Encyclopedia
An
optical fiber or
fibre is a thin, transparent fiber, usually made of
glass or
plastic, for transmitting
light.
Fiber optics is the branch of applied science and engineering concerned with such optical fibers.
Optical fibers are commonly used in
telecommunication systems, as well as in
illumination, sensors, and imaging optics.
Principle of operation
An optical fiber or fibre is a cylindrical
dielectric waveguide that transmits light along its axis, by the process of
total internal reflection. The fiber consists of a denser
core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in
step-index fiber, or gradual, in
graded-index fiber.
Multimode fiber
Fiber with large core diameter may be analyzed by
geometric optics. Such fiber is called
multimode fiber, from the electromagnetic analysis . In a step-index multimode 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 , greater than the
critical angle for this boundary, are completely reflected. The critical angle 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, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may be therefore be desirable.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. 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.
Singlemode fiber
Fiber with a core diameter less than about ten times the
wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the
electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as
speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined
transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or
mono-mode fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation . The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
The most common type of single-mode fiber has a core diameter of 8 to 10 µm and is designed for use in the
near infrared. 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 core diameters as small as 50 microns and as large as hundreds of microns.
Special-purpose fiber
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.
Materials
Glass optical fibers are almost always made from silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses are used for longer-wavelength infrared applications. Like other glasses, these glasses have a refractive index of about 1.5. Typically the difference between core and cladding is less than one percent.
Plastic optical fiber is commonly step-index multimode fiber, with core diameter of 1 mm or larger. POF typically has much higher attenuation than glass fiber , 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.
Fiber fuse
At high optical intensities, above 2 megawatts per square centimetre, when a fiber is subjected to a shock 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 propagates back toward the transmitter at 1–3 meters per second
,,. The open fiber control system, which ensures
laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse . In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to prevent damage.
Optical fiber 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 attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances , 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.
Transmission method
As the name indicates optical fiber communications use light as the method of communication. 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.
Since the refractive index of glass is around 1.5, the
speed of light in the fiber is around 200,000 km/s, or two thirds of the speed of light in a
vacuum.
Dispersion-limited communication
For modern glass optical fiber, the maximum transmission distance is limited not by attenuation 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.
Bandwidth-distance product
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 trade off 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 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.
WDM systems
Using
wavelength division multiplexing , 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 only a few wavelengths are used. One use of CWDM is to allow bidirectional communications over one fiber. Dense wavelength division multiplexing usually involves transmitting and receiving more than eight "windows" of light. Sixteen, 40, and 80 windowed systems are common. Mathematically, 111 windows are possible over a single pair of optical fibers at the wavelengths used today.
Fiber amplifiers
The range of long-range systems is extended by the use of
repeaters and
optical amplifiers. A repeater is essentially a back-to-back receiver and transmitter, which
regenerates the optical signal, eliminating or reducing the degradations resulting from transmission through the fiber. An optical amplifier is typically made by doping a length of fiber with the rare-earth mineral erbium, and
pumping it with light from a
laser with a shorter wavelength than the communications signal . Because of their greater reliability, amplifiers have largely replaced repeaters in new installations. These amplifiers are called Erbium Doped Fiber Amplifier .
Recent advances in fiber and optical communications technology have reduced signal degradation so far that
regeneration of the optical signal is only needed over distances of hundreds of kilometers. This has greatly reduced the cost of optical networking, particularly over undersea spans where the cost and reliability of repeaters is one of the key factors determining the performance of the whole cable system. The main advances contributing to these performance improvements are
dispersion management, which seeks to balance the effects of dispersion against non-linearity; and solitons, which use nonlinear effects in the fiber to enable dispersion-free propagation over long distances.
