Abstract

The utility of Raman spectroscopy as a means of characterizing the properties of pure and doped fused silica has been investigated. Laser-Raman spectra were obtained by forward scattering from solid optical fibers ~35 to 85 m in length using 514.5 nm excitation with an "image slicer" and a Cary model 81 instrument. Clad and unclad fibers of fused silica and doped fibers having SiO<sub>2</sub>-GeO<sub>2</sub> and SiO<sub>2</sub>-GeO<sub>2</sub>-B<sub>2</sub>O<sub>2</sub> cores were examined. Raman spectra were also obtained from bulk samples of glasses, including pure GeO<sub>2</sub>, pure B<sub>2</sub>O<sub>3</sub>, and various compositions of SiO<sub>2</sub>-GeO<sub>2</sub>, SiO<sub>2</sub>-B<sub>2</sub>O<sub>3</sub>, and SiO<sub>2</sub>-GeO<sub>2</sub>-B<sub>2</sub>O<sub>3</sub>. The addition of dopants to fused silica was found to alter the Raman spectrum both by the appearance of new bands, roughly proportional to dopant concentration and not common either to the fused silica or to the dopant alone, and by the marked alteration of other Raman bands, which is indicative of changes in the local intermolecular order. Thus, addition of GeO<sub>2</sub> produces new Raman bands at ~675 and ~1000 cm<sup>−1</sup>; and of B<sub>2</sub>O<sub>3</sub>, new bands at ~940 and ~1350 cm<sup>−1</sup>. Addition of GeO<sub>2</sub> and/or B<sub>2</sub>O<sub>3</sub> weakens the relatively sharp Raman lines near 485 and 600 cm<sup>−1</sup> (and a similar but small effect was also noted with increasing OH content). GeO<sub>2</sub> and B<sub>2</sub>O<sub>3</sub> together also produce observable narrowing of the broad intense 440 cm<sup>−1</sup> Raman contour. These spectral effects are interpreted, respectively, in terms of a decrease in the concentrations of [Equation] and [Equation] defects produced by dopant addition and of a concomitant reordering of the silica structure. Raman spectroscopy thus appears to be a useful optical technique for elucidating the properties of dopants that have been especially chosen for good optical transmission and hence are not easily detectable by absorption measurements.

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