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Fibers

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Buyer's Guide

212 suppliers for optical fibers are listed.

Among them:

NKT Photonics

Crystal Fibre line of photonic crystal fibres, with nonlinear fibers for supercontinuum generation, active and passive large mode area fibers and hollow-core fibers

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RP Photonics has a detailed expertise on optical fibers and offers a powerful simulation software for fiber devices. Apart from this encyclopedia, Dr. Paschotta has authored the SPIE Field Guide to Optical Fiber Technology and a comprehensive tutorial "Passive Fiber Optics".
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Definition: a kind of long and thin optical waveguides which can be bent to some degree

German: Fasern

(British spelling: fibres)

Optical fibers are a kind of waveguides, which are usually made of some kind of glass, can potentially be very long (hundreds of kilometers), and are – in contrast to other waveguides – fairly flexible. The most commonly used glass is silica (quartz glass, amorphous silicon dioxide = SiO2), either in pure form or with some dopants. Silica is so widely used because of its outstanding properties, in particular its potential for extremely low propagation losses (realized with ultrapure material) and its amazingly high mechanical strength against pulling and even bending (provided that the surfaces are well prepared).

Most optical fibers used in laser technology have a core with a refractive index which is somewhat higher than that of the surrounding medium (called the cladding). The simplest case is that of a step-index fiber, where the refractive index is constant within the core and within the cladding. The index contrast between core and cladding determines the numerical aperture of the fiber (see below), and is typically small, so that optical fibers are weakly guiding. Light launched into the core is guided along the core, i.e., it propagates mainly in the core region, although the intensity distribution may extend somewhat beyond the core. Due to the guidance and the low propagation losses, the optical intensity can be maintained over long lengths of fiber.

launching light into a glass optical fiber

Figure 1: Simple setup for launching light into an optical fiber (not to scale). A collimated laser beam is focused into the fiber core. The light propagates along the core and leaves the other fiber end as a divergent beam. The fiber core and cladding are made of glass. A polymer jacket protects the glass fiber.

A less frequently used principle of guiding light is based on a photonic bandgap (→ photonic bandgap fibers). For example, this can be realized with concentric rings of different refractive index, forming a kind of two-dimensional Bragg mirror.

The term specialty fibers is used for many different kinds of optical fibers with special properties, and is thus not very specific.

Applications of Optical Fibers

There are many important applications of fiber optics. Some of the most important ones are:

Therefore, fiber optics has become a particularly important area within the technology of photonics.

Fiber Modes – Single-mode versus Multimode Fibers

A optical fiber can support one or several (sometimes even many) guided modes, the intensity distributions of which are located at or immediately around the fiber core, although some of the intensity may propagate within the fiber cladding. In addition, there is a multitude of cladding modes, which are not restricted to the core region. The optical power in cladding modes is usually lost after some moderate distance of propagation, but can in some cases propagate over longer distances. Outside the cladding, there is typically a protective polymer coating, which gives the fiber improved mechanical strength and protection against moisture, and also determines the losses for cladding modes. Such buffer coatings may consist of acrylate, silicone or polyimide, for example. At the fiber ends, the coating often has to be stripped off.

An important distinction is that between single-mode and multimode fibers:

modes of a fiber

Figure 2: Electric field amplitude profiles for all the guided modes of a fiber with a top-hat refractive index profile (→ step index fiber). The two colors indicate different signs of electric field values. The lowest-order mode (l = 0, m = 1, called LP01 mode) has an intensity profile which is similar to that of a Gaussian beam. In general, light launched into a multimode fiber will excite a superposition of different modes, which can have a complicated shape.

Long-range optical fiber communication systems usually use single-mode fibers, because the different group velocities of different modes would distort the signal at high data rates (→ intermodal dispersion). For shorter distances, however, multimode fibers are more convenient as the demands on light sources and component alignment are lower. Therefore, local area networks (LANs), except those for highest bandwidth, normally use multimode fiber.

Single-mode fibers are also normally used for fiber lasers and amplifiers. Multimode fibers are often used, e.g., for the transport of light from a laser source to the place where it is needed, particularly when the light source has a poor beam quality and/or the high optical power requires a large mode area.

Different modes of an optical fiber can be coupled via various effects, e.g. by bending or often by irregularities in the refractive index profile. These may be unwanted or purposely introduced, e.g. as fiber Bragg gratings. Waveguide theory shows that an important factor for the coupling between different fiber modes is the difference in their wavenumbers, which for efficient coupling has to match the spatial frequency of a coupling disturbance.

Main Parameters

The design of a step-index fiber can be characterized with only two parameters, e.g. the core radius a and the refractive index difference Δn between core and cladding. Typical values of the core radius are a few microns for single-mode fibers and tens of microns or more for multimode fibers.