Comparison with electrical transmission
The choice between optical fiber and electrical transmission for a particular system is made based on a number of trade-offs. Optical fiber is generally chosen for systems with higher
bandwidths or 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. Fiber is also much lighter than copper: 700 km of
telecommunications copper cabling weighs 20 tonnes. If the same cable run were made with fiber, it would use only 7 kg of glass. One further benefit of fiber is that even when run alongside each other for long distances, fiber cables 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, where large quantities are 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:
...
s .
- High electrical resistance, making it safe to use near high-voltage equipment or between areas with different earth potentials.
- Lighter weight, important, for example, 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.
- Much smaller cable size - important where pathway is limited.
Governing standards
In order for various manufacturers to be able to develop components that function compatibly in fiber optic communication systems, a number of standards have been developed. The
International Telecommunications Union publishes several standards related to the characteristics and performance of fibers themselves, including
- ITU-T G.651, "Characteristics of a 50/125 µm multimode graded index optical fibre cable"
- ITU-T G.652, "Characteristics of a single-mode optical fibre cable"
Other standards, produced by a variety of standards organizations, specify performance criteria for fiber, transmitters, and receivers to be used together in conforming systems. Some of these standards are the following:
- 10 gigabit Ethernet
- FDDI
- Fibre Channel
- Gigabit Ethernet
- HIPPI
- Synchronous Digital Hierarchy
- Synchronous Optical Networking
Fiber optic sensors
Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters. The small size and the fact that no electrical power is needed at the remote location gives the fiber optic sensor advantages to conventional electrical sensor in certain applications.
Optical fibers are used as hydrophones for seismic or
SONAR applications. Hydrophone systems with more than 100 sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom mounted hydrophone arrays and towed streamer systems are in use. The German company
Sennheiser developed a microphone working with a
laser and optical fibers.
Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it is functioning at temperatures too high for semiconductor sensors .
Another use of the optical fiber as a sensor is the
optical gyroscope which is in use in the
Boeing 767 and in some car models and the use in Hydrogen microsensors.
Other uses of optical fibers
Fibers are widely used in illumination applications. They are 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. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building . Optical fiber illumination is also used for decorative applications, including
signs,
art, and artificial
Christmas trees.
Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source.
Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures . Industrial endoscopes are used for inspecting anything hard to reach, such as jet engine interiors.
An optical fiber doped with certain
rare-earth elements such as erbium can be used as the gain medium of a
laser or
optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fibre into a regular optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is
stimulated emission.
Plastic optical fiber is commonly used as a digital audio cable, to connect digital sources to digital receivers. The most common format is
TOSLINK.
Optical fibers doped with a wavelength shifter are used to collect scintillation light in
physics experiments.
Optical fiber can be used to supply a low level of power to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.
Manufacturing
Optical fiber is made by first constructing a large-diameter
preform, with a carefully controlled refractive index profile, and then
pulling the preform to form the long, thin optical fiber. The preform is commonly made by three
chemical vapor deposition methods:
inside vapor deposition,
outside vapor deposition, and
vapor axial deposition.
With
inside vapor deposition, a hollow glass tube approximately 40 cm in length known as a "preform" is placed horizontally and rotated slowly on a lathe, and gases such as
silicon tetrachloride or
germanium tetrachloride 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
kelvins, where the tetrachlorides react with oxygen to produce silica or germania particles. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called
modified chemical vapor deposition.
The oxide 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 . 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, resulting in precise control of the finished fiber's optical properties.
In outside vapor deposition or vapor axial deposition, the glass is formed by
flame hydrolysis, a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water in an oxyhydrogen flame. In outside vapor deposition the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short
seed rod is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid perform by heating to about 1800 kelvins.
The preform, however constructed, 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 controlled to maintain the fiber thickness.
This manufacturing process is accomplished by several fiber optic companies, including
3M,
Corning Inc., and
Molex. In addition, various fiber optic component manufacturers, assembly houses, and custom fiber optic providers exist.
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
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. In
loose-tube construction the fiber is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself. This protects the fiber from tension during laying and due to temperature changes. Alternatively the fiber may be embedded in a heavy polymer jacket, commonly called "tight buffer" construction. These
fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.
