UCSD Photonics
Optical Amplification in High Capacity Networks
UCSD Photonics
As of 1998 …
UCSD Photonics
OUTLINEErbium Doped Fiber Amplification
•Technology•Modeling•Design •Behavior•Noise Properties•Other Impairments•Architectures
Raman and Hybrid Optical Amplification
•Principles•Model•Impairments•Architecture
Future Amplification Technologies
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Types Of Optical Amplification
Unidirectional
Fiber Loss ~ 0.2dB/km
Typical terrestrial span: 80km
Loss: 16-17dB + Passives
Commercial spans 20 – 32dB
Long reach spans up to 50dB(Off-shore applications)
Amplifier Node
Bidirectional
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EDFA Technology Revolution: Science in 1980s, Engineering in 1990s
1990
•First practical devices ~ 1990
•Killed off Coherent Architectures
•Europe paid the heaviest price
•Pioneering Contributions:
• Stanford University• Crawford Hill (Bell)• Murray Hill (Bell)• BNR• Southampton Univ. (UK)
UCSD Photonics
λ1λ2
λN-2λN-1λN
MU
X
1
P1
2
VOA
4 5GFF
PBP
M1
OSC/BS
P2
OSC/BS
M2
P5 P6P4
PREAMP
BOOSTER
D-COMPENSATION/λMANAGEMENT
DC/WADM
PBP 3
P3
λ1λ2
λN-2λN-1λN
MU
X
1
P1
2
VOA
4 5GFF
PBP
M1
OSC/BS
P2
OSC/BS
λ1λ2
λN-2λN-1λN
MU
X
1
P1
2
VOA
4 5GFF
PBP
M1
OSC/BS
P2
OSC/BS
M2
P5 P6P4
PREAMP
BOOSTER
D-COMPENSATION/λMANAGEMENT
DC/WADM
PBP 3
P3
Generalized Amplifier:
Much More Than A Repeater
Provides:
•Monitoring
•DC Management
•Transient Controls
•Add/Drop Management
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Optical Architectures
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Signal Propagation in EDFA: Spatial and Spectral Nonuniformity
Stage 1 Stage 2 Stage 3 Stage 4
Difficult analysis – much tougher synthesis
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Erbium Doped Fiber
Er doping in Silica results in creation of Er3+ Ion; 26s and 4f electrons removed
Er is poorly soluble in pure silica – modifiers such as Al are added to modify the structure
Addition of Al broadens the emission spectrum allowing for amplification in 1520-1620nm window.
Operational Bands
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Erbium Model
Simple Model: Two-Level Approximation More Complex Model: Stark Splitting, Inhmogeneous Broadening …
4I11/2~ 1µs
980nmSignal: 1520-1620nm
310ms
2
1480nm
980nm
4I13/2
1530nm1
4I15/2
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Almost all practical models are two-level approximations
Assumptions: •Decay level of first excited state is much longer than any other•Level 3 empties much faster than level 2•ESA, Quenching and upconversion neglected
~ 1µs
980nmSignal: 1520-1620nm
1
2
310ms
1480nm
Very accurate predictions for practical applications (Pump < 1W, absorption < 50dB/m)
N2
N3 0
Upper Level
N1 + N2 = NTOT
N1 Ground Level
UCSD Photonics
z=0 z=LP tiIN b g P ti
OUT b g
P tjOUT b g P tj
IN b g
P z ti ,b g
P z tj ,b g
( ) ( ) ( )2 2
10
, , ,1 Ni
ii
dN z t N z t dP z tu
dt S dzτ ρ == − − ∑
Normalized population inversion
Spontaneous decay from the upper level
Pump – Signal Contribution
Excited Ion Population:
dP z tdz un
n n n n nN z t P z t,
, ,d i c h c h c h= + −γ α α2
Signal Amplification Signal Absorption
Photon Propagation:
UCSD PhotonicsIntuitive Rules
Inversion varies with different pumping/signal architectures
Signal Signal
N z2c hPump Pump
N z2c h
UCSD PhotonicsGiven the same input signal, stronger pump will add to upper population,thus increasing the local inversion
Signal
Pump
Signal
N z2c h
N z2c h
PumpWeak Pump
Strong Pump
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EDFA Inversion: Higher with higher pump
Pump P
ower
10mW
50mW500mW
Very difficult to fully invert the amplifier – high pump powers, short Er coil required.
