ULTRAVIOLET-ABSORPTION IN MODIFIED CHEMICAL-VAPOR-DEPOSITION PREFORMS

Using a simple and effective technique, we have systematically studied the UV absorption in a number of fiber preforms made by modified chemical vapor deposition. The 242-nm absorption band due to geranium-related oxygen-deficient centers (GODC's) is found to be approximately linearly dependent on germania concentration and, after a small initial increase, is almost constant throughout the preform collapse process. However, we found that the band can be increased by 700% when a reducing atmosphere (He + 1%D-2) is substituted for O-2. Several codopants have been evaluated to determine their effect on the GODC concentration. Phosphorus, boron, aluminum, and fluorine all reduce the 242-nm band; phosphorus causes the largest change.