Post on 20-Feb-2018
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Optical Amplifiers
Chapter 10
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Optical Amplifiers: amplifies in optical domain without O-E or E-O.E.g. SOA, EDFA, Raman Amplifier
Regenerators regenerate by converting O-E, re-time, re-shape
and E-O conversion.
As signal propagates through fiber channel, it gets attenuated due
to absorption, scattering, etc. and gets broadened due to dispersion.
Optical Amplifiers
Advantages of Optical Amplifiers:1. Insensitive to data rate
2. Large gain bandwidths
Disadvantages:
1. Do not regenerate,2. amplifies noise,3. no dispersion compensation
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Semiconductor Optical Amplifiers (SOA)
Semiconductor optical amplifiers are similar in constructionto semiconductor lasers.
The main difference is that SOAs aremade with layers of antireflection coatings
to prevent light from reflecting back intothe circuit.
semiconductor material such as indium phosphide.
Optical gain occurs as excited electrons in the semiconductormaterial are stimulated by incoming light signals; when current is
applied across the p-n junction the process causes the photons to
replicate, producing signal gain.
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SOA operation
The DC current applied to the device results in electrons being pumped intothe (normally empty) conduction band and removed from the (normally full)valence band. This creates the population inversion which is a pre-cursor tooptical gain. When signal photons travel through the device they cause
stimulated emission to occur when an electron and hole recombine.
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By adjusting the chemical composition of III-V semiconductors (typicallyGaInAsP) the band gap can be adjusted to give optical gain in the
telecommunications windows of interest.
The devices are typically 250mm long although devices of up to1mm have been made. In general the longer devices canachieve higher gain and wider bandwidths.
be as large as 100 nm and the fact that the energy source is a DC
electrical current makes these devices look promising as opticalamplifiers.
A major advantage of SOAs is that they can be integrated with othercomponents on a single planar substrate. For example, a WDMtransmitter device may be constructed including perhaps 10 lasersand a coupler all on the same substrate. In this case an SOA could beintegrated into the output to overcome some of the coupling losses.
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SOA: Semiconductor Optical Amplifier
The configuration of a SOA uses the familiar Double-Hetero (DH) Structure.Requirements are different than ILD.
1. Band-gap of active layer < surrounding layers2. RI of active layer > surrounding layers3. Efficient coupling of signal photons4. Optical feedback to be suppressed (AR coatings)
Optical Gain
The optical power P propagating through amplifiers is described as
PgPdzdP
eff=
where, g is gain coefficient and eff is effective loss coefficient.
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( )trg NNg =
If1. N is carrier concentration per unit volume,
2. Ntr is carrier conc. at transparency (when gain is unity),3. g is the gain cross-section (differential gain coefficient dg/dN)4. is the confinement factor,the gain coefficient is written as,
The carrier population rate change with injection current I and signal power P.
Ah
gPN
eV
I
dt
dN
c =
Ah
gP
eV
IN cc
=
Total number of carriersper unit volume Carrier loss per unit volumedue to non-radiative processesc- carrier life time
Carrier loss due to
stimulated emission
Under-steady state, dN/dt = 0,
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+
=
cg
trcg
Ah
P
Nev
I
g
1
Hence, The Gain Coefficient,
If P is so small that may be neglected,cg
sat
AhP
=&,
= trc
g NevIg 0
+
=
satP
Pgg
1
0
Then,
&,
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PP
P
P
g
dz
dPeff
sat
+
=
1
0
==
+
L
z
P
Pdzg
P
P
dPout
in 0
0
1
Substituting value of g,
Neglecting eff, & integrating,
satP
=
=+L
z
P
Psat
P
P
dzgdPPP
dP out
in
out
in 0
0
1
[ ] [ ] LgPPP
PP inoutsat
inout 0
1lnln =+
=
sat
inout
in
out
P
PPLg
P
P0ln
or,
or,
or,
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== sat
inout
P
PPLg
in
out eP
PG
0
inout PP >> Lg
eG 00=&,
sat
out
PPGG =
0lnln
Thus, The amplifier Gain, G is
If,
Then a good approximation is,
-
( )
sat
dBsat
P
PG
G = 30
0 ln2
ln
( )cg
satdBsatAhPP
== )2ln()2ln(3
,
or,
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Travelling Wave
Semiconductor Laser
Amplifier (SLA)
Angled-facet ortilted-stripe the
reflected beam atthe facet isphysicallyseparated from theforward beam
Mirror
Effect of optical reflections
Fabry-Perot
Semiconductor
Laser Amplifier
(SLA)
-
window facet theoptical beamspreads in thetransparentwindow
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( )( )
( ) )(sin4111
2
21
2
21
21
RRGRRG
GRR
P
PG
ss
s
in
out
+
==
Ln )(20
=
where, G is measured gain, Gs is single pass gain,& phase shift the wave goes on traversing length L.
