Download Ans: Optical Fiber Communication System

UNIT 1
What are the various elements of an optical communication system? Explain each
element in brief?
Ans: Optical Fiber Communication System:
The figure 1.1 shows a block schematic of the different elements in an optical fiber
communication system. The carrier is modulated using analog information signal. The
variation of light emitting from the optical source is a continuous signal. The information
source provides an electrical signal to the transmitter. The transmitter comprises electrical
stage. The electrical stage (circuits) drives an optical source. The optical source output is a
light which is intensity modulated by the information. The optical source converts the
electrical signal into an optical signal. The source may be either semiconductor laser or Light
Emitting Diode (LED). The intensity modulated light signal is coupled to fiber. The fiber
which is made up of a glass acts as a channel between the transmitter and receiver.
At the receiver the optical signal is detected by the optical detectors such as PIN diode and
Avalanche photodiode. Sometimes photo transistors and photo conductors are used for
converting an optical signal into electrical signal. The electrical signal is again processed and
given to the transducer to get the original information.
Give the block diagram of a digital optical communication system and explain the
function of each block?
Ans: Digital Fiber optical Communication System
Figure shows a schematic of a typical digital optic fiber link. The input is given as digital
signal from the information source and it is encoded for optical transmission in the encoder.
The encoder, encodes or modulates the digital signal as in the case of simple communication
system where we are using a message signal in which the signal is in analog form, but here
the signal is in digital form which is encoded i.e., modulated in the encoder. The laser drive
circuit directly modulates the intensity of semiconductor laser with the encoded digital signal.
Hence a digital optical signal is launched into the optical fiber cable. At the receiver we have
to decode the digital optical signal for which we are using another Avalanche Photo Diode
(APD) as detector. The avalanche photo diode detector is followed by a front-end amplifier
and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth
reductions. Then the signal is passed through the decoder to get original digital information
which is transmitted
Distinguish
between
optical
fiber
communication
system
and
conventional
communication system? And List out the advantageous and disadvantage of optical
fiber communication?
Ans:
Optical Fiber Communication System
Conventional
Communication
System
1. Requires a bandwidth of 1013 to 1016
1. Requires a bandwidth of 500 MHz
Hz.
2. Light weight
2. Heavier in weight.
3. Immune to R.F. interference
3. Needs external shielding.
4. Electrical isolation.
4. Exhibits earthing problems.
5. Low loss of about 0.2 dB/km.
5. Loss of about 10dB/km.
6. Secure signal propagation.
6. Signal can be tapped easily.
7. Due to increased bandwidth higher data
7. Low data rates compared to optical
fiber.
Advantageous Of Optical Fibers Communication:
1. Information bandwidth is more.
2. Optical fibers are small in size and light weighted.
3. Optical fibers are more immune to ambient electrical noise, electromagnetic interference.
4. Cross talk and internal noise are eliminated in optical fibers.
5. There is no risk of short circuit in optical fibers.
6. Optical fibers can be used for wide range of temperature.
7. A single fiber can be used to send many signals of different wavelengths using Wavelengths
Division Multiplexing (WDM).
8. Optical fibers are generally glass which is made up of sand and hence they are cheaper than
copper cables.
9. Optical fibers are having less transmission loss and hence less number of repeaters are used.
10. Optical fibers are more reliable and easy to maintain.
Disadvantageous Of Optical Fibers Communication:
1. Attenuation offered by the optical fibers depends upon the material by which it is made.
2. Complex electronic circuitry is required at transmitter and receiver.
3. The coupling of optical fibers is difficult.
4. Skilled labors are required to maintain the optical fiber communication.
5. Separated power supply is required for electronic repeaters at different stages.
Compare the advantages and disadvantages of guided optical communication lines with
that of microwave systems?
Ans:
Optical Communication System
Microwave System
1. Uses glass optical fibers or plastic
1. Uses co-axial cable or microwave
optical fibers for transmission.
waveguides for transmission.
2. Low weight, hence large transmission
2. Heavier than optical fibers.
distance or same weight of microwave
link.
3. Large bandwidth of range 1013 to
3. Bandwidth is lesser in the range of 108
l016Hz.
to 1010Hz.
4.Electrically isolated, hence no shielding
4. Prone to electrical disturbances and
is required.
hence,
shielding
for
reducing
RE
interference.
5. Low loss of 0.2dB/km.
5. A considerable loss of 5 dB/km.
6. Large spacing between repeaters about
6. Spacing distance between repeaters is
1 in 300 km.
less, is suitable only for short distance if
waveguides are used.
7. Because large bandwidth, higher data
7. Data rates of mega bits per second can
rate of the order of terabits per second.
be obtained.
8. Message security is obtained.
8. Signal can be tapped easily.
9. No cross talk, hence many fiber
9. If shielding is not done properly, cross
communication channels can be packed
talk is introduced.
inside one single cable.
Disadvantages
1. Expensive transmitter and receiver.
1. Simple and less expensive transmitter
and receiver.
2. Difficult coupling.
2. Easy coupling.
3. Power transmission depends upon the
3. Output power is directly coupled to the
quantum efficiency of light source (LED
transmission line.
or LASER).
4. Unable to excite the terminal device
4. Able to operate the terminal device
directly.
directly.
Write in detail about ray optics?
Ans: Ray optics is used for representing the mechanism of a ray which propagates through an
ideal multimode step index optical waveguide. There are two types of rays, the skew rays and
meridional rays which propagate through a fiber.
The path of meridional can be tracked very easily as they are confined to a single plane.
Meridional are described in two classes. They are,
(i)
Bound rays
(ii)
Unbound rays
Bound rays are those rays which are trapped in a core and they move along the fiber whereas
unbound rays are those rays which get refracted out of the fiber.
Skew rays are those rays which follow helical path but they are not confined to a single plane.
We know that skew rays are not confined to a particular plane so they cannot be tracked
easily.
Analyzing the meridional rays is sufficient for the purpose of result, rather than skew rays,
because skew rays lead to greater power loss.
Now coming to ray theory, we need to consider meridional rays. Representation of
meridional rays is given below.
From the medium of refractive index 'n' which is at an angle ‘θ0’with respect to fiber axis, the
light enters the fiber core. If the light strikes at such an angle then it gets reflected internally
and the meridional ray moves in a zig zag path along the fiber core, passing through the axis
of the guide. Now by using Snell's law the minimum angle ‘фmin’ supports total internal
reflection for meridional ray is given by
If the ray strikes the core-cladding interface at an angle less than фmin then they get refracted
out of the core and they will be lost from the cladding.
By applying Snell’s law to the air-fiber face boundaries, we get θmax
nsin θmax = n1 sin θc = (n n )1/2
Where θc = П/2 – θ0 (From the figure)
So, the rays whose entrance angle ‘θ0’ is less than the ‘θmax’ will be reflected back in to core
cladding interface.
Numerical aperture for a step index is given by the formula N.A = n sin θmax
(n n )1/2 = n1√2
An optical fiber has a NA of 0.20 and a cladding refractive index of 1.59 Determine
(i)
The acceptance angle for the fiber in water which has a refractive index of 1.33
(ii)
Critical angle at the core cladding interface.
Ans:
Given NA = 0.2 n1 =1.59
(i) The acceptance by the water is Refractive index for water n =1.33 NA = n sin θa
θa= sin-1(NA/n) = sin-1(0.2/1.59) = 8.640
Therefore the acceptance angle is = 8.640
(ii) Critical angle at core cladding interface is
We know that
NA= (n12-n22)1/2
We know that,
NA = 0.2 and n1=1.59 0.2 = (1.592-n22)1/2
0.447= (1.592- n22)
n22 = 2.081
θc= n sin-1(n2/ n1) = 1.33 sin-1(1.44/ 1.59) = 86.330
Define an optical fiber. Explain in detail different types of optical fibers giving neat
sketches?
Ans: A dielectric waveguide that operates at optical frequencies is known as optical fiber. It
is generally available in cylindrical form.
Fiber Types:
There are two fiber types
(i) Step index fiber
(ii) Graded index fiber.
(i) Step Index Fiber
Step index fiber is further divided in two types,
1. Single mode step index fiber
2. Multi mode step index fiber.
Single mode step index fiber is shown below,
The typical dimension of core is 8 to 12 μm and cladding is 125 μm.
In step index fiber, the refractive index of the core is uniform and at the cladding boundary, it
undergoes a step change.
In single mode step index fiber, there is only one mode of propagation. The multimode step
index fiber is shown below,
In multimode step index fiber, hundreds of modes are present.
The typical dimension of core is 50 to 200 μm and cladding is 125 to 400 μm. Multimode
fiber has several advantages, which includes, the transmitting the light directly in to fiber
using LED.
Graded Index Fiber
Graded index fiber also contains single mode and multimode. The multimode graded index
fiber is shown below,
In graded index fiber, the refractive index of the core is made to vary as a function of radial
distance taken from the center of the fiber.
The dimension of its core is 50 to 100 μm and cladding is 125 to 140 μm.
In both cases (step index and graded index) multimode has several advantages. When
compared with single mode, however, multimode has a drawback, that is, it suffers from inter
model dispersion.
Compare the fiber structure and numerical aperture in step index and graded index
fiber?
Ans: Fiber structure:
A fiber consists of a single solid dielectric cylinder of radius V and refractive index n{ called
as core of the fiber. The core is surrounded by a solid dielectric cladding with refractive index
n2 that is less than n1 The variation of material composition of core give rise to the two
commonly used fiber types (i). If the refractive index of the core is uniform throughout and
undergoes an abrupt change at the cladding boundary then such a fiber is called step index
fiber (ii). If the core refractive index gradually varies along the radial distance from the centre
of the fiber and becomes equal to the refractive index of the cladding at the boundary, then
such a fiber is called graded-index fiber.
The step-index and graded-index fibers are further divided into single mode and multimode
fibers The core radius in single mode fiber is very small hence only one mode of propagation
is possible and laser diode is-required to launch the light beam m the fiber. Multimode fibers
has larger core radius and hence supports many hundreds of modes of propagation. Due to
larger core radius a CED is sufficient to launch the light beam into fiber making it less
expensive than single mode fibers. But multi mode fibers suffer from inter model dispersion.
Numerical Aperture:
There are two types of rays that can propagate through fiber, they are meridional rays and
skew rays. Meridional rays are confined to the meridian planes of fiber which contains core
axis whereas skew rays are not confined to a single plane, but instead tend to follow a helical
path along the fiber. To obtain the general condition of ray propagation through fiber
meridional rays are considered.
(i) Step‐index Fiber
Consider a step index fiber with core radius ‘a’ and refractive index n1 and with a
cladding of refractive index n2 which is lower than n1, then we can say
n2 = n1(1- Δ )
Where 'A' is called the core-cladding index difference, when a light ray enters the fiber core
from a medium of refractive index at an angle θ and strikes the core-cladding boundary at a
normal
angle Φ such that it results m total internal reflection. Then the angle Φ should not be less
Φmin than given by Snell’s law,
Sin Φmin= n2/ n1
By applying Snell's law to air-fiber face boundary and using equation (1) it can be related to
maximum entrance angle Φmax given by,
n sin Φimax = n1sin Φc = (n12-n22)1/2
where Φc = П/2 – Φ
Therefore for step index the numerical aperture is given by,
NA = n sin Φimax = (n12-n22)1/2
= n1√2Δ
(ii) Graded-Index Fiber
For a graded index fiber the refractive index difference is given by,
is approximately equal in both step-index fiber and graded index fiber.
Numerical aperture of graded index fiber is a function of position across, the case end face,
whereas, NA is step-index is constant across the core. The light incident on the fiber core at
position r will propagate through fiber only if it is within the local numerical aperture of the
fiber at that position given by,
Where, r is the radial distance from the centered the fiber V is the radius of core a is
dimensionless parameter defining the shape of index profile and NA(0) is axial numerical
aperture defined as,
NA(0) = (n2(0) - n )1/2
from centre to core-cladding boundary i.e., at centre NA is equal to that of step index and
gradually reduces until it becomes zero at the core-cladding boundary.
Give three applications of optical fiber in instrumentation and explain them with
necessary figure?
Ans: Optical fibers are used as sensing-elements (sensors) in instrumentation
applications. Since, they have the advantage of efficient telemetry and control
communication they can also work in electrically harsh environments and are free from EM
interference.
The optical fiber sensor system modulates a light beam either directly or indirectly by the
parameters like temperature, pressure, displacement, strain etc. Modulation is done in the
modulation zone of the optical fiber sensor system as shown in figure 9.1. The light beam is
modulated in any of its parameters, which includes optical intensity, phase, polarization,
wavelength and spectral distribution.
(i) Optical Fluid Level Detector
Figure (9.2) shows the functioning of a simple optical fluid level detector. It contains an
optical source, optical detector, optical dipstick and fluid. The optical dipstick is formed by
glass (with refractive index μ1) and fluid has a refractive index μ2. The refractive index of
fluid is greater than refractive index of optical dipstick (μ1>μ2). When the fluid does not
touch the optical dipstick the light beam from optical source passes through the glass as
shown in figure 9.2(a). When the fluid touches the chamfered end, total internal reflection
halts and the light is transmitted into the fluid as shown in figure 9.2(b). As a result, an
indication of the fluid level is acquired at the optical detector.
(ii) Optical Displacement Detector
This is also implemented as extrinsic device. The received light ray is modulated by
intensity. The reflected light from the target is received and the intensity of received light is
proportional to distance/displacement of target. Thus, displacement is measured.
(iii) Optical Fiber Flow Meter
This is implemented as intrinsic device, where the flow rate itself causes the modulation of light.
A multimode fiber is placed along the cross-section of flow pipe, so that liquid flow pass
the fiber. Presence of fiber causes turbulence in the liquid flow as a result fiber oscillates and
frequency of oscillation is directly proportional to flow rate. This oscillation gives a modulated
light at the receiver. Thus, flow rate is measured
A single Mode step index fiber has a core diameter of 7μm and core refractive index of
1.49.Estimate the shortest wavelength of light which allows single mode operation when the
refractive index difference for the fiber is 1%?
Ans; Given that
For a single mode step index fiber, n1 = 1.49
2a = 7μm => a = 3.5 μm = 0.01
We have
n2 = n1 (1-Δ )
= 1.49(1-0.01)
= 1.4751 Therefore n2 =1.48
OPTICAL FIBRE SYSTEM
An optical fiber (or optical fiber) is a flexible, transparent fiber made of extruded glass (silica)
or plastic, slightly thicker than a human hair. It can function as a waveguide, or “light pipe”, to
transmit light between the two ends of the fiber. The field of applied science and engineering
concerned with the design and application of optical fibers is known as fiber optics.
Optical fibers are widely used in fiber-optic communications, where they permit transmission
over longer distances and at higher bandwidths (data rates) than wire cables. Fibers are used
instead of metal wires because signals travel along them with less loss and are also immune to
electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles
so that they may be used to carry images, thus allowing viewing in confined spaces. Specially
designed fibers are used for a variety of other applications, including sensors and fiber lasers.
Optical fibers typically include a transparent core surrounded by a transparent cladding material
with a lower index of refraction. Light is kept in the core by total internal reflection. This causes
the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes
are called multi-mode fibers (MMF), while those that only support a single mode are called
single-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter, and are used
for short-distance communication links and for applications where high power must be
transmitted. Single-mode fibers are used for most communication links longer than 1,000 meters
(3,300 ft).
How a Fiber Optic Communication Works?
Unlike copper wire based transmission where the transmission entirely depends on electrical
signals passing through the cable, the fiber optics transmission involves transmission of signals
in the form of light from one point to the other. Furthermore, a fiber optic communication
network consists of transmitting and receiving circuitry, a light source and detector devices like
the ones shown in the figure.
