chapter 5 wdm 1 (10-12-12)1
TRANSCRIPT
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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)