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UNIT V
SATELLITE AND OPTICAL COMMUNICATION
INTRODUCTION:
 A satellite is a celestial body that orbits around a planet.
 Artificial satellites can be launched into orbit for a variety of purposes.
 Non synchronous satellites rotate around the earth in an elliptical or circular path.
 In a circular orbit, speed or rotation is constant.
 In elliptical orbit, the speed depends on the height the satellite is above the earth.
 A communication satellite is a microwave repeater in the sky that consists of the following
components:
1. Transmitter
2. Receiver
3. Amplifier
4. Regenerator
5. Filter
6. Onboard computer
7. Multiplexer
8. De-multiplexer
9. Antenna
10. Waveguide
 A satellite radio repeater is called a transponder.
 A satellite system consists of
 one or more satellite space vehicles
 a ground-based station to control the operation of the system
 a user network of earth stations that provides the interface facilities for the
transmission and reception of communication traffic through the satellite system.
 Transmissions to and from satellites are categorized as either bus or pay load.
 The bus includes control mechanisms that support the pay load operation.
 The pay load is the actual user information conveyed through the system.
KEPLER’S LAW:
Kepler’s law simply states that,
(1) The planet move in ellipses with the sun at one focus
(2) The line joining the sun and a planet sweeps out equal areas in equal intervals of time and
(3) The square of the orbital period of any planet is proportional to the cube of the semimajor
axis of the elliptical orbit
 Kepler’s law can be applied to any two bodies in space that interact through gravitation.
 The larger the two bodies is called primary and the smaller is called the secondary or
satellite.
KEPLER’S FIRST LAW:
 Kepler’s first law states that a satellite will orbit a primary body (like earth) following an
elliptical path.
 An ellipse has two focal points (foci) and the center of mass of a two body system is
always centered in one of the foci.
 Because the mass of earth is substantially greater than that of the satellite, the center of
mass will always coincide with the center of earth.
 The geometric properties of the ellipse are normally referenced to one of the foci which is
logically selected to be the one at the center of earth.
 For the semi major axis (α) and the semi minor axis (β), the eccentricity (abnormality) of
the ellipse can be defined as,
ϵ = SR(α2- β2) / α
KEPLER’S SECOND LAW:
 Kepler’s second law is also known as law of areas.
 It states that for equal intervals of time a satellite will sweep out equal areas in the orbital
plane, focused at the bary center.
 For a satellite travelling distances D1 and D2 meters in 1 second, areas A1 and A2 will be
equal.
 Because of the equal area law, distance D1 must be greater than distance D2 and therefore
velocity V1 must be greater than velocity V2.
 The velocity will be greatest at the point of closest approach to earth (known as perigee)
and the velocity will be least at the farthest point from the earth (known as apogee).
KEPLER’S THIRD LAW:
 Kepler’s third law is also known as harmonic law.
 It states than the square of the periodic time of orbit is proportional to the cube of the
mean distance between the primary and the satellite.

This means distance is equal to the semi major axis; thus, kepler’s third law stated
mathematically as,
α = AP2/3
where A= constant (unit less)
α = semi major axis (kilometers)
P= mean solar earth days

P is the ratio of the time of one side real day (ts = 23 hrs and 56 min) to the time of one
revolution of earth on its own axis (te=24 hrs).
Thus P = ts/te = 1436/1440 = 0.9972

SATELLITE ORBITS:
 Satellites are sometimes called as orbital satellites, which are non synchronous.
 Non synchronous satellites rotate around earth in an elliptical or circular pattern.
 In a circular orbit, the speed or rotation is constant.
 In elliptical orbits the speed depends on the height the satellite is above earth.
 The speed of the satellite is greater when it is close to earth’s rotation.
PROGRADE:
 If the satellite is orbiting in the same direction as earth’s rotation (counter clockwise)
and at an angular velocity greater than that of earth (ωs>ωe), the orbit is called a prograde
or posigrade orbit.
 Most non synchronous satellites revolve around the earth in a prograde orbit.
RETROGRADE:
 If the satellite is orbiting in the opposite direction as earth’s rotation or in the same
direction with an angular velocity less than that of earth (ωs<ωe), the orbit is called as
retrograde orbit.
ADVANTAGES OF ORBITAL SATELLITES:
 Propulsion rockets are not required on board the satellites to keep them in their respective
orbits.
DISADVANTAGES OF ORBITAL SATELLITES:
 Need for complicated and expensive tracking equipment at the earth stations.
SATELLITE ELEVATION CATEGORIES:
Satellites are generally classified as
1. Low earth orbit (LEO)
2. Medium earth orbit (MEO)
3. Geo synchronous orbit (GEO)
Low earth orbit (LEO):
 LEO satellites operate in 1.0 GHz to 2.5 GHz frequency range.
 Low Earth orbit (LEO) is generally defined as an orbit below an altitude of 2,000
kilometers
Advantages:
 The path loss between earth stations and space vehicles is much lower than for satellites
revolving in medium or high altitude orbits.
 Less path loss equates to lower transmit powers, smaller antennas and less weight.
 Low propagation delay.
 Better coverage.
 Efficient use of frequency spectrum.
 Has less complexity.
Disadvantages:
 Requires long period of deployment.
 Defective satellites need to be replaced regularly.
 Routing mechanisms are complex.
Medium earth orbit (MEO):
 MEO satellites operate in the 1.2 GHz to 1.66 GHz frequency band and orbit between
6000 miles and 12,000 miles above earth.
 Medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), is
the region of space around the Earth above LEO and GEO.
Advantages:
 It provides global coverage applications.
 Medium propagation delay.
Geo synchronous orbit (GEO):
 It is a high altitude earth orbit satellites operating in the 2 GHz to 18 GHz frequency
spectrum with orbits 22,300 miles above earth’s surface.
 Most of the commercial satellites are in geo synchronous orbit.
 Geo synchronous or geo stationary satellites are those that orbit in a circular pattern with
an angular velocity equal to that of earth and following earth’s rotation.
 Geo stationary satellites have an orbital time of approximately 24 hours, the same as
earth.
 Thus geo synchronous satellites appear to be stationary.
Clarke orbit:
 Geo synchronous earth orbit is also called as the Clarke orbit.
 Clarke orbit meets the concise set of specifications for geo synchronous orbits.
 Located directly above the equator.
 Travel in the same direction as earth’s rotation at 6840 mph.