When the input data, in the form of electrical signals, is given to the transmitter circuitry, it
converts them into light signal with the help of a light source. This source is of LED whose
amplitude, frequency and phases must remain stable and free from fluctuation in order to have
efficient transmission. The light beam from the source is carried by a fiber optic cable to the
destination circuitry wherein the information is transmitted back to the electrical signal by a
receiver circuit.
The Receiver circuit consists of a photo detector along with an appropriate electronic circuit,
which is capable of measuring magnitude, frequency and phase of the optic field. This type of
communication uses the wave lengths near to the infrared band that are just above the visible
range. Both LED and Laser can be used as light sources based on the application.
Basic Elements of a Fiber Optic Communication System
There are three main basic elements of fiber optic communication system. They are
Compact Light Source
Low loss Optical Fiber
Photo Detector
Accessories like connectors, switches, couplers, multiplexing devices, amplifiers and splices are
also essential elements in this communication system.
1. Compact Light Source
Laser Diodes
Depending on the applications like local area networks and the long haul communication
systems, the light source requirements vary. The requirements of the sources include power,
speed, spectral line width, noise, ruggedness, cost, temperature, and so on. Two components are
used as light sources: light emitting diodes (LED’s) and laser diodes.
The light emitting diodes are used for short distances and low data rate applications due to their
low bandwidth and power capabilities. Two such LEDs structures include Surface and Edge
Emitting Systems. The surface emitting diodes are simple in design and are reliable, but due to
its broader line width and modulation frequency limitation edge emitting diode are mostly used.
Edge emitting diodes have high power and narrower line width capabilities.
For longer distances and high data rate transmission, Laser Diodes are preferred due to its high
power, high speed and narrower spectral line width characteristics. But these are inherently nonlinear and more sensitive to temperature variations.
LED Vs Laser Diodes
Nowadays many improvements and advancements have made these sources more reliable. A few
of such comparisons of these two sources are given below. Both these sources are modulated
using either direct or external modulation techniques.
2. Low Loss Optical Fiber
Optical fiber is a cable, which is also known as cylindrical dielectric waveguide made of low loss
material. An optical fiber also considers the parameters like the environment in which it is
operating, the tensile strength, durability and rigidity. The Fiber optic cable is made of high
quality extruded glass (si) or plastic, and it is flexible. The diameter of the fiber optic cable is in
between 0.25 to 0.5mm (slightly thicker than a human hair).
Fiber Optic Cable consists of four parts.
Core
Cladding Buffer
Jacket
Core
The core of a fiber cable is a cylinder of plastic that runs all along the fiber cable’s length, and
offers protection by cladding. The diameter of the core depends on the application used. Due to
internal reflection, the light travelling within the core reflects from the core, the cladding
boundary. The core cross section needs to be a circular one for most of the applications.
Cladding
Cladding is an outer optical material that protects the core. The main function of the cladding is
that it reflects the light back into the core. When light enters through the core (dense material)
into the cladding(less dense material), it changes its angle, and then reflects back to the core.
Buffer
The main function of the buffer is to protect the fiber from damage and thousands of optical
fibers arranged in hundreds of optical cables. These bundles are protected by the cable’s outer
covering that is called jacket.
JACKET
Fiber optic cables jackets are available in different colors that can easily make us recognize the
exact color of the cable we are dealing with. The color yellow clearly signifies a single mode
cable, and orange color indicates multimode.
2 Types of Optical Fibers
Single-Mode Fibers: Single mode fibers are used to transmit one signal per fiber; these fibers
are used in telephone and television sets. Single mode fibers have small cores.
Multi-Mode Fibers: Multimode fibers are used to transmit many signals per fiber; these signals
are used in computer and local area networks that have larger cores.
3. Photo Detectors
The purpose of photo detectors is to convert the light signal back to an electrical signal. Two
types of photo detectors are mainly used for optical receiver in optical communication system:
PN photo diode and avalanche photo diode. Depending on the application’s wavelengths, the
material composition of these devices vary. These materials include silicon, germanium, InGaAs,
etc.
Basic optical laws
Refraction of light
As a light ray passes from one transparent medium to another, it changes direction; this
phenomenon is called refraction of light. How much that light ray changes its direction depends
on the refractive index of the mediums.
Refractive Index
Refractive index is the speed of light in a vacuum (abbreviated c, c=299,792.458km/second)
divided by the speed of light in a material (abbreviated v). Refractive index measures how much
a material refracts light. Refractive index of a material, abbreviated as n, is defined as
n=c/v
Snell’s Law
In 1621, a Dutch physicist named Will ebrord Snell derived the relationship between the
different angles of light as it passes from one transparent medium to another. When light passes
from one transparent material to another, it bends according to Snell's law which is defined as:
n1sin( 1) = n2sin( 2)
where:
n1 is the refractive index of the medium the light is leaving
1
is the incident angle between the
light beam and the normal (normal is 90° to the interface between two materials)
n2 is the refractive index of the material the light is entering 2 is the refractive angle between the
light ray and the normal
Note:
For the case of 1 = 0° (i.e., a ray perpendicular to the interface) the solution is 2 = 0° regardless of
the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never
bent.
The above is also valid for light going from a dense (higher n) to a less dense (lower n) material;
the symmetry of Snell's law shows that the same ray paths are applicable in opposite direction.
Total Internal Reflection
When a light ray crosses an interface into a medium with a higher refractive index, it bends
towards the normal. Conversely, light traveling cross an interface from a higher refractive index
medium to a lower refractive index medium will bend away from the normal.
This has an interesting implication: at some angle, known as the critical angle c, light traveling
from a higher refractive index medium to a lower refractive index medium will be refracted at
90°; in other words, refracted along the interface.
If the light hits the interface at any angle larger than this critical angle, it will not pass through to
the second medium at all. Instead, all of it will be reflected back into the first medium, a process
known as total internal reflection.
The critical angle can be calculated from Snell's law, putting in an angle of 90° for the angle of
the refracted ray 2. This gives 1:
Since n2 = 90°
So
sin(n 2) = 1
Then
c
=
1
= arcsin(n2/n1)
For example, with light trying to emerge from glass with n1=1.5 into air (n2 =1), the critical
angle c is arcsin(1/1.5), or 41.8°.
For any angle of incidence larger than the critical angle, Snell's law will not be able to be solved
for the angle of refraction, because it will show that the refracted angle has a sine larger than 1,
which is not possible. In that case all the light is totally reflected off the interface, obeying the
law of reflection.
What is Fiber Mode?
An optical fiber guides light waves in distinct patterns called modes. Mode describes the
distribution of light energy across the fiber. The precise patterns depend on the wavelength of
light transmitted and on the variation in refractive index that shapes the core. In essence, the
variations in refractive index create boundary conditions that shape how light waves travel
through the fiber, like the walls of a tunnel affect how sounds echo inside.
We can take a look at large-core step-index fibers. Light rays enter the fiber at a range of angles,
and rays at different angles can all stably travel down the length of the fiber as long as they hit
the core-cladding interface at an angle larger than critical angle. These rays are different modes.
Fibers that carry more than one mode at a specific light wavelength are called multimode fibers.
Some fibers have very small diameter core that they can carry only one mode which travels as a
straight line at the center of the core. These fibers are single mode fibers. This is illustrated in the
following picture.
Optical Fiber Index Profile
Index profile is the refractive index distribution across the core and the cladding of a fiber. Some
optical fiber has a step index profile, in which the core has one uniformly distributed index and
the cladding has a lower uniformly distributed index. Other optical fiber has a graded index
profile, in which refractive index varies gradually as a function of radial distance from the fiber
center. Graded-index profiles include power-law index profiles and parabolic index profiles. The
following figure shows some common types of index profiles for single mode and multimode
fibers.
Multimode Fibers
As their name implies, multimode fibers propagate more than one mode. Multimode fibers can
propagate over 100 modes. The number of modes propagated depends on the core size and
numerical aperture (NA).
As the core size and NA increase, the number of modes increases. Typical values of fiber core
size and NA are 50 to 100 micrometer and 0.20 to 0.29, respectively.
Single Mode Fibers
The core size of single mode fibers is small. The core size (diameter) is typically around 8 to 10
micrometers. A fiber core of this size allows only the fundamental or lowest order mode to
propagate around a 1300 nanometer (nm) wavelength. Single mode fibers propagate only one
mode, because the core size approaches the operational wavelength. The value of the normalized
frequency parameter (V) relates core size with mode propagation.
In single mode fibers, V is less than or equal to 2.405. When V = 2.405, single mode fibers
propagate the fundamental mode down the fiber core, while high order modes are lost in the
cladding. For low V values (<1.0), most of the power is propagated in the cladding material.
Power transmitted by the cladding is easily lost at fiber bends. The value of V should remain
near the 2.405 level.
Multimode Step Index Fiber
Core diameter range from 50-1000 m .Light propagate in many different ray paths, or modes,
hence the name multimode Index of refraction is same all across the core of the fiber Bandwidth
range 20-30 MHz . Multimode Graded Index Fiber The index of refraction across the core is
gradually changed from a maximum at the center to a minimum near the edges, hence the name
“Graded Index” Bandwidth ranges from 100MHz-Km to 1GHz-Km
Pulse dispersion in a step index optical fiber is given by
where
is the difference in refractive indices of core and cladding.
is the refractive index of core
is the length of the optical fiber under observation
Graded-Index Multimode Fiber
Contains a core in which the refractive index diminishes gradually from the center axis out
toward the cladding. The higher refractive index at the center makes the light rays moving down
the axis advance more slowly than those near the cladding. Due to the graded index, light in the
core curves helically rather than zigzag off the cladding, reducing its travel distance. The
shortened path and the higher speed allow light at the periphery to arrive at a receiver at about
the same time as the slow but straight rays in the core axis. The result: digital pulse suffers less
dispersion. This type of fiber is best suited for local-area networks.
Pulse dispersion in a graded index optical fiber is given by
where
is the difference in refractive indices of core and cladding,
is the refractive index of the cladding,
is the length of the fiber taken for observing the pulse dispersion,
is the speed of light, and
K = is the constant of graded index profile
Historical Development
Fiber optics deals with study of propagation of light through transparent dielectric waveguides. The fiber optics
are used for transmission of data from point to point location. Fiber optic systems currently used most
extensively as the transmission line between terrestrial hardwired systems.
The carrier frequencies used in conventional systems had the limitations in handling the volume and rate of the
data transmission. The greater the carrier frequency larger the available bandwidth and information carrying
capacity.
First generation
The first generation of light wave systems uses GaAs semiconductor laser and operating region was near 0.8
μm. Other specifications of this generation are as under:
i) Bit rate : 45 Mb/s
ii) Repeater spacing : 10 km
Second generation
i) Bit rate: 100 Mb/s to 1.7 Gb/s
ii) Repeater spacing: 50 km
iii) Operation wavelength: 1.3 μm
iv) Semiconductor: In GaAsP
Third generation
i) Bit rate : 10 Gb/s
ii) Repeater spacing: 100 km
iii) Operating wavelength: 1.55 μm
Fourth generation
Fourth generation uses WDM technique.
i) Bit rate: 10 Tb/s
ii) Repeater spacing: > 10,000 km
Iii) Operating wavelength: 1.45 to 1.62 μm
Fifth generation
Fifth generation uses Roman amplification technique and optical solitiors.
i) Bit rate: 40 - 160 Gb/s
ii) Repeater spacing: 24000 km - 35000 km
iii) Operating wavelength: 1.53 to 1.57 μm
Linearly Polarized Modes
�The exact analysis of the modes of a fiber is mathematically very complex
� Fortunately, the analysis may be simplified when using weakly guiding
approximation
� weakly guiding approximation : refractive index difference Δ << 1
� Type of modes:
TE νm TM νm HE νm EH νm , however, at this approximation,
the propagation constants for some modes are almost identical :
- HE ν+1,m and EH ν−1,m are degenerated
- TE 0,m , TM 0,m , HE 2m are degenerated
The superposition of these degenerating
modes corresponds to particular Linearly
Polarized (LP) modes regardless of their
HE, EH, TE or TM
Relationship between traditional exact
modes and LP l m mode: 2.4.8 Linearly Polarized Modes
The exact analysis of the modes of a fiber is mathematically very complex
Fortunately, the analysis may be simplified when using weakly guiding
approximation
weakly guiding approximation : refractive index difference Δ << 1
Type of modes: TE νm TM νm HE νm EH νm , however, at this approximation,
the propagation constants for some modes are almost identical :
- HE ν+1,m and EH ν−1,m are degenerated
- TE 0,m , TM 0,m , HE 2m are degenerated
The superposition of these degenerating
modes corresponds to particular Linearly
Polarized (LP) modes regardless of their HE, EH, TE or TM
Relationship between traditional exact modes and LP l m mode:
V- Number
HE11 has no cut off, but we have single-mode condition which is:
2.5.1 Mode-Field Diameter (MFD)
�HE11 has no cut off, but we have single-mode condition which is:
�MFD specifies the transverse extent of the fundamental modal field. For typical
Single mode fiber, the modal field extends far into the cladding.
For a Gaussian distribution, the MFD is given by the 1/e2 width of the optical power
E ( r ) = E ex p( r /W )
UNIT- 2
What are the basic attenuation mechanisms in the optical fiber communication? Explain in brief on
what factors this mechanism depends?
Ans: Attenuation
When a decrease in light power occurs during light propagation along an optical fiber then such a
phenomenon is called attenuation. The major causes for attenuation in fiber optic communications are,
Bending loss
Scattering loss
Absorption loss
1. Bending Loss
Bending loss is further classified into,
(i) Macro bending loss-and
(ii) Micro bending loss.
(i) Macro bending Loss
The light travels in fiber due to occurrence of total internal reflection inside the fiber at the interface of core
and cladding. However the light beam forms a critical angle with the fiber's central axis at the fiber face.
When the fiber is bend and the light beam travelling through fiber strikes at the boundary o f core at an angle
greater than critical angle then the beam fails to achieve total internal reflection. Hence this beam is lost
through the cladding.
(ii) Micro bending Loss
Micro bending loss is caused by micro-deformations of the fiber axis. The beam which travels at the critical
propagation angle before incident on micro-deformations will change the angle of propagation after being
reflected by the imperfection of fiber and hence the condition for total internal reflection is lost and the beam
escapes from the fiber through cladding.
2. Scattering Loss
A light beam propagating through the fiber core at critical angle or less will change its direction after hitting
on an obstacle in the core region. The obstacle can be any particle in core that may have diffused inside the
core at the time of manufacturing when the light beam hits the particle it
get scattered and due to this total internal reflection is not achieved hence, the beam is lost through the
cladding.
3. Absorptions Loss
Whenever a beam of light photon having energy equal to energy band gap then the light photon is absorbed
by the material resulting in absorption loss. Absorption loss occur due to presence of anions OH~ in silica
fibers and due to metallic ions like Iron (Fe), Chromium (Cr) and Nickel (Ni). The absorption loss peak is
observed in the region of 2700 nm and 4200 nm wavelength with low-loss at 7200 nm, 9500 nm and 13800
nm wavelength windows.
Explain in detail about ultra sonic absorption, infrared absorption and ion resonance absorption
losses in the pure and doped SiO2 at various levels?
Ans: An absolutely pure silicate glass has little intrinsic absorption due to its, basic material structure in the
near infrared region. However it does have two major intrinsic absorption mechanisms at optical
wavelengths as illustrated in the following figure which shows a possible optical attenuation against
wavelength characteristic for absolutely pure glass (i.e., SiO2). There is a fundamental absorption edge, the
peaks of which are centered in the ultraviolet wavelength region. This is due to the stimulation of electrons
transitions within the glass by higher energy excitation. The tail of this peak may extend into the window
region at the shorter wavelengths. Also in the infrared and far-infrared, normally at wavelengths above 7μm.
Absorption bands from the interaction of photons with molecular variations within the glass occur. These
give absorption peaks which again extend into the window region. Hence, above 1.5μm, the tails of these
largely far-infrared absorption peaks tend to increase the pure glass losses.
