Functional Devices for all Optical Networks

Functional Devices, P.G., ENST 1

Functional Devices

for all Optical NetworksEurOpnet 2001 Tutorial, January 30

Philippe GallionEcole Nationale Supérieure des Télécommunications

URA CNRS 82046, rue Barrault, 75634 Paris


New Optical Functional Devices

• 1 - Interest in All Optical Signal Processing• 2 - Physics Phenomena• 3 - Wavelength Conversion• 4 - Nonlinear Mirror Demultiplexing• 5 - 3R Regeneration• 6 - Conclusion


1 - Interest in All Optical Signal Processing

• Bandwidth requirement• OTDM versus (D)WDM• From Optoelectronic to «ÊAll OpticalÊ» Signal Processing Ê


Bandwidth requirement

• Bandwidth: 1nm @1.55 µ = 125Ghz @ 193THz- Today radio-frequency range < 100GHz- Overall optical fiber bandwidth: few 10THz btwn 1.3 & 1.5µ- Erbium amplifier bandwidth: few THz btwn1.53 & 1.56µ- Today needs : 100 Gbit/s to 1 Tbit/s per fiber?- Electronic bandwidth : few 10GHz

• Up to now:- Optical transmissions- Commutation, Routing & Processing in electronic domain- O/E & E/O conversions : slow, expensive (packaging), noisy, scrambling of the

optical phase, lack of transparency...

• How to increase the bandwidth?


The first reported WDMexperiment:

Rê Horaky(the sun at noon)

transmits aWDM light beam

to the praying Tapéret(800-900 B.C.)

Le Louvre, Paris


OTDM versus (D)WDM

Mux R2

R1

Ri

OpticalLink

E/O

R2

R1

Ri E/O

E/O

λ1

λ2

λι

λ1

λ2

λ3

E/O

E/O

E/O

λ1 λ2 λι

Demux

• Wavelength Division Multiplexing (WDM)

• Optical Time Domain Multiplexing (OTDM)

Ri

Mux R2

R1Optical

Link

E/O

R2

R1

Ri E/O

E/O

λ

λ

λ

λ

λ

λ

E/O

E/O

E/Oλ Demuxλ

Single λOpticalSource


From Optoelectronic to «ÊAll OpticalÊ» Signal Processing

• Transmission– OTDM versus (D)WDM– Pulse generation & pulse shaping– Dispersion compensation...

• Routing– Wavelength Conversion– Fix and programmable Optical Add & Drops (OAD)– Restauration– Optical Cross Connect (OXC)…

• Optical signal processing– Optical Demultiplexing– Address label reading– Clock (phase) Recovery– Optical Regeneration : Re-amplification, Re-timing, Re-shaping (3R)...

• Supervision: “Êwave watcherÊ”• ...


2 - Physical Phenomena

• Semiconductors Inter-band Dynamics• Semiconductors Intra-band Dynamics• Non-linearities in optical fiber


Semiconductors Inter-band Dynamics• Gain and refractive index dependent on the carrier density

Gain saturation

• Typical time constant from 0,1 to 1 ns determined by the effective carrierlifetime

• Strong effect: power < 1mw, interaction length ≈ few 100µ• Phase-amplitude coupling :

α = few units

τeff−1 = τs

−1

Differential spontaneous emission rate

1 2 3 + ∂ GP( ) ∂N

Differential stimilated emission rate

1 2 4 4 3 4 4

τ s = Spontaneous carrier lifetime (↓ when I ↑) P = Photon density

α = ∆ ′ n ∆ ′ ′ n with n = ′ n − j ′ ′ n

G(N) = A(N − N0 )A = Differential gainG = Optical gainN0 = Carrier density for transparency


ChirpingÊ in a SOA

• When the optical Power increases :- The carrier density decreases- The refractive index increases (plasma effect)- The optical frequency decreases


Semiconductors Intra-band Dynamics

• A strong stimulated rate disturbs quasi-equilibrium

– ε gain compression factor– P photons density

• ps time constant determined by intra-band thermalizations• Gain compression (or compression)

