PRINCIPLES OF COMMUNICATION P.C. JAIN DEPARTMENT OF PHYSICS & ASTROPHYSICS UNIVERSITY OF DELHI DELHI 110 007
<ul><li>COMMUNICATION - Transmission of information /message from one point to another Communication enters our lives in many ways: - Telephone – makes us talk to any person anywhere. - Radio and television – entertain and educate. - Communication signals as navigational aids – ships, aircrafts and satellites . - Weather forecasting – conditions measured by a multitude of sensors are communicated to forecasters. - Videophones, voicemail and satellite conferencing – enable seeing live images instantly and communicate directly with people located far away. - Digital data transmission and retrieval – has made realization of e-mail, FAX and internet possible. We communicate through speech. In modern communication systems, the information is first converted into electrical signals and then sent electronically. </li></ul>
<ul><li>COMMUNICATION SYSTEM 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. A communication system consists of three basic components: - Transmitter (source) - Communication channel (link – medium) - Receiver 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 </li></ul>
Message signal Transmitter Communication channel Receiver Out put signal CUMMUNICATION SYSTEM
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.
<ul><li>Designing a Communication System In designing a communication system we have to focus our attention to the following questions: - In what form is the information that is to be conveyed. - How can the transmitter use this information? - How does the transmitter feed the information to the communication channel? - What effects does the communication channel have on the information? - In what form the receiver should present the information to the outside world? - How does the received information differ from the original information? What cause the difference? To what extent can the two be allowed to differ? </li></ul>
<ul><li>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 </li></ul>
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.
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
Modulator Amplifier Transmitter Receiver Tunable Amplifier Demodu-lator Audio Amplifier Signal To Speaker Antenna Antenna
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.
Simplest form of an analog signal is a sinusoidal signal having a single frequency g( t ) = A sin t Signals generated by different sources have their own characteristics - amplitude, frequency or nature. Nature – Simple single frequency or a complex superposition of several frequency components The signals associated with music or speech are complex; can be considered as superposition of several sinusoidal signals of varying amplitudes and frequencies. The range over which the frequencies in a signal vary is called the bandwidth (B) (the frequency range between the lowest and highest frequency components). Bandwidth for audio signals is 20 Hz to 20 kHz.
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 .
An analog signal can be converted into a digital signal – A/D conversion. A device performing this operation is called A/D converter. A discrete or digital signal can also be converted into an analog signal – D/A conversion. A device performing this operation is called a D/A converter. Advantages of transmitting information in the digital form are many. In a digital communication system, the receiver has to detect simple pulses, which have the same shape and height. It has only to recognize whether such a pulse is present or not in any prescribed time interval. The signal to noise ratio (S/N) is high. In digital data communication, the rate at which the data is communicated is very important. It is expressed in bits per second (bps)
<ul><li>Modulation We have seen: - Direct transmission of audio frequency signals is not possible – associated wavelengths are large. - Quite often, the message signals require a carrier and a communication channel for transmission from source to the receiver. - An efficient utilization of the communication channel requires a shift of the range of base band frequencies to another frequency range suitable for transmission. A shift of the range of frequencies in a signal is accomplished by employing a process called modulation. In this process the signal is superimposed over the carrier wave. The frequency, f c, of the carrier wave must be much higher than the highest frequency component of the message signal . The form of modulation depends on the specific characteristics of the carrier wave, nature of the message signal/data and the communication channel. The carrier wave may be (I) continuous or (ii) pulse. The signal may be analog or digital. </li></ul>
Forms of Modulation Amplitude Modulation Analog Signals Angle/Frequency Modulation Pulse Modulation Amplitude shift keying (ASK) Digital Signals Frequency shift keying (FSK) Phase shift keying (PSK)
ANALOG SIGNALS Amplitude & Frequency Modulation A sinusoidal wave conveys no information. To transmit information by the usual sinusoidal waveform, the characteristics of the wave must be varied in some manner. A sinusoidal carrier wave C( t ) is defined by C ( t ) = A c cos ( c t + o ) The modulation of the carrier wave can be accomplished in two ways: (1) The amplitude of the carrier wave is varied about a mean value, linearly with the baseband signal m ( t ), the angular frequency c remaining constant. This mode of modulation is termed as amplitude modulation. (2) The phase angle of the carrier wave is varied according to the baseband signal, the amplitude of the carrier wave being kept constant. This mode of modulation is termed as angle modulation. There are two variations of angle modulation - phase modulation (PM) and frequency modulation (FM).
