1 24051-ADVANCED COMMUNICATION SYSTEM QUESTION BANK RANGANATHAN POLYTECHNIC COLLEGE, COIMBATORE. Prepared by A.Indhumathi, Lecturer/ECE UNIT-I RADAR AND NAVIGATIONAL AIDS 1. Expand RADAR. RAdio Detection And Ranging 2. What is RADAR? RADAR is basically a means of gathering information about distant objects or targets, by sending electromagnetic waves. 3. What are the types of RADAR? According to the principle of operation, the RADAR is classified into 2 types: they are, Pulsed Radar system Continuous Radar system. 4. What are the applications of RADAR? Radars used in defense weapons system & in safety 7 navigation applications Used to locate enemy missiles, planes & ships Used as altimeters to measure height 5. Give the expression for RADAR range equation. = 0 2 42 1 4 ; 0 = 2 4Therefore, = 2 2 43 1 4 6. Give any two factors that influencing maximum range. By increasing the transmitted power, the range can be increased. The maximum range is achieved by reducing the minimum detectable signal power. 7. What are the display methods of radar receiver? A Scope Plan Position Indicator Direct feeding of a computer 8. What are the limitations of PPI? Distortion of true map position will occur if PPI is on an aircraft & its antenna is not pointing straight down.
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24051-ADVANCED COMMUNICATION SYSTEM QUESTION BANK
RANGANATHAN POLYTECHNIC COLLEGE, COIMBATORE.
Prepared by A.Indhumathi, Lecturer/ECE
UNIT-I RADAR AND NAVIGATIONAL AIDS
1. Expand RADAR.
RAdio Detection And Ranging
2. What is RADAR?
RADAR is basically a means of gathering information about distant objects or
targets, by sending electromagnetic waves.
3. What are the types of RADAR?
According to the principle of operation, the RADAR is classified into 2 types:
they are,
Pulsed Radar system
Continuous Radar system.
4. What are the applications of RADAR?
Radars used in defense weapons system & in safety 7 navigation
applications
Used to locate enemy missiles, planes & ships
Used as altimeters to measure height
5. Give the expression for RADAR range equation.
𝑟𝑚𝑎𝑥 = 𝑃𝑡𝑆𝐴0
2
4𝜋𝜆2𝑃𝑚𝑖𝑛
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; 𝐴0 = 𝐴𝑝𝜆2
4𝜋
Therefore, 𝑟𝑚𝑎𝑥 = 𝑃𝑡𝑆𝐴𝑝
2 𝜆2
4𝜋 3𝑃𝑚𝑖𝑛
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6. Give any two factors that influencing maximum range.
By increasing the transmitted power, the range can be increased.
The maximum range is achieved by reducing the minimum detectable signal
power.
7. What are the display methods of radar receiver?
A Scope
Plan Position Indicator
Direct feeding of a computer
8. What are the limitations of PPI?
Distortion of true map position will occur if PPI is on an aircraft & its antenna is
not pointing straight down.
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9. What are the applications of PPI?
PPI display can be used in search radars and especially when conical scanning
is employed.
10. What is Automatic Target detection?
The function of the radar operator viewing the ordinary radar display is to
recognize the presence of target & extract their location. When the function is
performed by electronic decision circuitry without intervention of an operator, the
process is known as automatic target detection.
11. What are the other names of automatic target detector?
The automatic detector is also called as plot extractor & data extractor.
12. What is the advantage of automatic target detector?
It is used to overcome the limitations of an operator due to fatigue, boredom &
overload.
13. What is the use of automatic target detector?
It allows the radar output to be transmitted over telephone lines rather than by
more expensive broadband microwave links.
14. What are the basic aspects of automatic target detection?
The four basic aspects are:
Integration of pulses from target
Detection decision & determination of target location
Range
Azimuth
15. What is direction finding?
Radar direction finding is a useful navigational aid. In this system, the waves
propagate away from the transmitter along a great circle, which makes radio direction
finding. This system operates at low & medium frequency ranges.
16. What is radio range?
A navigation system that establishes a course through space along which
aircraft can navigate from one location to another is termed as a radio range.
17. What is radio compass?
An instrument which is used in ships & aircrafts for finding the directions is
called compass. If this system is operated in radio signal, it is called radio compass.