In practical optical fibers prepared by conventional melting techniques, a major source of signal attenuation
is extrinsic (doped) absorption from transition metal element impurities. Certain impurities, namely
Chromium and Copper, in their worst valence state can cause attenuation is excess of 1 dB/km in the near
infrared region. Transition element contamination may be reduced to acceptable levels i.e., one part is 1010
by glass refining techniques such as vapor -phase oxidation. It may also be observed that the only significant
absorption band in the region below a wavelength of 1urn is the second overtone at 0.95 am which causes
attenuation of about 1 dB/km for one part per million (ppm) of hydroxyl. At longer wavelengths the first
overtone at 1.38 µrn and its side band at 1.24 am are strong absorbers giving attenuation of about 2 dB/km
ppm and 4 dB/km ppm
Since most resonances sharply peaked, narrow window exist in the longer wavelength region around 1.3 and
1.55μm which are essentially unaffected by OH absorption, once the impurity, level has been reduced below
one part in 107. This situation is illustrated in. figure (b) which shows the attenuation spectrum of an ultralow-loss single mode fiber. It may be observed that the lowest attenuation for this fiber occurs at a
wavelength of 1.55μm and is 0.2dB/km. This approaching is the minimum possible attenuation of around
0.18 dB/km at this wavelength.
Explain in detail about signal distortion and attenuation in optical fiber?
Ans: Signal Distortion in Optical Fibers
One of the important property of optical fiber is signal attenuation. It is also known as fiber loss or signal
loss. The signal attenuation of fiber determines the maximum distance between transmitter and receiver. The
attenuation also determines the number of repeaters required, maintaining repeater is a costly affair.
Another important property of optical fiber is distortion mechanism. As the signal pulse travels along the
fiber length it becomes broader. After sufficient length the broad pulses starts over lapping with adjacent
pulses. This creates error in the receiver. Hence the distortion limits the information carrying capacity of
fiber
Attenuation
Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses
propagate through the length of the fiber.
In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses.
Absorption is because of fiber material and scattering due to structural imperfections within the fiber. Nearly
90% of total attenuation is caused by Rayleigh scattering only. Micro bending of optical fiber also
contributes to the attenuation of signal.
Attenuation Units As attenuation leads to a loss of power along the fiber, the output power is significantly
less than the coupled power. Let the coupled optical power is P (0) i.e. at origin (z = 0) Then the power at
distance z is given by
This parameter is known as fiber loss or fiber attenuation. Attenuation is also a function of wavelength.
Optical fiber wavelength as a function of wavelength is shown in fig 5.1
Explain the following
(i) Mode field diameter
(ii) Modal Birefringence
Ans: Mode field diameter:
Mode field diameter is a primary parameter of single-mode fibers. It is obtained from the mode field
distribution of the fundamental mode.
The figure shows, the distribution of light in a single mode fiber.
In order to find the MFD for field intensity E2(r) must be calculated by using E2(r) MFD can be calculated
as,
MFD = 2ω0 = 2
Where 2ω0 = spot size
To avoid complexity, E(r) can be taken as,
E(r) = E (0) exp (r2/ ω02)
Where r= radius
E (0) = field at (r=0)
By using this relation, we can write
MFD = 1/e2 width of optical power.
(ii) Modal Birefringence
The propagation of two approximately degenerate modes with orthogonal polarizations is allowed in single
mode fibers with nominal circular symmetry about the core axis. Thus, these are referred as bimodal
supported and modes. Here, the super scripts x and y denotes the principle axes and are calculated using the
symmetry elements of the fiber cross section. The difference in the effective refractive indices and phase
velocities for these orthogonally polarized modes makes the fiber to function as a birefringent medium. The
independency of fiber cross section with the fiber length in the z-direction yields the expression for modal
birefringence BF as,
Where,
βx= Propagation constant for the mode‘x’
βy= Propagation constant for the mode 'y'λ = Optical wavelength.
The difference in phase velocities is responsible for linear retardationΦ (z) exhibited by the fiber. The
expression for linear retardation is given by,
Φ (z) = (βx – βy) L
Where,
L = Length of the fiber.
If the coherence time of the source is greater than the delay between the two transit times then only, the
phase coherence of the two mode components is achieved. However, the expression for coherence time of
the source is given by,
tc = 1/ δf
Where, δf = Uncorrelated source frequency width
Then, the length of fiber over which birefringent coherence is maintained is given by
Where,
c = Velocity of light in vacuum, δλ = Source line width
Figure 6.1 illustrates the variations of polarization state periodically along the fiber
The characteristic length LB corresponding to the above process is referred as beat length and is given by,
Based on the above observation of beat length, we can determine the modal birefringence BF.
Commonly available single mode fiber have beat length in the range 10cm<L P<2m .What rate of
refractive index difference does this corresponds to for λ =1300nm?
Ans:
Give that
For a single mode fiber,
Beat length LP = 10cm to 2cm
Operating wavelength λ = 1300nm
The refractive index difference is known as birefringence and is denoted by βf.
βf.= 2П/Lp
Case 1
For Lp =10cm
βf= 2П/10,
βf =62.83 m-1
Case 2
For Lp =2cm
βf=2П/2, βf= 3.14 m-1
Therefore, the range of refractive index differences is 3.14m-1<βf<62.83m-1
A 10 km length of fiber is 100 μW and the average output power is 25 (J.W. Calculate,
(i) The signal attenuation in dB through the fiber. It is assumed that there are no connectors or
splices
(ii) Signal attenuation per km of the fiber
(iii) Overall signal attenuation for the 11 km optical link using the same fiber with 3 splices, each
having an attenuation of 0.8 dB
(iv) Numerical value of the ratio between input and output power.
Ans:
Given that
L = 10Km
Pinput =100μm
Poutput = 25μm
(i) Attenuation
(αdB) = 10 log10 (Pinput/ Poutput)
αdB= 10 log10
αdB= 6.02dB.
(ii) The signal attenuation per Km of the fiber is,
αdB.L =6.02 αdB= 6.02/10
αdB =0.602dBKm-1
A 10 km length of fiber is 100 μW and the average output power is 25 (J.W. Calculate,
(i) The signal attenuation in dB through the fiber. It is assumed that there are no connectors or splices
(ii) Signal attenuation per km of the fiber
(iii) Overall signal attenuation for the 11 km optical link using the same fiber with 3 splices,
having an attenuation of 0.8 dB
(iv) Numerical value of the ratio between input and output power.
Ans:
Given L=10
Pinput = 100μm
Poutput =25μm
(i) Attenuation
( α) dB =10 log10 (Pinput/Poutput)
α(dB) = 10 log10(100*10-6/25*10-6)
α (dB) =6.02 dB
(ii) The signal Attenuation per Km of the fiber is
α (dB) .L =6.02
α (dB) = 6.02/10 dBKm-1
= 0.602 dBKm-1
(iii) Attenuation per unit length α (dB)
The loss produced along 11Km of the fiber is,
α (dB) .L = 0.602*11(Km* dBKm-1)= 6.622dB
The number of splices are 3, each having attenuation of 0.8 dB
Therefore Total loss due t splices is 0.8*3 =2.4
Therefore Total signal attenuation = 6.622 dB + 2.4dB
α (dB) = 9.022dB
each
(iv) Numerical values of the ratio between input and output power is,
A graded index fiber with a parabolic refractive index profile core has a refractive index at the core axis
of 1.5 and a relative index difference of 1%. Estimate the maximum possible core diameter which allows
single mode operation at a wave length of 1.3μm?
Ans: Given that,
For a graded index fiber with parabolic refractive index profile,
Refractive index of core is n1=1.5.
Relative index difference, = 1% = 0.01
Operating wave length, λ =1.3μm
Maximum possible core diameter = 2a =?
Where a is the radius of the core.
Therefore the maximum possible diameter of the of the core is given by,
2amax = 4.692μm
Write notes on broadening of pulse in the fiber dispersion?
Ans: The dispersion of the transmitted optical signal causes distortion for both digital and analog transmission
along optical fibers. If we consider the major implementation of optical fiber transmission which involves some
form of digital modulation, then the dispersion technique within the fiber causes broadening of the transmitted
light pulses as they travel along the channel. This phenomenon is depicted in figure (a), where it may be
observed that each pulse broadens and coincides with its neighbors, eventually becoming indistinguishable at
the receiver input
The effect of overlapping of pulses shown in figure (a)' is called Inter Symbol Interference
(ISI). Thus, ISI
becomes more pronounced when increasing numbers of errors are encountered on the digital optical channel
For no overlapping of pulses down on an optical fiber link, the digital bit rate BT must be less than the
reciprocal of the broadened pulse duration through dispersion (2τ) and hence,
BT≤ 1/2τ…………….. (1)
Equation (1) assumes that the pulse broadening due to dispersion on the channel is T which follows the input
pulse duration which is also τ.
Another more accurate estimate of the maximum bit rate for an optical channel with dispersion may be obtained
by considering the light pulses at the output to have a Gaussian shape with an r.m.s. width of τ.
Explain group delay or time delay in fiber optics?
Ans: Modulating signal of an optical source enhances all the modes of fiber equally. This results in carrying of
equal amount of energy by each and every mode of fiber. Since each mode contains all the spectral components
in the wavelength band over which the source emits, hence the modulating signal modulates every spectral
component equally. The signals propagating through these spectral components experience a time delay or
group delay per unit length in the direction of propagation and it is given as,
Here D = Distance travelled by the pulse
β = Propagation constant along fiber axis
k = 2П/λ
V = Velocity with which the energy in a pulse travels along a fiber
From equation (2) we can say that group delay is a function of wavelength 'λ', therefore each spectral
component of any particular mode takes different time to travel a particular distance.
This causes difference in time delays and spreading of pulse with time as it travels along the fiber.
The variations in group delay causes pulse spreading. If the spectral width of the optical source is quite wide,
then the delay difference per unit wavelength over the propagation path is given as
The total delay 8x over a distance 'D' for the spectral components which are δλ apart and
δλ/2 above and below a central wavelength 'λc’ is,
A multi mode graded index fiber exhibits the pulse broadening of 0.2μs over a distance of
15Km.Estimate,
(i) Optimum bandwidth of the fiber
(ii) Dispersion per unit length
(iii) Band width length product
Ans:
Given that,
For a multimode graded index fiber,
Total pulse boarding, τ = 0.2μs
Distance, L =15km
(i) The maximum possible optical bandwidth is equivalent to the maximum possible bit rate assuming no inter
symbol interference (ISI) and is given by
Bopt =BT= 1/2τ
= 1/ (2*0.2*10-6)
= 2.5 MHz
Therefore Bopt = 2.5MHz
(ii) The dispersion per unit length may be acquired by dividing the total dispersion by total length of the fiber
i.e,
Dispersion per unit length = Total dispersion / Total length of fiber
=τ/L
= (0.2*10-6)/15 =13.33ns Km-1
Therefore Dispersion per unit length =13.33ns Km-1
(iii) The band width length product may be obtained by simple multiplying the maximum band width for the
link by its length as,
Bopt .L= 2.5 *106 *15
= 37.5 MHz Km
Alternately, the band width product may be obtained from the dispersion per unit length as,
Bopt .L = 1/ (2*Dispersion per unit length
= 1/ (2*1.33*10-9)
= 37.5MHz.Km
Compare the optical parameters of free space with dispersive and non dispersive mediums?
Ans: The basic optical properties of light are,
(i) Propagation: Light travels along a straight line in a uniform median.
(ii) Reflection: It occurs at the surface or boundary of a medium.
(iii) Refraction (or bending): It may occur where a change of speed is experienced.
(iv) Interference: It is found where two waves are superposed.
(v) Diffraction: It happens around a corner when a wave passes the edge of one obstacle.
Dispersive Medium
Glass is an example of dispersive medium. Non uniform bending of different wavelength of light when it travels
along the medium is called dispersion. And the medium in which dispersion takes place is called dispersive
medium.
For a uniform dispersive medium the light wave travels along a straight line. If the medium is non-uniform and
if its density increases as the light wave travels, then the light waves tends to bend itself towards the normal and
vice-versa. Hence, refraction of light waves takes place in dispersive non uniform medium. Also, in dispersive
medium the group velocity and phase velocity is not constant as a result of interference of different wave fronts
travelling at an angle to each other. Hence refraction takes place. If a plane wave hits an obstacle placed
perpendicular to the direction of its propagation with a pin hole in the centre then the plane wave front is
changed into spherical wave front due to diffractions of light wave through the pin hole
Non-dispersive Medium
Vacuum is an example of non-dispersive medium. In vacuum the light wave travels in straight line. The group
velocity as well as phase velocity of the light wave ii constant. Hence no refraction takes place in phase
vacuum. As the waves travel in straight lines and are parallel to each other no interference takes place vacuum
is free of obstacles. Hence, no diffraction of light wave takes place.
Derive the expression for the phase velocity, group velocity using electric field distribution along the
fiber?
Ans: Phase velocity: All electromagnetic waves which travel along a waveguide have points of constant phase.
As a monochromatic light wave propagates along a waveguide in z-direction, this point of constant phase travel
at particular velocity termed as phase velocity. It is denoted by
Where ω = angular frequency of the wave and
β = Phase propagation constant
Group Velocity:
Group of waves with closely similar frequencies propagate along the waveguide so that there exists a resultant
in the form of packet of waves. This wave packet moves at a velocity termed as group velocity
The formation of wave packets from combination of waves of nearly equal frequencies is,
Propagation constant can be given as,
β = n1(2П/λ) = (n1ω)/c
where n1= Refractive index of medium
Phase velocity can be given as
Write a short notes on dispersion shifted fiber and dispersion compensating fiber?
Ans: Dispersion Shifted Fiber
Single mode fibers which are designed to offer simultaneously zero dispersion and minimum attenuation at λ =
1.55μ m is called dispersion shifted fibers. The dispersion classifications of various fibers are shown in figure
8.1, which depicts the shifting of zero dispersion wavelength from λ = 1.33 um to λ= 155 mm. This can be
achieved by changing the fiber parameters, namely, the refractive index dispersion shifted fiber.
For example, by reducing the fiber core diameter from 8-10μm to 4.5μm and increasing the refractive index
difference between core and cladding from 0.003 to greater than 0.01 yields zero dispersion wavelength shifted
from 1.33μm to 1.55μm. This may lead to substantial excess loss. Triangular core profile also yields dispersion
shifted fibers and moreover it solves the above excess loss problem. So, for better results we have to modify the
triangular profile. These Profiles are shown in below figure
The above figure shows that the convex index profile also gives the dispersion shifted fiber.
Dispersion shifted fibers have the advantage of increased guiding strength, increase in the cut-off wavelength of
second order mode and better resistance to bending losses Such dispersion shifted fibers have been produced by
BTRL and others and are now commercially available from any glass company.
Table (1) compares the characteristics of triangular refractive index profile dispersion shifted fiber with that of
simple step index fiber.
Step-index Fiber
Dispersion shifted fiber(Triangle RI)
Attenuation
0.200-0.22 (dB/Km)
Dispersion
17-180.1 ps(km-nm)
Table (1): Fiber parameters at 1.55 mm
Dispersion Compensating Fiber
The process of dispersion compensation and the fiber loop is referred as dispersion compensating fiber. A large
base of dispersion shifted fiber has been installed throughout the world for use in the single wavelength
transmission systems. For these kinds of links the complexity arises from Four Wave Mixing (FWM), when one
attempt to upgrade them with high speed dense WDM technology in which the channel spacing is less than 100
GHz and the bit rates are in excess of 2.5 Gb/s. By using the passive dispersion compensation technique we can
reduce the effect of FWM (four wave mixing). This consists of inserting into the link a loop of fiber having a
dispersion characteristic that negates the accumulated dispersion of the transmission fiber. This process is
known as dispersion compensation. If the transmission fiber has a low positive dispersion, the dispersion
compensating fiber will have a large negative dispersion. By using this technique, the total accumulated
dispersion will become zero after some distance, but the absolute dispersion per length is non-zero at all points
along the fiber.