Amplitude Modulation: - Employed for commercial broadcasting of voice signals. Carrier frequencies – 0.5 to 20 MHz. - Broadcast noisy – noise signals created by atmospheric static or man made electric discharges also get amplitude modulated. 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
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
Digital Signals / DATA Three modulation techniques are employed for transmitting digital signals / data. There is a step change in amplitude, frequency or phase. 1. Amplitude – shift keying (ASK) – used for transmitting data over optical fibre 2. Frequency – shift keying (FSK) – Less susceptible to errors. 3. Phase – shift keying (PSK
<ul><li>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. </li></ul>
<ul><li>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. </li></ul>
<ul><li>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. </li></ul>
<ul><li>Data Transmission and Retrieval – FAX and Modem Data : - The term data is applied to representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means. - In case of e-mail, the data consists of the message ( m ) to be sent. The input device and the transmitter are components of a PC. The user activates the e-mail package on the PC and enters the message via the keyboard (input device). The PC is connected to some transmission medium or channel by an I/O device. The input data are transferred to the transmitter as a sequence of bits [ g ( t ) ] on some communications cable. The transmitter is connected to the medium and converts the incoming bits into a signal [ s ( t ) ] suitable for transmission [Modem I] . At the receiving end the signal received from the medium is converted back into a sequence of bits [ r ( t ) ][Modem II]. These bits are sent to the output PC, where they are briefly buffered in memory as a block of bits or characters. These data are then presented to the user via an output device such as printer or screen. The message [ m ] as viewed by the user will usually be an exact copy of the original message. </li></ul>
Modem - Digital data can also be represented by analog signals by use of a modem (modulator/demodulator). - The modem coverts a series of binary pulse into an analog signal by encoding the digital data into a carrier frequency. - The resulting signal occupies a certain spectrum of frequency centred about the carrier and propogated across the a medium suitable for that carrier. - At the end of the line, the modem demodulates the signal to recover the original data. FAX - Facsimile or FAX means exact reproduction of a document at the receiving end. - The document to transmitted is first converted into digital data form. A process called ‘scanning’, which normally is carried out by optical means, does this. The device , which does scanning is called a ‘scanner’. - The digital data representing the document is then transmitted to the destination by using a suitable medium. At the receiving end the digital data is then used to reconstruct the original document.
<ul><li>Space communication - The communication process utilizing the physical space as communication channel/medium is termed as space communication radio, television and satellite communication fall under this category. - In this type of communication, electromagnetic waves of varying frequencies are used as carrier waves. These waves travel in open space or the atmosphere. - The interaction of electromagnetic waves with earth’ atmosphere, therefore, plays an important role. Electromagnetic radiations and the earth’s atmosphere - The atmosphere surrounding the earth is complex, it has many constituents and their relative proportions vary as we go higher and higher. The air thins out gradually as we go up. - The atmosphere has no sharp boundaries; it has several layers of regions with different properties and names. Except for layer in the upper atmosphere, called ionosphere, which is composed of ionized matter I.e. electrons and positive ions, the rest of the atmosphere is mostly composed of neutral molecules. </li></ul>
- Water vapour is concentrated in the lowest layer. - Ozone in the atmosphere is confined to the ozone layer, some 50 – 80 km above the ground. - The ionosphere, which extends from 60 – 350 km, plays an important role in space communication. It is subdivided into layers as C, D, E, F1, F2 Communication in space - In space communication, a signal is emitted from the antenna of a transmitter in the form of an electromagnetic wave, which travels through the intervening space and received by another antenna at the receiver. - An electromagnetic wave after being radiated by the transmitting antenna may be divided into various parts. One part travels along the surface of the earth and is called surface wave ( ground wave ). The remainder part moves upwards towards the sky and is called the sky wave . - A signal after being transmitted from the antenna of a transmitter can be received by the antenna of the receiver in two ways; (a) directly by the surface wave or (b) by the sky wave after it bounces back from the atmosphere
EARTH’S ATMOSPHERE Ionosphere Troposphere Stratosphere Mesosphere Ozone Layer 12 km 50 km 80 km EARTH
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
- 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.
