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Dept of TelecommunicationAtria Institute of technology

INTRODUCTION TO OPTICALNETWORKS

Telecommunication network

First generation Optical Networks

Multiplexing techniques

Second generation Optical Networks

Network Evolution

Non-linear effects SPM, CPM, Four wave mixing

Solitons

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INTRODUCTION TO OPTICAL NETWORKS

Telecommunication Networks

Service providers are called carriers Carriers, used to be telephone companies but now, they are different

breeds operating under different business models Carriers provide bulk bandwidth, virtual carriers also exist Building fiber links requires right of way privileges Local exchange carriers offers local services in metro areas and inter-

exchange carriers long distance services, but this distinction is blur Carriers are classified as metro carriers and long-haul carriers Private networks LAN, MAN, WAN, rely on capacity provided by public

network, can have topologies such as ring, mesh The public network consists of Long haul inter-exchange network,

Metropolitan inter-office network and metropolitan access network Another network provides handoff between them if operated by different

carriers

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First generation Optical networks

Offers higher bandwidth than copper cables

Less susceptible to various kinds of electromagnetic interferences

High cost

Used essentially for transmission and simply to provide capacity, lower Bit

Error Rates and higher capacities Switching and other intelligent network functions were handled by

electronics

Example : SONET, SDH, ESCON ( Enterprise Serial Connection)

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Multiplexing Techniques

Multiplexing transmit data at higher rates over a single fiber than at lowerrates over multiple fibers

Two ways to increase transmission capacity TDM and WDM

TDM lower-speed data streams are multiplexed into higher-speed stream

by means of electronic TDM. The multiplexer interleaves lower speedstreams to obtain the higher speed stream.

In TDM, up 10Gbps stream are commercially available, higher rates(250Gbps) are achieved by OTDM, where mux-demux are done optically

WDM is same as FDM, data transmitted simultaneously at multiple carrierwavelengths over fiber, provides virtual fibers, used in long haul and under

sea networks and also in the metro networks WDM and TDM are complementary to each other, networks use a

combination of both

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Second generation Optical Networks In first generation networks, the electronics at a node must handle data for

that node and data through that node. The latter is routed through in theoptical domain in the second generation optical networks.

Network is called Wavelength Routing Network (WRN) provides lightpaths to its users.

At intermediate nodes , light paths are routed and switched from one link to

another, may be converted to another wavelength. Different light paths in a WRN can use the same wavelength as long as theydo not share any common links.

All light paths will use same wavelength on every link in their path, if thereare no wavelength conversion capabilities with in the network.

Key network elements are Optical Line Terminals (OLT), Optical Add/Drop

Multiplexers (OADM), and Optical Cross connects (OXC). OLT does mux-demux of wavelengths and are used at ends of point topoint WDM link, OADM adds/drops wavelengths selectively, OXC performssimilar function as OADM but at a much larger sizes.

Both OADM and OXC may incorporate wavelength conversion capabilities

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Second generation Optical Networks

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System and Network EvolutionEarly days - Multimode fiber: (mid 1960s) glass fiber was used as waveguide Focused for a reasonable distance without scattering, received with

sufficient strength and decoded, proved optical transmission feasible. OFC was cylindrical glass waveguide consisting of core and cladding.

1970s low-loss OFC using silica and regenerators over several 10s of KM

were used Multimode fibers with 50 85m of diameters and larger thanwavelength. LED and semiconductors lasers used as sources, photo detectors were

used, lasers were multi longitudinal mode Fabry-Perot lasers. Distancebetween Regenerators limited by inter modal dispersion energy in pulsetravels in different modes with different speed in a multi mode fiber, at theend of fiber different modes arrive at slightly different times resulting in

smearing of the pulse. Bit rates from 32 140 Mbps with regenerators every 10 KM. Still used for low-cost computer interconnection at a few 100 Mbps over a

few KM

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System and Network EvolutionSingle-Mode fiber: 1984, used single mode fiber eliminating inter modal dispersion along with

MLM Fabry-Perot lasers in 1.3m wavelength band.

