the fiber optic association

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The Fiber Optic Association - Tech Topics

Cable Plant Link Loss Budget Analysis

Loss budget analysis is the calculation and verification of a fiber opticsystem's operating characteristics. This encompasses items such aselectronics, transceiver wavelengths, fiber type, and link length. Attenuationand bandwidth/dispersion are the key parameters for budget loss analysis.

FOA has a free app for smartphones and tablets that will calculate loss budgets for thecable plant you are designing or testing. See the app store for your device for details.

Analyze Link Loss In The Design Stage

Prior to designing or installing a fiber optic system, a loss budget analysis isreccommended to make certain the system will work over the proposed link.Both the passive and active components of the circuit have to be included inthe budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don't forget any couplers or splitters in the link. Activecomponents are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with asource and FO power meter to ensure that it is within the loss budget.

The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over thespecifications! Don't use the best possible specs for fiber attenuation or connector loss - give yourself some margin!


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The best way to illustrate calculating a loss budget is to show how it's done for a 2 km multimode link with 5 connections (2 connectors at each end and 3connections at patch panels in the link) and one splice in the middle. See thedrawings below of the link layout and the instantaneous power in the link atany point along it's length, scaled exactly to the link drawing above it.

Cable Plant Passive Component Loss

Step 1. Fiber loss at the operating wavelength

Cable Length 2.0 2.0Fiber Type Multimode SinglemodeWavelength (nm) 850 1300 1310 1550Fiber Atten. dB/km 3 3.5 1 1.5 0.4 1/0.5 0.3 1/0.5Total Fiber Loss 6.0 7.0 2.0 3.0


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(All specs in brackets are maximum values per EIA/TIA 568 standard . For singlemode fiber, a higher loss is allowed for premises applications. )

Step 2. Connector Loss

Multimode connectors will have losses of 0.2-0.5 dB typically. Singlemodeconnectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as highas 0.5-1.0 dB. Let's calculate it at both typical and worst case values.Remember that we include all the components in the complete link, includingthe connectors on each end.

Connector Loss 0.3 dB (typical

adhesive/polish conn)

0.75 dB (TIA-568

max acceptable)Total # of Connectors 5 5

Total Connector Loss 1.5 dB 3.75 dB

(All connectors are allowed 0.75 max per EIA/TIA 568 standard )

Remember that we include all the components in the complete link, including the connectors on each end. In our example above, the link includes

patchcords on each end to connect to the electronics. We need to assess thequality of these connectors, so we include them in the link loss budget and if we test the link end to end, including the patchcords, these connectors will beincluded in the test results when connected to launch and receive referencecables. On some links, only the permanently installed link, not including the

patchcords, will be tested. Again, we still need to include the connectors onthe end as they will be included when we test insertion loss with reference test cables on each end.

Step 3. Splice Loss

Multimode splices are usually made with mechanical splices, although somefusion splicing is used. The larger core and multiple layers make fusionsplicing abut the same loss as mechanical splicing, but fusion is more reliablein adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being agood average for an experienced installer. Fusion splicing of singlemode fiber will typically have less than 0.05 dB (that's right, less than a tenth of a dB!)
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Typical Splice Loss 0.3 dBTotal # splices 1Total Splice Loss 0.3 dB

(All splices are allowed 0.3 max per EIA/TIA 568 standard )

Step 4. Total Passive System Attenuation

Add the fiber loss, connector and splice losses to get the link loss.

Typical TIA 568 Max850 nm 1300 nm 850 nm 1300 nm

Total Fiber Loss

(dB)6.0 2.0 7.0 3.0

Total Connector Loss (dB) 1.5 1.5 3.75 3.75

Total Splice Loss(dB) 0.3 0.3 0.3 0.3

Other (dB) 0 0 0 0Total Link Loss(dB) 7.8 3.8 11.05 7.05

Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

Equipment Link Loss Budget Calculation: Link loss budget for networkhardware depends on the dynamic range, the difference between thesensitivity of the receiver and the output of the source into the fiber. You needsome margin for system degradation over time or environment, so subtractthat margin (as much as 3dB) to get the loss budget for the link.

