Laser Components and Fibre Optic Connectors PDF

Components of LASER Every LASER consists of three basic components. These are – Lasing material or active medium. External energy source. Optical resonator.  The active medium is excited by the external energy source(pump source) to produce the population inversion. In the gain medium th...

Components of LASER Every LASER consists of three basic components. These are – Lasing material or active medium. External energy source. Optical resonator.  The active medium is excited by the external energy source(pump source) to produce the population inversion. In the gain medium that spontaneous and stimulated emission of photons takes place, leading to the phenomenon of optical gain, or amplification. Semiconductors, organic dyes, gases (He, Ne, CO2, etc), solid materials (YAG, sapphire (ruby) etc.) are usually used as lasing materials and often LASERs are named for the ingredients used as a medium. The excitation source, pump source provides energy which is needed for the population inversion and stimulated emission to the system. Pumping can be done in two ways – electrical discharge method and optical method. Examples of pump sources are electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions etc. Resonator guide basically provides the guidance about the simulated emission process. It is induced by high-speed photons. Finally, a laser beam will be generated.  Types of LASER There are many types of LASERs available for different purposes. Depending upon the sources they can be described as below. 1. Solid State LASER In this kind of LASERs solid state, materials are used as active medium. The solid state materials can be ruby, neodymium-YAG (yttrium aluminum garnet) etc. 2. Gas LASER These LASERs contain a mixture of helium and Neon. This mixture is packed up into a glass tube. It acts as active medium. We can use Argon or Krypton or Xenon as the medium. CO2 and Nitrogen LASER can also be made. 3. Dye or Liquid LASER In this kind of LASERs organic dyes like Rhodamine 6G in liquid solution or suspension used as active medium inside the glass tube. 4.Excimer LASER Excimer LASERs (the name came from excited and dimers) use reactive gases like Chlorine and fluorine mixed with inert gases like Argon or Krypton or Xenon. These LASERs produce light in the ultraviolet range. 5.Chemical LASER A chemical laser is a LASER that obtains its energy from a chemical reaction. Examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the hydrogen fluoride laser, deuterium fluoride laser etc 6.Semiconductor LASER In these lasers, junction diodes are used. The Semiconductor is doped by both the acceptors and donors. These are known as injection laser diodes. Whenever the current is passed, light can be seen at the output.  Einstein A and B Coefficients Einstein found that the emission of a photon is possible by two different processes, spontaneous and stimulated emission, and that the coefficients describing the three processes—absorption, stimulated and spontaneous emission—are related to each other (Einstein relations) In 1917, about 9 years before the development of the relevant quantum theory, Einstein postulated on thermodynamic grounds that the probability for spontaneous emission, A, was related to the probability of stimulated emission, B, by the relationship A/B = 8πhν3/c3 From the development of the theory behind blackbody radiation, it was known that the equilibrium radiation energy density per unit volume per unit frequency was equal to ρ(ν) = 8πhν3/c3  Einstein A and B Coefficients Einstein argued that equilibrium would be possible, and the laws of thermodynamics obeyed, only if the ratio of the A and B coefficients had the value shown above. This ratio was calculated from quantum mechanics in the mid 1920's. In recognition of Einstein's insight, the coefficients have continued to be called the Einstein A and B coefficients. While applicable in many situations, the A and B coefficients received particular attention in the period in which lasers were being developed. The nature of the coefficients is such that you cannot use the radiation in a cavity to elevate electrons preferentially into an upper state, producing the population inversion necessary for laser action. The particular ratio between the coefficients suggests that the presence of the light to "pump" electrons into upper states will have the same probability of stimulating an already elevated electron to make the downward transition, so that laser action cannot be achieved with any two-level system. The achievement of laser action was obtained by three-level systems like that in the helium- neon laser where the population of the upper neon level could be achieved by a non-radiative transfer from the helium gas pumping energy to the neon atoms.  The implication of the Einstein A and B coefficients is that these two processes will occur at equal rates, so that no population inversion can be attained in a two-level system like that depicted here. In the helium-neon laser, for example, this limitation is overcome by collisional transfer from the helium gas to the neon gas, achieving the necessary population inversion for laser action.  Population Inversion Population inversion, in physics, the redistribution of atomic energy levels that takes place in a system so that laser action can occur. Normally, a system of atoms is in temperature equilibrium and there are always more atoms in low energy states than in higher ones. Although absorption and emission of energy is a continuous process, the statistical distribution (population) of atoms in the various energy states is constant. When this distribution is disturbed by pumping energy into the system, a population inversion will take place in which more atoms will exist in the higher energy states than in the lower. So,Population inversion occurs when more electrons, in a particular situation, are in a higher energy state than in a lower energy state. Population inversion can be thought of as an inversion from the standard, since electrons are typically located in lower energy states.  