Complete one revolution is 24 hours.
Have an altitude of 22,300 miles above earth.
Advantages:
 Remain almost stationary with respect to a given earth stations.
 Expensive tracking equipment is not required at the earth stations.
 Not necessary to switch from one geo synchronous satellite to another. Due to switching
times, there is no transmission break occurs.
Disadvantages:
 Require sophisticated and heavy propulsion devices onboard to keep them in a fixed
orbit.
 Transmission power needed is relatively high.
 Cannot be suitable for small mobile phones.
 High altitude geo synchronous satellites introduce much longer propagation delays.
Near synchronous orbits:
 Satellites in high elevation, non synchronous circular orbits between 19,000 miles and
25,000 miles above earth are said to be in near synchronous orbits.
 When the near synchronous orbit is slightly lower than 22,300 miles above earth, the
satellite’s orbital time is lower than earth’s rotational period.
 Therefore the satellite is moving slowly around earth in a west to east direction.
 This type of near synchronous orbit is called sub synchronous.
 If the orbit is higher than 22,300 miles above earth, the satellite’s orbital time is longer
than earth’s rotational period.
 Therefore the satellite will appear to have a reverse motion from east to west.
Comparison of LEO, MEO and GEO:
Parameters
GEO
1. Frequency range 2 GHz to 18 GHz
35,862 km
2. Altitude range
3. Visibility duration Permanent
No
variation,
low
4. Elevation
angles at high altitudes
16,000 km
5.Instantaneous
ground coverage
MEO
1 GHz to 2.5 GHz
500 to 1500 km
15 to 20 min/pass
Rapid variation, high
and low angles
6000 km
SATELLITE ORBITAL PATTERNS:
Basic understanding of some terms used to describe orbits is as follows:
1. Apogee  the point in an orbit that is located farthest from earth.
2. Perigee  the point in an orbit that is located closest to earth.
LEO
1.2 GHz to 1.66 GHz
8000 to 18000 km
2 to 8 hrs/pass
Slow variations, high
angles
12,000 to 15,000 km
3. Major axis  the line joining the perigee and apogee through the center of earth;
sometimes called line of apsides.
4. Minor axis  the line perpendicular to the major axis and halfway between the perigee
and apogee.
Half the distance of minor axis is called semi minor axis.
Although there is an infinite number of an orbital path, only three are useful for
communication satellites.
(i)
Inclined
(ii)
Equatorial
(iii)
Polar
All satellites rotate around earth in an orbit that forms a plane that passes through the center of
gravity of earth called the geo center.
(i) Inclined orbits:
 Inclined orbits are virtually all orbits except those that travel directly above the
equator or directly over the north and south poles.
 The angle of inclination is the angle between the earth’s equatorial plane and the
orbital plane of a satellite measured counterclockwise at the point in the orbit where it
crosses the equatorial plane travelling from south to north. This point is called the
ascending node.
 The point where a polar or inclined orbit crosses the equatorial plane travelling from
north to south is called the descending node.
 The line joining the ascending and descending nodes through the center of earth is
called the line of nodes.
 Angle of inclination vary between 0○ and 180○.
 To provide coverage to regions of high latitudes, inclined orbits are generally
elliptical.
(ii) Equatorial orbits:
 Equatorial orbit is when the satellite rotates in an orbit directly above the equator,
usually in a circular path.


With an equatorial orbit, the angle of inclination is 0○, and there are no ascending or
descending nodes and hence, no line of nodes.
All geo synchronous satellites are in equatorial orbits.
(iii) Polar orbit:
 Polar orbit is when the satellite rotates in a path that takes it over the north and south
poles in an orbits perpendicular to the equatorial plane.
 It follows the low altitude path that is close to earth and passes over and very close to
both the north and south poles.
 The angle of inclination of a satellite in a polar orbit is nearly 90○.
 100 % of earth’s surface can be covered with a single satellite in a polar orbit.
 Satellites in polar orbit rotate around earth in a longitudinal orbit while earth is
rotating on its axis in a latitudinal rotation.
 Earth is not a perfect sphere, as it bulges at the equator.
 An important effect of the earth’s equatorial bulge is causing elliptical orbits to rotate in a
manner that causes the apogee and perigee to move around the earth.
 This phenomena is called rotation of the line of apsides.
 However, for an angle of inclination is 63.4○, the rotation of the line of apsides is zero.
ANTENNA LOOK ANGLES:
 To optimize the performance of a satellite communications system, the direction of
maximum gain of an earth station antenna must be pointed directly at the satellite.
 To ensure that the earth station antenna is aligned two angles must be determined
1. Azimuth angle
2. Elevation angle
 Azimuth angle and elevation angle are jointly referred to as the antenna look angles.
1. Azimuth angle:
 Azimuth is the horizontal angular distance from a reference direction, either the
southern or northern most point of the horizon.
 Azimuth angle is defined as the horizontal pointing angle of an earth station
antenna.
 For navigation purposes, azimuth angle is usually measured in a clockwise
direction in degrees from true north.
 For satellites earth stations in the northern hemisphere and satellite vehicles in geo
synchronous orbits, azimuth angle is generally referenced to true south.
2. Angle of elevation:
 Angle of elevation is also called as elevation angle, is the vertical angle formed
between the direction of travel of an electromagnetic wave radiated from an earth
station antenna pointing directly towards a satellite and the horizontal plane.



