Practical aspects in the drawing of an optical fiber
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Practical aspects in the drawing of an optical fiber S. Roy Choudhury and Y. Jaluria Department of Mechanical and Aerospace Engineering, Rutgers University, New Brunswick, New Jersey 08903 (Received 13 May 1996; accepted 25 July 1997)
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.
I. INTRODUCTION
The preforms for optical waveguide fibers are typically made of doped silica glass, which is subsequently drawn to a desired fiber size by heating peripherally in an induction or resistance furnace,1 by laser 2 or by an oxy-hydrogen torch.3 Among these, the most popular method for fiber drawing is heating the preform in a concentric, cylindrical graphite furnace, where radiative and convective heat exchange with the wall and with the inert fluid inside the heating zone of the furnace are the chief modes of energy transport. As the preform proceeds through the heating zone of the furnace, its temperature and viscosity vary strongly in the axial direction. The softening, along with the extensional deformation caused by the draw tension, give rise to the “neck-down” profile inside the furnace. The neck-down shape depends on the drawing conditions as well as on the physical and process variables. It is within the neck-down region that the profile of the index of refraction may be altered, thus affecting the optical bandwidth of the fiber. Also the defect concentration within the fiber is dependent on the thermal and material flow processes in this neckdown region. Therefore, a rigorous study of the physical processes within this region is desirable in order to optimize and control the fiber-drawing system, as well as the quality of the final product. Modeling of the
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