– qlq % (heterostructure)– 10% (quantum wells )

G(N, P) = A(N − N0 )(1 − εP)

Pump

Gain suppression dip

Gain saturation level


Cross Gain Modulation (XGM)

Gain

Gain Max

Pinput

3dB

Psaturation

E1 = I1 exp jϕ1 E2 = I2 exp jϕ2 with I2 << I1

Two beams with intensity I1

and I2 in the same SOA

When I1 saturates the gain, I2experiments also a reduced gain :

XGM

E2 experiments also the index changeassociated to the gain change (XPM)

XGM


Nonlinear Index and Cross Phase modulation (XPM)

• Carrier heating– Large bandwidth (<10nm) , asymmetrical– High nonlinear index (α near 1)

• Spectral hole burning– Low bandwidth(<10nm) symmetrical– Low nonlinear index (α near 0)

Gain

Frequency

Index

FrequencyLasingFrequency

LinearNonlinear

Kramers-Kronig’s

Transformation(Hilbert’s

Transformation)E1 = I1 exp jϕ1

E2 = I2 exp jϕ2

XPM


Four Wave Mixing (FWM) in Semiconductor Optical Amplifier (SOA)

• Physical origins:– Interband for low frequencies : carrier density modulation,– Intraband for high frequencies : nonlinear gain i.e. carrier heating , spectral-hole-

burning, 2 photon absorption, Kerr effect• Application to Wavelength conversion, 3R regeneration, Spectrum inversion• Spurious effect in WDM systems


FWM New Frequencies Generation in WDM Systems

Input Output


Nonlinearities in optical fiber• Optical Kerr effect

– Polarization:

– Index:

• Self Phase Modulation (SPM) & Cross Phase Modulation (XPM)• Low effect but transverse confinement and longitudinal integration

• Very fast effects but:– Walk off problems– Other concurrent nonlinear effects

P(E) = ε0 χ(1)Elinear1 2 3 + 2χ (2)E2

=0 (symetry)1 2 4 3 4 + 4χ

(3)E3

Kerr effect1 2 4 3 4 + ....

n(ω, I) = n(ω)dispersion{ + n2 I

Kerr effect { with I = E2 2Z0

∆ϕ =2πλ0

n2LS

Ρ ∆ϕ = π for PL = 1W.kmL = length ≈ kmS = cross section ≈ 50µ2λ0 = wavelength =1,5µP = optical poower


Self Phase Modulation (SPM)& Cross Phase Modulation (XPM)

• Self Phase Modulation (SPM)

• Cross Phase Modulation (XPM)

E = I exp jϕ

E1 = I1 exp jϕ1E2 = I2 exp jϕ2

with I2 << I1

SPM

XPM


Nonlinear Propagation: Soliton• Linear Propagation:

• Nonlinear Propagation:

k(ω) =ω0

vϕ+ (ω − ω0 )

vG

+ (ω − ω0 )2

2!∂∂ω

1vG

Phase Enveloppe Enveloppe Propagation Propagation Distorsion

k(ω) =ω0

vϕ+ (ω − ω0 )

vG

+ (ω − ω0 )2

2!∂∂ω

1vG

+ ... + γI

Phase Enveloppe Enveloppe Nonlinearities Propagation Propagation Distorsion


3 - Wavelength Conversion• Wavelength converter• Cross Gain Modulation (XGM) in an SOA• Cross Phase Modulation (XPM) in an SOA• Gain Modulation in Semiconductor Laser (SCL)• Wavelength conversion in bistable Laser Structure• Four Wave Mixing (FWM) in a SOA


Wavelength Converter

• Wavelength routing• Wavelength re-use• WDM network reconfiguration• Optical circuit & packet (ATM…IP) switching

↓ tuning control

λin →

Modulated optical input

λout →

Modulatedoptical output

↑ gain control

Wavelengthconverter


ATM Matrix for wavelength routing

1

N

1

K

K

1

K 1

0xT

(K-1)xT

N''

1'

