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SMR 1829 - 14
Winter College on Fibre Optics, Fibre Lasers and Sensors
12 - 23 February 2007
Limits of Fiber-Optic
Communications Systems
Hugo L. Fragnito
Optics and Photonics Research Center UNICAMP-IFGW
Brazil
Hugo L. Fragnito
Optics and Photonics
Research Center
UNICAMP-IFGW
Tel. (xx55-19) 3521-5430
HUGO@IFI.UNICAMP.BR
Limits of FiberLimits of Fiber--OpticOptic
Communications SystemsCommunications Systems
Limits of FiberLimits of Fiber--OpticOptic
Communications SystemsCommunications Systems
Goals:
Present an overview of the basic physical mechanisms that limit fiber
optic system performance
Understand physical limits of fiber capacity
An introduction to nonlinear processes in Fiber Optic
Communications Systems
333 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Transmission Capacity of Optical FibersTransmission Capacity of Optical Fibers
100 THz Bandwidth (loss < 0.5 dB/km)
Capacity = BxL (Bit rate x Distance)
• Coaxial (copper) cable:
25 Mb/s x km
Limited by loss
(skin depth effect)
• Optical Fiber:
> 1 Pb/s x km (?)
Limited by dispersion (?)
Over 1 million km transmission demonstrated with optical fibers.
BxL product is not a good specification for fiber capacity
How far can we go with fibers? (for a given bit rate)
L (
km
)
B (Gb/s)
Lo
ss,
(dB
/km
)Wavelength, (µm)
0.8 1.0 1.2 1.4 1.6 1.80.1
0.2
0.4
1.0
2
4
Water peaks
XXI century fiber
XX century fiber
96 THz
444 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
TopicsTopics
• Physical Limits from Linear Effects
• Attenuation and Dispersion limits
• Nonlinear optical effects
• Limits imposed by optical nonlinearities
ICTP 2007 H. Fragnito - Unicamp 5
LINEAR EFFECTS
Attenuation – Sensitivity
Chromatic Dispersion
Polarization Mode Dispersion
Effects limiting capacity of optical fiber transmission
666 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Attenuation/Detection limitAttenuation/Detection limit
Minimum number of photons/bit at the receiver for a given error probability
(BER = 10-9)
p N( ) exp( )0 10 9
• Poisson statistics
• Quantum limit:
NQ = 21 photons
• Practical receivers:
N > 1000 photons
(Sensitivity)
Maximum fiber length to receive N photons/bit
for a given launched power, Ptx
0.1 1 10 100
100
70
80
90
300
200
N = 200
N = 21 (quantum limit)
L(k
m)
N = 2000
Ptx = 0 dBm
= 0.2 dB/km
= 1550 nm
B (Gb/s)
BER: Bit Error Rate
777 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
DispersionDispersion
DL
1
Measurement of Group Velocity Dispersion (GVD):
Received pulsesInput pulses(different
wavelength)
time
Single mode fiber, length L
++
time
Lo
ss
(d
B/k
m)
0.1
0.2
0.4
0.8
1.0
Wavelength (µm)
1.
3
1.
4
0
1.
5
0
Standard Fiber
Dispersion Shifted Fiber
(DSF)-10
0
20
10
S0= 0.075 (ps/nm
2)/km
1.
