Transparent silica gels have been fabricated for some time now with bulk densities ranging from less than 10% to near 100%. What is remarkable is that these materials in bulk form have high visible light transmission over this wide range of consolidation. The conditions under which the sol-gel process are carried out, the rates of chemical reactions, drying procedures and heat treatments all influence the glass structure at the submolecular level and all play a role in determining whether or not the gel is transparent.

Radial gradient index (GRIN) glass rods can be readily made by sol-gel methods. For the preparation of Ti02-Si02 GRIN glass, a sol-gel technique has been used in which a Ti02- Al203-Si02 gel is leached in acid to obtain a radial composition gradient of titania within the gel.?

Experimental measurement of modal attenuation in solution-derived Si02-Ti02 waveguides is compared to the total waveguide attenuation predicted by surface and volume scattering models. The surface scatter model utilizes measurements of the film index and thickness, and the rms roughness and correlation lengths (measured by Atomic Force Microscopy) of the film and substrate surfaces, to facilitate a realistic comparison of theoretical and experimental waveguide attenuation. Theoretical attenuation, calculated by means of a perturbation technique, in conjunction with a stationary phase method, is used to demonstrate the influence of the most important waveguide and microstructural parameters. The plots provide guidelines for acceptable surface roughness and bulk refractive index fluctuations to fabricate low- loss waveguides. Volume-induced scattering is shown to be the dominant loss mechanism in these waveguides.

Sol-Gel derived Si02-Ti02 films were densified by scanning a C02 laser beam across the surface. The effects of laser power and scanning rate on the morphology and optical properties of the coatings were studied. At low laser powers a depression, with rough sidewalls and approximately gaussian profile was formed in the coatings. As the laser power was increased the channels became wider and smoother, and flattened out on the bottom. At high laser powers a pair of ridges were formed in the bottom of the channel, apparently due to melting. The refractive index of the laser densified films was higher than for films fired in a furnace. Most laser densified areas supported waveguide modes but exhibited high loss. By processing at powers just below the damage threshold loss was reduced to a measurable value, 12.4 dB/cm for the TM0 mode for one sample. Annealing the films after laser processing also reduced the loss.

ORMOCER's with high optical performance and chemical stability are synthesized via the sol-gel process. Methacrylate group substituated silanes are copolymerized with methacrylic acid which acts as a chemical link between Zr as second inorganic network former. Chemical stable complex formation is proved by IR analysis and 13c NMR investigations. This material is laser patternable by the addition of a suitable photoinitiator and is characterized by high dielectric strength and low optical loss for applications in optics and non-linear optics.

Dried sol-gel silicate materials containing metal ions (2Th02-2M20-96Si02; where M = Na, K, or Cs) were ion-exchanged at 25 °C in selected regions using microelectronic masking methods and ionic solutions. After heat densification, the compositions and properties of the exchanged and unexchanged regions of the gel were investigated by microscopy, SEM-EDAX, and micro- Raman spectroscopy. Comparison of the spectra with those of melt-quench glasses of the same stoichiometries, exchanged identically, shows the effectiveness of the gel exchange.

The application of solution derived Si02-Ti02 thin films for use in integrated optics has been of considerable interest in recent years. Optical filters,waveguides and chemical sensors5 have been fabricated by a variety of researchers, using several different chemical routes to synthesize the deposition solution.

Three-layered thin-film assemblies of polymethysiloxane (PMSO) glass were prepared by fast reaction of methyltrimethoxysilane with water, using stoichiometric water-to-siloxane ratios, at elevated temperatures. Following hydrolysis, partial polymerization and distilling-out of the produced methanol, the viscous polymer of each layer was spin-cast onto the support. The layered assemblies comprised a bottom and top layers made of pure PMSO, and a middle layer embodying the laser-dye. A specific tuning of the synthetic route was maintained to ensure proper adherence between the layers, since PMSO surface rearranges itself to become hydrophilic or hydrophobic, depending on the environment it is facing. An additional tuning facilitated molecular dispersion of dye in the glass, even at high concentrations (>10-2 M). The absorbance and fluorescence spectra of the thus-embodied dyes showed typical features, while their absorbance - fluorescence gap was substantially narrowed. The indices of refraction in the three-layered assemblies were lower in the base and top layers, and a little higher in the dye-loaded middle layer. Fluorescence line-narrowing of the emission from the glass-edge could be observed upon excitation of the dye surface with an excimer laser at 308 nm. This lasing of the sol-gel glass- film was characterized by line width narrowing by 25-40% and an angular width of ca. 6 deg. The quickness and simplicity of preparing multilayered assemblies of varying indices of refraction by this method and its capability to maintain high concentrations of discrete dye molecules (>10-2 M) in the middle glass-layer offer a promising route for two-dimensional laser systems and for non-linear optics applications in general.