Effect of optical reflections
==
2/1
21
211
0
)4(
1sin
2
)(2
RRG
RRG
nL
c
s
s
2
21
21
1
1
+=
RRG
RRGG
s
s
where, n is RI of Active region material, is incident signal frequency,0 is frequency of resonant mode.
3 dB spectral Bandwidth,
Peak trough ratio of passband ripple,
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Limitations of SOA:
The carrier lifetime is of order of 0.1 ns, hence gain recovery timeis short w.r.t to GHz data rate
Therefore, different levels of signal intensity will experience different gains,
leading to signal distortion.
This becomes significant when SOA is operating near saturation.This sets an upper limit on maximum amplifier output power.
Semiconductor Layers are sensitive to polarization.
Non-linearities leads to inter-channel cross talk.
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What is EDFA
An optical fiber heavily doped with Er3+ at core that serves asan optical amplifier.
EDFA Stands for Erbium Doped Fiber Amplifier Used to boost the intensity of optical signals being carried
through a fiber optic communications system
Works on the concept of stimulated emission Operates near 1550 nm
Other Rare-earth elements:1. Holmium (Ho),
2. Neodymium (Nd)3. Samarium (Sm)4. Ytterbium (Yb)
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Why Erbium?
Erbium has several important properties that make it anexcellent choice for an optical amplifier
Erbium ions (Er3+) have quantum levels that allows them to bestimulated to emit in the 1540nm band, which is the band that
has the least power loss in most silica-based fiber.
r um s quan um eve s a so a ow o e exc e y a s gna
at either 980nm or 1480nm, both of which silica-based fiber
can carry without great losses
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Basic block diagram of EDFA
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EDFA Configuration
Co-propogating (Forward) Pump EDFA
Counterpropogating (Backward) Pump EDFA
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EDFA Configuration
Forward pump= lower noise but lower output power
Backward pump = higher output power but higher noise
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Amplification between 1.53 and 1.56 um.
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Advantages Insensitivity to light polarization state High gain
Low noise figure: 4.5 dB to 6dB No distortion at high bit rates Simultaneous amplification of wavelength division multiplexed
Immunity to cross talk among wavelength multiplexedchannels Do not require high speed electronics
Independent of bit rate (Bit rate transparency)
Commercially available in C-band & L-band.
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Drawbacks Pump laser necessary Need to use a gain equalizer for multistage amplification
Difficult to integrate with other components Dropping channels can give rise to errors in surviving channels
EDFA: Mature technology1. New materials (Fluoride, Tellurite)
2. New dopant (Pr, Tm) ~PDFA, TDFA to exhibit
broader and flatter gain
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Band Name Wavelengths Description
O-band 1260 1360 nm Original band, PON upstream
E-band 1360 1460 nm Water peak band
S-band 1460 1530 nm PON downstream
C-band 1530 1565 nm
Lowest attenuation, original DWDMband, compatible with fiber amplifiers,CATV
L-band 1565 1625 nmLow attenuation, expanded DWDMband
U-band 1625 1675 nm Ultra-long wavelength
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1660 nm1640162016001580156015401520150014601440 1480
Fluoride EDFA 62 nm
EDFA 52 nm
EDFA ~47 nm
Tellurite EDFA 76 nm]
TDFA 37 nm
Rare earth-Doped Fiber Amplifiers
Erbium-Doped Fiber Amplifiers (EDFA) : C, L-BandThulium-Doped Fiber Amplifiers (TDFA) : S-BandPraseodymium-Doped Fiber Amplifiers (PDFA) : O-Band
1660 nm1640162016001580156015401520150014601440 1480
TDFA 35 nm
Raman + Fluoride EDFA 80 nm
Dist. Raman + Fluoride EDFA 83 nm
Raman + TDFA 53 nm
Raman 18 nm
Raman 40 nm
Raman 100 nm
Raman 132 nm
C-Band L-BandS-Band U-BandE-Band
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Traditional Optical
Communication SystemLoss compensation: Repeaters at every 20-50 km
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Optically Amplified SystemsEDFA = Erbium Doped Fibre Amplifier
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Typical Packaged EDFA
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Erbium
