Outline

1
Outline
1. Introduction to Reconfigurable
Networks 2. Degrading Effects in
Systems 3. Optical Amplifiers 4. Dispersion
Compensation 5. Polarization Mode
Dispersion 6. Modulation Formats 7. Performance
Monitoring 8. Optical Switching
2
Signal Degradation in Optical Transmission
Dispersion
Noise
Tx
Rx
Bit errors without SNR degradation
Nonlinearities
In reconfigurable, dynamic optical networks,
there are a number of non-catastrophic effects
that can degrade the quality of a bit stream
without taking down the network or affecting the
SNR.
3
Degrading Effects Outline

• Chromatic Dispersion
• Nonlinear Effects SPM, XPM, FWM
• Polarization Mode Dispersion
• Crosstalk
• Noise

4
Origin of Dispersion and Nonlinearities
The refractive index of the fiber depends upon
both the frequency and the power of the signal

n(? , P)

Nonlinearities
Chromatic Dispersion

• Self-phase Modulation (SPM)
• Cross-phase Modulation (XPM)
• Four-wave Mixing (FWM)
• Stimulated scattering

5
Origin of Chromatic Dispersion
Speed of Light in a Vacuum
Photon Velocity ( )
Index of Refraction ( )
vj
Information Bandwidth of Data
vk
vi
Fourier
vvelocity
transform
fCarrier
freq.
Temporal Pulse Spreading
F distance, (bit rate)2
6
Chromatic Dispersion in Legacy Fiber
No distortion of output bit stream
2.5 Gbit/s
Optical fiber
Distance 0 km
100 km
Large distortion of output bit stream
10 Gbit/s
Optical fiber
Distance 0 km
100 km
7
Bit-Rate Dependence of Chromatic Dispersion
ps
Dispersion
nmkm
When the bit rate doubles
Time domain
Frequency domain
the bit duration is halved
the spectral width is doubled
ps/2
ps
nm2
nm
T/2
-1/T
1/T
0
-1/2T
1/2T
T
4x dispersion penalty
8
Chromatic Dispersion in Fiber
While different types of fiber may have different
zero-dispersion points, the refractive index (and
hence dispersion) varies with frequency for all
types of common commercially-available optical
fiber.
Tingye Li, Lightwave Communications, 1998
9
Origin of Fiber Nonlinearities
n(? , P)
n n0(?) n2(? ,P)
nonlinear index coefficient
fiber core index
(3.2x10-16 cm2/W for silica)
The glass a photon sees is dependent on how
many other photons are in the vicinity (i. e. the
optical power/intensity)
10
Self-phase Modulation
n2
Transmission through fiber
n1
n3
time
time
The intensity variation in the pulse causes phase
modulation since different parts of the pulse see
a different refractive index, which leads to
pulse broadening.
11
Cross-phase Modulation
v1
l1
l1
l1
n1
v2
n1
l2
l2
l2
n1
v3
l3
l3
l3
time
time
time
The glass that a photon in the ?3 pulse sees
changes as other channels (with potentially
varying power) move to coincide with the ?3 pulse.
12
Four-wave Mixing
I ? (? E)2
E-field mixing between two channels at f1 and f2
f1
f2
2f1 f2
2f2 f1
frequency
Mixing products may interfere with other channels
f1
f2
f3 2f2 f1
2f1 f2
frequency
13
Phase-matching and Nonlinearities
Nonlinear effects are most prominent when signals
are phase-matched
Nonlinear effects
Ch. 1
Ch. 2
Tx
Dispersion disrupts phase matching
No phase-matching fewer nonlinearities generated
Nonlinear effects
Ch. 1
D
D
Ch. 2
Tx
Chromatic dispersion is necessary!
14
Cross-phase Modulation and Dispersion
Phase shift due to cross phase modulation DB
2gLe (dP/dt) µ1/(DDl)
DB Pulse broadening in frequency domain, g
Nonlinear coefficient Le Effective nonlinear
length, D Dispersion, Dl Channel spacing
Performance vs. Dispersion
Performance vs. Channel Spacing
D. Marcuse et al, JLT, 1994
15
Four-wave Mixing and Dispersion
Out of
Fiber
Into Fiber
w
w
w1
w2
2w2-w1
2w1-w2
w1
w2
DSF D 0
Degradation on Bit Stream
Degradation on Optical Spectrum
(Equal Channel Spacing)
(Unequal Channel Spacing)
-10
D -0.2
(ps/nm/km)
(dBm)
-20
D -1
(ps/nm/km)
-30
Power
D -2
-40
(ps/nm/km)
1542
1544
1546
1548
16 ns
21
ns
26 ns
Wavelength (nm)
D. Marcuse et al, JLT, 1994
16
Dispersion Management (DM)
The maximum (1-dB penalty) allowable accumulated
dispersion at the receiver is given by DLps/nm
lt 10-5/R2 Where R is the bit rate in Gbit/s
Dispersion management ensures

• That the total accumulated dispersion between
  the transmitter and receiver is near zero.
• That the local dispersion at any given point is
  non-zero.

17
Dispersion Management Concept
D ? 0
D ? 0

Dtotal 0
Chromatic dispersion
time
time
SMF
D 0
time
XPM and FWM
DCF
SMF
time
time
All effects are minimized
DSF
18
Dispersion Management Techniques
Dispersion-compensating fiber
TX
RX
Transmission fiber
100
Total accumulated dispersion, ps/nm
0
50
100
150
200
Distance, km
19
Dispersion Management Techniques (2)
Lumped Dispersion Compensation
Distributed Dispersion Compensation
Total accumulated dispersion, ps/nm
Total accumulated dispersion, ps/nm
Distance, km
Distance, km
OK for CRZ and Soliton
Required for NRZ
20
Polarization-mode Dispersion (PMD)

• PMD is a result of a slight asymmetry in the
  fiber core.
• Different polarizations can propagate at
  different speeds.

PMD is a random, stochastic, time-varying,
temperature-dependent, and frequency-dependent
effect!
21
PMD Effects on 10 Gbit/s Data
100 km low-PMD fiber PMD 0.1 ps/km0.5
0
100 km high-PMD fiber PMD 10 ps/km0.5
0
time ns
0
0.4
0.8
1.2
1.6
22
Crosstalk in Optical Systems
12
10
N 100
8
Penalty dB
6
4
N 1
N 2
N 4
2
N 6
N 8
0
-50
-45
-40
-35
-30
-25
-20
-15
-10
Crosstalk dB
N number of crosstalk paths

As little as .1 total crosstalk in a multi-hop
network can lead to penalties gt3 dB!
Goldstein et al, PTL, 1994
23
In-band and Coherent Crosstalk
?1 ?2
Signal distortion with crosstalk levels of only
15 dB
Switch
Switch
l1
l1
l1 ?2l2
Switch
Switch
?l2
Switch
Switch
l2
l2
Switch
Switch
l2
Crosstalk in a switching system may lead to two
channels on equal wavelengths mixing at an output
port resulting in in-band beating and
interference.
Goldstein et al, PTL, 1994
24
Noise In Optical Systems

• Amplified spontaneous emission (ASE) noise from
  EDFAs.
• Thermal noise
• Signal-spontaneous beat noise
• Spontaneous-spontaneous beat noise
• Signal shot noise
• ASE shot noise
• and thats not all!!!