Optical Fiber and Preform Manufacturing Process

One advance bail we may like to take is for preparing this article we have referred some articles of some European fiber drawing tower manufacturers. Utilization of optical fibers in the field of communication has already been playing a predominant role over other long distance transmission media like coaxial cables microwave etc.

When put in simple words, an optical fiber is a thin cylindrical glass rod. Optical fibers are dielectric. Optical fibers consists of core and cladding having different refractive indices for total internal reflection of light so as the light to propagate through the optical fiber.

There are two steps involved in the manufacturing process of low loss optical fiber. Preform fabrication and Fiber drawing process from preform. Preform fabrication and Fiber drawing from performs are carried out at Optical fiber manufacturing plant.

The most commonly employed methods for optical fiber preform fabrication can be broadly categorized into the following types:

Inside Vapor Deposition Method ( IVD)

Out side Vapor Phase Oxidation (OVPO)

Inside vapor deposition method is further categorized as shown below:

Modified Chemical Vapor Deposition Method - MCVD

Plasma Chemical Vapor Deposition - PCVD

Plasma Enhanced MCVD - PMCVD

The outside vapor phase oxidation method is further categorized as shown below:

Outside Vapor Deposition Method - OVD

Vapor Axial Deposition Method – VAD

In all vapor deposition processes mentioned above, high purity glass to which different doping elements such as Germanium, Phosphorous, Boron, Fluorine etc. are added. This doping is done to modify the refractive indices. This refractive index difference is the basic characteristic of light propagation through an optical fiber. Generally, Germanium and Phosphorous are used to increase the refractive index of Silica. Boron and Fluorine are used to reduce the refractive index.

Basic raw materials used for making perform are in the form of halides, such as SiCl4, GeCl4, POCl3 etc. These are liquids. BCL3, SF6 etc. are vapors. These raw materials of Optical fiber will be converted into vapor phase. By converting into vapor phase, transition metal impurities like Ferrous, Nickel, Cobalt etc. will left un-vaporized. This happens because of the large difference in vapor pressure between the halides and the transitional metal impurities.

The raw material vapors are mixed with oxygen to form the reactant vapor stream. Reactant generation control of composition and transport is done with a vapor delivery system. Carrier gas will be bubbled through the liquid raw materials in order to convert them to vapor by the vapor delivery system. This output is transported to the heat zone where the reaction takes place in the heat zone of the tube and soot particles consisting of SiO2. GeO2, P2O5 etc. are formed which finally deposit on the inside glass surface.

The above is the procedure for making performs. The next process is drawing optical threads and applying coating over it in order to make them practically usable in the field.
A good fiber drawing process will ensure the following requirements.

* Draw optical fibers of desired diameter with very precise diameter control.

* Refractive index profile created at the preform stage should not be altered.

* The perform is made with a core-clad dimensional ratio. The fiber drawing should ensure the same ratio through the process of optical fiber drawing.

Optical characteristics should not be degraded during drawing and the suface quality shall be maintained ad high intrinsic tensile strength of glass shall be retained.

The perform is fed in to the high temperature furnace by a high precise feeding mechanism. The melted fiber is pulled by a capstan whose speed is automatically controlled by a fast response feed back loop from a scanning laser diameter guage to maintain an online optical fiber diameter value. The laser monitor gauge is capable of providing positional information which will be utilized to serve precision stepper motor driven X-Y slides on the perform feed to automatically maintain a fixed fiber line into the acrylate coating applicator.

As a current practice two protective coatings are applied on line by polymerization of concentrically applied liquid coating curable either UV radiation. Optical fiber Drawing tension is monitored as a function of time. The Optical fiber thus formed is reeled at low tension into fiber spools.