Pump laserWDM Fibre coupler
Interior of an Erbium Doped Fibre Amplfier (EDFA)
Fibreinput/output
doped fibre
loop
Source: Master 7_5
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Erbium doped aluminiumoxide spiral waveguide
1 mm square waveguide
Pum ed at 1480 nm
Miniature Optical Fibre Amp
Low pump power of 10 mW
Gain only 2.3 dB at present
20 dB gain possibleFOM Institute Amsterdam and
University of Holland at Delft
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Raman
Source: Master 7_5
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Raman Fibre Amplifiers (RFAs) rely on an intrinsic non-
linearity in silica fibre
Variable wavelength amplification:
Raman Amplifiers
For example pumping at 1500 nm produces gain at about 1560-1570nm
RFAs can be used as a standalone amplifier or as a
distributed amplifier in conjunction with an EDFA
Source: Master 7_5
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Raman Effect AmplifiersStimulatedStimulated RamanRaman ScatteringScattering (SRS)(SRS) causes a new signal (a Stokeswave) to be generated in the same direction as the pump wave down-
shifted in frequency by 13.2 THz (due to molecular vibrations) providedthat the pump signal is of sufficient strength.In addition SRS causes the amplification of a signal if it's lower in
.
difference in wavelengths is around 13.2 THz.The signal to be amplified must be lower in frequency (longer inwavelength) than the pump.It is easy to build a Raman amplifier, but there is a big problem:we just can't build very high power (around half a watt or more) pumplasers at any wavelength we desire! Laser wavelengths are veryspecific and high power lasers are quite hard to build.
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Distributed Raman Amplification (I)
Raman pumping takes place backwards over the fibre
Gain is a maximum close to the receiver and decreases in the
transmitter direction
Long Fibre Span
Source: Master 7_5
TransmitterOptical
ReceiverEDFA
RamanPumpLaser
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With only an EDFA at the transmit end the optical power level decreases
over the fibre length
With an EDFA and Raman the minimum optical power level occurs towardthe middle, not the end, of the fibre.
Distributed Raman Amplification (II)
Source: Master 7_5
Distance
OpticalPower
+
Raman
EDFA
only
Animation
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Raman amplification can provides very broadbandamplification
Multiple high-power "pump" lasers are used to producevery high gain over a range of wavelengths.
Broadband Amplification using Raman
Amplifiers
93 nm bandwidth has been demonstrated with just twopumps sources
400 nm bandwidth possible?
Source: Master 7_5
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Advantages
Variable wavelength amplification possible
Compatible with installed SM fibre
Can be used to "extend" EDFAs
Advantages and Disadvantages of Raman
Amplification
an resu t n a ower average power over a span, goo or ower crossta
Very broadband operation may be possible
Disadvantages
High pump power requirements, high pump power lasers have only recently
arrived
Sophisticated gain control needed
Noise is also an issue
Source: Master 7_5
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The Need of Optical Amplification
Erbium-Doped Fiber Amplifiers (EDFAs) application in long haul. Todays
amplifier of choice. Erbium-Doped Waveguide Amplifiers (EDWAs) application in metro andaccess networks
Raman Amplifiers application in DWDM
Why? Extend distance light signal can travelwithout regeneration
Semiconductor Optical Amplifiers (SOA) not fiber based type, application inmetro and access networks
Standard FiberAmplifier
Pump Lasers
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General Application of OpticalAmplification
In-line amplifierPreamplifier
LAN booster amplifier
Standard FiberAmplifier
Pump Lasers
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In-lineAmplifier
Fibre Link
Transmitter
Optical Amplifiers
Fibre Link
OpticalReceiver
Optical Amplifier Applications
Amplifier
Preamplifier
Transmitter
Transmitter
Optical Amplifier
Optical Amplifier
Fibre Link
Optical
Receiver
OpticalReceiver
Source: Master 7_5
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60 mW
30
35
40 Boosteramplifier
preamplifier
Amplifier OperationPoints
30 mW
10
15
20
-10 -5 0 5 10 15
Output power[dBm]
n- ne
amplifier