Fiber Fabrication | previous | next | feedback |
Most optical fibers are nowadays fabricated by pulling from a so-called preform in a fiber-drawing tower, an apparatus which is typically several meters high. The preform is a glass rod of a few centimeters diameter and roughly 1 m in length. Along its axis, the preform contains a region with increased refractive index, which will form the fiber core. When the preform is heated close to the melting point in a furnace (oven) at the top of the drawing tower, a thin fiber can be pulled out of the bottom of the preform. The fiber from a single preform can be many kilometers long. During the pulling process, the fiber diameter is held constant by automatically adjusting the pulling speed (and possibly the furnace temperature) with an automatic feedback system (containing a diameter monitor below the furnace).
Before the fiber is wound up, it usually receives a polymer coating for mechanical and chemical protection. Such coatings often consist of two or more different layers for optimum suppression of microbends. Additional PVC or similar protective coatings can be made by extrusion after the drawing process.
It is also possible to write type II fiber Bragg gratings into the fiber before it is coated.
Fabrication of Fiber Preforms
Most fiber preforms are fabricated with a process called modified chemical vapor deposition (MCVD or just CVD). This method (and some variations of it) were developed in the 1970s, with pioneering contributions from the University of Southampton (UK), Bell Telephone Laboratories (Bell Labs), and Corning. Here, a mixture of oxygen, silicon tetrachloride (SiCl4) and possibly other substances (e.g. germanium tetrachloride (GeCl4) and rare earth dopants → fiber core) is passed through a rotating silica glass tube, which is heated from outside to ∼1600 °C with a traversing flame. Chemical reactions in the gas form a fine soot of silica (and possibly other substances) which coats the inner surface of the glass tube and is sintered into a clear glass layer. Towards the end of the process, the gas mixture is modified to form a layer with higher refractive index, the precursor of the fiber core. Finally, the tube is collapsed by heating it to ∼2000 °C. The main advantage of MCVD is that extremely low losses down to below 0.2 dB/km can be achieved, because very high-purity materials can be used and contamination is avoided. In particular, SiCl4 and GeCl4 are easily purified by distillation, as they are liquid at room temperature. Due to the absence of hydrogen, the water content of such preforms is very low, avoiding a strong loss peak at 1.4 μm, which can also affect the telecom bands (→ optical fiber communications).
For cases where MCVD can not be applied, the rod-in-tube technique is another option. Here, a usually doped rod of a glass with higher refractive index is inserted into a glass tube with lower refractive index. Both can be reasonably well connected by heating. There are also methods where the molten core glass is poured into the cladding tube, or sucked into the tube using a vacuum pump.
The preforms for multimode fibers, particularly for large core fibers, are often fabricated using plasma outside deposition (POD), where an outer fluorine-doped layer with depressed refractive index, later forming the fiber cladding, is made with a plasma torch. The core can then be made of pure silica, without any dopant.
Preforms for photonic crystal fibers, containing small holes throughout, are usually fabricated by stacking capillary tubes and/or rods, in most cases made of pure fused silica. It is easy, of course, to introduce rare-earth-doped rods for active fiber devices.
The fabrication of rare-earth-doped fibers involves various special aspects, regarding both the doping itself and the geometry e.g. in the case of double-clad fibers.
Alternative Fabrication Methods
To begin with, there are variations of the MCVD method. For example, the silica soot can be deposited on the outside surface of a glass mandrel (→ outside vapor deposition, OVD); the mandrel is removed after deposition, and the preform is then collapsed by heating it to a high temperature. Also, there are methods using a plasma, generated with a radio-frequency coil around the deposition tube (→ plasma chemical vapor deposition = PCVD and plasma modified chemical vapor deposition = PMCVD). Another method is vapor-phase axial deposition (VAD), where glass soot is deposited on the bottom end of a rotating silica rod. The preform is continuously pulled upwards during the process, according to the growth at the bottom end.
For special core dopants, such as rare earth ions, modified fabrication techniques are often required, as discussed in the next section.
Not all fibers are actually produced in drawing towers. Soft glass fibers are often fabricated without a preform with the double crucible method, where core and cladding are simultaneously drawn from the crucible. The crucible has a reservoir for the core glass, leaving a small opening at the center, and one (or several) reservoirs for cladding glass. The double crucible method is older than the MCVD/preform method and is still used e.g. for soft glasses. Compared with drawing from a preform, it has a higher tolerance for different glass material parameters, but it is less suitable for producing ultrapure fibers with very low losses.
Some fibers, e.g. polymer fibers, are produced in a simple extrusion process. Such fibers are interesting for applications in mass markets, but do not reach top-level performance.
Special Fiber Core Dopants
The core of a fiber can be doped with laser-active ions, normally rare earth ions of erbium, neodymium, ytterbium, or thulium. If these ions are excited with suitable pump light, optical amplification results, which is the basis of fiber lasers or amplifiers. Other dopants can modify the refractive index, improve the solubility for rare earth ions, or modify the photosensitivity. The article on fiber core contains more details.
Not all dopants can be incorporated with methods based on convective transport. In particular, precursors for rare earth dopants usually have a too low vapor pressure. A common technique for such substances is solution doping, where a porous frit is deposited on the inner side of a hollow silica tube. This frit is then soaked with a solution containing a rare earth salt (e.g. a chloride). Later on, the preform needs to be further processed to form a dry and compact rare earth oxide layer. An alternative technique is direct nanoparticle deposition from some aerosol. This method allows for high doping concentrations with good homogeneity and accurate control of the doping profile.
Bibliography
| [1] | S. Nagel et al., "An overview of the modified chemical vapor deposition (MCVD) process and performance", IEEE J. Quantum Electron. 18 (4), 459 (1982) |
| [2] | W. A. Gambling, "The rise and rise of optical fibers" (an informative review on the development of glass fibers), IEEE J. Sel. Top. Quantum Electron. 6 (6), 1084 (2000) |
See also: fibers, rare-earth-doped fibers, fiber core, photonic crystal fibers
Category: fibers and other waveguides