6
0
Dis
pers
ion
(p
s/n
m/k
m)
0 = 1550 nm
EDFA
SdD
d
0 = 1310 nm
Ga
in (
dB
)
888 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Dispersion and ChirpDispersion and Chirp
Instantaneous frequency tc
Dzt
40
2
02
)(
0 t
0
t
Dispersion produces a frequency chirp in the bit pulse
999 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Beyond the dispersion limitBeyond the dispersion limit
– Dispersion Compensation
tD > 0 tDc < 0
tDc < 0D > 0
L1L2 DcL2 + DL1 = 0
– Dispersion Compensating fibers introduce losses (use EDFAs)
101010 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Fiber BirefringenceFiber Birefringence
Imperfections in fabrication
Stress induced during cabling, installation
Polarization fluctuations (mechanical vibrations, temperature,…)
Beat Length LB = /(nx – ny)
Typically, nx – ny ~ 10-7
LB ~ 15 m @ 1550 nm
BEAT LENGTH
Slow axis
Fast axis
y
x
111111 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Slo
w a
xis
Input pulse
Output pulse
Polarization Mode Dispersion Polarization Mode Dispersion -- PMDPMD
Fast axis
DGD (differential group delay)
121212 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
PMD in Long FibersPMD in Long Fibers
• In long fibers the birefringence axes are not preserved in orientation but vary, in average, after one “coherence length” (Lc)
• DGD follows a random walk process
(analogous to Örnstein-Uhlembeck process in Brownian motion theory)
Lc
cLLDGD
131313 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Attenuation, DispersionAttenuation, Dispersion,, and PMD Limitsand PMD Limits
0.001 0.01 0.1 1 10 100 10001
10
100
1000
Quantum limit for 1 mW transmitter2000 photons/bit
Z-DSF
Fib
er
len
gth
(km
)
Bit rate (Gb/s)
SDF
PM
D=
1p
s/
km
0.1
ps/
km
Attenuation limits for 1mW transmitter, = 0.25 dB/km and BER < 10-9
Quantum limit = 20 photons/bitSDF = Standard dispersion fiber (D = 17ps/nm/km)Z-DSF = Zero-Dispersion Shifted Fiber (D = 0, S = 0.07 ps/nm2/km)
141414 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
PMD compensationPMD compensation
Polarization state varies randomly but slowly (> minutes)
Several techniques involving feedback on Dynamical Polarization Controller (DPC)
S. Yao, WDM, Nov. 2000
TX DPCt
PBS
Feedback
PMD
analyzer
TX DPCPBS
Feedback
PMD
analyzer
+
t
TX: TransmitterPBS: Polarization Beam Splitter
t: delay line
151515 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Linear effects limiting fiber capacity Linear effects limiting fiber capacity REVIEWREVIEW
• Attenuation – related to sensitivity of receiver
– Minimum number of photons per bit for BER < 10-9
– Typically 2000 photons (Quantum limit: 21 photons)
– Compensated by using optical amplifiers
• Dispersion – related to index profile and material dispersion
– Pulse (bit) dispersion given by spectral width of laser
– Standard dispersion fiber: D = 17 ps/nm/km @ 1550 nm
– Dispersion Shifted fiber : D = 0; Dispersion Limit given by Dispersion Slope
– Compensated using Dispersion Compensation Fibers (for WDM dispersion slope must also be compensated)
• Polarization Mode Dispersion (PMD) – related to fiber birefringence
– Orthogonal polarizations travel at different speeds
– Polarization fluctuates slowly with time (vibrations, temperature, stress)
– PMD can be compensated using polarization controller and feedback circuit
ICTP 2007 H. Fragnito - Unicamp 16
Nonlinear optical
effects in fibers
Stimulated Scattering SBS: Stimulated Brillouin Scattering
SRS: Stimulated Raman Scattering