The smaller the angle of elevation, the greater the distance a propagated wave
must pass through earth’s atmosphere.
If the angle of elevation is too small and the distance the wave travels through
earth’s atmosphere is too long, the wave may deteriorate to the extent that it no
longer provides acceptable transmission quality.
Generally 5○ is considered as the minimum acceptable angle of elevation.
SATELLITE ANTENNA RADIATION PATTERNS:
FOOTPRINTS:
 The geographical representation of a satellite antenna’s radiation pattern is called a
footprint or sometimes a footprint map.
 A footprint of a satellite is the area on earth’s surface that the satellite can receive from
or transmit to.
 The shape of the satellite footprint depends on the satellite orbital path, height and the
type of antenna used.
 The higher the satellite, the more of the earth’s surface it can cover.
 Downlink satellite antennas broadcast microwave frequency signals to a selected
geographic region within view (line of sight) of the spacecraft.
 The effective power transmitted is called effective isotropic radiated power (EIRP) and is
generally expressed in dBm or dBw.
 A footprint map is constructed by drawing continuous lines between all points on a map
with equal EIRP’s.
 The pattern of the contour lines and power levels of a footprint are determined by
precise details of the downlink antenna design as well as by the level of microwave
power generated by each onboard channel.
 Each transponder is physically separate electronic circuit; signals from multiple
transponders are typically down linked through the same antenna.
 The shape, size and orientation of a satellite downlink antenna and the power generated
by each transponder determine geographic coverage and EIRPs.
 Radiation patterns from a satellite antenna are generally categorized as,
o Spot
o Zonal
o Hemispherical
o Earth or global
1. Spot and zonal beams:
 Smallest beams are spot beams followed by zonal beams.