UCSD PhotonicsGiven the same pump, stronger signal will deplete upper population,thus decreasing the local inversion
Signal
Pump
Signal
N z2c h
N z2c hPumpWeak Input Signal
Strong Input Signal
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EDFA Inversion: Higher input signal lowers average inversion
Increasing Input Signal
-30dBm-20dBm+0dBm
Pump Power 100mW, Er Coil Length 10m
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Total Number of Er Ions is conserved: N Nz t z t2 1 1, ,c h c h+ =
Average ion inversion introduced by averaging along the fiber length
ddt SN z t dz P t P t
liOUT
iIN
i
N+ = −
FHGG
IKJJ
LNM
OQPz −
=∑1 1
0 120τ ρ,d i d i d iIntegrate the ion population:
ddt l S P tN t i
IN
i
Neg ln+ = −
FHGG
IKJJ −
=∑1 1
0 12 1τ ρc h e j
N t N z t dzl
l2 20
1d i d i= z , eg ln P tP tiOUT
iIN= d id i
Introduce Average Inversion: Introduce average gain:
Attempt to eliminate average gain …
UCSD Photonics
Average gain can be extracted by integrating photon propagation relation:
( ) ( ) ( ) ( ) 02 , ,, l
n n n n nn dzN z t P z t
dP z tudz γ α α⎡ ⎤
⎢ ⎥⎣ ⎦+ −= ∫
( ) ( ) ( )2n n n nt N tg γ α α+ −= Golden EDFA Rule
Which allows to write a Master EDFA Equation only in terms of average inversion:
τ ςγ α α
0 1 12
2 11
ddt l P t eN t i
IN
i
Ni i t iN l+ = −
FHG
IKJ
LNMM
OQPP
+ −LNM OQP −=∑c h e j e j b g
CW operation becomes particularly simple to describe:
N t l P t eiIN
i
Ni i t iN l
21 2 1
1c h e j e j b g= − + −LNM OQP −L
NMMOQPP=
∑ςγ α α
UCSD Photonics
The importance of Golden EDFA rule eclipses all others, at least in designing for a targeted gain response
If one knows a single number ( ), the Golden Rule allows calculation of EDFAgain AT ANY WAVELENGTH OF INTEREST:
N2
G ln nn n Nλ γ α αλ λ λFHIK
FH IKFHG
IKJ= ×+ −d i d i e j2
Implication: Measuring a gain at single wavelength (λn), one can CALCULATE the gain at any other wavelength (λm):
G mm m
n nn n mG l lλ
γ λ α λ
γ λ α λλ α λ α λ
FHGIKJ
FH IKFH IK
FH IK= +
+× + × − ×
e j e je j e j
e j e j e j
Caution: SHB or inaccurate Er parameters will lead to bad predictions!
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Gain Tilt Function
Gain ratio between two different wavelengths is independent of average inversion level
Assume that EDFA gain at certain wavelength (say, 1542nm) is known:
G lN1542 1542 1542 2 1542FH IK FH IKFHG
IKJ= ×+ −γ α αc h c h e j
And that we have lost (or gained) gain due to pump decrease (increase):
∆ ∆G N l1542 1542 1542 2FH IK FH IK= + ×γ αe j e j
The gain difference (tilt) at any other wavelength (λ) is simply calculated using:
∆ ∆ ∆G G T Gλγ λ α λ
γ αλF
HIK
FHG
IKJ
FH IKFH IK FH IK= = ×
+
+
e j e je j e j
e j1542 1542
1542 1542
Black Box predictions: measurements from any two wavelengths can be used to determine T(λ) –Full spectrum can then be characterized, WITHOUT EVEN KNOWING THE EDFA LENGTH.
