1 Confidential © 1999, Cisco Systems, Inc. Design of the physical layer in Metro DWDM networks...

1Confidential © 1999, Cisco Systems, Inc.

Design of the physical Design of the physical layer in Metro DWDM layer in Metro DWDM

networksnetworks

Alessandro BarbieriAlessandro [email protected]

bock-bock.cisco.com/~abarbier

mailto:[email protected]

2© 1999, Cisco Systems, Inc. Cisco Systems Confidential

AgendaAgenda

• WDM system overview

•Loss Management: The problem The solution The limitation

•Dispersion Management: The problem The solution The limitation

• The role of PMD and nonlinear effects in Metro Optical Networks design: The problem The solutions The limitations


Basic Elements of a WDM system


Wavelength Division Multiplexing Wavelength Division Multiplexing SystemsSystems

λ1

λ3λ3

1310nm

850nm

Mu

x/DeM

ux

OEO

OEO

OEO

PumpPump PumpPump

Transponder-BasedWDM System

Client Equipment

EDFA


Managing Optical Power Loss


800 900 1000 1100 1200 1300 1400 1500 1600

UV AbsorptionUV Absorption

OH- Absorption Peaks inActual Fiber Attenuation Curve

OH- Absorption Peaks inActual Fiber Attenuation Curve

RayleighScattering

RayleighScattering IR AbsorptionIR Absorption

Wavelength in Nanometers (nm)Wavelength in Nanometers (nm)

0.2 dB/Km

0.5 dB/Km

2.0 dB/Km

Loss (dB)/km vs. WavelengthLoss (dB)/km vs. WavelengthS-Band:1460–1530nmS-Band:1460–1530nm

L-Band:1565–1625nmL-Band:1565–1625nm

C-Band:1530–1565nmC-Band:1530–1565nm

Loss Management: Loss Management: ProblemProblemFiber AttenuationFiber Attenuation


METASTABLE STATE

Pump Photon980 or 1480 nm SIGNAL PHOTON

1550 nm SIGNAL PHOTON 1550 nm

Loss Management: Loss Management: SolutionSolutionErbium Doped Fiber AmplifierErbium Doped Fiber Amplifier

FUNDAMENTAL STATEFUNDAMENTAL STATE

EXCITEDSTATE

TRANSITION

AmplifiedSignal1550 nm

AmplifiedSignal1550 nm


Erbium Doped Fiber Amplifier Erbium Doped Fiber Amplifier

“Simple” device consisting of four parts:

• Erbium-doped fiber

• An optical pump (to invert the population).

• A coupler

• An isolator to cut off backpropagating noise

Isolator Coupler IsolatorCoupler

Erbium-DopedFiber (10–50m)

PumpLaserPumpLaser

PumpLaserPumpLaser


Loss Management: Loss Management: LimitationsLimitationsErbium Doped Fiber AmplifierErbium Doped Fiber Amplifier

• Each amplifier adds noise, thus the optical SNR decreases gradually along the chain; we can have only have a finite number of amplifiers and spans and eventually electrical regeneration will be necessary

• Gain flatness is another key parameter mainly for long amplifier chains

Each EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input

Noise Figure > 3 dBTypically between 4 and 6

Noise Figure > 3 dBTypically between 4 and 6


Loss Management: Loss Management: LimitationsLimitationsEDFA (Cont.)EDFA (Cont.)

In an ADD/DROP WDM Ring Topology Noise Can Build up (Positive Feedback) Until It Overcomes All the Signals If the Overall Attenuation Is Not Bigger Than the Gain Provided by the Amplifier Chain

San Francisco

Phoenix San Diego

= EDFA

= DWDMEquipment

Constraint: Ge-αL < 1Constraint: Ge-αL < 1


Loss Management: Loss Management: SolutionSolution PhotodetectorsPhotodetectors

• A photodetector is a device that measure optical power by converting the energy of the absorbed photons into electrical current

• Photodetectors for optical communication are basically semiconductor diodes (pn junctions)

• The link budget can be tuned by choosing the appropriate type of photoreceiver:P-I-N or Avalanche Photo Diode


Photodiode Basic PrinciplePhotodiode Basic Principle

E=hc/λ

Photon

ΔE<hc/λAbsorption

HoleHole

Electron

O-E ConverterThe Electron-Hole Pair Give Rise

to an Electrical Current

Valence BandValence Band

Conduction Band


Photodetectors TypesPhotodetectors Types

There are two type of photodiodes:

• P-I-N photodiodes: This type employs an intrinsic (not doped) layer of semiconductor between the p-doped and n-doped side in order to extend the usable area to receive photons

