Analysis of coating thickness variation during optical fiber Processing

During the mass production of silica-based optical fibers, a large fiber preform is softened in a high-temperature furnace and is drawn to a small fiber with a diameter of about 125 /spl mu/m. The hot fiber exiting the furnace is cooled rapidly by the surrounding air or by blowing a gas, and is subsequently coated with a polymer layer to provide a protection of the fiber surface. The overall quality of the fiber depends on the uniformity of the coating layer, which is strongly influenced by the manufacturing conditions. While the average thickness of the coating layer is extensively investigated in the literature, the studies on the coating thickness fluctuation lack a sound fundamental basis. In this paper, a linear perturbation analysis is adopted to predict the coating thickness variation under different processing conditions. An experimental correlation is developed to determine the initial amplitude of the thickness disturbance. Numerical results are presented for the first time to directly link the processing and geometric parameters with the coating thickness fluctuation in the final product. The results provide guidelines for selecting coating materials, system designs, and processing parameters to achieve uniform fiber coating layers.

Nomenclature

Dimensionless fiber radius defined in (10).

Initial amplitude.

Final amplitude.

Coefficients in (13).

.

in (10).

.

Amplitude growth rate.

Dimensional amplitude growth rate .

.

Gravitational acceleration .

Local coating thickness [m].

Average coating thickness [m].

Length of the coating die [m].

.

Pressure at the applicator inlet [Pa].

Atmosphere pressure [Pa].

Fiber radius [m].

Inner radius of the applicator die [m].

.

in (10).

Maximum Reynolds number for an acceptable final amplitude.

Radial coordinate [m].

.

Time [s].

Drawing velocity [m/s].

Maximum velocity for the fully developed flow [m/s].

Axial coordinate for an inertial reference frame with velocity [m].

Dimensionless axial coordinate for a reference frame at rest.

Greek symbols

wavenumber defined in (10).

Critical wavenumber.

Dimensionless coating thickness variation.

Surface wave length [m].

Dynamic viscosity .

Stream function .

Density .

Surface tension [N/m].