UV spectroscopy of optical fibers and preforms

Ultraviolet loss spectra of optical fibers and preforms have been measured over the wavelength range from 200 nm to 600 nm. The observed spectra consist of a number of well-known absorption bands, most of which have been associated with germanosilicate related defects. We have found that the size of the absorption peaks in fibers is typically many orders of magnitude smaller than the corresponding absorption peaks in the preforms from which the fibers were drawn. In particular, the 240 nm absorption band, so evident in preform spectra, is very much weaker in the fiber's spectrum. Exposure of the fibers to broad-band uv radiation increases the size of the absorption peak at 325 nm, and also produces new bands at 380 nm and 305 nm, not previously ascribed to any defects. Fibers exposed to high power uv-laser radiation to form Bragg reflection gratings in the infrared, show the appearance of a previously unassigned band at 300 nm. Applying the Kramers-Kronig transformation to the measured changes in the absorption spectrum of the fiber between 200 nm and 600 nm, gives a calculated index change which is not sufficient to explain the observed refractive index change of approximately equals 10-4.