Raman investigation of the drawing effects on Ge-doped fibers

We have investigated the Raman activity of various germanosilicate fibers and their associated preforms. Our data indicate an enhancement in small rings' (3-member rings) concentration in the silica-based matrix of the fibers during the drawing process. The generation of such rings appears compatible with an increase of the sample density and fictive temperature. The data regarding the drawing effects on the fiber stress appear less clear, and it is possible to suggest that in some cases the drawing could lower the tensile stress. Finally we have also provided evidence that changing the drawing conditions within the usual range of application leads to no significant changes of the matrix structure as can be observed using Raman spectroscopy.

Research Highlights
► We report an experimental investigation regarding the structural modification induced by the drawing of the preforms to produce fibers. In particular, we have recorded micro-confocal Raman spectra in fibers and their original preforms. The investigated samples are Germanium doped fibers with different Ge profiles. The data shows that the drawing conditions induce the greater modifications in the fiber core central part, and that these modifications consist in a possible densification or, from another point of view, in a decrease of the starting tensile stress.
► Our data supply new information regarding the difference between fibers and preforms. They are interesting also because we have studied three different fibers produced starting from the same preform changing the drawing conditions inside the usual range used in the applicative fields.
► We have also provided evidence that changing the drawing conditions within the usual range of application leads to no significant changes of the matrix structure as it can be observed using Raman spectroscopy.