fiber fabrication (original) (raw)
Definition: the fabrication of optical fibers
Alternative term: fiber manufacturing
Category: 
fiber optics and waveguides
Related: fibersfiber preformsrare-earth-doped fibersfiber corephotonic crystal fibersoptical fabrication
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DOI: 10.61835/7ol Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
Introduction
There is a wide range of methods for the fabrication of optical fibers. In a first step, these can be divided into methods involving a so-called fiber preform and methods of direct fiber production. Preform-based methods are most important at least in the context of glass fibers (which are mostly silica fibers), whereas direct methods, e.g. based on extrusion, are common for plastic optical fibers and for some specialty glasses. In the following, this article first discusses how a fiber can be made from a preform, then how preforms are fabricated, and finally methods not requiring a preform.
Fiber Pulling from a Preform
Figure 1: A fiber drawing tower.
Most glass fibers are 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 with a diameter between about 1 cm and 10 cm and roughly 1 m to 2 m length. Along its axis, the preform contains a region with increased refractive index, which will form the fiber core.
When the preform is heated in a furnace (oven) at the top of the drawing tower, such that its bottom part becomes soft, a thin fiber can be pulled out of the bottom of the preform and cooled to become solid. It is then wound up on a spool, using a spinning machine. 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 just below the furnace). This also leads to a quite constant fiber core diameter, which is important for various reasons.
Before the fiber is wound up, one usually applies a thin polymer coating for mechanical and chemical protection. For that purpose, the fiber can (after having been cooled down enough) be fed through a coating applicator, followed by a curing oven and/or a UV irradiator for curing the coating. Multiple coating layers may be applied using a suitable sequence of coating cups (applicators) and curing facilities. Coatings e.g. for telecom fibers frequently consist of two or more different layers for optimum suppression of microbends. Typical coating materials used are acrylate, silicone and (for fibers with high temperature resistance) polyimide, but there are also carbon and metal coatings for special applications. Additional PVC or similar protective coatings can be made by extrusion after the drawing process. For more details, see the article on fiber coatings.
The fiber from a single preform can be many kilometers long. Particularly telecom fibers are needed with long length, and it is desirable to obtain a high pulling speed of typically well over 10 m/s or even above 20 m/s. Simpler machines for smaller production volumes may only use pulling speeds of the order of 1 m/s.
Particularly for fibers which need to have a small polarization mode dispersion, it is common to spin either the preform (around its axis) or the fiber itself during the pulling process. Such spun fibers have become common for optical fiber communications with high data rates and are also used for certain fiber-optic sensors.
Fiber pulling works quite well for the usual silica fibers, since silica has a rather broad glass transition, i.e., a large range of temperatures in which the viscosity is in a suitable range. Other materials, as used e.g. for fluoride fibers, have a much smaller temperature range suitable for pulling, and the method is accordingly more delicate.
Additional measures may be required for drawing photonic crystal fibers. For example, one sometimes applies an overpressure to the preform to prevent the collapse of the tiny air holes and/or to control the thickness of the resulting tiny silica strands.
It is also possible to write type-II fiber Bragg gratings into the fiber during the fabrication process. For that purpose, ultraviolet nanosecond laser pulses are shot at the fiber through some phase mask just before the fiber is coated.
Concerning the fabrication of fiber preforms, see the article on fiber preforms.
Fabrication Methods Not Involving a Preform
Soft glass fibers are often fabricated with the double crucible method, where core and cladding are simultaneously drawn from the crucible. The crucible has a reservoir for the molten 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 vapor deposition methods and is still used e.g. for soft glasses. Compared with drawing from a preform, it can be adapted more easily to different glass materials. However, it is less suitable for producing ultrapure fibers with very low losses, as it is difficult to avoid any contamination with material from the crucible.
Some fibers, e.g. plastic optical fibers, are produced in a simple extrusion process, which is similar to the double crucible method. Such fibers are interesting for applications in mass markets, but do not reach top-level performance.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
How are optical glass fibers typically fabricated?
Most fibers are made by pulling them from a heated glass rod, called a preform, in a tall apparatus known as a fiber-drawing tower. The preform already contains the core and cladding structure of the final fiber.
What is a fiber preform?
A fiber preform is a glass rod, typically between 1 cm and 10 cm in diameter and up to 2 m long, which serves as the source material for drawing an optical fiber. It contains a region with an increased refractive index that will become the fiber core.
Why are optical fibers coated during the drawing process?
A thin polymer coating is applied to the fiber for mechanical and chemical protection immediately after it is drawn and cooled. This coating is crucial for the fiber's durability and can also help suppress losses from microbends.
How is the fiber diameter kept constant during fabrication?
An automatic feedback system is used, which includes a diameter monitor just below the furnace. This system constantly measures the fiber's thickness and adjusts the pulling speed to maintain a constant diameter.
What are spun fibers?
Spun fibers are created by spinning either the preform or the fiber itself around its axis during the pulling process. This technique is used to produce fibers with very low polarization mode dispersion, which is important for high-data-rate optical fiber communications.
How are fibers made without a preform?
Methods like the double crucible technique are used, where molten core and cladding glass are drawn simultaneously from separate reservoirs. Another method is extrusion, which is common for plastic optical fibers.
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Bibliography
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| [6] | L. Cognolato, “Chemical vapour deposition for optical fibre technology”, J. Phys. IV France 5, C5-975 (1995); doi:10.1051/jphyscol:19955115 |
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| [8] | M. Leich et al., “Highly efficient Yb-doped silica fibers prepared by powder sinter technology”, Opt. Lett. 36 (9), 1557 (2011); doi:10.1364/OL.36.001557 |
(Suggest additional literature!)
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