Core diameter 8 10m, increase in bit rates (100s of Mbps) anddistance between regenerators (40 KM),

Chromatic dispersion was limiting factor for increasing bit rates, smearing of

pulse in single mode fiber of 1.55m wavelength window Dispersion shifted fiber, designed to have zero dispersion in 1.55 m

wavelength window, were used.

Technique of reduced width of spectrum of the transmitted pulse was used,greater was the smearing for wider the spectrum of transmitted pulse

Distributed feedback (DFB) laser ( eg. of Single Longitudinal Mode Laser)was used as MLM had many spectral lines.

Bit rate was more than 1Gbps

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System and Network EvolutionOptical Amplifiers and WDM: Early 90s, Erbium-doped fiber amplifiers (EDFA) were used, EDFA were optic fiber doped with rare earth element erbium. Erbium atoms

in the fiber are pumped from their ground state to an excited state at ahigher energy level using pump source, an incoming photon triggers theseatoms to come down to their ground state, in the process each atom emits aphoton resulting in optical amplification.

EDFA are capable of amplifying signals at many wavelengthssimultaneously, thereby increasing system capacity, rather than increasingthe bit rate, keeping the same bit rate and use more than one wavelength(WDM). EDFA brought down the cost of long-haul transmission system andincreased their capacity, achieving bit rate 10Tbps

Distance between regenerators were 600 KM, transmitted spectrum was

reduced by external modulation ( using external device to turn laser on/off.Chromatic dispersion compensation techniques were used. Limitations - Non-linear effects in fiber such as four wave mixing, non flat

gain spectrum of EDFA, various polarization related effects

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System and Network Evolution

Beyond transmission links to networks:

100Mbps Fiber Distributed Data Interface (FDDI) for data communicationsin MAN, 200Mbps Enterprise Serial Connection (ESCON) to interconnectmainframe computers

Standardization and mass deployment of SONET and SDH networks, highspeed optical interfaces on IP routers and ATM switches,

OADM, OXC, are available as commercial products.

Broadband services internet access and video on demand is acceleratingthe deployment of optical networks

Large businesses requiring very high capacities are served by SONET/SDH

Research activity on optical packet-switched networks and local area opticalnetwork, optical ring and mesh networks to provide light paths on demand,

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Non-linear effectsSelf Phase Modulation: SPM arises because the refractive index of the fiber has an intensity-

dependent component. Non-linear refractive index causes and induced phase shift proportional to

the intensity of the pulse, different parts of the pulse undergo differentphase shift giving rise to chirping of the pulses which in turn enhances pulse

broadening effects of chromatic dispersion. The chirping effect is proportional to the transmitted signal power and SPMis more pronounced in the systems using high transmitted powers. ( chirp frequency of the launched pulse change with time).

The trailing edges of the pulse undergoes a negative frequency shift and theleading edges a positive (compressed).

SPM causes positive chirping of pulse and causes enhanced, monotone,pulse broadening in the chromatic dispersion regime. SPM can actuallyreduce the pulse broadening effect of chromatic dispersion.

When the effects of chromatic dispersion and SPM are equal, the pulseremains stable, i.e. doesnt broaden further, after undergoing some initialbroadening.

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Non-linear effectsEffects of SPM:Consider a single channel system where electric field isE(z,t) = Ecos(0t- 0z) ; sign of phase shift due to SPM is negativeFor a monochromatic plane wave, nonlinear dielectric polarization is given byPNL(r,t) = 0 (3) E3 cos3 (0t - 0z)

= 0 (3) E3 { cos(0t- 0z) + cos(0t- 0z) } Nonlinear dielectric polarization has a new frequency at 30; The electric

field generated as a result of nonlinear dielectric polarization at 30 haspropagation constant 30; 0= (0); In ideal dispersion-less fiber, = n/c; n is constant In real fibers (30) will be different from 3(0) Because of lack of phase match ( mismatch between the two propagation

constants) electric field component 30is negligible.