Pumping For maintaining a state of population inversion atoms have to be raised continuously to excited state. It requires energy to be supplied to the system. The process of supplying energy to the medium with a view to transfer it into state of population inversion is known as pumping. Commonly used pumping types are : — Optical pumping: light is used to raise the atoms to higher energy states. Chemical pumping: chemical reactions are used to raise the atoms. Electrical pumping: A strong field is applied to the atomic system with the use of high voltage power supply. The high energy electrons collide with the atoms and transfer their kinetic energy to the later. As a result, atoms rise to the higher states  Components of laser Components of Lasers 1. Active Medium :It is the material in which the laser action takes place. The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium / Neon, or semiconductors such as GaAs. This medium decides the wavelength of laser radiation. Active mediums contain atoms which can produce more stimulated emission than spontaneous emission and cause amplification they are called “Active Centers”. 2. Energy Source (Excitation Mechanism): Energy Source (Excitation mechanisms) pumps the active centers from ground state to excited state to achieve population inversion. The pumping by energy source can be optical, electrical or chemical depending on the active medium. 3. Resonance Cavity: Resonance cavity consists of active medium enclosed between two mirrors one is highly reflective mirror (100% reflective) and the other is partially transmissive mirror (99% reflective).  Fibre optic connectors There are many occasions when it is necessary to connect a fibre optic cable to another item. It may be that the fibre optic cable needs to be connected to another cable, or to an electronic interface device where the optical signal is converted to an electrical signal or to a light source. It is necessary that the fibre optic cable is correctly interfaced so that the minimum amount of light is lost. To achieve this it is necessary to use the correct form of fibre optic connector. In these cases fibre optic connectors are required. While fibre optic connectors offer a very convenient method of connecting fibre optic cables, they should only be used where necessary. They introduce a loss at each connection. Typically the value is between 10 and 20 percent. Against this they make reconfiguring systems very much easier.  Connector basics The fibre optic connector basically consists of a rigid cylindrical barrel surrounded by a sleeve. The barrel provides the mechanical means by which the connector is held in place wit the mating half. A variety of methods are used to ensure the connector is held in place, ranging from screw fit, to latch arrangements. The main requirement si that the end of the fibre optic cable is held accurately in place so that the maximum light transfer occurs. As it is imperative that the optical fibre is held securely and accurately in place, connectors will normally be designed so that the fibre is glued in place, and in addition to this strain relief is also provided Fibre ends may also be polished. For single mode fibre, the ends may be polished with a slight convex curvature so that the centres of the cables from the two connectors achieve physical contact. This approach reduces the back reflections, although the level of loss may be slightly higher.  Fibre optic connector types Fibre optic connectors (fiber optic connectors) come in a variety of formats. These different fibre optic connectors may be used in slightly different applications or under different circumstances, as each type has its own capabilities. When choosing a fibre optic connector, it is necessary to ensure that its properties meet the needs of the particular application in question. Some fibre optic connectors may be suitable for different optical fibres, and this needs to be taken into consideration. There is a wide variety of different fiber optic connectors available. A selection of some is given below: FC/PC This form of fibre optic connector is used for single-mode fiber optic cable. It provides very accurate positioning of the single-mode fiber optic cable with respect to transmitter (optical source) or the receiver (optical detector). SC This form of connector is mainly used with single-mode fiber optic cables. The connector is simple low cost and reliable. The location and alignment is provided using a ceramic ferrule. It also has a locking tab to enable it to be mated and removed without fear of it accidentally falling loose. Plastic fiber optic cable connectors As the name implies, these fibre optic cable connectors are only used with plastic fibre optic cabling.  Fibre optic splicing Rather than using optical fibre connectors, it is possible to splice two optical fibres together. An fibre optic splice is defined by the fact that it gives a permanent or relatively permanent connection between two fibre optic cables. That said, some manufacturers do offer fibre optic splices that can be disconnected, but nevertheless they are not intended for repeated connection and disconnection. There are many occasions when fibre optic splices are needed. One of the most common occurs when a fibre optic cable that is available is not sufficiently long for the required run. In this case it is possible to splice together two cables to make a permanent connection. As fibre optic cables are generally only manufactured in lengths up to about 5 km, when lengths of 10 km are required, for example, then it is necessary to splice two lengths together.  