Nonlinear refractive index (phase modulation)Phase Modulation
Solitons
Four Wave Mixing
171717 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
• Effective area
Effective Area and Effective LengthEffective Area and Effective Length
• Effective Length
•• Nonlinear effects depend on light intensity (I = Power/Area)Nonlinear effects depend on light intensity (I = Power/Area)
Le
eff
L1
I 2dA
IdAAeff
80 µm2 for standard fibers
55 µm2 for DSF
Aeff
L0distance
po
wer
L for L << 1
1/ for L >> 1Leff
(ex., 0.22 dB/km,
1/ 20 km)
•• Nonlinearities are important over a length of fiber where intenNonlinearities are important over a length of fiber where intensity is high sity is high
181818 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Stimulated Raman Scattering (SRS)Stimulated Raman Scattering (SRS)
• Co-propagating scattering
• Coupling to molecular
vibrations of silica
• High power threshold
example: Pth 1.8 W
(Aeff = 80 µm2, Leff = 20km)
laser - vibr
laser
Pump Laser
0
2
4
6
6 12 180
Raman Shift (THz)
Ram
an
Ga
in (
10
-12
cm
/W)
191919 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Raman in Raman in DDWDM systemsWDM systems
SRS manifests as tilting of WDM spectrum
higher loss for channels with smaller ’s
wavelength
Po
wer,
dB
m
202020 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Stimulated Brillouin Scattering (SBS)Stimulated Brillouin Scattering (SBS)
laser - B
laser• Precludes long distance bi-
directional transmission
Strongest nonlinearity
Counter-propagating scattering
But easy to eliminate
• Threshold depends on laser linewidth
Example (20 km, 80 µm2): Pth = 8.4 mW (narrow line lasers) Frequency, GHz
Sig
na
l
10.6 10.8
B
B
g 4x10-9 cm/W
B
laser
eff
eff
thv
v
gL
AP 184
212121 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Phase ModulationPhase Modulation
• Self phase modulation
• Modulation of phase of the wave changes instantaneous frequency (t)
• Broadens pulse spectrum(t )P(t )
time, t
n = n0 + n2P(t)/Aeff
• Accelerates pulse
spreading by dispersion
n = n0 + 2n2 Pk(t)/Aeff• Cross phase modulation
222222 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Phase modulation and dispersionPhase modulation and dispersion
Spectral broadening is larger in fibers with lower dispersion [bad for WDM]
1542 1543 1544 1545 1546
-20
-15
-10
-5
0
5
10
15
OUTPUT (50 km)
INPUT
Inp
ut
po
wer
[dB
m]
Wavelength [nm]
-45
-40
-35
-30
-25
-20
-15
Ou
tpu
t p
ow
er
[dB
m]
D = 0
D = 17ps/nm/km
-150 -100 -50 0 50 100 1500
5
10
15
Ou
tpu
t p
ow
er,
mW
Time, ps
D = 17 ps/nm/km
4 mW
20 mW
But pulse broadening is larger in fibers with higher dispersion [bad for high bit rate systems]
Trade off between phase modulation and dispersion
(general philosophy in modern NZ-DSFs)
232323 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Limitations from Phase ModulationLimitations from Phase Modulation
Self Phase Modulation (SPM)
• Important in single channel systems
Cross Phase Modulation (XPM)
• Important in WDM systems
• Phase of one channel is modulated by the power of all other channels
– broadens pulse spectrum
– accelerating pulse broadening by dispersion
– Limits pulse duration and power ( P/ t)
– Pulse Duration limits the bit rate (B)
– Power limits the distance (L)
• Ultimate limit of DWDM systems?P.P. Mitra and J.B. Stark, Nonlinear limits to the information capacity of optical
fibre communications, Nature, 411, 1027-1030 (June 2001)