UCSD Photonics
Practical EDFA design: What is the minimum length of Er coil to reach, say G = 25dB of gain between 1532 and 1560nm?
Is this a fair question?No, since we were not told what pump power is available or what is the maximum allowed variance of gain (Gmax – Gmin).
With varying inversion levels, the unit Er fiber gain (g) changes according to:
( ) ( )2n nn n Ng λ λγ λ α λ α⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠
+ −=
Many inversion curves can satisfy the (vague) design criterion given here:
-5
-4
-3
-2
-1
0
1
2
3
4
5
1510 1530 1550 1570 1590 1610
Wavelength (nm)
Gai
n (d
B/m
)
n = 1.0n = 0.7n = 0.5n =0.3n =0.0
Increasing Inversion
UCSD Photonics
Once we choose inversion level, we must determine the minimum unit gain (gmin)provided by such:n = 0.7
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
1510 1530 1550 1570 1590
Wavelength (nm)
Gai
n (d
B/m
)gmin
gmax
Say, we choose N2 with a minimum at λ =1542nm and unit length gain of 1.21dB/mRequired EDFA coil length is then: 25dB/1.21dB/m = 20.67m
EDFA designed in this manner will, however, have a peak gain of L*gmax = 20.67mx1.67dB/m = 34.52dB.
If we are to equalize this amplifier, we will have to provide a filter that selectively attenuates ∆G ~10dB.
What would happen if we choose a lower (or higher) inversion level?
Which one is likely to require more pumping power?
The argument gets complicated … How about noise performance?
UCSD Photonics
Inversion model provides realistic model, with 0.1-0.5dB accuracy in most cases.
Pump 100mWPump 10mW
Pump 500mWPump 50mW
10m Er-Coil Length, Input Signal -25dBm
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Inversion Model: Reservoir Analogy
Higher Gain … … Comes at the Pump Expense
What happens to ASE Powers?
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Pump Depletion – Fixed Pump (100mW)
Increasing the Input Signal … … Depletes the Pump
-20dBm
-30dBm
-10dBm
0dBm
What happens to ASE Powers?
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Change Pumping Architecture
Choose Length Partitionwith best performance and mostefficient/affordable pump
Partition total length (L1+L2)and calculate:
Required PumpActual GainNoise Performance
Predict Equalization Filter shape
Estimate lengthwith assumed inversion
Required Gain
Two Coil Design:A Complex Synthesis
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EDFA Noise
Besides amplifying the incident signal, EDFA generates amplified spontaneous emission (ASE)
Spontaneously emitted photons are uncorrelated to each other and to the incident signal
ASE is formed by a large number of emitters (Er ions) with random phase and position: Central Limit Theorem
Statistics of emitted noise is zero-mean Gaussian, well known and easily characterized
ASE is assumed to be polarization invariant (not always true) and assumed to have same powers in both polarization states
UCSD Photonics
ASE spectral representationn = 1.0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
1510 1530 1550 1570 1590 1610
Wavelength (nm)
Spec
tral
Den
sity
ρ(f)∆fMost commonly used observable is Spectral Power Density: ρASE(f)
ρASE(f) represents total noise power in frequency range (f, f+∆f)
Easily measured by any OSA, and is used to characterized noise properties of any EDFA