Step 5. Data From Manufacturer's Specification for Active Components(Typical 100 Mb/s link)

Operating Wavelength (nm) 1300Fiber Type MMReceiver Sens. (dBm@ required BER) -31
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Average Transmitter Output (dBm) -16Dynamic Range (dB) 15Recommended Excess Margin (dB) 3

Step 6. Loss Margin Calculation

Dynamic Range (dB) (above) 15 15

Cable Plant Link Loss (dB) 3.8 (Typ) 7.05(TIA)Link Loss Margin (dB) 11.2 7.95

As a general rule, the Link Loss Margin should be greater than approximately3 dB to allow for link degradation over time. LEDs in the transmitter may age

and lose power, connectors or splices may degrade or connectors may getdirty if opened for rerouting or testing. If cables are accidentally cut, excessmargin will be needed to accommodate splices for restoration.

History

Daniel Colladon first described this light fountain or light pipe in an 1842 article titled Onthe reflections of a ray of light inside a parabolic liquid stream . This particular illustration comesfrom a later article by Colladon, in 1884.
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Fiber optics, though used extensively in the modern world, is a fairly simple, and relatively old,technology. Guiding of light by refraction, the principle that makes fiber optics possible, wasfirst demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. JohnTyndall included a demonstration of it in his public lectures in London , 12 years later .3 Tyndallalso wrote about the property of total internal reflection in an introductory book about the nature

of light in 1870: "When the light passes from air into water, the refracted ray is bent towards the perpendicular ... When the ray passes from water to air it is bent from the perpendicular... If theangle which the ray in water encloses with the perpendicular to the surface be greater than 48degrees, the ray will not quit the water at all: it will be totally reflected at the surface.... Theangle which marks the limit where total reflection begins is called the limiting angle of themedium. For water this angle is 4827', for flint glass it is 3841', while for diamond it is2342'. "45 Unpigmented human hairs have also been shown to act as an optical fiber .6

Practical applications, such as close internal illumination during dentistry, appeared early in thetwentieth century. Image transmission through tubes was demonstrated independently by theradio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s.

The principle was first used for internal medical examinations by Heinrich Lamm in thefollowing decade. Modern optical fibers, where the glass fiber is coated with a transparentcladding to offer a more suitable refractive index , appeared later in the decade .3 Developmentthen focused on fiber bundles for image transmission. Harold Hopkins and Narinder SinghKapany at Imperial College in London achieved low-loss light transmission through a 75 cmlong bundle which combined several thousand fibers. Their article titled "A flexible fibrescope,using static scanning" was published in the journal Nature in 1954 .78 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 thegastroscope, 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 1880 Alexander Graham Bell and Sumner Tainter invented the 'Photophone ' at the VoltaLaboratory in Washington, D.C., to transmit voice signals over an optical beam .9 It was anadvanced form of telecommunications, but subject to atmospheric interferences and impracticaluntil the secure transport of light that would be offered by fiber-optical systems. In the late 19thand early 20th centuries, light was guided through bent glass rods to illuminate body cavities .10 Jun-ichi Nishizawa , a Japanese scientist at Tohoku University , also proposed the use of opticalfibers for communications in 1963, as stated in his book published in 2004 in India .11 Nishizawainvented other technologies that contributed to the development of optical fiber communications,such as the graded-index optical fiber as a channel for transmitting light from semiconductor lasers .1213 The first working fiber-optical data transmission system was demonstrated by German

physicist Manfred Brner at Telefunken Research Labs in Ulm in 1965, which was followed bythe first patent application for this technology in 1966 .1415 Charles K. Kao and George A.Hockham of the British company Standard Telephones and Cables (STC) were the first to

promote the idea that the attenuation in optical fibers could be reduced below 20 decibels per kilometer (dB/km), making fibers a practical communication medium .16 They proposed that theattenuation in fibers available at the time was caused by impurities that could be removed, rather
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than by fundamental physical effects such as scattering. They correctly and systematicallytheorized the light-loss properties for optical fiber, and pointed out the right material to use for such fibers silica glass with high purity. This discovery earned Kao the Nobel Prize in Physics in 2009 .17

NASA used fiber optics in the television cameras that were sent to the moon. At the time, the usein the cameras was classified confidential , and only those with the right security clearance or those accompanied by someone with the right security clearance were permitted to handle thecameras .18

