Opto Electronics
Syed Abdul Rehman Rizvi
The Islamia University of Bahawalpur
Opt0 Electronics
Syed Abdul Rehman Rizvi
E-mail: [email protected]
New Building, U.C.E.T
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To provide an understanding of the structure,
operating principles and underlying physical
concepts of optical communication systems
(particularly fiber links), having emphasis on
fundamental aspects, but taking care of
engineering issues as well.
Text Book/Reference Books
Fiber-Optic Communication Systems by Govind P. Agrawal
Fiber Communication, principles and practices (2nd
edition) by John M. Senior
Understanding optical communication by Harry Dutton.
communication (Willy series in telecommunication
and signal processing) by Robert M.Gagliardi, Sherman
Communication Systems by John Gawar
The starting point !
For thousands of years we have used light to communicate.
Even in these high-tech days of satellite communications, ships still
carry powerful lamps for signaling at sea.
It was a well known ‘fact’ that, as light travels in straight lines, it is
impossible to make it follow a curved path.
Boston, USA, 1870. An Irish physicist by the name of John Tyndall
gave a public
demonstration of an experiment which not only
disproved this belief but gave birth to a revolution in communications
Expected !
 His
idea was very
simple. He filled a
container with water
and shown a light into
it in dark room.
 It was expected that
the light would shine
straight out of the
hole and the water would
would Curve downward
as shown in Figure.
what actually happened !
The light stayed inside the water
column and followed the curved path.
He had found a way to guide light!
The basic requirements still remain
the same today — a light source and
a clear material (usually plastic or
glass) for the light to shine through.
The light can be guided around any
complex path as in Figure .
Applications of light guiding
Road signs- A single light source can be used to power many
optic fibers.
Hazardous areas.
All at sea.
Flexible lighting.(marking escape routes for fire fighters,
mountain and mine rescue, underwater routes for divers,
helicopter landing zones, oil refineries, planes, ships, tunnels.
The list is almost endless)
Back ground - Need for Optical Fiber
The development of worldwide telephone networks during 20th
Century necessitated the use of coaxial cables instead of pairs wires
for increased capacity.
A 3-MHz system capable of transmitting 300 voice channels was put
in to use in 1940.
Then arises the frequency-dependent cable losses, which increase
rapidly for frequencies beyond 10 MHz.
This limitation led to the development of microwave communication
systems in which an electromagnetic carrier wave with frequencies in
the range of 1-10 GHz is used to transmit the signal by using suitable
modulation techniques.
The first microwave system operating at the carrier frequency of 4
GHz was put into service in 1948. Since then, both coaxial and
microwave systems have evolved considerably and are able to
operate at bit rates ~100 Mb/s.
A severe drawback of such high-speed coaxial systems was their
small repeater spacing (~ 1 km) -- expensive to operate.
Microwave communication systems -allow larger repeater spacing.
Figure of merit for communication systems is the bit rate-distance
product, BL, where B is the bit rate and L is the repeater spacing
BL Development
An increase of several orders of magnitude in the BL product would be
possible if optical waves were used as the carrier -- But neither a
coherent optical source nor a suitable transmission medium was
available during the 1950s.
The invention of the laser and its demonstration in 1960 solved the first
Attention was then focused on finding ways for using laser light for
optical communications.
It was suggested in 1966 that optical fibers might be the best choice, as
they are capable of guiding the light in a manner similar to the guiding
of electrons in copper wires
The main problem was the
high losses of optical fibers.
fibers available during the
1960s had losses in excess of
1000 dB/kmm.
A breakthrough occurred in
1970 when fiber losses could
be reduced to below 20
dB/km in the wavelength
region near 1 µm.
The reduction of loss made it
possible to use optical fibers
for communication. Which
was further reduced to
0.2 Around 1975. The enormous progress
was realized !
db/km in 1979.
At about the same time, GaAs
semiconductor lasers, operating at room
temperature, were demonstrated .
The simultaneous availability of compact
optical sources and a low-loss optical
fibers led to a worldwide effort for
developing fiber-optic communication
Figure shows the increase in the capacity
of lightwave systems realized after 1980
The commercial deployment of lightwave
systems followed the research and
development phase closely. The progress
has indeed been rapid as evident from an
increase in the bit rate by a factor of
100,000 over a period of less than 25
Transmission distances have also
increased from 10 to 10,000 km over the
same time period. As a result, the bit
lightwave systems can exceed by a factor
of 107 compared with the first-generation
lightwave systems.
Optical Comm Systems
Optical Communication Systems
Optical communication systems differ in principle from microwave
systems only in the frequency range of the carrier wave used to carry
the information i.e. 200 THz & 1 GHz respectively.
An increase in the information capacity is expected i.e. 1o,ooo times.
