Optical fiber

An optical fiber (also spelled fibre) is a transparent thin fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of science and engineering concerned with such optical fibers.

Optical description

File:Singlemode-optical-fibre.png

A typical single-mode optical fiber, showing diameters of the component layers.

Optical fiber is a cyclindrical structure that transmits light along its axis. The fiber consists of a core surrounded by a cladding layer. Like other glasses, the glass used in optical fiber has a refractive index of about 1.5. For the fiber to guide the optical signal the refractive index of the core must be slightly higher than that of the cladding, though typically the difference is less than one per cent. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.

A diagram which illustrates the propagation of light through a multi-mode optical fiber.

Fiber with large core diameter, called multi-mode fiber (from the electromagnetic analysis, see below), may be analyzed by geometric optics. In a step-index fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary) are completely reflected. The minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, where they are not useful for conveying light along the fiber. In this way, the minimum angle for total internal reflection determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture makes it easier to efficiently couple a transmitter or receiver to the fiber. However, by allowing light to propogate down the fiber in rays both close to the axis and at various angles, a high numerical aperture also increases the amount of multi-path spreading, or dispersion, that affects light pulses in the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This reduces multi-path dispersion because the high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. Index grading also causes light rays to bend smoothly as they approach the cladding, rather than reflect abruptly from the core-cladding boundary. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Fiber with a core diameter narrower than a few wavelengths of the light carried, is analyzed as an electromagnetic structure. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propogates in multi-mode fiber. The optical fiber is seen as a cylindrical dielectric wave guide. This wave guide supports one or more confined transverse modes by which light can propagate along its axis. Fiber supporting only one mode is called single-mode or mono-mode fiber, while fiber that supports more than one mode is called multi-mode fiber. By the waveguide analysis, it is seen that the light energy in the fiber is not completely confined in the core, but, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.

The common type of single-mode fiber, as described in Federal Standard 1037C, has a core diameter of 8 to 10 micrometres. It is notable that the mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with a core diameter of 50 µm, 62.5 µm, or larger.

Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.

At high optical powers, above one watt, when a fiber is subjected to a shock or or is otherwise suddenly damaged, a fiber fuse can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propogates back toward the transmitter at 1–3 meters per second [1][2] [3] [4]. A "Fiber fuse" protection device at the transmitter can break the circuit to prevent damage, if the extreme conditions for this are deemed possible.

Optical fiber communication

(See also: Optical communication) The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical absorption. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

The light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use. The fiber absorption is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission. A local minimum of absorption is found near 850 nm, a wavelength for which low cost transmitters and receivers can be designed, and this wavelength is often used for short distance applications. Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction.

For modern glass optical fiber, the maximum transmission distance is limited not by absorption but by dispersion, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of multi-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated. For single-mode fiber performance is limited by chromatic dispersion, which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters has nonzero spectral width. Polarization mode dispersion, which can limit the performance of single-mode systems, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver.

Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by its bandwidth-distance product, often expressed in units of MHz×km. This value is a product of bandwidth and distance because there is a tradeoff between the bandwidth of the signal and the distance it can be carried. For example, a common multimode fiber with bandwidth-distance product of 500 MHz×km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.

In single-mode fiber systems, both the fiber characteristics and the spectral width of the transmitter contribute to determining the bandwidth-distance product of the system. Typical single-mode systems can sustain transmission distances of 80 to 140 km (50 to 87 miles) between regenerations of the signal. By using an extremely narrow-spectrum laser source, data rates of up to 40 gigabits per second are achieved in real-world applications.

Using Wavelength division multiplexing (WDM), the bandwidth carried by a single fiber can be increased into the range of terabits per second. This is accomplished by transmitting many wavelengths at once on the fiber. Wavelength division multiplexers and demultiplexers are used to combine and split up the wavelengths at each end of the link. In coarse WDM (CWDM) only a few wavelengths are used. One use of CWDM is to allow bidirectional communications over one fiber.

Recent advances in fiber technology have reduced losses so far that no amplification of the optical signal is needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost reliability of amplifiers is one of the key factors determining the performance of the whole cable system. In the past few years several manufacturers of submarine cable line terminal equipment have introduced upgrades that promise to quadruple the capacity of older submarine systems installed in the early to mid 1990s.

Longer-range systems still have to use optical amplifiers.

Considerations in choosing optical fiber

The choice between optical fiber and electrical (or "copper") transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems with higher bandwidths, spanning longer distances, than electrical cabling can provide. The main benefits of fiber are its exceptionally low loss, allowing long distances between amplifiers or repeaters; and its inherently high data-carrying capacity, such that thousands of electrical links would be required to replace a single high bandwidth fiber. One further benefit of fiber is that fiber cables run alongside each other, experience effectively no crosstalk, in contrast to some types of electrical transmission lines.

