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A general method that predicts the thermal behavior of optical fibers during the cooling stage of the drawing process was developed. The method can be used for thin diameter D < 200 jam, medium (200 /xm < D < 500 jam), and thick (0.5 mm < D < 2 mm) single as well as core-clad fibers. A two-dimensional analysis implementing a finite difference method combined with the Karman-Pohlhausen technique was performed to obtain the temperature profiles in thick fibers. This method accounted for axial and radial heat conduction, and can also be applied to thin and medium fibers. The case of the core-clad fibers was investigated to obtain the temperature profiles both in the radial and the axial directions. All results are presented in graphical form and can be used for optimization of the drawing process.

I. INTRODUCTION

Optical fibers, used in numerous applications in biomedical, communications, space, hazardous environments, and other areas, are primarily manufactured by the drawing process. During a typical drawing process of a single fiber, a glass rod is placed in a cylindrical furnace and is heated to a softening temperature, which depends on the type of glass processed.1 At that temperature the glass is soft and it can be pulled downward to form a glass fiber (Fig. 1). As the fiber exits the furnace, it enters the cooling stage, where convection from the fiber to the air cools the fiber. In the case of thin fibers, the drawn fiber is attached to a rotating drum that winds it. In the case of thick fibers, the drawn fiber is cut in straight pieces using a glass cutting instrument. The drawing process of optical fibers has been the subject of numerous investigations over the past two decades. These studies were motivated by the need to improve the quality and increase the yield of optical fibers and optical multifiber systems. The majority of the investigations have focused on the optical properties and performance of the optical fibers. Although heat transfer effects during the process play a very important role, significantly less work has been done in the area of thermal modeling of optical fiber drawing processes. Most fiber drawing speeds, furnace operating temperatures, and cooling schemes are determined by time-consuming and costly trial and error techniques. Thermal modeling of the process can provide researchers and industrial production lines with simula-

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Address correspondence to this author. J. Mater. Res., Vol. 6, No. 1, Jan 1991

Preform

\ HEATING STAGE

Furnace

Fiber

COOLING STAGE

FIG. 1. Optical fiber drawing process.

tion tools that will assist them in process optimization and new product design. Previous investigations in the area of thermal analysis of the cooling region of the fiber drawing process focus on thin fibers used primarily for communications. Different studies present results for the cooling rates, the convective heat transfer coefficients, and the temperature distribution in drawn fibers. Oh2 predicted the cooling rates of thin fibers drawn by different heating methods. The cool