Optical Fiber Theories

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Optical fiber

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A bundle of optical fibers. Theoretically, using advanced techniques such as DWDM, the

modest number of fibers seen here could have sufficientbandwidth to easily carry the sum of

all types of current data transmission needs for the entire planet. (~100 terabits per second per

fiber[1])

An optical fiber orfibre is a thin, transparentfiber, usually made ofglassorplastic, fortransmitting light. Fiber optics is the branch ofapplied 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.

Contents

1 Principle of operation

o 1.1 Materials 2 Optical fiber communication

o 2.1 Comparison with electrical transmission

o 2.2 Governing standards

3 Fiber optic sensors

4 Other uses of optical fibers

5 Manufacture

6 Optical fiber cables

7 Termination and splicing

8 History

9 Notes

10 References

11 See also

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12 External links

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Principle of operation

An optical fiber (American spelling) or fibre (British spelling) is a cylindrical dielectric

waveguide that transmits light along its axis, by the process of total internal reflection. The

fiber consists of a densercore surrounded by a claddinglayer. For total internal reflection 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, instep-

index fiber, or gradual, ingraded-index fiber.

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

Fiber with large (greater than 10 m) core diameter may be analyzed by geometric optics.

Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). 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 normalto the

boundary) are completely reflected. The minimum angle for total internal reflection isdetermined 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 coreinto 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 propagate 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, ordispersion, 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 causes light rays to bend smoothly as they approach the cladding,rather than reflect 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 aparabolic 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

anelectromagnetic structure, by solution ofMaxwell'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 coherentlight propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confinedtransverse

modes by which light can propagate along its axis. Fiber supporting only one mode is called

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single-mode ormono-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 anevanescent wave.

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

The common type of single-mode fiber has a core diameter of 8 to 10m. 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 m, 62.5 m, 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 includepolarization-

maintaining fiberand fiber designed to suppress whispering gallery mode propagation.

At high optical powers, above one watt, when a fiber is subjected to a shock or is otherwise

suddenly damaged, afiber 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 13 meters per second [2],[3],[4]. 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 [5]. 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.

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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(POF) is commonly step-index multimode fiber, with core diameter of 1

mm or larger. POF typically has much higher attenuation than glass fiber (that is, the

amplitude of the signal in it decreases faster), 1 dB/m or higher, and this high attenuation

limits the range of POF-based systems.

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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 orglass, 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 (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 nmlight 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 lightin the fiber is around

200,000 km/s, or two thirds of the speed of light in a vacuum.

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 ofmulti-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 twodifferent 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

MHzkm. 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 MHzkm could carry a

500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km.

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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 ofterabits 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 allowbidirectional

communications over one fiber. Dense Wavelength Division Multiplexing (DWDM) 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.

The range of long-range systems is extended by the use ofrepeaters 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, andpumpingit with light from a laserwith a shorter wavelength than the

communications signal (typically 980 nm). Because of their greater reliability, amplifiers have

largely replaced repeaters in new installations.

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, particularlyover 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 nonlinearity; and solitons, which use nonlinear effects in the fiber

to enable dispersion-free propagation over long distances.

[edit]

Comparison with electrical transmission

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 higherbandwidths, 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 even when run alongside each other for long distances, fiber cables experience

effectively nocrosstalk, 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.

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Ease ofsplicing.

Capability to carry electrical poweras 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 thosescales 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 nuclearelectromagnetic pulses

(although fiber can be damaged by alpha andbeta 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.

Much smaller cable size - important where pathway is limited.

[edit]

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 m multimode graded index optical fibre

cable"

ITU-T G.652, "Characteristics of a single-mode optical fibre cable"

Other standards, produced by a variety ofstandards 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

SDH

SONET

[edit]

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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 orSONARapplications. 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 laserand optical fibers[6].

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 (for navigation purposes).

[edit]

Other uses of optical fibersFibers are widely used in illumination applications. They are used aslight 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 (see non-imaging optics). Optical fiber illumination is also

used fordecorative applications, including signs, art, and artificialChristmas trees.

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 (endoscopy). Industrial endoscopes (seefiberscope orborescope) areused for inspecting anything hard to reach, such as jet engine interiors.

An optical fiberdoped with certain rare-earth elements can be used as the gain medium of a

laseroroptical amplifier.

[edit]

Manufacture

Optical fiber is made by first constructing a large-diameterpreform, with a carefullycontrolled refractive index profile, and thenpullingthe 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.

