Optical Fiber Drawing Research Articles

Neck Down and Thermally Induced Defects in High-Speed Optical Fiber Drawing

The drawing speeds employed in the manufacturing of optical fibers have been rising in recent years due to growing worldwide demand. However, increasing speeds have placed stringent demands on the manufacturing process, mainly because of large temperature gradients that can generate thermally induced defects and undesirable variations in fiber characteristics. Heat transfer and glass flow that arise in drawing fibers of diameters 100–125 microns from cylindrical silica preforms of diameters 5–10 cm play a critical role in the success of the process and in the maintenance of fiber quality. This paper presents an analytical and numerical study of the optical fiber drawing process for relatively large diameter preforms and draw speeds as high as 20 m/s. The free surface, which defines the neck-down profile, is not assumed but is determined by using a balance of forces. An iterative numerical scheme is employed to obtain the profile under steady conditions. The transport in the glass is calculated to obtain the temperature, velocity and defect distributions. A zone radiation model, developed earlier, is used for calculating radiative transport within the glass. Because of the large reduction in the diameter of the preform/fiber, the velocity level increases dramatically and the geometry becomes complicated. A coordinate transformation is used to convert the computational domains to cylindrical ones. The numerical results are compared with experimental and numerical results in the literature for smaller draw speeds for validation. The effects of high draw speeds and of other physical variables on defects generated in the fiber, on the neck-down profile, and on the feasible domain for the process are determined. [S0022-1481(00)02302-1]

Newtonian glass fiber drawing: Chaotic variation of the cross-sectional radius

A model of Newtonian glass fiber drawing at fixed temperature in the unsteady range (the draw ratio E>20.22) is considered. In this range under steady boundary conditions, as is well known, the draw resonance instability sets in, resulting in self-sustained oscillations. These oscillations lead to a periodic variation of the cross-sectional radius of the fiber. In the present work we consider the case where the spinline radius varies periodically. Such a variation may result from flow oscillations in the fiber forming channels in the direct-melt process, or from the variation of the preform cross-sectional radius in drawing of optical fibers. When this variation takes place in the range E>20.22, the self-sustained periodic oscillations of the draw resonance are replaced by quasiperiodic and periodic (mode-locked) subharmonic or (under the appropriate conditions) chaotic oscillations (strange attractors). The routes to chaos found in the present work include (1) smooth period doubling bifurcation of (any) mode-locked periodic solution, (2) abrupt explosions of quasiperiodic solutions. The predicted chaotic variation of the spinline radius at the winding mandrel may result in a similar variation of the cross-sectional radius of the solidified fibers.

Free Drawing and Polymer Coating of Silica Glass Optical Fibers

The paper is primarily to highlight the current issues concerning fiber drawing and coating. The main emphasis is on high-volume production of silica-based optical fibers by using a large preform and a high-speed drawing. The commercial application of these processes has led to increasing the productivity in fiber manufacturing and resulted in a low cost of produced fibers. In order to systematically address the problems associated with the fiber manufacturing process, the fiber drawing system was divided into three major functional sections: heating, cooling, and coating zones. The governing equations at each section were formulated to describe the process mechanics and to identify the key control parameters for drawing and coating. These process parameters are the basic elements of implementing a streamline production system of optical fibers.

Thermal Transport and Flow in High-Speed Optical Fiber Drawing

The thermal transport associated with optical fiber drawing at relatively high drawing speeds, ranging up to around 15 m/s, has been numerically investigated. A conjugate problem involving the glass and the purge gas regions is solved. The transport in the preform/fiber is coupled, through the boundary conditions, with that in the purge gas, which is used to provide an inert environment in the furnace. The zonal method, which models radiative transport between finite zones in a participating medium, has been employed to compute the radiative heat transfer in the glass. The flow of glass due to the drawing process is modeled with a prescribed free-surface neck-down profile. The numerical results are compared with the few that are available in the literature. The effects of important physical variables such as draw speed, purge gas velocity and properties, furnace temperature, and preform diameter on the flow and the thermal field are investigated. It is found that the fiber drawing speed, the furnace temperature, and the preform diameter have significant effects on the temperature field in the preform/fiber, while the effects of the purge gas velocity and properties are relatively minor. The overall heating of the preform/fiber is largely due to radiative transport in the furnace and the changes needed in the furnace temperature distribution in order to heat the glass to its softening point at high speeds are determined.

