Nondestructive concentration profiling of fiber optic preforms by analysis of Raman spectra

We have demonstrated that Raman scattering may be used to determine P and Ge concentration profiles of optical fiber preforms. For Ge, the technique has an absolute accuracy of about 0.5 mol.% Ge, and spatial resolution to 40 μm was achieved. When used in a nondestructive mode, spatial resolution of 100 μm is possible. The results clearly show that phosphorus as well as germanium burn out as the tube is collapsed, with the spatial extent of the phosphorus burnout region being about twice as great as that for germanium. Our measurements on a graded-P preform provide a determination of the Ge incorporation ratio across the preform with the surprising result that it increases toward the edge, where the temperature is higher and P content is lower.The spectra also provide clear evidence for two types of Ge sites in silica-germanium glasses, where one type of site, possibly associated with network defects, is predominantly occupied at low concentration. The 710 cm^?1 line associated with these defect sites is not detected when either P or B is present, and we speculate that this may be related to the dependence of the Ge incorporation ratio on the phosphorous concentration.