The optical fiber draw tower
In order to meet huge demand of optical fibre for telecommunications applications, many companies have had to build new plants and install new fibre facilities all around the world. Today the manufacturing process of optical fiber is quite similar regardless of the manufacturer. However, a certain know-how which remains rather secret is necessary to obtain an optical fiber with the best characteristics, performances and long lifetime. Then, at side of the quality organization of the producers, the performance of the measurement instruments and of the process control is decisive.
What's the fiber optic drawing tower made of ?
The optical fiber is obtained from a multi-layered pure tube glass, the preform. This large tube (generally 20cm round and 2m high) is vertically located at the top of a metallic tower. Because the final glass fiber is obtained by “drawing” down a droplet, it is commonly called the draw tower.
Draw tower with a height of between 30 and 45 meters are often found in the industry. Several equipments located from the top to the bottom are required to manufacture and control the drawing process. At the top, the furnace heats the base of the preform to melt the glass until a droplet slowly go down pulling the initial glass fiber. When the glass drawing starts, an helium cooling tube help to reduce temperature before coating application. The coating applications can be done in one or two steps and required curing oven to ensure the coating is cured before any contact with pulleys. More and more coating layers are cured with high-intensity UV LED (compared to bulb lamps) to reduce electric consumption and reduce space on the tower.
A drawing process under control
All along the draw tower, several measurement instruments provide all relevant parameters to optimize the process :
§ Diameter measurements of the bare fiber (top) and of the coated fiber (bottom)
§ Position of the fiber (X-Y) and more globally alignements of the devices all along the draw tower
§ Drawing tension measurement to adjust speed and temperature of the drawing process
§ Internal defects detection inside the glass but also inside the coating layers to avoid reduced performances of the optical fiber
§ Lump & Neckdown detection after the second coating layer
Digital inputs and outputs, analog outputs (voltage or current), RS232, field bus, are used to connect these devices to the tower PLC for loop requirements and measurement analysis. Globally, these specific instruments bring control capability to maintain, record and analyse product and tower parameters. Because production is 24/24 almost all year round, these devices need to be very reliable. As the increase in drawing speed is an indispensable parameter to improve the productivity of production plants, the accuracy and at high measuring rate of the devices are decisive to ensure 100% in-line fiber certification.