Instead of the refractive index difference, one usually uses the numerical aperture, defined as

numerical aperture

which is the sine of the maximum acceptable angle of an incident beam with respect to the fiber axis (considering the launch from air into the core in a ray-optic picture). The NA also quantifies the strength of guidance. Typical values are of the order of 0.1 for single-mode fibers, even though actual values vary in a relatively large range. For example, large mode area single-mode fibers can have low numerical apertures below 0.05, whereas some rare-earth-doped fibers have values of 0.3 and higher for a high gain efficiency. NA values around 0.3 are typical for multimode fibers. The sensitivity of a fiber to bend losses strongly diminishes with increasing NA, which causes strong confinement of the mode field to the core.

Another frequently used parameter is the V number

V number

which is a kind of normalized frequency. Single-mode guidance is achieved when the V number is below ≈ 2.405. Multimode fibers can have huge V values. The number of modes then scales with V2.

As a numerical example, consider a typical step-index silica fiber for single-mode operation in the 1.5-μm spectral region, with a cut-off wavelength of 1.3 μm and a numerical aperture of 0.1. The refractive index of the pure silica cladding at 1.5 μm is ≈ 1.444. The core index is ≈ 1.4475, i.e., the index difference is ≈ 0.0035. The core diameter is 10 μm, and the V number is 2.1.

Refractive Index Profiles

The refractive index profile of optical fibers often deviates substantially from that of a step-index profile (with constant refractive index within the core):

Note that the definitions of the numerical aperture and consequently of the V number become somewhat ambiguous for non-rectangular index profiles.

In addition, there are so-called photonic crystal fibers (see below), where the refractive index profile is strongly structured.

Propagation Losses

The power losses for light propagating in an optical fiber can be extremely small, particularly for single-mode silica fibers as used in telecommunications. The resulting attenuation is typically dominated by Rayleigh scattering for short wavelengths and by multiphonon absorption at long wavelengths. Rayleigh scattering results from refractive index fluctuations, which are to some extent unavoidable in a glass, but can be strongly increased by concentration fluctuations in fibers with high numerical aperture. Other loss contributions come from inelastic scattering (spontaneous Brillouin scattering and Raman scattering), from absorbing impurities, and from fluctuations of the core diameter.

For silica fibers, the loss minimum occurs around 1.5–1.6 μm and can be below 0.2 dB/km (≈ 4.5% per km), which is close to the theoretical limit based on Rayleigh scattering in an amorphous glass material. There is often some loss peak around 1.4 μm, which can be largely eliminated, however, by carefully optimizing the chemical composition of the core so as to reduce the OH content (i.e. the concentration of hydroxyl bonds). Interestingly, fibers with high OH content can exhibit lower losses for ultraviolet light, whereas they exhibit pronounced loss peaks in the infrared spectral region.

Multimode fibers, and in general fibers with high numerical aperture, tend to have significantly higher propagation losses, essentially because the higher doping level of the core increases the scattering losses. Rare-earth-doped fibers also have much higher losses, but as more than some tens of meters of such a fiber are rarely used, this usually does not matter for their applications.

Polarization Properties

Despite the typically cylindrical symmetry, fibers usually exhibit some amount of birefringence which can cause the polarization state of light to evolve in an uncontrolled way (→ polarization of laser emission). There are special polarization-maintaining fibers with a strong built-in birefringence to solve this problem. In addition, there are single-polarization fibers, which guide only light with one polarization direction. There are also various types of fiber polarization controllers, which allow one to adjust the state of polarization in a fiber.

Dispersion Properties

As a result of the waveguide properties, the chromatic dispersion of an optical fiber can deviate significantly from its material dispersion, particularly when the mode area is small (→ waveguide dispersion). This makes it possible to obtain unusual dispersion properties by engineering the waveguide properties. For example, dispersion-shifted fibers can have near zero dispersion in the 1.5-μm spectral region, and there are dispersion-flattened fibers with small dispersion over a large wavelength range or dispersion-decreasing fibers. A particularly high design freedom exists for photonic crystal fibers (see below).

The birefringence makes the group delay polarization-dependent; this is often called polarization mode dispersion. For multimode fibers, there is also intermodal dispersion, i.e., a dependence of the group velocity on the fiber mode, which may be minimized by choosing a suitable refractive index profile but is typically larger than the dispersion of single-mode fibers.

Fiber Fabrication

Most optical fibers are fabricated by pulling from a so-called preform, which is a glass rod with a diameter of a few centimeters and roughly 1 m length. Along its axis, the preform contains a region with increased refractive index, which will form the core. When the preform is heated close to the melting point in a furnace (oven), a thin fiber with a diameter of typically 125 μm and a length of many kilometers can be pulled from the bottom of the preform. Before the fiber is wound up, it usually obtains a polymer buffer coating for mechanical and chemical protection.

