A method for modeling the fabrication of small-scale hollow glass capillaries is developed. The model is based on an asymptotic analysis of the Navier-Stokes equations, which yields a simple closed-form solution for this problem. We demonstrate the validity of this approach using experimental data and use it to make predictions for a range of regimes of interest for the development of microstructured optical fiber technology.

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Modeling the fabrication of hollow fibers: capillary drawing

Microstructured optical fibres (i.e. fibres that contain holes) have assumed a high profile in recent years, and given rise to many novel optical devices. The problem of manufacturing such fibres by heating and then drawing a preform is considered for the particularly simple case of annular capillaries. A fluid-mechanics model is constructed using asymptotic analysis based on the small aspect ratio of the capillary. The leading-order equations are then examined in a number of asymptotic limits, many of which give valuable practical information about the control parameters that influence the drawing process. Finally, some comparisons with experiment are performed. For a limited set of experiments where the internal hole is pressurised, the theoretical predictions give qualitatively accurate results. For a much more detailed set of experiments carried out with a high-grade silica glass where no hole pressurisation is used, the relevant asymptotic solution to the governing equations is shown to give predictions that agree remarkably well with experiment.

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The mathematical modelling of capillary drawing for holey fibre manufacture

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.

Free Drawing and Polymer Coating of Silica Glass Optical Fibers

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.

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

Optimization of a thermal manufacturing process: Drawing of optical fibers

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.

Zhilong Yin

Papers

Thermal Transport and Flow in High-Speed Optical Fiber Drawing