• Avalanche Photo Diode (APD): This is a strongly biased (reverse biasing) pn diode that creates many electron-hole pairs per each photon received; an APD amplifies the signal, therefore it has improved sensitivity (+8/10dBm over a PIN), but even higher noise and saturates with less input power than the PIN diode


PIN PhotodiodePIN Photodiode

pp i nn

InPInP InPInPInGaAs

Transparent

Absorptive

VR

OpticalInput


Managing Chromatic Dispersion


Dispersion Management: Dispersion Management: ProblemProblemChromatic Dispersion (CD)Chromatic Dispersion (CD)

• The optical pulse tend to spread as it propagates down the fiber generating Inter-Symbol-Interference (ISI) and therefore limiting either the bit rate or the maximum achievable distance at a specific bit rate

• Physics behind the effect

The refractive index has a wavelength dependent factor, so the different frequency-components of the optical pulses are traveling at different speeds

Bit 1 Bit 2 Bit 1 Bit 2Bit 1 Bit 2Bit 1 Bit 2 Bit 1 Bit 2


Dispersion Management: Dispersion Management: ProblemProblem Fiber Dispersion CharacteristicFiber Dispersion Characteristic

Dis

per

sio

n C

oef

fici

ent

ps/

nm

-km

17

0

1310 nm 1550nm

Normal Single Mode Fiber (SMF) >95% of Deployed Plant

Dispersion Shifted Fiber (DSF)


Dispersion Management: Dispersion Management: ProblemProblem

Increasing the Bit Rate Increasing the Bit Rate • Higher Bit Rates experience higher signal

degradation due to Chromatic Dispersion:

OA10Gb/s Dispersion

16 Times GreaterDispersion

16 Times Greater

Dispersion Scales as (Bit Rate)2

Time Slot

OA2.5Gb/s DispersionDispersion

1)


Dispersion Management: Dispersion Management: SolutionSolutionDirect vs. External ModulationDirect vs. External Modulation

• Laser diode’s bias current is modulated with signal input to produce modulated optical output

• Approach is straightforward and low cost, but is susceptible to chirp (spectral broadening) thus exposing the signal to higher dispersion

• The laser diode’s bias current is stable

• Approach yields low chirp and better dispersion performance, but it is a more expensive approach

Electrical Signal in

Direct Modulation External Modulation

Iin

Optical Signal out

Electrical Signal inDC Iin

Mod. Optical Signal

Unmodulated Optical Signal

External Modulator


Dispersion Management: Dispersion Management: SolutionSolution Dispersion CompensationDispersion Compensation

In the Normal(1) Dispersion Regime Shorter Wavelengths Travel Slower

(BLUE Is Slower Than RED)

(1) In the Normal Dispersion Regime the Dispersion Coefficient Is D > 0While in the Anomalous Regime It Is D < 0

Note: f = c/


Dispersion Management: Dispersion Management: SolutionSolution

Dispersion Compensation (Cont.)Dispersion Compensation (Cont.)

• Dispersion Compensating Fiber:

By joining fibers with CD of opposite signs and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter

Note: Although the Total Dispersion Is Close to Zero, This Technique Can Also Be Employed to Manage FWM and CPM Since at Every Point We Have Dispersion Which Translates in Decoupling the Different Channels Limiting the Mutual Interaction


Dispersion Management: Dispersion Management: LimitationLimitationChromatic DispersionChromatic Dispersion

• CD places a limit on the maximum distance a signal can be transmitted without electrical regeneration:

For directly modulated (high chirp laser)

LD = 1/ B D (1)

D dispersion coefficient (ps/km-nm): 17ps/nm*km @1.55μm

source line width or optical bandwidth (nm): 0.5nm

B bit rate (1/T where T is the bit period): 2.5Gb/s

LD ~ 47 km (*)

For externally modulated (very low chirp laser f ~ 1.2B ) LD ~ 1000 km @ 2.5Gb/s (*)

LD ~ 61 km @ 10Gb/s (*) @1.55μm and 17ps/nm*km

(*) Source: Optical Fiber Communication IIIA, Chap. 7


The role of Polarization Mode Dispersinand

Nonlinear effects in WDM systems


Polarization Mode Dispersion Polarization Mode Dispersion (PMD)(PMD)

• The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more

• Physics behind the effectIf the core of the fiber lacks a perfect circular symmetry, the two components (along the x and y axis) of the electric field of the light pulse travel with different speeds

nx

nyEx

Ey

Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber


Transmission Limitations Due to Transmission Limitations Due to Polarization Mode DispersionPolarization Mode Dispersion