The crucial attenuation limit of 20 dB/km was first achieved in 1970, by researchers Robert D.Maurer , Donald Keck , Peter C. Schultz , and Frank Zimar working for American glass maker Corning Glass Works, now Corning Incorporated . They demonstrated a fiber with 17 dB/kmattenuation by doping silica glass with titanium . A few years later they produced a fiber withonly 4 dB/km attenuation using germanium dioxide as the core dopant. Such low attenuationushered in optical fiber telecommunication. In 1981, General Electric produced fused quartz

ingots that could be drawn into strands 25 miles (40 km) long .19

Attenuation in modern optical cables is far less than in electrical copper cables, leading to long-haul fiber connections with repeater distances of 70 150 kilometers (43 93 mi). The erbium-doped fiber amplifier , which reduced the cost of long-distance fiber systems by reducing or eliminating optical-electrical-optical repeaters, was co-developed by teams led by David N.Payne of the University of Southampton and Emmanuel Desurvire at Bell Labs in 1986. Robustmodern optical fiber uses glass for both core and sheath, and is therefore less prone to aging. Itwas invented by Gerhard Bernsee of Schott Glass in Germany in 1973 .20

The emerging field of photonic crystals led to the development in 1991 of photonic-crystal

fiber ,21

which guides light by diffraction from a periodic structure, rather than by total internalreflection. The first photonic crystal fibers became commercially available in 2000 .22 Photoniccrystal fibers can carry higher power than conventional fibers and their wavelength-dependent

properties can be manipulated to improve performance.

Uses

Optical fiber communication

Main article: Fiber-optic communication

Optical fiber can be used as a medium for telecommunication and computer networking becauseit is flexible and can be bundled as cables. It is especially advantageous for long-distancecommunications, because light propagates through the fiber with little attenuation compared toelectrical cables. This allows long distances to be spanned with few repeaters .

The per-channel light signals propagating in the fiber have been modulated at rates as high as111 gigabits per second (GBPS) by NTT ,2324 although 10 or 40 Gbit/s is typical in deployed
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iber#cite_note-23http://en.wikipedia.org/wiki/Optical_fiber#cite_note-23http://en.wikipedia.org/wiki/Nippon_Telegraph_and_Telephonehttp://en.wikipedia.org/wiki/Gigabit_per_secondhttp://en.wikipedia.org/wiki/Optical_communications_repeaterhttp://en.wikipedia.org/wiki/Computer_networkhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Optical_fiber#cite_note-22http://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Optical_fiber#cite_note-21http://en.wikipedia.org/wiki/Photonic-crystal_fiberhttp://en.wikipedia.org/wiki/Photonic-crystal_fiberhttp://en.wikipedia.org/wiki/Photonic_crystalhttp://en.wikipedia.org/wiki/Optical_fiber#cite_note-20http://en.wikipedia.org/wiki/Germanyhttp://en.wikipedia.org/wiki/Schott_AGhttp://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/w/index.php?title=Emmanuel_Desurvire&action=edit&redlink=1http://en.wikipedia.org/wiki/University_of_Southamptonhttp://en.wikipedia.org/wiki/David_N._Paynehttp://en.wikipedia.org/wiki/David_N._Paynehttp://en.wikipedia.org/wiki/Erbium-doped_fiber_amplifierhttp://en.wikipedia.org/wiki/Erbium-doped_fiber_amplifierhttp://en.wikipedia.org/wiki/Optical_fiber#cite_note-19http://en.wikipedia.org/wiki/Ingotshttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/General_Electrichttp://en.wikipedia.org/wiki/Germanium_dioxidehttp://en.wikipedia.org/wiki/Titaniumhttp://en.wikipedia.org/wiki/Silica_glasshttp://en.wikipedia.org/wiki/Doping_(semiconductor)http://en.wikipedia.org/wiki/Corning_Incorporatedhttp://en.wikipedia.org/wiki/Peter_C._Schultzhttp://en.wikipedia.org/wiki/Donald_Keckhttp://en.wikipedia.org/wiki/Robert_D._Maurerhttp://en.wikipedia.org/wiki/Robert_D._Maurerhttp://en.wikipedia.org/wiki/Optical_fiber#cite_note-18http://en.wikipedia.org/wiki/Optical_fiber#cite_note-17http://en.wikipedia.org/wiki/Nobel_Prize_in_Physics


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systems .2526 In June 2013, researchers demonstrated transmission of 400 GBPS over a singlechannel using 4-mode orbital angular momentum mode division multiplexing .27

