OBSERVATIONS OF CHANGES IN UV ABSORPTION-BANDS OF SINGLEMODE GERMANOSILICATE CORE OPTICAL FIBERS ON WRITING AND THERMALLY ERASING REFRACTIVE-INDEX GRATINGS

Refractive index gratings have been formed in singlemode germanosilicate core fibres via UV side-writing at 242 nm. Direct measurements of the fibre core UV transmission spectrum show a reduction in the intensity of the GeO band at 241 nm and the growth of a short wavelength absorption edge whose origin appears to lie below 200 nm. Thermal erasure of the grating results in a recovery of the 241 nm GeO band and the disappearance of the short wavelength edge absorption.