4. Communication is the transfer of information or message from point to another.

5. Two persons talking to each other constitute the simplest communication system. The person who speaks is the source, the person listening is the receiver and the intervening air is the communication link between them. COMMUNICATION SYSTEM

6. A communication system consists of three basic components: - Transmitter (source) - Communication channel (link – medium) - Receiver

7. Nature/details of these components depend on: 1. Nature of the signal/ message to be communicated. 2. Distance which separates the source and the receiver. - Direct talking is possible over short distances – sound waves attenuate fast. - Long distance communication requires the signal/message to be converted into an electrical signal/a set of signals /electromagnetic waves. - Long distance communication requires a link between the source and the receiver

8. Message signal Transmitter Communication channel Receiver Out put signal CUMMUNICATION SYSTEM

9. Transmitter: Transmits the message/signal over the communication channel. Quite often the original signal is not suitable for transmission over the communication channel to the receiver. It requires to be modified to a form suitable for transmission. Communication Channel: Provides a link between the transmitter and the receiver. It can be a transmission line (telephone and telegraphy), an optical fibre (optical communication) or free space in which the signal is radiated in the form of electromagnetic waves. Receiver: Reconstructs the original message/signal after propagation through the communication channel.

10. TRANSMITTER - We communicate through a message or a signal. - A transmitter transmits the message over the communication channel to the receiver. As a rule, the message produced by the source is not suitable for transmission over the communication channel. Accordingly, a suitable transducer converts it into a time varying electrical signal called the message signal. A transmitter, in its simplest form, is a setup which boosts the power of message signal and feeds it into the communication channel

11. Microphone Transmitter Amplifier Receiver Antenna Amplifier Loud speaker

12. ModulatorAmplifier Transmitter Receiver Tunable Amplifier Demodu- lator Audio Amplifier Signal To Speaker Antenna

13. Basic constituents of a transmitter are: 1. Message signal 2. Modulation 3. Antenna Message signal: A single valued function of time that conveys the information. Analog Signals Discrete or digital Analog Signal Is a continuous function of time, with the amplitude (instantaneous value of the signal) being continuous.

14. Elementary idea of Analog and Digital Communication There are two different modes of communication (1)Analog communication and (2) digital communication.

15. Time period; amplitude I 4 Time period; amplitude I 4 1) ANALOG COMMUNICATION Analog signal is a continuous variation of current or which is the representative of the signal or message. Analog signal is a continuous variation of current or which is the representative of the signal or message.

16. An alternating voltage which varies sinusoidal is an analog signal. During the transmission of analog signal, there is a chance for interference and distortion and analog communication is less reliable.

17. Discrete Signals Discrete signals are discontinuous in time; they are defined only at discrete times. In case of discrete signals the independent variable (time) takes only discrete values which are usually uniformly spaced. Consequently, discrete-time signals are described as sequences of samples whose amplitudes may take a continuum of values. When each sample of a discrete-time signal is quantized I.e. its amplitude is only allowed to take on a finite set of values (e.g. in a binary representation low and high signals are designated as 0 and 1) and then coded, the resulting signal is referred to as a digital signal.

19. signal which has two levels of current orPulse voltage. These two levels are high or low level. values of voltage or current represented by binary digits zero and one. The binary signals are easy to generate and process with a digital circuit. Digital communication is more reliable. level 0

20. The low frequency electric signal (audio frequency range] transmitted to very large distances. This is because the energy as s a low frequency signal will be very small (energy <* frequency). 1 audio frequency electric signal to transmits a large distances, a high frequency electric signal is used. Hence we superimpose the audio frequency (modulating signal) on a high frequency carrier wave. This process is modulation. Modulation

21. 1) The energy associated with the audio signal is very small (E = hv). Hence the audio frequency signal cannot be transmit such over long distances. Need for modulation: 3) If the low frequency signals are transmitted from different stations get mixed up. So the different signals cannot be distinguished. 2) For the maximum efficiency of the transmitting and receiving a height of the antenna must be of the order of X/4.

