Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz.

The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C.

Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes.

In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm−1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.

Raman and infrared spectra on silica gel evolving toward glass

A Raman spectroscopic study of glasses along the joins silica-calcium aluminate, silica-sodium aluminate, and silica-potassium aluminate

Procedures are presented for estimating critical powers for nonlinear optical processes in single-mode fiber transmission systems. Crosstalk due to Raman gain in multiplexed systems can appear at powers of a few mW. The effects of self-phase modulation and stimulated Brillouin scattering can appear around 100 mW while typical stimulated Raman threshold powers are a few watts.

Nonlinearity in fiber transmission

Solar furnace melting and fast-quench techniques have been used to prepare SiO2Al2O3 glasses to high alumina content (near 60 mol% Al2O3), which have been studied by Raman spectroscopy. These spectra may not be simply interpreted. The structures of crystalline compounds in the SiO2Al2O3 system are discussed in relation to their vibrational spectra. On the basis of this discussion and other considerations, a structural model for the silica-alumina glass system is proposed, which is consistent with the stable or metastable immiscibility suggested along this join. The essential features of this model include a modified silica structure at low alumina content, and “structure-broken” regions at high alumina compositions, with silicon in tetrahedral coordination, but aluminium assuming a variety of bonding geometries. These are proposed to include aluminate tetrahedra with higher polymerization than simple corner-sharing, and less well-defined polyhedra of higher average coordination number.

The structures and vibrational spectra of crystals and glasses in the silica-alumina system

Description d'un systeme dans lequel la lumiere laser et la diffusion Raman ont pour support des fibres optiques. Cette technique permet des analyses dans des environnements hostiles

Versatile, efficient Raman sampling with fiber optics

Intense spontaneous Raman radiation has been obtained from C6H6 and C2Cl, by passing the focused beam from an argon ion laser (4880 A, 5 to 250 mW) through filled hollow fused quartz optical fibers having core diameters of ∼75 μm and lengths from 10 to 25 meters. Spectral intensifications by factors of ∼102 to 103 compared to conventional sample techniques have been obtained with the fiber optics method. Raman spectra were recorded with a Cary model 81 spectrophotometer by collecting the divergent radiation emitted from the end of the optical fiber at the focus of the "image slicer." However, the intensification method is general and may be employed with any double or triple monochromator by placing the fiber end near the entrance slit. For C6H6 at least 33 Raman components were observed in the 3300 > Δν> 1650 cm−1 region including shifts due to overtones and combinations, by using a slit-width of 2 cm−1 with a 15 m fiber length and a laser power of ∼130 mW. The Raman spectra from C6H6 were found to be in good agreement with those recently reported by Schrotter and Bofilias. For C2Cl4 intense spectra were also obtained using 2 cm−1 slit-widths with fiber lengths of 25 m and power levels to 250 mW. These spectra augment those obtained earlier by Wittek and indicate five newly observed Raman and infrared coincidences that in several cases may result from the breakdown of D2h selection rules. Details of the fiber optics Raman technique are described.

Intensification of Spontaneous Raman Spectra by Use of Liquid Core Optical Fibers

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 SiO2-GeO2 and SiO2-GeO2-B2O2 cores were examined. Raman spectra were also obtained from bulk samples of glasses, including pure GeO2, pure B2O3, and various compositions of SiO2-GeO2, SiO2-B2O3, and SiO2-GeO2-B2O3. 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 GeO2 produces new Raman bands at ~675 and ~1000 cm−1; and of B2O3, new bands at ~940 and ~1350 cm−1. Addition of GeO2 and/or B2O3 weakens the relatively sharp Raman lines near 485 and 600 cm−1 (and a similar but small effect was also noted with increasing OH content). GeO2 and B2O3 together also produce observable narrowing of the broad intense 440 cm−1 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.

J. Stone

Papers

Intensification of Spontaneous Raman Spectra by Use of Liquid Core Optical Fibers

Raman Spectral Characterization of Pure and Doped Fused Silica Optical Fibers