18. What are the operating frequencies of radio telemetry systems?
The operating frequency is 216 to 220MHz & 2.2 to 2.3 GHz.
19. What are the electronic systems used for landing aircraft?
The types of aircraft landing systems are
Ground Controlled Approach
Instrument Landing System
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20. What is glide-path equipment?
The glide-path equipment or glide path facility provides an equi-signal path
type of guidance in the vertical plane by the equi-signal path of the localizer.
21. What are the two radars employed by GCA?
Airport Surveillance Radar (ASR)
Precision Approach Radar (PAR)
22. What is the use of ASR?
The ASR is used for control the traffic pattern of aircraft in the vicinity of the
landing field. It has a range of approximately 30miles. It is a high resolution short
range set. It is used to conduct the actual landing.
23. What is the advantage of GCA system?
No equipment is required in the aircraft other than an ordinary radio receiver &
the ground installation can be mobile.
24. What is the disadvantage of GCA system?
The disadvantages are that there are number of human links in the chain & that
the chief responsibility for making a successful landing is taken away from the pilot.
25. What is a telephone system?
The telephone is derived from the greek words; tele means Far, and phone
means sound. Telephony thus involves the conversion of sound signals into an audio
frequency analog electrical signal.
26. What are the classifications of telephone system?
One-way communication system (Simplex system)
Two-way communication system (Duplex system)
27. What is simplex system?
In this system, transmission takes place in only one direction at a time, but it
provides communication in either direction alternatively.
28. What is duplex system?
In this system, transmission takes place in both directions. A pair of wires used
for this purpose.
29. What are the requirements of telephone systems?
It must be able to transmit voice signals in both directions.
It must provide the means for signaling from each terminal towards the
other.
30. What is ISDN?
ISDN stands for Integrated Service Digital Network for providing an end to end
connectivity over a digital link. It is basically a circuit switched digital network. In
ISDN line a wide range of digital services such as voice, data, image, video, facsimile
etc. can be integrated.
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31. Expand PAR.
Precision Approach Radar
32. What is scanning?
The scanning process examines each line turn & extracting the value for each
pixel on the line in a sequential manner usually from left to right until entire image is
scanned.
33. What is a facsimile communication system?
A facsimile means an exact reproduction. In facsimiles transmission document
or picture is exactly reproduced at the receiving end.
34. Define index of cooperation.
The IOC is a number derived from the width-height ratio.
35. What is direct recording?
Direct recording means that the image is transferred directly to the copy paper,
no post processing is required to develop the image.
36. What are the four reference points in ISDN model?
U – Connection between ISDN exchange in the carrier‟s office & NT1.
T – Connector on NT1 provided to the customer
S – Interface between the ISDN PBX and the ISDN terminals
R – Connection between the terminal adapter and non-ISDN terminals.
37. Expand CCITT.
Consultative Committee for International Telegraphy and Telephony
38. What is the use of synchronization?
In synchronous system, it is necessary for the receiver scanner and the
transmitter scanner to run at exactly the same speed.
39. What are the features of ISDN?
High speed internet access
High flexibility
Video conferencing
Provides data rate of 3Mbps on a local link & 64 Kbps/128Kbps over a wide
area link
40. Expand GCA.
Ground Controlled Approach
41. Expand IOC.
Index Of Cooperation
42. Expand ILS.
Instrument Landing System
43. Expand PPI.
Plan Position Indicator
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44. Expand ISDN.
Integrated Service Digital Network
45. Expand ASR.
Airport Surveillance Radar
46. What are the uses of fax?
To transmit photographs, documents, weather maps and language texts etc.
47. What is ILS?
Instrument Landing System (ILS) is an electronic system used for Aircraft
Landing System (ALS). It consists of essential elements like Runway Localizer,
Glide-path Equipment & Marker Beacons.
UNIT-II DIGITAL COMMUNICATION
1. What is the use of data system?
The data system is required to transmit rectangular pulses at a rate of ranging from
100 to 500 Kbits per second. The typical uses are:
Computer to computer communication
Programming & data collection
Telemetry & alarm system
2. What are the types of digital communication?
On line system
Offline system
3. Define online system.
In an online system the data is transmitted directly to or from a computer.
4. Define offline system.
In an offline system the data is transmitted to or from an intermediate storage
point such as card, a paper tape punch, magnetic tape or disk.
