Bragg gratings in defect-free germanium-doped optical fibers.

Bragg gratings have been written in germanium-doped optical fibers that have been treated to remove the UV absorption bands associated with oxygen-deficient defects. When one is using high-intensity 193-nm light from an ArF excimer laser to fabricate the gratings, the refractive index increases and the grating transmission spectra are similar to those obtained in standard (untreated) fiber

The microstructure of borosilicate glasses containing elongated and oriented phase-separated crystalline particles

The microstructure of phase-separated and stretched silver halide particles, embedded in an aluminoborosilicate glass, is studied by small angle X-ray scattering. The data are quantitatively analysed, using a model of prolate ellipsoids of revolution generated by the elongation of a distribution of spheres. The average particle is 22 +/- 3 x 160 +/- 20 nm(2), but the distribution is very much skewed so that a fraction of much longer particles is also present. Chemical reduction in hydrogen causes the formation of metal particles inside the rigid glass cavities formed during drawing. Two thirds of the volume of these cavities remains empty, due to the loss of the halide atoms and to the higher density of the silver crystals. The longer particles are lost, each one giving rise to two or three smaller ones. These data are compared with transmission electron microscopy (TEM) results

TWO-DIMENSIONAL SMALL-ANGLE X-RAY SCATTERING INVESTIGATION OF STRETCHED BOROSILICATE GLASSES

A two-dimensional fit of a suitable model for interpreting small-angle X-ray scattering (SAXS) has been shown to be a valuable tool in obtaining quantitative microstructural information. The model is based on the hypothesis that dilute ellipsoidal particles are arranged parallel to each other. The method has been applied to two glasses containing oriented particles. The investigated materials are both alkali aluminoborosilicate glasses, thermally treated and redrawn at a temperature above their softening point. The Ag(Cl,Br) crystalline droplets, formed during the preliminary thermal treatment, assume, after drawing, a cigar-like shape, oriented in the stretching direction, and give the material birefringence properties. The volumetric particle distribution has an average of 22 x 70 nm and is skewed up to length values of about 1200 nm. The composition of the solid solution, determined by X-ray diffraction (XRD), is Ag(Cl-0.35, Br-0.65) When the Ag(Cl,Br) particles on the surface are chemically reduced to Ag, a material with polarizing properties is obtained (Polarcor(TM)). For this sample, two distributions of particles have been found: one with an average of Is x 230 nm and skewed up to 600 nm, and one, very sharp, of much shorter particles (14 x 30 nm

Redrawn phase-separated Borosilicate Glasses: a TEM Investigation

Two alkali aluminoborosilicate glasses containing oriented crystalline particles have been investigated by Transmission Electron Microscopy (TEM). Both materials have been thermally treated in order to phase-separate crystalline droplets and have been then redrawn at a temperature above their softening point to elongate and orient them. In one sample the crystalline phase is a Ag(Cl, Br) solid solution, which gives the material birefringence properties, while in the other sample the particles on the surface have been chemically reduced to Ag, giving the material polarizing properties (Polarcor (TM)). Most of the observed needle-like particles are built up by a small number of well grown crystallites often separated by voids. Both the crystallites and the voids occupy the whole width of the particles and can assume very different lengths. Crystallites inside a particle have different orientations