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Chapter 5

WDM SYSTEMS

Fiber-Optic Communications Systems, Third Edition.

Govind P. Agrawal

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Chapter Contents

+ WDM Lightwave Systems: High-Capacity Point-to-Point

Links, Wide-Area and Metro-Area Networks, Multiple-

Access WDM Networks

+ System Performance Issues: Four-Wave Mixing

+ Basic Concepts of optical amplifiers: Gain Spectrum and

Bandwidth, Gain Saturation, Amplifier Noise, Amplifier

Applications, Semiconductor Optical Amplifiers, Amplifier

Design

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+ Erbium-Doped Fiber Amplifiers: Pumping

Requirements, Gain Spectrum, Multichannel

Amplification

+ Raman Amplifiers: Raman Gain and Bandwidth,

Amplifier Characteristics, Amplifier Performance

+System Applications: Optical Preamplification, Noise

Accumulation in Long-Haul Systems

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The Arrival of Optical Revolution

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WDM N Channels

> WDM = economical solution to reach multiterabit/s capacity

STM-64

terminal16 x STM-4

STM-64terminal

16 x STM-4

16 channels

D

E

M

U

X

16 x STM-4 STM-64terminal

16 x STM-4STM-64

terminal

M

U

XN channels

Total Capacity =N x channel bit-rate

WDM channels

STM : SynchronousTransfer ModeMux : MultiplexerDemux

: Demultiplexer

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The Revolution

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WDM Optical components

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Optical Fiber

40 - 120 km

Up to 10,000 km

= 25 - 100 GHz

(0.4 or 0.8 nm @ 1500 nm)

Amp Amp

1

2

3

N

WDM

Mux

R

R

R

R

WDMDeMux

Frequency-registered

transmittersReceivers

WDM Optical System

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Need for amplifiers

Gain needed for :

compensation of link fiber loss

Increasing distance between electrical regenerators

Increasing signal power before receiver

Tx Rx Tx RxG G

Fiber lengthSignalP

ower

Receiver

sensitivity

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Base characterictics in FOCS

Optical Transmitter Comm. Channel Optical Receiver OutputInput

Modulation

Characteristics

Power

Wavelength

Loss Dispersion

4-Wave

Mixing

Noise

Crosstalks Distortion

Amplification

Bandwidth Responsivity

Sensitivity

Noise

Wavelength

Format

Bandwidth

Protocol

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Dense WDM Transmission Properties

Loss Attenuation

Dispersion Distortion

Nonlinearity New Frequencies

Gain Amplification &Noise

Cause Effect

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Optical Amplifier

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The Need for Optical Amplification

Repeaters can convert an optical signal into an electrical signal, amplify it

and reconvert the signal back to an optical signal.

This procedure has several disadvantages:

Costly

Require a large number over long distances

Noise is introduced after each conversion in analog signals (which

cannot be reconstructed)

Restriction on bandwidth, wavelengths and type of optical signals

being used, due to the electronics

By amplifying signal in the optical domain many of these disadvantages

would disappear!

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Optical Amplifiers - Types

There are mainly two types:

Semiconductor Laser Amplifier (SLA)

or Semiconductor Optical Amplifier (SOA)

Active-Fibre or Doped-Fibre

Erbium Doped Fiber Amplifier (EDFA)

Fiber Raman Amplifier (FRA)

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SLA - Principle Operation

Remember diode lasers?

Suppose that the diode laser has no mirrors:

- Setting the diode a population inversion condition

- Injecting photons at one end of the diode

By stimulated emission, the incident signal will be amplified!

By stimulated emission, one photon gives rise to another

photon: the total is two photons. Each of these two photons

can give rise to another photon: the total is then four

photons. And it goes on and on...

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SLA - Principle Operation

ASE Photons

1550 nm

Ground state

Excited state

Metastable state

Pump signal

@ 980 nm

Excited state

Metastable state

Pump signal

@ 980 nmStimulated

emission

1550 nmSignal photon

1550 nm

Ground state

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SLA - Disadvantages

Problems:

Poor noise performance: they add a lot of noise to thesignal!

Matching with the fibre is also a problem!

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Bandwidth of different Optical Amplifiers

[nm]

Pout

[dBm]

YDFA (doped Ytterbium), PDFA (doped Praseodymium),

TDFA (doped Thulium) v EDFA (doped Erbium).