242424 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Four Wave Mixing (FWM)Four Wave Mixing (FWM)
– Most important effect for WDM systems with low dispersion
– Due to nonlinear refractive index
Reduces the transmitted power in each channel
Produces cross talk
P
P1 P2
P
1 2
1 22 1
252525 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
FWM and dispersionFWM and dispersion
– FWM efficiency is larger in lower dispersion fibers
– Region around zero dispersion wavelength ( 0) extremely difficult for DWDM (“forbidden gap” )
– Non-Zero Dispersion Shifted (NZD) fibers alleviates the problem
-70
-60
-50
-40
-30
-20
-10
0
1530 1535 1540 1545 1550 1555 1560
1 (nm)
FW
M e
ffic
ien
cy,
(dB
)
0 = 1550 nm0 = 1570 nm
0 = 1310 nm
Forbidden gap
2
2
22
2
1
)2/(sin41
L
L
e
Le
DSF
NZD
SMF
262626 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Limitations by FWMLimitations by FWM
5% Power penalty
1
10
100
1000
0.001 0.01 0.1 1 10 100 1000
D (ps/nm/km) Ptx (dBm)
17 39
-1.4 26
0 6.3
D = 17 ps/nm/km
D = -1.4 ps/nm/km
D = 0
Dispersion limit
B (Gb/s)
L(k
m)
FWM limit
272727 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
SS -- ,C,C -- and Land L -- bands for DWDMbands for DWDM
Lo
ss (
dB
/km
)
0.1
0.2
0.4
0.8
1.0
Wavelength (µm)
-10
0
20
1.3 1.40 1.50
10
1.60
Dis
pers
ion
(p
s/n
m/k
m)
0 = 1320 nm
Dry Fiber
OH peak
1550 nm
S- C- L- band
NZ-DSF+
NZ-DSF low slope
DSFNZ-DSF-
Standard SM fiber
282828 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Raman amplificationRaman amplification
Ram
an
Gain
, d
B
Signal Wavelength, nm
1460 1500 1540 1580 16200
5
10
15
20
Raman Gain
40 nm centered in any
wavelength
>90 nm combining pump lasers
Distributed gain:
fiber now has gain instead of
loss
can use low power
transmitter (mitigates
nonlinearities)
Can be used for D = 0
Raman Pumps (14XX nm)
292929 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
SummarySummary
• Attenuation, Dispersion and PMD limits can be overcome with Optical Amplifiers, Dispersion Management and Compensation techniques
• Maximum capacity in practice: DWDM systems > 10 Tb/s x Mm (in the lab)
Limited by bandwidth of available amplifiers and nonlinearities
• Capacity of fiber transmission is limited by nonlinear optical effects– Moderate and High dispersion fibers: XPM
– Low dispersion fibers: FWM
• Nonlinearities can be reduced by – Large area fibers
– Dispersion management strategies (D large, average D = 0)
Compensate also for dispersion slope
– Operate the system in |D| > 2 ps/nm/km region (L band, S band,…)
– Distributed Raman Gain: loss-less fiber (no need to use high power)
303030 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Where to get more informationWhere to get more information
• G.P. Agrawal, Fiber-Optic Communication Systems, 3rd ed., Wiley, New York, 2002.
• WDM Solutions, PenWeell, free subscription: www.optoelectronics.world.com
• R. Ramaswami and K. N. Sivarajan, Optical Networks, 2nd ed., Morgan Kaufmann, San
Francisco, 2002.
• Optical Fiber Communications Conference Proceedings (OFC), Optical Society of America,
Washington, DC, (1973-2002).
• Optical Fiber Telecommunications, Vols. III-A, III-B, I.P. Kaminow and T. Koch, Eds. (1997);
Vols. IV-A and IV-B, I.P. Kaminow and T. Li, Eds., (2002), Academic Press, San Diego.
• E. Desurvire, D. Bayart, B. Desthieux, S. Bigo, Erbium-Doped Fiber Amplifiers: Device and
System Developments, Wiley, New York, 2002.
• E. Desurvire (Guest editor), C. R. Phyisique, Dossier: Optical Telecommunications, 4(1), 2003.
