Observations of changes in UV absorption bands of singlemode germanosilicate core optical fibres 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.