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In 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 (SiCl4) orgermanium 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 kelvins, where the tetrachlorides react with oxygen toproduce silica or germania (germaniumoxide) 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

(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 asolid 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 byflame hydrolysis,

a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction

with water (H2O) in anoxyhydrogen flame. In outside vapor deposition the glass is deposited

onto a solid rod, which is removed before further processing. In vapor axial deposition, a

shortseed rodis 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.

[edit]

Optical fiber cables

In practical fibers, the cladding is usually coated with a tough resin bufferlayer, which maybe further surrounded by ajacketlayer, 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 terminatedwith a specializedoptical fiber

connectorto 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 cableto 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

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jacket. Thesefiber units are commonly attached to 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 hydroxylions can diffuse into the fiber, reducing thefiber's strength and increasing the optical attenuation. Water is kept out of the cable by use of

solid barriers such as copper tubes, or water-repellant jelly 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 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, dual use as power lines [7], 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 forFiber to the Home (FTTH) installations.

[edit]

Termination and splicing

Optical fibers are connected to terminal equipment by optical fiber connectors. These

connectors are usually of a standard type such asFC, SC, ST, orLC.

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 fibre ends together with an electric arc. For

quicker fastening jobs, a "mechanical splice" is used. The fiber ends are aligned and heldtogether by a precision-made sleeve.

Various methods to align two fiber ends to each other or one fiber to an optical device

(VCSEL, LED,waveguide etc.) have been reported. They all follow either an active fiber

alignment approach or a passive fiber alignment approach.

[edit]

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

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development of fiber bundles for image transmission, with the primary application being the

medical gastroscope. The first fiber optic semi-flexible gastroscope was patented byBasil

Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of

Michigan, in1956. 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 thelow-index cladding material. A variety of other image transmission applications soon

followed.

In 1965, Charles K. Kao and George A. Hockham of the British companyStandard

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 dBper 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 makerCorning Glass Works. They manufactured a fiber with 17 dB opticattenuation per kilometer by dopingsilica glasswith titanium.

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 byDavid Payneof

the University of Southampton, and Emmanuel Desurvire at Bell Laboratoriesin 1986. The

two pioneers were awarded the Benjamin Franklin Medalin 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 ofphotonic crystals led to the development ofphotonic crystalfiber(Science (2003), vol 299, page 358), 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 [8]. 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 underMoore'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.

[edit]

Notes

1. The Risks Digest Volume 12: Issue 44. URL accessed on December 4, 2005.2. Optics Letters. URL accessed on December 4,2005.

3. Photonics Spectra. URL accessed on December 4, 2005.

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.wikipedia.org/wiki/Optical_fiber#ref_ol.osa.org.240%23ref_ol.osa.org.240http://ol.osa.org/abstract.cfm?id=72607http://en.wikipedia.org/wiki/December_4http://en.wikipedia.org/wiki/2005http://en.wikipedia.org/wiki/Optical_fiber#ref_www.photonics.com%23ref_www.photonics.comhttp://www.photonics.com/spectra/tech/XQ/ASP/techid.1576/QX/read.htmhttp://en.wikipedia.org/wiki/December_4http://en.wikipedia.org/wiki/2005


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4. Evaluation of High-power Endurance in Optical Fiber Links. URL accessed on

December 4,2005.

5. TP: Der Glasfaser-Schallwandler. URL accessed on December 4, 2005.

[edit]

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

[edit]

See also

List of fiber optic terms

Gradient index optics

Optical fiber connector

Plastic optical fiber

Submarine communications cables

cable jetting

SFP interface

ST, SC and MTRJ are types of fiber optic cable jacks or connectors.

TOSLINK

XENPAK

[edit]

External links

Optical Fiber Cabling Standards

The Fiber Optic Association

Lennie Lightwave's Guide To Fiber Optics

Fibers article in RP Photonics Encyclopedia of Laser Physics and Technology

How Fiber Optics Work

Retrieved from "http://en.wikipedia.org/wiki/Optical_fiber"

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Synchronous optical networking

(Redirected from SDH)

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The Synchronous optical network, commonly known as SONET, is a standard for

communicating digital information using lasers or light emitting diodes (LEDs) overoptical

fiberas defined by GR-253-CORE from Telcordia. It was developed to replace the PDH

system for transporting large amounts of telephone and data traffic and to allow for

interoperability between equipment from different vendors. The more recent Synchronous

Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built

on experience in the development of SONET. Both SDH and SONET are widely used today;

SONET in the U.S. andCanada, SDH in the rest of the world. SDH is growing in popularity

and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly

synchronized across the entire network, made possible byatomic clocks. This Telecom

Synchronization system allows entire inter-country networks to operate synchronously,

greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such

as the PDH standard, or used directly to support eitherATMor so-calledPacket over SONET

networking. As such, it is inaccurate to think of SONET as a communications protocol in and

of itself, but rather as a generic and all-purpose transport container for moving both voice and

data.