2.
3.
4.
5.
Spot beams concentrate their power to very small geographic areas and therefore,
typically have proportionately higher EIRPs than those targeting much larger
areas because a given output power can be more concentrated.
 Spot and zonal beams blanket less than 10% of the earth’s surface.
 The higher the downlink frequency, the more easily a beam can be focused into a
smaller spot pattern.
Hemispherical beams:
 Hemispherical downlink antennas typically target up to 20% of the earth’s
surface.
 Therefore have EIRPs that are 3 dB or 50% lower than those transmitted by spot
beams that typically cover only 10% of the earth’s surface.
Earth or global beams:
 Radiation pattern of earth coverage antennas have a beam width of approximately
17○ and are capable of covering approximately 42○ of earth’s surface, which is the
maximum view.
 Power levels are considerably lower with earth beams than with spot, zonal or
hemispherical beams.
 Large receiver dishes are needed to adequately detect video, audio and data
broadcasts.
Reuse:
 When an allocated frequency band s filled, additional capacity can be achieved by
reuse of the frequency spectrum.
 By increasing the size of the antenna, the beam width of the antenna is also
reduced.
 Thus different beams of the same frequency can be directed to different
geographical areas of earth.
 This is called frequency reuse.
Dual polarization:
 Another method of frequency reuse.
 Different information signals can be transmitted to different earth station receivers
using the same band of frequencies simply by orienting their electromagnetic
polarizations in an orthogonal manner.
 It is less effective because earth’s atmosphere has a tendency to reorient an
electromagnetic wave as it passes through.
 Reuse is simply another way to increase the capacity of a limited bandwidth.
SYSTEM LINK MODELS:
 A satellite system consists of three basic sections. They are
1. Uplink
2. Satellite transponder
3. Downlink
1. Uplink model:
 The primary component within the uplink section of a satellite system is the earth
station transmitter.
 A typical earth system consists of an IF modulator, an IF to RF microwave up
converter, a high power amplifier (HPA) and some means of band limiting the
final output spectrum (output band pass filters).
 The IF modulator converts the input base band signals to either a FM, a PSK or a
QAM modulated intermediate frequency.
 The up converter converts the IF to an appropriate RF carrier frequency.
 The HPA provides adequate gain and output power to propagate the signal to the
satellite transponder.
Baseband i/p
Modulator
Mixer
BPF
BPF
HPA
transponder
MW
Generator
2. Transponder:
 A typical satellite transponder consists of an input band limiting device (BPF), an
input low noise amplifier (LNA), a frequency translator, a low level power
amplifier, and an output band pass filter.
 This transponder is an RF to RF repeater.
 Other transponder configurations are IF and base band repeaters similar to those
used in microwave repeaters.
 The input BPF limits the total noise applied to the input of the LNA.
 The output of the LNA is fed to a frequency translator, which converts the high
band uplink frequency to the low band downlink frequency.
 The low level power amplifier, which is commonly a traveling wave tube,
amplifies the RF signal for transmission through the downlink to earth station
receivers.
 Each RF satellite channel requires a separate transponder.
Earth station
BPF
Low noise
amplifier
LNA
Mixer
MW Shift
oscillator
To other transponders
BPF
LNA
Earth station
3. Downlink model:
 An earth station receiver includes an input BPF, an LNA and an RF to IF down
converter.
 The BPF limits the input noise power to the LNA.
 The LNA is a highly sensitive, low noise device.
 The RF to IF down converter is a mixer/band pass filer combination that converts
the received RF signal to an IF frequency.
From satellite
transponder
BPF
Low noise
amplifier
LNA
Mixer
BPF
Demodulator
Base band out
MW
generator
4. Cross links:
 Occasionally, there is an application where it is necessary to communication
between satellites.
 This is done by using satellite cross links or inter satellite links (ISLs).
 Disadvantage of using an ISL is that both the transmitter and the receiver are
space bound.
OPTICAL COMMUNICATION SYSTEM:
INTRODUCTION:
 An optical communication system is the one that used light as the carrier of
information.
 Propagating light waves through earth’s atmosphere is difficult and often practical.
 Consequently optical fiber communication systems use glass or plastic fiber cables to
contain the light waves and guide them in a manner similar to the way
electromagnetic waves are guided through a metallic transmission medium.
HISTORY OF OPTICAL FIBER COMMUNICATION:
 In 1880, Alexander graham bell experimented with ah apparatus called a photo
phone.
 It was a device constructed from mirrors and selenium detectors that transmitted
sound waves over a beam of light.
 Transmission of light waves for any useful distance through earth’s atmosphere is
impractical because water vapor, oxygen and particulates in the air absorb and
attenuate the signals at light frequencies.
 The only practical type of optical communication systems is one that uses a fiber
guide.
 The fiber cables used in 1960 were extremely lossy, which limited optical
transmission to short distances.
Advantages of optical fiber cables:
1. Wider bandwidth and greater information capacity Optical fibers have greater
information capacity than metallic cables because of the wider bandwidths available with
optical frequencies.
 Optical fibers are available with bandwidths up to several thousand gigahertz.
2. Immunity to cross talk  Optical fibers are immune to cross talks because glass and
plastic fibers are non conductors of electrical current.
 Therefore fiber cables are not surrounded by a changing magnetic field, which is the
primary cause of cross talk between metallic conductors located close to each other.
3. Immunity to static interference  Because optical fibers are non conductors of electrical
current, they are immune to static noise due to electromagnetic interference caused by
lightning, electric motors, fluorescent lights and other electrical noise sources.
4. Environmental immunity  Optical fiber cables are more resistant to environmental
extremes than metallic cables. Because it can operate over a wider temperature range and
are less affected by corrosive liquids and gases.
5. Safety and convenience  Safer and easier to install and maintain than metallic cables.
 Because glass and plastic fibers are non conductors, there are no electrical voltages or
currents associated with them.
 Also they are more flexible, easier to work with, require less storage space, cheaper to
transport.
6. Lower transmission loss  Have less signal loss than metallic cables.
7. Security  It is impossible to tap into a fiber cable without user’s knowledge, and optical
cables cannot be detected with metal detectors.
8. Durability and reliability  Optical fiber cables last longer and are more reliable than
metallic facilities because fiber cables have a higher tolerance to changes in
environmental conditions and are immune to corrosive materials.
9. Economics  Cost of fiber cables is approximately the same as metallic cables.
 Fiber cables has less loss and require fewer repeaters which equates to lower
installation and system costs and has improved reliability.
Disadvantages of optical fiber cables:
1. Interfacing costs  To be practical and useful, optical cables must be connected to
standard electronic facilities which often require expensive interfaces.
2. Strength  It has significantly lower tensile strength than coaxial cable.
 This can be improved by coating the fiber with standard kevlar and a protective coat
of PVC.
3. Remote electrical power  it is necessary to provide electrical power to remote interface
or regenerating equipment.
 This can’t be accomplished with the optical cable, so additional metallic cables must
be included in the cable assembly.
4. Optical fiber cables are more susceptible to losses introduced by bending the cable
electromagnetic waves propagate through an optical cable by either refraction or
reflection.
Therefore bending the cable causes irregularities in the cable dimensions, resulting in a
loss of signal power
5. Specialized tools, equipments and training require special tools to splice and repair
cables and special test equipments to make routine measurements.
 Not only repairing fiber cables difficult and expensive, but technicians working on
optical cables also require special skills and training.
 Sometimes it is difficult to locate faults in optical cables because there is no electrical
continuity.
BLOCK DIAGRAM OF AN OPTICAL FIBER COMMUNICATION SYSTEM:
Three primary building blocks are the transmitter, the receiver, and the optical fiber cable.
 Transmitter is comprised of a voltage to current converter, a light source and a source to
fiber interface (light coupler).
 The fiber guide is the transmission medium, which is either an ultra pure glass or a plastic
cable.
 It may be necessary to add one or more regenerators to the transmission medium,
depending on the distance between the transmitter and receiver.
 The regenerator performs light amplification. But in reality the signal is not actually
amplified, it is reconstructed.
 The receiver includes a fiber to interface, a photo detector and a current to voltage
converter.
In the transmitter, the light source can be modulated by a digital or an analog signal.
 The voltage to current converter serves as an electrical interface between the input
circuitry and the light source.
 The light source is either an infrared light emitting diode (LED) or an injected laser diode
(ILD).
 The amount of light emitted by either an LED or ILD is proportional to the amount of
drive current.
 Thus, the voltage to current converter converts an input signal voltage to a current that is
used to drive the light source.
 The light outputted by the light source is directly proportional to the magnitude of the
input voltage.
 In essence, the light intensity is modulated by the input voltage.
The source to fiber coupler is a mechanical interface.
 Its function is to couple light emitted by the light source into the optical fiber cable.
 The optical fiber consists of a glass or plastic fiber core surrounded by a cladding and
then encapsulated in a protective jacket.
 The fiber to light detector coupling device is also a mechanical coupler.
 Its function is to couple as much light as possible from the fiber cable into the light
detector.
The light detector is generally a PIN (p-type intrinsic n-type) diode, an APD (avalanche
photodiode), or a phototransistor.
 All three of these devices convert light energy to current.
 A current to voltage converter is required to produce an output voltage proportional to the
original source information.

