Fiber Optical Communication
Examiner/lectures:
Prof. Peter Andrekson
peter.andrekson@ttu.ee, +372 – 5558 7388; +46 – 70 3088 606
Laboratory exercises:
Egon Astra
egon.astra@gmail.com, +372 – 5560 2230
Fiber Optical Communication
Lecture 1, Slide 1
Lecture 1
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Course information
Introduction to photonics and fiber optics
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Applications of photonics
Telecommunication history
System overview and terminology
Fiber basics
• Geometric description (ray optics) of optical fibers
Fiber Optical Communication
Lecture 1, Slide 2
Course outline
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Lectures:
- ca 13 in total + repetition
Lab exercises
Home assignments
Exam on xxx
Fiber Optical Communication
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The book that is used is very well
known…
…and in the fourth edition
Govind P. Agrawal: Fiber-Optic
Communication Systems, ISBN:
9780470505113
Lecture 1, Slide 3
Course objectives
After completion of this course, the student should be able to
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Describe the fundamental properties and limitations of fiber-optic systems
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Describe and analyze the most important system components and their
limitations: transmitters, fibers, receivers, optical amplifiers
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Evaluate a proposed system design and understand the trade-offs
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For optical transmitters: Understand different implementations, primarily using
external modulators
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For optical fibers: Quantify dispersion, attenuation, and to some extent
nonlinearities as well as their impact on signal transmission
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For optical receivers: Analyze receivers with and without optical pre-amplifier,
know all relevant noise mechanisms, evaluate the bit error rate
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For optical amplifiers: Understand the fundamental properties of erbium-doped
fiber amplifiers and quantify the impact of using them
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For systems: Evaluate different implementations such as wavelength division
multiplexing and time division multiplexing.
Fiber Optical Communication
Lecture 1, Slide 4
Plan for lectures
1.
Information and introduction
2.
General concepts, modulation and
detection
3.
Modes in optical fibers
4.
Dispersion and losses
5.
Nonlinear effects in optical fibers
6.
Optical transmitters
7.
Optical receivers
8.
Error probability, power penalties
9.
Multichannel systems
10. Optical amplification, gain
Please note:
• Some of the contents of the
lecture notes are not available in
the book
• The opposite is of course also
very true!
• Some sections of the book are
outside the scope of the course
• The section corresponding to a
slide has been indicated (when
possible)
11. Optical amplification, noise
12. Dispersion management
13. Advanced lightwave systems
Fiber Optical Communication
Lecture 1, Slide 5
Information about the homepage
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There is a course homepage http://lr.ttu.ee/irm0120/
Will eventually contain
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News
Lecture notes
Home assignments
Lab instructions
Fiber Optical Communication
Lecture 1, Slide 6
Some photonic applications
Blu-Ray disc
LED-TV
Environmental
monitoring
Fiber-optic gyroscope
LED light bulb
Optical communication
Fiber Optical Communication
Solar cell
Lecture 1, Slide 7
Some photonic applications
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Telecommunication
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Information and Communication Technology
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Lasers, modulators, fibers, detectors for communication systems
Free-space optical links
CCD and CMOS sensors for imaging
Data storage and retrieval (CD, DVD, BluRay)
Optical interconnects (mainly in high performance computing context today)
Sensors and spectroscopy
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”Smart cameras” for image processing/machine vision
Many, many applications, including sensors for measuring:
• Position, distance, thickness etc.
• Angular rate (ring laser/fiber gyroscopes)
• Gas concentration (using absorption)
Fiber Optical Communication
Lecture 1, Slide 8
Some photonic applications
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Security
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Lighting
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Solar cells
Biophotonics
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LEDs for indoor lighting
LEDs and Lasers for artistic lighting
Energy
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Intrusion detection
Laser radar (LIDAR)
Optical tweezers, optical scalpels
Optical tomography
Military
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Surveillance
Weapon guidance
Countermeasures and laser guns
Fiber Optical Communication
Lecture 1, Slide 9
The motivation: the Internet
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Figure shows number of hosts connected to the Internet
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Around 109 hosts...
Traffic grows quickly
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How to keep up with the increasing demand?
Fiber Optical Communication
Lecture 1, Slide 10
Optical fibers vs wireless communication
Fiber systems:
• High data rates
• Long distances
• One “ether” per system
Wireless systems:
• “Low” data rates
• Short distances
• Shared “ether”
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Static links
Expensive installation
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Limiting regulations
Cross-talk problems
Enables mobility
Easy and flexible installation
Wireless and optical fiber communication are complementary rather than
competing technologies
Fiber Optical Communication
Lecture 1, Slide 11
Transatlantic cables
The continents are today connected by fiberoptical communication links
A submarine cable
1.
2.
3.
4.
5.
6.
7.
8.
Polyethylene
Mylar tape
Stranded steel wires
Aluminum water barrier
Polycarbonate
