UV INDUCED DENSIFICATION DURING BRAGG GRATING INSCRIPTION GE-SIO2 PREFORMS

The only investigation of a Bragg grating in Ge:SiO2 in the direct spa ce known to date is a view from an optical microscope. Here, we show s urface level modulations associated with the writing of the grating. O ur conclusion is that a densification takes place. We estimate the cor responding peak to peak change of the refractive index to be 1.4-1.7 X 10(-4) whether a pulsed or a CW irradiation mode is used. Compared to the refractive index change obtained by optical measurement, this acc ounts for a nonnegligible part (7% for pulsed and 26% for CW irradiati on). We discuss the possible application of the observations to fiber and the differences in photo-chemical reactions according to the irrad iation mode.