Each fiber can carry many independent channels, each using a different wavelength of light(wavelength-division multiplexing (WDM)). The net data rate (data rate without overhead bytes)

per fiber is the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels (usually up to eighty in commercial dense WDM systems as of 2008). As of 2011 therecord for bandwidth on a single core was 101 Tbit/sec (370 channels at 273 Gbit/sec each) .28 The record for a multi-core fibre as of January 2013 was 1.05 petabits per second. 29 In 2009,Bell Labs broke the 100 (Petabit per second)kilometre barrier (15.5 Tbit/s over a single7000 km fiber) .30

For short distance application, such as a network in an office building, fiber-optic cabling cansave space in cable ducts. This is because a single fiber can carry much more data than electricalcables such as standard category 5 Ethernet cabling, which typically runs at 100 Mbit/s or 1Gbit/s speeds. Fiber is also immune to electrical interference; there is no cross-talk between

signals in different cables, and no pickup of environmental noise. Non-armored fiber cables donot conduct electricity, which makes fiber a good solution for protecting communicationsequipment in high voltage environments, such as power generation facilities, or metalcommunication structures prone to lightning strikes. They can also be used in environmentswhere explosive fumes are present, without danger of ignition. Wiretapping (in this case, fiber tapping ) is more difficult compared to electrical connections, and there are concentric dual corefibers that are said to be tap-proof .31

Fiber optic sensors

Main article: Fiber optic sensor

Fibers have many uses in remote sensing. In some applications, the sensor is itself an opticalfiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system.Depending on the application, fiber may be used because of its small size, or the fact that noelectrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensingthe time delay as light passes along the fiber through each sensor. Time delay can be determinedusing a device such as an optical time-domain reflectometer .

Optical fibers can be used as sensors to measure strain , temperature , pressure and other quantities by modifying a fiber so that the property to measure modulates the intensity , phase , polarization ,

wavelength , or transit time of light in the fiber. Sensors that vary the intensity of light are thesimplest, since only a simple source and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.

Extrinsic fiber optic sensors use an optical fiber cable , normally a multi-mode one, to transmitmodulated light from either a non-fiber optical sensor or an electronic sensor connected to anoptical transmitter. A major benefit of extrinsic sensors is their ability to reach otherwise
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inaccessible places. An example is the measurement of temperature inside aircraft jet engines byusing a fiber to transmit radiation into a radiation pyrometer outside the engine. Extrinsic sensorscan be used in the same way to measure the internal temperature of electrical transformers , where the extreme electromagnetic fields present make other measurement techniquesimpossible. Extrinsic sensors measure vibration, rotation, displacement, velocity, acceleration,

torque, and twisting. A solid state version of the gyroscope, using the interference of light, has been developed. The fiber optic gyroscope (FOG) has no moving parts, and exploits the Sagnaceffect to detect mechanical rotation.

Common uses for fiber optic sensors includes advanced intrusion detection security systems. Thelight is transmitted along a fiber optic sensor cable placed on a fence, pipeline, or communicationcabling, and the returned signal is monitored and analysed for disturbances. This return signal isdigitally processed to detect disturbances and trip an alarm if an intrusion has occurred.

Other uses of optical fibers

A frisbee illuminated by fiber optics

Light reflected from optical fiber illuminates exhibited model

Fibers are widely used in illumination applications. They are used as light guides in medical andother applications where bright light needs to be shone on a target without a clear line-of-sight

path. In some buildings, optical fibers route sunlight from the roof to other parts of the building
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(see nonimaging optics ). Optical fiber illumination is also used for decorative applications,including signs , art , toys and artificial Christmas trees . Swarovski boutiques use optical fibers toilluminate their crystal showcases from many different angles while only employing one lightsource. Optical fiber is an intrinsic part of the light-transmitting concrete building product,LiTraCon .

Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes alongwith lenses, for a long, thin imaging device called an endoscope , which is used to view objectsthrough a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures. Industrial endoscopes (see fiberscope or borescope ) are used for inspectinganything hard to reach, such as jet engine interiors. Many microscopes use fiber-optic lightsources to provide intense illumination of samples being studied.