Practical aspects in the drawing of an optical fiber

The transport processes in the furnace for the continuous drawing of optical fibers have been studied numerically and analytically. Practical circumstances and operating conditions are considered. A peripheral gas flow configuration has been modeled, along with irises at the ends, as employed in practical furnaces. The neck-down profile of the fiber is not chosen, but has been generated on the basis of a surface force balance. The results obtained are validated by comparisons with earlier experimental results. A detailed analysis has been carried out to determine the relative contributions of different forces during the drawing process. Even though the internal viscous stress is shown to be the major contributor to the draw tension, it is found that under certain operating conditions, the force due to gravity is significant, especially at the beginning of the neck-down region. For a peripheral flow configuration, the effect of flow entrance is found to be very important in determining the necking shape. However, the effect of the iris size on the fiber temperature field is found to be negligible. It is found that for a given furnace temperature and fiber radius, there is an upper limit for draw-down speed at which a fiber can be drawn without rupture. Practical ranges of draw speeds and furnace temperature conditions are identified for the process to be feasible.

Thermal transport due to material and gas flow in a furnace for drawing an optical fiber

The transport processes involved in the neck-down region for optical fiber drawing are numerically investigated. In this manufacturing process, a moving glass rod is heated in a furnace containing an inert gas environment and drawn into a thin optical fiber. The conjugate problem is solved considering both radiation and convection, with focus on the latter. Two different flow configurations, involving inert gas flow in the same as well as in the opposite direction as the moving preform/fiber, are considered in this study. A coordinate transformation is used to change the complicated computational domains in the gas and the fiber to cylindrical ones. The transport in the fiber is coupled with that in the gas through the boundary conditions. The radiative thermal transport is calculated using an enclosure model developed in an earlier study. The numerical results on convective flow and transport are validated by comparing with results available in the literaturefor simpler configurations. The effects of several important parameters such as fiber draw speed, inert gas velocity, furnace dimensions, and gas properties on the flow and temperature distributions are investigated. For the aiding flow case, in which the inert gases flow in the same direction as the fiber, heat transfer to the fiber increases as the gas velocity increases. For opposing flow, a recirculating region appears in the gas, close to the moving fiber surface, causing reduction in heat transfer as compared to the aiding case. The thickness of this recirculating zone decreases with increasing inert gas velocity. Radiation is found to be the dominant mode of heat transfer in the overall heating of the preform/fiber, with nitrogen as the inert gas. However, near the edges of the furnace, radiation heat transfer is relatively small and convection becomes very important. Also, the convective transfer rate is relatively large near the flow entrance because of the large temperature difference between the gas and the fiber. However, away from the entrance, the gas heats up and the temperature difference relative to the fiber decreases, resulting in a smaller convective heat transfer rate. The relevance of the results to various aspects of the fiber-drawing process is discussed.

Sol-Gel Fabrication and Properties of Silica Cores of Optical Fibers Doped with Yb3+, Er3+, Al2 O3 or TiO2

Optical cores of preforms for drawing optical fibers doped with Er3+ and Yb3+ were fabricated by the sol-gel method with the aim of increasing the thickness of glass layers coated in a single coating cycle and to determine the relation between the preparation conditions and optical properties of the fibers. Al2O3-P2O5-SiO2 and TiO2-P2O5-SiO2 glasses have been studied as matrices for entrapping the rare-earth elements. Input sols have been prepared from silicon and titanium alkoxides, AlCl3, ErCl3, YbCl3, POCl3, water and a modifier under acidic catalysis of HCl. The sols were coated on the inner wall of a silica substrate tube and the gel layers were sintered at high temperatures up to 2000°C after which the tube was collapsed into the preform. Continuous and homogenous glass films with the maximum thickness of about 8 μm were fabricated. The influence of high-temperature heat treatment of the layers on their composition and optical attenuation was observed. The amplified stimulated emission of Er3+ around 1.55 μm was measured under the excitation of the fibers by an Nd : YAG laser at 1.064 μm.