Another critical concern in cabling is to protect the fiber from contamination by water, because its component
hydrogen and hydroxyl ions can diffuse into the fiber, reducing the fiber's strength and increasing the optical attenuation. Water is kept out of the cable by use of solid barriers such as copper tubes, water-repellant jelly, or more recently water absorbing powder, surrounding the fiber.
Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power signals that are carried to power amplifiers or repeaters in the cable.
Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodates 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, dual use as power lines , 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 installations.
Termination and splicing
Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard type such as
FC,
SC,
ST, or
LC.
Optical fibers may be connected to each other by connectors or by
splicing, that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing, which melts the fiber ends together with an
electric arc. For quicker fastening jobs, a "mechanical splice" is used.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends are stripped of their protective polymer coating . The ends are
cleaved with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process is the major thing that makes fiber splicing more difficult than splicing copper wire.
Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear gel that enhances the transmission of light across the joint. Such joints typically have higher optical loss, and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. A fiber optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. It can be push and click, turn and latch, or threaded. A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick set glue is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the glue has set, the end is polished to a mirror finish. Various types of polish profile are used, depending on the type of fiber and the application. For singlemode fiber, the fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a "physical contact" polish. The curved surface may be polished at an angle, to make an angled physical contact connection. Such connections have higher loss than PC connections, but greatly reduced backreflection, because light that reflects from the angled surface leaks out of the fiber core.
Various methods to align two fiber ends to each other or one fiber to an optical device have been reported. They all follow either an active fiber alignment approach or a passive fiber alignment approach.
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, focused 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, researchers at the
University of Michigan, 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 company Standard Telephones and Cables 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 . 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.
On 22 April, 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics, at 6 Mbit/s, in Long Beach, California.
The
erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by eliminating the need for optical-electrical-optical repeaters, was invented by
David Payne of the
University of Southampton, and Emmanuel Desurvire at
Bell Laboratories in 1986. The two pioneers were awarded the Benjamin Franklin Medal in Engineering in 1998.
The first transatlantic telephone cable to use optical fiber was TAT-8, based on Desurvire optimized laser amplification technology. It went into operation in 1988.
In 1991, the emerging field of
photonic crystals led to the development of
photonic crystal fiber , which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 1996 . Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.
In the late 1990s through 2000, the fiber optics industry, including optical communications equipment makers in addition to the optical fiber makers themselves, became associated with the
dot-com bubble. Industry promoters, and research companies such as KMI and RHK predicted vast increases in demand for communications bandwidth due to increased use of the
Internet, and commercialization of various bandwidth-intensive consumer services, such as
video on demand. Internet protocol data traffic was said to be increasing exponentially, and at a faster rate than integrated circuit complexity had increased under
Moore's Law. From the bust of the dot-com bubble through 2006, however, the main trend in the industry has been consolidation of firms and offshoring of manufacturing to reduce costs.
See also
Notes
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
- Hecht, Jeff, City of Light, The Story of Fiber Optics, Oxford University Press, New York, 1999
- Hecht, Jeff, Understanding Fiber Optics, 4th ed., Prentice-Hall, Upper Saddle River, NJ, USA 2002
- Nagel S. R., MacChesney J. B., Walker K. L., "An Overview of the Modified Chemical Vapor Deposition Process and Performance", IEEE Journal of Quantum Mechanics, Vol. QE-18, No. 4, April 1982
- Ramaswami, R., Sivarajan, K. N., Optical Networks: A Practical Perspective, Morgan Kaufmann Publishers, San Francisco, 1998
External links
-
-
- article in RP Photonics Encyclopedia of Laser Physics and Technology
- In video
- Atikem Haile-Mariam, , International Lighting in Controlled Environments Workshop, T.W.Tibbitts , NASA-CP-95-3309
- , Mercury Communications Ltd, August 1992.
- , Mercury Communications Ltd, March 1993.
