Optical Fiber Drawing Research Articles

Effect of Furnace Thermal Configuration on Optical Fiber Heating and Drawing

ABSTRACT The thermal configuration of the draw furnace, which involves the wall temperature profile, the temperature level, and the length of the heated zone, is an important aspect in high-speed optical fiber manufacture. This article presents a computational study on the effect of the furnace thermal configuration on the draw process and on physical quantities such as velocity and temperature difference across the preform/fiber, tension, and neck-down profile. Considering a cylindrical graphite furnace, the study solves a conjugate problem, which involves both the moving silica glass rod or fiber and the inert gases in the furnace. The flow and heat transfer in the two regions is linked due to the boundary conditions at the surface of the glass. Force balance conditions are used to determine the neck-down profile. A fairly versatile finite-difference numerical scheme is employed to consider different temperature distributions along the furnace wall, as well as a range of heating-region lengths. Besides the flow and thermal transport, the tension in the fiber and thermally induced defects that affect fiber quality are also calculated. A range of fiber draw speeds is also considered, and the effect on the variables that determine the fiber characteristics is studied.

Optimization of a thermal manufacturing process: Drawing of optical fibers

The optimization of thermal systems and processes has received much less attention than their simulation and often lags behind optimization in other engineering areas. This paper considers the optimization of the important thermal manufacturing process involved in the drawing of optical fibers. Despite the importance of optical fibers and the need to enhance product quality and reduce costs, very little work has been done on the optimization of the process. The main aspects that arise in the optimization of such thermal processes are considered in detail in order to formulate an appropriate objective function and to determine the existence of optimal conditions. Using validated numerical models to simulate the thermal transport processes that govern the characteristics of the fiber and the production rate, the study investigates the relevant parametric space and obtains the domain in which the process is physically feasible. This is followed by an attempt to narrow the feasible region and focus on the domain that could lead to optimization. Employing standard optimization techniques, optimal conditions are determined for typical operating parameters. The study thus provides a basis for choosing optimal design conditions and for more detailed investigations on the feasibility and optimization of this complicated and important process.

Convection and radiation effects in hollow, compound optical fibers

A coupled model for the study of hollow, compound optical fiber drawing processes that accounts for the heat transfer in the preform and fiber and for the motion of the gases surrounding the preform and fiber by means of two-dimensional equations, employs a net radiative model for the radiative heat exchanges amongst the preform, fiber, irises and furnace walls, and uses asymptotic one-dimensional equations for the geometry, axial velocity component and temperature along the fiber for small Biot numbers is presented. It is shown that the coupled model predicts that radiative heat exchanges are about three times larger than forced convection effects, and free convection is not important. It is also shown that the fiber's geometry, axial velocity and temperature predicted by the coupled model are in remarkable good agreement with those obtained with only the one-dimensional model for hollow, compound fibers using a properly chosen constant Biot number. The results of the one-dimensional model for hollow, compound fibers show that, as the heat transfer losses from the fiber increase, the fiber's dynamic viscosity increases, the fiber exhibits a strong necking phenomenon and the fiber's axial velocity increases rapidly from its value at the die's exit to a constant value downstream and then remains constant. For the boundary conditions considered in this paper, it is shown that the activation energies of the viscosity laws for the inner and outer materials of the hollow, compound fiber do not have very strong effects on the fiber's geometry, axial velocity component and temperature, whereas the fiber's solidification point moves towards the die as the thermal Péclet number is decreased. It is also shown that the pre-exponential factor and activation energy of the dynamic viscosity law do not play a key role in determining the fiber's geometry and temperature for the conditions analyzed in this paper.

A study of environmental effects on the attenuation of chalcogenide optical fibre

The attenuation of a number of chalcogenide glass optical fibres has been studied with regard to their exposure the environment. We demonstrate that gallium lanthanum sulfide (Ga:La:S) based glasses appear to be as resilient if not more so than arsenic sulfide (As2S3) glass to the attack of moisture when stored uncoated in ambient conditions for various periods exceeding 1 year. The increase in the characteristic OH− attenuation peak was ∼3–4dB/m for all fibres following storage. Given the significant improvements achieved in As2S3 glass technology over recent years we believe that Ga:La:S based fibres can also be improved to at least match these levels with the advantage of being non-toxic and having a significantly higher melting temperature. Studies of the time-dependant attenuation of the optical fibres during immersion in water have also been carried out. These results show that the deleterious effect of moisture on these glasses occurs over a short time, ∼24h, thus having implications on the treatment and storage of fibre preforms prior to optical fibre drawing.