Wavelength encoder Spatial switch

MemoryStar coupler

CONTROL UNIT

Inpu

ts

Out

puts

Wavelength converter

Optical gateDelay line D

Coupler 1:K

FilterDetector

1''

N'

D

D

Europeen Project ACTS OPEN et KEOPSFunctional Devices, P.G., ENST 22

Expected Properties• All optical device:

– Transparent to modulation format– Bit rate independent: 155Mb/s à 10 Gb/s….40Gb/s– No clock recovery requirement

• Flexible implementation– Speed (10 Gbit/s and more)– Wide conversion range– Up conversion & down conversion allowed– Polarization independent– Input wavelength rejection

• No BER penalty (cascability)– High extinction (on /off) ratio– No jitter– No chirp– Amplification of the signal level– Pulse reshaping…

All of them together ?


Optoelectronic conversion v.s. all optical conversion

• Optoelectronic conversion– No bit rate transparency– Bit rate bottleneck– Noise– Cost (packaging)

• All optical conversion– Bit rate transparency– High bit rate– Possible pulse regeneration– Integration: low cost

λElectronicalprocessing

λ 1 2

OpticalProcessing

λ λ21


Cross Gain Modulation (XGM) in an SOA(TU Denmark)

The carrier depletioninduced by (λ1)modulated signalamplification reduce theavailable gain for theCW signal (λ2)amplification

CWλ2

λ1

filterλ 1

λ 2 λ2


Performances for XGM in an SOA

• Large conversion range:– Few10nm, i.e. few 1000GHz– Gain bandwidth limited

• Modulation bandwidth:– Few 10 Gbit/s– Carrier lifetime limited

• Low chirp

• But:– Inverted modulation (even cell number required)– External output carrier generation– High driving signal level: 0 to -10 dBm– Low on/off ratio: 5 to 10 dB


Cross Phase Modulation (XPM) in an SOA Mach Zendher Interferometer (MZI) Conversion

• Cross Phase Modulation (XPM)• Low control power ( only a phase shift π is required)• Accurate control power• Polarization independent• Possible up conversion and down conversion• Low chirp• 40 Gbit/s operation already demonstrated• Optical Cross Connect (OXC) 16x10 Gbit/s already demonstrated

SOA

SOA

CouplerCoupler

filterInput signal

CW signal

Converted signal


Gain Modulation in Semiconductor Laser (SCL) Oscillator

• The carrier depletion induced by (λ1) signal injection reduce the availablegain for the CW lasing (λ2) operation

• Gain and index changes result in simultaneous AM and FM (i.e. chirped)output

• Few10 Gbit/s, On/Off Ratio > 10dB, Pcontrol = few dBm

Bragg region

Ι1 Ι2 ΙB

filterλ 1

λ 2 λ 2

λ 1

AM and FM


Wavelength Conversion in Bistable Laser Structure

• Saturable absorption– Non injected (i.e. unpumped) region– Vanish out under light injection

• Operation up to 40GBit/s demonstrated• Possible re-timing operation by simultaneous optical or

electrical clock modulation

active region Bragg region

reff ( ω )

Ι 1 Ι 2 Ι B

active region

saturable absorber

λ2λ1


Output Wavelength Tuning

• Mode hopping• Progressive index saturation results in tuning efficiency (slope) reduction

0 20 40 60 80 1001551.0

1552.0

1553.0

1554.0

Injection current in the Bragg section (mA)

Lasing wavelength (nm)

Lasing wavelength vs. injection current in the Bragg section


Optical Bisability

The bistability improve the on/off ratio)Reshaping possibility

t

Pin

Pin

PoutPout


BER influence

10-10

10-8

10-6

10-4

10-2

-22 -20 -18 -16 -14 -12Received power (dBm)

Bistable laserReference

Bit-error-rate

Low degradation (or small improvement!) of the BER allows large scale cascability


Four Wave Mixing (FWM) in a SOA

1545 1555 1565

-40

-20

0

Wavelength (nm)

