Physical Impairments in Optical Systems and Networks(FIBER NON-LINEARITIES)

Prof. Manoj KumarDept. of Electronics and Communication Engineering

DAVIET Jalandhar

Outline

Problems posed by Chromatic Dispersion

Problems posed by Fiber Nonlinearities

Possible Solutions

Practical Issues

Electromagnetic spectrum

Transmission Impairments

900 1100 1300 1500 1700

0.5

1.0

1.5

2.0

2.5

OH Absorption

Att

en

uati

on

(d

B/k

m)

Wavelength (nm)

“Optical Windows” 2 3

1

Main cause of attenuation: Rayleigh scattering in the fiber core

45

AllWaveTM eliminatesthe 1385nm water peak

History of Optical Transmission

All-Optical Network(Terabits Petabits)

TDM DWDM

0

5

10

15

20

25

30

35

40

Ban

dw

idth

8@OC-484@OC-192

4@OC-48

2@OC-48

2@1.2Gb/s(1310 nm, 1550 nm)

10 Gb/s

2.4 Gb/s1.2 Gb/s565 Mb/s

1.8 Gb/s810 Mb/s405 Mb/s

EnablersEDFA + Raman AmplifierDense WDM/FilterHigh Speed Opto-electronicsAdvanced Fiber

1982

1984

1988

1994

1996

1998

2000

2002

1990

1986

1992

16@OC-192

40 Gb/s

32@OC-192

176@OC-192

2004

2006

TDM (Gb/s)

EDFA

EDFA +Raman Amplifier

80@ 40Gb/s

Bandwidth Evolutionary LandmarksBandwidth Evolutionary Landmarks

Multiplexing

Two ways to increase transmission capacity:

1. Increase the bit rate2. WDM: wavelength division multiplexing

1. High speed electronics, TDM & OTDM

2. 32 at 2.5 Gbit/s on 1 fiber (or less at 10Gbit/s)

Explosive GrowthExplosive Growthof Internet Trafficof Internet Traffic

Significantly Reduce Significantly Reduce the Cost per Bytethe Cost per Byte

Switch Traffic withSwitch Traffic withHigher GranularityHigher Granularity

Architecture ofArchitecture ofWDM Optical NetworksWDM Optical Networks

Wavelength Routed Wavelength Routed

Optical NetworksOptical Networks

Cost-EffectiveCost-Effective

ControlControl?

WDM DriversFaster Electronics Electronics more expensive

More Fibers

Slow Time to MarketExpensive EngineeringLimited Rights of WayDuct Exhaust

WavelengthDivisionMultiplexing

Fiber CompatibilityFiber Capacity ReleaseFast Time to MarketLower Cost of OwnershipUtilizes existing TDM Equipment

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

WavelengthConverter

Ch 1

Ch 2

Ch n

1

2

n

Mux &Demux

Mux &Demux

Mux &Demux

Mux &Demux

1

2

n

WDM System Function

Design Parameters of WDM Network

Number of Wavelengths Bit Rate per Wavelength Channel Spacing Useable Bandwidth Bandwidth Efficiency Span between Optical Amplifiers Transmission Span without Regeneration

Sources of WDM Network Degradation

Problem Posed by Chromatic Dispersion

Chromatic Dispersion Non-zero 2 at 1550nm (D=17ps/nm-km) Different frequencies travel at different

group velocities Results in pulse broadening causing ISI Sources of chromatic dispersion

Finite Laser line-width Laser Chirp due to direct modulation Finite Bandwidth of the bit sequence

Chromatic Dispersion (CD)

Effect and consequencesThe refractive index has a wavelength dependent factor, so the different frequency-components of the optical pulses are travelling at different speeds (the higher frequencies travel faster than the lower frequencies)The resulting effect is a broadening of the optical pulses and a consequent interference between these broadened pulses

CounteractionsCD compensation, Use of DS or NZDS fibres, combinations of these two techniques

SMF, DSF, NZDSF

SMF : Single Mode Fibercovered by ITU-T G.652 Recommendation

DSF : Dispersion Shifted Fibercovered by ITU-T G.653 Recommendation

NZDSF : Non-Zero Dispersion Shifted Fibercovered by ITU-T G.655 Recommendation

Chromatic Dispersion (CD)

The dispersion paradigm :Even if it is important to reduce Chromatic Dispersion in order to achieve longer transmission distances

... HOWEVER ...

too little dispersion means too high non-linear effects in the transmission fiber that can severely degrades Bit Error Ratio (BER)

Fiber Nonlinearities

As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium; that is the loss and refractive index are independent of the signal power

When optical power level gets fairly high, the fiber becomes a nonlinear medium; that is the loss and refractive index depend on the optical power

Single-channel

Multi-Channel/WDM

Self-phase modulation (SPM)signal optical phase modulated proportionally to signal power;conversion to intensity «noise» by GVD.