In spectroscopy , optical fiber bundles transmit light from a spectrometer to a substance thatcannot be placed inside the spectrometer itself, in order to analyze its composition. Aspectrometer analyzes substances by bouncing light off of and through them. By using fibers, a

spectrometer can be used to study objects remotely .323 334

An optical fiber doped with certain rare earth elements such as erbium can be used as the gainmedium of a laser or optical amplifier . Rare-earth doped optical fibers can be used to providesignal amplification by splicing a short section of doped fiber into a regular (undoped) opticalfiber line. The doped fiber is optically pumped with a second laser wavelength that is coupledinto the line in addition to the signal wave. Both wavelengths of light are transmitted through thedoped fiber, which transfers energy from the second pump wavelength to the signal wave. The

process that causes the amplification is stimulated emission .

Optical fibers doped with a wavelength shifter collect scintillation light in physics experiments .

Optical fiber can be used to supply a low level of power (around one watt )citation needed toelectronics situated in a difficult electrical environment. Examples of this are electronics in high-

powered antenna elements and measurement devices used in high voltage transmissionequipment.

The iron sights for handguns, rifles, and shotguns may use short pieces of optical fiber for contrast enhancement.

Principle of operation
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An overview of the operating principles of the optical fiber

An optical fiber is a cylindrical dielectric waveguide (nonconducting waveguide) that transmitslight along its axis, by the process of total internal reflection . The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the

optical signal in the core, the refractive index of the core must be greater than that of thecladding. The boundary between the core and cladding may either be abrupt, in step-index fiber , or gradual, in graded-index fiber .

Index of refraction

Main article: Refractive index

The index of refraction is a way of measuring the speed of light in a material. Light travelsfastest in a vacuum , such as outer space. The speed of light in a vacuum is about 300,000kilometers (186,000 miles) per second. Index of refraction is calculated by dividing the speed of

light in a vacuum by the speed of light in some other medium. The index of refraction of avacuum is therefore 1, by definition. The typical value for the cladding of an optical fiber is1.52 .35 The core value is typically 1.62 .35 The larger the index of refraction, the slower lighttravels in that medium. From this information, a good rule of thumb is that signal using opticalfiber for communication will travel at around 200,000 kilometers per second. Or to put it another way, to travel 1000 kilometers in fiber, the signal will take 5 milliseconds to propagate. Thus a

phone call carried by fiber between Sydney and New York, a 16,000-kilometer distance, meansthat there is an absolute minimum delay of 80 milliseconds (or around 1/12 of a second) betweenwhen one caller speaks to when the other hears. (Of course the fiber in this case will probablytravel a longer route, and there will be additional delays due to communication equipmentswitching and the process of encoding and decoding the voice onto the fiber).

Total internal reflection

Main article: Total internal reflection

When light traveling in an optically dense medium hits a boundary at a steep angle (larger thanthe critical angle for the boundary), the light is completely reflected. This is called total internalreflection. This effect is used in optical fibers to confine light in the core. Light travels throughthe fiber core, bouncing back and forth off the boundary between the core and cladding. Becausethe light must strike the boundary with an angle greater than the critical angle, only light thatenters the fiber within a certain range of angles can travel down the fiber without leaking out.

This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone isa function of the refractive index difference between the fiber's core and cladding.

In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is thenumerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice andwork with than fiber with a smaller NA. Single-mode fiber has a small NA.
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Multi-mode fiber

The propagation of light through a multi-mode optical fiber .

A laser bouncing down an acrylic rod, illustrating the total internal reflection of light in a multi-mode optical fiber.

Main article: Multi-mode optical fiber

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometricaloptics . Such fiber is called multi-mode fiber , from the electromagnetic analysis (see below). In astep-index multi-mode fiber, rays of light are guided along the fiber core by total internalreflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a linenormal to the boundary), greater than the critical angle for this boundary, are completelyreflected. The critical angle (minimum angle for total internal reflection) is determined by thedifference 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 lightand hence information along the fiber. The critical angle determines the acceptance angle of thefiber, often reported as a numerical aperture . A high numerical aperture allows light to propagatedown 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 asrays at different angles have different path lengths and therefore take different times to traversethe fiber.
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Optical fiber types.

In graded-index fiber, the index of refraction in the core decreases continuously between the axisand 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 differencein axial propagation speeds of the various rays in the fiber. This ideal index profile is very closeto a parabolic relationship between the index and the distance from the axis.