Copper or aluminum tube
Petroleum jelly
Optical fibers
Fibers are used for high speed, long haul communication, but also for:
• Intercity connects and city networks
• Providing high speed connections to terminals providing wireless services
• High speed (100 Mbit/s and above) services FTTx, where x = home etc.
Fiber Optical Communication
Lecture 1, Slide 12
The electromagnetic spectrum
Photon energy
1 keV
Frequency
1018 Hz
Wavelength
x-ray
1 nm
ultra-violet
1 eV
1015 Hz
visible
1 µm
infrared
1 meV
1 THz
mm-waves
The carrier frequency is
much higher in lightwave
systems than in
microwave systems
1 mm
Lightwave systems
typically use infrared
light
microwaves
10-6
eV
1 GHz
1m
radio waves
10-9
eV
1 MHz
Fiber Optical Communication
Frequency: ν ≈ 200 THz
Wavelength: λ, typ. 1.55 μm
Light velocity in vacuum:
1 km
c ≈ 2.998×108 m/s
Lecture 1, Slide 13
Properties of optical fibers
Advantages:
• Low attenuation (0.2 dB/km)
• Large bandwidth (1.55 μm–1.3 μm = 250 nm > 30 THz)
• Low weight, compact, flexible
• Isolated from the environment
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Low sensitivity to environmental conditions
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No crosstalk from other fibers or microwave sources
Can operate on the ocean floor
Immune to electromagnetic interference
Provides electrical isolation between terminals
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No ground loops, damage cannot cause sparking
Disadvantages:
• Not wireless, installation is costly and slow
• Hardware is expensive compared to mass-produced electronics
Fiber Optical Communication
Lecture 1, Slide 14
Early communication history (1.1)
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Ancient time: Fire beacons, smoke signals, etc.
”Optical telegraph”, 1794
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Telegraphy, 1830s
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Morse code
10 bit/s, 1000 km
Trans-atlantic cable
Telephone, 1876
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Coded signals using semaphore signaling
Used light: Otherwise you could not see the signals!
Analog modulation
Communication systems, 1940–1980
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Coaxial-cable systems
Microwave systems
Fiber Optical Communication
Lecture 1, Slide 15
Optical Communication History
1962
1966
1970
1970
1982
1986
1988
1995
1997
2004
2007
First semiconductor laser (GE, IBM, Lincoln Lab)
First optical fiber, loss: 1000 dB/km (Corning Glass)
Fiber with an optical attenuation of 20 dB/km (Corning Glass)
AlGaAs-lasers operating at room temperature
0.16 dB/km (theoretical limit) single-mode fiber
First erbium-doped fiber optical amplifier
Trans-Atlantic and trans-Pacific cable systems (565 Mbit/s)
Repeaterless trans-oceanic systems (5 Gbit/s)
Commercial WDM systems
Multiple band transmission (S + C + L)
“Advanced” nonbinary formats; 40 Gbit/s systems
Fiber Optical Communication
Lecture 1, Slide 16
Optical Communication History
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First generation, 1980
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Second generation
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Dispersion-shifted fibers
Single longitudinal mode lasers
Still using electrical repeaters
Fourth generation
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InGaAsP lasers, 1.3 μm (minimum dispersion), 100 Mbit/s
Single-mode fibers, 2 Gbit/s over 44 km in 1981
Third generation, 1.55 μm (minimum loss)
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GaAs lasers, 0.8 μm, 45 Mbit/s, using electrical repeaters
Optical amplification
Wavelength-division multiplexing (WDM)
Fifth generation
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Increased spectral range
Increased spectral efficiency
Fiber Optical Communication
Lecture 1, Slide 17
Progress in lightwave communication
The bit rate-distance product has increased eight orders of magnitude
Fiber Optical Communication
Lecture 1, Slide 18
Progress in lightwave communication
Efforts have been made to:
• Increase the data rate per
channel
• Increase the number of
channels
• Increase the distance
between repeaters
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Or at least keep it from
shrinking when
increasing other
parameters
Rapid progress has been made, laboratory systems show the way forward
Fiber Optical Communication
Lecture 1, Slide 19
optical transmitter
e
nics
optical
modulator
semiconductor
laser
optical
fiber
A fiber-optic communication link
optical transmitter
repeater
optical
transmitter
optical
receiver
electronics
information
source
optical
amplifier
optical
fiber
drive
electronics
semiconductor
laser
optical
fiber
repeater
optical
transmitter
electronics
optical
modulator
optical
fiber
optical
receiver
optical receiver
eiver
ronics
photodetector
optical preamplifier
optical
fiber
optical
amplifier
electrical signal
optical signal
optical receiver
information
receiver
Fiber Optical Communication
receiver
electronics
photodetector
electrical signal
optical preamplifier
Lecture 1, Slide 20
Channel multiplexing (1.2.2)
Wavelength-division multiplexing (WDM)
Multiplexing
in wavelength
(frequency)
Optical fiber
Time-division multiplexing (TDM)
O-MUX
Pulse source
Interleaving in
time
O-DEMUX
1,2...N
receivers
N x Gbit/s
transmission
Data encoders
x Gbit/s
Fiber Optical Communication
Timing control
x Gbit/s
clock
1,2...N
Lecture 1, Slide 21
Coherent fiber systems
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Were used before optical amplifiers (EDFA) for increased sensitivity
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Very hard to obtain stable phase-locked operation
Recently re-gained popularity
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Allows increased spectral efficiency (phase modulation, dual-polarization)
Fiber Optical Communication
Lecture 1, Slide 22
The dB units (1.4.2)
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Decibel (dB) expresses a power ratio according to
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 P1 
10 log 10  
 P2 
The photo current is proportional to the optical power
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Idet  Popt ⇒ Pel  Popt2
dBopt ≠ dBel (3 dB optical power diff. ⇒ 6 dB electrical power diff.)
dBm expresses the absolute power on a log scale relative to 1 mW
 P [ W] 

