A High Level Overview of

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A High Level Overview of Optical Fiber
Communications Systems A Presentation to ECE1001
Class of Electrical and Computer Engineering
Department at University of Minnesota
Duluth By Professor Imran Hayee
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Worldwide Fiber Deployment
Deploying Fiber at the speed of Mach 3
Optical Fiber
In 2001, fiber was deployed at a rate of 2000
miles every hour
T. Li A.R. Chraplyvy, 2001
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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Optical Fibers

Single Mode Fiber
Core diameter 8µm
r
0
A wonder of total internal reflection
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Low Attenuation and High Bandwidth
25 Tbit/sec

• Fiber attenuation is 0.2 dB/km at around 1550 nm
• The bandwidth around low attenuation is 25 Tb/sec

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Generic Optical Communication System
Transmission Fiber
Light Source
Modulator
Detector
Demodulator

Electronic Data
Electronic Data
Transmitter
Receiver
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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Old TDM Optical Fiber Transmission Systems
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Erbium-Doped Fiber Amplifiers
EDFA
Input
Output
Channels
Channels
Single light wavelength is amplified after
passing through EDFA
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Erbium-Doped Fiber Amplifiers
EDFA
Input
Output
Channels
Channels
Single light wavelength is amplified after
passing through EDFA not only that!
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Erbium-Doped Fiber Amplifiers
EDFA
Input
Output
Channels
Channels
Single light wavelength is amplified after
passing through EDFA not only that! Multiple
Wavelengths are amplified without affecting each
other
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Low Attenuation and High Bandwidth


About 3 Tbits/sec bandwidth could be used using
EDFAs which is still 10 of total fiber bandwidth
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Wavelength-Division-Multiplexing (WDM)
Optical Fiber
Gain Region
Data _at_ Different Wavelength
Optical amplifiers have opened the door to WDM
Communications onto the single fiber
Alan Willner, IEEE Spectrum, April 1997
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WDM Optical Fiber Transmission Systems
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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Limitations of Fiber Systems
Amplifiers
PMD
Crosstalk
Nonlinearities
Dispersion
25 Tb/s
We are approaching fundamental limits !!
Experimental
Capacity (Gb/s)
Commercial
80
82
84
86
88
90
92
94
96
98
00
02
Year
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Mathematical Model of Fiber
Nonlinear Schrödinger Equation

• A Envelope of Optical Wave
• b1 Accounts for Phase Delay
• b2 Accounts for Dispersion
• b3 Accounts for Dispersion Slope
• a Accounts for Loss
• g Accounts for Nonlinearities
• TR Accounts for Raman Gain

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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Origin of Fiber Dispersion
Different wavelengths in the fiber travel with
different speeds
vj
Information Bandwidth of Data
vi
vk
Fourier
vvelocity
0 1 1 0 1 0
transform
freq.
fCarrier
Time
Temporal Pulse Spreading
F distance, (bit rate)2
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Chromatic Dispersion and Achievable Bit Rate
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
Dispersion induced 1-dB Power Penalty
2.5 Gb/s 16,640 ps/nm 980 km
SMF 10 Gb/s 1,040 ps/nm
60 km SMF 40 Gb/s 65 ps/nm
4 km SMF
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Chromatic Dispersion Curve
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17 ps/nm.km
Conventional
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Nonzero-dispersion
Dispersion-shifted
10
2 ps/nm.km
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Dispersion (ps/nm.km)
0
ps/nm.km
0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
? (?m)
-5
-10
-15
-20
While different types of fiber may have different
zero-dispersion points, the dispersion of almost
all commercially available fibers varies with
wavelength.
Tingye Li, Lightwave Communications, 1998
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Fiber Bragg Grating
?B ?i
Cladding
?i
Core
?j
?B ? Bragg reflection wavelength
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Chirped FBG
f2
f3
f1
f2
f1
f3
Chirped FBG
?0
Dispersion comp. at
Relative