22. In its simplest form, the transmitter has following problems: 1. Size of the antenna or aerial For transmitting a signal we need an antenna. It should have a size comparable to the wavelength of the electromagnetic wave representing the signal ( at least /4) so that the time variation of the signal is properly sensed by the antenna. For an electromagnetic wave of frequency 20 kHz, the wavelength is 15 km. Obviously such a long antenna is not possible. Therefore, direct transmission of such a signal is not possible. If the frequency of the signal is 1MHz, the corresponding wavelength is 300m and transmission of such a signal is possible. Therefore, there is a need of translating the information contained in the original low frequency signal into high or radio-frequencies before transmission. 2. Effective power radiated by an antenna The power radiated from a linear antenna  For a good transmission we need high power hence there is need for high frequency transmission.

23. 3. Mixing up of signals from different transmitters Direct transmission of baseband signal leads to interference from multiple transmitters. Thus multiple user friendly communication is not possible. A possible solution is provided by employing communication at high frequencies and then allotting a band of frequencies to each user. The above arguments suggest that there is a need for translating the original signal ( low frequency) into a high frequency wave before transmission such that the translated signal continues to possess the information contained in the original signal. The high frequency wave carrying the information is called the carrier wave. The process of transformation is called Modulation. Modulation Transformation of the signal into a form suitable for transmission through a given communication channel

24. Types of Modulation The process of changing the amplitude or frequency or phase the carrier wave in accordance with the intensity of the signal is called modulation.

25. 1)Amplitude modulation (AM) 2) Frequency modulation (FM) 3) Phase Modulation (PM) Accordingly the different types of modulations are

26. Amplitude modulation The process of changing the amplitude of high frequency carrier wave in accordance with the intensity of the signal is called the amplitude modulation.

29. Amplitude change of the carrier = 2A - A = A Amplitude change of the carrier = 2A - A = A A.M. Wave A.M. Wave m™ *u j i * r *. amplitude change of carrier A. i«™/ Now the modulation factor, m = ——-———————*—————:— =— = 1 or 100% m™ *u j i * r *. amplitude change of carrier A. i«™/ Now the modulation factor, m = ——-———————*—————:— =— = 1 or 100% amplitude of normal earner A amplitude of normal earner A For effective modulation the degree of modulation should never exceed 100%. For effective modulation the degree of modulation should never exceed 100%. ur j i «. j j Signal wave amplitude Em /T,,... ^f Modulation index = —2-———————*~——— = —sl (Em= amplitude of ur j i «. j j Signal wave amplitude Em /T,,... ^f Modulation index = —2-———————*~——— = —sl (Em= amplitude of carrier wave amplitude Ec modulating wave, Ec= amplitude of carrier wave) carrier wave amplitude Ec modulating wave, Ec= amplitude of carrier wave) Modulation index =Emfl*"Emio, where Emax and E^ are the Modulation index =Emfl*"Emio, where Emax and E^ are the Emax + Emin Emax + Emin and minimum amplitudes of the modulated wave, and minimum amplitudes of the modulated wave, » width of modulation = 2fm, where fm is the modulating signal frequency width of modulation = 2fm, where fm is the modulating signal frequency : In amplitude modulation, the base band signal amplitude is half the er signal amplitude. Find the modulation index. : In amplitude modulation, the base band signal amplitude is half the er signal amplitude. Find the modulation index.

30. The process of changing the frequency of high frequency carrier wave in accordance with the intensity of the signal is called frequency modulation. Frequency Modulation

31. Frequency Modulation: - TV broadcast, VHF, UHF, SHF and EHF broadcasts. - Requires higher carrier wave frequencies. - Noise generated by atmospheric or man made electric discharges does no harm to intelligence. - Higher S/N ratio, quality of broadcast very good. FM Radio – 88 to 108 MHz VHF TV – 47 to 230 MHz UHF TV – 470 to 960 MHz

32. Comparison of Amplitude and Frequency modulation AMFM 1) Noisy Less noise 1) Noisy Less noise 2) Less band width More band width 2) Less band width More band width 3) low costcostly 3) low costcostly 4) Low efficiencyHigh efficiency 4) Low efficiencyHigh efficiency 5) Transmission and reception easier Transmission and reception com 5) Transmission and reception easier Transmission and reception com

33. Phase Modulation The process of changing the phase of the carrier wave accordance with the intensity of the signal is called phase modulation The process of changing the phase of the carrier wave accordance with the intensity of the signal is called phase modulation The wave form of the phase modulated wave is similar to that wave. Phase modulation uses a smaller bandwidth than FM. Hence information can be sent in a given band width. The wave form of the phase modulated wave is similar to that wave. Phase modulation uses a smaller bandwidth than FM. Hence information can be sent in a given band width.