Ways of Fibre optic splices Fibre optic splices can be undertaken in two ways: Mechanical splices:The mechanical splices are normally used when splices need to be made quickly and easily. To undertaken a mechanical fibre optic splice it is necessary to strip back the outer protective layer on the fibre optic cable, clean it and then perform a precision cleave or cut. When cleaving (cutting) the fibre optic cable it is necessary to obtain a very clean cut, and one in which the cut on the fibre is exactly at right angles to the axis of the fibre. Once cut the ends of the fibres to be spliced are placed into a precision made sleeve. They are accurately aligned to maximise the level of light transmission and then they are clamped in place. A clear, index matching gel may sometimes be used to enhance the light transmission across the joint. Mechanical fibre optic splices can take as little as five minutes to make, although the level of light loss is around ten percent. However this level of better than that which can be obtained using a connector.  Fusion splices: Fusion splices form the other type of fibre optic splice that can be made. This type of connection is made by fusing or melting the two ends together. This type of splice uses an electric arc to weld two fibre optic cables together and it requires specialised equipment to perform the splice. The protective coating from the fibres to be spliced is removed from the ends of the fibres. The ends of the fibre optic cable are then cut, or to give the correct term they are cleaved with a precision cleaver to ensure that the cuts are exactly perpendicular. The next stage involves placing the two optical fibres into a holder in the fibre optic splicer. First the ends if the cable are inspected using a magnifying viewer. Then the ends of the fibre are automatically aligned within the fibre optic splicer. Then the area to be spliced is cleaned of any dust often by a process using small electrical sparks. Once complete the fibre optic splicer then uses a much larger spark to enable the temperature of the glass in the optical fibre to be raised above its melting point and thereby allowing the two ends to fuse together. The location spark and the energy it contains are very closely controlled so that the molten core and cladding do not mix to ensure that any light loss in the fbre optic splice is minimised.  Mechanical and fusion splices The two types of fibre optic splices are used in different applications The mechanical splices are used for Fusion splices offer a lower level of applications where splices need to be loss and a high degree of made very quickly and where the permanence. However they require expensive equipment for fusion the use of the expensive fusion splices may not be available. Some of splicing equipment. In view of this the sleeves for mechanical fibre optic they tend to be used more for the splices are advertised as allowing long high data rate lines that are connection and disconnection. In this installed that are unlikely to be way a mechanical splice may be used changed once installed. in applications where the splice may be less permanent.  Fiber Couplers Fiber couplers belong to the basic components of many fiber-optic setups. Note that the term fiber coupler is used with two different meanings: It can be an optical fiber device with one or more input fibers and one or several output fibers. Light from an input fiber can appear at one or more outputs, with the power distribution potentially depending on the wavelength and polarization. It can also be a device for coupling (launching) light from free space into a fiber.  Two or more fibers can be thermally tapered and fused so that their cores come into intimate contact over some length of a few centimeters, for example. This can also be done with polarization- maintaining fibers, leading to polarization-maintaining couplers (PM couplers) or splitters. Some couplers use side-polished fibers, providing access to the fiber core. There are fiber-optic pump combiners and pump–signal combiners, which usually work with multimode pump fibers. There are planar lightwave circuits, containing things like branching waveguides, with fibers coupled to the inputs and outputs. Couplers can also be made from bulk optics, for example in the form of microlenses and beam splitters, which can be coupled to fibers (“fiber pig-tailed”).  Absorption Absorption is the process in which optical energy is converted to internal energy of electrons, atoms, or molecules. When a photon is absorbed, the energy may cause an electron in an atom to go from a lower to a higher energy level, thereby changing the internal momentum of the electron and the electron's internal quantum numbers.  Spontaneous Emission Spontaneous emission is an energy conversion process in which an excited electron or molecule decays to an available lower energy level and in the process gives off a photon. This process occurs naturally and does not involve interaction of other photons. The average time for decay by spontaneous emission is called the spontaneous emission lifetime. For some excited energy levels this spontaneous decay occurs on average within nanoseconds while in other materials it occurs within a few seconds As with absorption, this process can occur in isolated atoms, ionic compounds, molecules, and other types of materials, and it can occur in solids, liquids, and gases. Energy is conserved when the electron decays to the lower level, and that energy must go somewhere. The energy may be converted to heat, mechanical vibrations, or electromagnetic photons. If it is converted to photons, the process is called spontaneous emission, and the energy of the photon produced is equal to the energy divergence between the electron energy levels involved. The emitted photon may have any direction, phase, and electromagnetic polarization.  