Powe

r (dB

m) Pump

SignalConjugatedSignal

Beating at the frequencydifference ωSIGNAL-ωPUMP

SOAsignal

pump

• Applications :– Wavelength conversion– All optical clock recovery– Spectrum inversion

• Spurious effect:– Diaphotie in WDM systems


Conversion efficiencyη =

POUTCONJUGATED

PINPROBE SIGNAL


SOA Four Wave Mixing Efficiency(BT Labs)

104

20

101 102 103-40

-30

-20

-10

0

10

Eff

icie

ncy

(dB

)

Detuning (GHz)

20

Eff

icie

ncy

(dB

)

101 102 103 104-40

-30

-20

-10

0

10

Detuning (GHz)

simulationexperiment

• Physical origins:– Interband for low frequencies : carrier density modulation,– Intraband for high frequencies : nonlinear gain i.e. carrier heating , spectral-

hole-burning, 2 photon absorption, Kerr effect• Takes benefit of built-in gain

Interband Interband+

intraband


FWM Performances(ENST)

• Transparency for the modulation format• High bit rate• Wide conversion range (65nm) by multiple conversion• Polarization dependent


4 - Optical Demultiplexing by Nonlinear Mirror

• Optical Linear Mirror Loop• Optical Nonlinear Mirror Loop• Nonlinearity realization• SLALOM• 4x10GBit/s Demultiplexing


Optical Linear Mirror LoopLinear Optical Loop Mirror (LOLM)

COUPLEUR

BOUCLE LINEAIRE ET RECIPROQUE

(1−α)1/2

α1/2j

E

E

E

0

R

T

• Repartition coefficients for intensity :

• Repartition coefficients for amplitude :

• Transmitted reflected fields for E0 =1:

• In linear regime output port is the input port (mirror)

A= : (1 − α) and A× : α

a= : (1 −α )1/ 2 and a× : jα1/ 2

ER = 2a=a× = 2 jα1 / 2 (1 −α )1/ 2 = j for α = 1/ 2ET = a=

2 + a×2 = (1− α ) − α = 0 for α = 1/ 2

Incident

Transmitted

Reflected LinearReciprocal

LoopCoupler


Nonlinear Optical Loop Mirror (NLOLM)

• Clockwise & unclockwise equal intensities : NL phase shift are identical• Tuning for a nonlinear phase difference de phase equal to π• Tradeoff between contrast (α ≈ 1/2) & sensitivity (α ≠ 1/2)• High level signal (1) transmission (NL regime)• Low level signal (0) reflection (NL effects are negligible)

COUPLEUR BOUCLE NON-LINEAIRE

E

E

E

0

R

T

(1−α)1/2expj Φ NL1

expjα1/2j Φ NL2

Non LinearLoop

Coupler

Incident

Transmitted

Reflected


Non linearity realizationFiber or semiconductor amplifier

• 1 - Cross phase Modulation (XPM) in a fiber (NOLM)- Very fast(>100GHz)- High optical driving signal level (10 to 30dBm)- Long length(1km)

• 2 - Cross phase Modulation (XPM) in SC amplifier (SLALOM)Semiconductor Laser Amplifier in a Loop Optical Mirror

- Fast (<20GHz, inter-band & <100GHz intra-band)- Low optical driving signal level ((-10 à 10dBm)- Compact (<1mm)

• Utilizations :- Intensity Filter- Correlator- Demuliplexer ....


SLALOMSemiconductor Laser Amplifier in a

Loop Optical Mirror

≈Filtre

coupleur 50/50

coupleur 100/0

contrôle (pompe)

données démultiplexées

t

SOA

données multiplexéesInput data

Output data

Control data (pump)


4x10GBit/s Demultiplexing(HHI)


5 - 3R Regeneration

Re-amplification, Re-timing, Re-shaping (3R)

• Re-amplification• Pulse reshaping by nonlinear filtering• Re- timing (Re-synchronization)• Clock (phase) recovery• 3R Regeneration• MZI all optical regeneration• All optical regeneration using FWM in SOA