34. Pulse Modulation Modulation of a carrier wave may be accomplished by short pulses. Conventional telegraphy is the simplest example of this mode of modulation. Pulse systems are based on sampling of the information signal at periodic intervals, usually twice the maximum frequency present (2B). They transmit a very short pulse of radio-frequency carrier for each sample, with pulse characteristics varied in some manner proportional to the amplitude at the sampling instant. A general name given to these modes of modulation is the pulse modulation. The common pulse systems employed in pulse modulation of analog signals are: (i) Pulse – amplitude modulation (PAM) (ii) Pulse – position modulation (PPM) (iii) Pulse – duration/width modulation (PDM/PWM) (iv) Pulse – code modulation (PCM

36. DEMODULATION process of recovering audio frequency signal from the modulated carrier wave is known as demodulation or detection.

37. In demodulation, the two main operations involved are (1) Rectification of the modulated wave (2) Elimination of radio frequency components from the rectified wave.

38. PULSE MODULATION Pulse modulation is used in digital communication. In the pulse modulation the carrier is in the form of pulses

39. The different types of pulse modulation are 1) Pulse amplitude modulation (PAM) 2) Pulse width modulation (PWM) 3) Pulse position modulation (PPM)

40. The process of changing the amplitude of the pulse in accordance with the modulating signal is called pulse amplitude modulation. Pulse Amplitude Modulation (PAM)

42. Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) The process of changing the width of the pulse in accordance with the intensity of the modulating signal is called pulse width modulation.

43. Pulse Position Modulation (PPM) The process of changing the position of pulse with the modulating signal is called pulse position modulation

44. Pulse Code Modulation (PCM) The process of modulating the carrier pulse in accordance with the digitized modulating signal is called pulse code modulation.

46. The common modulation techniques used for digital data are 1) Amplitude shift keying (ASK) 2) Frequency shift Keying (FSK) 3) Phase shift keying (PSK)

47. DATA TRANSMISSION AND RETRIEVAL Before transmission the message signal is modulated with a carrier wave using a modulator. Modulated wave is amplified using a high frequency power amplifier and it is supplied to the transmitting antenna. The modulating signal is received using antenna at the receiving station. The signal demodulated and the message signal is retrieved.

48. Modem Modem is the abbreviation for modulator and demodulator. It’s used both at the transmission and the receiving ends. At the transmission modem accepts the digital data and converts it into analog signal modulation with a carrier signal. At the receiving end, the carrier wave demodulated to get the analog signal and the equivalent digital signal produced.

49. Fax (Facsimile) A fax is an electronic device for transmitting graphical information through wire or wireless. The printed material is scanned and converted into an electronic signal by fax machine.

51. There are two types of communication (1) communication through space called, space communication (2) communication through wire called line communication.

52. The communication utilizing the physical space surrounding the earth is called space communication. Space Communication

53. There are three modes of space communication (1) Ground wave propagation (2) Space wave or troposphere wave propagation (3) Sky wave propagation or ionosphere propagation.

54. In ground wave propagation the transmitted wave propagate along the surface of the earth. 1) Ground Wave Propagation

55. 2) Space Wave Propagation or Troposphere Wave Propagation In space wave propagation the radio waves from the transmitting antenna reaches the receiving antenna either directly or after reflecting the troposphere or ground.