The irradiation of optical fibers by intense optical fields from visible or ultraviolet laser sources creates color centers within the glass fiber. Such laser light irradiation usually has the detrimental effect of increasing the transmission loss of the optical fibers particularly in the visible spectral region. A concomitant effect of the light irradiation is that the refractive index of the glass is permanently changed even at wavelengths much longer than the wavelength of the irradiating light. This latter effect termed fiber photosensitivity is beneficial in that it provides a versatile means for fabricating periodic index gratings in the cores of optical fibers. Since the physical processes underlying fiber photosensitivity are not well understood, a purpose of this paper is demonstrate the importance of the phenomena in order to stimulate further research on the origin of the effect and developing new photosensitive fiber materials. The paper reviews briefly the phenomena of photosensitivity in glass fibers with germanium dopant in the fiber core. The methods used for detecting and characterizing fiber photosensitivity are applied to a new photosensitive fiber - Eu2+:A1203 - doped - core fiber. This fiber was manufactured at Brown University using MCVD with a novel aerosol delivery system for the transport of low pressure precursors. This is the first fiber reported that is free of germanium dopant and also exhibits fiber photosensitivity.

A perturbation analysis of stimulated backward Brillouin scattering (SBBS) in single mode optical fibers is presented. The nonlinear interaction between the pump and Stokes waves, optical loss and Bragg diffraction have been all considered. The analytic expressions of the threshold, output Stokes intensity, efficiency for the SBBS fiber amplifiers and lasers are derived. We show that the stronger Bragg diffraction will increase the Brillouin diffraction efficiency and decrease the threshold of SBBS. Effects of acoustic guidance conditions of optical fibers on the above SBBS parameters will be discussed.

Acoustic characterization of doped silica glasses with a Ge02, P205, F, Ti02, A1203 or B203 dopant having different concentrations is presented. The quantitative measurements are performed by a 225 MHz line-focus-beam scanning acoustic microscope. The acoustic velocity variation due to different dopant concentrations for each dopant is given. It has been found that the A1203 dopant increases but the other dopants decrease the acoustic velocity as compared to that of the pure fused silica. The fiber preforms having step and graded refractive index profiles also show step and graded acoustic velocity profiles respectively. We have also found that the acoustic velocity is more sensitive to the dopant concentration as compared with that of optical refractive index.

Air flow induced by the motion of the glass and by natural convection affects the heating and cooling of the material during the optical fiber drawing process. This flow is also responsible for impurity transfer on the surface of the fiber during processing. This paper presents results of a numerical simulation of the convective air currents in both the heating region (furnace) and the cooling region of the drawing process. For the heating region the case of an empty furnace, a furnace with a stationary glass preform and a furnace with a moving preform were examined. For the cooling region studies were performed for the case of pure forced, pure natural and mixed convection. Air velocity profiles are presented in graphical form.

Alteration of absorption spectra of fused silica fibers under delivery of high power UV laser radiation (4th harmonic of Nd:YAG laser, wavelength 266 nm) was studied in comparison with their photoluminescence properties. Tested fibers were produced by various technologies based on PMCVD methods. The nature of defects responsible for UV absorption in fibers and mechanisms of their photogeneration are discussed

Spectral changes induced by UV irradiation were studied in a variety of glasses. These include photosensitive glasses in which colloidal silver is formed, glasses in which the formation of V+ ions produces violet color, and glasses in which irradiation within a charge transfer band leads to the formation of Fe+2 ions and holes in the valence band. In all of these glasses, hole trapping by non-bridging oxygen atoms is required for the occurence of spectral changes.

The thermal relaxation of the tellurium halide based glasses has been studied at different temperatures. It was found that some of these glasses can be appreciably relaxed at room temperature and the glass transition temperature increases significantly. It is always possible to increase Tg of all these glasses by appropriate heat treatment. The fictive temperatures as a function of annealing time have been determined for several glasses. This study allows to optimize the heat treatment.

A new type of glases, either totally inorganic or composites of organic and inorganic materials are doped by silver aggregates, photostable organic dyes and semiconductor quantum dots. The experimental technique for preparation of such materials is outlined. Spectroscopic properties of these materials are presented and their application for fiber waveguides, tunable lasers and nonlinear optics materials demonstrated.

A thin film of SiO coated on abraded Si02 glass was found to improve the mechanical fatigue resistance of the glass in water. The improvement was attributed to an increase in the number of reaction sites on the coated surface, which convert the mobile H2O molecule to immobile SiOH and the consequent reduction of the effective diffusion rate of water into the glass.

Here are reported the results of an investigation on luminescence of Cu+ in borate and phosphate glasses. The former system exhibits a unique very broad and strong emission covering the whole visible region so that when excited in the ultra-violet region, the emission appears as a white fluorescence. The latter compound shows two different blue and red fluorescence bands.