The current to voltage converter transforms changes in detector current to changes in
voltage.
The analog or digital interfaces are electrical interfaces that match impedances and signal levels
between the information source and destination to the input and output circuitry of the optical
system.
OPTICAL FIBER TYPES:
Optical fiber construction:
 Optical cable is generally considered to include both the fiber core and its cladding.
 A special lacquer, silicone or acrylate coating is generally applied to the outside of the
cladding to seal and preserve the fiber’s strength helping maintain the cables attenuation
characteristics.
 The coating also helps protect the fiber from moisture, which reduces the possibility of
the occurrence of a detrimental phenomenon called stress corrosion cause by high
humidity.
 The protective coating is surrounded by a buffer jacket, which provides the cable
additional protection against abrasion and shock.
 Materials commonly used for the buffer jacket include steel, fiberglass, plastic, flame
retardant polyvinyl chloride (FR-PVC) and paper.
 The buffer jacket is encapsulated in a strength member, which increases the tensile
strength of the overall cable assembly.
 Finally, the entire cable assembly is contained in an outer polyurethane jacket.
There are three essential types of optical fibers commonly used today. All three varieties are
constructed of either glass, plastic or a combination of glass and plastic.
1. Plastic code and cladding
2. Glass core with plastic cladding (called as PCS fiber [plastic clad silica])
3. Glass core and glass cladding (called as SCS fiber [silica clad silica])
 Plastic fibers are more flexible than glass. Therefore, plastic cables are easier to install,
can better withstand stress, less expensive and less weight.
 But it has higher attenuation characteristics and do not propagate light as efficiently as
glass.
 Therefore, plastic fibers are limited to short cable runs, such as within a single building.
 Fibers with glass cores have less attenuation than plastic fibers, with PCS being slightly
better than SCS.
 PCS fibers are also less affected by radiation and therefore less immune to external
interfaces.
Optical fiber configuration:
Light can be propagated down an optical fiber cable using either reflection or refraction.
How the light propagates depends on the mode of propagation and the index profile of the fiber.
1. Mode of propagation:
 Mode simply means path in fiber optics terminology.
 If there is only one path for light rays to take down a cable, it is called single mode.
 If there is more than one path, it is called multimode.

