Gradient-Index Profile Control by Ion Exchange in Glass.
Abstract
Gradient-Index (GRIN) parameters are useful in the design of optical imaging systems, but fabrication technology has limited the application of these designs in the past. The control of large geometry index profiles using ion exchange in single alkali glass is investigated, with special attention paid to quantitative prediction of index profiles. The study focuses on a simple aluminosilicate glass and silver for sodium ion exchange from various chloride salt melts. The research strategy consists of two approaches. The first is characterization of the glass and salt melts and the resulting dopant distributions using well known methods. These include radiotracer diffusions, microprobe and wet chemical composition analysis and interferometry of the final index profiles. In the second approach, the index profiles are measured as they form during diffusion using a new instrument, an oven interferometer, designed and built for this purpose. Useful methods for controlling the maximum change in refractive index (Deltan) and the profile distribution were discovered. Additives in the salt melt can be used to reduce fabrication costs while maintaining high Deltan's. Various methods, including anneals and field-assisted ion exchange, were shown to be useful in controlling the dopant distribution. The results of the characterization of the glass and salts showed that the ion exchange model works for small index changes (Delta{n } < 0.01 at 0.6471 μm). However, the models for predicting both the concentration and index profiles do not apply for larger dopant levels. The oven interferometer can be used to monitor the formation of the index profiles. This instrument has a phase resolution of lambda/50 and can measure fringe densities up to 36 lines/mm. Evolution of the index profile under various fabrication conditions is demonstrated. GRIN materials must be made under conditions which lead to high Deltan's and large diffusion depths. The current single alkali glass model does not apply to these conditions. It is shown that realtime measurement of the index profile evolution is useful for process control and improving the understanding of ion exchange in glass.
- Publication:
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Ph.D. Thesis
- Pub Date:
- September 1987
- Bibcode:
- 1987PhDT.......108H
- Keywords:
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- Physics: Optics