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 and performances on the market.
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 fiber drawing process. At the top, the furnace with intense heat to the base of the preform to liquify until a droplet is obtained. When the glass starts to drop, an helium cooling tube help to reduce temperature before coating application. These operations can be done in one or two steps and required curing oven to ensure the fiber is dry before any contact with pulleys. More and more layers are cured with high-intensity UV LED light (compared to thermally cured) 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 :
§ Fiber diameter measurements for the bare fiber (top) and the coated fiber (bottom)
§ Position of the fiber (X-Y) and more globally concentricity of the devices all along the draw tower
§ Tension measurement to adjust speed and temperature of the drawing process
§ Internal defects measurement inside the glass but also inside the coating layers to avoid reduced performances of the optical fiber
§ Flaw detection after the second coating called Lump & Neckdown defect detection
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 reliable. As the increase in drawing speed is an indispensable parameter to improve the productivity of production plants, the accuracy and measuring rate of the devices are decisive to ensure 100% fiber certification.