Reservoir Analogy:
EDFA is a two-level system filled by pump and emptied by amplified signal and ASE
High ASE generationHigh pumping and small input signal
High pumping and high input signal Lower ASE generation
UCSD Photonics
ASE-induced impairment: detection process degradation
Optical Signal Electrical Signal + NoiseD
|x|2 τ I T E t t E t t dtDET SIG ASET
TtFHIK
FHIK
FHIK= + + +
−zη 12
2' ' '
ASE
Time averaged spectrum of the detector current:
F I IDET DETt tFHIK
FHIK
FHG
IKJ+τ
Results in three distinct components:
P P f P f f dfSIG ASE SIG ASE SIG ASEf ff f
SIG
SIG+ + +FHG
IKJFHGIKJ
FHG
IKJ
FHGIKJ−
+z2 22δ ρ ρ ' '
∆
∆
Averaged Received Power Spontaneous-Spontaneous Beat Noise
Signal-Spontaneous Beat Noise
UCSD Photonics
Signal is almost always limited by Signal-ASE Beat Noise
Increase in ASE means loss of definition in amplitude-modulated signals
-70
-60
-50
-40
-30
-20
1578 1582 1586 1590 1594Wavelength (nm)
(dB
)
ASE Level
Signal Level
-70
-60
-50
-40
-30
-20
1578 1582 1586 1590 1594Wavelength (nm)
(dB
)
ASE Level
Signal Level
Logical “One”
Logical “Zero”
32dB26dB
UCSD Photonics
Noise Figure: Single Stage AmplifierRepresents the most important figure of merit of any amplifier
F SNRSNR
INOUT
= F GG= −2 1
SNRINP
P hfBIN
IN el=
2
2
Ideal, limited by shot noise only
SNROUTP
P BOUT
OUT optASE
=2
2 ρ
Signal-ASE Beat limited
F GhfASE= ρ
ρ ASE G hf= −2 1d i
G >>1
F ~2F – linear Noise Figure unitsNF - decibel Noise Figure units “The best possible performance”
( NF ~ 3dB)
UCSD Photonics
In practice, one can estimate NF by reading the spectral density from the instrument only:
OSNR Pf
SIG
ASE Bopt
=×ρ ∆
OSNR GPf
GPhfGF f
IN
ASE Bopt
IN
Bopt
=× ×
=ρ ∆ ∆
NF P OSNRINhc= − − FHGIKJ10 2
3log ∆λλ
Example: λ=1550nm, measured OSNR of 38dB, and noise figure scales with the input power as:
NF P dB dBIN= − +38 58
With input power of –16dBm (typical of terrestrial amplification), Amplifier has NF of 4dB
What happens if input signal power falls below -20dBm? Do we have negative NF?
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Noise Figure: Addition of Lossy Elements
TIN TLOSS
Er POUT
P T G T POUT IN OUT IN= × × ×PIN
ρ ρASEOUT
ASE OUT sp OUTT hfn TG= × = ×−2 1e jPump
ρASE
SNRIN
PhfB
IN
el= 2
F SNRSNR
n GG
IN
OUT
sp
TIN= = × −1 2 1e jSNROUT
PB
T G T Phfn G T B
OUT
ASE el
IN OUT IN
sp OUT el= = × × ×
− ×2 4 1ρ e j FEDFA
In dB: NF NFEDFAdB
TIN= −
Remember that TINdB<0 Front-loss adds to NF dB for dB!
What is the impact of the output loss?
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G1 G2 GN-1 GN
NF1 NF2 NFN-1 NFN
T1 T2 TN-1 TN
PIN POUT
Assume that all lossy elements are identical (T), matched to amplifier Gains (G = 1/T), as in typical transmission link:
P G T P POUT
N N
IN ING T= × × =⎯ →⎯⎯⎯⎯1/Total Output:
ρ ASE sphfn G( )1 2 1= −e jNoise Propagation:
ρ ASE sp sphfn G hfn GT G( ) ( ) ( )2 2 1 2 1= − −× × +
Total Noise at the Output:
ρ ASEOUT
sp
N
i Nsphfn G hfn G NT TGT G T( )
,( ) ( )( ) /= − −×
=∑ =⎯ →⎯⎯⎯⎯2 1 2 1
1
1
How much is OSNR degraded after Nth amplifier?
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EDFA impairment mechanisms: not only ASE
Negative impact on system performance
Equivalent to optical echo - Signal interferes withdelayed/reflected pulse.