Modeling of silica nanowires for optical sensing

Based on evanescent-wave guiding properties of nanowire waveguides, we propose to use single-mode subwavelength-diameter silica nanowires for optical sensing. Phase shift of the guided mode caused by index change is obtained by solving Maxwell's equation, and is used as a criterion for sensitivity estimation. Nanowire sensor employing a wire-assembled Mach-Zehnder structure is modeled. The result shows that optical nanowires, especially those fabricated by taper drawing of optical fibers, are promising for developing miniaturized optical sensors with high sensitivity.

Feasibility of High Speed Furnace Drawing of Optical Fibers

The domain of operating conditions, in which the optical fiber-drawing process is successful, is an important consideration. Such a domain is mainly determined by the stresses acting on the fiber and by the stability of the process. This paper considers an electrical resistance furnace for fiber drawing and examines conditions for process feasibility. In actual practice, it is known that only certain ranges of furnace temperature and draw speed lead to successful fiber drawing. The results obtained here show that the length of the heated zone and the furnace temperature distribution are other important parameters that can be varied to obtain a feasible process. Physical behavior close to the boundary of the feasible domain is also studied. It is found that the iterative scheme for neck-down profile determination diverges rapidly when the draw temperature is lower than that at the acceptable domain boundary due to the lack of material flow. However, the divergence rate becomes much smaller as the temperature is brought close to the domain boundary. Additional information on the profile determination as one approaches the acceptable region is obtained. It is found that it is computationally expensive and time-consuming to locate the exact boundary of the feasible drawing domain. From the results obtained, along with practical considerations of material rupture, defect concentration, and flow instability, an optimum design of a fiber-drawing system can be obtained for the best fiber quality.

Determination of gaseous heat transfer coefficients at elevated temperatures

An experimental method of measuring gaseous heat transfer coefficients of two noble gases, as well as several binary mixtures of these gases, is described in this paper. The method employed during this study involved measuring the cooling rate of a “hot wire” that was surrounded by a nearly static atmosphere of either pure Helium, pure Argon, or a binary mixture of these gases. Initial wire temperature increases, in the presence of each gas or gas mixture tested, were created by a controlled electrical heating process just prior to the onset of the cooling process. The findings obtained during this study are believed to be of importance in providing a better quantitative understanding of the importance of using very low molecular weight gases during the cooling process that accompanies the drawing of optical fibers from heated glass preforms.

Effects of Drawing Condition on Drawing Tension and Neck-Down Profile in Optical Fiber Drawing from Silica Glass Preform

Numerical calculation of silica glass deformation in optical fiber drawing from preform has been carried out to reveal the effects of drawing condition on drawing tension and neck-down profile. Effects of drawing condition on drawing tension in optical fiber drawing are important in designing and setting of manufacturing process, because transmission characteristics of fiber is dependent on drawing tension when it is manufactured. Drawing condition is considered to have any effect on neck-down profile in drawing furnace, but it is not well known. This report presents numerical result of drawing tension and neck-down profile. It also shows how the distribution of value in drawing direction changes by drawing condition. The parameters of drawing condition are preform diameter, drawing velocity, heater temperature and its length. The ranges of them are 0.025-0.1m, 10-25m/ s, 2400-2550 K and 0.05-0.2m, respectively.

Dopant diffusion during optical fibre drawing

Diffusion of Ge and F was studied during drawing of silica optical fibres. Preforms were drawn using various draw conditions and fibres analysed using the etching and Atomic Force Microscope (AFM) technique. The results were confirmed by comparison with fibre Refractive Index Profiles (RIP). Both Ge and F were found to diffuse at high temperature, 2100 degrees C, and low draw speed, 10m/min. Diffusion simulations showed that most diffusion occurred in the neck-down region. The draw temperature and preform feed rate had a comparable effect on diffusion, whereas preform diameter did not significantly affect the diffusion.

