Rational design and optimization for the next generation of fiber optics requires fundamental knowledge of the processes at each step of production [1], not the least of which is the formation and deposition of the glass precursor particles. Currently, gas-phase synthesis dominates the industry, owing, in part, to the high purity possible for gaseous reagents. However, the production engineer has relatively little control over the microstructure of the boule from which the fiber is drawn because many complex mechanisms take part in the growth and thermophoretic deposition of the precursors. Although it is desirable, for example, to obtain a porous boule in order to facilitate the removal of deleterious hydroxyls, connected porosity is by no means guaranteed. The successful attainment of high porosity depends on a number of variables such as the size distribution [2], internal structure, shape distribution, viscosity, and surface tension of the particles at the instant of deposition.

Energetics and structure of germania doped silica prepared from high temperature melts and by flame hydrolysis were investigated using transposed temperature drop and solution calorimetry. Heat contents (H973-H298) are similar for first and second drops for the flame hydrolysis preforms reflecting the effect of temperature of deposition (1073 K) in creating a relaxed structure which is not easily rearranged. Heats of solution, at 973 K in molten lead borate, of fused glasses along the SiO2-GeO2 binary show ideal heats of mixing between the endmembers. This'indicates that fused glasses behave in a simple manner, with ideal substitution of Ge for Si in the tetrahedral framework. Flame hydrolysis samples (preform and annealed glasses) also show ideal heats of mixing. This suggests that the flame hydrolysis materials are energetically very similar to their bulk-melted counterparts. This insensitivity of energetics to preparation conditions is consistent with our previous studies on silica prepared at various temperatures [1].

The sol-gel process is a versatile process for the preparation of many types of glass and ceramics. This includes optical fibers. In this paper, the general principles of sol-gel processing are described first. This is followed by details of the specific methods that can be used for the fabrication of optical fibers. Finally, some conclusions are drawn as to the commercial potential of the various methods.

A feasibility study of continuous casting of particulate silica sols has been conducted. The process consists in measured pumping of the sol and a gelling agent into a heated mold where the sol gels. The gel is continuously pushed out of the cylindrical mold by the pressure of the pumped sol. Gel rods of 2.5 cm diameter, up to 150 cm long, were prepared by this method. Temperature dependence of the gelation was studied, and activation energies of the process were determined.

The nonlinear stability analysis of viscoelastic fibers is presented. The molten fiber is modeled as a Maxwellian viscoelastic fluid and the zeroth order equations governing its behavior given. Linear stability analysis is performed to determine the influence of winder speed and impedance as well as viscosity and elasticity. The results of numerical solution of the nonlinear equations are given.

A new general method for determining the local dimensionless heat convection coefficient (Nusselt number) and the temperature distribution in thin and thick optical fibers during the drawing process was developed. The axial heat transfer by conduction was included in the analysis as an addition to previously developed models. The developed model can serve as a general thermal solver for thin (<200μm in dia.) and thick (>200μm in dia.) fibers. The general method was compared with existing methods and it was found to be in agreement in the case of thin fiber analysis but not in the case of thick fiber analysis, where axial conduction effects are significant. Results of a parametric study to examine the effects of diameter of fiber and drawing speed on the temperature distribution are presented.

A novel apparatus for accelerated cooling of optical fiber has been used at different fiber speeds using Nitrogen and Helium as the cooling gases. The gas flow was counter to the direction of the fiber motion inside a small diameter tube. The experimental results show significant improvement over natural cooling, as well as over available transverse cooling.

Carbon coated optical fibers have recently been shown to have excellent resistance to both static fatigue and hydrogen induced losses. The deposition technique used to form the carbon coating strongly affects the coating's ability to resist these degradation mechanisms. The system developed by AT&T utilizes an atmospheric CVD chamber in which a hydrocarbon has is pyrolyzed on the fiber surface. The heat retained in the fiber from the fiber forming process is used to drive the reaction, and high draw speeds are typically used to attain the ˜900°C temperature required to deposit the hermetic form of the carbon coating. Deposition rates of ˜1μm/sec are required to produce the ˜500 Å coating.

Electronic structure calculations were performed on model silicate complexes in the presence of a perturbing dielectric field. Solid state dielectric effects were represented by an electrostatic representation of surrounding molecular subunits, which were developed from quantum mechanical electron density distributions. Only minor changes in the molecular geometry of the surface silicate model were noted; however, significant intramolecular charge redistribution and intermolecular interaction energy changes occur in the presence of the perturbing field. Molecular probes of surface absorption show a differential stabilization of donor and acceptor interactions for hydrogen bonding species.

The use of composite materials for aerospace applications has created an increased need for developing nondestructive methods for assessment of composite performance. Embedded optical fiber sensor technology provides the potential for monitoring parameters of interest during processing and testing of composite materials as well as the opportunity for tracking properties over the lifetime of composite parts in service. The successful development of this technology depends on designing optical fiber sensor systems suitable for embedding in composite structures.

This paper focuses on the role played by optical fiber coatings in the design of embedded sensor systems. The performance of different optical fiber coatings under typical composite processing conditions will be discussed. Photomicrographs of test specimens containing embedded sensors will be presented which show delamination occurring at the coating/optical fiber interface in preference to the coating/epoxy resin interface. Coating performance criteria will be outlined for use in the selection of fiber optic sensors for composite applications.

The crystallization of fluoride glass being in contact with oxide glass is investigated using thermal analyses to perform oxide glass coating for durability and strength improvement. The chemical reaction does not occur in the fiber drawing temperature range and begin at the melting temperature of fluoride glasses. The crystallization of fluoride glass is suppressed by contact with oxide glass. Smooth contact between fluoride and oxide glasses is confirmed.