Different propagation characteristics of primary andechoed pulses lead to unwanted collisions, interference
High levels of MPI suppression achieved via bandpassfilter suppresion.
Multiple Path Interference
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EDFA impairmet mechanisms: not only ASE
Four-Wave Mixing
Acute problem in L-Band amplifiers, since combined Er fiber length can approach 1km
Adjacent Channels Dropped Uniform Channel Load
500 ps/div 500 ps/div
1.5 dBInte
nsity
, a.u
.
Inte
nsity
, a.u
.
UCSD PhotonicsNot all Er Fibers are born equal: Er concentration, dispersion matters
FWM Penalty
10
12
14
16
18
20
22
-8 -4 0 4Channel Input (dBm)
SNR
Type_1
Type_2
UCSD Photonics
10
15
20
25
30
35
40
45
-8 -4 0 4
Power Stage Input Power (dBm)
OS
NR
(d
B)
OSNR = 10.60dB
OSNR = 42.50dB500 ps/div
500 ps/div
Back-Pumping
Front Pumping
Even the Pumping Scheme Matters
Total input/output power maintained for both topologies by adjusting the pumps:
Front Pumping: P980 = 160 mW, P1480 = 30mW
Back Pumping: P980 = 0 mW, P1480 = 100mW
OSNR = <I>/σ
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FWM in spectrally and spatially nonuniform distributed structure
PinPout
z0+Lz0 z z+∆z
G(ω ,z)
Ep(z0)
EF(z0+∆z)
Ep(z)
Eq(z)
Er(z)
EF(z+∆z)
Ep(z0+L)
Eq(z0+L)
Er(z0+L)
EF(z0+L)
Eq(z0)Er(z0)
- Solution along each section from:
- Total FWM solution obtained in a limit ∆z 0.
- Generating fields Epqr along each section assumed uniform∂
∂−
∂
∂−
∂
∂=
∂
∂
2
2 2
2
2 2
2
2
2
4z
E znc t
E znc t
E zc t
D E z E z E zF F F p q ra f a f a f a f a f a fα πχ
*
UCSD Photonics
FWM generation efficiency in L-EDFA:
η β= × zΩ ∆e dzi z G z G z G zG z
L p q r
F
e j e j e je j0
2
Ω=1024 0 0 02
2
2
4 2 2
πλχD P P P
n c ALp q r
effFGe j e j e j e j
∆ ∆ ∆∆ ∆β πλ λ λ λ
λ~ 2c ( ) + ( )2 f f D c f f dD
dpr qr
pr qr2
2 + ×LNMM
OQPPe j
FWM level suppressed by:
- Aeff increase- D(λ) increase- Shorter length- Minimal path integral governed by gain evolution function Gi(z)
UCSD Photonics
System Impact: Chaining FWM EDFA has a high cost
1577 .5 1580 .0 1582.5 1585 .0 1587 .5 1590.0 1592 .5W ave length (nm)
13 .5
14 .0
14 .5
15 .0
15 .5
16 .0
16 .5
17 .0
17 .5
OS
NR
( dB)
A mplifie r 1A mplifie r 2A mplifie r 3A mplifie r 4A mplifie r 5A mplifie r 6
C ha in P erformance
∆
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Toward Complete Design Rules
X-Talk/MPI
NL Distortion
Minimum Ripple
Maximum Power
Minimum NF
Pump Ceiling Optimum Performance
Lumped Design
Distributed Design
Interferences“Designer Circles of Hell”
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Interleaved
Banded
More Complex EDFAs: Bidirectionality
UCSD PhotonicsMixed Bidirectional Amplification
1 2
1
2
Divided Bands
Bidirectional Transport withinthe Band
Band Evolution Difficultafter (2) add-on.
1 2
1
1
2
2
1
1
2
2Divided Bands
Bidirectional Transport withinthe Band
Easy Multiple BandEvolution.