Time
Delay (ps)
?0
Wavelength (nm)
Linearly Chirped
Dispersion dT/d ? (ps/nm)
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FBG and Dispersion Compensation
Fiber Dispersion
FBG Disp. Comp.
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Outline

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

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Origin of Fiber Nonlinearities
n(? , P)
n n0(?) n2(P)
Index of Refraction
nonlinear index coefficient

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

A photons behavior in the glass is dependent on
how many other photons are in the vicinity (i. e.
the optical power or intensity)
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Self- and Cross-Phase Modulation
SPM
XPM
Both self- and cross-phase modulation cause
signal distortion and therefore impose a limit on
bit rate and transmission distance.
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Four Wave Mixing
Optical third-order inter-modulation.
FWM causes severe inter-channel interference and
therefore severely limits the WDM system
performance
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Four Wave Mixing
Optical third-order inter-modulation.
FWM causes severe inter-channel interference and
therefore severely limits the WDM system
performance
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What Does Fiber Nonlinearity Do?
Data
Encoder
FEC
Fiber/Amplifier Chain
Clock
Driver
Decoder
Receiver
DMUX D C U
MU X D C U
Fiber
Mod
Mod
PD LPF
Q
FEC
Gain Equalizer
Laser
Pump-Signal Combiner
RZ
NRZ
WDM Transmitter
WDM Receiver
Raman Pumps
Raman Pumps
Input Eye Diagram
Output Eye Diagram
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Q-Factor and BER
m
s
,
Eye Diagram
1
1
Decision
Level
Output Voltage
m
s
,
0
0
Sampling Time Interval
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What Does Fiber Nonlinearity Do?
Data
Encoder
FEC
Fiber/Amplifier Chain
Clock
Driver
Decoder
Receiver
DMUX D C U
MU X D C U
Fiber
Mod
Mod
PD LPF
Q
FEC
Gain Equalizer
Laser
Pump-Signal Combiner
RZ
NRZ
WDM Transmitter
WDM Receiver
Raman Pumps
Raman Pumps
Goal
Longer Distance More Channels
Input Eye Diagram
Output Eye Diagram
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What Does Fiber Nonlinearity Do?
Data
Encoder
FEC
Fiber/Amplifier Chain
Clock
Driver
Decoder
Receiver
DMUX D C U
MU X D C U
Fiber
Mod
Mod
PD LPF
Q
FEC
Gain Equalizer
Laser
Pump-Signal Combiner
RZ
NRZ
WDM Transmitter
WDM Receiver
Raman Pumps
Raman Pumps
Goal
Longer Distance More Channels
Input Eye Diagram
Output Eye Diagram
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Nonlinearity Limits the Transmission Distance
Back to Back
5000 km transmission
7500km transmission
Nonlinearity limits the system performance
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Nonlinearity Limits the Number of Channels
7500km _at_ 10Gb/s
Channel Spacing
With increasing channel spacing,
cross-phase-modulation and four-wave-mixing
decrease and system performance improves.
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Dispersion Management Techniques
Positive Dispersion Transmission Fiber
Negative Dispersion Element
Dtotal 0
Dispersion (ps/nm)
D
-D
D
-D
D
-D
Distance (km)
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How Does Dispersion help reduce Nonlinearities?
Fiber Dispersion
0 km
10 km
20 km
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Raman Amplifier vs. EDFAs
EDFA System
Raman System
Raman amplification allows 30 of fiber
bandwidth to be used
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How do Raman Amplifiers reduce Nonlinearities?
EDFA
Raman
Path average power in EDFA system is 1 dB larger
than that of Raman System
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System Performance Comparison
11.5Gb/s _at_ 7500km 75km Spans
Dispersion Managed Raman Q 15.0 dB
Dispersion managed EDFA Q 12.6 dB
Normalized Power
Time (ps)
Time (ps)
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Summary

• Why Optical Fiber?
• TDM to WDM
• Fundamental Limitations
• - Chromatic dispersion
• - Fiber nonlinearities

Questions?