Chapter8

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Connection-Oriented Networks - Harry Perros 1

Chapter 8:Optical Fibers and Components

TOPICS– WDM optical networks– Light transmitted through an optical fiber– Types of optical fibers– Impairments– Components: Lasers, optical amplifiers, couplers,

OXCs


WDM optical networks

A point-to-point connection

In-lineamplification

opticalfiber

opticalfiberλW

λ1

…

Tx

Wavelengthmultiplexer

Poweramplifier

λW

λ1

…

Tx

Rx

Rx

Wavelengthdemultiplexer

Pre-amplifier

2


An example of an optical network

Mesh network

Ring 1

Ring 2

Ring 4

Ring 3


How light is transmitted through anoptical fiber

Waves and electrical fields

Source

Electricfield

Wave

3


Core

Cladding

Cladding

Radial distance

Refra

ctiv

e ind

ex

n1

n2

Core

Cladding

Radial distance

n2

n1

Cladding

Core

Refra

ctiv

e ind

ex

a) Step-index fiber b) graded-index fiber

Core and cladding

An optical fiber


Refraction and reflection of a light ray

Incident ray Reflected ray

Refracted ray

θι θr

θf

n2

n1

4


Angle of launching a ray into the fiber

Core

Cladding

Cladding

θι θrθl

Cladding

Cladding

Core

Opticaltransmitter

Cladding

Cladding

Core


Multi-mode and single-mode fibers

• Core/diameter of a multi-mode fiber:– 50/125 µm,– 62.5/125 µm,– 100/140 µm

• Core/diameter of single-mode fiber– 9 or 10 / 125 µm

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Electric fields

Cladding

CladdingCore

1

A

B

2


Electric field amplitudes forvarious fiber modes

Cladding

Cladding

Core

m=0 m=1 m=2

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Propagation of modes

Cladding

Cladding

Cladding

a) step-index fiber

b) Graded-index fiber

Cladding


Single-mode fiber

Cladding

Cladding

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Impairments

• The transmission of light through anoptical fiber is subjected to opticaleffects, known as impairments.

• There are:– linear impairments, and– non-linear impairments.


Linear impairments

• These impairments are called linear becausetheir effect is proportional to the length ofthe fiber.

• Attenuation:– Attenuation is the decrease of the optical power

along the length of the fiber.• Dispersion

– Dispersion is the distortion of the shape of apulse.

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Attenuation

800 1000 1200 1400 1600 1800

0.5

1.0

1.5

2.0

2.5

Wavelength, nm

Atte

nuat

ion,

dB


Dispersion

• Dispersion is due to a number ofreasons, such as– modal dispersion,– chromatic dispersion,– polarization mode dispersion.

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Modal dispersion

• In multi-mode fibers some modes travel a longerdistance to get to the end of the fiber than others

• In view of this, the modes have different delays,which causes a spreading of the output pulse

Power Power

TimeTime

Power

Time


Chromatic dispersion

• It is due to the fact that the refractive indexof silica is frequency dependent. In view ofthis, different frequencies travel at differentspeeds, and as a result they experiencedifferent delays.

• These delays cause spreading in theduration of the output pulse.

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• Chromatic dispersion can be corrected usinga dispersion compensating fiber. The lengthof this fiber is proportional to the dispersionof the transmission fiber. Approximately, aspool of 15 km of dispersion compensatingfiber is placed for every 80 km oftransmission fiber.

• Dispersion compensating fiber introducesattenuation of about 0.5 dB/km.


Polarization mode dispersion (PMD)• It is due to the fact that the core of the fiber is not

perfectly round.• In an ideal circularly symmetric fiber the light gets

polarized and it travels along two polarizationplanes which have the same speed.

• When the core of the fiber is not round, the lighttraveling along the two planes may travel atdifferent speeds.

• This difference in speed will cause the pulse tobreak.

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Non-linear impairments

• They are due to the dependency of therefractive index on the intensity of theapplied electrical field. The most importantnon-linear effects in this category are: self-phase modulation and four-wave mixing.

• Another category of non-linear impairmentsincludes the stimulated Raman scatteringand stimulated Brillouin scattering.


Types of fibers

• Multi-mode fibers: They are used in LANsand more recently in 1 Gigabit Ethernet and10 Gigabit Ethernet.

• Single-mode fiber is used for long-distancetelephony, CATV, and packet-switchednetworks.

• Plastic optical fibers (POF)

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Single-mode fibers:• Standard single-mode fiber (SSMF): Most

of the installed fiber falls in this category. Itwas designed to support early long-haultransmission systems, and it has zerodispersion at 1310 nm.

