Neck down and thermally induced defects in high-speed optical fiber drawing

The drawing speeds employed in the manufacturing of optical fibers have bee n rising in recent years due to growing worldwide demand. However, increasi ng speeds have placed stringent demands on the manufacturing process, mainl y because of large temperature gradients that can generate thermally induce d defects and undesirable variations in fiber characteristics. Heat transfe r 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 draw ing 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 as sumed but is determined by using a balance of forces. An iterative numerica l scheme is employed to obtain the profile under steady conditions. The tra nsport in the glass is calculated to obtain the temperature, velocity and d efect distributions. A zone radiation model, developed earlier, is used for calculating radiative transport within the glass. Because of the large red uction in the diameter of the preform/fiber, the velocity level increases d ramatically and the geometry becomes complicated A coordinate transformatio n is used to convert the computational domains to cylindrical ones. The num erical 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 de termined.