UCSD PhotonicsGeneralized Bidirectional EDFA: Complex Designs
To WADM/DCM
From WADM/DCM
To WADM/DCM
From WADM/DCM
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Bidirectional Penalties 1
PINWE
PXWE
GxPINWE
RxGxPINWE
GxRxGxPINWE
In-Band (Coherent) X-Talk
In-Band Round Trip:
P = G R G R PX INWEWE × × × ×
-14dB in-band X-TalkExample: Rayleigh Limit R ~ -32dB, G ~ 25dB
UCSD Photonics
Bidirectional Penalties 2 50GHz Spaced 10Gb/s Channel Plan
-40
-30
-20
-10
0
10
1580.5 1581 1581.5 1582 1582.5 1583 1583.5 1584 1584.5 1585 1585.5
WEST
50GHz
PINEW
PINWE
PXWE
GxPINEW
Out-of-Band (Incoherent) X-Talk
Out-of-Band Cross Talk
-35
-30
-25
-20
-15
-10
-5
0
5
1581 1581.5 1582 1582.5 1583 1583.5 1584 1584.5 1585
WEST
EAST
Out-of-Band Round Trip:
P =XWE R G PIN
EW× ×
Example: R ~ -32dB, G ~ 25dB -7dB out-of-band X-Talk
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Bidirectional Penalties 3
Round Trip Gain R G R G 1= × × × <
Self-Oscillation
Q-Switching
Distributed Rayleigh MirrorsCatastrophic (Physical) Failure
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Raman Amplification
Raman amplification largely neglected until late nineties
Relatively small efficiencies: ~ 0.01dB of Gain per mW of optical pump requires strong pump (~1W)
Advent of pumping technologies and new (higher confinement) transmission fiber justifies Raman use.
Main advantages:
Optical amplification ANYWHERE within the transmission window
Increased system performance due to distributed amplification
Equalized gain by pump wavelength selection
UCSD PhotonicsPrinciples I
The pump photon scatters off the material molecule, transferring the energy to higher vibrational state
The vibrational energy can be transferred to a new photon via either stimulated or spontaneous process
The probability for the energy transfer depends on the material host: structure, impurities
Crystalline material is characterized by narrow Raman spectra, while fiber (amorphous) has bands exceeding 20THz, making the amplification process practical
Virtual Upper State
Pow
er (a
.u.) ~100nm
Signal
Pump
Phonon
λ
UCSD PhotonicsPrinciples II
Raman amplification is:
•Polarization dependent: pump scrambling or multiplexing IS required (unlike EDFA)
•Ultrafast: pump variations (and transients) are transferred much faster than bit rate
•Direction invariant: pump propagation direction does not matter
Peak Raman gain efficiency depends strongly on confinement factor of the fiber and varies between 0.3 to over 3.0 (Wkm)-1 – it falls off rapidly for pump-signal separation larger than 120nm.