Figure depicts the Dispersion Compensating Fiber (DCF) which can be inserted at either the starting (or) the
end of an installed fiber span between two optical amplifiers. A third option is to have DCF (Dispersion
Compensating Fiber) at both ends.
The above figure shows that the convex index profile also gives the dispersion shifted fiber.
Dispersion shifted fibers have the advantage of increased guiding strength, increase in the cut-off wavelength of
second order mode and better resistance to bending losses.
In pre-compensation schemes, the DCF is located just after the optical amplifier and just before the transmission
fiber. Where as in post compensation schemes, the DCF is located just after the transmission fiber and just
before the optical amplifier. Above Figure also depicts the plots of accumulated dispersion and optical power
level as functions of distance along the fiber. This figure is known as dispersion and power maps respectively.
Material Loss
(a) Due to impurities: The material loss is due to the impurities present in glass used for
making
fibers. Inspite of best purification efforts, there are always impurities like Fe, Ni, Co, Al which are
present in the fiber material. The Fig. shows attenuation due to various molecules inside glass as a
function of wavelength. It can be noted from the figure that the material loss due to impurities reduces
substantially beyond about 1200nm wavelength.
(b) Due to OH molecule: In addition, the OH molecule diffuses in the material and causes absorption of
light. The OH molecule has main absorption peak somewhere in the deep infra-red wavelength region.
However, it shows substantial loss in the range of 1000 to 2000nm
(c) Due to infra-red absorption : Glass intrinsically is a good infra-red absorber. As we increase the
wavelength the infra-red loss increases rapidly.
SCATTERING LOSS
The scattering loss is due to the non-uniformity of the refractive index inside the core of the fiber. The refractive
index of an optical fiber has fluctuation of the order of 10
4
over spatial scales much smaller than the optical
wavelength. These fluctuations act as scattering centers for the light passing through the fiber. The process is,
Rayleigh scattering. A very tiny fraction of light gets scattered and therefore contributes to the loss.
The Rayleigh scattering is a very strong function of the wavelength. The scattering loss varies as 4 . This loss
therefore rapidly reduces as the wavelength increases. For each doubling of the wavelength, the scattering loss
reduces by a factor of 16. It is then clear that the scattering loss at 1550nm is about factor of 16 lower than that
at 800nm. The following Fig. shows the infrared, scattering and the total loss as a function of wavelength.
It is interesting to see that in the presence of various losses, there is a natural window in the optical spectrum
where the loss is as low as 0.2-0.3dB/Km. This window is from 1200nm to 1600nm.
There is a local attenuation peak around 1400nm which is due to OH absorption. The low-loss window
therefore is divided into sub-windows, one around 1300nm and other around 1550nm. In fact these are the
windows which are the II and III generation windows of optical communication.
MICRO-BENDING LOSSES
While commissioning the optical fiber is subjected to micro-bending as shown in Fig.
The analysis of micro-bends is a rather complex task. However, just for basic understanding of how the loss
takes place due to micro-bending, we use following arguments. In a fiber without micro-bends the light is
guided by total internal reflection (ITR) at the core-cladding boundary. The rays which are guided inside the
fiber has incident angle greater than the critical angle at the core-cladding interface. In the presence of microbends however, the direction of the local normal to the core-cladding interface deviates and therefore the rays
may not have angle of incidence greater than the critical angle and consequently will be leaked out.
A part of the propagating optical energy therefore leaks out due to micro-bends.
Depending upon the roughness of the surface through which the fiber passes, the micro-bending loss varies.
Typically the micro-bends increase the fiber loss by 0.1-0.2 dB/Km.
RADIATION OR BENDING LOSS
While laying the fiber the fiber may undergo a slow bend. In micro-bend the bending is on micron scale,
whereas in a slow bend the bending is on cm scale. A typical example of a slow bend is a formation of optical
fiber loop.
The loss mechanism due to bending loss can be well understood using modal propagation model.
As we have seen, the light inside a fiber propagates in the form of modes. The modal fields decay inside the
cladding away from the core cladding interface. Theoretically the field in the cladding is finite no matter how
far away we are from the core-cladding interface. Now look at the amplitude and phase distribution for the
fibers which are straight and which are bent over an circular arc as shown in Fig.
Phase Fronts in a Straight Fiber
Cladding
Core
Field Amplitude
Phase fronts
It can be noted that for the straight the phase fronts are parallel and each point on the phase front travels with
the same phase velocity.
Phase Fronts for a Bent Fiber
Phase Fronts
However, as soon the fiber is bent (no matter how gently) the phase fronts are no more parallel. The phase
fronts move like a fan pivoted to the center of curvature of the bent fiber (see Fig.). Every point on the phase
front consequently does not move with same velocity. The velocity increases as we move radically outwards
the velocity of the phase front increases. Very quickly we reach to a distance xc from the fiber where the
velocity tries to become greater than the velocity of light in the cladding medium.
since the velocity of energy cannot be greater than velocity of light, the energy associated with the modal
field beyond xc gets detached from the mode and radiates away. This is called the bending or the radiation
loss.
Following important things can be noted about the bending loss.
1. The radiation loss is present in every bent fiber no matter how gentle the bend is.
2. Radiation loss depends upon how much is the energy beyond xc .
3. For a given modal field distribution if xc reduces, the radiation loss increases. The xc reduces as the
radius of curvature of the bent fiber reduces, that is the fiber is sharply bent.
4. The number of modes therefore reduces in a multimode fiber in presence of bends.
UNIT 3
Explain about fiber optic connectors and types of connectors in detail?
Ans: Fiber Optic Connectors:
Connectors are mechanisms or techniques used to join an optical fiber to another fiber or to a fiber optic
component. Different connectors with different characteristics, advantages and disadvantages and
performance parameters are available. Suitable connector is chosen as per the requirement and cost.
Various fiber optic connectors from different manufacturers are available SMA 906, ST, Biconic, FC, D4,
HMS-10, SC, FDDI, ESCON, EC/RACE,
Principles of good connector design
1. Low coupling loss.
5. Low cost.
2. Inter-changeability.
6. Reliable operation.
3. Ease of assembly.
7. Ease of connection.
4. Low environmental sensitivity.
Connector Types Connectors use variety of techniques for coupling such as screw on, bayonet-mount,
push-pull configurations, butt joint and expanded beam fiber connectors.
Butt Joint Connectors
Fiber is epoxies into precision hole and ferrules arc used for each fiber. The fibers are secured in a
precision alignment sleeve. Butt joints are used for single mode as well as for multimode fiber systems.
Two commonly used butt-joint alignment designs are:
1. Straight-Sleeve.
2. Tapered-Sleeve/Bi conical.
In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of
two fibers. Below Fig. shows straight sleeve alignment mechanism of fiber optic connectors
In tapered sleeve or bi conical connector mechanism, a tapered sleeve is used to accommodate tapered
ferrules. The fiber end separations are determined by sleeve length and guide rings. The below figure
shows tapered sleeve fiber connectors
A multi mode graded index fiber exhibits the pulse broadening of 0.2μs over a distance of
15Km.Estimate,
(i) Optimum bandwidth of the fiber
(ii) Dispersion per unit length
(iii) Band width length product
Ans:
Given that,
For a multimode graded index fiber,
Total pulse boarding, τ = 0.2μs
Distance, L =15km
(i) The maximum possible optical bandwidth is equivalent to the maximum possible bit rate assuming no
inter symbol interference (ISI) and is given by
Bopt =BT= 1/2τ
4.1/ (2*0.2*10-6)
5.2.5 MHz
Therefore Bopt = 2.5MHz
(ii) The dispersion per unit length may be acquired by dividing the total dispersion by total length of the
fiber i.e,
Dispersion per unit length = Total dispersion / Total length of fiber
= τ/L
= (0.2*10-6)/15
=13.33ns
Km-1
Therefore Dispersion per unit length =13.33ns Km-1
(iii) The band width length product may be obtained by simple multiplying the maximum band width for
the link by its length as,
Bopt .L= 2.5 *106 *15
= 37.5 MHz Km
Alternately, the band width product may be obtained from the dispersion per unit length as,
Bopt .L = 1/ (2*Dispersion per unit length
= 1/ (2*1.33*10-9)
How to connect two fibers in a low manner? Explain?
Ans: Interconnection of Two Fibers in a Low Loss Manner
The major factor in any fiber optic system is the requirement to interconnect fibers in a low loss manner.
These interconnections occur in three stages namely.
1. At the optical source
2. At the photo detector
3. At intermediate points.
1. Optical Sources
The optical sources such as Light Emitting Diodes (LEDs), Solid state lasers and semiconductor
injection lasers are used because of their efficiency, low cost, longer life, sufficient power output,
compatibility and ability to give desired modulations.
2. Photo Detectors
Photo detectors such as semiconductor photodiodes are used because of their high quantum efficiency,
adequate frequency response, low dark current and low signal impedance.
3. Intermediate Points
The two fibers are joined at intermediate points with two cables within a cable.
The two major methods for the interconnection of fibers in a low loss manner are as follows,
(i) Fiber Splices
(ii) Simple Connectors.
(i) Fiber splices
In this, the fiber splices are the semi permanent (or) permanent joints which are mostly used for
interconnection in optic-telecommunication system.
(ii) Simple Connectors
Simple connectors are the removable joints which allow easy, fast manual coupling of fibers.
We can say that losses in interconnection of two fibers depend on factors like input power
distribution to joints, length of fiber between optical source and joint, wave characteristics of two fibers
at joint and fiber end face qualities.
If these factors are satisfied low-loss in the interconnection of two fibers is achieved.
Explain about losses in end separation, connecting different fibers when joining two fibers?
Ans: When an optical fiber communication link is established, interconnections occur at the optical source,
at the photo detector, at intermediate points within a cable where two fibers are joined and at intermediate
points in a link where two cables are connected. If the interconnection is permanent bound then it is
generally referred to as splicing whereas a demountable joint is known as connector. At every joint optical
power loss takes place depending on input power distribution to the joint, the length of the fiber between the
optical source and the joint, the geometrical and waveguide characteristics of the two fiber ends at the joint
and the fiber end face qualities. These losses are classified into (i) Intrinsic losses (ii) Extrinsic losses and
(iii) Reflection loss.
3.
Intrinsic Losses
Intrinsic losses occur when a mismatch occurs between two connecting fibers. Mismatch occurs
when fiber's mechanical dimensions are out of tolerance limit. The mismatch can occur due to the
following.
(a) Core-Diameter Mismatch
If the core of two joining, fibers has different diameter then core-diameter mismatch occurs.
The loss will be more if the light is travelling from larger core into a smaller core than if it is in
reverse direction.
For a gradient multi mode fiber the loss due to core-diameter mismatch is given by,
(b) Numerical Aperture Mismatch
The light beam from emitting fiber fills the entire exit aperture of the emitting fiber. The
receiving fiber has to accept all the optical power emitted by the first fiber. If there is a mismatch in
waveguide characteristics of the two fibers resulting in smaller NA per second fiber, then it results in
optical power loss. This loss is called numerical aperture mismatch loss given by,
Mode-Field-Diameter (MFD) Mismatch or Refractive Index Profile (α) Mismatch:
This loss takes place only in graded-index fiber where the index profile emitting fiber is different
From the index profile of receiving fiber
(ii) Extrinsic Losses
Extrinsic losses occur due to mechanical misalignment at point of joints. They are,
(a) Lateral Misalignment
This misalignment occurs when the, fibers are displaced along the face of fiber and hence the core
overlapping area is reduced from circular to elliptical form hence power loss from emitting fiber to the
receiving is given below,
b) Angular Misalignment
For a perfectly matched fiber, if point of joint at which core axis of fiber 1 is at an
angle with the
core axis of fiber 2 then angular misalignment occurs and the result is same as due to numerical aperture
mismatch.
(c) End separation Misalignment
When two fibers are separated longitudinally by a gap of ‘S’ between them, then longitudinal end separation
misalignment occurs.
(iii) Reflection Loss
At the surface of contact some light will be reflected back. This is called Fresnel reflection. This
reflection changes the amount of power transmitted towards a receiver. The loss caused by reflection is
called Fresnel loss. If the transmitted power is Ptran and input power at the source is Pin and reflected power
is Pref then they are related by,
Hence reflection loss is given by,
Ptrans =Pin –Pref
Hence reflection loss is given by,
Where R = R l+ R 2 - 2 cos(4П/λ) S = The separation between two fiber. R = Total reflection. R1 and R2 are
reflections at two interfaces
Draw the schematic of edge emitting double hetero junction LED and explain its working in
detail?
Ans: Double Hetero junction Laser
If a single p-n junction diode is fabricated from suitable single crystal semiconductor material it
exhibits photo emissive properties. It is known as homo junction' p-n diode. However the emissive
properties of a junction diode can be improved considerably by the use of 'hetero junction'. A hetero
junction is an interface between two adjoining crystal semiconductors having different values of band
gap energies. Devices are fabricated with hetero junction are said to have hetero structures.
They are of two types,
6. Isotopes such as n-n or p-p type
7. Anisotropy such as p-n type.
The isotope p-p junction has a potential barrier within the structure. The structure is capable of confining
min carriers to small active region called cavity. It effectively reduces the diffusion length of the carrier
and thus the volume of the structure where radioactive recombination may occur.
Figures show the schematic layer structure, energy band diagram and refractive index profile, for
a double hetero junction injection laser diode with biasing. The laser oscillations take place in the
central p-type GaAs region which is known as active layer.
There is hetero junction at the both sides of the active layer. A forward bias voltage is applied by
connecting the positive electrode of the power supply voltage to the P-side of the structure and
negative electrode to the n-side when a voltage which is almost equal to the band gap energy. The
hetero junctions are used to provide potential barrier in the injection laser. In this structure it is
possible to obtain both carrier and optical containment to the active layer.
Broad Area Double Heterojunction Laser (DH Laser)
The above figure represents the layer structure of a broad area DH. The GaAs layers acts as active
layer which is sandwiched between p-type AZGaAs and n-A/GaAs layer and these two layers act
as the confinement layers. Light is emitted from the central GaAs active layer through the front
and back side of the device.
In the case of the DH broad area laser structure, the optical confinement in the vertical direction is
achieved by the refractive index at the hetero junction interfaces between the active layer and the
containment layers, but the laser .action takes places across the whole width of the device. In a
broad area laser the sides of the cavity are formed by simple roughening the ends of the device to
reduce the unwanted emission and limit the horizontal transverse modes.
Stripe Geometry Laser
In order to overcome the difficulties in a broad area laser structure, the stripe geometry laser
structure has developed and in this structure the active area does not enter upto the edges of the
device.
A common method is used to introduce the stripe geometry to the structure which provides the
optical contaminant in the horizontal plane as shown in the figure above. The stripe geometry is
usually formed by creating a high resistance area on either side of the stripe by 'Proton
bombardment' technique or by oxide oscillation. The stripe usually acts as a guiding mechanism
which avoids all major difficulties encountered in the case of a broad area laser. The contact stripe
provides the balance of guiding single transverse mode operation in a direction parallel to the
junction plane, whereas broad area devices allow multiple mode operation in this horizontal plane.
The width of the stripe generally ranges from 2.0 to 65 mm and stripe laser find wide application
in fiber communications.
7. What is population inversion and how it can be achieved? And discuss the requirements of
population inversion in order that stimulated emission may dominate over spontaneous
emission. Illustrate your answer with energy level diagram of laser?
Ans: Population Inversion
The lifetime of an atom in excited state is of the order of 10~8 seconds. So. before an excited atom
can be stimulated to emit a photon, it is most likely to make a spontaneous emission. The photons
emitted by a spontaneous emission are not coherent.
