion depth with time seems to be independent of the C0. Presence of an external field causes the effective depth of diffusion to increase. In fact, for large fields, the profile can accurately be described by where where μ is the ionic mobility of Ag⁺in glass. We define a new diffusion depth as the distance from the surface to the concentration point, and for large fields, varies linearly with and . Experimental results yielded a value of 15.55 μm²/ min forμ. As before, square root dependence of with and the linear variation of versus C0for MF, with saturating for MF, were observed in the case of field assisted diffusion. A two-step process, where a surface waveguide formed in the first step with either equal to zero or some finite value, is modified by performing a second diffusion in pure sodium nitrate to produce a buried, symmetrical fiber-like profile. This process is also studied in detail.