Fiber Effective Area (µm2) CR (Wkm)-1
SMF
NZDSF
DCF
80
55
15
0.32
0.64
15
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Superiority of Distributed Amplification
Lumped (Discrete) Amplification
Distributed (Raman) Amplification
UCSD PhotonicsS+(L)
Sign
al P
ower
(dB
)
Distance (m)
S+(0)
0 LS-(L)S-(0)
zdRaman pumping:
•Forward
•Backward
•Bidirectional
UCSD PhotonicsOn-Off Raman Gain: Making the Transmission Fiber Transparent
POUTPSIG
PPUMP
dP zdz
SIG P z C P z P zSIG PUMP SIGR S p( ) ( ) ( , ) ( ) ( )= − +α λ λ
dP zdz
PUMP P z C P z P zPUMP PUMP SIGSIG
PUMPR S p
( ) ( ) ( , ) ( ) ( )= − −α λ λλλ
GRamanR P eff
P LP
SIGSIG
e eC P L L= = −( )( )
( )
00 α L eeff
L= − −( )1 α
Gon off−
UCSD Photonics
Raman Model
Simple … … ComplexP+ P+
S+ S+
S- S-
P- P-
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Fully Bidirectional Raman Model
( ) ( ) ( ) ( ),
, , ,dS z
S z S z S zdzω
α ω γ ω γ ω−− + −= + + − + ( ) ( ) ( ) ( )
,, , ,
dS zS z S z S zdz
ωα ω γ ω γ ω+
+ − += − + − +
( ) ( ) ( ) ( ), , , ,S z S z S z dω
ω + − +Ω>
⎡ ⎤⎣ ⎦Ω Ω + Ω Ω Ω−∫ g( ) ( ) ( ) ( ), , , ,S z S z S z d
ωω + − −
Ω>
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω Ω−∫ g
( ) ( ) ( ) ( ), , , ,S z S z S z dω
ω + − −Ω<
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω Ω+∫ g ( ) ( ) ( ) ( ), , , ,S z S z S z dω
ω + − +Ω<
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω Ω+∫ g
( ) ( ) ( ), , ,h S z S z dω
ω ωπ + −Ω>
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω−∫ g
( ) ( ) ( ), , ,h S z S z dω
ω ωπ + −Ω<
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω∫ g
( ) ( ) ( ), , ,h S z S z dω
ω ωπ + −Ω>
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω−∫ g
( ) ( ) ( ), , ,h S z S z dω
ω ωπ + −Ω<
⎡ ⎤⎢ ⎥⎣ ⎦
Ω Ω + Ω Ω∫ g
.
UCSD Photonics
Forward pumped unidirectional transmission Backward pumped unidirectional transmission
-20 -15 -10 -5 0 5 10 15 20
1.000
0.875
0.750
0.625
0.500
0.375
0.250
0.125
Input Signal Power (dBm)
Pum
p Po
wer
(W)
-35 -25 -15 -5
On-Off Gain (dB)
-20 -15 -10 -5 0 5 10 15 20
1.000
0.875
0.750
0.625
0.500
0.375
0.250
0.125
Input Signal Power (dBm)Pu
mp
Pow
er (W
)
-35 -25 -15 -5
On-Off Gain (dB)
NZDSF is 200km long, has a loss of 0.2dB/km, an effective area of 50µm2 and a Rayleigh scattering coefficient of 0.7 m-14
UCSD Photonics
Bidirectionaly pumped unidirectional transmission Bidirectionally pumped bidirectional transmission
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Input Signal Power (dBm)
Pum
p Po
wer
(W)
-35 -25 -15 -5
On-Off Gain (dB)
-20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Input Signal Power (dBm)
Pum
p Po
wer
(W)
-35 -25 -15 -5
On-Off Gain (dB)
NZDSF is 200km long, has a loss of 0.2dB/km, an effective area of 50µm2 and a Rayleigh scattering coefficient of 0.7 m-14
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Spectrally Multiplexed Pumps Create Flat, Wideband Gain
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Banded, Raman Bidirectional Transmission
S+
P-P+
S-
λ
Pow
er
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Raman Penalty Mechanism: Signal-Pump-Signal Cross Talk
Und
eple
ted
Pum
p
Undepleted Pump -No Pump-Signal Modulation
Dep
lete
d Pu
mp
Pump is depleted by strong signal λ1 Modulated pump in turn modulates signal at λ2
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Higher Order Raman Amplification
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Hybrid (EDFA/Raman) Bidirectional Transmission:Extending the reach of EDFA-amplified systems
Data
Raman Pump
W EEDFA
Data Data