2/15/2007 Hugo Fragnito 1
Prof. Hugo L. Fragnito
Optics and Photonics Research Center – CePOF-Unicamp
Physics Institute, State University of Campinas – UNICAMP-IFGW
Campinas, SP, Brazil
HUGO@IFI.UNICAMP.BR
UNICAMP - IFGW
Fiber-Optic Parametric
Devices
222 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Optical Amplifiers for DWDMOptical Amplifiers for DWDM
EDFA (1510-1620 nm) Erbium
TDFA (1440-1510 nm) Thulium
SOA (60 nm bandwidth)
Semiconductor Optical Amplifier
Raman Amplifiers (40 nm bandwidth;
choice of spectral region)
1520 1540 1560 1580 1600 1620 16400
10
20
30
40
Gain
(dB
)
Wavelength (nm)
C-band L-band
EDFAs
90 nm
Limited by Nature to ~ 100 nmPhysics: Quantum energy levels
Chemistry: Special materials
Ram
an
Gain
, d
B
Signal Wavelength, nm
1460 1500 1540 1580 16200
5
10
15
20
Raman Gain
333 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
The ideal optical amplifierThe ideal optical amplifier
• Explore the full capacity of silica fibers
(1250-1650 nm) with a single device
54.5 THz bandwidth1164 channels @ 40 Gb/s = 46.6 Tb/sM x 46.6 = 140 Tb/s (M = 3) for M-ary format
• 400 nm bandwidth, flat gain spectrum, high output power
• Is it possible?
• Do we need it? Do we want it?
• If yes, probably the only option is the FOPA (Fiber-Optic Parametric Amplifier)
• But FOPDs can do much more than just amplify signals…L
os
s (
dB
/km
)
Wavelength (µm)
0.8 1.0 1.2 1.4 1.6 1.8
0.1
0.2
0.5
1.0
2
4
55 THz
444 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
WDM NetworkingWDM Networking
• WDM networks need new functionalities
– Add and Drop Channels Optically
– Wavelength Routing Assignment
– Wavelength reuse
• Wavelength Conversion
• Wavelength Exchangers
• FOPDs met the needs of WDM networking
OADMOptical Add & Drop Multiplexer
2/15/20072/15/20072/15/2007 Hugo FragnitoHugo FragnitoHugo Fragnito 555
FOPAs
• Spectral region not limited by energy levels of the medium – can be
optimized by engineering waveguide dispersion
• Large gain, flat gain spectrum, large bandwidth, low noise
• Based on ~ phase matched FWM
• Idler wave generated
• can be used as wavelength converter
• Typical pump power, fiber length, gain, bandwidth, noise figure:
• P = 0.1 – 1 W,
• L = 0.2 – 5 km,
• G = 20 – 40 dB (up to 70 dB demonstrated)
• 3dB = 20 – 60 nm (100 nm demonstrated)
• NF = 3.5 – 4.5 dB (0 dB in phase sensitive mode)
Fiber-Optic Parametric Amplifiers
p si
i = 2 p - s
2/15/20072/15/20072/15/2007 Hugo FragnitoHugo FragnitoHugo Fragnito 666
Single pump or
1P-FOPA
Double pump or
2P-FOPA
Two Types of FOPA
)(ˆ)(ˆ)(ˆ *2)3(
043
ipsNL EEP
• Broader gain spectrum• Flatter gain spectrum• Narrow-band idler
s p
p si
i s
1 si 2
)(ˆ)(ˆ)(ˆ)(ˆ *
21
)3(
023
isNL EEEP
• Simpler
2/15/20072/15/20072/15/2007 Hugo FragnitoHugo FragnitoHugo Fragnito 777
FOPA Basic Components
Single pump FOPA
Double pump FOPA
L
Pump laser
Pump-signal
coupler
Signal ( s)
p
Nonlinear, low
dispersion fiberInput Output
Filter
Pump out
L
Pump-signal
coupler
Signal ( s)Nonlinear, low
dispersion fiberInput Output
Pump lasersp1
p2 Filter
2/15/2007 Hugo Fragnito 8
Gain spectrum
Tunable Laser
1P-FOPA
EDFA
OSA
0
0
Tunable Lasers
1
2
MUX
OSA
2P-FOPA
2/15/2007 Hugo Fragnito 9
Gain and Phase Matching
W1.0P1 =
nm16001 =
W0.50P2 =
W0.50P1 =
nm1628.6252 =
nm15721 =
0
5
10
15
20
25
30
35
40
G, d
B
Parametric Gain
1P-FOPA 2P-FOPA
1550 1560 1570 1580 1590 1600 1610 1620 1630 1640 1650
Wavelength, nm
T(
km
-1)
-50
-40
-30
-20
-10
0.0
Wavevector mismatch
W-1/km2.4=
dB/km0.20=
km2.0L =
nm1600ZD =
ps/nm2/km0.065SZD =
Parametric processes depend critically on phase matching conditions
2 – 1 = 52 nm, P1 = P2 = 2.2 W
0 = 1568.2 nm, L = 0.95 km, = 4.0 dB/km, (Leff = 0.63 km)= 2.1 W-1/km, 3 = 0.11 ps3/km, 4 = -5.5x10-4 ps4/km
Tuning the central pump wavelength of a 2P-FOPA
102/15/2007 Hugo Fragnito 10
1568.2 nm
Parametric Gain
0.00
10.00
20.00
30.00
40.00
50.00
60.00
1520 1530 1540 1550 1560 1570 1580 1590 1600 1610 1620
Wavelength, nm
G(d
B)
