Download Lecture 1

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1 nm
1 µm
1 mm
Microwaves
30 kHz
30 MHz
3 GHz 30 GHz
THz
Gap
Photonics
3 THz
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Optics and photonics is essentially concerned with the study of light and its useful
application.
Hence the aim of this course is to enhance your understanding of optical phenomena,
materials and devices and to see how they are applied in engineering.
The basic questions we will seek to ask are:
1. What is light? (And by extension, what is optics and photonics?)
2. How does light interact with matter?
3. How do we generate light?
5. How do we detect light?
6. How can we use light?
3 PHz
Frequency
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4. How do we manipulate or modulate the properties of light?
X-ray
Far-infrared
Optics
Electronic techniques
• HMY 645
• Lecture 01
• Spring Semester 2015
10-8 10-9
Ultraviolet
10-4 10-5 10-6 10-7
Visible
10-2 10-3
Mid-infrared
1 cm
10-1
Sub-Mm-wave
1
Microwave
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Extremely
ultrashortwave
Longwave
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Ultrashortwave
Stavros Iezekiel
Department of Electrical and
Computer Engineering
University of Cyprus
[email protected]
Wavelength (m)
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Shortwave
104
Mediumwave
ECE 645 – Optics and Photonics
Lecture 01 - Introduction
Mm-wave
1 km
• In this course we will consider the part of the electromagnetic spectrum between about
3 THz and 300 THz, which covers the domain of optics and photonics:
HOW CAN WE USE LIGHT?
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Applications of Photonics
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Displays and lighting
Photonics is now an important part of the world economy, with multiple applications,
including:
Data Storage
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Biomedical
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Manufacturing
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Telecommunications
Energy
Probably the biggest application for photonics is telecommunications, and more
specifically the internet, which would not be possible without fibre-optic technology:
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IT departments in financial capital markets are facing huge growth in the volume of
market data, while they are also under pressure to improve trade execution times. In
these environments, one extra millisecond of trade execution latency can mean as much
as $100 million in lost trades per year. [Source: Mellanox.]
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http://www.arthitectural.com/wp-content/uploads/2013/04/02-TABLE-OF-OPTICKS-SIR-ISAAC-NEWTON-1704.jpg
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Facebook invests in 55-terabit intra-Asia
submarine cable system
Facebook has joined a consortium that will build by far
the fastest intra-Asia submarine fiber optic network,
the Asia Pacific Gateway (APG).
Facebook is the only American company involved with
the venture, which will see 10,000km (6,000 miles) of
prime fiber laid between Malaysia and Japan (pictured
above), with branches landing in almost every country
along the way (Singapore, Thailand, Vietnam, China,
Taiwan, and South Korea).
When the cable goes online in 2014, it is slated to use
40Gbps channels, for a total capacity of 55 terabits
per second, or a transfer speed of 6.9 terabytes (138
Blu-ray discs) per second. When the various routers
and repeaters are upgraded to 100Gbps-per-channel,
the cable will have a total capacity of well over
100Tbps. The members of the consortium have put
forward a total of $450 million so far, which makes it
one of the most expensive submarine cable systems in
the world.
WHAT IS OPTICS?
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Optics is one of the oldest branches of science.
It is concerned with the generation, propagation, manipulation and detection of light. For
many centuries, the development of optical sources and optical detectors was very slow,
hence progress was strongest in studies of light propagation and light manipulation, e.g.:
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By the late 19th century, the theoretical work of Maxwell and the experiments of Hertz had
resulted in the electromagnetic view of light, in which it holds that light consists of
coupled time-varying electric and magnetic fields that satisfy a wave equation (which itself
can be derived from Maxwell’s equations):
c = speed of light = 2.998 × 10-8 ms-1 in vacuo
c = fλ
Solution is a travelling-wave:
Refraction (ray optics)
Interference (wave optics)
k=
Polarisation (electromagnetic optics)
2π
λ
ω=
2π
T
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Light as photons (“particles”):
The ultraviolet catastrophe
However, the development of modern physics (and especially the work of Planck and
Einstein) led to the photon view of light.
Radiation modes in a hot cavity provide a test of quantum
theory.
In classical physics, probability of modes being occupied is
equal, which predicts increasing radiated energy as frequency
is increased. This leads to the “ultraviolet catastrophe”.
Energy of a photon:
E = hf =
Planck showed that by assuming the modes
were quantised in energy, quantum theory
would match measured results.
hc
λ
h = Planck’s constant = 6.626 × 10-34 J·s
 Hyperphysics
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Light as photons: Photoelectric effect
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Light as photons
The Compton Effect
• Increasing the intensity of the light increases the number of photoelectrons, but not their
maximum kinetic energy.
• Red light will not cause the ejection of electrons, no matter what the intensity.