Optical communication system consists of a transmitter, a
commmmunication channel and a receiver.
Optical communication systems can be classified as guided and
In the guided lightwave systems the optical beam emitted by the
transmitter remains confined, using optical fibers.
In the unguided optical communication systems the optical beam
emitted by the transmitter spreads in space, similar to spreading of
Unguided optical systems are less suitable for broadcasting
applications than microwave systems because optical beams spreads
mainly in the forward direction because of their short wavelength.
Fiber-optic communication
 This is a method of transmitting information from
one place to another by sending light through an
optical fiber.
 The light forms an electromagnetic carrier wave
that is modulated to carry information.
Fiber-optic communication
The process of communicating using fiber-optics
involves the following basic steps:
 Creating the optical signal using a transmitter,
 relaying the signal along the fiber, ensuring that
the signal does not become too distorted or weak,
 and receiving the optical signal and converting it
into an electrical signal.
Evolution of Fiber
1880 – Alexander Graham Bell
1930 – Patents on tubing
1950 – Patent for two-layer glass wave-guide
1960 – Laser first used as light source
1965 – High loss of light discovered
1970s – Refining of manufacturing process
1980s – OF technology becomes backbone of long
distance telephone networks in NA.
An optical fiber (or fibre) is a glass or plastic fiber
that carries light along its length.
Light is kept in the "core" of the optical fiber by
total internal reflection.
Advantages of Optical Fibre
 Thinner
 Less Expensive
 Higher Carrying Capacity
 Less Signal Degradation
 Light Signals
 Non-Flammable
 Light Weight
Advantages of fiber optics
Much Higher Bandwidth (Gbps) - Thousands of
channels can be multiplexed together over one
strand of fiber
electromagnetic interference (EMI).
Safety - Doesn’t transmit electrical signals,
making it safe in environments like a gas
High Security - Impossible to “tap into.”
Advantages of fiber optics
Less Loss - Repeaters can be spaced 75 miles apart
(fibers can be made to have only 0.2 dB/km of
Reliability - More resilient than copper in extreme
environmental conditions.
Size - Lighter and more compact than copper.
Flexibility - Unlike impure, brittle glass, fiber is
physically very flexible.
Areas of Application
 Telecommunications
 Computer network
 Cable TV
 Optical Fiber Sensors
 Nuclear plant instrument
 Industrial process control
Fiber Optic Cable
Core – thin glass center of the
fiber where light travels.
Cladding – outer optical
material surrounding the core
Buffer Coating – plastic
coating that protects
the fiber.
• The core, and the lower-refractive-index cladding,
are typically made of high-quality silica glass,
though they can both be made of plastic as well.
Fiber Optic Layers
• consists of three concentric sections
plastic jacket
glass or plastic
fiber core
Fiber Optic Cable
App. Of Fiber Optic Cable
 Relatively new transmission medium used by telephone companies
in place of long-distance trunk lines
 Also used by private companies in implementing local data
 It require a light source with injection laser diode (ILD) or lightemitting diodes (LED)
Five Generations of Light wave Systems
First generation
Operating near 800 nm and used GaAs semiconducor
laser, commercially available in 1980
Operated at bit rate of 45 Mbps and repeater spacing of
about 10 km (larger compared that of coaxial cable)
Dec the instl and maintenance cost
Second generation
Operating near 1300 nm where fiber loss is 1 db/km
(typically 0.5 db/km) and fiber exhibit minimum
Uses InGaAsP semiconductor lasers and detectors. (newly
Available in early 80s
By 1987 commercially available systems were operating at
bit rates of up to 1.7 Gbps and repeater spacing of about
50 km(SMF).
Third generation
 Fiber has minimum loss at 1550
nm (realized in
1979 but
dispersion was considerably
 Displayed
more dispersion
arround 1550nm
Dispersion shifted fibers could
problem , designed to have
minimum dispersion around
1550 nm.
In 1990 commercially available
systems were operating at 2.5
Gbps and capable of operating
at 10 Gbps. (DSF with singlelongitudinal-mode lasers)
Typical repeaters
around 60-70 km
Fourth generation
 A drawback of third generation
1.55µmis that the signal is
regenerated periodically by using electronic repeater.
 The fourth generation makes use of optical amplifiers(1989) for
increasing the repeater spacing and WDM for increasing the bit
 It employs erbium-doped fiber amplifiers(1990), 60 - 100 km
 Several WDM systems were deployed across the Atlantic and
Pacific oceans during
1998-2001 in response to the Internetinduced increase in the data traffic; they have increased the total
capacity by orders of magnitudes.
Fifth generation
Concerned with finding the fiber dispersion problems
Optical amplifiers have solved the loss problem but made the
dispersion problem worse
Solution is based on the concept of optical solitons - optical pulses
that preserve their shape during propagation by counteracting the
effect of dispersion through the fiber nonlinearity.