Spontaneous emission processes may be classified based on the source of energy which excites the electrons, If the initial source of energy for spontaneous emission is supplied optically, the process is called photoluminescence. Glow in the dark materials emit light by this process. If the initial form of energy is supplied by a chemical reaction, the process is called chemiluminescence. Glow sticks produce spontaneous emission by chemiluminescence. If the initial form of energy is supplied by a voltage, the process is called electroluminescence. LEDs emit light by electroluminescence. If the initial form of energy is caused by sound waves, the process is called sonoluminescence. If the initial form of energy is due to accelerated electrons hitting a target, this process is called cathodoluminescence. If spontaneous emission occurs in a living organism, such a firefly, the process is called bioluminescence.  Stimulated Emission Stimulated emission is the process in which an excited electron or molecule interacts with a photon, decays to an available lower energy level, and in the process gives o a photon. As with the other processes, this process can occur in isolated atoms, ionic compounds, organic molecules, and other types of materials, and it can occur in solids, liquids, and gases. If an incoming photon, with energy equal to the difference between allowed energy levels, interacts with an electron in an excited state, stimulated emission can occur. The energy of the excited electron will be converted to the energy of a photon. The stimulated photon will have the same frequency, direction, phase, and electromagnetic polarization as the incoming photon which initiated the process  Characteristics of Laser Laser light has four unique characteristics that differentiate it from ordinary light: these are 1) Coherence 2) Directionality 3)Monochromatic 4) High intensity  Coherence We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition. In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase. In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light. Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.  Directionality In conventional light sources (lamp, sodium lamp and torchlight), photons will travel in random direction. Therefore, these light sources emit light in all directions. On the other hand, in laser, all photons will travel in same direction. Therefore, laser emits light only in one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading. If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.  Monochromatic Monochromatic light means a light containing a single color or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths or colors. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths. On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Hence, the light waves of laser have single wavelength or color. Therefore, laser light covers a very narrow range of frequencies or wavelengths.  High Intensity You know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions. If you look at a 100 Watt lamp filament from a distance of 30 cm, the power entering your eye is less than 1/1000 of a watt. In laser, the light spreads in small region of space and in a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light. If you look directly along the beam from a laser (caution: don’t do it), then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp. Conclusion: Thus, these four properties of laser beam enable us to cut a huge block of steel by melting. They are also used for recording and reproducing large information on a compact disc (CD).  What are Dielectrics? Dielectrics, in general, can be described as materials that are very poor conductors of electric current. They are basically insulators and contain no free electron. Dielectrics can be easily polarized when an electric field is applied to it. Thus, their behaviour in an electric field is entirely different from that of conductors as would be clear from the following discussion.  Dielectric Materials With respect to the atomic view, dielectric materials are classified into two categories  Polar Molecules A polar molecule is one in which the ‘centres of gravity’ of the positive charges (i.e., protons) and negative charges (i.e., electrons) do not coincide. Such molecules are called permanent electric dipoles as these have permanent dipole moments. Some common polar molecules are HCl, H2O, N2O, NH3, H2S, C2H5OH, SO2. In a molecule of HCl, there is an excess positive charge on the H-ion and an equal negative charge on the Cl-ion. The molecule, therefore, has a dipole moment at every instant and is a polar molecule. Another interesting example of polar molecules is H2O. In the water molecule, two O-H bonds are not placed opposite to each other (unlike the CO2 molecule) but are inclined at an angle of about 105°. The hydrogen ion forms a dipole moment with each of the oxygen ion, and there is a net dipole moment. [Fig. (a)].  