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

UCSD Photonics

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

UCSD Photonics

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

UCSD Photonics

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

UCSD Photonics

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!

UCSD Photonics

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

UCSD Photonics

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

UCSD Photonics

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?

UCSD Photonics

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?

UCSD Photonics

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?

UCSD Photonics

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

UCSD Photonics

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

UCSD Photonics

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

∆

UCSD Photonics

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”

UCSD Photonics

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

UCSD Photonics

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

UCSD Photonics

Bidirectional Penalties 3

Round Trip Gain R G R G 1= × × × <

Self-Oscillation

Q-Switching

Distributed Rayleigh MirrorsCatastrophic (Physical) Failure

UCSD Photonics

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

UCSD Photonics

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-

UCSD Photonics

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

UCSD Photonics

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

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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

UCSD Photonics

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