Contents

[hide]

1 Structure of SONET/SDH signals

2 SONET/SDH and relationship to 10 Gigabit Ethernet

3 SONET/SDH data rates

4 SONET Physical Layer

5 SONET/SDH system management protocols

6 SONET Equipment

7 SONET Network Architectures 8 SONET Synchronization

9 Next Generation SDH

10 See also

11 External links

[edit]

Structure of SONET/SDH signals12
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The basic unit of transmission for SONET is a signal that operates at 51.840 Mbit/s,

designated STS-1 (Synchronous Transport Signal one). This differs from SDH's basic unit,

the STM-1 (Synchronous Transport Module-level 1), which operates at 155.52 Mbit/s.

The two major components of the STS-1 frame are the transport overhead and thesynchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the

section overhead and line overhead. These bytes are used for signalling and measuring

transmission error rates. The SPE is comprised of two components: the payload overhead (9

bytes, used for end to end signalling and error measurement) and the payload of 774 bytes.

The STS-1 payload is designed to carry a full DS-3 frame. When the DS-3 enters a SONET

network,path overhead is added, and that SONET network element is said to bepath

terminating. Where multiple DS-3 paths are multiplexed, the SONET NE is said to be line

terminating. Note that wherever the line or path is terminated, the section is terminated also.

SONET Regenerators (see below) terminate the Section but not the path or line.

The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125microseconds on a fiber-optic circuit designated OC-1 (optical carrier one). In practice, the

terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers

to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s:

an OC-3 can be said to contain 3 STS-1s.

Three OC-1 (STS-1) signals are multiplexed by time-division multiplexing to form the next

level of the SONET hierarchy, the OC-3(STS-3), running at 155.52 Mbit/s. The multiplexing

is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame,

containing 2430 bytes and transmitted in 125 microseconds.

Higher speed circuits are formed by successively aggregating multiples of slower circuits,their speed always being immediately apparent from their designation. For example, four OC-

3 or STM-1 circuits can be aggregated to form a 622.08 Mbit/s circuit designated as OC-12 or

STM-4.

The highest rate that is commonly deployed is the OC-192 orSTM-64 circuit, which operates

at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under

evaluation. Where fiber exhaust is a concern, multiple SONET signals can be transported over

multiple wavelengths over a single fiber pair by means ofDense Wave Division Multiplexing

(DWDM). Such circuits are the basis for all modern transatlantic cable systems and other

long-haul circuits.

[edit]

SONET/SDH and relationship to 10 Gigabit Ethernet

Another fast growing circuit type amongst data networking equipment is 10 Gigabit Ethernet

(10GbE). This is similar in rate to OC-192/STM-64, and, in its wide area variant, encapsulates

its data using a light-weight SDH/SONET frame so as to be compatible at low level with

equipment designed to carry those signals.

However, 10 Gigabit Ethernet does not explicitly provide any interoperability at the bitstreamlevel with other SDH/SONET systems. This differs from WDM System Transponders,

including both Coarse- and Dense-WDM systems (CWDM, DWDM) that currently support

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OC-192 SONET Signals, which can normally support thin-SONET framed 10 Gigabit

Ethernet.

[edit]

SONET/SDH data rates

SONET/SDH Designations and bandwidths

SONET

Optical

Carrier Level

SONET

Frame

Format

SDH level

and Frame

Format

Payload

bandwidth[1]

(kbit/s)

Line Rate

(kbit/s)

OC-1 STS-1 STM-0 48 960 51 840

OC-3 STS-3 STM-1 150 336 155 520

OC-12 STS-12 STM-4 601 344 622 080

OC-24 STS-24 STM-8 1 202 688 1 244 160

OC-48 STS-48 STM-16 2 405 376 2 488 320

OC-192 STS-192 STM-64 9 621 504 9 953 280

OC-768 STS-768 STM-256 38 486 016 39 813 120

OC-1536 STS-1536 STM-512 76 972 032 79 626 120

OC-3072 STS-3072 STM-1024 153 944 064 159 252 240

^ Payload bandwidth is the actual data carrying capacity.