2/15/2007 Hugo Fragnito 11
FOPD examples
1550 nm to 500 nm (visible) wavelength conversion
375 THz (1020 nm) translation!
20 m PCF with = 70 W-1/km,
01 = 750 nm, and 02 = 1260 nm R. Jiang et. al., OFC 2006
S. Radic et al., OFC 2003
Conversion with phase conjugation (dispersion compensation)
360 km transmission over standard (D = 16 ps/nm/km) fiber
5 DWDM channels at 10 Gb/s
2P-FOPA (HNLF, 1 km), orthogonal pumps, counter-phase
modulation
80 km 160 kmz = 0 km 240 km 320 km
FOPA*
2/15/2007 Hugo Fragnito 12
Wavelength conversion
Simulation vs.
measurements
0 2 4 6 8 10 12 14 16 180
10
20
30
40
50
60
70
80
90
100
En
erg
y C
on
vers
ion
, %
Input Power, mW
SimulationMeasurements
J.M. Chavez Boggio et al., PTL 15, 1528 (2003)
• FOPDs can convert wavelengths with gain
• Broadband, tunable
• Transparent to bit rate/protocol to > 1000 Gb/s
• L-C-S-… Band conversion
• Almost 100% energy conversion (pump to signal+idler)
2/15/2007 Hugo Fragnito 13
All-optical regenarion
High order idlers generated by cascaded FWM can
reduce noise (fluctuations in levels “0” and “1”)
S. Radic et al., PTL 2003
Wavelength ExchangerWavelength Exchanger
)()()()( 32*
1)3(
023
4 EEEPNL
Input Output
Po
we
rd
en
sit
y
Frequency, (THz)
192.8 193.0 193.1192.9
Simultaneous exchange of the
modulation signals carried by
two different channels
(without ADD or DROP devices)
)2sin()2cos()(
002010 PLeAiPLAA
i
sii
K. Uesaka et al., IEEE-JSTQE 8, 560 (2002)
)(ˆ)(ˆ)(ˆ)(ˆ21
*)3(
023
isNL EEEP
if 2 PL = /2 10
2/15/2007 Hugo Fragnito 15
Spurious FWM under control
Short fibers
Uniform fibers
Up to 16 mW per channel (x10 channels) can be
extracted from a 0.8 km DSF 2P-FOPA pumped at 4.4 W
(22 dBm total signal output power)
J.M.C. Boggio et. al., PTL 17, 1842-1844 (2005);Opt. Comm. 259, 94-103 (2006)
J.L. Blows Opt. Comm. 236,115 (2004).
(G = 40 dB)
FW
M P
ow
er
(dB
m) 4
-12
-8
4
0
12 13 14 15 16 17 18
Output signal Power (dBm)
1550 1560 1570 1580
1 10.....