Recent work has made it possible to deposit hermetic carbon coatings on optical fibers during the drawing process. These coatings are used to protect the silica portion of the fiber from undesirable loss increases and strength reductions caused by H2 and H2O, respectively. The hermetic properties of the carbon films have been evaluated using accelerated test conditions where the coated fibers are exposed to H2 at elevated temperatures and hydrogen pressures. In-situ spectral loss monitoring has made it possible to measure changes in the characteristic optical loss features associated with either molecular H2or with species such as OH which form when hydrogen reacts with defects in the silica. By using long lengths of fiber it is thus possible to optically measure the extremely small amounts of hydrogen that penetrate the carbon films during accelerated tests. At temperatures in the range of 100 to 145°C the diffusion of H2 is readily modeled using classical diffusion theories for a composite cylinder, allowing calculation of the diffusion coefficient and the solubility for H2 in the carbon. At higher temperatures the diffusing H2 is partially depleted by reaction with defects in the glass. For these conditions the inward diffusion of the H2 and its reaction at defect sites tend to balance each other, giving rise to a constant, but extremely low, concentration of H2 in the fiber.

An amorphous carbon coating has been developed to improve the resistance of silica optical fibers to static fatigue and hydrogen permeation. The carbon coated optical fibers are characterized by electron paramagnetic resonance spectroscopy. A Lorentzian lineshape is observed, centered at the g value of 2.002. Intensity measurements as a function of temperature suggest that localized spin centers contribute to the spin resonance. It is shown that atmospheric control of the carbonaceous environment results in the removal of these EPR active sites.

Statistically significant correlations have been established between certain fabrication parameters of matched clad single mode fiber waveguides and the induced attenuation and recovery kinetics following exposure to an ionizing radiation dose.

Recent studies have indicated that the birefringent-inducing stress of polarization-maintaining (PM) fibers decreases the long term, permanent loss induced by ionizing radiation and that light polarized along the two orthogonal axes of PM fibers may be attenuated differently by exposure to irradiation sources. This paper reports the results of specific studies of this differential attenuation induced in a series of PM fibers by both steady state and transient irradiations. It has been found that the response to ionizing radiation depends on the materials properties of the fiber, i.e. the core and clad dopants and/or degree of stress, and that the magnitude of the differential attenuation is small relative to the total or one-axis incremental loss.

Exicimer laser induced luminsecence characteristics of type III fused silica was investigated. The luminescence characteristics in the ultraviolet wavelength region are strongly depending on the intensity of red luminescence at 650 nm. A sample without red luminescence shows a strong emission band at 280 nm. We found that the 280 nm band is corresponds to a absorption band at 225 nm. To clearfy the nature of the 280 nm band, we studied the difference of luminescence of samples which had been kept on heating under various atmosphares, i.e., N2, O2, He, H2, and air. By these results we suspect that the 280 nm band is related to hydrogen atom which are weakly bounded to the glass network.

photoluminescence in the visible range in high purity silica rods at room temperature was studied using 6.4 eV photons as the excitation source. Electron spin resonance (ESR) was used to monitor the induced paramagnetic centers. Broad luminescence bands at 2.7 eV, 2.2 eV and 1.9 eV were observed. The relative intensities of these bands were found to be intensity dependent. The fluence dependence of these bands were also studied. The results are discussed in relationship to the excitonic mechanism for defect generation in silica.

The fabrication of optical fibres based on fluoride glasses has required the development of novel preparation techniques. Some of these techniques are based on methods used to prepare multicomponent oxide glass fibres although some have been developed specially for fluoride glasses. This paper will review such processes as glove-box melting, reactive atmosphere processing, and rapid quenching; show how these have been used to prepare fluoride glass fibres with losses down to 2.6 dB/km at 2.6 μm; and discuss remaining problems with the glass.

Different preparation processes for fluorozirconate glasses (ZBLAN) have been investigated by infrared spectroscopy (IR) and x-ray diffraction (XRD). Glasses have been prepared with different NH4 F.HF amount (20–10–5–0% weight) or with NF30 as fluorinating agent. The presence of NF3 in the fluorination step of the preparation is useful to improve the glass quality in terms of oxide and hydroxil content: electron spin resonance (ESR) on these glasses (γ-irradiated, 20 Mrad) does not detect any signal.

The effects of compositional modifications, specifically oxide and p-block fluoride additions, and melt processing, via “on-situ oxidation”, have been investigated on bulk samples of ZBLAN glasses. Molecular light scattering in the Mie and Rayleigh regime has been measured as a function of scattering angle at wavelengths between 0.6 and 2.6 μm. From an analysis of the angular scattering of both vertically and horizontally polarized beams the Rayleigh ratios and dissymmetry factors are determined as a function of wavelength. The average size of the scattering sites can be estimated in the range of 0.02 and 2.0 μm. Relatively high oxide substituted ZBLAN glasses exhibit light scattering losses only moderately higher than the ZBLAN base glass, and do not show any evidence of gross phase separation or precipitation. InF3 containing samples also exhibit scattering comparable to the ZBLAN base glass. The scattering of ZBLAN at 2.6 μm is larger than the scattering predicted by extrapolating data taken at 0.647 μm using a λ−4 dependence. This added scattering may interpreted as due to either a wavelength independent contribution or a result of a difference in the wavelength dependence of the dispersion between the host and the scattering center.