- Microwaves have frequencies ~ 100 – 300 GHz and therefore wavelengths in the range of few millimeter. Because of short wavelength, they have directional properties and better suited for beaming signals in particular direction. This possibility was first employed in radar. Microwave communication is extensively employed in telecommunication as it provides larger bandwidth. - The waves in HF band are reflected back by the ionosphere. Therefore, employing sky wave does the broadcasting in this band. - The ionosphere consists of positively charged ions and electrons. Such a system is known as plasma. It has a characteristic frequency called ‘plasma frequency’ given by where N is the electron density. When a radiation of frequency fp reaches the region of electron density N at normal incidence, it will be reflected. Thus the radio waves of different frequencies sent from earth will be reflected back the appropriate layers of the ionosphere. The critical frequency for reflection is, therefore, given by
- There exists a distance from the transmitter, measured along the surface of the earth, to the point where the sky wave returns to the earth after reflection from the ionosphere. This distance is known as ‘skip distance’ for a single hop. Using multiple hops in which the wave is reflected between ionosphere and earth several times or beaming at different angles can increase the range of transmission. Satellite Communication - With increasing demands of information technology there has been pressure on increasing the bandwidth and therefore the carrier frequency. - Beyond a certain frequency (>30 MHz) the ionosphere bends any EM wave but does not reflect it back towards earth. - A new concept of communication, the communication via the satellite has revolutionized the communication technology. - Signals from an earth station are beamed up to a satellite in space, which acts as a microwave link repeater. The signal is amplified and returned to earth at a different frequency to avoid interference between the up link and down link. - Microwave frequencies have to be used to penetrate ionosphere because all practical satellites orbit well above the atmosphere.
- 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.
<ul><li>Remote Sensing - Remote sensing is the science and art of obtaining information about an object, area, or phenomenon, acquired by a sensor that is not in direct contact with the object. - Any photography is a kind of remote sensing. - If we want to cover large areas for which information is required we have to take photograph from longer distances. Aerial photography was first introduced in World War I for military uses and later extended to areas such as archaeology, cartography, resource surveys, town and country planning etc. - It has now to share with satellite imagery. - A satellite equipped with appropriate sensors to acquire data can be placed in an orbit around earth at any height having a period of revolution. It takes photographs or collect any other information desired and transmits it back to an earth station. This is known as remote sensing . - A LANDSAT series of satellites designed for land-use applications are employed for this purpose. These satellites fly in near-polar orbits at an altitude of 918 km. The instrument on board is a multi-spectral scanner. This instrument scans a swath of width about 185 km. The satellite travels in a direction slightly west of south . In a single day there will be 14 southbound passes. The distance between successive passes is more than the swath width. Thus different parts of earth are scanned. The data obtained is transmitted to the receiving station on earth. </li></ul>
‘ Taking photograph’ of any object relies on the reflected wave from the object as we use visible light in normal photography. - Selection of the wavelength of the radiation depends on the effect of atmosphere including ionosphere and the nature of the objects to be scanned. - The visible and near infrared spectral bands are chosen to amplify or separate specific earth features such as vegetation and water. - Data from thermal infrared bands are of high interest, particularly due to the fact that thermal infrared data is a measure of surface temperature and can also be obtained at night. - Microwave data are of particular relevance for certain hydrological variables such as soil moisture and precipitation. They can also be obtained at night and are not restricted to cloud free conditions. - The spatial resolution of remote sensing data varies with the form of sensor employed and height of the satellite. The resolution also depends on the wavelength of radiation used. Low altitude satellites are capable of resolving 10 m while the resolution of geo-stationary satellite is ~ 5 km. The temporal resolution depends on the nature of the orbit. The temporal resolution of geo-stationary satellite can be half an hour or less while that of polar orbits could be 15 days. - Some applications of remote sensing include meteorology, climatology, oceanography and coastal studies. Archeology, geological surveys, water resource surveys, urban land use surveys, agriculture and forestry etc