Wavelength, nm
Po
we
r, d
Bm
-10
10
20
0
10 1.....Signals Idlers
Slope 3
1555 1560 15651550
-5
0
5
10
15
Po
wer,
dB
m
Wavelength, nm
2/15/2007 Hugo Fragnito 16
Stimulated Brillouin Scattering
Big problemUsually solved by broadening pump spectrum
But this induces gain fluctuations
idler broadening
Sensitiveness to 0(z) fluctuations
-4 -2 0 2 4
-80
-75
-70
-65
-60
-55
-50
-45
Po
wer
sp
ectr
um
(d
Bm
)
Relative frequency (GHz)-4 -2 0 2 4
(nm)
0.16 nm
=1566.9 nm0
s= 1560 nm
0.25 nmD (
ps/
nm
/km
)
J.M. Chavez Boggio et al., Opt. Comm. 249, 451 (2005)
( 0 = 0.1 nm)
2/15/2007 Hugo Fragnito 17
Mitigating SBS
Varying elastic properties along fiber length
Stain (or temperature) modifies the sound velocity, producing a shift in Brillouin frequency
A strain (or temperature) distribution along the length of the fiber (inhomogeneously) broadens the Brillouin spectrum, increasing the SBS threshold.
J. Hansryd et al., JLT 19, 1691 (2001)
4.8 dB
5 10 15 20 25 30
-60
-50
-40
-30
-20
-10
0
10
20Without strain distributionWith strain distribution
Back
sca
tte
red
po
wer
(dB
m)
Input power (dBm)
10.7 dB
10 dB
J.D. Marconi e al., ECOC’2005
2/15/2007 Hugo Fragnito 18
SBS: B(z) and 0(z)
Brillouin frequency B = 2 optV/c
Sound velocity V =(stress/density)½
Temperature d B/dT = 1 MHz/K
Strain d B/dS = 460 MHz/%
Zero-dispersion wavelength also varies:Temperature d 0/dT = 0.03 nm/K
Strain d 0/dS = 2 nm/%
Can be used together to suppress SBS and control 0(z)
No
rma
lize
d p
ow
er
(dB
)
Frequency shift (GHz)
705 MHz
9.4 9.6 9.8 10.0 10.2
unstrained
strained
-10
-8
-6
-4
-2
0J.D. Marconi et. al., ECOC’2005
2/15/2007 Hugo Fragnito 19
Example: Wavelength Converter
Strain suppressed SBS
70 nm signal tuning range (1548 – 1618 nm)
Narrow line idler (100 kHz pump width)
J.D. Marconi et al., ECOC’2005
2/15/2007 Hugo Fragnito 20
Low SBS fibers
Photonic Crystal Fibers
Acoustic and optical confinement
Strong interaction but low coupling between optic
mode and acoustic modes
P. Dainese et al., Nature Phys. 2, 388, 2006
Optical mode
Acoustic mode
+-
+
• Standard telecom-compatible fibers: 4 dB SBS threshold increaseM.V. Vaughin et al., JON 5, 40 (2006)
SBS threshold increase by 7 dB
Core diameter
(1 µm)
• FOPAs for DWDM can be superior to Semiconductor, Erbium, and Raman
amplifiers
– in bandwidth
– in gain flatness
– in noise figure
• Parametric processes can be used in a variety of functions for optical
networking (wavelength conversion/exchange) and signal processing
(phase conjugation, power limiting, all optical switching, fast tunable
filters…)
• Fibers for further progress: uniformity, tailored dispersion, highly
nonlinear but low SBS
• As new fibers (photonic crystal or conventional) become available with
optimized designs, FOPDs may revolutionize optical communications
ReviewReview
222222 H. Fragnito UNICAMP H. Fragnito UNICAMP H. Fragnito UNICAMP ––– IFGWIFGWIFGW
Optical Communications LaboratoryOptical Communications Laboratory
Students 4 André Guimarães
Andrés Rieznik
Gustavo Wiederhecker
Henri Okajima
Res. Assoc. 1 José Manuel Chávez Boggio
Pos-Doc 1 Diego Marconi
Technicians 1 Vagner Cardoso
Faculty 1 Hugo Fragnito
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