5. What is real time system & non real time system?
If the system requires rapid response then it is a real time system.
In non real system the data is much delayed.
6. What is simplex, semi-simplex & full-duplex connection?
A one-way channel connection is known as simplex connection. An either way
transmission requires semi-duplex connection & a full-duplex is the one in which the
data can be sent in both directions simultaneously.
7. What is a digital signal?
The digital signal does not provide the continuous representation of the original
signal. Instead, the digital signal represents data as a series of digits such as a number.
This digital representation can be considered as a code which approximates the actual
value.
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8. What are the advantages of digital communication?
Relatively inexpensive digital circuits can be used.
Privacy is preserved by using data encryption
Greater dynamic range
In long distance systems, noise does not accumulate from repeater to
repeater.
9. What are the disadvantages of digital communication?
More bandwidth is required.
Synchronization is required.
10. What is the frequency range of telephone channel?
The frequency range of telephone channel is if 300 to 3400Hz, this referred to
within the industry as a 4 KHz channel.
11. What is the frequency range of HF radio & submarine cables?
The HF radio and submarine cables frequency range is 300-2800Hz.
12. What are the characteristics of digital communication?
Bandwidth requirement
Speed & Noise
Crosstalk & Equalizers
Baud rate &Distortion
Echo compressors
13. Define speed.
The transmission speed of a communication channel is described in „baud rate‟.
Baud rate is the unit of signaling speed.
14. Define Nyquist rate.
The effect of the noise on the data channel can be reduced by increasing the
signal to noise ratio for an ideal channel the nyquist rate twice the bandwidth.
15. Define baud rate.
Baud rate is the unit of signaling speed generally the information may transfer
at a rate equal to or different from the baud rate.
16. Define cross talk.
Any transmission system which conveys more than one signal simultaneously
can experience cross talk.
17. Define distortion.
Signals of different frequencies can be passed by a channel with different values
of amplitude attenuation & at different propagation speeds. This results in distortion.
18. Define equalizers.
Phase distortion & attenuation can be reduced by using equalizer on the
channel.
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19. What are the common codes used by computers?
Baudot code, Binary code, ASCII code, EBCDIC code
20. What is adaptive equalization?
The equalizer automatically varies its delay characteristics, while sampling the
return signal to determine when the channel delay plus equalizer delay reaches proper
tolerances. At that time data transmission commences. The data is thereafter sampled
during transmission to ensure that equalization settings are modified in appropriate
level. This type of equalization is called adaptive equalization.
21. Define echo compressors.
Echo compressors or echo suppressors are used on long distance circuits to
overcome echoes caused by circuit imbalances.
22. Mention the uses of baudot code.
Used in teletypewriter machines incorporating a paper to be punched & reader
mechanism.
23. Define Baudot code.
The baudot code is a 5 bit code which has been used in telegraphy & paper tape
systems. There are totally 32 different code combinations (25=32). This is not
sufficient to represent the alphabets (26), numerals (10) and other characters (+,-,>, <,)
hence we increase the total number of combination by incorporating the shift codes.
24. Mention the drawbacks of baudot code.
It does not provide extra combination of bits to code punctuation & various
codes.
All the five bits are used for coding the data. Hence error correction using parity
technique is not possible.
25. Expand CRC.
Cyclic Redundancy Code
26. Define ASCII code.
ASCII stands for American Standard Code for Information Interchange. It is a
seven bit code. The seven bits are formed, based on a standard binary propagation.
27. Mention the limitations of baudot code.
Only 5bits are available.
The numbers of combinations are limited only upto 25=32 codes.
This code is not sequential &It is not used for error detection & correction.
28. What are the advantages of ASCII code?
Error detection can be achieved by increasing the total numbers of bits to 8.The
parity bit is added as the 8th bit, usually the MSB.
It can be easily used in a computer. A modern computer uses hexadecimal code
for internal computations. Since ASCII is an 8 bit code with parity bit, it can be
easily accommodated in computer as 8 bit data.
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29. What is the use of ASCII code?
It is widely used in modern computers.
30. Define EBCDIC code.
EBCDIC code stands for Extended Binary Coded Decimal Interchange Code. It
is a 8 bit fixed length code. Here all bits are used for representing the information.
