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Optical Fiber connector:

An optical fibre connector terminates theend of an optical fiber, and enables quickerconnection and disconnection than splicing.The connectors mechanically couple andalign the cores of fibers so that light canpass.

Connector Types:

In todays local area networks (LANs),there are two primary legacy optical fiberconnector types (the ST-style and the SCDuplex) and two primary small form factor(SFF) connectors (MT-RJ and LC). Allfour of these connector types have been inuse for several years and have a proventrack record for the performance andreliability desired for local area networks.However, there are some significant

differences between these connector types.


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The ST-style Connector:

This connector type, sometimes referred toas the BIFOC connector, is a simplexfiber connector that means one fiber inone ferrule with one 2.5mm cylindricalferrule. To get a duplex ST-style

connection, four connectors and twoadapters are required. The housing of theST includes a push-and-twist, spring-loaded latching mechanism that isrelatively large by todays standards,

particularly when consideration for finger space (the space around the connector thatis needed to get fingers in to grip, push andtwist the connector) is considered. Thisconnector was one of the first high-performance, robust optical fiberconnectors and, as such, was widelyadopted in the telco market. This popularityspilled over into the LANs that

incorporated optical fiber cabling and was


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widely used. To this day, it is still a verypopular optical fiber connector.

The SC Duplex Connector:

While the ST-style remains a completelyfunctional connector, some of its propertiesare not well-suited for the LAN market.

Since most all LANs are based on duplexoptical fiber runs, a simplex connector isless desirable than a duplex. For thisreason, the SC Duplex was introduced.While the base component (the ferrule) isthe same as the ST-style, the housing iscompletely different. The SC connector hasa housing that features a push-pull latchmechanism, making it easier to mate and

de-mate, and reducing the finger spaceneeded. The SC Duplex connector,sometimes referred to as the TIA568Aconnector, consists of two SC connectorsyoked together, which can both be mated or

de-mated with the same push or pull action.


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These features helped the SC Duplexbecome the recommended connector in

LAN standards both in North America (inEIA/TIA-568-A) and internationally (inISO 11801). Even though thisrecommendation was initiated almost tenyears ago, only recently has the number of SC connectors installed in LANs equalledthe number of ST-style connectors.

Small Form Factor Connectors:

Although the SC Duplex solved some of the LAN-related issues for fiberconnectiv ity, it didnt solve one of the mostimportant issues: density. Because of thesingle-fiber ferrules, the large housings and

the finger space, the fiber connectiondensity was still twice that of traditionalcopper terminations (RJ-45). The industryreally needed a duplex fiber connector thathad the same basic size as the RJ-45 copper

connector - a small form factor connector.


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an RJ-45 plug. When combined with theno-epoxy/no-polish termination technology

(discussed later), this connector type offershigh density and ease-of-installationadvantages.

The LC Connector:

Recognizing that fiber ferrule size was thelimiting factor in fiber connection density,the LC connector was developed using areduced-diameter, simplex fiber ferrule.Instead of the 2.5mm ferrule, the LCconnector uses a 1.25mm cylindricalferrule. The smaller ferrule allows for asmaller housing and thus a smallerconnector. Small enough that two LC

connectors yoked together take up aboutthe same port space as an RJ-45. The LCconnectors also feature a latchingmechanism similar to the RJ-45, and thesmaller ferrule also reduces the time


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required for the polishing step forepoxy/polish connector termination.

Optical Fibre Couplers:

A fiber optic couple r is a device used inoptical fiber systems with one or moreinput fibers and one or several output

fibers. Light entering an input fiber canappear at one or more outputs and its powerdistribution potentially depending on thewavelength and polarization. Such couplerscan be fabricated in different ways, forexample by thermally fusing fibers so thattheir cores get into intimate contact. If allinvolved fibers are single-mode (supportingonly a single mode per polarization

direction for a given wavelength), there arecertain physical restrictions on theperformance of the coupler. In particular, itis not possible to combine two or moreinputs of the same optical frequency into

one single-polarization output without


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significant excess losses. However, such arestriction does not occur for different input

wavelengths: there are couplers which cancombine two inputs at differentwavelengths into one output withoutexhibiting significant losses. Such couplersare used. Wavelength-sensitive couplers areused as multiplexers in wavelength-divisionmultiplexing (WDM) telecom systems tocombine several input channels withdifferent wavelengths, or to separate

channels. A coupler is passive and bidirectional.Because the coupler is not a perfect device,excess losses can occur. These losseswithin fibers are internal to the coupler andoccur from scattering, absorption,reflections, misalignments, and poorisolation. Excess loss does not includelosses from connectors attaching fibers to

the ports. Further, since most couplers


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contain an optical fiber at each port,additional loss can occur because of

diameter and NA mismatches between thecoupler port and the attached fiber.

Some fiber optic data links require morethan simple point-to-point connections.

These data links may be of a much morecomplex design that requires multi-port orother types of connections. Figure 4-23shows some example system architecturesthat use more complex link designs. In

many cases these types of systems requirefiber optic components that can redistribute(combine or split) optical signalsthroughout the system

One type of fiber optic component thatallows for the redistribution of opticalsignals is a fiber optic coupler. A fiberoptic coupler is a device that can distributethe optical signal (power) from one fiber

among two or more fibers. A fiber optic


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coupler can also combine the optical signalfrom two or more fibers into a single fiber.

Fiber optic couplers attenuate the signalmuch more than a connector or splicebecause the input signal is divided amongthe output ports. For example, with a 1 X 2fiber optic coupler, each output is less thanone-half the power of the input signal (overa 3 dB loss).

Fiber optic couplers can be either active orpassive devices. The difference between

active and passive couplers is that a passivecoupler redistributes the optical signalwithout optical-to-electrical conversion.Active couplers are electronic devices thatsplit or combine the signal electrically anduse fiber optic detectors and sources forinput and output.

A basic fiber optic coupler has N inputports and M output ports. N and M

typically range from 1 to 64. The number


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of input ports and output ports varydepending on the intended application for

the coupler. Types of fiber optic couplersinclude optical splitters, optical combiners,X couplers, star couplers, and tree couplers.

Figure 4-24. - Basic passive fiber opticcoupler design.

X coupler combines the functions of theoptical splitter and combiner. The Xcoupler combines and divides the optical

power from the two input fibers betweenthe two output fibers. Another name for theX coupler is the 2 X 2 coupler.

Star and tree couplers

are multiport couplers that have more thantwo input or two output ports. A starcoupler is a passive device that distributesoptical power from more than two inputports among several output ports. A treecoupler is a passive device that splits the


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optical power from one input fiber to morethan two output fibers. A tree coupler may

also be used to combine the optical powerfrom more than two input fibers into asingle output fiber.

Fiber optic couplers should prevent the

transfer of optical power from one inputfiber to another input fiber. Directionalcouplers are fiber optic couplers thatprevent this transfer of power betweeninput fibers. Many fiber optic couplers are

also symmetrical. A symmetrical couplertransmits the same amount of powerthrough the coupler when the input andoutput fibers are reversed.

Passive fiber optic coupler fabricationtechniques can be complex and difficult tounderstand. Some fiber optic couplerfabrication involves beam splitting usingmicrolenses or graded-refractive-index

(GRIN) rods and beam splitters or optical


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mixers. These beam splitter devices dividethe optical beam into two or more separated

beams. Fabrication of fiber optic couplersmay also involve twisting, fusing, andtapering together two or more opticalfibers. This type of fiber optic coupler is afused biconical taper coupler. Fusedbiconical taper couplers use the radiativecoupling of light from the input fiber to theoutput fibers in the tapered region toaccomplish beam splitting.