Note that the typicaldata rate progression starts at OC-3 and increases by multiples of 4. As

such, while OC-24 and OC-1536, along with other rates such as OC-9, OC-18, OC-36, and

OC-96 may be defined in some standards documents, they are not available on a wide-range

of equipment.

As of 2006, OC-3072 is still a work in progress. It has not yet been manufactured.

[edit]

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SONET Physical Layer

The "SONET Physical Layer" actually comprises a large number of layers within it, only one

of which is the optical/transmission layer (which includes bitrates, jitter specifications, optical

signal specifications and so on). The SONET and SDH Standards have within them a host offeatures for isolating and identifying signal defects and their origins.

[edit]

SONET/SDH system management protocols

SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom

language for managing and reconfiguring SONET network elements. TL1 (or whatever

command language a SONET Network Element utilizes) must be carried by other

management protocols, including SNMP, CORBA, and XML.

SONET Network Management is a large, difficult, and arcane subject, but there are some

features that are fairly universal. First of all, most SONET NEs have a limited number of

management interfaces defined. These are:

Electrical Interface. The electrical interface (often 50 ) sends SONET TL1

commands from a local management network physically housed in the Central Office

where the SONET NE is located. This is for "local management" of that NE and,

possibly, remote management of other SONET NEs.

Craft Interface. Local "craftspersons" can access a SONET NE and issue commandsthrough a dumb terminal or terminal emulation program running on a laptop.

SDH has dedicated Data Communication Channels DCCs for management traffic.

According toITU-T G. 7712, there are three modes used for management:

IP-only stack, using PPP as data-link

OSI-only stack, using LAP-D as data-link

Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate

between stacks.

An interesting fact about modern SONET NEs is that, to handle all of the possible

management channels and signals, most NEs actually contain a router for routing the network

commands and underlying (data) protocols.

The main functions of SONET Network Management include:

SONET Network and NE Provisioning. In order to allocate bandwidth throughout a

SONET Network, each SONET NE must be configured. Although this can be done

locally, through a craft interface, it is normally done through a Network Management

System (sitting at a higher layer) that in turn operates through the SONET/SDH

Network Management Network.

Software Upgrade. SONET NE Software Upgrade is in modern NEs done mostly

through the SONET/SDH Management network.

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Performance Management. SONET NEs have a very large set of standards for

Performance Management. The PM criteria allow for monitoring not only the health

of individual NEs, but for the isolation and identification of most network defects or

outages. Higher-layer Network monitoring and management software allows for the

proper filtering and troubleshooting of network-wide PM so that defects and outagescan be quickly identified and responded to.

[edit]

SONET Equipment

With recent advances in SONET and SDH chipsets, the traditional categories of SONET NEs

are breaking down. Nevertheless, as SONET Network architectures have remained relatively

constant, even newer SONET Equipment (including "Multiservice Provisioning Platforms")

can be examined in light of the architectures they will support. Thus, there is value in viewing

new (as well as traditional) SONET Equipment in terms of the older categories.

SONET Regenerator. Traditional SONET Regenerators terminate the SONET Section

overhead, but not the line or path. SONET Regens extended long haul routes in a way

similar to most regenerator, by converting an optical signal that has already traveled a

long distance into electrical format and then retransmitting a regenerated high-power

signal.

Since the late 1990s, SONET regenerators have been largely replaced by Optical

Amplifiers. Also, some of the functionality of SONET Regens has been absorbed by

the Transponders ofWavelength Division Multiplexing systems. SONET Add-Drop Multiplexer. SONET ADMs are the most common type of SONET

Equipment. Traditional SONET ADMs were designed to support one of the SONET

Network Architectures, though new generation SONET systems can often support

several architectures, sometimes simultaneously. SONET ADMs traditionally have a

"high speed side" (where the full line rate signal is supported), and a "low speed side",

which can consist of electrical as well as optical interfaces. The low speed side takes

in low speed signals which are multiplexed by the SONET NE and sent out from the

high speed side, or vice versa.

SONET Digital Cross Connect System. Recent SONET Digital Cross Connect

systems (DCSs or DXCs) support numerous high-speed signals, and allow for crossconnection of DS1s, DS3s and even STS-3s/12c and so on, from any input to any

output. Advanced SONET DCSs can support numerous subtending rings

simulataneously.

[edit]

SONET Network Architectures

Currently, SONET (and SDH) have a limited number of architectures defined. These

architectures allow for efficient bandwidth usage as well as protection (i.e. the ability to

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transmit traffic even when part of the network has failed), and are key in understanding the

almost worldwide usage of SONET and SDH for moving digital traffic. The three main

architectures are:

Linear APS (Automatic Protection Switching): This involves 4 fibers: 2 workingfibers in each direction, and two protection fibers.