• PMD accumulates with a squared root dependence on the fiber length:

= DPMD L

differential delay between the x and y component of the electric field

DPMD PMD coefficient

• The distance versus bit rate limit can be determined using:

B2L ~ 0.02/(DPMD)2 (*)

DPMD typical values between 0.5 and 2 ps/km (**)

If DPMD = 1.4 ps/km and B = 10Gb/s L is limited to 100km

If DPMD = 0.14 ps/km and B = 10Gb/s L is limited to 10000km

(*) Source: Optical Fiber Communication IIIA, Chap. 6(**) Source: Optical Networks, Chap. 5


Fiber NonlinearitiesFiber 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


Cross Phase ModulationCross Phase Modulation

• In WDM systems intensity fluctuation of one channel can affect the phase of other channels CPM induced chirp dispersion induced distortion

• Chromatic dispersion limit the effect of CPM because the interfering pulses of different channel tend to “walk away” from each other limiting the reciprocal interaction

• To limit CPM distortion, channel power should be below 10mW for 5 channels and below 1mW for 50 channels(*)

• Decreasing the number of channels reduces CPM effects

• Increasing the channel spacing reduces CPM effects

• Dispersion management can also be used by dispersion management techniques

(*) Application of Nonlinear Fiber Optics, Chap. 7


Four Wave MixingFour Wave Mixing

• FWM is the dominant source of crosstalk and loss in WDM systems; the beating among many channels generates new tones as sidebands; in the worst case of equally spaced channels most new frequencies coincide with existing channels and generates interference; in the best case the WDM channels experience just a power depletion

fijk - fi = fj - fk (i,j <> k)

1 2 3

f113f113 f112f112

f123f123

f213f213

f223f223 f132f132

f312f312

f221f221 f332f332

f321f321

f231f231

f331f331


FWM Performance ImpactFWM Performance Impact

• Like CPM in the presence of dispersion FWM is less efficient because of the “walk away” effect of different channels

• Using Dispersion Shifted Fiber greatly enhances the FWM process

• Reducing the channel count and the channel spacing also reduces FWM penalties

• Adopting an unequal channel spacing limits FWM cross-talk but the channel power depletion is still present


Wavelength

Dis

per

sio

n p

s/n

m-k

m 20

0

1310 nm

Normal Single Mode Fiber (SMF) >95% of Deployed Plant

Dispersion Shifted Fiber (DSF)

Managing CPM and FWM:Managing CPM and FWM:Non-Zero Dispersion Shifted FiberNon-Zero Dispersion Shifted Fiber

Nonzero Dispersion Shifted Fibers (NZDSF)

~+3ps/nmkm

~-3ps/nmkm

1550nm


NZDSF Flavors:NZDSF Flavors:Lucent TrueWave vs. Corning LEAFLucent TrueWave vs. Corning LEAF

• TrueWave fibers: Small amount of chromatic dispersion throughout the EDFA band (~1550nm); this dispersion prevents phase matching among the various signals reducing CPM and FWM

• Corning LEAF: Similar to TrueWave as far as dispersion goes; however it has a large effective area design that reduced the light intensity and therefore all the nonlinear effects


Appendix


Definitions: Definitions: refractive index & propagation refractive index & propagation

constantconstant

• Relationship between frequency and wavelength in electromagnetic radiations: c =x

•The refractive index (n) of a material is the ratio of the speed of light in the vacuum to the speed of light in that material: n=c/v. • The propagation speed of an electromagnetic radiation depends on the refractive index and the wavelength and it is determined by the propagation constant: β=2πn/λ


The physics behind Chromatic The physics behind Chromatic DispersionDispersion

• An optical pulse S is composed of a series of monochromatic waves:

S(cos(2πc λ1t - β1z) + cos(2πc λ2t - β2z) + … Where β1 != β2!= … the different waves composing the pulse propagates at different speed


Calculating Transponders CD Calculating Transponders CD distance limitationdistance limitation

• Transponder Dispersion Tolerance (TDT) is usually expressed in: [ps/nm]

• The dispersion coefficient D is expressed in [ps/nm*km]

•To calculate the distance: LMAX = TDT/D [ps/nm * nm*km/ps = km]

Eg. ONS 15540 has 1800ps/nm of TDT on regular SMF D = 18 ps/nm*km

LMAX = 1800/ 18 = 100km

1 Confidential © 1999, Cisco Systems, Inc. Design of the physical layer in Metro DWDM networks...

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Transcript of 1 Confidential © 1999, Cisco Systems, Inc. Design of the physical layer in Metro DWDM networks...