Re-amplification

PIN

POUT Non linear transfert function

Signal

Signal

Noise

Noise

• Re-amplification is limited by accumulated ASE and limited amplifier outputpower• On/off ratio improvement by nonlinear response• Different processing for weak (0) & strong (1) signals• Act as digital electronics

PIN

POUT Non linear transfert function

Noise

Noise


AmplifiedSpontaneous

Emission (ASE)Suppression

(France Telecom)


Noise Suppression & Intensity Modulation(Tokyo Institute of Technology)

• Intensity noise of a spectrum-sliced incoherent source

• Gain saturated SOA with currentmodulation

• Beat noise reduction• Noise error floor observed with

LiNbO3 linear modulation isremoved


Pulse reshaping by nonlinear filtering

PIN

POUT

tt

Input

Outputs

reflected transmitted

Non linear transfert function


Re-timing (Re-synchronization)

Clock Recovery

OpticalGate

Input

Output

Input

GateOpening

Outputafter the Gate

Output after PowerControl

de la porteFunctional Devices, P.G., ENST 48

Clock (phase) recovery

- Large wavelength deviation (13 nm)- Low time jitter (11 ps)- High bit-rate (3.8 GHz).

Mode locked laser

Clock signal

1 1 1 0 1 0

Self-Pulsating laser

Clock signalR.Z. Data Sequence

1 1 1 0 1 0


Self Pulsating Laser

Laser DFB Autopulsant

I1 I2

t

puissance optique

TAP = 1/f AP

• Self pulsation origins:– Instabilities in longitudinal carrier or field distributions (Spatial-Hole Burning)– Dispersive self Q-switching associated to the negative slope of the Bragg grating

Self pulsating laser

Ouput power


Synchronization time(HHI)

• 10 Gbit/s• 1ns (10 “one” bits)

locking time• Synchronization resist

to > 100 “zero” bits

• Compatible with IPpacket switching

a - data signalb - clock signalc - recovered clock signal


Jitter & AmplifiedSpontaneous

Emission (ASE)Suppression

(France Telecom)


3R Regeneration

Amplification

Power Control

ClockRecovery

OpticalGate

Non-linear Filter

Input

Output

Reamplification

Reshaping

Retiming


MZI all optical regeneration - 1

SOA

SOA

Coupler Coupler

filterClock signalRegenerated signal

Input signal

Input signal

• XPM is the key phenomena• Up to 40Gbit/s operation demonstrated


MZI all optical regeneration - 2(Alcatel Corporate Research Center)

• 2 cascaded MZI to improve nonlinear response• 20 and 40 Gbit/s operation• 1.2 dB penalty• Polarization insensitivity and10 dB power dynamic range


All optical regeneration using FWM in SOA(ENST)

• PUMP = SIGNALPROBE =CLOCK

• The squaring improvedthe On/off ratio

• The pulse overlap is thecorrelation process betweenclock and signal

4

6

8

10

12

14

2 4 6 8 10 12

Input extinction ratio (dB)

Oup

ut e

xtin

ctio

n ra

tio

(dB

)

0 dB improvement

Theoretical limitPFWM ∝ PPUMP

2 .PPROBE

l Psignal/P clock = 20 dB∆ Psignal/P clock = 10 dB


Nonlinear Fiber Devices

Semiconductor Optical Devices(Intraband dynamics)

Semiconductor Optical Devices(Interband dynamics)

Fast electronical Devices

High IntegrationElectronical Devices

10 7 10 8 109 10 10 1011 10 12 (Hz)

Conclusion - 1

IntegrationPossibility


Conclusion - 2• Nonlinearity allows light with light interaction

–They are the key phenomena for all optical devices

• Nonlinearity is spurious in analog devices and systems• Digital electronics takes benefit if it at each step

–They are the key phenomena for optical regeneration

• The all optical processing do not exist!– It a user (system) point of view– Electrons play a key role in light- matter interaction–The speedy user have no time left to look at it

• Wide range of functional devices is today available• Are this grapes too unripe? (Jean de La Fontaine)

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Functional Devices for all Optical Networks