The number of modes possible for a multi mode fiber cable depends on the frequency
(wavelength) of the light signal, the refractive indexes of the core and cladding and
the core diameter.
 Mathematically, the number of modes possible for a given cable can be approximated
by the following formula:
N≈((Πd/λ)SQ(n12- n22))2
Where N = number of propagating modes
D= core diameter (meters)
λ = wavelength (meters)
n1 = refractive index of core
n2 = refractive index of cladding
2. Index profile:
 The index profile of an optical fiber is a graphical representation of the magnitude
of the refractive index across the fiber.
 The refractive index is plotted on the horizontal axis and the radial distance from the
core axis is plotted on the vertical axis.
There are two basic types of index profiles. They are
1. Step index
2. Graded index
1. Step index:
 It has a central core with a uniform refractive index.
 An outside cladding that also has a uniform refractive index surrounds the
core; however the refractive index of the cladding is less than that of the central
core.
2. Graded index:
 There is no cladding and the refractive index of the core is non uniform.
 It is the highest in the center of the core and decreases gradually with distance
towards the outer edge.
 The index profile shows a core density is maximum in the center and decreases
symmetrically with distance from the center.
OPTICAL FIBER CLASSIFICATIONS:
Although there are a wide variety of combination of modes and indexes, there are only three
practical types of optical fiber configurations. They are
1. Single mode step index
2. Multi mode step index
3. Multi mode graded index
1. Single mode step index:
 A single mode step index fiber has a central core that is significantly smaller in
diameter.
 The diameter is sufficiently small that there is essentially only one path that light may
take as it propagates down the cable.
 In the simplest form of single mode step index fiber, the outside cladding is simply
air.
 The refractive index of the glass core (n1) is approximately 1.5 and the refractive
index of the air cladding (n2) is 1.
 A single mode step index fiber has a wide external acceptance angle, which makes it
relatively easy to couple light into the cable from an external source.
2. Multi mode step index:
 Multi mode step index fibers are similar to the single mode step index fibers except
the center core is much larger with the multimode configuration.
 This type of fiber has a large light to fiber aperture and consequently, allows more
external light to enter the cable.
 The light rays that strike the core/cladding interface at an angle greater than the
critical angle are propagated down the core in a zigzag fashion, continuously
reflecting off the interface boundary.
 The light rays that strike the core/cladding interface at an angle less than the critical
angle ray enter the cladding and are lost.
 As a result, all light rays don’t follow the same path and consequently, don’t take the
same amount of time to travel the length of the cable.
3. Multi mode graded index:
 These are characterized by a central core with a non uniform refractive index.
 Thus cable density is its maximum at the center and decreases gradually toward the
outer edge.
 Light rays propagate down this type of fibers through refraction rather than
reflection.
 As a light rays propagates diagonally across the core toward the center, it is
continually intersecting a less dense to more dense interface.
 Light rays enter the fiber at many different angles. As the light rays propagate down
the fiber, the rays travelling in the outer most area of the fiber travel a greater distance
than the rays traveling near the center.
 Because, the refractive index decreases with distance from the center and the velocity
is inversely proportional to refractive index, the light rays travelling farthest from the
center propagate at a higher velocity.
OPTICAL FIBER COMPARISON:
1. Single mode step index:
Advantages:
1. Minimum dispersion: All rays propagating down the fiber take approximately the
same path; thus take approximately the same length of time to travel down the cable.
2. Because of high accuracy in reproducing transmitted pulses at the receive end, wider
bandwidths and higher information transmission rates are possible.
Disadvantages:
1. Because the central core is very small, it is difficult to couple light into and out of this
type of fiber.
2. Again, because of the small central core, a highly directive light source, such as a
laser is required to couple light into a single mode step index fiber.
3. These are expensive and difficult to manufacture.
2. Multi mode step index:
Advantages:
1. These are relatively inexpensive and simple to manufacture.
2. It is easier to couple light into and out of multi mode step index fibers because they
have a large source to fiber aperture.
Disadvantages:
1. Light rays may take different paths down the fiber, which results in large differences
in propagation times.
2. This type of fiber has a tendency to spread out. A pulse of light propagating down a
multi mode step index fiber is distorted more than with the other types of fibers.
3. The bandwidths and rate of information transfer rates possible with this type of cable
are less than that possible with the other types of fiber cables.
3. Multi mode graded index:
There are no outstanding advantages and disadvantages of this type of fiber.
1. These are easier to couple light into and out of than single mode step index fibers but
more difficult than multi mode step index fibers.
2. Distortion due to multiple propagation paths is greater than in single mode step index
fibers but less than in multi mode step index fibers.
3. This multimode graded index fiber is considered an intermediate fiber compared to
the other fiber types.
LOSSES IN OPTICAL FIBER CABLES:
Power loss:
 Power loss is often called attenuation and results in a reduction in the power of the
light wave as it travels down the cable.
 Attenuation has several adverse effects on performance, including reducing the
system’s bandwidth, information transmission rate, efficiency and overall system
capacity.
 The standard formula for expressing the total power loss in an optical fiber cable is
A(dB) = 10 log (Pout/Pin)
Where A(dB) = total reduction in power level, attenuation (unit less)
Pout = cable output power (watts)
Pin = cable input power (watts)
 The optical power in watts measured at a given distance from a power source can be
determined mathematically as,
P=Pt *10-Al/10
Where P = measured power level (watts)
Pt = transmitted power level (watts)
A= cable power loss (dB/km)
l = cable length (km)
 Likewise, the optical power in decibel units is
P(dBm) =Pin (dBm) – Al(dB)
Where P = measured power level (dBm)
Pin = transmitted power (dBm)
Al= cable power loss, attenuation (dB)
1. Absorption loss:
 Absorption losses in optical fibers are analogous to power dissipation in copper
cables; impurities in the fiber absorb the light and convert it to heat.
 There are 3 factors that contribute to the absorption losses in optical fibers.
o Ultraviolet absorption
o Infrared absorption
o Ion resonance absorption
Ultraviolet absorption:
 Ultra violet absorption is caused by valence electrons in the silica material from which
fibers are manufactured.
 Light ionizes the valence electrons into conduction.
 This ionization is equivalent to loss in the total light field and contributes to the
transmission losses of the fiber.
Infrared absorption:
 Infrared absorption is a result of photons of light that are absorbed by the atoms of the
glass core molecules.
 The absorbed photons are converted to random mechanical vibrations typical of heating.
Ion resonance absorption:
 Ion resonance absorption is caused by OH- ions in the material.
 The source of OH- ions is water molecules that have been trapped in the glass during the
manufacturing process.
 Iron, copper and chromium molecules also cause ion absorption.
2. Material, or Rayleigh, Scattering Losses:
 During manufacturing, glass is drawn into long fibers of very small diameter.
 During this process, the glass is in a plastic state. The tension applied to the glass causes
the cooling glass to develop permanent submicroscopic irregularities.



When light rays propagating down a fiber strike one of these impurities, they are
diffracted.
Diffraction causes the light to disperse or spread out in many directions. Some of the
diffracted light continuous down the fiber, and some of it escapes through the cladding.
The light rays that escape represent a loss in light power. This is called Rayleigh
scattering loss.
3. Chromatic or wavelength, dispersion:
 LEDs emit light containing many wavelengths. Each wavelength within the
composite signal travels at a different velocity when propagating through glass.
 Consequently, light rays that are simultaneously emitted from a LED and propagated
down an optical fiber do not arrive at the far end of the fiber at the same time,
resulting in an impairment called chromatic distortion.
 Chromatic distortion can be eliminated by using a monochromatic light source such
as an injection laser diode (ILD).
 Chromatic distortion occurs only in fibers with a single mode of transmission.
4. Radiation losses:
 Radiation losses are cause mainly by small bends in the fiber. There are two types of
bends.
 Micro bends  Occurs as a result of differences in the thermal contraction rates between
the core and cladding material.
 It is miniature bend or geometric imperfections along the axis of the fiber
and represents a discontinuity in the fiber where Rayleigh scattering can
occur.
 Constant radius bends  Caused by excessive pressure and tension and generally occur
when fibers are bent during handling or installation.
5. Modal dispersion:
 Also called as pulse spreading is caused by the difference in the propagation times of
light rays that take different paths down a fiber.
 Modal dispersion can occur only in multi mode fibers. It can be reduced considerably by
using graded index fibers and almost entirely eliminated by using single mode step index
fibers.
 For multimode propagation, dispersion is often expressed as a bandwidth length product
(BLP) or bandwidth distance product (BDP).
 BLP indicates what signal frequencies can be propagated through a given distance of
fiber cable and is expressed mathematically as the product of distance and bandwidth.
 BLP is expressed in MHz – km units. The length of optical cable inversely proportional
to the bandwidth.
6. Coupling losses:
 Coupling losses are caused by imperfect physical connections. In fiber cables, coupling
losses can occur at any of the following three types of optical junctions.
 Light source to fiber connections
 Fiber to fiber connections
 Fiber to photo detector connections
 Junction losses are most often caused by one of the following alignment problems.
 Lateral misalignment
 Gap misalignment
 Angular misalignment
 Imperfect surface finishes
1. Lateral displacement:
 It is the lateral or axis displacement between two pieces of adjoining fiber cable.
The loss is generally negligible if the fiber axes are aligned to within 5 % of the
smaller fiber’s diameter.
2. Gap displacement:
 Gap displacement is sometimes called end separation. When splices are made in
optical fibers, the fibers should actually touch.
 The farther apart the fiber, the greater the loss of light. If two fibers are joined
with a connector, the ends should not touch because the two ends rubbing against
each other in the connector could cause damage to either or both fibers.
3. Angular displacement:
 Also called as angular misalignment. If it is less than 2 degree, the loss will
typically less than 0.5 dB.
4. Imperfect surface finish:
 The ends of the two adjoining fibers should be highly polished and fit together
squarely.
 If the fiber ends are less than 3 degree off from perpendicular, the losses will
typically be less than 0.5 dB.
Light Sources:
 Light sources used for optical fiber systems must be at wavelengths efficiently
propagated by the optical fiber.
 The range of wavelengths must be considered because the wider the range, the more
likely the chance that chromatic dispersion will occur.