The core of a fiber can be doped with laser-active ions, normally rare earth ions of erbium, neodymium, ytterbium, or thulium. When these ions are excited with suitable pump light, optical amplification occurs, which can be used in fiber lasers or amplifiers.

More details are given in the article on fiber fabrication.

Fiber-optic Cables

fiber connector

Figure 3: A fiber connector at the end of a fiber cable. The photograph has been kindly provided by NKT Photonics.

Glass fibers are amazingly robust, considering that glass is known as a particularly fragile material. However, additional protection is often required when fibers are used in an environment which is e.g. accessible by operators. For laboratory use, e.g. for sending light from a telecom setup to some diagnostic instrument and also in large industrial assemblies, it is convenient to use connectorized fiber cables (fiber patchcords, see Figure 3), where the actual fiber is surrounded by additional protective layers. While the bare glass fiber may have a typical diameter of 125 μm, and the polymer buffer and jacket increase this to a few hundred micrometers, the total diameter of the fiber cable may be several millimeters. Apart from considerably strengthening the cable, the (typically yellow) cable material also makes it much easier for operators to recognize the fiber, thus avoiding too harsh treatment in the first place.

Thicker fiber cables are used for the delivery of high-power beams, e.g. from fiber-coupled diode lasers to a solid-state laser head or to some material processing equipment. For power levels of hundreds of watts to many kilowatts, the fiber cable may have a diameter of several centimeters. High-power fiber cables may also contain sensors for detecting damage to the cable, so that the laser source can be immediately switched off when there is a risk that high-power laser radiation exits the cable at a damaged point. Such precautions can be very important for laser safety.

Fiber cables for long-haul optical fiber communications are also fairly thick, because they often have to pass through harsh environments and must be protected accordingly. In extreme cases, such cables may be lying on the sea bed or are slightly buried there. A high level of protection is required against the mechanical stress both during installation and at later times.

Of course, a fiber cable can contain multiple fibers. In this way, the already huge data transmission capability of a single fiber can be multiplied to enormously high levels.

Fiber-optic Components

Many optical components can be directly made from fibers. Some examples are:

Other fiber-optic components contain bulk elements with attached fiber connections. Examples of so-called fiber-pigtailed devices are:

Fiber collimators provide a connection between fiber optics and free-space optics. Essentially, such a device contains a collimation lens, transforming the strongly divergent beam from a fiber end into a collimated beam. Finally, there are mechanical splices, providing semi-permanent connections between fibers.

Polishing, Cleaving, and Splicing

Clean and smoothly shaped fiber ends can be produced with polishing techniques. These can also be used to produce end faces which are not perpendicular to the fiber axis. With a tilt angle of the order of 10° (angle polishing), reflections from fiber ends can be effectively eliminated from the beam path, so that e.g. reflection-sensitive lasers are well protected.

A much faster technique for preparing fiber ends is cleaving. Here, one typically pulls the fiber while scratching it from a side, e.g. with a vibrating diamond blade. This makes the fiber break with normally fairly smooth end faces – at least around the core region. By twisting the fiber during this process, angle cleaves can be fabricated, but the results are less reproducible than for polishing techniques.

Optical fibers (particularly those made of silica) can also be spliced together. One may use the technique of fusion splicing for making permanent fiber joints. A simpler technique is mechanical splicing, where the fiber ends are firmly held together by some mechanical means, but not fused. Here, however, the splice losses are typically higher, even when reduced with an index-matching gel between the surfaces.

There are also many types of fiber connectors which allow one to obtain good mechanical contact (as in a mechanical splice), but also to disconnect the fibers easily as required.

In general, the handling of fiber ends is fairly delicate, compared with the handling of electrical connections. Apart from problems with dust, grease and the like, fiber ends are relatively sensitive and are easily scratched. Their handling often requires very expensive equipment (e.g. high-quality fusion splicers), particularly when reliable results are required under field conditions, i.e., in a comparatively dirty environment. On the other hand, a fair comparison with electrical cables has to take into account the much higher transmission capacity of a fiber.

Safety Issues

Laser safety in terms of eye safety is a serious issue with high-power fiber devices. Very harmful high-power light could exit a damage fiber cable; therefore, such cables must be well protected against damage and possibly monitored with built-in sensor systems.

In optical fiber communications, the optical power levels are often small enough to avoid eye safety problems, particularly when using the eye-safe wavelength region around 1.5 μm. However, dangerous power levels can sometimes occur, e.g. in cable TV applications, where a high-power amplifier creates sufficiently signal power for splitting signals into many fibers.

Another risk for eyes is not associated with laser radiation, but with the sharp scraps of fiber ends, as are obtained e.g. when cleaving fibers. These scraps are extremely sharp, may be transported into eyes e.g. when they stick to a finger, and may also penetrate the skin. They should also not be ingested. For such reasons, one should carefully dispose fiber scraps into a properly marked container immediately when they occur, take precautions to make them well visible in the working area, and avoid any eating or drinking near the work area.