Effects of Radiative Transfer Modeling on Transient Temperature Distribution in Semitransparent Glass Rod

This paper presents a method of modeling the radiative energy transfer that takes place during the transient of joining two concentric, semitransparent glass cylinders. Specifically, we predict the two-dimensional transient temperature and heat flux distributions to a ramp input which advances the cylinders into a furnace at high temperature. In this paper, we discretize the fully conservative form of two-dimensional Radiative Transfer Equation (RTE) in both curvilinear and cylindrical coordinate systems so that it can be used for arbitrary axisymmetric cylindrical geometry. We compute the transient temperature field using both the Discrete Ordinate Method (DOM) and the widely used Rosseland’s approximation. The comparison shows that Rosseland’s approximation fails badly near the gap inside the glass media and when the radiative heat flux is dominant at short wavelengths where the spectral absorption coefficient is relatively small. Most prior studies of optical fiber drawing processes at the melting point (generally used Myers’ two-step band model at room temperature) neglect the effects of the spectral absorption coefficient at short wavelengths λ<3μm. In this study, we suggest a modified band model that includes the glass absorption coefficient in the short-wavelength band. Our results show that although the spectral absorption coefficient at short wavelengths is relatively small, its effects on the temperature and heat flux are considerable.

Nano-scale compositional lamination of doped silica glass deposited in surface discharge plasma of SPCVD technology

The kinetics of metal halogenide mixture transformation into oxides was studied with the help of optical emission spectroscopy (OES) in the microwave-induced discharge of surface plasma chemical vapor deposition (SPCVD) process. The structure of germanium-doped silica layer deposited in plasmachemical synthesis of preforms for optical fiber drawing was also investigated. It was shown that deposited glass has a periodical structure of alternate layers with different glass composition. Observation experiments on the oxide formation kinetics in plasma column were performed for the interpretation of the lamination effect. It was demonstrated that the lamination effect is related to a non-uniform distribution of the corresponding monoxides along the plasma column. This lamination results from the competition of different chemical oxidation reactions for oxygen. On the basis of the results obtained, a set of chemical reactions responsible for the formation of multicomponent glasses in SPCVD-type processes is proposed.

The Fabrication of a Photonic Crystal Fiber and Measurement of its Properties

In this paper, we describe the fabrication process of a photonic crystal fiber and present the measured optical properties of the photonic crystal fiber. The fabrication of the photonic crystal fiber involves stacking, jacketing, collapsing, and drawing using a conventional drawing tower The photonic crystal fiber drawing needs higher tension to maintain the uniform air hole structure. Thus, the temperature of the photonic crystal fiber drawing is lowered by a few hundred degrees Celsius than for the case of conventional optical fiber drawing. The optical properties of the fabricated photonic crystal fiber such as mode profile, optical loss, transmission spectrum, bending loss, and polarization dependent loss are measured.

Numerical Computation of Optical Fiber Drawing from Silica Glass Preform in Steady State

Numerical computation of silica glass deformation in optical fiber drawing from preform has been carried out to reveal thermal and dynamic characteristics of the process. This report presents numerical procedure of the calculation and results of two cases. One case is that continuity and momentum equation are solved one-dimensionally in drawing direction and energy equation is solved axi-symmetrically, and the other case is that these three equations are solved one-dimensionally. As a result, two things are implicated. 0ne thing is that one-dimensional calculation, as the later case, would be enough to compare neck-down profile and drawing tension from practical viewpoint, because the discrepancy of them in these two cases is not so large. Another thing is that inertia force, surface tension and viscous dissipation are not essential in the process, because they have little effect on drawing tension.

Numerical Computation of Diameter Fluctuation in Optical Fiber Drawing from Silica Glass Preform by Perturbation Method

One-dimensional numerical model of optical fiber drawing to predict diameter fluctuation caused by periodical fluctuation of heat transfer between silica glass preform and furnace gas, has been developed by using perturbation method based on steady solution. It is assumed that heat transfer coefficient is fluctuating in neck-down region and its frequency range is 0.001∼100 Hz. As a result, it is revealed that relationship between fluctuation of heat transfer coefficient and that of diameter can be divided into three modes by the order of diameter gradient and velocity gradient at heat transfer fluctuating point. The diameter fluctuation of first mode is large in low frequency range and that of third mode is so in high frequency range. As drawing velocity become higher, the fluctuation of third mode becomes larger, because velocity gradient becomes higher. And as heater becomes longer, the region of second mode becomes wider, because velocity gradient of heater region decreases.