Zonal Method to Model Radiative Transport in an Optical Fiber Drawing Furnace

Precise modeling of radiative heat exchange between the furnace and the glass preform is a very important part of the modeling of the fiber drawing process in a high temperature furnace. Most earlier studies on this process have used the optically thick approximation, i.e., the radiative heat exchange is assumed to depend only on the preform surface temperature while the transmission, emission, and absorption within the preform are approximated as a diffusion process. The validity of this approximation in the modeling of the fiber drawing process is dubious since the diameter of the preform undergoes a drastic reduction during the drawing process. The objectives of this research are to use a more accurate approach—the zonal method—to replace the optically thick approximation for computing the radiative heat exchange between the furnace and the preform, and to determine if the optically thick approximation is valid for this process. In applying the zonal method, the preform surface is assumed to be diffuse to both transmission and reflection. An enclosure analysis is performed for the radiative exchange between the furnace and the outer surface of the preform and the zonal method is employed to consider the radiative exchange within the glass preform. The emissivity for the glass preform has been calculated based on the diffuse surface assumption and applied to the computation of radiative heat flux with the optically thick approximation for the purpose of comparison with the present work. The results obtained by the zonal method show that the radiative heat flux is strongly influenced by the radial temperature variation within the preform, while those obtained by the optically thick approximation do not show this effect, as expected. Comparisons of the results obtained by these two approaches reveal that the optically thick approximation predicts the radiative heat flux satisfactorily for a range of axial temperature variations, but only when the radial temperature variation within the preform is small. The diameter change in the neck-down region has almost no effect on the validity of the optically thick approximation.

A novel approach for drawing optical fibers from disparate core/clad. glasses

We report a new idea, based on the Wollaston wire technique, which is to draw optical fibers of core and cladding glasses such that the fiber-drawing temperature is: (a) above the liquidus of the core glass yet (b) matches the normal fiber drawing viscosity of the cladding glass. Property compatibility of the disparate glasses is discussed with particular reference to making fibers with a fluorozirconate glass core and oxide glass cladding, but realization of such fiber has been unsuccessful, so far, because of the aggressive attack of molten halides upon most oxide glasses. A demonstration of the Wollaston wire method involving fabrication of multimode fiber with a lead borate glass core and a Pyrex optical cladding is described. In addition, forming the oxide tube for the optical preform via extrusion is discussed.

Effects of Variable Properties and Viscous Dissipation During Optical Fiber Drawing

The axisymmetric free-surface flow and thermal transport of fused silica during optical fiber drawing was considered with variable properties, prescribed heat flux, and neck shape. Experimental data from previous researchers were adapted or used as the basis for assumptions in order to enable a realistic analysis. The main objectives were to model the neck-down process in order to clarify the effects of the variable properties and the associated viscous dissipation. Due to the large changes in dimension and viscosity, this system poses severe nonlinearities, and a new solution algorithm was necessarily developed. Validation was achieved and several important results were obtained. Among these, it was shown that the viscous dissipation has considerable impact on the fiber temperature due to its localization to a small volume near the fiber section. Also, it was shown that a variable viscosity generated vorticity, which was localized to the region where the preform radius undergoes large changes.

On Nonstationary von Kármán Equations

AbstractIn der vorliegenden Arbeit wird ein Anfangswert‐ und Randwertproblem für eine visko‐elastische dünne Platte mit großen Biegungen formuliert und studiert. Es wird ein System nichtlinearer pseudoparabolischer und Volterrascher Integro‐Differential Gleichungen für die Biegungs‐ und die Airy‐Spannungs‐Funktion formuliert. Das vorliegende System ist eine Verallgemeinerung der von Kármá‐Gleichungen für den elastischen Fall. Die schwache Formulierung in Sobolev‐Räumen und das kanonische Anfangswertproblem für die Biegungsfunktion werden betrachtet. Die Existenz einer Lösung wird mittels elliptischer Regularisierung bewiesen.