<ul><li>Line Communication - Line communication is point-to-point communication. - It requires the use of a guided medium as communication channel. - In a guided medium as a signal travels, it gets attenuated. The signal, therefore, needs periodic amplification and retransmission at a repeater station. - In line communication, the response of the communication medium towards the signal plays an important role. - The transmission characteristics of a guided medium and the quality of transmission depend upon: 1. Nature of the medium (wire/co-axial cable/optical fibre) 2. Nature of the signal (frequency, strength/power etc.) - A given medium can accommodate only a limited band of frequencies which depends on its characteristics. Guided Media - Guided medium has finite boundaries . - Examples of such media are provided by: (a) Twisted pair (b) Co-axial Cables (c) Optical Fibre </li></ul>
Transmitter Guided Medium Repeater station Receiver Point-to-point Physical Connection LINE COMMUNICATION
Twisted Pair - Two insulated copper wires arranged in a spiral pattern form a twisted pair - Twisting of wires minimizes electromagnetic interference. - A number of such wires are bundled together into a thick cable. - Copper conductor wires in the twisted pair provide a very low cost medium. - Is used for transmitting both, analog as well as digital information. - In local telephone services, individual telephone sets from a house are connected to the local telephone exchange box on the street using a twisted pair. Each one of these boxes is connected underneath by a thick cable, which carries several twisted pairs, to the main telephone exchange and all around the city. - Transmission of many signals at a time in a single twisted pair is done by transmitting the signal as a modulated wave with a fixed carrier frequency. An analog voice signal of bandwidth 3 kHz, requires a bandwidth of 6 kHz, if such a signal is to be transmitted in the form of a modulated wave. If we wish to send 100 such signals over the same wire we would then require a bandwidth of 600 kHz.
<ul><li>Loss due to finite resistance. - At high frequencies the current in a conductor tends to flow on the surface (Skin effect). Therefore the with increasing frequency the effective resistance per unit length of the conductor also increases. - The upper limit to frequency in this type of transmission line is 1 MHz and the repeater spacing limit is ~ 6 km. Coaxial Cable - A coaxial cable consists of a hollow outer cylindrical conductor, which surrounds a coaxial single inner conductor. </li></ul>
- The inner conductor is held at the centre by a solid dielectric (insulating) material all around. - The outer conductor is normally connected to ground and thus provides an electrical shield to the signals carried by the central conductor. - Most of the power is carried by the electromagnetic waves and only a relatively small amount in the form of conduction electrons in the central conductor. - Very high frequencies can be transmitted before the attenuation becomes too severe. - Reduced attenuation in co-axial cables increases the repeater spacing to about 20 km and permitting much larger bandwidths (20 MHz). Employing digital transmission further enhances the bandwidth. - Co-axial cables are commonly used for long distance high frequency transmission. They can carry both analog and digital signals. They are extensively used in local area computer network for high speed data transmission.
Optical Communication - Communication is through optical (light) signals. - Communication medium employed is an optical fibre - The schematics of the essential components of an optical communication link are as shown. - This mode of communication provides a very large bandwidth and therefore carry large amount of information. - Light is a form of electromagnetic wave with frequencies lying in the range 10 12 to 10 16 Hz and corresponding wavelengths in the range 0.03 to 300 m. - The light emitted from He-Ne laser has = 0.63 m, or a frequency of ~ 4.7 10 14 Hz. If we use this light and employ only 1% of this frequency bandwidth I.e. 4700 GHz for an optical communication channel, it offers tremendous capacity for transmission. - To telecast pictures through a TV channel we need an approximate bandwidth of 4.7 MHz per channel. The number of TV channels which can be accommodated in a bandwidth of 4700 GHz is ~ 10 6
Transmitter Optical source Input signal Modulation Optical fibre cable Receiver Optical detector Demodulation Output signal Optical Communication
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.