• Weak violet light will eject only a few electrons, but their maximum kinetic energies are
greater than those for intense light of longer wavelengths!
Explained by Planck relationship:
E = hf =
hc
λ
• Incoming photon gives part of its energy to electron.
• The scattered photon then has less energy, which according to the Planck
relationship means it will have a lower frequency (i.e. longer wavelength).
• Experimental observations (using X-rays and a carbon target) helped to
convince people of the photon nature of light.
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Source: EPSRC
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WHAT IS PHOTONICS?
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Photonics as a word has similar roots to the word electronics.
http://www.coltecnica.com/images/transistor.jpg
Electronics: is concerned with the behaviour and control of electrons in materials (e.g.
semiconductors) and devices (e.g. transistors) and circuits (e.g. microchips). It is
commonly divided into analogue and digital applications in areas such as
communications and computing. (ICT – Information and Communication Technology.)
The versatility of the transistor and the ability to
integrate large numbers of transistors into a single chip
is responsible for the electronics revolution.
The transistor supports many functions such as
amplification, oscillation, signal mixing, active filtering
and switching.
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Moore’s law has successfully predicted both the increasing number of transistors per
chip and the reduced cost per transistor:
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As a consequence, electronics has led to a computing revolution:
The term photonics was coined in 1967 by Pierre Aigrain, a French scientist, who gave the
following definition:
'Photonics is the science of the harnessing of light. Photonics encompasses the generation
of light, the detection of light, the management of light through guidance, manipulation,
and amplification, and most importantly, its utilisation for the benefit of mankind.‘
Notice that no explicit mention is made of generation or control of photons. This is because in some
areas of photonics we use the wave model of light instead of the photon model.
http://spie.org/x39920.xml
Unlike electronics, there is no single device
which performs many different functions.
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Gilder’s law has successfully predicted increases in bandwidth (bit rates) over optical
fiber networks:
However, the development of the laser is
credited as having started the modern field
of photonics. This in turn encouraged the
development of optical fiber and other
components for use in optical
communications.
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CISCO predicts that by 2015 we will be in the zettabyte era (i.e. global IP traffic to exceed 1
zettabyte per year).
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DEFINITIONS
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One of the problems with this field is that different words are used to describe it, for
example:
• Electro-optics: used for optical devices in which electrical effects play a role (e.g. lasers
and modulators).
• Optoelectronics: mainly used for devices that are essentially electronic in nature but
involve optical effects (e.g. photodiodes, light emitting diodes).
• Quantum electronics: mostly used to describe devices that rely on the interaction
between light and matter (e.g. lasers)
• Lightwave technology: Used to describe devices and systems used for optical
communications (e.g. lasers, optical fibre, photodetectors).
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ELECTRONICS
OPTICS
ELECTRO-OPTICS
OPTOELECTRONICS
LIGHTWAVE TECHNOLOGY
QUANTUM ELECTRONICS
PHOTONICS
All of the above involve the interaction between the two core technologies of:
• Optics –implies the use of exclusively optical components and techniques
• Electronics – implies the exclusive use of electrical effects
• Some people regard photonics as exclusively optical, while others also include
electronic interactions. We will take this second point of view.
• Although photonics has many applications, the main use to date has been for
optical communications, and this is what has driven many advances in photonics
devices and systems.
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The key elements of this course (ECE 645)
Optics
Photonics
• Ray optics
• Optical fibre
• Laser resonators
• Wave optics
• Electromagnetic optics
• Polarization
• Wave-guiding
•
•
•
•
Optical fibre
Lasers
Photodiodes
Modulators
OPTICAL COMMUNICATIONS
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The most basic optical communication link:
Optical
source
Modulation
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Optical communications has a long history, having been used by many civilizations. One
example is the friktories of ancient Greece:
Channel
Optical
detector
This was a very early example of digital
optical communications.
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Bell’s photophone 1880
- Analogue optical link
http://www.ec-lyon.fr/tourisme/Chappe/
Digital optics, 1793-1852:
•
•
•
•
•
Claude Chappe’s Optical Telegraph (France)
Based on a semaphore system
Repeater spacing ≈ 6 miles
Message could cover 100 miles in 30 minutes
Bit rate < 1 bit/s
Transmitter
Receiver
• Light modulated by vibrating mirror
• Light is photodetected using selenium
(i.e. opto-mechanical)
(resistance decreases with increasing light
intensity)
• First example of optoelectronic receiver
© Alexander Graham Bell Foundation
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One of the problems with these early systems was the fact that there was no guided
channel between the transmitter and receiver, in other words the channel was free-space
optics.
For some applications, such as satelliteto-satellite free space optical links, this is
not a problem.