The ratio of the number n' of excited atoms to that of n in the ground state is given by the
Boltzmann's equation.
n’/n =e-W/kT
W = Energy difference between excited state and ground state;
K = Boltzmann constant
T = Kelvin temperature.
Consider three level system in which three active energy levels E1 , E2 and E3 are present and
population in those energy levels are N, $1 and 7V3 respectively. In normal conditions E 1< E2 <
E3 and N1> N2 >N 3
E1 is the ground state, its lifetime is unlimited.E3 is highest energy state, its lifetime is very less
and it is the most unstable state. E. is in excited state and has more life time. Hence E2 is a meta
stable state. When suitable form of energy is supplied to the system in a suitable way, then the
atoms excite from ground state (E1) to excited states (E2 and E3). Due to un stability, Excited
atoms will come back to ground state after the Life time of the respective energy states E2 and E3
If this process is continued then atoms will excite continuously to E2 and E3
Because E3 is the most unstable state, atoms will fall into E2 immediately. At some stage the
population in E2, will become more than the population in ground state. This situation is called
population inversion and is shown in figure 4.11
There are several ways of pumping a laser and producing population inversion necessary for
stimulated emission to occur. Most commonly used methods are as follows
There are several ways of pumping a laser and producing population inversion necessary for
stimulated emission to occur. Most commonly used methods are as follows.
= Optical pumping
= Electric discharge
= Inelastic atom to atom collision
= Direct conversion
= Chemical reactions.
The emission process can occur in two ways
(i)
Spontaneous Emission
The electrons in the excited state E2 are unstable. They return back to the lower
energy state without any external influence. This process leads to spontaneous
emission.
(ii)
Stimulated Emission
When an external photon of energy which is equal to the energy difference between two
states (E2 – E1) hit the excited electron, this excited electron return to the ground state
(E1) by emitting a photon of energy hv12. This emission is known as stimulated emission.
Semiconductor laser diodes are preferred over LED for the optical fiber communication
systems requiring bandwidth greater than approximately 200 MHz.
Laser diodes have
(iii)
Response time less than 1 ns
(iv)
Optical bandwidth of 2nm
(v)
High coupling efficiency
For the efficient functioning of laser the method of exciting electrons in atoms is from,
i Lower energy levels to higher energy levels.
ii Lower energy levels to large population inversion high energy level.
The stimulated emission in semiconductor laser arises from optical transitions between distributions
of energy state in the valence and conduction bands. Stimulated emission is achieved in an intrinsic
semiconductor by the injection of electrons into the material. Figure represents the electron states for
an intrinsic direct band gap semiconductor at absolute zero.
The population when the conduction band contains no electrons are injected into the material, fill the
lower energy states in the conduction band gap upto the injection energy or quasi fermi level for
electrons.
Since the charge is neutrally conserved with the material, an equal density of holes is created in the
top of the valence band by the absence of electrons as shown in the figure 1(b). Since more electrons
are there in valence band than the conduction band population inversion is achieved.
Obtain the expression for the 3dB modulation bandwidth of LED and discuss the importance of
radiative recombination life time?
Ans: The expression for the 3 dB modulation bandwidth of LED in optical communication maybe
obtained in either electrical and optical terms. If we consider the associated electrical circuitry in an
optical fiber communication system to use the electrical definition, where the electrical signal power
has dropped to half of its constant value due to the modulated portion of the optical signal. Hence, this
corresponds to the electrical 3 dB frequency at which the output electrical power is reduced by 3 dB
with respect to the input electrical power. We can also consider the high frequency 3 dB point, when
the optical source operates down to D.C.
The expression for the electrical bandwidth can be obtained from the ratio of the electrical output
power to the electrical input power in decibels and is given as
REdB = 10log10(Electrical output power /Electrical input power)
= 10log10
= 10 log10
The electrical 3 dB points occur when the ratio of electrical powers shown in above expression is √2 Hence, it
follows that this must Occur when,
Thus this expression depicts that the bandwidth of the electrical regime may be defined by the frequency when
the output current has dropped to /√2 (or) 0.707 of the input current of the system.
Optical bandwidth can be obtained from the ratio of the optical power output to optical power input in decibels
ROdB is given by
ROdB = 10 log 10
= 10 log10
Hence, the optical 3 dB points occur when the currents is equal to 0.5,hence
Therefore In optical regime the bandwidth is defined by the frequency at which the output current has dropped
to 0.5 of the input current to the system.
The Modulation bandwidth of LED is generally determined by three methods , They are
1.
The doping level in the active layer,
2.
Due to the injected carriers, the reduction in radiative lifetime.
3.
The parasitic capacitance of the device.
If we assume that the parasitic capacitance is negligible, then the speed at which an LED can be
directly current modulated is fundamentally limited by the recombination lifetime of the carriers,
P
= Optical output power of the device
ω=Angular modulation frequency.
τ =Injected carrier lifetime in the recombination region
(iii)
D.C. optical output power for the same drive current
Discuss the major requirements of an optical source for use in optical communication systems?
Ans: The development of efficient semiconductor optical sources along with low-loss optical fibers, led to
substantial improvements in fiber optic communications. Semiconductor optical sources have the physical
characteristics and performance properties necessary for successful implementations of fiber optic systems.
It is desirable that optical sources must be,
I.
Compatible in size to low-loss optical fibers by having a small light emitting which are
capable of launching light into fiber.
II.
Launch sufficient optical power into the optical fiber to overcome fiber attenuation and
connection losses allowing for signal detection at the receiver.
III.
Emit light at wavelengths that minimize optical fiber loss and dispersion.
IV.
Optical sources should have a narrow spectral width to minimize dispersion.
V.
Allow for direct modulation of optical output power.
Maintain stable operation in changing environmental conditions (such as temperature). Cost less and
be more reliable than electrical devices, permitting fiber optic communication systems to compete
with conventional systems. Semiconductor optical sources suitable for fiber optic systems range
from inexpensive Light Emitting Diodes (LEDs) to more expensive semiconductor lasers.
Semiconductor LEDs and laser diodes are the principle light sources used in fiber optics.
Semiconductor sources are designed to operate at wavelengths (i.e., 850 nm, 1300 nm and 1500 nm)
that minimize optical fiber absorption and maximum system bandwidth. By designing an optical
source to operate at specific wavelengths, absorption from impurities in the optical fiber such as
hydroxyl ions (OH-) can be minimized. Maximizing system bandwidth involves designing fibers and
sources that minimize chromatic and inter modal dispersion at the intended operational wavelength.
Compare the advantageous and disadvantageous of LED and Explain the key process involved
in the LASER operation?
Simple Fabrication: There are no mirror facets and is some structures no striped geometry
Cost: The simpler construction of LED leas to much reduced cost which is always likely to be
maintained.
Reliability: The LED does not exhibit catastrophic degradation and has proved to be less sensitive to gradual
degradation than the injection laser.
Simpler Drive Circuitry: This is due to lower drive currents and reduced temperature dependence which
makes temperature compensation circuits unnecessary.
Linearity: Ideally, the LED has a linear light output against current characteristics unlike the injection laser.
Disadvantage
An LED radiates rather dispersed light, which makes coupling this light into an optical fiber a
problem.
The key processes involved in laser action are as given below.
(i) Absorption.
(ii) Spontaneous emission.
(iii) Stimulated emission.
These three key processes are represented by 2-energy level diagrams.
Where, E1= Energy of ground state.
E2 = Energy of excited state.
Absorption
When transition occurs between two states, then it involves the emission and absorption of energy in the form
of photon energy hvn = E2 – E 1
In the above figure we can see that electron in 'E1’ absorbs the photon energy and is excited to state
l
E2' when photon of energy hvn is incident on the system.
(ii)
Spontaneous Emission
Charge carriers are unstable in excited state so they try to come back in stable state and this is possible
by emission of radiation. This emission takes place when energy hvv is released. As it occurs without
any external stimulation, it is known as spontaneous emission.
(iii)
Stimulated Emission
Here in this type of emission when a photon of energy hv12 is striking on system while the electron is
still in its excited state, then the electron is stimulated so that it drops on to ground state and gives a
photon of energy hvn and the emitted photon will be in phase with incident photon. The resultant
emission is called as stimulated emission.
Light Emitting Diodes:INTRODUCTION
Over the past 25 years the light-emitting diode (LED) has grown from a laboratory curiosity to a broadly used
light source for signaling applications. In 1992 LED production reached a level of approximately 25 billion
chips, and $2. 5 billion worth of LED-based components were shipped to original equipment manufacturers.
This article covers light-emitting diodes from the basic light-generation processes to descriptions of LED
products. First, we will deal with light-generation mechanisms and light extraction. Four major types of device
structures—from simple grown or dif fused homojunctions to complex double heterojunction devices are
discussed next , followed by a description of the commercially important semiconductors used for LEDs , from
the pioneering GaAsP system to the AlGaInP system that is currently revolutionizing LED technology . Then
processes used to fabricate LED chips are explained—the growth of GaAs and GaP substrates; the major
techniques used for growing the epitaxial material in which the light-generation processes occur ; and the steps
required to create LED chips up to the point of assembly . Next the important topics of quality and reliability—
in particular, chip degradation and package-related failure mechanisms—will be addressed. Finally, LED-based
products, such as indicator lamps, numeric and alphanumeric displays, opto-couplers, fiber-optic transmitters,
and sensors, are described. This article covers the mainstream structures, materials, processes, and applications
in use today. It does not cover certain advanced structures, such as quantum well or strained layer devices. The
reader is also referred to for current information on edge-emitting LEDs, whose fabrication and use are similar
to lasers.
Schematic:
Theory:
A Light emitting diode (LED) is essentially a pn junction diode. When carriers are injected across a forwardbiased junction, it emits incoherent light. Most of the commercial LEDs are realized using a highly doped n and
a p Junction.
Figure 1: p-n+ Junction under Unbiased and biased conditions
To understand the principle, let’s consider an unbiased pn+ junction (Figure1 shows the pn+ energy band
diagram). The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n-side
to the Ec on the p-side, called the built-in voltage, V0. This potential barrier prevents the excess free electrons
on the n+ side from diffusing into the p side. When a Voltage V is applied across the junction, the built-in
potential is reduced from V0 to V0– V. This allows the electrons from the n+ side to get injected into the p-side.
Since electrons are the minority carriers in the p-side, this process is called minority carrier injection. But the
hole injection from the p side to n+ side is very less and so the current is primarily due to the flow of electrons
into the p-side. These electrons injected into the p-side recombine with the holes. This recombination results in
spontaneous emission of photons (light). This effect is called injection electroluminescence. These photons
should be allowed to escape from the device without being reabsorbed.
The recombination can be classified into the following two kinds
• Direct recombination
• Indirect recombination
Direct Recombination:
In direct band gap materials, the minimum energy of the conduction band lies directly above the maximum
energy of the valence band in momentum space energy. In this material, free electrons at the bottom of the
conduction band can recombine directly with free holes at the top of the valence band, as the momentum of the
two particles is the same. This transition from conduction band to valence band involves photon emission (takes
care of the principle of energy conservation). This is known as direct recombination. Direct recombination
occurs spontaneously GaAs is an example of a direct band-gap material.
Figure 2: Direct Bandgap and Direct Recombination
Indirect Recombination:
In the indirect band gap materials, the minimum energy in the conduction band is shifted by a k-vector relative
to the valence band. The k-vector difference represents a difference in momentum. Due to this difference in
momentum, the probability of direct electronhole recombination is less.
In these materials, additional dopants(impurities) are added which form very shallow donor states. These donor
states capture the free electrons locally; provides the necessary momentum shift for recombination. These donor
states serve as the recombination centers. This is called Indirect (non-radiative) Recombination.
Nitrogen serves as a recombination center in GaAsP. In this case it creates a donor state, when SiC is doped
with Al, it recombination takes place through an acceptor level. when SiC is doped with Al, it recombination
takes place through an acceptor level.
The indirect recombination should satisfy both conservation energy, and momentum. Thus besides a
photon emission, phonon emission or absorption has to take place. GaP is an example of an indirect
band-gap material.
Figure 3: Indirect Bandgap and NonRadiative recombination
The wavelength of the light emitted, and hence the color, depends on the band gap energy of the materials
forming the p-n junction.
The emitted photon energy is approximately equal to the band gap energy of the semiconductor.
The following equation relates the wavelength and the energy band gap.
hν = Eg
hc/λ = Eg λ
= hc/ Eg
Where h is Plank’s constant, c is the speed of the light and Eg is the energy band gap Thus, a semiconductor
with a 2 eV band-gap emits light at about 620 nm, in the red. A 3 eV band-gap material would emit at 414 nm,
in the violet.
LED Materials:
An important class of commercial LEDs that cover the visible spectrum are the III-V. ternary alloys based on
alloying GaAs and GaP which are denoted by GaAs1-yPy. InGaAlP is an example of a quarternary (four
elements) III-V alloy with a direct band gap. The LEDs realized using two differently doped semiconductors
that are the same material is called a homojunction. When they are realized using different band gap materials
they are called a heterostructure device. A heterostructure LED is brighter than a homoJunction LED.
LED Structure:
The LED structure plays a crucial role in emitting light from the LED surface. The LEDs are structured to
ensure most of the recombination’s takes place on the surface by the following two ways.
• By increasing the doping concentration of the substrate, so that additional free minority
charge carrier’s
electrons move to the top, recombine and emit light at the surface.
• By increasing the diffusion length L = √ Dτ, where D is the diffusion coefficient and τ is the carrier life time.
But when increased beyond a critical length there is a chance of re-absorption of the photons into the device.
The LED has to be structured so that the photons generated from the device are emitted without being
reabsorbed. One solution is to make the p layer on the top thin, enough to create a depletion layer. Following
picture shows the layered structure. There are different ways to structure the dome for efficient emitting.
LED structure
LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its
surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/ InGaN
also use sapphire substrate.
LED efficiency:
A very important metric of an LED is the external quantum efficiency ηext. It quantifies the efficiency of the
conversion of electrical energy into emitted optical energy. It is defined as the light output divided by the
electrical input power. It is also defined as the product of Internal radiative efficiency and Extraction efficiency.
ηext = Pout(optical) / IV
For indirect bandgap semiconductors ηext is generally less than 1%, where as for a direct band gap material it
could be substantial.
ηint = rate of radiation recombination/ Total recombination
The internal efficiency is a function of the quality of the material and the structure and composition of the layer.
Applications: LED have a lot of applications. Following are few examples.
• Devices, medical applications, clothing, toys
• Remote Controls (TVs, VCRs)
• Lighting
• Indicators and signs
• Opto isolators and opto couplers
• Swimming pool lighting
Optocoupler schematic showing LED and phototransistor
Advantages of using LEDs:
• LEDs produce more light per watt than incandescent bulbs; this is useful inbattery powered or energy-saving
devices.
• LEDs can emit light of an intended color without the use of color filters that traditional lighting methods
require. This is more efficient and can lower initial costs.
• The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often
require an external reflector to collect light and direct itin a usable manner.
• When used in applications where dimming is required, LEDs do not change their color tint as the current
passing through them is lowered, unlike incandescent lamps, which turn yellow.
• LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that
burn out more quickly when cycled frequently, or High Intensity Discharge (HID) lamps that require a long
time before restarting.
• LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and
incandescent bulbs are easily broken if dropped on the ground.
• LEDs can have a relatively long useful life. A Philips LUXEON k2 LED has a life time of about 50,000
hours, whereas Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at
1,000–2,000 hours.
• LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• LEDs light up very quickly. A typical red indicator LED will achieve full brightness in
Microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states "less than 100ns." LEDs
used in communications devices can have even faster response times.
• LEDs can be very small and are easily populated onto printed circuit boards.
• LEDs do not contain mercury, unlike compact fluorescent lamps.
Disadvantages:
• LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional
lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive
circuitry and power supplies needed. However, when considering the total cost of ownership (including energy
and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten the future
existence of compact fluorescent lamps.