Effect of Technological Parameter Variations on the Stability of Optical Fiber Drawing

A unidimensional model is developed including the equations of continuity and force equilibrium and a physical equation describing the state of glass melt in the fiber formation zone. Perturbations related to variations in the technological parameters are introduced into the solutions of equations determining a stable process, and possible deviations of the fiber radius are found.

Fabrication and mechanical behavior of dye-doped polymer optical fiber

The purpose of this article is to study the materials physics behind dye-doped polymethyl metharcylate (PMMA) that is important for the optical fiber drawing process. We report effects of the fabrication process on the mechanical properties of the final fiber. The qualitative degree of polymer chain alignment is found to increase with the drawing force, which in turn decreases with the drawing temperature and increases with the drawing ratio. The chain alignment relaxes when the fibers are annealed at 95 °C with a commensurate decrease in fiber length and increase in diameter. The annealed fiber has higher ductility but lower strength than the unannealed fiber. Both the yield and tensile strengths are dependent on the strain rate. The relationship between tensile strength, σb, and fiber diameter, d, is found empirically to be σb∝d−0.5. The yield strength appears to be less sensitive to the fiber diameter than the tensile strength. For PMMA doped with disperse red 1 azo dye, the yield strength, tensile strength, and Young’s modulus peak at a dye concentration of 0.0094 wt %. These results are useful for designing polymer optical fibers with well-defined mechanical properties.

EFFECT OF DRAW FURNACE GEOMETRY ON HIGH-SPEED OPTICAL FIBER MANUFACTURING

Optimal design of the draw furnace is particularly desirable to meet the need of high-volume production in the optical fiber industry. This article investigates the thermal transport and flow in optical fiber drawing at high draw speeds in a cylindrincal graphite furnace. A conjugate problem involving the glass and the purge gases is solved. The transport in the two regions is coupled through the boundary conditions at the free glass surface. The neck-down profile of the preform at steady state is determined by a force balance, using an iterative numerical scheme. To emphasize the effects of draw furnace geometry, the diameters of the preform and the fiber are kept fixed. Only the length and the diameter of the furnace are changed. For the purposes of comparison, a wide domain of draw speeds, ranging from 5 m/s to 20 m/s, is considered, and the form of the temperature distribution at the furnace surface remains unchanged. The dependence of the preform/fiber characteristics on the furnace geometry are demonstrated quantitively. Based on these numerical results, an optimal design of the draw furnace can be developed.

ADVANCED SIMULATION OF OPTICAL FIBER DRAWING PROCESS

Precise modeling of the optical fiber drawing process is extremely important for identifying optimum drawing conditions in a furnace that would produce high-quality fibers at a low cost. In this study, a numerical approach for detailed simulation of thermal transport in an optical fiber drawing furnace is developed by implementing a primitive variable computational fluid dynamics algorithm. To accurately simulate the underlying fluid dynamics, the complete Navier–Stokes equations are solved for both glass and external gas, which are coupled by the conjugate boundary conditions at the interface. The governing equations are discretized by a finite volume approach and the solution algorithm for the discretized equations is based on the semi-implicit method for the pressure linked equations revised (SIMPLER) method instead of the traditional streamfunction approach. Radiative heat transfer is the most dominant mode of heat transfer during optical fiber drawing and it is modeled by the finite volume method. The gas-preform interface is treated as a Fresnel surface instead of a diffuse surface. To validate the present numerical approach and to examine the effects of the Fresnel interface condition on the preform temperature, a benchmark optical fiber drawing problem containing a prescribed neck-down profile is investigated with various fiber drawing speeds, furnace wall temperatures, and preform diameters. For the diffuse interface, the present prediction in temperature is found to match the available other solution very well. For the Fresnel interface, the present prediction is usually higher in comparison with that of the same approach with the diffuse interface. However, the temperature difference between two interfaces is found to be small, implying that the error caused by the diffuse assumption may be not significant.

