Raman investigation of germanium- and phosphorus-doping effects on the structure of sol–gel silica-based optical fiber preforms.

This works reports the structural investigation of homogeneously doped sol–gel-derived silica optical fiber preforms. Crack-free germanosilicate (2.7 wt% Ge) and phosphosilicate (2.5 wt% P) glasses, suitable for optical fiber fabrication, have been prepared using the polymeric sol–gel route combined with the sol-doping technique. Space homogeneity of the doping oxide (GeO 2 or P 2 O 5 ) in the cylindrical preforms has been checked by chemical analysis. The structure of these glasses have been studied using Raman spectroscopy and compared to pure silica glasses. It is shown that phosphorus has a much more sensitive effect on the glassy structure than germanium, both at small and medium scales. Indeed, in the low-wavenumber region (7−100 cm −1 ), the phosphorus-doping was found to affect the intensity of the boson peak, suggesting an enhancement of heterogeneity at the nanometer scale. At a shorter scale, both germanium- and phosphorus-doping were found to reduce the number of three- and four-membered rings in the silica glass network. [ABSTRACT FROM AUTHOR]