• LED performance largely depends on the ambient temperature of the operating environment. Over-driving the
LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device
failure. Adequate heat-sinking is required to maintain long life .
• LEDs must be supplied with the correct current. This can involve series resistors or current-regulated power
supplies.
• LEDs do not approximate a "point source" of light, so they cannot be used in applications needing a highly
collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with
commercial ruby lasers with divergences of 0.2 degrees or less. However this can be corrected by using lenses
and other optical devices.
Laser diodes:Laser diodes (also called .injection lasers.) are in effect an specialized form of LED. Just like a LED, they.re a
form of P-N junction diode with a thin depletion layer where electrons and holes collide to create light photons,
when the diode is forward biased.
The difference is that in this case the .active. part of the depletion layer (i.e., where most of the current flows) is
made quite narrow, to concentrate the carriers. The ends of this narrow active region are also highly polished, or
coated with multiple very thin reflective layers to act as mirrors, so it forms a resonant optical cavity.
The forward current level is also increased, to the point where the current density reaches a critical level where
carrier population inversion occurs. This means there are more holes than electrons in the conduction band, and
more electrons than holes in the valence band. or in other words, a very large excess population of electrons and
holes which can potentially combine to release photons. And when this happens, the creation of new photons
can be triggered not just by random collisions of electrons and holes, but by the influence of passing photons.
Passing photons are then able to stimulate the production of more photons, without themselves being absorbed.
So laser action is able to occur: Light Amplification by Stimulated Emission of Radiation. And the important
thing to realize is that the photons that are triggered by other passing photons have the same wavelength and are
also in phase with them. In other words, they end up .in sync and forming continuous-wave coherent radiation.
Because of the resonant cavity, photons are thus able to travel back and forth from one end of the active region
to the other, triggering the production of more and more photons in sync with themselves. So quite a lot of
coherent light energy is generated.
And as the ends of the cavity are not totally reflective (typically about 90-95%), some of this coherent light can
leave the laser chip to form its output beam. Because a lasers light output is coherent, it is very low in noise and
also more suitable for use as a carrier for data communications. The bandwidth also tends to be narrower and
better defined than LEDs, making them more suitable for optical systems where light beams need to be
separated or manipulated on the basis of wavelength.
The very compact size of laser diodes makes them very suitable for use in equipment like CD, DVD and
MiniDisc players and recorders. As their light is reasonably well collimated (although not as well as gas lasers)
and easily focused, they are also used in optical levels, compact handheld laser pointers, barcode scanners etc.
There are two main forms of laser diode: the horizontal type, which emits light from the polished ends of the
chip, and the vertical or .surface emitting type. They both operate in the way just described, differing mainly in
terms of the way the active light generating region and resonant cavity are formed inside the chip. Because laser
diodes have to be operated at such a high current density, and have a very low forward resistance when lasing
action occurs, they are at risk of destroying themselves due to thermal runaway. Their operating light density
can also rise to a level where the end mirrors can begin melting. As a result their electrical operation must be
much more carefully controlled than a LED. This means that not only must a laser diodes current be regulated
by a constant current circuit rather than a simple series resistor, but optical negative feedback must generally be
used as well to ensure that the optical output is held to a constant safe level.
To make this optical feedback easier, most laser diodes have a silicon PIN photodiode built right into the
package, arranged so that it automatically receives a fixed proportion of the lasers output. The output of this
monitor diode can then be used to control the current fed through the laser by the constant current circuit, for
stable and reliable operation. Fig.6 shows a typical horizontal type laser chip mounted in its package, with the
monitor photodiode mounted on the base flange below it so the diode receives the light output from the rear of
the laser chip.
Fig.7 (page 3) shows a simple current regulator circuit used to operate a small laser diode, and you can see how
the monitor photodiode is connected. The monitor diode is shunting the base forward bias for transistor Q1,
which has its emitter voltage fixed by the zener diode. So as the laser output rises, the monitor diode current
increases, reducing the conduction of Q1 and hence that of transistor Q2, which controls the laser current. As a
result, the laser current is automatically stabilized to a level set by adjustable resistor VR.
Laser diode parameters Perhaps the key parameter for a laser diode is the threshold current (ITH), which is the
forward current level where lasing actually begins to occur. Below that current level the device delivers some
light output, but it operates only as a LED rather than a laser. So the light it does produce in this mode is
incoherent. Another important parameter is the rated light output (Po), which is the highest recommended light
output level (in mill watts) for reliable continuous operation. Not surprisingly there is an operating current level
(IOP) which corresponds to this rated light output (Fig.8). There is also the corresponding current output from
the feedback photodiode, known as the monitor current level (Im). Other parameters usually given for a laser
diode are its peak lasing wavelength, using given in nanometers (nm); and its beam divergence angles (defined
as the angle away from the beam axis before the light intensity drops to 50%), in the X and Y directions
(parallel to, and normal to the chip plane).
Laser safety Although most of the laser diodes used in electronic equipment have quite low optical output levels
typically less than 5mW (mill watts) their output is generally concentrated in a relatively narrow beam. This
means that it is still capable of causing damage to a human or animal eye, and particularly to its light-sensitive
retina.
Infra-red (IR) lasers are especially capable of causing eye damage, because their light is not visible. This
prevents the eye is usual protective reflex mechanisms (iris contraction, eyelid closure) from operating. So
always take special care when using devices like laser pointers, and especially when working on equipment
which includes IR lasers, to make sure that the laser beam cannot enter either your own, or anyone else is eyes.
If you need to observe the output from a laser, either use protective filter goggles or use an IR-sensitive CCD
type video camera. Remember that eye damage is often irreversible, especially when it is damage to the retina.
•Light Emitting Diode
•Light is mostly monochromatic (narrow energy spread comparable to the distribution of
electrons/ hole populations in the band edges)
•Light is from spontaneous emission (random events in time and thus phase). •Light diverges
significantly
LASER
•Light is essentially single wavelength (highly monochromatic)
•Light is from “stimulated emission” (timed to be in phase with other photons
•Light has significantly lower divergence (Semiconductor versions have more than gas lasers though).
Spontaneous Light Emission
• We can add to our understanding of absorption and spontaneous radiation due to random recombination
another form of radiation – Stimulated emission.
• Stimulated emission can occur when we have a “population inversion”, i.e. when we have injected so many
minority carriers that in some regions there are more “excited carriers” (electrons) than “ground state” carriers
(holes).
• Given an incident photon of the band gap energy, a second photon will be “stimulated” by the first photon
resulting in two photons with the same energy (wavelength) and phase.
• This phase coherence results in minimal divergence of the optical beam resulting in a directed light source.
Spontaneous Vs Stimulated Light Emission:
The power current curve of a laser diode. Below threshold, the diode is an LED. Above threshold, the
population is inverted and the light output increases rapidly.
LASER Wavelength Design:
Adjusting the depth and width of quantum wells to select the wavelength of emission is one form of band-gap
engineering. The shaded areas indicate the width of the well to illustrate the degree of confinement of the mode.
Advanced LASER Wavelength Design:
(a). GRINSCH structure helps funnel the carriers into the wells to improve the probability of recombination.
Additionally, the graded refractive index helps confine the optical mode in the near well region. Requires
very precise control over layers due to grading Almost always implemented via MBE
(b). A multiple quantum well structure has improves carrier capture. Sometimes the two are combined to
give a “digitally graded” device where only two compositions are used but the well thicknesses are
varied to implement an effective “index grade”.
Draw the schematic diagram of high radiance surface emitting LED and explain
the
working in detail?
Ans: High radiance is obtained by restricting the emission to a small active region within the
device. A well is etched in a substrate (GaAs) to avoid the heavy absorption of the emitter
radiation and to accommodate the fiber. These structures have a low thermal impedance in the
active region and hence radiance emission into the fiber. Double hetero structures are used to get
increased efficiency and less optical absorption. The structure of a high radiance etched well DH
(Double Hetero structure) surface emitter which is also known as burrus type LED is as shown in
figure (4.5).
This structure emits light in band of 0.8 to 0.9 um wavelength. The plane of the active light emitting
region is made perpendicular to the fiber axis. The fiber is cemented in a well matched through the
substrate of the fiber so that maximum emitted light is coupled to the fiber. Due to large band gap
conjoining area, the internal absorption is less and the reflection coefficient at the back crystal face is
high, hence forward radiance is good. The active area in circle is of 50μ m in diameter and up to 2.5
μm thick. The emission from this active area is isotropic with 120° half power beam width is used for
practical purpose. Isotropic pattern from a surface emitter is lambertian pattern.
The source is equally bright when viewed from any direction but power diminishes as cosΦ where $ is
the angle between viewing direction and to the normal to the surface. Power is down to 50%, when Φ
= 60°, so that the total half power beam width is 120°. The power coupled into a multimode step index
fiber may be estimated from the relationship.
PC =П(1-r)ARD (NA)2………..(1)
Where,
PC = Power coupled into fiber
r = Fresnel reflection coefficient
A = Emission area of source
RD = Radiance of the source
NA = Numerical aperture
Power coupled into the fiber depends on
(i) Distance and alignment between emission area and the fiber.
(ii) Medium between the emitting area and the fiber.
(iii)Emission pattern of SLED
Addition of Epoxy resin in the etched well reduces the refractive index mismatch and increases the external
power efficiency of the device. Hence the power coupled in the double hetero structure surface emitters are
more than Pc(optical power) that is given by equation (1), For graded index fiber-direct coupling requires the
source diameter of about one half the fiber core diameter
Briefly explain about source output pattern in power launching from source to fiber.
Ans: Source Output Pattern
Consider the following figure 5.1.1, which shows a spherical coordinate system characterized by R,θ and φ
with the normal to the emitting surface being the polar axis. The radiance may be a function of both θ and φ,
and can also vary from point to point on the emitting surface. Surface emitting LEDs are characterized by
their lambertian output pattern, which means the source is equally bright when viewed from any direction.
The power delivered at an angle 'θ', measured relative to a normal to the emitting surface, varies as cos θ
because the projected area of the emitting surface varies as cos θ with viewing direction. The emission
pattern for a lambertian source thus follows the relationship.
B(θ,φ) =B0 cos θ
Figure 5.1.2, shows the radiation pattern for a lambertian source. The complexity of emission pattern is
still increases, when we consider edge-emitting LEDs and laser diodes. In the planes parallel and normal
to the emitting junction plane of the device. The radiances of these devices are given by, B(θ, 0°) and
B(θ, 90°). Generally, these radiances can be approximated as,
Where,
T = Transverse power distribution coefficient
L = Lateral power distribution coefficient.
For edge emitter L=1 and T is significantly large value.
Briefly explain about the concept of equilibrium numerical aperture.
Ans: Equilibrium Numerical Aperture
Generally, the source is perfectly coupled into a system fiber by supplying a light source with short fiber
fly lead. This fly lead should be connected to a system fiber with identical NA and core diameter. At this
junction, around 0.1 to 1 dB optical power is lost. An excess power loss will occur in the system fiber in
addition to the coupling loss, which is due to the non-propagating modes scattering out of the fiber as the
launched modes come to an equilibrium condition. This loss has a severe effect on surface-emitting
LED's (i.e., the power is launched into all modes of the fiber) but, the fiber coupled lasers (i.e., the power
is launched into fewer non-propagating fiber modes) are less prove to this effect.
Because of the variation in effect of excess power loss on different type of fibers, it can be analyzed
carefully in any system design. Figure shows the plot of excess power loss in terms of the fiber numerical
aperture. The optical power in the fiber after the launched modes have come to equilibrium is,
Where,
P50 = Power expected in the fiber at 50m point based up on the launch NA Equilibrium numerical
aperture
(iv) Launch Numerical Aperture
The power coupled into the fiber, when the light emitting area of the LED is less than the cross-sectional
area of the fiber-core is given by,
PLED = P50 (NA)2
Where NA=NAin
The degree of mode coupling is primarily a function of core-cladding index difference, which may vary
significantly from one fiber to another. The value of NA has great importance in launching optical power
in telecommunication systems as most of fibers attain 8090% of equilibrium numerical aperture after
about 50 m.
Explain about lensing schemes for coupling efficiency improvement.
Ans: Practically much of the light emitted from LEDs is not coupled into the narrow acceptance angle of
the fiber It has been found that greater coupling efficiency may be obtained if lenses are used to collimate
the emission from the LED, particularly when the fiber core diameter is significantly larger than the
width of the emission region There are several lens coupling configurations which include spherically
polished structures, spherical ended or tapered fiber coupling, truncated spherical micro lenses, GRINrod lenses and integral lens structures. The below figure (5.3.1) shows a GaAs/ AlGaAs spherical ended
fiber coupled LED.
It consists of a planar surface emitting structure with the spherical ended fiber attached to the cap by
epoxy resin. An emitting diameter of 35 μm is fabricated into the device and light is coupled into fibers
with core diameters or 75 μm and 110 μm. For increased coupling efficiency
it is necessary that the active diameter of the device be substantially less than the fiber core diameter by a
factor of 2. In this case a coupling efficiency of 6% is obtained. Another lens coupling technique employs
a truncated spherical micro lens. This is shown in figure (5.3.2).
Efficient coupling is obtained when diameter of emission region is much smaller than the core diameter
of the fiber. In this case the best results are obtained with a 14 μm active diameter and an 85 μm core
diameter step index fiber with a numerical aperture of 0.16. The coupling efficiency was increased by a
factor of 13. The integral lens structure has a useful power coupling strategy for use with surface
emitters. In this technique a low absorption lens is formed at the exit face of the substrate material instead
of it being fabricated in glass and attached to a planar sLED with Epoxy. This method provides an
advantage that the semiconductor epoxy lens interface is eliminated which can limit the maximum lens
gain of LEDs. Lens coupling can also be usefully employed with edge emitting devices. Practically lens
attached to the fiber ends or tapered fiber lenses are widely used to increase coupling efficiency.
Explain about laser diode-to-fiber coupling.
Ans: We know that the edge emitting laser diodes have the emission pattern that nominally has a FullWidth at Half-Maximum (FWHM) of 30° - 50° in the perpendicular plane to the active area junction and
an FWHM of 5° to 10° in the parallel plane to the junction. Since the fiber acceptance angle is smaller
than the angular output distribution of the laser and since the fiber core is much greater than the laser
emitted area, spherical (or) cylindrical lenses (or) optical fiber tapers can also be used to improve the
coupling efficiency between edge emitting laser diodes and optical fibers. This phenomenon also works
well for Vertical Cavity Surface Emitting Lasers (VCSELs). Here the outcome 35% of coupling
efficiencies to multimode fibers for mass-produced connections of laser arrays to parallel optical fibers
are possible by direct coupling from a single Vertical Cavity Surface Emitting Lasers (VCSELs) source
to a multimode fiber.
The measured FWHM value of the laser output beams are,
1. For near field parallel to junction lies between 3 and 9 urn.
2. For field perpendicular to the junction lies between 30° and 60°.
3. For field parallel to the junction lies between 15° and 55°.
In practice, the coupling efficiencies range between 50% and 80%.
Differentiate between Lambertian and monochromatic optical sources in terms of power coupling
into a single mode fiber.
Ans: Lambertian optical sources emits equal brightness when viewed from any direction. Surface
emitting LED's exhibit Lambertian output pattern. The power delivered at an angle G, measured with
respect to a line drawn perpendicular to emitting surface varies as a function of'cost)'. The Lambertian
source emission pattern is given as,
B(θ,Φ) = B0cosθ
Where,
B0 – Radiation along θ=00 i.e., perpendicular line from emitting surface.
The monochromatic optical sources has a narrow output beam in a particular direction i.e., high
directivity is exhibited by monochromatic sources. Laser's exhibit monochromatic pattern. Because of
high directivity, the monochromatic source couples more power into single mode fibers than Lambertian
sources.
The patterns for Lambertian and monochromatic sources are shown in the following figure.