In short distance and relatively low bandwidth applications, electrical transmission is often preferred because of its

Lower material cost, when cabling is not required.
Lower cost of transmitters and receivers.
Ease of splicing.
Capability to carry electrical power as well as signals.

Because of these benefits of electrical transmission, optical communication is not common in short box-to-box, backplane, or chip-to-chip applications; however, optical systems on those scales have been demonstrated in the laboratory.

In certain situations fiber may be used even for short distance or low bandwidth applications, due to other important features:

Immunity to electromagnetic interference, including nuclear electromagnetic pulses (although fiber can be damaged by alpha and beta radiation).
High electrical resistance, making it safe to use near high-voltage equipment or between areas with different earth potentials.
Low weight, important in aircraft.
No sparks, important in flammable or explosive gas environments.
Not electromagnetically radiating, and difficult to tap without disrupting the signal, important in high-security environments.

Other uses of optical fibers

Fibers can be used as light guides in medical and other applications where bright light needs to be brought to bear on a target without a clear line-of-sight path.
Lasers and optical amplifiers can use doped optical fiber as a gain medium.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters.
Bundles of fibers are used along with lenses for long, thin imaging devices called endoscopes, which are used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
In some high-tech buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics).
Optical fibers have many decorative applications, including signs and art, artificial Christmas trees, and lighting.
The German company Sennheiser developed a microphone working with a Laser and optical fibers. German article about this microphone

Manufacture

One technique for the manufacture of optic fibers is the Modified Chemical Vapour Deposition technique, where a hollow glass tube approximately 40cm in length known as a "preform" is placed horizontally and rotated slowly on a lathe. Gases such as Silicon Tetrachloride (SiCl4) or Germanium Tetrachloride (GeCl4) are injected with oxygen in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1900 Kelvin, where the tetrachlorides react with oxygen to produce silica or germania oxide particles.

These particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outwards (this is known as thermophoresis). The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited, for each layer the composition can be varied by varying the gas composition. This composition change results in a change in the finished fiber's optical properties.

This preform is then placed in a device known as a drawing tower, where the preform tip is heated and the optic fiber is pulled out as a string. By measuring the resultant fiber width, the tension on the fiber can be used to maintain the fiber thickness.

Optical fiber cables

In practical fibers, the cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties.

For indoor applications, the jacketed fiber is generally enclosed, with a bundle of flexible fibrous polymer (e.g. Kevlar) strength members, in a lightweight plastic cover to form a simple cable. Each end of the cable may be terminated with a specialized optical fiber connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.

For use in more strenuous environments, a much more robust cable construction is required.

Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodate even today's demands for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fiber buried in recent years is actually 'lit'.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole mounted cables has greatly decreased due to the high Japanese and South Korean demand for Fiber to the Home (FTTH) installations.

History

The history of dielectric optical lightguides goes back to Victorian times, when the total internal reflection principle was used to illuminate streams of water in elaborate public fountains. Later development, in the early-to-mid twentieth century, focussed on the development of fiber bundles for image transmission, with the primary application being the medical gastroscope. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed.

In 1965, Charles K. Kao and George A. Hockham of the British Post Office were the first to recognize that attenuation of contemporary fibers was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. They demonstrated that optical fiber could be a practical medium for communication, if the attenuation could be reduced below 20 dB per kilometer (Hecht, 1999, p. 114). By this measure, the first practical optical fiber for communications was invented in 1970 by researchers Robert D. Maurer, Donald Keck, Peter Schultz, and Frank Zimar working for American glass maker Corning Glass Works. They manufactured a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium.

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The first transatlantic telephone cable to use optical fiber was TAT-8, which went into operation in 1988.

Many optical fiber cables carrying internet traffic are buried under railroad rights of way in the United States.

References

Gambling, W. A., "The Rise and Rise of Optical Fibers", IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, pp. 1084-1093, Nov./Dec. 2000

Gowar, John, Optical Communication Systems, 2 ed., Prentice-Hall, Hempstead UK, 1993 (ISBN 0136387276)

Hecht, Jeff, City of Light, The Story of Fiber Optics, Oxford University Press, New York, 1999 (ISBN 0195108183)

Nagel S. R., MacChesney J. B., Walker K. L., "An Overview of the Modified Chemical Vapor Deposition (MCVD) Process and Performance", IEEE Journal of Quantum Mechanics, Vol. QE-18, No. 4, April 1982

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