In our last blog post we talked about glass, the material from which most optical fibers are made. In particular, glass fibers are usually made of silica (silicon dioxide, SiO2) although other glasses can also be employed, including phosphate glass or fluoride glass. In this post, we are going to focus on how optical fibers are made.
There are always two main steps in the manufacturing process of optical fibers: the preform production, and the drawing process. The preform is a solid glass rod that already has a core and a cladding, but their dimensions are far larger than in the final fiber. In the drawing process, the preform is heated and stretched by pulling with a tractor belt, forming this way the optical fiber.
The preform manufacturing process is usually based on the chemical reaction of gases SiCl4 (silicon tetrachloride) and GeCl4 (germanium tetrachloride) with O2 (oxygen) to produce SiO2 (silica) and GeO2 (germanium dioxide), also liberating Cl2 (chlorine). Gas ratio can be modified depending on the desired final composition. Generally, the cladding is made of pure silica while the core has a higher refractive index due to the presence of GeO2. It must also be considered that multimode fibers typically have a higher content in GeO2 than single mode fibers.
Right now, there are three main techniques to produce the preform: 1) modified chemical vapor deposition (MCVD), 2) outside vapor deposition (OVD), and 3) vapor axial deposition (VAD). Although there are differences among these three techniques, please consider that all of them include “vapor deposition” in their names. This means that they are processes that involve the deposition of materials in a vapor state on a substrate.
In the case of MCVD, gases (SiCl4, GeCl4) flow through a rotating tube of pure silica (which will be the cladding of the fiber), while heat is applied to the tube using a torch, see Figure 1. Heat and oxygen cause the gases to react, leading to the formation of solid submicron particles, called “soot”, which deposit on the inner wall of the tube. The heat then transforms this “soot” into glass (SiO2, GeO2), starting to form the core region of the optical fiber. This process is repeated for hours to create the successive layers of the core. The parameters can be varied along the process to obtain fibers with different composition and properties. Once enough core layers have been manufactured, the temperature is increased so the tube collapses into the preform.
Regarding OVD, the manufacturing setup is very similar to that of the MCVD technique, but in this case, we start with a rotating inert rod instead of a tube of silica, see Figure 2. Again, gases are flowed along the rod and heat is applied to it with a torch. In this case, successive layers will be formed on the outside of the rod, first corresponding to the core, and then to the cladding. Once enough layers have been deposited, the rod is removed and heat is applied so the tube collapses into the preform.
Finally, in the case of VAD, a rotating inert rod is also employed, as it was done in OVD, but here it is vertically placed. The preform is manufactured of one of the tips of the rod, see Figure 3, which is pulled upwards during the process. Two torches are employed, the one located in the lower part enables the deposition of the core glass, while the one placed above allows the deposition of the cladding glass.
As it has been previously mentioned, once the preform has been manufactured, the fiber is drawn. The speed of the fiber draw depends on the preform, fiber type and available equipment, and it can vary between a few meters per minute to up to more than 1 kilometer per minute. During the drawing process the diameter of the fiber is controlled with high precision. Then, a protective jacket made of two layers (a soft inner coating and a hard outer one) is added to the fiber. Finally, different tests are performed to check everything is correct, including optical (attenuation, bandwidth, numerical aperture,…), mechanical (tensile strength) or environmental (temperature dependence, aging) properties.
In conclusion, in this post we have revised the manufacturing process of an optical fiber, which includes the preform production, the fiber draw and the performance tests. There are several techniques to fabricate the preform: modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), and vapor axial deposition (VAD), which have several points in common as they are all vapor deposition processes.
Written by J.J. Imas