Light sources must also produce sufficient power to allow the light to propagate through
the fiber without causing distortion in the cable itself or in the receiver.
Optical sources:
 There are two types of practical light sources used to generate light for optical fiber
communication systems:
 LED
 ILD
 Both devices are constructed from semiconductor materials and have advantages and
disadvantages.
LED:
 A LED is a p-n junction diode, usually made from a semiconductor material such as
aluminium-gallium-arsenide (AlGaAs) or gallium-arsenide-phosphide (GaAsP).
 LEDs emit light by spontaneous emission – light emitted as a result of the recombination
of electrons and holes.
Function:
 With forward biased, minority carriers are injected across the p-n junction.
 Once across the junction, these minority carries recombine with majority carriers and
give up energy in the form of light.
 This process is the same as in a conventional semiconductor diode except that in LEDs
certain semiconductor materials and dopants are chosen such that the process is radiative;
that is the photon is produced.
 A photon is a quantum of electromagnetic wave energy. Photons are particles that travel
at the speed of light but at rest have no mass.
 The energy gap of the material used to construct an LED determines the color of light it
emits and whether the light emitted by it is visible to human eye.
1. Homojunction LED:
 A p-n junction made from two different mixtures of the same types of atoms is
called homojunction structure.
 The simplest LED structures are homojunction and epitaxially grown, or they are
single diffused semiconductor devices.
Epitaxially grown LED:
 It is generally constructed of silicon-doped gallium-arsenide.
 A typical wavelength of light emitted from this construction is 940 nm, and a typical
output power is approximately 2 mW at 100mA of forward current.
Planar diffused homojunction LED:
 This type of LEDs output approximately 500 micro watts at a wavelength of 900 nm.
 The primary disadvantage of homojunction LEDs is the non-directionality of their light
emission, which makes them a poor choice as a light source for optical fiber systems.
Heterojunction LEDs:
 These are made from p-type semiconductor material of one set of atoms and an n-type
semiconductor material from another set.
 These devices are layered (usually two) and the junction is manufactured on a substrate
backing material and then sandwiched between metal contacts that are used to connect
the device to a source of electricity.
 With heterojunction devices, light emitted from the edge of the material and are therefore
often called edge emitters.
Planar heterojunction:
 A planar heterojunction LED is quite similar to the epitaxially grown LED except that the
geometry is designed such that the forward current is concentrated to a very small area of
the active layer.
Advantages of heterojunction devices over homojunction devices:
 Increase in current density generates a more brilliant light spot.
 The smaller emitting area makes it easier to couple its emitted light into a fiber.
 The smaller effective area has a smaller capacitance, which allows the planar
heterojunction LED to be used at higher speeds.
Burrus Etched-Well Surface-Emitting LED:
 For more practical applications, data rates in excess of 100 Mbps are required.
 For these applications, the etched will LED was developed. It is a surface emitting LED
that emits light in many directions.
 The etched well helps concentrate the emitted light to a very small area. Also domed
lenses can be placed over the emitting surface to direct the light into a smaller area.
 These devices are more efficient than the standard surface emitters, and they allow more
power to be coupled into the optical fiber.
 But they are also more difficult and expensive to manufacture.
Edge emitting LED:
 These LEDs emit a more directional light pattern than do the surface-emitting LEDs.
 The concentration is similar to the planar and burrus diodes except that the emitting
surface is a stripe rather than a confined circular area.
 The light emitted from an active stripe and forms an elliptical beam.
 Surface-emitting LEDs are more commonly used than edge emitters because they emit
more light.
ILD:
 The type of laser used most often for fiber-optic communications is the semiconductor
laser.
 The ILD is similar to the LED. Above the threshold current, an ILD oscillates; lasing
occurs.
 As current passes through a forward biased p-n junction diode, light emitted by
spontaneous emission at a frequency determined by the energy gap of the semiconductor
material.
 When a particular current level is reached, the number of minority carriers and photons
produced on either side of the p-n junction reaches a level where they begin to collide
with already excited minority carriers.
 This causes an increase in the ionization energy level and makes the carriers unstable.
 When this happens, a typical carrier recombines with an opposite type of the carrier at an
energy level that is above its normal before collision value.
 In the process, two photons are created; one is stimulated by another.
 For this to happen, a large forward current that can provide many carriers is required.
 The construction of ILD is similar to that of an LED except that the ends are highly
polished.

