Manufacturing of Optical Fibers

Optical fiber is a single, hair fine filament drawn from molten silca glass. Today, this has replaced metal wires in high speed communication.In a fiber optic communications system, cables made of optical fibers connect datalinks that contain lasers and light detectors. To transmit information, a datalink converts an analog electronic signal into digital pulses of laser light. These travel through the optical fiber to another datalink, where a light detector reconverts them into an electronic signal. It makes people to wonder how something only 1/8 of a mm - 0.005 inches - in diameter can be made with such precison. So here is breif explanation of how optical fiber is made.

Raw Materials

Optical fiber is mostly made from silicon dioxide(SiO 2 ) but some little amount of other materials such as fluorozirconate, fluoroaluminate, and chalcogenide glasses as well as crystalline materials like sapphire, are used for longer-wavelength infrared or other specialized applications.Chemical compounds such as germanium tetrachloride (GeCl₄ ) and phosphorus oxychloride (POCl₃ ) can be used to produce core fibers and outer shells, or claddings, with function-specific optical properties.

why silca?

Silica, which be drawn into fibers at reasonably high temperatures, has a fairly broad glass transformation range. One other advantage is that fusion splicing and cleaving of silica fibers is relatively effective. Silica fiber also has high mechanical strength against both pulling and even bending, provided that the fiber is not too thick and that the surfaces have been well prepared during processing. Even simple cleaving (breaking) of the ends of the fiber can provide nicely flat surfaces with acceptable optical quality. Silica is also relatively chemically inert. In particular, it is not hygroscopic (does not absorb water) also it can be doped with various materials. Silica fiber also exhibits a high threshold for optical damage. But, pure silca is not best suitable for optical fiber, because it exhibits a low solubility for rare earth ions. This can lead to quenching effects due to clustering of dopant ions. These properties makes silca most widely use material for optical fibers.

Process of Manufacutring

There are two main steps in the process of converting raw materials into optical fiber ready to be shipped.

Manufacturing of the pure glass preform
Drawing of the preform

The first step in manufacturing glass optical fibers is to make a solid glass rod, known as a preform. Ultra-pure chemicals -- primarily silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) -- are converted into glass during preform manufacturing. These chemicals are used in varying proportions to fabricate the core regions for the different types of preforms. The basic chemical reaction of manufacturing optical glass is:

SiCl4 (gas) + O2 > SiO2 (solid) + 2Cl2 (in the presence of heat)
GeCl4 (gas) + O2 > GeO2 (solid) + 2Cl2 (in the presence of heat)

The core composition of all standard communication fibers consists primarily of silica, with varying amounts of germania added to increase the fiber's refractive index to the desired level. Single-mode fibers typically have only small amounts of germania and have a uniform composition within the core. Multimode fibers typically have a much higher refractive index, and therefore much higher germania content. Also, the core composition and the refractive index of graded-index multimode fibers changes across the core of the fiber to give the refractive index a parabolic shape.

Today Three methods are used to manufacture perform.

Modified Chemical Vapor Deposition(MCVD)

First, a cylindrical preform is made by depositing layers of specially formulated silicon dioxide on the inside surface of a hollow substrate rod. The layers are deposited by applying a gaseous stream of pure oxygen to the substrate rod. Various chemical vapors, such as silicon tetrachloride (SiCl 4 ), germanium tetrachloride (GeCl 4 ), and phosphorous oxychloride (POCl 3 ), are added to the stream of oxygen. As the oxygen contacts the hot surface of the rod-a flame underneath the rod keeps the walls of the rod very hot-silicon dioxide of high purity is formed. The result is a glassy soot, several layers thick, deposited inside the rod. This soot will become the core. The properties of these layers of soot can be altered depending on the types of chemical vapors used.

Fig.Illustration of MCVD (inside) process

After sufficient layers are built up, the tube is collapsed into a solid glass rod referred to as a preform. It is now a scale model of the desired fiber, but much shorter and thicker. The preform is then taken to the drawing tower, where it is pulled into a length of fiber up to 10 kilometers long.

Outside Vapor Deposition(OVD)

One of many variations of vapour deposition technique for fabricating optical fiber. Here an inert rod is layered with core and cladding glass deposits built up on the outside. Once enough layers are in place, the rod is removed and the layers consolidated into a solid preform which can be drawn into fiber. Silicon chloride, SiCl4 and germanium chloride, GeCl4 are oxidised to form silica and germania particles for the deposition.

Vapour Phase Axial Deposition (VPAD)

In this diagram, we see how the preform is made. A seed rod is slowly rotated and pulled upward. As the seed rod is pulled, two burners deposit fine glass soot. The lower burner in this diagram is depositing the core glass material, and above it is a burner depositing the cladding glass. The rate at which the seed rod is pulled is carefully controlled by servo mechanisms. After deposition the glass soot rod is dehydrated and sintered into a solid preform in a furnace.