31. What are the advantages of EBCDIC code?
It is similar to ASCII code. It can be readily used in computer.
Total number of combinations is higher.
32. What are the disadvantages of EBCDIC code?
Here all the 8 bits are used for data encoding.
There is no provision for parity bit.
Here error correction is not possible.
33. What is a parity check code?
The most widely used approach for detecting errors that arise in storing &
moving words in the use of a parity check bit added to each character code group.
Codes of this type are called parity check codes.
34. What are the two types of parity check codes?
Odd parity & even parity.
35. Define redundant code.
Most error detection system use some form of redundancy to check whether the
received data contains errors. That means, additional data is sent with the basic data.
The redundancy takes the form of transmitting the information twice & comparing the
two sets of data to see that are same.
36. Define CRC.
The cyclic redundancy check is a more reliable error detection scheme. It is a
mathematical technique which is applied to the transmitted data. It is effective in
catching 99.9% or more of transmission errors.
37. What is constant ratio code?
The 2 out of 5 and 4 out of 8 codes depend upon the ratio of 1s to 0s, to indicate
that errors have occurred. So they are called constant ratio codes.
38. Mention the types of error correction methods.
Retransmission
Forward error correcting code
Hamming code.
39. Define error correction codes.
Error correction is an important aspect of data transmission. This process
involved with error correction normally results in an increase in the number of bits per
second which are transmitted, & naturally this increases the cost of transmission.
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40. Define hamming code.
Hamming code is an interesting error detection code. This code adds several
parity check bits to a data word. The hamming code is capable of locating a single
error but it fails if multiple errors occur in a single data block.
41. What is dibit?
Each of the 2-bit elements is called a dibit. Each of the 2bit pairs is converted to
a phase values of in the data set.
42. Define ASK modulation.
In amplitude shift keying (ASK) modulation, the digital signal is switched in
between amplitude levels of digital signal. The carrier signal is on & off accordance
with the digital binary input signal. The carrier signal is transmitted during the high
level input signal & blocked during low level signal. This method is also called as
ON-OFF keying (OOK).
43. Define QPSK modulation.
QPSK is a Quaternary Phase Shift keying or Quadrature PSK which is an angle
modulated constant amplitude digital modulation. It transmits higher bit rate.
44. Expand EBCDIC code.
Extended Binary Coded Decimal Interchange Code
45. What is even parity & odd parity?
Even parity means adding an extra bit to the group of bits to make the group of
bits to make the whole number of 1‟s as even. In case of odd parity, add one more bit on
the end of character, to make the whole word as odd number of 1‟s.
46. Define FSK.
Frequency shift keying is that which shifts carrier frequency to indicate a mark
or a space for a common digital signal. In this, the unmodulated carrier frequency
corresponds to mark condition & space by downward frequency shift.
47. Define PSK.
PSK is a system in which the carrier may be phase shifted by +90° for a mark &
-90° for a space. In this, the unmodulated carrier frequency corresponds to mark
condition & space by downward frequency shift.
UNIT-3 OPTICAL COMMUNICATION
1. What is fiber?
Optical fibers are replaced by wire transmission lines in communication
systems. Light is effectively the same as radio frequency radiation, but its frequency is
high. The information carrying capacity of fiber is very much greater than microwave
radio systems.
2. What is the frequency range of an optical waveguide?
The optical carrier frequency is in the range of 1013
to 1015
Hz, while the radio
frequency is about 106Hz and the microwave frequency range is about 10
10Hz.
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3. What are the materials used in fibers?
The material used in fiber is silica glass or silicon dioxide, which costs lower
than other wire lines.
4. What are the advantages of optical fiber?
Wider bandwidth
Low transmission loss
Dielectric waveguide or Electrical isolation
Immunity to crosstalk & EMI
Signal security,
Small size & weight
5. What are the limitations of optical fiber?
Limited application & Low power
Distance & Modulation
Nuclear radiation & Fragility
6. What is light pipe?
When light enters one end of a glass fiber under the light condition most of the
light will propagate or move down the length of the fiber and exit from the far end. A
small part of light will be lost due to internal absorption but a portion of light will be
contained and guided to the far end such a fiber is called light pipe or light guide.
7. What are the conditions for total internal reflection?
The glass inside the fiber core must have a slightly higher index of refraction
“n1” than the index of refraction “n2” of the material(cladding) surrounding the
fiber core.