Photonic Devices Photonics is a subject that deals with the generation and detection of photons. The devices which generate and detect photons are called photonic devices. Such devices are extensively employed in optical communication. To understand the functioning of these devices we need to know the different interaction mechanisms between photons and electrons in a solid. There are three important processes; (a) Absorption (b) Spontaneous emission (c) Stimulated emission
Optical Light Source and Optical Detector Light Source - In optical communication light source plays a pivotal role. - It generates a beam of light. This light wave can be modulated directly to produce light pulses, by applying an appropriate electrical signal to the optical source. Another alternative is to modulate the light beam externally using an electro-optic modulator. - The light source to be used in optical communication is required to have some special characteristics. - Its physical size should be small so that it can be coupled to the thin optical fibre. - Monochromatic light sources which can produce light of desired wavelength (0.85, 1.3 and 1.55 m) are preferred. - Rapid switching. Some of these fundamental requirements are met by light emitting devices such as solid state semiconductor LASER diodes.
Light emitting diode A light emitting diode (LED) works by the process of spontaneous emission, when a p-n junction is forward biased. In forward bias, electrons from the n – region migrate to the p – region and the holes from the p – region move towards the n – region. Holes injected into the n – region quickly encounter free electrons and recombine. Electrons injected into the p – region encounter holes and recombine. When each electron – hole pair recombines a single photon is released which carries with it the energy required to liberate an electron from the valency bond. The wavelength and frequency of light emitted are determined by the band gap energy. The intensity of light produced is proportional to the forward current conducted by the junction that controls the number of holes and electrons crossing the junction to be recombined..
n n p Light h = E g Visible – Phos. Doped GaAs Infrared – Al doped GaAs LED Operated in forward bias
Laser - LASER is an acronym and stands for l ight a mplification by s timulated e mission of r adiation. - An important optical and electronic device. - The laser is a source of highly directional , monochromatic and coherent light. - Laser action has been obtained using different materials, including gases, such as neon, helium, carbon dioxide, and solids such as ruby and semiconductors. - The action of a laser is based on the principle of stimulated emission.
When light is absorbed, electrons from the ground state E 1 are excited to the band of level designated as E 3 . These excited levels are highly unstable, and the electrons decay rapidly to level E 2 . The energy difference E 3 E 2 is given up in the form of heat. The level E 2 is very important for the stimulated emission because this level is metastable having a mean life of few mili-seconds. If the electrons are excited from E 1 to E 3 at a rate faster than the rate E 2 back to E 1 , the population of the metastable state E 2 becomes larger than the ground state E 1 . Such a situation is called population inversion . The population inversion is very crucial for laser action. Now if a photon of energy E 2 E 1 enters this system and interacts with one of the inverted population atoms. This photon can now actually stimulate the electron to fall from state E 2 to state E 1 and emit a photon of energy E 2 E 1 . Thus the first photon has the emission of another photon of same energy and multiplying the number of photons by a factor of two. We thus have light amplification by stimulated emission of radiation or LASER E 2 E 1 .
The construction of a semiconductor laser diode is achieved by polishing the sides of the p-n junction. A highly polished surface acts as a partially reflecting surface and also enables a back reflection process. Stimulated photons assist in the process of combining the electrons and holes by stimulating the electrons to fall quickly .
The structure of a modern semiconductor LASER designed for optical communication is very complex as shown below. The lasing material is gallium arsenide, an alloy of gallium and arsenic. It has the advantage that the material can be made n- type or p -type by varying the relative proportion of the two elements. It has also the band gap whose corresponding wavelength lies in the first low loss window at 0.86 m.
Optical Detectors - Light pulses that propagate in the fibre and reach the receiver end become weak, due to some unavoidable losses along the way. - The optical detector should highly sensitive. - The optical detector should have a quick time response. It should respond quickly to the changing light pulses that are switching ON and OFF. - The physical size of the detector should be very small, so that it can be properly coupled to the thin optical fibre. Several kinds of photosensitive devices that generate an electrical signal when light falls on them are used as optical detectors. These include silicon photo-diodes, avalanche photo-diodes (APD), photo-transistors, and photo-resistors .
Light p n p – n operated in reverse bias I V A