TECHNIQUE DEVELOPED FOR CHARACTERIZATION OF PHOTOSENSITIVE GLASS.

Ultraviolet light can induce a permanent change in the refractive index of a glass material. While this UV photosensitivity has a detrimental effect on photolithography optics, it is very useful in fabricating integrated optic and fiber Bragg grating devices. Fiber Bragg gratings are wavelength-selective reflectors that can be written into the core of optical fiber. They are very useful as wavelength filters and dispersion compensators in the new wavelength division multiplexed optical fiber communication systems and also make excellent strain sensors that can be networked to obtain distributed strain measurements of large structures, such as bridges and ships. Despite the development of numerous techniques to control the phenomenon, a fundamental understanding of UV photosensitivity remains incomplete.

To facilitate a study of the formation mechanisms, NIST scientists developed a technique for measuring UV-induced index change using a Michelson interferometer. The technique was applied to compositions of germanium-doped glass that had been exposed to periodic spatial patterns of ultraviolet light. The resulting index pattern in the samples was passed through one arm of an interferometer, which produced a fluctuation in amplitude response due to the optical path difference. The measured fluctuations were interpreted as index changes based on the phase relationship of the interfering beams at the measurement wavelength. Index changes as small as (2 [+ or -] 0.5) X [10.sup.-5] have been measured, with the potential for much higher sensitivities. With this measurement technique, the effect of glass composition, fabrication processes, and irradiation conditions on photosensitivity can be studied quantitatively.