Cross-phase modulation (XPM)Signal optical phase modulated proportionally to power of neighboring channels; conversion to intensity «noise» by GVD.

Modulation instability (MI)(anomalous dispersion regime only)selective amplification of noise.

Stimulated Brillouin scattering (SBS)Retrodiffusion of energy;increases fibre loss.

Four-wave mixing (FWM)Generation of new spectral components;crosstalk when overlap with other channels.

Kerr effect

Other interactions with medium

Stimulated Raman scattering (SRS)Energy transfer from lower-wavelength channels to higher-wavelength ones.

n = n() + n2

P(t)

Aeff

Limitations :short list of fibre nonlinearities

Effects of Nonlinearites

Stimulated Raman Scattering (SRS)

1) Effect and consequencesSRS causes a signal wavelength to behave as a “pump” for longer wavelengths, either other signal channels or spontaneously scattered Raman-shifted light. The shorter wavelengths is attenuated by this process, which amplifies longer wavelengthsSRS takes place in the transmission fiber

2) SRS could be exploited as an advantageBy using suitable Raman Pumps it is possible to implement a Distributed Raman Amplifier into the transmission fiber. This helps the amplification of the signal (in co-operation with the localized EDFA). The pumps are depleted and the power is transferred to the signal

f fTransmission Fiber

Non Linear Effects:Cross Phase Modulation (XPM)

XPM acts as a crosstalk penalty, which increases with increasing channel power level and system length and with decreasing channel spacingXPM causes a spectral broadening of the optical pulses and thus reduces the dispersion tolerance of the systemAt 10 Gbps, its penalty is minimized by distributing dispersion compensation at each line amplifier site If dispersion is compensated only at the terminal ends, there will be a residual penalty due to XPM

FIBER EFFECTIVE LENGTH

•Nonlinear interaction depends on transmission length and cross-sectional area of the fiber

•The longer the length, the more the interaction and the worse the effect of the nonlinearity.

•BUT, signal propagates along link and experiences loss (from fiber attenuation) …...complicated to model.

Simple model: Assume power is constant over a certain effective length

P denotes power transmitted into fiber. L denotes actual fiber length

P(z) = P e-z power at distance z along link.

L

e

L

z

e

eL

dzzPPL

1

)(0

Typical: = 0.22 dB/km at 1.55umif L>>1/ ,then Le approx 20 km

EFFECTIVE CROSS SECTIONAL AREA

Effect of nonlinearity grows with intensity in the fiber. This is inversely proportional to the area of the core (for a given power).

Power not evenly distributed in the cross section.Use effective cross sectional area (for convenience).

A = actual cross sectional areaI(r, ) = actual cross sectional distribution of the intensity.

),(

),(

2

rIrdrd

rIrdrd

A

r

re

Most cases of interest:

fiber mode single of areaeA

•The phonons are acoustic phonons.•Pump and Stokes wave propagate in opposite directions.•Does not typically cause interaction between different wavelengths.•Creates distortion in a single channel.•Depletes the transmitted signal. •The opposite traveling Stokes wave means the transmitter needs an isolator

Meaning: If we launched 1.05mW = 0.2dBm, fiber loss alone would cause the receiverto receive 0.2dBm-(0.2dB/km)(20km) = -3.8dBm. However, if SBS is present, the Stokesand signal powers are equal in threshold condition; therefore the receiver gets -3.8dBm- 3dB = -6.8 dBm. The backwards Stokes wave has power of -6.8 dBm.

SBS

•If two or more signals at different wavelengths are injected into a fiber, SRS causespower to be transferred from the lower wavelength channels to the higher-wavelengthchannels.•Has a broadband effect (unlike SBS)•Gain coefficient gR much less than SBS gain coefficient gB.

•Both forward and reverse traveling Stokes wave.•Coupling between channels occurs only if both channels sending a “1”. SRS penaltyis therefore reduced by dispersion.

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)/10)(50(16

:

16

13

262

dBmWattmWm

mmmP

example

Lg

AP

th

effR

effth

SRS generally does not contribute to fiber systems.