Lambertian sources exhibit more losses like coupling power due to numerical aperture variation, small
angle of incidence etc. Monochromatic source exhibit very less loss, because we can couple large powers
by steering the fiber such that it points to the laser sources (monochromatic source) in the direction of
maximum directivity.
Write short notes on power coupling from a vertical cavity surface Emitting Laser(VCSEL) diode
to a single mode fiber?
Ans: The development of Vertical Cavity Surface Emitting Laser (VCSEL) is done inorder to allow high
data rate transmission. It is a single crystal nano wire, used as a single mode optical wave guide, like an
optical fiber. The important issue is the light coupling from VCSEL array to a single mode fiber for
increasing optical power coupling efficiency and brightness in micro-optical system. In order to obtain a
highly optical power coupling efficiency, micro lens arrays are used. Let us consider the light coupling
from a 4 x 4 nano-scale VCSEL array to a single mode fiber. The overall size of our considered 4 x 4
nano-scale VCSEL array is setup as 8 μm2 with 2μm core pitch and 850nm wavelength.
The figure (6.1) shows the schematic diagram of the array lens for propagating 16 elements of
VCSEL in a single mode fiber, The distance between VCSEL array to micro lens is set as 640 nm. The input
end of single-mode fiber is placed at 890 nm far from micro lens output end. The detector is set as 10 p.m 2
behind 1000 mm single mode fiber. The core pitch between lenses is also 2 um. The core diameter of silica is
10 μm and the cladding diameter of fibers is 125 Jim. The figure (2) shows the transmission loss from the
laser source.
The transmission loss is 1.9 dB at 1000 mm far from VCSEL array. The spot size of laser source is 360 nm.
But here, the spot size is 2000 nm at 500 mm far from the VCSEL array, and the spot size is still below 2000
nm at 1000 mm far from the VCSEL array. Therefore, using the array lens, we can increase the irradiation
field distribution of VCSEL array in a single-mode fiber.
Figure (6.3) shows the irradiance distribution and pattern of the 4 x 4 VCSEL array light output after
propagating 1000 mm long of single-mode fiber through 4 x 4 micro lens array.
array using 4 x 4 BK7 micro lens array, we can simultaneously convey each laser beam, from 360 nm to
2000 nm into a 1000 mm long single mode fiber.
List the factors involved in launching optical power from a light source to a optical fiber?
Ans: Source To Fiber power Launching:
Launching optical power from source into fiber needs te following considerations:
(i) Numerical Aperture
(ii) Core Size
(iii)Refractive index profile
(iv) Core cladding index difference to the fiber
(v) Radiance
(vi) Angular power distribution of the optical source
' A measure of the amount of optical power emitted from a source that can be coupled into a fiber usually
given by the coupling efficiency is defined as
PF/PS
Where PF is the power coupled into the fiber and Ps is the power emitted from the light source.
The launching or coupling efficiency depends on the type of fiber that is attached to the source and on the
coupling process, many source suppliers offer devices with short length of optical fiber (lm or less)
attached in an optimum power configuration.
This section of fiber is generally referred to as fly lead devices. These fly lead sources reduces many
power-launching problems and make the coupling easier.
The effects to e considered are:
(i) Fiber misalignment
(ii) Different core sizes
(iii) Numerical apertures
(iv) Core refractive index
(V ) The need for clean and smooth fiber end faces that are perpendicular to the fiber axis.
While considering the source to fiber power coupling efficiency, the radiance (spatial distribution of
optical power) is important rather than the total output power.
For an optical source having refractive index of 3.6 coupled to a fiber of 1.48 refractive index.
Considering the medium between fiber and source has similar index as that of fiber. Calculate
Fresnel reflection and loss of power in dBs.
Ans:
Given data,
Source refractive index, n1 = 3.6 Fiber refractive index, n = 1.48
UNIT-IV
1. What is splicing? Explain about fusion splicing?
Ans: Splicing
A permanent joint formed between two individual optical fibers in the field is known as splicing. The
fiber splicing is used to establish optical fiber links, where smaller fiber lengths are needed to be
joined and where there is no requirement for repeated connection and disconnection.
Splicing can be divided into two broad categories depending on the splicing technique utilized. These
are fusion-splicing, mechanical or welding splicing.
Fusion Splicing
Fusion splicing of single fibers involves the heating of the two prepared fiber ends to their fusing
point with sufficient axial pressure between the two optical fibers. It is essential that the stripped fiber
ends are adequately positioned and clamped with the aid of inspection microscope.
The most widely used heating technique is an electric arc. This technique offers advantage of
consistent, easily controlled heat with adaptability for use under field conditions.
The welding of 2 fibers can be shown as illustrated in the following figure.
The figure shows basic arc fusion process, which involves the rounding of the fiber ends with a low energy
discharge before pressing the fibers together and fusing with a stronger arc.
This technique is called perfusion. It removes the requirement for fiber end preparation. It has been
utilized with multimode fibers giving average splice losses of 0.09 dB.
Fusion splicing of single mode fibers with arc diameters between 5 and 10 um present problems of
more critical fiber alignment (lateral offsets of less than 1 um are required for low loss joints).
Splice uncertain losses below 0.3 dB may be achieved due to self alignment phenomenon which
partially compensates for any lateral offset.
The drawback with fusion splicing is that the heat necessary to fuse the fibers may weaken the fiber
in the vicinity of the splice.
The tensile strength' of the fused fiber may be as low as 30% as that of the uncoated fiber before
fusion. The fiber fracture occurs in the heat affected zone adjacent to the fused joint. The reduced
tensile strength is attributed, to the combined effects of surface damage caused by handling, surface
defect growth during heating and induced stresses due to changes in chemical composition. Hence it
is necessary that splice is packaged so as to reduce tensile loading upon the fiber in the vicinity of the
splice.
Explain about adhesive splicing?
Ans: Adhesive Splicing:
A common method involves the use of ah accurately produced rigid alignment tube into which the prepared
fiber ends are permanently, bonded. This snug tube splice may utilize a glass or ceramic capillary with an inner
diameter just large enough to accept the optical fibers. Transparent adhesive (e.g. epoxy resin) is injected
through a transverse bore in die capillary to give mechanical sealing and index matching of the splice.
However, in general, snug tube splices exhibit problems with capillary tolerance requirements.
A mechanical splicing-technique which avoids the critical tolerance requirements of the snug tube
splice is shown in figure 4.3.This loose tube splice uses an over sized square section metal tube
which easily accepts the prepared fiber ends. Transparent adhesive is first insulated in the tube
followed by the fibers. The splice is self aligning when the fibers are curved in the same plane,
forcing the fiber ends simultaneously into the same corner of tube.
Other common splicing techniques involve the use of grooves to secure the fibers to be jointed. A simple
method utilized a V-groove into which the two prepared fiber ends are pressed. The V-groove splice which
is shown in figure gives alignment of the prepared fiber ends through insertion in the groove. The splice is
made permanent by securing the fibers in the V-groove with epoxy resin (i.e., transparent adhesive).
Explain about Multiple splices?
Ans: Multiple splices:
Multiple simultaneous fusions splicing of an array of fibers in a ribbon cable has been
demonstrated for both multimode and single mode fibers. In both cases a five fiber ribbon was
prepared by scoring and breaking prior to pressing the fiber ends on to a contact plate to avoid
difficulties, with varying gaps between the fibers to be fused
The most common technique employed for multiple simultaneous splicing involves mechanical splicing of
an array of fibers, usually in a ribbon cable. A V-groove multiple splice secondary element comprising
etched silicon chips that has been used extensively for splicing multimode fibers. In this technique a twelve
fiber ribbon splice is prepared by stripping the ribbon and coating material from the fibers. Then the twelve
fibers are laid into the trapezoidal grooves of a silicon chip using a comb structure, as shown in figure. The
top silicon chip is then applied prior to applying epoxy to the chip-ribbon interface. Finally, after curing, the
front end face is grounded and polished.
Major advantages of this method are the substantial reduction splicing time (by more than
a
factor of 10) per fiber and the increased robustness of the final connection.
With schematic representation explain the working principle of pin photo diode.
Ans: PIN Photodiode
PIN diode consists of an intrinsic semiconductor sandwiched between two heavily doped p-type and n-type
semiconductors as shown in Fig. 6.1.1. Sufficient reverse voltage is applied so as to keep intrinsic region
free from carriers, so its resistance is high, most of diode voltage appears across it, and the electrical forces
are strong within it. The incident photons give up their energy and excite an electron from valance to
conduction band. Thus a free electron hole pair is generated these are called as photo carriers. These carriers
are collected across the reverse biased junction resulting in rise in current in external circuit called
photocurrent.
In the absence of light, PIN photodiodes behave electrically just like an ordinary rectifier diode. If forward
biased, they conduct large amount of current. PIN detectors can be operated in two modes, Photovoltaic and
photoconductive. In photovoltaic mode, no bias is applied to the detector. In this case the detector works very
slow, and output is approximately logarithmic to the input light level. Real world fiber optic receivers never
use the photovoltaic mode. In photoconductive mode, the detector is reverse biased. The output in this case is a
current that is very linear with the input light power. The intrinsic region somewhat improves the sensitivity of
the device. It does not provide internal gain. The combination of different semiconductors operating at different
wavelengths allows the selection of material capable of responding to the desired operating wavelength.
What are the requirement of photo detector and why photodiode is preferred in fiber optic
communication system?
Ans: Photo detector
Photo detector is an essential component of an optical fiber communication system. Its function is to convert
the received optical signal into an electrical signal which is amplified before further processing. The
requirements for photo detector can be given as follows.
(v)
They should have high sensitivity at operating wavelength.
(vi)
It should have high fidelity to reproduce the original waveform effectively.
(vii)
It should produce maximum electrical signal for a given amount of optical power.
(viii)
It should have short response time to obtain a suitable bandwidth.
(ix)
Dark currents, leakage currents, shunt conductance should be low.
(x)
Performance characteristics should be independent of changes in ambient conditions.
(xi)
The physical size of detector must be small for effective coupling.
(xii)
Detector should not require excessive bias voltages or currents.
(xiii)
It must be highly reliable and must be capable of continuous stable operation at room temperature.
(xiv)
It should be economical.
The detector must satisfy the above requirements of performance and compatibility.
Photodiode
Photodiodes are preferred for photo detection in optical system. The photodiodes provide good performance
and compatibility with relatively low cost. These photodiodes are made from semiconductors such as silicon,
germanium and an increasing number of III-V alloys. Internal photoemission process may take place in both
intrinsic and extrinsic semiconductors. The intrinsic absorption process is preferred as they have fast response
coupled with efficient absorption of photons.
These photodiodes are sensitive, have adequate speed, negligible shunt, conductance, low dark current, long
term stability. Thus they are widely used. Avalanche photodiodes are also widely employed in fiber
communication system. They have very sophisticated structure.
With schematic representation explain the working principle of Avalanche Photodiode.
Ans: Avalanche Photodiode
When a p-n junction diode is applied with high reverse bias breakdown can occur by two separate mechanisms
direct ionization of the lattice atoms, zener breakdown and high velocity carriers causing impact ionization of
the lattice atoms called avalanche breakdown. APDs use the avalanche breakdown phenomena for its
operation. The APD has its internal gain which increases its responsivity. Fig. 6.3.1 shows the schematic
structure of an APD. By virtue of the doping concentration and physical construction of the n+ p junction, the
electric field is high enough to cause impact ionization. Under normal operating bias, the I-layer (the p~ region)
is completely depleted.
This is known as reach through condition, hence APDs are also known as reach through APD or RAPDs.
Similar to PIN photodiode, light absorption in APDs is most efficient in I-layer. In this region, the E-field
separates the carriers and the electrons drift into the avalanche region where carrier multiplication occurs. If the
APD is biased close to breakdown, it will result in reverse leakage current. Thus APDs are usually biased just
below breakdown, with the bias voltage being tightly controlled. The multiplication for all carriers generated in
the photodiode is given as
Where,
IM = Average value of total multiplied output current.
IP = Primary unmultiplied photocurrent.
Responsivity of APD is given by
What are the requirements of optical receiver? Using a flow chart explain the receiver design.
Ans: The Optical Receiver Circuit
flow chart depicting the receiver design is shown above. The optical signal received is linearly converted into
an electrical signal at the detector, it is amplified to obtain a suitable signal level. Initially the amplification is
performed in the preamplifier circuit, where it is important that the additional noise is kept to a minimum in
order to avoid corruption of the received signal. As the noise source within the preamplifier may be dominant,
its configuration and design are major factors in determining receiver sensitivity. The main amplifier provides
additional low noise amplification of the signal to give an increased signal level for the following circuits. Even
though the optical detectors are linear devices and do not provide any distortion. Other components in the
system may exhibit nonlinear behavior to remove this i.e., to compensate for the distortion, equalizers are used.
The function of the filter is to maximize the acquired signal to noise ratio without destroying the essential
features of the signal.
Explain various errors that occur in the detection mechanism.
Ans:
Errors in detection mechanism can arise from various noises and disturbances associated with the signal detection
system, as shown in figure. The term noise is used customarily to describe unwanted components of an electric
signal that tend to disturb the transmission and processing of the signal in a physical system, and over which we
have incomplete control.
The noise sources can be either external to the system or internal to the system. The two most common noises are
shot noise and thermal noise. Shot noise arises in electronic devices since of the discrete nature of current flow in
the device. Thermal noise arises from the random motion of electrons in a conductor. The random arrival rate of
signal photons produces a quantum or shot noise at the photo detector. Since this noise depends on the signal level,
it is of particular importance for pin receivers that have large optical levels and for avalanche photodiode receivers.
An additional photo detector noise comes from the dark current and leakage current. These are independent of
photodiode illumination and can generally be made very small in relation to other noise currents by a judicious
choice of components. Thermal noises arising from the detector load resistor and from the amplifier electronics
tend to dominate in application with low signal-to-noise ratio when a pin photodiode is used. When an avalanche
photodiode is used in low Optical signal level applications the optimum avalanche gain is determined by a design
tradeoff between the thermal noise and gain-dependent quantum noise. A further error source is attributed to inter
symbol interference (ISI) which results front pulse spreading in the optical fiber. When a pulse is transmitted in a
given time slot most of the pulse energy will arrive in the corresponding time slot at the receiver. The presence of
this energy in adjacent time slots results in an interfering signal, hence the terms inter symbol interference.
Describe with relevant diagram about the signal path through optical data link via transmitter, fiber and
receiver giving the nature of the signal waveform.
Ans: The optical fiber transmission link is shown below,
Here a two level binary signal is used for transmission purpose. The two levels are represented by 1 and 0
respectively. Each level has finite time duration known as bit period Tb. The stream of l’s and 0's are
transmitted using amplitude shift keying modulation technique. In ASK the voltage has two levels which are V
volts for binary 1 and 0 volts for binary 0. Corresponding to these voltage levels the optical source will produce
pulses of optical power. These pulses are coupled to an optical fiber and are transmitted. The signal gets
attenuated for various reasons and therefore it is distorted. The below figure 6.2.2 shows a block diagram of an
optical receiver.
At the receiver the distorted signal is coupled to a photo detector generally a pin diode which produces an
electric current which is equivalent to the incoming signal. The amplifier and filter removes noise and amplifies
the signal.
The decision making device gives output binary 1 for a voltage V and 0 for a voltage 0 respectively. At the
signal processing circuiting the signal is demodulated thus producing the desired output.
Describe important specifications of semiconductor photo diode to be suitable for fiber optic
communications.
Ans: Specifications of a Semiconductor Photo Diode
The important performance parameters of a semiconductor photodiode are described below.
1. Responsivity
The ratio of generated photocurrent to incident light power typically expressed in A/W when used in photo
conductive mode. The responsivity may also be expressed as quantum efficiency or the ratio of the number of
photo generated carriers to incident photons and thus, a unit less quantity. The typical values for responsivity are,
0.65 for A/W silicon at 900 nm, 0.45 A/W for germanium at 1.3 μm and 0.9 A/W for InGaAs at 1.3 μm.