PdBm  10 log10 
 1 mW 
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Examples:
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1 mW = 0 dBm, 2 mW = 3 dBm, 4 mW = 6 dBm, 8 mW = 9 dBm
0.5 mW = –3 dBm, 1 μW = –30 dBm
100 mW = 20 dBm, 400 mW = 26 dBm
Fiber Optical Communication
Lecture 1, Slide 23
Optical fibers (2.1)
Typical core sizes:
• Single-mode fibers: d ≈ 5–10 µm
• Multi-mode fibers: d ≈ 50–200 µm
cladding, n2
core, n1
d
n1 > n2
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Typical attenuation:
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0.2 dB/km@1.55 µm
4% power loss per km@1.55 µm
Available bandwidth:
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Attenuation (dB/km)
15 THz
20 THz
0.2
>30 THz in modern fibers
1.3
1.55
Wavelength (µm)
Fiber Optical Communication
Lecture 1, Slide 24
Fiber basics
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Wave-guiding: n1 > n2
A finite number of modes can
2a
propagate in the fiber
Modes are solutions to Maxwell's
equations + boundary conditions
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One mode: single-mode fiber
Several modes: multi-mode fiber
Most commonly used fiber
material is silica (SiO2)
To change index of refraction
dopants are added
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Dopants can increase or
decrease the index of refraction
Can dope either the core or the
cladding
Fiber Optical Communication
2b
n1
core
refractive index
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n2
cladding
GeO2
1.48
1.46
F
1.44
protective coating
0
B2O3
5
10
15
dopant addition [mol %]
20
Lecture 1, Slide 25
Geometrical-optics description
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The fractional index change is
  (n1  n2 ) / n1  1
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Δ ≈ 1–3% for
MM fibers
Δ ≈ 0.1–1% for
SM fibers
n2 = n1(1 – Δ)
n0
(normally = 1)
i
ray
unguided
r

