Formation and characteristics of graded-index optical waveguides buried in glass

Abstract

Low-loss optical waveguides have been prepared through thermally induced ion exchange, where sodium ions in glass are replaced by silver ions from an unstirred silver nitrate melt. The index distributions of the inhomogeneous guides produced in the glass are deduced by analysing the measured mode spectra with the WKB-method. The increase in the refractive index caused by the presence of the silver is found to have a nearly Gaussian form with a peak of about 0.082. The index profile is correlated to the two parameters of the preparation process, the diffusion time and the operating temperature. In a typical case (9 hrs and 221°C) the index takes on its maximum value 2.6 μm below the surface of the glass and one obtains an 8-mode guide with five modes buried within the glass and three modes extending to the air-glass interface.A single-mode guide is formed in less than 5 min.

The measurements yield an activation energy for the diffusion of Ag⁺ in glass of 8.4·10⁴J/mole and a diffusion coefficient at 221°C of 0.67·10⁻¹² cm²/s, in good agreement with the experimental results reported by other authors. The present observations suggest that a potential gradient is set up in the glass when silver diffuses into it so that the silver ions drift into the glass with a velocity of about 0.3μm/hr.