“The ordinary man will find difficulty in
comprehending how sunbeams are to be
used.
Does Prof. Bell intend to connect Boston and
Cambridge with a line of sunbeams hung on
telegraph posts?”
New York Times, 30 Aug. 1880.
But for terrestrial free space
optical communications,
weather conditions have to be
considered:
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Kao and Hockham proposed the use of optical fibres for communications 1966
An efficient way of guiding light is essential to modern optical communications….
At this stage, however, losses are way too high (1000 dB/km for glass, as opposed
to tens of dB/km at most for coaxial cable).
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However, work at Corning in the early 1970’s led to fiber losses of 20 dB/km, and
over time these have been reduced to as low as 0.2 dB/km (at 1550 nm).
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Egyptian
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Venetian
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Optical
Loss
(dB/km)
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Optical fibre
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Optical glass
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1
0.1
3000 BC 1000 AD
1900
1966
1979
1983
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Fibre offers a lot of bandwidth!
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Optical Fibres: Basic Structure
•
•
Advantages of optical fibre
dielectric waveguides that operate at optical wavelengths; mostly made from silica
glass, but plastic versions (for multimode) also available
•
confine electromagnetic energy in the form of light within core and guide the light
parallel to the longitudinal axis:
CORE
CLADDING
•
•
•
•
Very wide bandwidth compared to metallic transmission lines, i.e. potentially
thousands of GHz
Very low loss (as low as 0.2 dB/km)
Can achieve low dispersion (depends on wavelength of source and fibre type)
Small size and weight
Electrical isolation (glass and plastic)
BUFFER COATING
Not to scale!
•A circular core of refractive index n1 is surrounded by cladding with a slightly lower
value of refractive index (n2 < n1). The fibre is encapsulated by the buffer and
additional layers as appropriate.
A fiber-optic cable (right) containing 144 tiny glass
fibers is compared with a cross section of a
conventional copper cable.
• Light is confined to the core of the fibre by total internal reflection – TIR at the
core-cladding interface.
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However, even though fibre itself is small in cross-section, in some applications the overall
cable is not so small or light:
A lot of optical fibre is installed in
undersea (submarine) systems,
and must be well protected.
The basic ingredients of optical communications include:
• coherent oscillator (i.e. laser)
• mixer (e.g. directly modulated laser)
• envelope detector (e.g. photodiode)
However, there are other components (analogous to microwave
components) that are also used:
• amplifiers (Erbium-doped fibre)
• couplers, combiners and splitters
• wavelength selective components – filters, multiplexers
• isolators and circulators
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Basic architecture of an optical fibre link
λ1
λ1
Laser 1
Fibre
Detector 2
EDFA
MUX
λ3
Laser 3
λ2
DEMUX
λ2
Laser 2
λ3
Detector 3
λN
λN
Laser N
Example
WDM link
Detector 1
λ1, λ2, λ3 ..... λN
λ1 = c/f1
λ0
λ2 = c/f2
≈1550nm
≈1530nm
≈1570nm
Detector N
∆f ≈
c ∆λ
= 5 THz
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1310 nm
Fibre attenuation
(dB/km)
850 nm
1550 nm
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Three main wavelength
windows
λ20
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Most fibre links are digital, and consequently we worry about bit rate – distance
products and bit error rates:
1
700
900
1100
GaAlAs
1300
1500
InGaAsP
Wavelength (nm)
Optical sources
Semiconductor optical amplifiers
Optical fibre amplifiers
EDFA
1.0
© G.D. Keiser
Photodiode
responsivity (A/W)
PDFA
InGaAs
Si
0.5
Ge
700
900
1100
1300
1500
Wavelength (nm)
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More wavelengths, higher modulation speeds
Figures of merit
1000
•
Using polarization multiplexing
2008
Bit-rate - repeater spacing product (bits/s - km)
2001
2003
2001
1998
100
10
Improving photonics
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•
The designers of a long distance high-bit rate fibre link have a number of
objectives.
One is to achieve as high a bit rate as possible.
However, it is also important to maximise the distance between optical amplifiers
or repeaters (i.e. the repeater spacing).
The two figures are multiplied to give a key figure of merit used to assess link
performance:
Number of wavelength channels
•
1998
2006
2003
1996
1995
1993
1989
1977
1995
1
1983
1986 1987 1991
Improving electronics
0.1
0.01
0.1
10
1
Data rate per channel (Gb/s)
100
1000
Total
capacity
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Bit rate – distance product improvements
Closing Remarks
• Optics is one of the oldest sciences
• Photonics is some ways the successor of optics, and is a very fast moving field
with many applications.
• So far, the highest impact applications of photonics have been in the ICT
(information communications technology) sector
• Without photonics, we would have no internet
• Without photonics, computing itself will start to run into problems