SRS

Non Linear Effects:Four Wave Mixing (FWM)

1) Effect and consequencesFWM is the generation of new optical waves (at frequencies which are the mixing products of the originator signals). This is due to interaction of the transmitted optical waves. The created mixing products interfere with the signal channels causing consequent eye closing and BER degradation Decreasing channel spacing and chromatic dispersion will increase FWM

N channels N2(N-1)/2 side bands are created, causing Reduction of signals Interference Cross talk

2) CounteractionsAvoid use of ITU-T G.653 (DSF) fiber, Use of ITU-T G.652 (SMF) fiber and ITU-T G.655 (NZDSF) fiberUnequal channel spacing will cause the mixing products to be created at different frequencies which do not interfere with the signal channels

Non Linear Effects:Four Wave Mixing (FWM) cont…

Consider a simple three wavelength (1, 2 & 3) Let’s assume that the input wavelengths are l = 1551.72 nm, 2 = 1552.52 nm & 3 = 1553.32 nm. The interfering wavelengths that are of most concern in our hypothetical three wavelength system are:

1 + 2 - 3 = 1550.92 nm 1 - = 1552.52 nm 2 + 3 = 1554.12 nm 21 - 2 = 1550.92 nm 21 - 3 = 1550.12 nm 22 - 1 = 1553.32 nm 22 - 3 = 1551.72 nm 23 - 1 = 1554.92 nm 23 - 2 = 1554.12 nm

Critical Issues

Receiver Sensitivity (Minimum Power @ RX)Fiber Chromatic DispersionFiber PMDNon-linear EffectsMode partition Noise

Mode partition Noise

Mode Partition Noise is a problem in single mode fiber operationThe problem is that fiber dispersion varies with wavelength.With changes in the wavelength of the laser, the group velocity also changes.Thus instead of getting an even dispersion as we might if all wavelengths were produced simultaneously, we get random and unpredictable variations in the received signal strength – even during a single bit timeThis is a form of noise and degrades the quality of the received signal

Polarization-Mode Dispersion

Singlemode actually has two orthogonal componentsReal fiber is not completely symmetric Recall geometry data in sheets

Components propagate at different velocitiesThus, another form of dispersion (PMD)Small, but significant when other forms of dispersion are suppressed

Polarizations of fundamental mode

Two polarization states exist in the fundamental mode in a single mode fiber

Polarization Mode Dispersion (PMD)

Each polarization state has a different velocity PMD

PMD Pulse Spreading

DPMD does not depend on wavelength

Typical value: 0.5 pskmTherefore, 5 ps for a 100 km fiber

LDt PMDPMD

Bit Rate of Singlemode Fiber

Recall the bit rate formula

)4 (i.e., 4

1tT

tBR

For chromatic dispersion

LDBR

)(4

1

For polarization-mode dispersion

LDBR

PMD4

1

Dispersion compensation techniques

Postcompensation Precompensation Hybrid/Symmetrical Compensation Optical Equalization Filters Optical Phase Conjugation Fiber Bragg gratings Dispersion Compensation Fibers

Tools to combat Impairments

Power per Channel Dispersion Compensation Channel Spacing Wavelength or Frequency Choice

Increasing Total Throughput of WDM Systems

- Channel selection and stabilization multiplexing / demultiplexing- WDM nonlinearities (FWM, XPM, Raman)

Wavelength

Wavelength

Wavelength

- Higher-speed electronics required- Polarization mode dispersion (PMD) group-velocity dispersion (GVD) self-phase modulation (SPM)

Initial configuration

Upgrade strategies: B’tot, R’, ’

Per channel bit rate: RChannel spacing:

Bandwidth Btot

Wavelength

- increase in the per channel bit rate

- decrease in the channel spacing

- increase in the total WDM bandwidth

R’ > RB’tot = Btot and ’ =

’ < B’tot = Btot and R’ = R

B’tot > Btot with ’ = and R’ = R higher channel count

Limitations: - Technology - Physical effects within line fiber

- Broadband amplifiers- WDM nonlinearity (Raman)

Capacity Increase via Increase inPer-Channel Bit Rate: 40-Gbit/s Channel

Scalable, transparent, flexible and cost-optimized access to the backbone: 40-Gbits/s system as a tributary of the Alcatel WDM platforms NO management of STM-256 framing and synchronization

transparent 4:1 concentration of 10-Gbit/s plesiochronous sources embedded scalable 10Gbit/s OXC connectivity

flexible bandwidth optimization and network protection

Other SDHADM

ATMOtherSDHADM

Up to40

Up to40

40-Gbit/spoint-to-

pointtopology

10G TRIB

10-Gbit/sswitch

40-Gaggr.

40-Gaggr.

40-Gtransp.

40-Gtransp.

10-Gbit/s trib. WD

M

10-Gbit/sswitch

WD

M9.95-Gbit/stributary

9.95-Gbit/stributary

40-Gtransp.

40-Gtransp.

40-Gaggr.

40-Gaggr.

10-Gbit/s trib.

Fixedconnectivity

(in a first step)

ATMIP

IP

IP

IP

Standard Bit Rates

Future: Traffic Growth

Future: Computing Power

Thank You!