The light must absorb the wall with an angle of incidence Ф (between the ray
path & the normal to the fiber wall) that is greater than the critical angle Фc,
which is defined as sin Фc= 𝑛2
𝑛1
8. Define critical angle.
The minimum angle at which the external light rays may enter the air/glass
interface and still propagate down the fiber is called critical angle θc
9. Define numerical aperture.
Numerical aperture (NA) is closely related to acceptance and is the figure of
merit commonly used to measure the magnitude of the acceptance angle.
10. Define Snell’s law.
Snell‟s law states that the incidence angle θ0 is related to the refraction angle by
θ1 by the relationship.
sin 𝜃0 = (𝑛1
𝑛0) sin 𝜃1
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11. What is meridonial ray?
The ray has an angle of incidence Ф at the interface which is greater than the
critical angle and is reflected at the same angle to the normal. The light ray is known
as meridonial ray as it passes through the axis of the fiber core.
12. Define acceptance angle.
Acceptance angle 𝜃𝑎 is the maximum angle in which external light rays may
strike the air/glass interface and still propagate down the fiber. Acceptance angle is the
maximum angle.
13. Define core & cladding.
The cylindrical shaped central part of the fiber is known as the core of the fiber.
The core is surrounded by a solid dielectric cladding. The cladding is not necessary for
the light to propagate along the fiber.
14. Define skew rays.
When a light input of the fiber is non-uniform, skew rays will therefore tend to
have a smoothing effect on the distribution of the light as it is transmitted, giving a
more uniform output. The amount of smoothing is dependent on the number of
reflections encountered by the skew rays.
15. What is the purpose of cladding?
The cladding also reduces scattering loss & adds mechanical strength.
16. Define step index fiber.
Step index fiber: In step index fibers the index of refraction of fiber of the fiber
core and the cladding interface are sharply defined in step it means that the core has
one constant index of refraction N1 while the cladding has another constant index of
refraction N2.
17. Define Graded indexfiber
Graded indexfiber: In graded index fiber the index of refraction of the core is
not constant instead, the index of refraction varies smoothly & continuously over the
diameter of the core.
18. Define single mode fiber.
Single mode fiber: in single mode fiber, light follows a single path through the
core.
19. Define multimode fiber.
Multimode fiber: in this the light takes many paths through the fiber.
20. What are the types of optical fibers?
Single mode step index fiber
Multi-mode step index fiber
Multi-mode graded index fiber
21. Expand LASER.
Light Amplification by Stimulated Emission of Radiation
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22. What is the advantage of multimode step index fiber?
The main advantage of a multimode step index fiber is the large size. Typical
core diameters are in 50-1000um range. Such large diameter cores excellent gathering
light & transmitting it efficiently. It is an inexpensive light source to produce the light
source.
23. What are the mechanisms responsible for attenuation in optical fiber?
Absorption loss&Scattering loss
Bending loss
Core & cladding loss
24. What are the types of absorption losses?
Ultraviolet absorption
Infrared absorption
Ion resonance absorption
25. What is ion resonance absorption?
During manufacturing, minute quantities of water molecules are trapped in the
glass. These water molecules make OH impurity ion in the material. These ions absorb
light energy. This is called ion resonance absorption.
26. Define infrared absorption.
When light propagates in the fiber, the photons of light energy are absorbed by
the atoms within the glass molecules. It is then converted to random mechanical
vibrations, with a result that heat is produced. It is called infrared absorption.
27. Define ultraviolet absorption.
When light propagates in the fiber of pure fused silica, the valance electrons can
be ionized into conduction band. The energy for this ionization is drawn from the
light, propagate through it. It produces a transmission loss called ultraviolet
absorption.
28. What are the types of scattering losses?
Linear scattering losses
Rayleigh scattering
Mie scattering
Non-linear scattering losses
Stimulated brilliouin scattering
Stimulated remand scattering
29. What is bending loss? Give the types of bending losses.
Whenever the optical fiber contains bends, then the bends produce radiative
losses.
Types: 1) Micro bending losses
2) Macro bending losses.
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30. What is intermodal dispersion?