UPSR (Unidirectional Path Switched Ring): In a UPSR, two redundant (path-level)

copies of protected traffic are sent in either direction around a ring. A selector at the

egress node determines the higher-quality copy and decides to use the best copy, thus

coping if deterioration in one copy occurs due to a broken fiber or other failure.

UPSRs tend to sit nearer to the edge of a SONET network and, as such, are sometimes

called "collector rings".

BLSR (Bidirectional Line Switched Ring): BLSR comes in two varieties, a 2-fiber

BLSR and 4-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does

not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to

the failure reroute the traffic "the long way" around the ring. BLSRs trade cost andcomplexity for bandwdith efficiency as well as the ability to support "extra traffic",

which can be pre-empted when a protection switching event occurs. BLSRs can

operate within a metropolitan region or, often, will move traffic between

municipalities.

[edit]

SONET Synchronization

Like management, Synchronization of SONET and SDH networks is a difficult and arcanesubject. Remember that a SONET NE will transport and/or multiplex traffic that has

originated from a variety of different clock sources. In addition, a SONET NE may have a

number of different synchronization options to choose from, which in some cases it will do so

dynamically based on Synch Status Messages and other indicators.

As for Synchronization sources available to a SONET NE, these are:

Local External Timing. This is generated by an atomic Cesium clock or a satellite-

derived clock by a device located in the same central office as the SONET NE. the

interface is often a DS1, with Sync Status Messages supplied by the clock and placed

into the DS1 overhead. Line-derived timing. A SONET NE can choose (or be configured) to derive its timing

from the line-level, by monitoring the S1 sync status bytes to ensure quality.

Holdover. As a last resort, in the absence of higher quality timing, a SONET NE can

go into "holdover" until higher quality external timing becomes available again. In

this mode, a SONET NE uses its own timing circuits to time the SONET signal.

An interesting and hard-to-troubleshoot issue in SONET Networks is the existence of "timing

loops". With a timing loop, SONET NEs in a network are each deriving their timing from

another NE, and back again to initial NE, like a snake biting its own tail. This network loop

will eventually see its own timing "float away" from any external SONET networks, causing

mysterious bit errors, the source of which can be hard to find (unless the presence of thetiming loop is detected). In general, a SONET Network that has been properly configured will

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never find itself in a timing loop, but it is sometimes hard to avoid this without sophisticated

network management tools.

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Next Generation SDH

SONET/SDH was originally developed primarily to transport multiple DS1s (ie T1s), DS3s

(ie, T3s), and other groups of multiplexed 64 kbit/spulse-code modulated voice traffic. The

ability to transport ATM (Asynchronous Transfer Mode) traffic was another early application.

In order to support large ATM bandwidths, the technique of concatenation was developed,

whereby smaller SONET multiplexing containers (eg, STS-1) are inversely multiplexed to

build up a larger container (eg, STS-3c) to support large data-oriented pipes. SONET is

therefore able to transport both voice and data simultaneously.

One problem with traditional concatenation, however, is inflexibility. Depending on the data

and voice traffic mix that must be carried, there can be a large amount of unused bandwidth

left over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s

Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to considerable wastage.

Virtual Concatenation (VCAT) allows for a more arbitrary gluing-together of lower order

multiplexing containers to build larger containers of fairly arbitrary size (e.g. 100 Mbit/s),

without the need for intermediate SONET NEs to support that particular form of

concatenation. Virtual Concatenation now often leveragesX.86orGeneric Framing

Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually

concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the

bandwidth via dynamically virtually concatenating multiplexing containers based on short-

term bandwidth needs in the network.

[edit]

See also

Generalized Multiprotocol Label Switching Internet Drafts: IETF - Common Control

and Measurement Plane (ccamp)

Framework for Generalized Multi-Protocol Label Switching (GMPLS)-based Control

of Synchronous Digital Hierarchy/Synchronous Optical Networking (SDH/SONET)

Network: rfc4257.txt

Why SDH/SONET Will Not Disappear Tomorrow (pdf) in rfc4257: Framework for

Generalized Multi-Protocol Label Switching (GMPLS)-based Control of Synchronous

Digital Hierarchy/Synchronous Optical Networking (SDH/SONET) Networks

[edit]

External links

Optical Hub A useful resource site on optical networks.

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