• Non-zero dispersion fiber (NZDF): Thisfiber has zero dispersion near 1450 nm.


• Negative dispersion fiber (NDF): This typeof fiber has a negative dispersion in theregion 1300 to 1600 nm.

• Low water peak fiber (LWPF): The peak inthe attenuation curve at 1385 nm is knownas the water peak. With this new type offiber this peak is eliminated, which allowsthe use of this region.

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Plastic optical fibers (POF)• Single-mode and multi-mode fibers have a high

cost and they require a skilled technician to installthem.

• POFs on the other hand, are very low-cost and theycan be easily installed by an untrained person.

• The core has a very large diameter, and it is about96% of the diameter of the cladding.

• Plastic optic fibers find use in digital homeappliance interfaces, home networks, and cars


Components

• Lasers• Photo-detectors and optical receivers• Optical amplifiers• The 2x2 coupler• Optical cross connects (OXC)

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Light amplification by stimulatedemission of radiation (Laser)

• A laser is a device that produces a very strong andconcentrated beam.

• It consists of an energy source which is applied toa lasing material, a substance that emits light in alldirections and it can be of gas, solid, orsemiconducting material.

• The light produced by the lasing material isenhanced using a device such as the Fabry-Perotresonator cavity.


Fabry-Perot resonator cavity.It consists of two partially reflecting parallel flatmirrors, known as facets, which create an opticalfeedback that causes the cavity to oscillate.Light hits the right facet and part of it leaves thecavity through the right facet and part of it isreflected.

Left facet Right facet

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• Since there are many resonant wavelengths,the resulting output consists of manywavelengths spread over a few nm, with agap between two adjacent wavelengths of100 to 200 GHz.

• A single wavelength can be selected byusing a filtering mechanism that selects thedesired wavelength and provides loss to theother wavelengths.


Tunable lasers

• Tunable lasers are important to opticalnetworks

• Also, it is more convenient to manufactureand stock tunable lasers, than makedifferent lasers for specific wavelengths.

• Several different types of tunable lasersexist, varying from slow tunability to fasttunability.

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Modulation

• Modulation is the addition of informationon a light stream

• This can be realized using the on-off keying(OOK) scheme, whereby the light stream isturned on or off depending whether we wantto modulate a 1 or a 0.


WDM and dense WDM (DWDM)

• WDM or dense WDM (DWDM) are terms usedinterchangeably.

• DWDM refers to the wavelength spacingproposed in the ITU-T G.692 standard in the 1550nm window (which has the smallest amount ofattenuation and it also lies in the band where theErbium-doped fiber amplifier operates.)

• The ITU-T grid is not always followed, since thereare many proprietary solutions.

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The ITU-T DWDM grid

Channelcode

λ (nm) Channelcode

λ (nm) Channelcode

λ (nm) Channelcode

λ (nm)

18 1563.05 30 1553.33 42 1543.73 54 1534.2519 1562.23 31 1552.53 43 1542.94 55 1533.4720 1561.42 32 1551.72 44 1542.14 56 1532.6821 1560.61 33 1590.12 45 1541.35 57 1531.9022 1559.80 34 1550.12 46 1540.56 58 1531.1223 1558.98 35 1549.32 47 1539.77 59 1530.3324 1558.17 36 1548.52 48 1538.98 60 1529.5525 1557.36 37 1547.72 49 1538.19 61 1528.7726 1556.56 38 1546.92 50 1537.40 62 1527.9927 1555.75 39 1546.12 51 1536.6128 1554.94 40 1545.32 52 1535.8229 1554.13 41 1544.53 53 1535.04


Photo-detectors and optical receivers

• The WDM optical signal is demultiplexedinto the W different wavelengths, and eachwavelength is directed to a receiver.

• Each receiver consists of a– photodetector,– an amplifier, and– signal-processing circuit.

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Optical amplifiers

• The optical signal looses its power as itpropagates through an optical fiber, andafter some distance it becomes too weak tobe detected.

• Optical amplification is used to restore thestrength of the signal


Amplifiers:power amplifiers,in-line amplifiers,pre-amplifiers

In-lineamplification

opticalfiber

opticalfiberλW

λ1

…

Tx

Wavelengthmultiplexer

Poweramplifier λW

λ1

…

Tx

Rx

Rx

Wavelengthdemultiplexer

Pre-amplifier

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1R, 2R, 3R

• Prior to optical amplifiers, the optical signalwas regenerated by first converting it intoan electrical signal, then apply– 1R (re-amplification), or– 2R (re-amplification and re-shaping) or– 3R (re-amplification, re-shaping, and re-timing)and then converting the regenerated signalback into the optical domain.