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USABLE SPECTRUM OF SILICA FIBER

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Optical Amplifiers Bandwidth Bandwidth name

YDFA1060-1140

PDFA 1260 -1340 O

TDFA 1460 -1530 S

EDFA (1530 -1565) +(1565 -1625)

C + L

L: near future

Raman 1460-1675 S+C + L+U

Bandwidth of different Optical Amplifiers

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Energy Diagram of Ion Er3+

2/112H

2/3

4S

2/9

4F

2/9

4I

2/11

4I

2/13

4

I

2/15

4I

1530 nm

980 nm

800 nm

650 nm

550 nm

520 nm

GSA ESA

790 nm

850 nm

1140 nm

1689 nm

2700 nm

Excited State Absorption

= 0.001 ms

= 10 ms

Hnh 3.21 Gin nng lng ca Ion Er+.(GSA: Ground State Absorption, ESA: Excited State Absorption)

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Erbium Doped Fibre Amplifier (EDFA)

EDFA is an optical fibre doped with

erbium. Erbium is a rare-earth element which

has some interesting properties for fibre

optics communications.

Photons at 1480 or 980 nm activate

electrons into a metastable state Electrons falling back emit light at 1550

nm.

By one of the most extraordinary

coincidences, 1550 nm is a low-loss

wavelength region for silica opticalfibres.

This means that we could amplify a

signal by using stimulated emission.

1480

980

820

540

670

Ground state

Metastable

state

1550 nm

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Erbium Doped Fiber Amplifier

Simple device consisting of four parts:

Erbium-doped fiber

A coupler

An isolator to cut off backpropagating noise

An optical pump (to invert the population).

+ p = 1480 nmpump efficiency: 6dB/mW+ p = 980 nmpump efficiency: 10dB/mW more efficient = 980 nm used more popular

Isolator Coupler IsolatorCoupler

Erbium-Doped

Fiber (1050m)

Pump

Laser

Pump

Laser

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Bandwidth of EDFA

(nm)

PoutC: (1530 -1565)nmn

EDFA Gain vs Wavelength

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G i EDFA L h

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Gain vs EDFA Length

+ Higher pump power, larger gain G

+ With a given pump power (Eg: 4mW) Gain (030m)

quite linear but G (3050m) due to N2

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Gain of EDFA: Saturation

EDFA Gain is funtion of Pout corresponding to pump

power

Signal Power: Pout (dBm)

Gain(dB)

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Optical Amplification

Amplification gain: Up to a factor of 10,000 (+40 dB)

In WDM: Signals within the amplifiers gain (G) bandwidth are

amplified, but not to the same Gain

It generates its own noise source known as Amplified

Spontaneous Emission (ASE) noise.

Optical

Amplifier

(G)

Weak signal

Pin

Amplified signal

Pout

ASE ASE

Pump Source

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Optical Amplification - Noise Figure

Required figure of merit to compare amplifier noise performance

Defined when the input signal is coherent

)(rationoisetosignalOutput

)(rationoisetosignalInput(NF)FigureNoise

o

i

SNR

SNR

NF is a positive number, nearly always > 2 (I.e. 3 dB)

Good performance: when NF ~ 3 dB

NF is one of a number of factors that determine the overall BER of a

network.

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Noise in EDFA

1,48 1,50 1,52 1,54 1,56 1,58

-40

-30

-20

-10

0 SignalPump

Wavelength (m)

Output Power (dBm)

Power spectrum of pump; signal power and noise power

sp

sp

out

in nG

GnNSNSFNF 2)1(21

)/()/(

12

2

nn

nnsp

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Optical Amplification - Spectral Characteristics

Wavelength

Power

(u

namplifiedsign

al)

Wavelength

Power

(amplifiedsigna

l)

ASE

Wavelength

Power

(unamplified

signal)

Wavelength

Power

(amplifiedsignal)

ASE

Single channel

WDM channels

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Optical Amplification - Spectral Characteristics

How can to reducePASE?

PASE(f) = mthnsp[G(f) 1]B0

Wavelength

Power

(

amplifiedsignal)

ASE

Wavelength

Power

(amplifiedsigna

l)

ASE

B0 B0

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ASE Power at the output of EDFA

)()()( fPfGfPinout

0)1)(()( BfGhfnmfP SPtASE

mt

: number of polarization modes

nSP : Spontaneous Emission factor

G(f): gain of EDFA at frequencyf

Bo: optical filter bandwidth

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Example

=1550 nm; G(f)=1000; mt=2; nSP=1.5; Bo=10GHz;

Pin(f)=-20dBm

1) Calculate signal power Pout(f) in W and in dBm

2) Calculate PASE(f)

3) Calculate SNR(f)