Thus the nonlinear dielectric polarization can be written asPNL(r,t) = (0(3) E2 ) Ecos(0t- 0z)________________

0= 0/c 1 + (1) + (3) E2 ; n2= 1 + (1)

_________________ Hence 0= 0n/c 1 + 1/n2(3) E2

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Non-linear effectsEffects of SPM: Since (3) is very small for silica fibers we can approximate this by

0 = 0/c( n + 3/8n(3) E2) Thus the electric field E(z,t) = Ecos(0t- 0z) is a sinusoid whose phase

changes as E2z. This phenomenon is referred to as SPM. The intensity of the electric field corresponding to a plane wave with

amplitude Eis I = 0cnE2. Thus the phase change due to SPM is

proportional to the intensity of the electric field. This phase changeincreases as the propagation distance z increases. The above expression for0 can be interpreted as an intensity-dependent

refractive index n~(E) = n + n`I for fiber in the presence of nonlinearities. Here I = 0cn|E|2is the intensity of the field and is measured in units of

W/m2. the quantity n` = 2/ cn0 3/8n 0(3) is called the nonlinear indexcoefficient.

In general, all non linear effects in optical fiber are weak and depend onlong interaction lengths to build up to significant levels, so any mechanismthat reduces the interaction length decreases the effect of nonlinearity.

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Non-linear effectsCross Modulation: Consider WDM system with 2 channels E(r,t) = E1cos(1t- 1z) + E2cos(2t- 2z).

The nonlinear dielectric polarization is given byPNL(r,t) = 0(3) (E1 cos(1t 1z) + E2cos(2t 2z) )3

= 0(3) [ ( E1 3 + 3/2 E22E1)cos(1t 1z) +( E32+ 3/2E12E2)cos(2t 2z) + E12E2cos((21 - 2)t (21 2)z)

+ E22E1 cos((22- 1)t (22 1)z) + E12E2cos((21+2)t (21 +2)z)+ E22E1cos((22+ 1)t (22 1)z) + E13cos(31t 31z)+ E23cos(31t 31z)]

The terms at 21 + 2, 22+ 1, 31 and 32can be neglected since thephase-matching condition will not be satisfied owing to the presence ofchromatic dispersion

The component of the nonlinear dielectric polarization at the frequency at1 is 0(3) (E12+ 2E22) E1cos(1t- 1z).

Thus the electric field has a sinusoidal component at 1 whose phasechanges in proportion to (E12+ 2E22)z, the effect of 2nd term is CPM

IfE1= E2the effect of CPM is twice as bad as that of SPM. In dispersion shifted fiber the pulses in different channels do not walk away

from each other since they travel approximately with same group velocityand CPM can be significant in high speed WDM systems over DSF

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Non-linear effects

Four Wave Mixing: In a WDM system using the angular frequencies 1,, n, the

intensity dependence of refractive index not only reduces phaseshifts within a channel but also gives rise to signals at newfrequencies 2i- Jand i+ J k. This phenomenon is called

FWM. In contrast to SPM and CPM, which are significant mainly for high bit

rate systems, FWM effect is independent of bit rate but is criticallydependent on the channel spacing and fiber chromatic dispersion.

Decreasing the channel spacing or decreasing chromatic dispersion

increases the FWM effect. Thus the effects of FWM must be considered even for moderate bit

rate systems when the channels are closely spaced and/ordispersion shifted fibers are used

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Non-linear effectsFour Wave Mixing: Consider a WDM signal that is the sum of n monochromatic plane waves.

