The optical fiber production process
A glass preform for the fiber manufacture
Everything starts with silica sand. High purified materials are then combined to manufacture a glass tube named Preform: a multi layered pure glass of around 20 centimetres round. Two perfect concentric tubes with different glass index for light transmission. Several different horizontal processes exist (IVD, OVD, VAD) to manufacture the preform. Objective is the same : allow the best performance characteristics to the final drawn fiber. Preform is then mounted to a tower and its base heated in a furnace (more than 2000°C) to obtain a fine droplet that drop by gravity and will be draw to obtain a narrow and uniform diameter of several microns. High-purity, oxygen-free Argon gas is cycled through the furnace to protect the graphite element at high temperatures. The core/clad ratio of the preform is maintained during the draw process. The resulting optical fiber has this same core/clad ratio. In fact, the preform is a bigger solid version of a fiber.
From the preform to the final optical fiber
During the process the bare fiber is first cooled down to apply two layers of coating material such as acrylade or polyimide that provide protection and bending properties. Fiber temperature needs to be reduced for proper coating application.
A capstan at the bottom of the draw tower pull and wind continuously the glass fiber. The speed of the fiber draw depends on the preform, fiber type and available equipments. It can be a few meters per minute up to 3000 meters per minute for up to date telecommunication fiber production. The draw speed is carefully monitored to control the final fiber diameter.
Concentricity of all equipments are required to ensure a perfect round filament of glass. Optical fiber dimensions are directly dependant on that.
The drawing force applied on the fiber during drawing must be accurately controlled in order to optimize the light propagation into the fiber.
Several measuring devices located all along the drawing tower provide many data to monitor final dimensions and global quality of the fiber.
Sort and test of the optical fiber
The finished optical fiber is wound onto spools of several kms until preform is over. Several mechanical tests are performed to avoid any future breaking risk. Then, smaller reels are used to sort and classify the optical fibers according to their future applications. Globally, we can distinguish the fiber in two categories: single mode fiber (long distance usage) and multi-mode fiber (shorter distances). Typically, the final diameter for both fibers is 250µm. After drawing optical fibers are tested to verify all other properties are within the specification and customer requirements.