The explosive growth in the use of fiber optic cable for telecommunications has put unprecedented pressures on manufacturing processes. CFD is being used increasingly to better understand these processes, design ways of increasing production rates, identify defects, and improve quality.

Optical glass fiber production is a wonder of modern engineering. First, a preform of ultra pure silica is produced, usually by vapor deposition. By carefully controlling the concentrations of dopant species during deposition, such as germanium oxide, the refractive index profile across the diameter of the preform can be tailored to give the desired light-carrying properties. The preform is heated to its softening point in a furnace and drawn into a very thin fiber at high speed. The drawdown ratio can be extremely high (from an initial preform diameter of 70 mm or more, to a final fiber diameter on the order of 125 microns) producing a continuous fiber of several hundred to more than a thousand kilometers in length. To give the fiber additional strength and to protect it from surface contamination, the fiber is immediately coated with one or more polymer layers. The coating die must supply a continuous uniform coating without entraining air bubbles or other defects. This is particularly challenging since fiber speeds through the coating die are typically in excess of 15 to 20 m/s.

Modified chemical vapor deposition (MCVD) is one process used to create glass preforms. An oxy-hydrogen torch is passed underneath a silica tube, which is spinning in a lathe. Silicon tetrachloride, oxygen, and dopant species are introduced at one end of the tube, and a reaction takes place, generating silica particles. These particles are forced to deposit on the inner wall of the tube by thermophoresis. Production of preforms is limited by the deposition rate. Since the velocity and temperature fields largely determine the particle deposition rate and efficiency, a predictive CFD model can be very valuable in identifying and testing process improvements. FLUENT has been successfully used to model the MCVD process and is being used to study how changes in operating conditions affect particle deposition efficiency.

Temperature (top) and SiO 2 concentration (bottom) during an MCVD process

Precise control of the fiber diameter and prevention of surface flaws caused by contamination are just two of the many manufacturing issues associated with the fiber drawing furnace. FIDAP has been used to simulate the heating of the preform due to radiation and convection in the furnace, along with the rapid (and large) diameter attenuation that occurs during drawing. The model also accounts for the flow of inert gas around the fiber. FIDAP can accurately predict the fiber diameter profile for a given drawing velocity. It also reports the drawing force. The effect of different gas flow configurations and furnace heating profiles can easily be studied.

Fiber temperature and gas velocity inside a drawing furnace

After exiting from the furnace, the fiber is coated with one or more protective layers. To achieve higher production speeds, fiber coating dies are now highly pressurized to assure stable coating conditions. A FIDAP analysis of the flow within a fiber coating die shows how the location and shape of the upstream meniscus (free surface) dictates the range of fiber speeds over which stable coating can be expected.

Streamlines in a fiber coating die

Fluent software is being used to analyze other manufacturing processes associated with optical glass fiber and cable production, including ribbon coating and cable jacket extrusion.

The telecommunications and internet boom will obviously continue, and an ever-growing percentage of this data traffic will be carried on optical glass fiber. Fluent CFD software will continue to be an indispensable tool for engineers and designers in this industry striving to meet the demands for higher production and quality.

"FLUENT has been a valuable tool for helping us improve the efficiency of our glass fiber production."
Chris D. Shay, CVD Engineer
Alcatel (Telecommunications) Cable

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