Intermodal dispersion arises due to the variation of group velocity for each
mode at a single frequency
31. What is dispersion? Mention its types.
A pulse of light with a given width & amplitude transmitted into one end of a
fiber should theoretically arrive at the far end with its shape & width unchanged. But
its amplitude is reduced by losses. Types of dispersions are: Intermodal dispersion
&intramodel dispersion
32. What is intramodel dispersion?
Intramodel dispersion arises due to the dependence of group velocity on the
wavelength. Further it increases with the increase of special width of the optical
source. This spectral width is the range of wavelengths emitted by the optical source.
33. What is material dispersion?
Material dispersion is sometimes referred to as chromatic dispersion, because it
produces the same effect by which a prism spreads out a spectrum. This causes a
wavelength dependence of the group velocity of any given mode, that is pulse
spreading occurs even when different wavelength follow the same path.
34. What is waveguide dispersion?
The waveguide dispersion arises due to the finite frequency bandwidth and the
dependence of the mode group velocity with frequency of light. Higher the frequency
bandwidth of the transmitted pulse, higher will be the waveguide dispersion.
35. What is LED?
A light emitting diode works by the process of spontaneous emission when it is
forward biased and conducting current.
36. What are the types of diodes or detectors used in communication applications?
PIN photodiode & Avalanche photodiode
37. What is PIN photodiode?
An ordinary “PN” diode may be used as a photo detector. It has sufficient
speed, but its sensitivity is very low. The sensitivity of the pn photodiode can be
improved by including a lightly doped n layer between the junction and the more
heavily doped n-contact region to form the pin diode.
38. What are the disadvantages of fushion splicing techniques?
During fusion chemical reaction may take place which causes a change in the
refractive index of two fibers
It develops thermal stresses at the fusioned region
39. What is APD?
It consists of four regions p+
-i –p –n+ in order to develop a very high electric
field in the intrinsic region. It is used to produce new electron pairs by impact
ionization. This impact ionization leads to avalanche breakdown in the reverse biased
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diode. So the APD‟s have high sensitivity & high response over pin diodes due to the
avalanche multiplication.
40. What are the types of fiber coupling devices?
Fiber splices (Permanent joint)
Fiber connectors (Temporary joint)
41. Define connector & splices.
Two fibers are connected by a permanent bond called splice, connected by a
demountable joint called connector.
42. What are the types of couplers?
Diffusion couplers
Radiative couplers
Area splitting couplers
Beam splitter coupler
43. What are the types of splicing techniques?
Fusion splice, V-groove splice
Elastic tube splice,Rotary splice
Mechanical tube splice
44. What are the advantages of fushion splicing techniques?
Low splice loss
There is perfect bonding between two fibers
45. What are the light sources used in optical fiber system?
Light emitting diodes & Semiconductor lasers are used in optical fiber system.
46. Mention advantageof ILD over LED.
The radiant output power from an ILD is greater than that for a LED.
47. How scattering loss occurs?
Linear scattering may cause the attenuation of operating mode by means of
transferring power to leaky or radiation mode. Nonlinear scattering occurs when the
refractive index of the medium depends upon the optical intensity of the signal.
48. Mention the principle of LASER.
The principle of operation of Laser action is the result of 3 key processes: they are:
Photon absorption
Spontaneous emission
Stimulated emission
49. What is lasing?
The ends of ILD are highly polished. The mirror like ends trap the photons in te
active region and as they reflect back and forth, stimulate free electrons to recombine
with holes at a higher than normal energy level. This is called lasing.
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50. What are the requirements of good connector?
Low coupling losses & Interchangeability
Ease of assembly & Ease of connection
Low environmental sensitivity
Low cost & reliable construction
51. What is fiber splice & fiber coupler?
The permanent joint formed between two individual optical fibers in the field or
factory is known as fiber splice.
The fiber couplers are branching devices that split all the light from a main fiber
into two or more fibers.
52. What is the function of photo detector?
The photo detector converts the current changes into voltage changes.
53. Outline the primary building blocks of fiber optical system.
The primary building blocks of fiber optic system are the transmitter, receiver
and optical fiber cables.
54. What are the disadvantages of APD?
Carriers in the intrinsic region have a long transit time to the junction, which
slows the response as in the pin diode.
The avalanche multiplication factor tends to fluctuate randomly to add noise to
the signal.
55. What are the applications of optical fiber communications?