Raman Pump Raman Pump
W EEDFA
UCSD Photonics
Bidirectional Penalty Mechanisms: Rayleigh Backscatter Increase
Forward and Backscattered Gain
0
2
4
6
8
10
12
-12 -10 -8 -6 -4 -2 0 2 4Input Signal Power (dBm)
Gai
n (d
B)
Signal Gain
RB Gain
Pump Depletion (dB)
80km TWRS, ~240mW 1480nm CoPump40 Channels (1579-1595nm)
~2dB
~2.4dB-80
-70
-60
-50
-40
-30
-20
1588 1588.5 1589 1589.5 1590
Wavelength (nm)
(dB
)
With Raman PumpWithout Raman Pump
X
G
GRB
SignalPump
Scatter
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Bidirectional Penalty Mechanisms: Required Suppression
LF
GWE+ GWE−
PWEL
E
WE EW
W
P L PWEIN
WEL
IL WE F WE ILG L G L= × + −× × × ×
P L R G L PEWX
IL WE IL EWOUT= × × × × ×−κ
Signal:
Out-of-Band Cross Talk:
Example:
R dB G dBWE=− =−31 1 5, ~ , ,κ
⇒ =L dBF 36
Signal = X-talk
PWEIN
PEWX
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Raman/EDFA AdvantageWE EW
40km40km 40km 40km
VOA VOA
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
2 4 6 8 10 12 14 16 18 20
VOA (dB)
∆Q
(dB
)
EDFA Only, EW/WE Traffic
EDFA/Raman, EW/WE Traffic
EDFA/Raman, WE Traffic Only
~1.2dB
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Future: Parametric Amplification
• Long regarded as the most insidious WDM impairment (FWM)• Holds a promise for all-optical network construction• Practical platforms: Highly Nonlinear Fiber, Semiconductor, PPLN
1 Reduced Noise Signal Regeneration:(Long way out –requires polarization management)
2 Amplification anywhere within the transmission window
∆ ∆E E1 2 1= 3 Wavelength/Band conversion:
INOUT
E1
E2
G1=G2=G
2 1G −b g
IN OUT
E1
E2
G1=1/G2=G
G
1/G
4 Signal Conjugation: Transport Penalty Reversal
5 Fast (Packet) Switching/Routing
6 Signal Regeneration
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Conventional PA
ω1ω1- ω1+
ω
ω0ω2-ω1- ω1+ ω2+ω2ω1
ω
ω0P1 P2
P
Modulational Coupling PA
JQE 2002-60
-40
-20
0
1565 1575 1585 1595
λ0 (nm)
(dB)
-75
-55
-35
-15
5
1560 1570 1580 1590 1600
λ0(dB)
(nm)
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-60
-50
-40
-30
-20
1550 1560 1570 1580 1590 1600Wavelength (nm)
(dB
)
Pump Wavelength Tuning
- 380/178mW- 220/107mW- 189/85mW
RB = 0.5nm
-60
-50
-40
-30
-20
-10
0
10
1560 1570 1580 1590 1600 1610
Wavelength (nm )
(dB
)
Pump Power Tuning
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Current Record
Pump Powers: 200/600mW
Small Signal Gain: Pin ~ -25dBm
Idler generation within +/- 1dBof the amplified signal.
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Signal at 1577.8nm, Pump 1: 1569.02nm, 225mWPump 2: 1599.92, 220mWHNLF 2500m
A Nonlinear Device
-10-505
101520
-45 -35 -25 -15 -5
Input Signal (dBm)
(dB
m)
Output SignalOutput Idler
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Nonlinear Amplifier can Regenerate
Clean Signal PA Amplification (1)
Noisy Signal PA Amplification (1)
Noisy Signal PA Amplification (2)
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Conclusion
•EDFA and Raman technologies to support network backbone in foreseeable future
•EDFA is superior lumped amplifier, with operational band approaching 100nm
•New materials (ZBLAN) likely to extend the life of the technology indefinitely
•Raman amplification: inefficient, but distributed in nature
•Raman fiber pumping makes the span transparent: uni- and bidirectional
•The most flexible systems combine EDFA and Raman (Hybrid Amplification)
•Future Technologies: Parametric amplification
•Parametric amplifier is a nonlinear processing device, rather than mere amplifier
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