2. Dark Current
The current through the photodiode in the absence of light, when it is operated in photoconductive m dark current
includes photocurrent generated by background radiation and the saturation current of the semiconductor junction.
Dark current must be accounted for calibration if a photodiode is used to make an accurate optical measurement. It
is also a source of noise when a photo diode is used in an optical communication system.
3. Noise Equivalent Power (NEP)
The minimum input optical power to generate photocurrent, equal to the r.m.s noise current in a 1 Hz bandwidth.
The related characteristic directivity DIS the inverse of NEP i.e., 1/NEP and the specific directivity D* is the
directivity normalized to the area, A of the photodiode i.e.,
D*=D√A
4. Quantum Efficiency
The photodiode's capability to convert light energy to electrical energy is referred as quantum efficiency, it can be
also described as the ratio of number of electron-hole pairs generated to the number of incident photons. In a
practical photodiode, the quantum efficiency of the detector ranges from 30 to 95%.
5. Sensitivity
It is a measure of the effectiveness of a detector in producing an electrical signal at the peak sensitivity
wavelength.
6. Rise time
The time required for a detector output to reach from 10 to 90% of its final value.
With the help of a suitable block diagram explain the functioning of every element of a fiber optic receiver.
Ans: Fiber Optic Receiver
The terra receiver at the output end of the fiber optic cable refers to both a light detecting transducer and its related
electronics, which provides any necessary signal conditioning to restore the signal to its original shape at the input,
as well as additional signal amplification. To interface the receiver with the optical fiber, the proper match between
light source, fiber optic cable and light detector is required. In the AM transmission system, the optical power
input at the fiber is modulated so that the photo detector operating in the photocurrent mode must provide good
linearity, speed and stability.
The photodiode produces no electrical gain and is therefore followed by circuits that amplify electrical voltage and
power to drive the coaxial cable. Figure below illustrates the block diagram for the optical fiber receiver unit.
As light enters from the receiver end of an optical fiber, it spreads out with a divergence approximately equal to
the acceptance cone angle at the transmitter end of the fiber. Photodiodes are packaged with lenses on their
housings so that the lens collects this output energy and focuses it down onto the photodiode-sensitive area. The
most common fiber optic receiver uses a photodiode to convert the incident light from the fiber into electrical
energy. After the light energy is converted into an electrical signal by the photodiode, it is linearly amplified and
conditioned to be suitable for transmission over standard coaxial cable to a monitor or recorder.
a) Differentiate between the photodiode parameters, ‘quantum limit’ and ‘dark current’.
b) Define sensitivity and Bit Error Rate (BER) with reference to a fiber optic receiver.
Ans:
a). Quantum Limit
The minimum received optical power required for a particular bit error rate performance using an ideal photo
detector (which has zero dark current and unity quantum efficiency) is referred as quantum efficiency. In this case,
the performance of the system will depend only on the photo detection statistics as the remaining all the system
parameters are considered to be ideal. Due to several nonlinear distortions and noise effects in the transmission
link, the practical values of most of the receiver sensitivities will be around 20 dB higher than the quantum limit. It
is also very important to differentiate average power and peak power while specifying he quantum limit.
Dark Current
In the absence of light, the current that flows continuously through the basic circuit of the device is referred as dark
current or the leakage current that flows when the photodiode is in the dark and a reverse voltage is applied across
the junction is referred as dark current. This voltage may be low as 10 mV or as high as 50 V and the dark current
may vary from pA to uA depending on the junction area and the process used.
The dark current is temperature dependent. The rule of thumb is that the dark current will approximately double
for every 10°C increase in ambient temperature. However, specific diode types can vary considerably from this
relationship.
b) Sensitivity
The sensitivity of the receiver is defined as the minimum amount of optical power required to achieve a specific
receiver performance. The receiver takes many signals such as synchronous signals (to recover the clock signal
similar to that is used at transmitter), decoded data and errors. So, in order to generate a correct signal in the
presence of all these signals, a receiver should have high sensitivity. If it has high sensitivity it will be even able to
detect low level optical signals. Thus, the higher the sensitivity, the more efficiently the receivers can detect the
attenuated on low level signals. The sensitivity of receiver can be sketched taking the data rate and optical power
that the receiver can detect.
Bit Error Rate (BER)
In practice, that there are several standard ways of measuring the rate of error occurrences in a digital data stream.
One common approach is to divide the number Ne of errors occurring over a certain time interval t by number Nt
of pulses (ones and zeros) transmitted during this interval.
This is called either the error rate of the bit error rate, which is commonly abbreviated as BER. Thus, we have
BER = Ne/Nt
A pin photodiode on average generates one electron hole pair per three incident photons at a wavelength of
0.8 μ m. Assuming all the electrons are collected, calculate
1.
The quantum efficiency of the device
2.
Its maximum possible band gap energy
3.
The mean output photocurrent when the received optical power is 10-7 W.
Ans Given that,
For a pin photodiode one electron hole pair generated for every three incident photons.
Operating wavelength, λ = 0.8 μm
i) Quantum efficiency of the device, = ?
(ii) Maximum possible band gap energy, Eg = ?
(iii) Received optical power, P0 = 10-7 W
(iv)
Mean output photocurrent, Ip = ?
Photodetectors:These are Opto-electric devices i.e. to convert the optical signal back into electrical impulses. The light
detectors are commonly made up of semiconductor material.
When the light strikes the light detector a current is produced in the external circuit proportional to the intensity of
the incident light.
Optical signal generally is weakened and distorted when it emerges from the end of the fiber, the
photodetector must meet following strict performance requirements.
A high sensitivity to the emission wavelength range of the received light signal. A minimum
addition of noise to the signal.
A fast response speed to handle the desired data rate. Be insensitive to
temperature variations.
Be compatible with the physical dimensions of the fiber.
Have a Reasonable cost compared to other system components. Have a long
operating lifetime.
Some important parameters while discussing photodetectors:
Quantum Efficiency
It is the ratio of primary electron-hole pairs created by incident photon to the photon incident on the diode
material.
Detector Responsivity
This is the ratio of output current to input optical power. Hence this is the efficiency of the device.
Spectral Response Range
This is the range of wavelengths over which the device will operate.
Types of Light Detectors
_ PIN Photodiode
_ Avalanche Photodiode
The Pin Photodetector:The device structure consists of p and n semiconductor regions separated by a very lightly n-doped intrinsic (i)
region.
In normal operation a reverse-bias voltage is applied across the device so that no free electrons or holes exist in
the intrinsic region.
Incident photon having energy greater than or equal to the bandgap energy of the semiconductor material,
give up itsenergy and excite an electron from the valence band to the conduction band.
The high electric field present in the depletion region causes photo generated carriers to separate and be collected
across the reverse – biased junction. This gives rise to a current flow in an external circuit, known as
photocurrent.
Photo carriers:
Incident photon generates free (mobile) electron-hole pairs in the intrinsic region. These charge carriers are
known as photo carriers, since they are generated by a photon.
Photocurrent:
The electric field across the device causes the photo carriers to be swept out of the intrinsic region, thereby
giving rise to a current flow in an external circuit. This current flow is known
as the photocurrent.
Energy-Band diagram for a pin photodiode:
An incident photon is able to boost an electron to the conduction band only if it has an energy that is greater
than or equal to the bandgap energy. Thus, a particular semiconductor material can be used only over a limited
wavelength range.
As the charge carriers flow through the material some of them recombine and disappear.
The charge carriers move a distance Ln or Lp for electrons and holes before recombining. This distance is known
as diffusion length
The time it take to recombine is its life time _n or _p respectively.
Where Dn and Dp are the diffusion coefficients for electrons and holes respectively.
Photocurrent:As a photon flux penetrates through the semiconductor, it will be absorbed.If Pin is the optical power falling on
the photo detector at x=0 and P(x) is the power level at a distance x into the material then the incremental change
be given as
Where _s(_) is the photon absorption coefficient at a wavelength _. So that
Optical power absorbed, P(x), in the depletion region can be written in terms of incident optical power, Pin :
Absorption coefficient as (l) strongly depends on wavelength. The upper wavelength cutoff for any
semiconductor can be
Taking entrance face reflectivity into consideration, the absorbed power in the width of depletion region, w,
becomes:
Avalanche Photodiode (APD):
APDs internally multiply the primary photocurrent before it enters to following circuitry. In order to carrier
multiplication take place, the photo generated carriers must traverse along a high field region. In this region, photo
generated electrons and holes gain enough energy to
ionize bound electrons in VB upon colliding with them. This multiplication is known as impact ionization. The
newly created carriers in the presence of high electric field result in more ionization called avalanche effect.
Responsivity of APD:The multiplication factor (current gain) M for all carriers generated in the photodiode is
defined as:
where IM is the average value of the total multiplied output current & Ip is the primary photocurrent.
The responsivity of APD can be calculated by considering the current gain as:
Photo detector Noise & S/N:Detection of weak optical signal requires that the photo detector and its following amplification circuitry be
optimized for a desired signal-to-noise ratio.
It is the noise current which determines the minimum optical power level that can be detected. This minimum
detectable optical power defines the sensitivity of photo detector. That is the optical power that generates a
photocurrent with the amplitude equal to that of the total noise current (S/N=1)
Structures for InGaAs APDs:-
UNIT-V
Fiber Optic Link Budget
The FOL budget provides the design engineer with quantitative performance information about the
FOL. It is determined by computing the FOL power budget and overall link gain.
Fiber Optic Power Budget
The FOL power budget (PB) is simply the difference between the maximum and minimum
signals that the FOL can transport.
Fiber Optic Link Gain:
FOL link gain is a summation of gains and losses derived from the different elements of the FOL
as shown in above figure. Gains and losses attributed to the Tx, Rx, optical fiber and connectors,
as well as any additional in-line components such as splitters, multiplexers, splices etc, must be
taken into accounts when computing the link loss budget.
In the case of a simple point-to-point link described in above figure, and resistively matched (50
ohms) components, the link gain (G) is expressed as:G = T + R - 2LO (1)
Where T is the gain of the Tx, R is the gain of the Rx, and LO is the insertion loss attributed to
the fiber link. Note the factor of two in this last optical term, meaning that for each dB optical
loss there is a corresponding 2dB RF loss.
To calculate LO the following information is needed.
Standard Corning SMF28 single mode fiber has an insertion loss 0.2dB/km at 1310nm and
0.15dB/km at 1550nm. Optical connectors such as FC/APC typically have an insertion loss of
0.25dB. Optical splices introduce a further 0.25dB loss. Refer to TIA 568 standard for Inter
facility and Premise cable specifications.
Output Noise Power
The output noise power of an analogue FOL must also be considered when quantifying the
overall link budget. The measured output noise power is defined as:Output Noise Power = ONF + 10log10 (BW)
Where ONF (Optical Noise Floor) is the noise output of the link on its own, defined in a
bandwidth of 1Hz,and BW is the bandwidth of the service transported over fiber. In a real
installation, the NF, or Noise Figure is used to define the noise performance of the fiber optic
link and is related to the output noise floor as follows:
ONF = -174dBm + NF + G (3)
-174dBm, is the noise contribution from an ideal 1ohm resistive load at zero degrees Kelvin.
The measured output noise power is given as:= -174dBm + NF + G + 10log10 (MBW)
Fiber Optic System Design:There are many factors that must be considered to ensure that enough light reaches the receiver.
Without the right amount of light, the entire system will not operate properly.
Fiber Optic System Design- Step-by-Step:Select the most appropriate optical transmitter and receiver combination based upon the signal to
be transmitted. Determine the operating power available (AC, DC, etc.). Determine the special
modifications (if any) necessary (Impedances, bandwidths, connectors, fiber size, etc.). Carry out
system link power budget.
Carry out system rise time budget (I.e. bandwidth budget).
If it is discovered that the fiber bandwidth is inadequate for transmitting the required signal over
the necessary distance, then either select a different transmitter/receiver (wavelength)
combination, or consider the use of a lower loss premium fiber
Link Power Budget:-
Total loss LT = αf L + lc + lsp
Pt − Po = LT + SM
Po = Receiver sensitivity (i.e. minimum power requirement)
SM = System margin (to ensure that small variation the system operating parameters do not
result in an unacceptable decrease in system performance)
Link Power Budget - Example 1:-
Link Power Budget - Example 2:-
Link Power Budget - Example 2 contd.:Link-Power Budget - Example 3:-
Rise Time Budget:The system design must also take into account the temporal response of the system components.
The total loss LT (given in the power budget section) is determined in the absence of the any
pulse broadening due to dispersion.
Finite bandwidth of the system (transmitter, channel, receiver) may results in pulse spreading
(i.e. intersymbol interference), giving a reduction in the receiver sencitivity. I.e. worsening of
BER or SNR
The additional loss penalty is known as dispersion equalisation or ISI penalty.
Transmission Distance -1st window:-
Transmission Distance -3rd window:Analogue System:The system must have sufficient bandwidth to pass the HIGEST FREQUENCIES. Link Power
budget is the same as in digital systems Rise Time budget is also the same, except for the system
bandwidth which is defined as:
Describe eye pattern analysis for assessing the performance of digital fiber optic link. Is it possible
to estimate BER also from eye patterns?
Ans: Eye pattern is a simple and powerful technique in measuring the capacity and performance of a
digital transmission system. The measurements are in time domain and the waveform distortion can be
seen on the CRO immediately. These eye patterns are formed by superimposing the 2N possible
combinations of N-bit long NRZ patterns.
As said, we need a variety of word patterns to measure the performance of a system using eye-pattern
technique, these word patterns are provided by pseudo random bit generator. This pseudo random data (bit)
pattern generator produces a random data signal that contains 1's and 0's in a random fashion providing uniform
data rate. The random data from pseudo random data pattern generator is applied to the vertical input of CRO
and the data rate triggers the horizontal sweep. This generates an eye-pattern. For example, consider 8 possible
3-bit NRZ patterns as shown in following figure 8.10.2.
An eye pattern is obtained by superimposing the above 8 patterns as shown in figure 8.10.3
4. The eye-width opening defines the Inter Symbol Interference (ISI) error free sampling rate of signal.
5. The best sampling rate is obtained when the height of eye is maximum.
6. One cannot recognize 1's and 0's if height of eye is reduced.
7. The eye height at a particular sampling period gives noise margin. It is given by,
Noise margin = X 100%
The system's sensitivity to time is determined by the closing rate of eye for a variation in sampling
period.
P One can also get rise and fall times of the system from the pattern.
P Bit Error Rate (BER) is also estimated from the patterns and it can be reduced by inserting a small
amount of redundancy into the transmitted pulse train.
Discuss about the choice of different components in designing an optical fiber link. Discuss the
shortcomings of decency of each component.
Ans: The system designer has many choices when selecting components for an optical fiber
system. The major components choices are,
Optical Fiber Type and Parameters
Multimode or single mode, size, refractive index, attenuation, dispersion, mode coupling, strength,
joints etc.
Source Type
Laser or LED, optical power launched into the fiber, rise and fall time, stability etc.,
Transmitter Configuration
Design for digital or analog, input impedance, supply voltage, dynamic range, feedback etc.
Detector Type and Characteristics
p-n-p in or avalanche photodiode, response time, active diameter, bias voltage, dark current etc.
Receiver Configuration
Preamplifier design, BER, SNR, range etc.
Modulation and Coding
Source intensity modulation, pulse frequency modulation, PWM and PPM transmission.
(vii) Digital transmission or analog transmission such as bi-phase scheme and FM respectively.
These decisions will be taken depending on the system performance, ready availability of suitable
components and cost.
The short comings of the components can be mentioned as follows,
LED may appear ideally suitable for analog transmission most of the LED display some degree of
non-linearity in their output.