Special Types of Fibers

So-called double-clad fibers can have a single-mode core and a multimode inner cladding, the latter transporting the pump light e.g. of a high-power fiber laser or amplifier.

There are various kinds of polarization-maintaining fibers, mostly realized on the basis of strong birefringence. The linear polarization of light is preserved provided that the initial polarization axis is aligned with a birefringent axis of the fiber. In addition, there are also single-polarization fibers (polarizing fibers) where one polarization direction experiences strong losses.

A special kind of optical fibers is the photonic crystal fiber (PCF), also called microstructure fiber or holey fiber. Such fibers typically consist only of a single material (usually silica), containing very small air holes with diameters well below 1 μm. Fabrication of such fibers is possible by using preforms with holes, made e.g. by stacking capillary tubes. By varying the arrangement of air holes, fibers with extremely different properties can be made, e.g.

Photonic crystal fibers are now attracting strong interest for a wide range of applications, including extremely nonlinear fiber devices, soliton fiber lasers operating at short wavelengths, and high-power fiber amplifiers.

Although most fiber cores consist of some variant of silica (e.g. germanosilicate or aluminosilicate glass), other glass materials can also be used. Examples are

Low-cost multimode fibers can be made of polymers (plastic optical fibers, POF), which are cheap materials, allow simple production by extrusion, and are robust and flexible even when made with larger diameters. In some application areas, they allow for substantially cheaper solutions than possible with glass fibers. Even photonic crystal fibers can nowadays be produced from polymers. Some polymer fibers can also be used to guide terahertz waves.

In some cases, fibers are made of crystalline materials such as sapphire, but these fibers are usually not flexible and can be seen as thin rods using waveguide propagation (with or without a core structure at the center). They can be used for very high-power fiber lasers and amplifiers.

Damage of Fibers

Optical fiber devices can be damaged in various ways during operation. Various aspects are relevant in this context:

end cap on a photonic crystal fiber rod

Figure 4: A core-less end cap on a photonic crystal fiber rod. The photograph has been kindly provided by NKT Photonics.

Comparison of Optical Fibers with Electric Cables

In some technical areas, such as optical data transmission over long distances or between computer chips, optical fibers (or other waveguides) compete with electric cables. Compared with the latter, they have a number of pronounced advantages:

On the other hand, fibers also have their disadvantages:

Bibliography

[1]K. C. Kao and G. A. Hockham, “Dielectric-fiber surface waveguides for optical frequencies”, Proc. Inst. Elect. Eng. 113, 1151 (1966)
[2]K. C. Kao and T. W. Davies, “Spectrophotometric studies of ultra low loss optical glasses – I: Single beam method”, J. Phys. E 2 (1), 1063 (1968)
[3]D. Gloge, “Weakly guiding fibers”, Appl. Opt. 10 (10), 2252 (1971)
[4]W. A. Gambling, “The rise and rise of optical fibers”, IEEE J. Sel. Top. Quantum Electron. 6 (6), 1084 (2000) (an informative review on the development of glass fibers)
[5]A. W. Snyder, “Guiding light into the millennium”, IEEE J. Sel. Top. Quantum Electron. 6 (6), 1408 (2000)
[6]Timbercon's fiber-optic keywords
[7]A. W. Snyder and J. D. Love, Optical Waveguide Theory, Chapman and Hall, London (1983)
[8]J. Hecht, City of Light, The Story of Fiber Optics, Oxford University Press, New York (1999)
[9]J. A. Buck, Fundamentals of Optical Fibers, Wiley, Hoboken, New Jersey (2004)
[10]W. Koechner, Solid-State Laser Engineering, 6th edn., Springer, Berlin (2006)
[11]F. Mitschke, Fiber Optics: Physics and Technology, Springer, Berlin (2010)
[12]R. Paschotta, Field Guide to Optical Fiber Technology, SPIE Press, Bellingham, WA (2010)
[13]R. Paschotta, tutorial on "Passive Fiber Optics"

See also: fiber optics, silica fibers, plastic optical fibers, rare-earth-doped fibers, double-clad fibers, single-mode fibers, multimode fibers, modes, LP modes, photonic crystal fibers, large mode area fibers, specialty fibers, effective mode area, mode size converters, cut-off wavelength, tapered fibers, polarization-maintaining fibers, dispersion-decreasing fibers, dispersion-shifted fibers, fiber Bragg gratings, fiber-optic sensors, power over fiber, fiber lasers, waveguides, cladding modes, polarization mode dispersion, numerical aperture, fiber joints, cleaving of fibers, fusion splicing of fibers, core-less end caps, fiber fuse, Spotlight article 2006-12-03

Category: fibers and other waveguides


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

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