56. 3) Sky wave Propagation or Ionosphere Propagation The atmosphere surrounding the earth at a height of about 80km to 300km is called ionosphere. This region is full of ions produced by ultraviolet and high energy radiations coming from the sun. This region has the property of reflecting e.m waves coming from earth. By using this reflecting property of ionosphere electromagnetic waves coming from transmitter can be sent to far away points as shown in fig. 10.11

57. IONOSPHERE 60 100 200 300 Height (km) C Layer D Layer n ~10 8 (m  3 ) n ~ 10 9 (m  3 ) n ~ 10 11 (m  3 ) n ~ 5  10 11 (m  3 ) n ~ 8  10 11 (m  3 ) E Layer F1 Layer F2 Layer

59. - Surface wave propagation – used for medium wave band and TV broadcasting which is done in the frequency rang 100 – 200 MHz. In this transmission the reception is possible only when the receiver antenna directly intercepts the signal. Thus, if the broadcast is made from a tower of height h above the ground, due to the curvature of earth no reception is possible beyond certain points. - To get larger coverage TV broadcast are made from tall antennas. Further, the power transmitted also decreases nearly as the inverse square of the distance hence the signal becomes weak as the distance increases, which limits the range of transmission by this mode. - The ground wave attenuation increases with frequency, so the transmission via this mode is in practice possible only for frequencies up to about 1500 kHz or wavelengths greater than 200m. - Below 200m wavelength, the communication in AM band is via sky wave. - The diffraction of electromagnetic waves also affects their propagation.The frequencies of the waves employed for radio and television broadcast lie in the range 5 – 1000 MHz and the corresponding wavelengths are in the range of 30 cm – 200 m. At these wavelengths the diffraction effects are considerable and therefore these waves lose their directional properties.

60. Antenna - An antenna is a vital component of any communication system. It is employed both at the transmitting end as well as at the receiving end. - An antenna is a length of conductor, its length is such that it acts as a resonant circuit at the frequency of operation. l = /2. - It acts as a conversion device. The first conversion takes place at the transmitter where electrical energy is converted into electromagnetic waves. The second conversion occurs at the receiving end where the electromagnetic waves are transformed into electrical signal that is applied to the input of the receiver. Two types of antenna: 1. Dipole antenna – Length of dipole = /2 ; Omni directional. 2. Dish antenna – A spherical or parabolic dish is employed as a reflector or collector. The resonant element is placed at the focus. It is highly directional.Antenna - An antenna is a vital component of any communication system. It is employed both at the transmitting end as well as at the receiving end. - An antenna is a length of conductor, its length is such that it acts as a resonant circuit at the frequency of operation. l = /2. - It acts as a conversion device. The first conversion takes place at the transmitter where electrical energy is converted into electromagnetic waves. The second conversion occurs at the receiving end where the electromagnetic waves are transformed into electrical signal that is applied to the input of the receiver. Two types of antenna: 1. Dipole antenna – Length of dipole = /2 ; Omni directional. 2. Dish antenna – A spherical or parabolic dish is employed as a reflector or collector. The resonant element is placed at the focus. It is highly directional.

62. Communication Channel - In a communication system, the communication channel or the transmission medium is the physical path between the transmitter and receiver. - Transmission media can be classified into two broad categories: (a) Guided - Point – to – point communication (i) Twisted pair (ii) Coaxial cable (iii) Optical fibre (b) Unguided – Free space - Characteristics and quality of transmission are determined both by the nature of the signal as well as the medium. - In guided media, the nature of the medium is more important; in unguided media, the spectrum or the frequency band of the signal transmitted by the transmitter is more important. Characteristics of a Communication Channel : Band width, Modulation and Data rate. Communication Channel - In a communication system, the communication channel or the transmission medium is the physical path between the transmitter and receiver. - Transmission media can be classified into two broad categories: (a) Guided - Point – to – point communication (i) Twisted pair (ii) Coaxial cable (iii) Optical fibre (b) Unguided – Free space - Characteristics and quality of transmission are determined both by the nature of the signal as well as the medium. - In guided media, the nature of the medium is more important; in unguided media, the spectrum or the frequency band of the signal transmitted by the transmitter is more important. Characteristics of a Communication Channel : Band width, Modulation and Data rate.

64. Receiver - Reconstructs the original message or data after its propagation through the communication channel - The process consisting of decoupling of the carrier wave and the modulating signal is broadly termed as demodulation. - The design of the receiver depends on the modulation process employed in the transmitter. - The antenna receives the modulated wave transmitted from the transmitter, which is then amplified by a suitable amplifier and fed to the demodulator or decoder. - The demodulator or decoder extracts the original signal. The process of demodulation provides a means of recovering the original signal from the modulated wave. In effect, demodulation is reverse of modulation: therefore, it depends on the modulation process used.Receiver - Reconstructs the original message or data after its propagation through the communication channel - The process consisting of decoupling of the carrier wave and the modulating signal is broadly termed as demodulation. - The design of the receiver depends on the modulation process employed in the transmitter. - The antenna receives the modulated wave transmitted from the transmitter, which is then amplified by a suitable amplifier and fed to the demodulator or decoder. - The demodulator or decoder extracts the original signal. The process of demodulation provides a means of recovering the original signal from the modulated wave. In effect, demodulation is reverse of modulation: therefore, it depends on the modulation process used.