Single-mode fiber

The structure of a typical single-mode fiber . 1. Core: 8 m diameter 2. Cladding: 125 m dia.3. Buffer: 250 m dia.4. Jacket: 400 m dia.

Main article: Single-mode optical fiber

Fiber with a core diameter less than about ten times the wavelength of the propagating lightcannot 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 thatoccur 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 multi-mode fiber can also be modeled using the wave equation, which shows thatsuch fiber supports more than one mode of propagation (hence the name). The results of suchmodeling 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 thecore. Instead, especially in single-mode fibers, a significant fraction of the energy in the boundmode travels in the cladding as an evanescent wave .
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The most common type of single-mode fiber has a core diameter of 8 10 micrometers and isdesigned for use in the near infrared . The mode structure depends on the wavelength of the lightused, so that this fiber actually supports a small number of additional modes at visiblewavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as50 micrometers and as large as hundreds of micrometers. The normalized frequency V for this

fiber should be less than the first zero of the Bessel function J 0 (approximately 2.405).

Special-purpose fiber

Some special-purpose optical fiber is constructed with a non-cylindrical core and/or claddinglayer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.Polarization-maintaining fibers are unique type of fibers that is commonly used in fiber opticsensors due to its ability to maintain the polarization of the light inserted in it.

Photonic-crystal fiber is made with a regular pattern of index variation (often in the form of

cylindrical holes that run along the length of the fiber). Such fiber uses diffraction effects insteadof or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.

Mechanisms of attenuation

Light attenuation by ZBLAN and silica fibers Main article: Transparent materials

Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of thelight beam (or signal) as it travels through the transmission medium. Attenuation coefficients infiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass

that confines the incident light beam to the inside. Attenuation is an important factor limiting thetransmission of a digital signal across large distances. Thus, much research has gone into bothlimiting the attenuation and maximizing the amplification of the optical signal. Empiricalresearch has shown that attenuation in optical fiber is caused primarily by both scattering andabsorption .

Light scattering
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Specular reflection

Diffuse reflection

The propagation of light through the core of an optical fiber is based on total internal reflectionof the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light raysto be reflected in random directions. This is called diffuse reflection or scattering , and it istypically characterized by wide variety of reflection angles.

Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatialscales of visibility arise, depending on the frequency of the incident light-wave and the physicaldimension (or spatial scale) of the scattering center, which is typically in the form of somespecific micro-structural feature. Since visible light has a wavelength of the order of onemicrometer (one millionth of a meter) scattering centers will have dimensions on a similar spatialscale.

Thus, attenuation results from the incoherent scattering of light at internal surfaces andinterfaces . In (poly)crystalline materials such as metals and ceramics, in addition to pores, mostof the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regionsof crystalline order. It has recently been shown that when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the
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scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials .

Similarly, the scattering of light in optical quality glass fiber is caused by molecular levelirregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of

thought is that a glass is simply the limiting case of a polycrystalline solid. Within thisframework, "domains" exhibiting various degrees of short-range order become the building blocks of both metals and alloys, as well as glasses and ceramics. Distributed both between andwithin these domains are micro-structural defects that provide the most ideal locations for lightscattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes .36

At high optical powers, scattering can also be caused by nonlinear optical processes in thefiber .3738

UV-Vis-IR absorption

In addition to light scattering, attenuation or signal loss can also occur due to selective absorptionof specific wavelengths, in a manner similar to that responsible for the appearance of color.Primary material considerations include both electrons and molecules as follows:

1) At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized")such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency inthe ultraviolet (UV) or visible ranges. This is what gives rise to color.

2) At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed its atoms or molecules are, and whether or not

the atoms or molecules exhibit long-range order. These factors will determine the capacity of thematerial transmitting longer wavelengths in the infrared (IR), far IR, radio and microwaveranges.

The design of any optically transparent device requires the selection of materials based uponknowledge of its properties and limitations. The Lattice absorption characteristics observed at thelower frequency regions (mid IR to far-infrared wavelength range) define the long-wavelengthtransparency limit of the material. They are the result of the interactive coupling between themotions of thermally induced vibrations of the constituent atoms and molecules of the solidlattice and the incident light wave radiation. Hence, all materials are bounded by limiting regionsof absorption caused by atomic and molecular vibrations (bond-stretching)in the far-infrared

(>10 m).