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Apply Snell's law at input
n0 sin i  n1 sin  r
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Minimum critical angle φc for total internal reflection
n1 sin c  n2 sin( / 2)  sin c  n2 / n1
cladding, n2
core, n1
guided ray
• Relate to maximum entrance angle
n0 sin i ,max  n1 sin  r ,max  n1 sin( / 2  c )  n1 cos c  n1 1  sin 2 c  n12  n22
Fiber Optical Communication
Lecture 1, Slide 26
Numerical aperture
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The numerical aperture (NA) is a measure of the light-gathering power of
an optical system
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The term originates from microscopy
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For fibers, we have
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NA  n0 sin i ,max  n12  n22  n1 2
Clearly, a higher NA is always better!?!
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No, we get problems with dispersion
n0
(normally = 1)
i
Fiber Optical Communication
ray
unguided
r

cladding, n2
core, n1
guided ray
Lecture 1, Slide 27
Modal dispersion (multi-mode fiber)
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Compare the propagation times along the slowest and the fastest path
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 n1
 n1  n1 L n12
Lslow  Lfast  L
T 
 
 L   L  1 

c / n1
sin

c
n
c
c
n
c
2
 2 


For this approximate model, use the condition TB = 1/B > ΔT
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Means: Propagation time difference < bit slot
We get a limit on the bit rate-distance product
n c
BL  22
n1 
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Do not confuse ΔT with Δ
c
cladding, n2
core, n1
i,max
Fiber Optical Communication
fastest ray path
slowest ray path
Lecture 1, Slide 28
Modal dispersion, example
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(Hypothetical) fiber without cladding:
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n1 = 1.5, n2 = 1
BL < 0.4 (Mbit/s) × km
Large index-step:
• Wide “acceptance cone”
– Easy to get the light into the fiber
• Very small bandwidth!
Typical communication fiber: Δ = 0.002 ⇒ BL < 100 (Mbit/s) × km
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1 Gbit/s over 100 m
Estimate is a bit too conservative
Modal dispersion is a severe limitation
Use single-mode fibers when possible
Remember that the geometrical-optics description has limited accuracy
Fiber Optical Communication
Lecture 1, Slide 29
Basic fiber types
Single-mode step-index: Multi-mode step-index: Multi-mode gradedindex:
• No intermodal
• Large core radius,
dispersion gives
easy to launch light • Reduced intermodal
highest bandwidth • Intermodal
dispersion increases
bandwidth
• Small core radius,
dispersion reduces
difficult to launch
the bandwidth
light
n
n2

2a: 5-12 m
2b: 125 m
Fiber Optical Communication
n1 n
n
a
b
2a: 50-200 m
2b: 125-400 m


2a: 50-100 m
2b: 125-140 m
Lecture 1, Slide 30
Other types of fiber
Polarization preserving fibers
• Using birefringence
Fiber Optical Communication
Photonic crystal fibers
• Air capillaries inside fiber
Lecture 1, Slide 31
Fiber fabrication (2.7)
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Preform is made from glass (2–20 cm thick cylinder)
Heated and pulled to 125 µm diameter
Adding coating for mechanical protection
Fiber Optical Communication
Lecture 1, Slide 32
A 2 inch wafer with optical components
Fiber Optical Communication
Lecture 1, Slide 33
Optical modulator (LiNbO3)
Fiber Optical Communication
Lecture 1, Slide 34
A complete transponder
Fiber Optical Communication
Lecture 1, Slide 35