66. A communication satellite is an electronic device. It has a receiver and transmitter which are together called transponders. The signal transmitted from the earth is up linked receiver in the satellite. The receiver amplifies the signal and download the ground station by its transmitter. The uplink frequency and the download frequency are different in order to avoid confusion. The satellite communication is possible by using a geostationary satellite. Geostationary satellite appears to be stationary relative to the earth. This is possible if the satellite is at a height of about 36000 km from the earth.

68. - The satellite communication first started in 1962 with the satellite Telstar. The first commercially operated satellite was launched in 1965. Since then numerous communication satellites have been launched for the services of point-to-point telecommunication circuits, vide area TV coverage, direct broadcasting by satellite, navigational communications to ships and aircrafts. - Most of the satellites orbit at heights greater than 600 km to minimize atmospheric drag. - The choice of orbit is of fundamental importance, as it determines the transmission path loss and delay time, the earth coverage area and time period the satellite is visible from a given area. - The orbits of communication satellites are conventionally classified as inclined elliptical, polar circular and geo-stationary. - The geo-stationary orbit is the most widely used orbit for communication satellites.

70. Polar Satellite Polar satellite revolves round the earth at a height of about 1000km from earth. Their orbits are around the earth over the poles in geographical meridian with a speed such that it reaches over a place when it is just under the sun. Hence they are called the polar satellites. Because of the earth's spinning the polar satellite is able to scan the entire surface of the earth, Polar satellite revolves round the earth at a height of about 1000km from earth. Their orbits are around the earth over the poles in geographical meridian with a speed such that it reaches over a place when it is just under the sun. Hence they are called the polar satellites. Because of the earth's spinning the polar satellite is able to scan the entire surface of the earth,

71. Remote sensing and spy work. USE OF POLAR SATTELLITES

72. Remote sensing Remote sensing is one of the applications of the satellite communication. A satellite can take photograph of an object or some area the earth or collect any other information and transmit it back to an earth station. This is known as remote sensing.

73. The process of taking photograph from satellites is generally called satellite imagery. It is mainly used resource survey, town and country planning and for archeology etc.

74. Applications of remote sensing (1) meteorology (2) climate (3) o ceanography (4) archeology (5) geological surveys (6) agriculture (7) forestry (8) predicting natural disaster etc.

75. LINE COMMUNICATION Line communication is a mode of communication from one point to another through a wire or cable. Etc…

76. There are three different types of line communication. 1)Two wire transmission line 2) Co-axial cable 3) Optical Fibre

77. In two wire transmission line and co-axial cable audio frequency ultra high frequency (UHF) are used. In two wire transmission line and co-axial cable audio frequency ultra high frequency (UHF) are used. But in optical fibres, optical frequencies are used.

78. Two wire Transmission Lines The best example for two wire transmission line is a telephone: figure shows the transmission through two wires.

79. The signal flowing through the wires produces electric field E and magnetic field B as given in the fig. (b). The signal flowing through the wires produces electric field E and magnetic field B as given in the fig. (b). Each portion of the transmission can be considered as a small inductor, resistor and capacitor a. fig. (c). Hence each length of transmission line will have a ch impedance. Because of this, for maximum power transfer, impede receiving unit should have an impedance equal to the ch; impedance. This is also called load matching.

80. Twisted pair of wires A twisted pair" of wires consists of two insulated wires twisted around each other to form a spiral shape as shown in figure. The electromagnetic interference from other sources can be reduced by the use of twisted wires. The twisted wires are widely used in telephone and computer networking

81. Co-axial cable Mesh conductor A co-axial cable consists of a single inner conductor made of copper, surrounded by solid conductor or a braded mesh of fine wires. An ms medium like teflon or poly ethylene separates the two cables as shown in figure. The outer conductor is normally connected to the ground hence it provides an electrical shield to the signal carried by the conductor, reducing radiation loss. The outer conductor is further covered with a polymer for protection. A co-axial cable has least interference from other signals.