The mirror like ends trap the photons in the active region and as they reflect back and
forth, stimulate free electrons to recombine with holes are a higher than normal energy
level. This process is lasing.

The radiant output light power of a typical ILD is shown below. It can be seen that very
little output power is realized until the threshold current is reached; then lasing occurs.
The radiation patterns of an LED and an ILD is shown below. Because light is radiated
out the end of an ILD in a narrow concentrated beam, it has more direct radiation pattern.

Advantages of ILD over LED:
 ILD emit coherent (orderly) light, whereas LED emits incoherent (disorderly) light.
 Therefore ILDs have a more direct radian pattern, making it easier to couple light emitted
by the ILD into an optical fiber cable.
 This reduces the coupling losses and allows smaller fibers to be used.
 The radiant output power from an ILD is greater than that for an LED.
 ILDs can be used at higher bit rates than LEDs.
 ILDs generate monochromatic light, which reduces chromatic or wavelength dispersion.
Disadvantages of ILD:
 These are typically 10 times more expensive than LEDs.
 Because ILDs operate at higher powers, they typically have a much shorter lifetime than
LEDs.
 ILDs operate more temperature dependent than LEDs.
Light Detectors:
 There are two devices commonly used to detect light energy in the fiber optic
communication receivers. They are
 PIN diodes
 APDs
PIN diodes:
 A PIN diode is a depletion-layer photodiode and is probably the most common device
used as a light detector in fiber optic communication systems.
 A very lightly doped layer of n-type semiconductor material is sandwiched between the
junction of the two heavily doped n and p-type contact areas.
 Light enters the device through a very small window and falls on the carrier void intrinsic
material.
 The intrinsic material is made thick enough so that most of the photons that enter the
device are absorbed by this layer.
 The PIN photodiode operates just the opposite of an LED.
 Most of the photons are absorbed by electrons in the valence band of the intrinsic
material.
 When the photons are absorbed, they add sufficient energy to generate carriers in the
depletion region and allow the current to flow through the device.
Photoelectric effect:
 Light entering through the window of a PIN diode is absorbed by the intrinsic material
and adds energy to cause electrons to move from the valence band into the conduction
band.
 The increase in the number of electrons that move into the conduction band is matched
by an increase in the number of holes in the valance band.
 To cause current to flow in a photodiode, light of sufficient energy must be absorbed to
give valence electrons enough energy to jump the energy gap.
 The energy gap for silicon is 1.12 eV. Mathematically the operation is as follows:
For silicon, the energy gap (Eg) equals to 1.12 eV:
1 eV =1.6*10-19J
 Thus the energy gap for silicon is
Eg = (1.12 eV)( 1.6*10-19J/eV) = 1.792 *10-19J



And energy (E) = hf
Where h = planck’s constant =6.6256 *10-34 J/Hz
f = frequency (Hz)
Rearranging and solving for f yields
f = E/h
For a silicon photodiode,
f = 1.792 *10-19J / 6.6256 *10-34 J/Hz = 2.075 *1014Hz
Converting to wavelength yields,
λ = c/f = (3*108 m/s) / (2.075 *1014Hz) = 1109 nm/cycle
APDs:
 An APD is a pinp structure.
 Light enters the diode and is absorbed by the thin, heavily doped n-layer.
 A high electric field intensity developed across the i-p-n junction by reverse bias causes
impact ionization to occur.
 During impact ionization, a carrier can gain sufficient energy to ionize other bound
electrons.
 These ionized carriers, in turn, cause more ionization to occur.
 The process continues as in an avalanche and is effectively, equivalent to an internal gain
or carrier multiplication.
 APDs are more sensitive than PIN diodes and require less additional amplification.
 The disadvantages of APDs are relatively long transit times and additional internally
generated noise due to the avalanche multiplication factor.
Characteristics of light detectors:
The most important characteristics of light detectors are the following:
1. Responsivity: A measure of the conversion efficiency of a photo detector. It is the ratio of
the output current of a photo diode to the input optical power and has the unit of
ampheres per watt. Responsivity is generally given for a particular wavelength or
frequency.
2. Dark current: The leakage current that flows through a photodiode with no light input.
Thermally generated carriers in the diode cause dark current.
3. Transit time: The time it takes a light induced carrier to travel across the depletion region
of a semiconductor. This parameter determines the maximum bit rate possible with a
particular photodiode.
4. Spectral response: The range of wavelength values that a given photodiode will respond.
Generally, relative spectral response is graphed as a function of wavelength or frequency.
5. Light sensitivity: The minimum optical power a light detector can receive and still
produce a usable electrical output signal. Light sensitivity is generally given for a
particular wavelength in either dBm or dBμ.
Solved Problems:
1. A glass fiber with nc =1.5 and nc1 = 1.45 has a core diameter of 9μm and is used at λ = 1.3
μm. Find its normalized frequency.
Solution:
Given:
n1 = nc = 1.5
n2 = nc1 =1.45
d = 9 *10-6 m
λ = 1.3 * 10-6
Normalized frequency:
V = Π (d/ λ) (SR (n12 – nc12 )
= Π (9 *10-6 m /1.3 * 10-6) (SR (1.52 -1.452)
= 8.35