DWDM System
Imagine shining a flashlight. The light waves spread out along its beam.
 As we move further from the light source, the wavefront gets straighter
and straighter.
 At
a long distance from the light source, the wavefront would be virtually
 In a short interval of time each end of the wavefront would move forward
a set distance.
 If we look at a single ray of light moving through a clear material the
distance advanced by the wavefront would be quite regular.There is a
widely held view that light always travels at the same speed.
 This ‘fact’ is simply not true. The speed of light depends upon the
material through which it is moving. In free space light travels at its
maximum possible speed, close to 300 million meters or nearly eight
times round the world in a second
Refractive index !!
through a clear material,
it slows down by an
amount dependent upon
a property of the
Refractive index.
For most materials
that we use in optic
the refractive
index is in the region of
Refractive Index = Speed of light in free space/Speed of light in material
Lower refractive index = higher speed
If a ray of light enters from a material
refractive index to another
material with a lower index,
in which it would move
faster. W e can see that the
between the
successive wave crests, or
increase as soon as the light
moves into the second
The direction that the light
approaches the boundary
between the two materials
is very significant.
Snell’s law
Willebrord Snell, a Dutch astronomer,
discovered that there was a relationship
between the refractive indices of the
materials and the sine of the angles. He
made this discovery in the year 1621.
Snell’s law states the relationship as:
n1sin φ1 = n2sin φ2
Where: n1 and n2 are the refractive indices
of the two materials, and sin φ1 and sin φ2
are the angles of incidence and refraction
Snell's law will apply to the refraction of
light in any situation, regardless of what the
two media are.
Calculate the angle shown as φ2 ,The first material has a refractive
index of 1.51 and the angle of incidence is 38° and the second
material has a refractive index of 1.46.
Starting with Snell’s law:
n1sinφ1 = n2sinφ2
Critical angle - Light Guiding
As the angle of incidence in the first material is increased, there will come
a time when, eventually, the angle of refraction reaches 90° and the light
is refracted along the boundary between the two materials. The angle of
incidence which results in this effect is called the critical angle.We can
calculate the value of the critical angle by assuming the angle of
refraction to be 90° and transposing Snell’s law:
n1sin φ1 = n2sin90°
As the value of sin90° is 1, we can now transpose to find sin φ1, and hence φ1,
(which is now the critical angle):
φ Critical
= arcSin ⎜
⎛n 2
A worked example
Total internal reflection
The critical angle is well-named as its
value is indeed critical to the operation
of optic fibers.
At angles of incidence less than the
critical angle, the ray is refracted.
However, if the light approaches the
boundary at an angle greater than the
critical angle, the light is actually
reflected from the boundary region
back into the first material. The
boundary region simply acts as a
mirror. This effect is called total
internal reflection (TIR).
The effect holds the solution to the
puzzle of trapping the light in the
fiber. If the fiber has parallel sides,
and is surrounded by a material with a
lower refractive index, the light will be
reflected along it at a constant angle shown as ø in the Figure .
Any ray launched at an angle
greater than the critical angle
will be propagated along the
optic fiber
Electromagnetic spectrum
Numerical aperture
The numerical aperture of a fiber is a figure which represents
its light gathering capability.
The acceptance angle also determines how much light is able to
enter the fiber and so we must expect an easy relationship
between the nummerical aperture and the cone of acceptance as
they are both essentially measurements of the same thing.
The formula for the numerical aperture is based on the
refractive indices of the core and the cladding.
NA = n
Aceptance angle=sin-1 NA
Let’s try the short cut and see how it works out using values of ncore = 1.5, and
n cladding = 1.48
What will happen if incident angle is more than coneH1of acceptance ?
Geometrical- Optics description
In its simplest form an optical fiber
consists of a cylindrical core of silica
glass surrounded by a cladding
whose refractive index is lower than
that of the core.
Because of an abrupt index change
at the core-cladding interface, such
fibers are called step-index fibers. In
a different type of fiber, known as
graded-index fiber, the refractive
index decreases gradually inside the
NA - Step index fiber
Numerical Aperture is a measure of the light gathering power of the fiber.
 The acceptance angle for an optical fiber is maximum angle to the axis at
which light may enter the fiber in order to be propagated.
It gives a relationship between the acceptance angle and the refractive
indices of the three media involved, namely the core, cladding and air.
The ray enters the fiber from a medium (air) of refractive index n0 , and the
fiber core has a refractive index n1 , which is slightly greater than the
cladding refractive index n2.
using Snell’s law
no sinθi = n1 sinθr
Details are in class lecture
A silica optical fiber with a core diameter large enough to be considered by ray
theory analysis has a core refractive index of 1.50 and a cladding refractive
index of 1.47.