MODELING OF RADIATIVE TRANSFER IN OPTICAL FIBER DRAWING PROCESSES WITH FRESNEL INTERFACES

Radiative heat transfer is the most dominant mode of heat transfer in an optical fiber drawing process, and its accurate prediction is the key to a realistic simulation of flow and heat transfer in the system. In this study, the finite-volume method (FVM ) is developed to model radiative heat transfer in a gas enclosure as well as inside a glass preform with a Fresnel boundary at the gas?glass interface. Unlike diffuse boundaries, the reflections and transmissions at Fresnel boundaries are governed by Fresnel?s relation and Snell?s law, and they are strongly dependent on the angular direction. During the implementation of the FVM in semitransparent media, control-angle overlap may occur, and it is treated by assuming that the flux of radiant energy across a control angle is determined only by the discrete direction over this control angle and the corresponding radiative intensity. At the Fresnel boundary, a reflective or refractive intensity may not coincide with any discrete direction, and it is approximated by an intensity whose discrete direction makes the smallest angle with the corresponding reflective or refractive intensity. To validate the present FVM in semitransparent media with Fresnel boundaries, two benchmark problems are examined and the present solutions are found to be very accurate with a fine angular discretization. After that, the present model is applied to investigate an optical fiber drawing process with different boundary conditions and temperature distributions. For the diffuse boundary at the gas?preform interface, the results from the FVM are very close to those from the zonal method in each case. For the Fresnel boundary, the FVM provides a solution that tends to be higher in the upper neck-down region and lower in the low neck-down region than that from the same method with the diffuse boundary. The maximum difference betweentwo solutions varies and may reach 10?15% at some cases.

Fluid Flow Phenomena in Materials Processing—The 2000 Freeman Scholar Lecture

There has been an explosive growth in the development of new materials and processing techniques in recent years to meet the challenges posed by new applications arising in electronics, telecommunications, aerospace, transportation, and other new and traditional areas. Semiconductor and optical materials, composites, ceramics, biomaterials, advanced polymers, and specialized alloys are some of the materials that have seen intense interest and research activity over the last two decades. New approaches have been developed to improve product quality, reduce cost, and achieve essentially custom-made material properties. Current trends indicate continued research effort in materials processing as demand for specialized materials continues to increase. Fluid flow that arises in many materials processing applications is critical in the determination of the quality and characteristics of the final product and in the control, operation, and optimization of the system. This review is focused on the fluid flow phenomena underlying a wide variety of materials processing operations such as optical fiber manufacture, crystal growth for semiconductor fabrication, casting, thin film manufacture, and polymer processing. The review outlines the main aspects that must be considered in materials processing, the basic considerations that are common across fluid flow phenomena involved in different areas, the present state of the art in analytical, experimental and numerical techniques that may be employed to study the flow, and the effect of fluid flow on the process and the product. The main issues that distinguish flow in materials processing from that in other fields, as well as the similar aspects, are outlined. The complexities that are inherent in materials processing, such as large material property changes, complicated domains, multiple regions, combined mechanisms, and complex boundary conditions are discussed. The governing equations and boundary conditions for typical processes, along with important parameters, common simplifications and specialized methods employed to study these processes are outlined. The field is vast and it is not possible to consider all the different techniques employed for materials processing. Among the processes discussed in some detail are polymer extrusion, optical fiber drawing, casting, continuous processing, and chemical vapor deposition for the fabrication of thin films. Besides indicating the effect of fluid flow on the final product, these results illustrate the nature of the basic problems, solution strategies, and issues involved in the area. The review also discusses present trends in materials processing and suggests future research needs. Of particular importance are well-controlled and well-designed experiments that would provide inputs for model validation and for increased understanding of the underlying fluid flow mechanisms. Also, accurate material property data are very much needed to obtain accurate and repeatable results that can form the basis for design and optimization. There is need for the development of innovative numerical and experimental approaches to study the complex flows that commonly arise in materials processing. Materials processing techniques that are in particular need of further detailed work are listed. Finally, it is stessed that it is critical to understand the basic mechanisms that determine changes in the material, in addition to the fluid flow aspects, in order to impact on the overall field of materials processing.