LED coupling In Single Mode Fiber:

In the early years of optical fiberapplications, LED's were traditionally onlyconsidered for multimode-fiber

systems.However,around 1985 researchersrecognized that edge-emitting LED's canlaunch sufficient optical power into a singlemode fiber for transmission at data ratesunto 560Mb/s over several kms.The interest

in this arouse because of the advantage and


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reliability advantages of LED's over laserdiodes. Investigators have used edge

emitting LED's rather than surface-emittingLEDs because the edge-emitter s have alaser like output pattern in the directionperpendicular to the junction plane.

To rigorously evaluate the couplingbetween an LED and a single-mode fiberwe need to use the formalism of electromagnetic theory rather thangeometric optics, because of the monomode

nature of the fiber.Here we will use the analysis of Reigth andShumate to look at the following two cases:

(a) direct coupling of an LED into a single-

mode fiber.

(b) coupling into a single-mode fiber froma multimode flylead attached to the LED.


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The efficiency of direct coupling of asource into an optical fiber is affected by

three different factors. These factors are :geometrical losses, Fresnel losses andangular losses.

GEOMETRICAL LOSSES :

The light emitted by a source of area (Asource) can be butt-coupled into an opticalfiber which is larger the the source area.

The light incident outside the core area willnot be guided by the fiber. There is threepossible configurations for emitter andfiber area:

First, if the source dimensions (H, L) aresmaller than fiber core diameter, all thelight will be coupled into the fiber.

- Second, if the source is larger than fiber

core area , the geometrical factor for


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coupling efficiency is given by the ratio of light captured by the fiber.

- Third, if fiber core diameter is larger thanone side of the source, but is smaller than

second side. That can happened with laserdiodes that are typic ally 1 m height bysome hundred microns long.

FRESNEL LOSSES:

The light incident on an interface betweentwo dielectric media of different refractiveindex is partially transmitted and partiallyreflected (assuming linear, homogeneous

and isotropic media). The proportion of light reflected and transmitted depends onlight polarisation and incidence angle andcan be calculated from. Fresnel equationsbased on electromagnetic theory. In thisdocument, we do not present the


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development leading to the result we need,for a complete theoretical treatment, refer

to references [1] or [2]. The powerreflectance at normal incidence is thengiven by:

Coupling Improvement:

Practically much of the light emitted fromLEDs is not coupled into the narrowacceptance angle of the fiber. It has been

found that greater coupling efficiency maybe obtained if lenses are used to collimatethe emission from the LED, particularlywhen the fiber core diameter issignificantly larger than the width of theemission region There are several lenscoupling configurations which includespherically polished structures, sphericalended or tapered fiber coupling, truncated


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spherical micro lenses, GRIN-rod lensesand integral lens structures.

It consists of a planar surface emittingstructure with the spherical ended fiberattached to the cap by epoxy resin. Anemitting diameter of 35 m is fabricated

into the device and light is coupled intofibers with core diameters or 75 m and110 m. For increased coupling efficiency.

Transmitter and Receiver requirements

in WDM Network:

In fiber-optic communications, wavelength-division multiplexing (WDM) is atechnology which multiplexes a number of

optical carrier signals onto a single opticalfiber by using different wavelengths (i.e.colours) of laser light. This techniqueenables bidirectional communications overone strand of fiber, as well as multiplication

of capacity.


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The term wavelength-division multiplexingis commonly applied to an optical carrier

(which is typically described by itswavelength), whereas frequency-divisionmultiplexing typically applies to a radiocarrier (which is more often described byfrequency). Since wavelength andfrequency are tied together through asimple directly inverse relationship, the twoterms actually describe the same concept.

A WDM system uses a multiplexer at the

transmitter to join the signals together, anda demultiplexer at the receiver to split themapart. With the right type of fiber it ispossible to have a device that does bothsimultaneously, and can function as anoptical add-drop multiplexer. The opticalfiltering devices used have conventionallybeen etalons, stable solid-state single-frequency Fabry Prot interferometers in

the form of thin-film-coated optical glass.


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The concept was first published in 1970,and by 1978 WDM systems were being

realized in the laboratory. The first WDMsystems combined only two signals.Modern systems can handle up to 160signals and can thus expand a basic 10Gbit/s system over a single fiber pair toover 1.6 Tbit/s.

