A fiber preform is a typically cylindrical piece of optical glass which is used for drawing an optical fiber in a fiber drawing tower. Typically, preforms are e.g. 40 cm long and have a diameter of a couple of centimeters, but there are also much longer preforms and some with a large diameter of e.g. 20 cm. The article on fiber fabrication describes the drawing process and details of the used drawing towers.
The drawn fiber has a much smaller diameter than the preform, and all features of the preform are getting correspondingly smaller in the fiber. In particular, this holds for the refractive index profile, including the structure made for the fiber core.
Plastic optical fibers can be drawn from preforms in a similar process as often used for silica fibers, only with a much lower temperature (e.g. 200 °C) of the preform.
In most cases, the fiber preforms are fabricated on the same sites where the fiber drawing tower is operated. However, there are also suppliers of various kinds of fiber preforms, which are then to be drawn into fibers elsewhere. Also, some companies offer to apply the cladding glass to received core rods.
Fabrication of Standard Fiber Preforms
Here, we cover only the fabrication of glass preforms, and mostly on those for silica fibers. In this section, the fabrication of standard fiber preforms is explained, while special preforms for various types of specialty fibers are discussed later on.
Vapor Deposition Methods
Many fiber preforms are fabricated with a process called modified chemical vapor deposition (MCVD or just CVD). This method was developed for silica telecom fibers 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 generated, and chemical reactions in the gas (e.g. combustion of hydrogen) produce a fine white “soot” of (often doped) silica which is deposited on the preform and later on sintered into a clear glass layer at ≈1500 °C. During that viscous sintering, the preform is held in a gas atmosphere, which can be oxidizing or reducing, and influences the deviation from perfect stoichiometry. The process results in a fully dense and very clear glass.
Instead of conventional MCVD, one can use plasma activated chemical vapor deposition (PCVD). The difference to MCVD is that microwaves instead of a burner are used for heating the deposition region. The deposition is slow, but very precise.
A modified method with particularly high precision is plasma impulse chemical vapor deposition (PICVD), where short microwave pulses are used.
There is also plasma-enhanced chemical vapor deposition (PECVD), operating at atmospheric pressure with fairly high deposition rate.
The general advantage of vapor deposition methods is that extremely low propagation 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. Particularly when no hydrogen is present (e.g. as fuel gas), the water content of such preforms is very low, avoiding a strong loss peak at 1.4 μm, which would also affect the telecom bands (→ optical fiber communications).
The different vapor deposition methods differ in many respects, e.g. concerning the possible material purity, the degree, precision and flexibility of refractive index control, the mechanical strength of the fabricated fibers, and the deposition efficiency and speed.
Different fabrication strategies have been developed:
- Inside vapor deposition (IVD) is the most common process. Here, the deposition of material occurs inside a rotating silica glass tube, which is heated with a slowly moving gas torch from outside to ≈ 1600 °C with a flame. The burner is continuously moved back and forth along the tube. 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 more than 2000 °C; surface tension of the glass at the inner wall drives that collapse. The special deposited glass on the inner side then forms the region which will become the fiber core.
- Outside vapor deposition (OVD) is a process where the silica soot is deposited on the outer surface of some target rod (e.g. a glass mandrel), rather than inside a tube as with MCVD. Together with the material precursors such as SiCl4, a fuel gas such as hydrogen or methane is supplied to a burner which is again moved along the rotating rod. After the deposition, which increases the rod diameter, the target rod is removed, and the preform is consolidated at ≈1800 °C in a furnace, where it is also purged with a drying gas for lowering the hydroxyl content. Outside vapor deposition is used e.g. for making multimode fibers with a pure silica core and a fluorine-doped cladding; only the cladding is made by vapor deposition.
- Vapor phase axial deposition (VAD or AVD) is similar to OVD, but again uses a modified geometry, where the deposition occurs at the end of the target rod (growth in axial direction). The rod is continuously pulled away from the burner, and very long preforms can be made. Consolidation of the material can be done in a separate zone melting process. An important difference to OVD and IVD is that the doping profile is determined only by the burner geometry, rather than by a variation of the gas mixture over time.