82. Inner conductor Outer cover

83. Telephone links can be established by using two line wire waves, sky waves, microwaves, co-axial cables and optical fiber individual telephone sets are connected to local telephone exchange twisted pair of wires. The satellite linking with telephones are don’t figure. The message from the telephone goes to the earth satellite through local exchange, main exchange, earth satellite station and transmitter. The message from the geostationary satellite is n another microwave receiver kept far away. The message from this receiver finally goes to another telephone set through local exchange in figure. Satellite In Geostationary Coaxial Cable Telephone I/inks

84. Optical communication Optical communication is made by using optical carrier high frequency ranging from 1O12 Hz to 1016 Hz. Hence very large bands and- large number of channels are possible. Optical communication possible in the past due to the following reasons. 1. Non availability of carrier optical signal. 2. Non availability of a cable carrying optical signal. The optical communication technology is developing fast invention of optical fibers carrying optical signals, semiconductor like LED, diode laser etc arid detectors like photocjiode. The main advantages

85. Transmitter Optical source Input signal Modulation Optical fibre cable Receiver Optical detector Demodulation Output signal Optical Communication

86. Optical Fibre - An optical fibre is a thin fibre of glass. Its diameter is about the same as that of a human hair ~ 10 to 100  m. - Light can be guided in such a fibre by launching it at one end, using an intense and focused light source, and allowing it bounce down to the other end by a series of reflections (total internal reflections) from the sides. - An optical fibre essentially consists of an inner cylinder of glass known as the core, having a refractive index n 1, and an outer cylinder of a different glass, called the cladding having a refractive index n 2,  n = n 1  n 2 ~ 10  3. - For use in a telecommunications system, many fibres are usually incorporated into a cable structure for pulling into underground ducts.

88. The main advantages of optical communication are Wide bandwidth and a large number of channels. Wide bandwidth and a large number of channels. Low transmission losses. Low transmission losses. Cladding with refractive index, Cladding with refractive index, Medium of refractive index, n Medium of refractive index, n Core wi Core wi refractive index, ni refractive index, ni Buffer coating Buffer coating

89. OPTICAL FIBRE Point to point communication of light signal can be realized by using optical fibers. For this a transparent medium like glass, polymer or a dielectric material is required. Point to point communication of light signal can be realized by using optical fibers. For this a transparent medium like glass, polymer or a dielectric material is required. The principle of on optical fiber is based on. the phenomenon of multiple total internal reflections. The optical fiber can be considered as a light pipe like a water pipe. The principle of on optical fiber is based on. the phenomenon of multiple total internal reflections. The optical fiber can be considered as a light pipe like a water pipe. The important parts of an optical fibre are The important parts of an optical fibre are 1) core, 2) cladding 3)buffer.

90. 1) Core: The core is the cylindrical central part of the optical fibre, made up of glass or polymer of refractive index, m. The diameter of the core is 10-100 1) Core: The core is the cylindrical central part of the optical fibre, made up of glass or polymer of refractive index, m. The diameter of the core is 10-100 2) Cladding: The cladding is a covering on the core, made of glass or plastic, refractive index, n2 where n2 < n\. The difference of refractive indices m, very small of the order of 0.001. 2) Cladding: The cladding is a covering on the core, made of glass or plastic, refractive index, n2 where n2 < n\. The difference of refractive indices m, very small of the order of 0.001. 3) Buffer: Buffer is a plastic coating made on the cladding for providing si and strength. 3) Buffer: Buffer is a plastic coating made on the cladding for providing si and strength.

91. Principle of an optical fibre The principle of an optical fibre is based on the phenomenon of multiple total internal reflections. The total internal reflection at core-clad< boundary will take place when the light travels from a denser to a rarer medium at an angle of incidence greater than the critical angle. The dimci of the core is so small that the light entering into the core will have an incident angle always greater than the critical angle. The multiple reflections of Ugt1 opposite boundaries will continue and the light passes through the core light pipe. The principle of an optical fibre is based on the phenomenon of multiple total internal reflections. The total internal reflection at core-clad< boundary will take place when the light travels from a denser to a rarer medium at an angle of incidence greater than the critical angle. The dimci of the core is so small that the light entering into the core will have an incident angle always greater than the critical angle. The multiple reflections of Ugt1 opposite boundaries will continue and the light passes through the core light pipe.