The role and benefits of using statistical design in experiments for the development and optimization of glass and ceramic products are examined. It is shown that considerable benefits can be derived by following an appropriate design in which a number of factors (processing parameters) are varied simultaneously while the system response such modulus of rupture, toughness, optical loss, etc is monitored. If it is assumed that ccnplex factor interactions (e.g., having greater than four factors) can be ignored, it is further shown that essentially the same conclusions can be drawn from an examination of only a fraction of the whole factorial matrix. As a consequence, significantly less trials are required to determine (screen and rank) the various single- and multi-factor effects (having up to four factors) on the measured property. Examples drawn from recent work in materials development are presented.

The Gas Film Levitation (GFL) technique, based on aerostatic lubrication on a porous membrane, allows containerless processing and shaping of large material samples. This method is applied to the preparation of ZBLAN fluoride glasses, which are promising candidates for low-loss infrared fibre applications. Fluoride glass samples of high optical quality are obtained starting directly from a powder mixture of the different fluorides. Laser Scanning Tomography and light scattering measurements are used to characterize the as-made samples.

For the Li3Sc2-xFex(PO4)3 superionic solid solutions it is shown that dynamic disorder of lithium ions observing at room temperature transforms into static disorder under cooling down to the liquid helium temperature. So, a glassy state in the lithium sublattice is suggested by analogy with "protonic glasses" in the ADP-RDP, KDP-ADP, and similar systems.

Lithium phosphate glasses with different iron oxide contents were prepared using a rapid quenching technique and their glass transition temperatures (Tg), densities (d) and refractive indexes (n) were measured.X-ray photoelectron spectroscopy(XPS), infrared spectroscopy(IR) and Mossbauer spectroscopy have been used to study structure of the glasses and oxidation states of iron. These results indicate that both the Fe2+ and Fe3+ ions are present in the phosphate glasses, and that Fe2+ ions are coordinated with both four and six oxygen, but Fe2+ ions are only in six coordination, and that the physical and optical properties depended on the Fe2+/Fe ratio and Fe3+(4-coordinated)/Fe ratio. The XPS spectra and Mossbauer effect measurements showed that an increase in the Fe203 content causes a decrease of Fe2+/Fe ratio and an increase Fe3+(4-coordinated)/Fe ratio. The infrared measurements showed that an increase in the Fe203 content causes a change in coordination from Fe06 to FeO4.

AgCl/AgBr crystallites with diameters of about 40 nm were synthesized within a sodium alumo borosilicate glass matrix by the sol-gel process. Ag was introduced into the sol through the addition of an ethanolic AgN03 solution. After gelation, AgClxBr1- x was generated by treating the gel with NaCl and CuBr solutions. The gel can be densified to a glassy powder that shows a darkening effect with a saturation behaviour after UV irradiation and recovers at elevated temperatures (> 150 °C) . The best recovery kinetics were obtained for a silver halide content of about 12 mole/g.

X-ray amorphous, calcium aluminate glass fibers can be made by one of three melt processing methods. This review compares products and processes, and points to a potentially promising future. Selected amorphous fibers with 23-47% A1203 have high melt viscosities (>100 Pa-s) and can either be drawn from supercooled melts or spun above the liquidus through an orifice. The vast majority of fibers, especially those with 50-100% A1203 have low viscosities (<1 Pa-s) and can only be made by inviscid melt spinning, a process whereby the molten low viscosity jet is ejected through an orifice above the liquidus, and chemically stabilized in a reactive environment. As-spun fibers with 50-81% A1203 were x-ray amorphous and strong, but polycrystalline and weak with 82-100% A1203. Fibers by either process were aimed at structural uses (fiber reinforced composites). Recent work shows that x-ray amorphous fibers have sapphire-like infrared transmission spectra and have greater potential in optical than in structural applications. Thus new non-silica optical fibers can now be explored by any of the three processes; all promise to afford lower cost fibers at higher production rates than possible with slow processes (e.g., single crystal fiber growth) yielding costly specialty non-silica optical fibers (e.g., sapphire).

An optical fiber whose core is an aluminosilicate glass doped with Eu+2 and Eu+3 has been made using MC VD (Modified Chemical Vapor Deposition) and a novel aerosol delivery technique developed at Brown University. The UV- visible absorption of the preform has been studied in 200-700 nm range as well as the excitation and emission spectra between 250 and 720 nm. Three broad UV absorption bands with maxima at 255, 302 and 316 nm have been observed. The 302 nm band is attributed to the energy transfer from Eu+2 to Eu+3, while the 255 and 316 nm bands are correlated with the 4f7->4f65d1(t2g) and 4f7->4f65d1(eg) transition of Eu+2. For Eu+2 emission, a linear relation between excitation and maximum emission wavelength has been observed as the excitation wavelength increases beyond 375 nm.