WDM systems are popular withtelecommunications companies becausethey allow them to expand the capacity of

the network without laying more fiber. Byusing WDM and optical amplifiers, theycan accommodate several generations of technology development in their opticalinfrastructure without having to overhaulthe backbone network. Capacity of a givenlink can be expanded simply by upgrades tothe multiplexers and demultiplexers at eachend.


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This is often done by use of optical-to-electrical-to-optical (O/E/O) translation at

the very edge of the transport network, thuspermitting interoperation with existingequipment with optical interfaces.

Most WDM systems operate on single-

mode fiber optical cables, which have acore diameter of 9 m. Certain forms of WDM can also be used in multi-mode fibercables (also known as premises cables)which have core diameters of 50 or

62.5 m.

Early WDM systems were expensive andcomplicated to run. However, recentstandardization and better understanding of

the dynamics of WDM systems have madeWDM less expensive to deploy.

Optical receivers, in contrast to lasersources, tend to be wideband devices.Therefore the demultiplexer must provide


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the wavelength selectivity of the receiver inthe WDM system.

WDM systems are divided into differentwavelength patterns, conventional/coarse(CWDM) and dense (DWDM).Conventional WDM systems provide up to

8 channels in the 3rd transmission window(C-Band) of silica fibers around 1550 nm.Dense wavelength division multiplexing(DWDM) uses the same transmissionwindow but with denser channel spacing.

Channel plans vary, but a typical systemwould use 40 channels at 100 GHz spacingor 80 channels with 50 GHz spacing. Sometechnologies are capable of 12.5 GHzspacing (sometimes called ultra denseWDM). Such spacings are today onlyachieved by Free space technology. Newamplification options (Ramanamplification) enable the extension of the


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usable wavelengths to the L-band, more orless doubling these numbers.

Coarse wavelength division multiplexing(CWDM) in contrast to conventional WDMand DWDM uses increased channelspacing to allow less sophisticated and thus

cheaper transceiver designs. To provide 8channels on a single fiber CWDM uses theentire frequency band between second andthird transmission window (1310/1550 nmrespectively) including both windows

(minimum dispersion window andminimum attenuation window) but also thecritical area where OH scattering mayoccur, recommending the use of OH-freesilica fibers in case the wavelengthsbetween second and third transmissionwindow should also be used. Avoiding thisregion, the channels 47, 49, 51, 53, 55, 57,59, 61 remain and these are the most

commonly used.


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WDM, DWDM and CWDM are based onthe same concept of using multiple

wavelengths of light on a single fiber, butdiffer in the spacing of the wavelengths,number of channels, and the ability toamplify the multiplexed signals in theoptical space. EDFA provide an efficientwideband amplification for the C-band,Raman amplification adds a mechanism foramplification in the L-band. For CWDMwideband optical amplification is not

available, limiting the optical spans toseveral tens of kilometres.

We consider broadcast WDM networkswith nodes equipped with rapidly tunabletransmitters and slowly tunable receivers.The rapidly tunable transmitters provideall-optical paths among the network nodesby creating logical connections that can bechanged on a packet-by-packet basis. The

ability of receivers to tune, albeit slowly, is


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invoked only for reallocating the bandwidthin response to changes in the overall

pattern. Since this variation in trac isexpected to take place over larger timescales, receiver retuning will be a relativelyinfrequent event, making slowly tunabledevices a cost effective solution. Assumingan existing assignment of receivewavelengths and some informationregarding the new trac demands, we presenttwo approaches to obtaining a new

wavelength assignment such that (a) thenew trac load is balanced across thechannels, and (b) the number of receiversthat need to be retuned is minimized. Oneof our contributions is an approximation

algorithm for the load balancing problemthat provides for tradeo selection, using asingle parameter , between theseconnecting goals. This algorithm leads to ascalable approach to recon guring thenetwork since, in addition to providing


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guarantees in terms of load balancing, forcertain values of parameter, the expected

number of retunings scales with the numberof channels, not the number of nodes in thenetwork.

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