Thus the electric field of this signal can be written asn

E(r,t) = E1cos(it- iz)i=1

n n n

PNL(r,

t) =

0

(3)

Eicos(it- iz) EJcos(Jt- Jz) Ekcos(kt- kz)i=1 J=1 k=1

n

= 0(3) [Ei2+2 EiEJ] Eicos(it- iz) ----1i=1 J I

n

+ 0(3) Ei3 cos(3it- 3iz) ----2i=1

n

+ 0(3) Ei2EJcos((2i- J)t (2i- iJ)z) -----3i=1 J I

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Non-linear effectsn

+ 0(3) Ei2EJcos((2i+ J)t (2i+iJ)z) -------4i=1 J I

n n n

+ 6/4 0(3) EiEJEki=1 J>i k>J

[cos(i+J+k)t (i+ J+ k)z -------5+ cos((i+ J- k)t (i+ J- k)z --------6

+ cos((i J+ k)t (i- J+ k)z) --------7+ cos((i J- k)t (i i- J)z)] --------8

Thus the nonlinear susceptibility of the fiber generates new fields at thefrequencies i J k. This phenomenon is termed Four Wave Mixing,i.e. 3 waves with frequencies i,J, kcombine to generate a 4th wave atfrequency i J k.

For equal frequency spacing, and certain choices ofI, j, and k, the fourthwave contaminates i

Ex: for a frequency spacing , taking 1, 2, and kto be successivefrequencies, i.e. 2= 1+ and 3= 1+ 2 ;we have 1- 2+ 3=2and 22- 1= 3.

The first term in the above equation represents the effect of SPM and CPM.

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Non-linear effects

The 2nd 4th and 5th terms can be neglected because of the lack of phasematching. Under suitable conditions, the phase matching for the other termscan be satisfied.

Compact expression for FWM terms iJk= i+ J- kthe degeneracy factor diJk= 3, for i = j; and 6 for i j;

Then the nonlinear dielectric polarization term at iJkcan be written asPNL(r,t) = 0(3)diJkEiEJEkcos(i+ J - k)t (i+ J k)zIf the signals propagate as plane waves over an effective cross-sectional area

Ae within the fiber, it can be shown that the power of the signal generated atthe frequency iJkafter traversing a fiber length of L,

PiJk= [ iJk (3) diJk]2 PiPJPkL2

[8Aeneffc]2

where Pi

PJ

Pk

are the input powers at Ei

EJ

Ek

cos(i

, J

and k

in terms ofnonlinear refractive index n ; PiJk= [ iJk n diJk]2 PiPJPkL2

[3Aec]2

In practice the signals generated by FWM have lower powers due to lack ofperfect phase matching and attenuation due to fiber loss.

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Solitons

Solitons are narrow pulses with high peak powers and special shapes.(refer book for figure)

They take advantage of nonlinear effects in silica (SPM) to overcome thepulse broadening effects of group velocity dispersion and can travel for longdistances with no change in pulse shape

If the relative effects of SPM and Group velocity dispersion parameter

(GVD) are controlled just right, and the appropriate pulse shape chosen, thecompression effect undergone by the chirp ed pulses can exactly offset thepulse broadening effect of dispersion.

The pulse shapes for which this balance between pulse compression andbroadening occurs so that the pulse either undergoes periodic changes inshape ( Higher order Solitons) or no change in shape ( Fundamental

Solitons) only are called Solitons. Significance of solitons they overcome the detrimental effects of chromatic

dispersion completely.

Solitons and optical amplifiers, when used together, offer very high bit rate,repeaterless data transmission over very large distances

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Solitons

Solitons can also be used in conjunction with WDM, but significantimpairments arise when 2 pulses at different wavelengths overlap in timeand position in the fiber. Such collisions adds timing jitter to pulses.

Commercial deployment of Solitons based systems are not widespreadbecause they require new dispersion shifted fiber with a small value ofanomalous dispersion (0 < D < 1 ps/nm-km) and they require amplification

every 20 KM or so.. High bit rate transmission , with reasonable amplifier spacing, has been

achieved through a combination of (1) using pulses narrower than a bitperiod but much wider than solitons and (2) dispersion compensation of thefiber plant at periodic intervals to keep the average dispersion low.

The pulses used are Chirped Return to Zero pulses

In such a dispersion managed system a specific Chirped Gaussian pulseshape (dispersion managed solitions) will be transmitted through it with onlyperiodic changes in shape.

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