(a) The critical angle at the core-cladding interface.
(b) The NA for the fiber.
(c) The acceptance angle in air for the fiber.
(a)The critical angle φc at the core- cladding interface is given by Eq.
sinφc = n2 / n1
φc = sin-1n2 / n1
= sin-1 1.47/1.50
= 78.50
(b); From Eq. The numerical aperture is
NA = (n12 - n22) ½
= (1.502 - 1.472) ½
=(2.25 - 2.16) ½
(c): Considering Eq the acceptance angle in the air θa is given by:
θa=sin-1 NA
= sin-1 0.30
Intermodel dispersion (Multimode
The extent of pulse broadening can be estimated by considering the
longest and shortest ray paths. The shortest path occurs for θ i = 0, and X
is just equal to the fiber lenght 'L'.The longest path occurs for
shown previously and has a lenght 'L/sin Φc .
v = c / n1,
the time delay is given by ;
∆T =T Max −TMin
s x−L
Ln n −n
= 1 1 2n1
= =
cn2 n1
Ln 21 ∆
cn 2
When ∆ <<1
X= L/SinΦc
SinΦc= n2/n1
X = L n1/n2
The tim e delay between the two rays taking the shortest and longest
paths is a measure of broadening experienced by an impulse launched
at the fiber input.
We can relate ∆T to the information-carrying capacity of the fiber
measured through the bit rate B.,
Requirement for minimal inter symbol interference:
B ∆T < 1
B = bit rate
Names given to different rays
The position and the angle at which the ray
strikes the core will determine the exact path
taken by the ray. There are three possibilities,
called the skew, meridional and the axial ray as
shown in Figure . If light enters a fiber from a
practical light source, all three rays tend to
occur as well as those outside of the cone of
acceptance .
The skew ray never passes through the center
of the core. Instead it reflects off the
core/cladding interface and bounces around the
outside of the core. It moves forward in a shape
reminiscent of a spiral staircase built from
straight sections.
The meridional ray enters the core and
passes through its center. Thereafter, assuming
the surfaces of the core are parallel, it will
always be reflected to pass through the center.
The axial ray is a particular ray that just
happens to travel straight through the center of
the core.
Graded index fibers
The refractive index - is not constant.
Decreases gradually from its maximum value n1 at the core center to
its minimum value n2 at the core-cladding interface.
Most graded-index fibers are designed to have a nearly quadratic
decrease and are analyzed by using α-profile, given by
• where ‘a’ is the core radius. ‘ρ’ is the radial distance.
• The parameter α determines the index profile.
• A step-index - large α. A parabolic-index fiber corresponds to α= 2.
Intermodal or multipath dispersion is
reduced for graded-index fibers.
Figure shows schematically paths for
three different rays.
The path is longer for more oblique
rays. However, the ray velocity
changes along the path because of
variations in the refractive index.
The ray propagating along the fiber
axis takes the shortest path but
travels most slowly as the index is
largest along this path.
Oblique rays have a large part of their path in a medium of lower
refractive index.
Suitable choice of the refractive-index profile leads to non-dispersive
pulse propagation. The trajectory of a ray is obtained by
where ρ is the radial distance of the ray from the axis. For ρ< a with α
= 2, Eq. above reduces to an equation of harm onic oscillator and has
the general solution;
where p = (2∆/a2)1/2 and ρ0 and ρ’0 are the position and the direction
of the input ray, respectively. All rays recover their initial positions
and directions at distances z = 2mπ/p, where m is an integer.
Such a complete restoration of the input implies that a parabolicindex fiber does not exhibit intermodal dispersion.
The quantity ∆T/L, where ∆T is the maximum multipath delay in a fiber of
length L, is found to vary considerably with α. Figure shows this variation
for n1 = 1.5 and ∆ = 0.01. The minimum dispersion occurs for α= 2(1−∆)
and depends on ∆ as
The limiting bit rate-distance product
is obtained by using the criterion
∆T < 1/B and is given by
• The BL product of such fibers is improved by nearly three orders of
magnitude over that of step-index fibers.
• Indeed, the first generation of lightwave systems used graded-index
fibers. Further improvement is possible only by using single-mode fibers..
• Graded-index fibers are rarely used for long-haul links. They have
relatively large core, resulting in a high numerical aperture and high
coupling efficiency - but exhibit high losses .
• They can be used to transmit data at bit rates >1 Gb/s over short
distances of 1 km or less (LAN).
- Profile
The figures below expressing the range of refractive index profile of the
fiber core as a variation of α. Allows representation of the step index
fiber when α = ∞, a parabolic profile when α =2 and a triangular profile
when α =1.

File - Electrical Engineering

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