92. There are two types of optical fibres. 1. Step index fibre: In step index fibre, there is an abrupt cl refractive index at core- cladding interface. 2. Graded index fibre: In Graded index fibre, the refractive inde fibre decreases radially outwards. There is no abrupt change in n index.

93. Advantages of fibre communication 1. High bandwidth. Hence more information can be sent 1. High bandwidth. Hence more information can be sent 2. High signal security 2. High signal security 3. Size and weight are smaller compared to metal conductors 3. Size and weight are smaller compared to metal conductors 4. Noise free 4. Noise free 5. Negligibly small transmission loss 5. Negligibly small transmission loss

94. LASER Laser is the abbreviated form for light amplification by stimulated emission of radiation. Laser is a highly coherent light. Laser is the abbreviated form for light amplification by stimulated emission of radiation. Laser is a highly coherent light. Principle of production of Laser Principle of production of Laser E3 E3 Short lived state Short lived state The atoms in the ground energy state are irradiated to higher energy state. The atoms in the ground energy state are irradiated to higher energy state.

95. Ground state This is achieved by optical pumping. The life span This is achieved by optical pumping. The life span the excited state is in the order of 10-8 s. Therefore the atoms jump down the metastable state (where life span is 10-3 s). the excited state is in the order of 10-8 s. Therefore the atoms jump down the metastable state (where life span is 10-3 s). population inversion place when the number of atoms in the metastable state is larger the atoms in the ground state.

96. When a photon of energy hv, equal to the difference in the energy of metastable state and the ground state is incident, all the atoms in metastable state come simultaneously to the ground state emitting photons. This is called stimulated emission. All the emitted photons are in phase with the incident photon and the emitted beam of light is called laser. When a photon of energy hv, equal to the difference in the energy of metastable state and the ground state is incident, all the atoms in metastable state come simultaneously to the ground state emitting photons. This is called stimulated emission. All the emitted photons are in phase with the incident photon and the emitted beam of light is called laser. Different types of lasers are Ruby laser (694 nm), CO2 laser (10600 mnjft He - Ne laser (5430/633 mn) and Argon laser (488/514 nm) Different types of lasers are Ruby laser (694 nm), CO2 laser (10600 mnjft He - Ne laser (5430/633 mn) and Argon laser (488/514 nm)

97. ELEMENTARY PRINCIPLE OF LIGHT MODULATION Light can be modulated with the information signal. This is achieved‘ by changing the intensity, frequency or phase of the light- signal in accordant with the intensity of the information signal. Light can be modulated with the information signal. This is achieved‘ by changing the intensity, frequency or phase of the light- signal in accordant with the intensity of the information signal. Light modulation can be done in two ways. Light modulation can be done in two ways. 1) Direct modulation 2) Indirect or External modulation ; The direct modulation can be achieved by modulating the direct curiw supplying the light source. This is obtained by changing the laser biasing current above and below the threshold value to make the laser on and^on. This causes a change in frequency. Thus Direct modulation can be used to modulate the frequency of light wave. The direct modulation can be achieved by modulating the direct curiw supplying the light source. This is obtained by changing the laser biasing current above and below the threshold value to make the laser on and^on. This causes a change in frequency. Thus Direct modulation can be used to modulate the frequency of light wave. An indirect modulator consists of a material like lithium (UNbO3) crystal whose refractive index can be controlled with the help applied Electric field. This control on the refractive index brings the con phase of the signal. When light passes through the applied electric suffers cumulative phase change. The desired modulation can be ach| nrnnprlv hv switrhinor the electric field on and off. The external An indirect modulator consists of a material like lithium (UNbO3) crystal whose refractive index can be controlled with the help applied Electric field. This control on the refractive index brings the con phase of the signal. When light passes through the applied electric suffers cumulative phase change. The desired modulation can be ach| nrnnprlv hv switrhinor the electric field on and off. The external