Each strategy may be combined with different methods of deposition, i.e., of forming the gas phase from which silica soot is generated.
In some cases, one uses an additional overcladding process. Here, one inserts the glass rod into a capillary tube (typically consisting of synthetic silica) which is then collapsed by heating, forming an additional outer layer to the original rod.
Preforms for Specialty Fibers
Various kinds of specialty fibers require preforms with special features, and thus adapted methods of preform fabrication:
For active fiber devices such as fiber lasers and fiber amplifiers, rare-earth-doped fibers are required. Here, the fiber core is doped with rare earth ions e.g. of erbium, neodymium, ytterbium, or thulium. Additional dopants can modify the refractive index, improve the solubility for rare earth ions, or modify the photosensitivity.
Not all dopants can be easily incorporated with vapor deposition methods, requiring convective material transport. In particular, precursors for rare earth dopants usually have a too low vapor pressure. One possibility to overcome this problem is to expose the source of rare earth ions to a higher temperature. For example, a glass tube as used for MCVD may contain an additional dopant chamber or a porous silica part soaked with a rare earth salt, which is heated with an additional burner.
Another common technique is solution doping, where initially a porous silica frit (made at a lower temperature, not yet sintered and not yet containing rare earth ions) 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.
Sintering techniques  are also frequently employed for fabricating rare-earth-doped fiber cores.
For materials where vapor deposition can not be applied, because suitable gaseous raw precursors are not available (e.g. for fluoride fibers), the rod-in-tube technique is another option. Here, a 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, but great care is required to avoid bubbles and other disturbances.
There are also casting methods where the molten core glass is poured into the cladding tube, or sucked into the tube using a vacuum pump.
Photonic Crystal Fibers
Preforms for photonic crystal fibers are typically fabricated by stacking glass tubes (often made of pure fused silica) and capillaries inside a larger glass tube. In the fiber drawing process, the capillary holes and the voids between those glass elements are transformed into tiny air holes in the fiber, having only nanometer dimensions. Limitations arise from the need to achieve a sufficiently high mechanical stability (also during the fiber drawing process) and avoid the collapse of needed air holes during the drawing. The the article on photonic crystal fibers for more details on how to fabricate their preforms.
Preforms for multi-core fibers cannot be made with the normal preform fabrication techniques, but there are several options:
- The rod-in-tube method can be used, with one rod for each fiber core.
- One may use some number of single-core preforms and assemble them together to obtain a bunch fiber.
- In case of photonic crystal fibers, it is easy to assemble a preform containing multiple cores.
Polarization-maintaining fibers are often of PANDA type (see Figure 1, left side). Here, one needs to drill two very precise holes parallel to the reform axis, insert to accurately fitting stress rates of borosilicate glass and finally process the preform ends. The fiber drawing process also needs to be optimized to work well with such preforms.
Another common kind of polarization-maintaining fibers, called bow-tie fibers (Figure 1, right side), use stress elements of different geometrical shape, which are also made with a quite different technique. They are made with chemical vapor deposition, just as most other parts of the preform. It is common to first deposit such material all around the core region but then to remove some of it into areas opposite the fiber core with chemical etching. Further material with different properties is then deposited there.
A similar approach is used for elliptical-stress-layer fibers, where the stress element is also deposited with chemical vapor deposition, but in that case shaped with mechanical machining rather than with chemical etching. Two flat surfaces are machined into the preform, thus not having a cylindrical shape; nevertheless, it can be used for pulling a fiber with circular cross-section when using a carefully optimized drawing process.
Besides, there are elliptical core fibers, where one primarily exploits form birefringence rather than stress birefringence. An elliptical core in the preformed can be obtained with an asymmetric vapor deposition process.
An entirely different approach can be chosen for polarization-maintaining photonic crystal fibers. Here, one can simply introduce an asymmetry, causing stress or form birefringence, by appropriately arranging the glass elements of the preform.
The RP Photonics Buyer's Guide contains 3 suppliers for fiber preforms.
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