The Erbium-doped fiber amplifier(EDFA)

Laser850 nm

Signal to be amplified1550 nm

Isolator

Coupler

Erbium-dopedfiberIsolator

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Two-stage EDFA

Signal to beamplified1550 nm

Laser850 nm

Isolator

Coupler

Erbium-dopedfiber

Laser850 nm

Coupler

Isolator


The 2x2 coupler

The 2x2 coupler is a basic device in optical networks, andit can be constructed in variety of different ways. Acommon construction is the fused-fiber coupler.

Couplingregion

Taperedregion

Taperedregion

Input 1 Output 1

Output 2

Fiber 1

Fiber 2Input 2

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3-dB coupler

A 2x2 coupler is called a 3-dB coupler when theoptical power of an input light applied to, sayinput 1 of fiber 1, is evenly divided betweenoutput 1 and output 2.


• If we only launch a light to the one of thetwo inputs of a 3-dB coupler, say input 1,then the coupler acts as a splitter.

• If we launch a light to input 1 and a light toinput 2 of a 3-dB coupler, then the twolights will be coupled together and theresulting light will be evenly dividedbetween outputs 1 and 2.

• In the above case, if we ignore output 2, the3-dB coupler acts as a combiner.

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A banyan network of 3-dB couplers

λ1λ2

λ3

λ6

λ4

λ5

λ7λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8

λ1,λ2..,λ8


……

…

Optical cross connects (OXCs)

A logical diagram of an OXC

CPUInputfibers

. . .

Output fibers

Fiber 1

Fiber N Fiber N

…

λW

λW

λW

λ1

λW

λ1

. . .

Switch fabric

λ1

λ1

Fiber 1

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OXC functionality

• It switches optically all the incomingwavelengths of the input fibers to theoutgoing wavelengths of the output fibers.

• For instance, it can switch the optical signalon incoming wavelength λi of input fiber kto the outgoing wavelength λi of outputfiber m.


• Converters:If it is equipped with converters, it can switch theoptical signal of the incoming wavelength λi ofinput fiber k to another outgoing wavelength λj ofthe output fiber m.This happens when the wavelength λi of theoutput fiber m is in use.Converters typically have a limited range withinthey can convert a wavelength.

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• Optical add/drop multiplexer (OADM):

An OXC can also be used as an OADM.That is, it can terminate the optical signal of anumber of incoming wavelengths and insert newoptical signals on the same wavelengths in anoutput port.The remaining incoming wavelengths areswitched through as described above.


Transparent and Opaque SwitchesTransparent switch:The incoming wavelengths are switched to theoutput fibers optically, without having to convertthem to the electrical domain.Opaque switch:The input optical signals are converted toelectrical signals, from where the packets areextracted. Packets are switched using a packetswitch, and then they are transmitted out of theswitch in the optical domain.

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Switch technologies

Several different technologies exist:– micro electronic mechanical systems (MEMS)– semiconductor optical amplifiers (SOA)– micro-bubbles– holograms– Also, 2x2 directional coupler , such as the

electro-optic switch, the thermo-optic switch,and the Mach-Zehnder interferometer, can beused to construct large OXC switch fabrics


2D MEMS switching fabric

Down

Actuator Mirror

Up… …

… …

…

i

j

Inputports

Outputports

…

… ………

……

……

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A 2D MEMS OADM

λ1,λ2..,λW

…

Addwavelengths

Terminatewavelengths

λ1,λ2..,λW

Addwavelengths

iλ1,λ2..,λW λ1,λ2..,λW

… …

… …

…

… …

Drop wavelengths

…

……

… …

… …

Logical design 2D MEMS implementation


3D MEMS switching fabric

MirrorInside

ringx

axis

y axis

Output wavelengths

Input wavelengths

MEMSarray

MEMSarray

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Semiconductor optical amplifier (SOA)

• A SOA is a pn-junction that acts as anamplifier and also as an on-off switch

p-type n-type

Current

Optical signal


Α 2x2 SOA switch• Wavelength λ1 is split into two optical signals, and each

signal is directed to a different SOA. One SOA amplifiesthe optical signal and permits it to go through, and theother one stops it. As a result λ1 may leave from either theupper or the lower output port.

• Switching time is currently about 100 psec.

Polymerwaveguides

PolymerwaveguidesSOAs

λ1

λ2

Chapter8

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