Evaluation of Optical Glasses for Low-Loss Fibers: Optical Attenuation and Fiber Drawing Ability

Optical attenuation of commercial optical glasses with low refractive indices is evaluated on the basis of the measurements for light scattering and infrared absorption. Furthermore, these glasses are drawn into fibers to examine the stability against crystallization in the fiber drawing process. All glasses exhibit light scattering intensity higher than that of silica glass. Fluorophosphate glasses are crystallized during fiber drawing. The present evaluation reveals that silicate glass, C7, is suited for low-loss fibers. The intrinsic minimum attenuation is predicted to be 0.8 dB/km at 1.2 µm, which is somewhat larger than that of silica glass. In addition, the estimations for density and concentration fluctuations indicate that concentration fluctuation dominates the light scattering in multicomponent optical glass.

Surface-tension-driven flows at low Reynolds number arising in optoelectronic technology

Some methods of formation of preforms for drawing of polarization-maintaining optical fibres are based on utilization of the surface tension of glass in the liquid state. Under the action of surface tension non-circular glass articles begin to flow, which results in formation of an anisotropic internal structure of the preforms. The hydrodynamic analysis of two such methods is given in the paper. Analytical solutions of the Stokes equations with linearized boundary conditions for the corresponding creeping surface-tension-driven flows of liquid glass are obtained. By means of these solutions a processing strategy may be predetermined with a view to a specific internal structure of the fibre, as well as to the required value of birefringence. The theoretical results are compared with experimental data and agreement is fairly good.

Distributed strain sensing with fibre Bragg grating arrays embedded in CRTM™ composites

Fibre Bragg grating (FBG) sensor arrays fabricated during optical fibre drawing have been embedded in fibre-resin composite panels made by the continuous resin transfer molding™ (CRTM™) process to accurately measure distributed strain.

Influence of the gas flow rate on the drawing of optical fibers from fluoride glasses

The influence of the mass flow rate and temperature of the gas on the dynamics of the formation of a fluoride glass fiber is investigated on the basis of the steady-state conjugate problem of gas flow in the furnace duct

Investigation of elemental diffusion during the drawing and heat treatment of glass optical fibres

Using high-resolution X-ray analysis, elemental mobility during fibre drawing and the subsequent heat treatment were studied in germanium-doped, erbium-doped and cadmium-sulpho-selenide-doped optical fibres. It was found that all but the monovalent cations were effectively immobile in the silica matrix. There was also some evidence for reduced erbium volatility due to aluminium doping in some fibres.

New fluoroindate glass compositions

The limits of glass formation of new fluoroindate glass compositions have been determined for the basic systems InF3–BaF2–GdF3−20ZnF2−20SrF2–2XFn where × = Na, La and InF3–BaF2−20ZnF2−20SrF2−2GdF3–XFn, where × = Ca and Y. The incorporation of small amounts of GaF3 and/or GdF3 increases strongly the thermal glass stability. All the fluoroindate compositions studied are highly transparent in the mid-infrared range; their transmission edge is shifted beyond 7 μm and the theoretical attenuation coefficient, extrapolated at the CO laser wavelength emission, is typically α = 100 dB/km. Large bulks and preforms have been prepared allowing the drawing of optical fibers.

Drawing of optical fibers under nonsteady heat exchange

Drawing of optical fibers under nonsteady heat exchange

Noncontact measurement of optical fiber draw tension

A simple technique for measuring the tension on an optical fiber during the draw process without contacting the fiber surface is reported. It is based on detecting the resonant vibrational frequency of the fiber during draw using the position output of the fiber diameter monitor and requires no specialized circuitry. The tension, determined from the fundamental resonant frequency of the length of fiber between the neckdown region in the furnace and the coating cup, is accurate to within 1% and repeatable to >

Thermal Modeling of the Optical Fiber Drawing Process

AbstractA 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.