Thus, multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipolescan absorb energy from the incident radiation, reaching a maximum coupling with the radiationwhen the frequency is equal to the fundamental vibrational mode of the molecular dipole (e.g.Si-O bond) in the far-infrared, or one of its harmonics.
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The selective absorption of infrared (IR) light by a particular material occurs because theselected frequency of the light wave matches the frequency (or an integer multiple of thefrequency) at which the particles of that material vibrate. Since different atoms and moleculeshave different natural frequencies of vibration, they will selectively absorb different frequencies(or portions of the spectrum) of infrared (IR) light.

Reflection and transmission of light waves occur because the frequencies of the light waves donot match the natural resonant frequencies of vibration of the objects. When IR light of thesefrequencies strikes an object, the energy is either reflected or transmitted.

Manufacturing

Materials

Glass optical fibers are almost always made from silica , but some other materials, such asfluorozirconate , fluoroaluminate , and chalcogenide glasses as well as crystalline materials likesapphire , are used for longer-wavelength infrared or other specialized applications. Silica andfluoride glasses usually have refractive indices of about 1.5, but some materials such as thechalcogenides can have indices as high as 3. Typically the index difference between core andcladding is less than one percent.

Plastic optical fibers (POF) are commonly step-index multi-mode fibers with a core diameter of 0.5 millimeters or larger. POF typically have higher attenuation coefficients than glass fibers,1 dB/m or higher, and this high attenuation limits the range of POF-based systems.

Silica

Silica exhibits fairly good optical transmission over a wide range of wavelengths. In the near-infrared (near IR) portion of the spectrum, particularly around 1.5 m, silica can have extremelylow absorption and scattering losses of the order of 0.2 dB/km. Such remarkably low losses are

possible only because ultra-pure silicon is available, it being essential for manufacturingintegrated circuits and discrete transistors. A high transparency in the 1.4- m region is achieved

by maintaining a low concentration of hydroxyl groups (OH). Alternatively, a high OHconcentration is better for transmission in the ultraviolet (UV) region.

Silica can be drawn into fibers at reasonably high temperatures, and has a fairly broad glasstransformation range . One other advantage is that fusion splicing and cleaving of silica fibers isrelatively effective. Silica fiber also has high mechanical strength against both pulling and even

bending, provided that the fiber is not too thick and that the surfaces have been well preparedduring processing. Even simple cleaving (breaking) of the ends of the fiber can provide nicelyflat surfaces with acceptable optical quality. Silica is also relatively chemically inert . In

particular, it is not hygroscopic (does not absorb water).

Silica glass can be doped with various materials. One purpose of doping is to raise the refractiveindex (e.g. with Germanium dioxide (GeO 2) or Aluminium oxide (Al 2O3)) or to lower it (e.g.with fluorine or Boron trioxide (B2O3)). Doping is also possible with laser-active ions (for
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example, rare earth -doped fibers) in order to obtain active fibers to be used, for example, in fiber amplifiers or laser applications. Both the fiber core and cladding are typically doped, so that theentire assembly (core and cladding) is effectively the same compound (e.g. an aluminosilicate , germanosilicate, phosphosilicate or borosilicate glass ).

Particularly for active fibers, pure silica is usually not a very suitable host glass, because itexhibits a low solubility for rare earth ions. This can lead to quenching effects due to clusteringof dopant ions. Aluminosilicates are much more effective in this respect.

Silica fiber also exhibits a high threshold for optical damage. This property ensures a lowtendency for laser-induced breakdown. This is important for fiber amplifiers when utilized for the amplification of short pulses.

Because of these properties silica fibers are the material of choice in many optical applications,such as communications (except for very short distances with plastic optical fiber), fiber lasers,fiber amplifiers, and fiber-optic sensors. Large efforts put forth in the development of various

types of silica fibers have further increased the performance of such fibers over other materials .394 0414 2434 44546

Fluoride

Fluoride glass is a class of non-oxide optical quality glasses composed of fluorides of variousmetals . Because of their low viscosity , it is very difficult to completely avoid crystallization while processing it through the glass transition (or drawing the fiber from the melt). Thus,although heavy metal fluoride glasses (HMFG) exhibit very low optical attenuation, they are notonly difficult to manufacture, but are quite fragile, and have poor resistance to moisture andother envir

the fiber optic association

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