Effect of electric Field (1) In the absence of an electric field, the electric dipole moments of these polar molecules point in random directions [Fig. (b)] and cancel each other. Therefore, even though each molecule has a dipole moment, the average moment per unit volume is zero. (2) On the application of an electric field, the dipole moments of these molecules align themselves parallel, to the direction of the electric field as shown in figure (c). But this alignment is incomplete due to the thermal vibrations of the molecules. It is obvious that the alignment of the molecules with the applied field increases if: (i) The electric intensity of the field is increased. (ii)Temperature is decreased. Note-It should be noted that increased electric intensity may also increase the dipole moment. It is due to the reason that with increased electric intensity, the distance between the centres of gravity of the positive and negative charges increases which results in an increase in the dipole moment.  Non-polar Molecules A non-polar molecule is one in which the centres of gravity of positive charges (i.e., protons) and negative charges (i.e., electrons) coincide. These molecules, thus, do not have any permanent dipole moment. Some common examples of non-polar molecules are CO2, CCl4, oxygen (O2), nitrogen (N2), hydrogen (H2), methane (CH4) and ethane (C2H6). In a molecule of CO2, the oxygen ions are symmetrically placed with respect to the carbon ion, hence the dipole moment is zero [Figure (a)]. If the molecule is placed in an electric field E along the line joining the ions, the oxygen ions get displaced with respect to the carbon ion and the net dipole moment induced is along the direction of electric field E [Figure. (b)]. If the electric field E is applied perpendicular to the line joining the ions, the directions of the induced dipole moment is again along the field E as shown in figure (c)  Conclusion Thus, in general, when a non-polar molecule is placed in an electric field, the centres of positive and negative charges get displaced and the molecule is then said to have been polarized as shown in fig. (d). Such a molecule is then called the induced electric dipole and its electric dipole moment is called the induced electric dipole moment. As soon as the electric field is removed, the induced electric dipole moment disappears. The induced electric dipole moment is proportional to the applied electric field but is almost independent of temperature. Further, the induced dipole is parallel to the electric field right at the time of its creation. The main difference between the polar and the non-polar molecules is the temperature dependence of dipole moment in case of polar molecules, and no such dependence in case of non-polar molecules.  Dielectric Polarisation A dielectric may be made up of polar or non-polar molecules. But the net effect of an external field is almost the same, i.e., the external field will compel the molecules to align their dipole moments along its own direction. Let us consider a dielectric slab in an electric field which is acting in the direction shown in the figure. The arrangement of charges within the molecules of the dielectric in the electric field is as shown in the figure. The positive charges move in the direction of the field and the negative charges in the opposite direction. In other words, the electric dipoles align themselves with the direction of the field. In this state, the entire dielectric and its molecules are said to be polarised. Dielectric slab in an electric field  The alignment of the dipole moments of the permanent or induced dipoles with the direction of the applied electric field is called polarisation. Within the two extremely thin surface layers indicated by shaded regions, there is an excess negative charge in one layer and an excess equal positive charge in the other layer. The induced charges on the surfaces of the dielectric are due to these layers. These charges are not free but each is bound to a molecule lying in or near the surface. That is why these charges are called bound charges or fictitious charges. Within the remaining dielectric, the net charge per unit volume remains zero. Thus, although the dielectric is polarized, yet as a whole, it remains electrically neutral.  Conclusion Obviously, the positive induced surface charge must be equal in magnitude to the negative induced surface charge. Thus, in polarisation, the internal state of the slab is characterised not by an excess charge but by the relative displacement of the charges within it. Polarisation can thus also be thought of as a phenomenon in which an alignment of positive and negative charges takes place within the dielectric resulting in no net increase in the charge of the dielectric. Applications of Laser  Types of lasers Types of lasers Lasers are classified into 4 types based on the type of laser medium used: Solid-state laser Gas laser Liquid laser Semiconductor laser  He-Ne Laser Discovery- Dr Ali and his Collegues at Bell Laboratory in 1961. Definition- It is a gas laser in which a mixture of He and Ne gas is used as active medium or laser medium.  Laser medium is mixture of Helium and Neon gases in the ratio 10:1 Medium excited by large electric discharge, flash pump or continuous high power pump In gas, atoms characterized by sharp energy levels compared to solids Actual lasing atoms are the Neon atoms  Pumping action: Electric discharge is passed through the gas Electrons are accelerated, collide with He and He atoms and excite them to higher energy levels  Construction of He – Ne laser: Construction of He – Ne laser: A quartz tube including a mixture of helium and neon in ratio 10: 1 at low pressure of nearly 0.6 mm Hg. Two plane or concave parallel mirrors which are perpendicular to the tube axis, one has a reflection coefficient of nearly 99.5 %, while the other mirror is semi-transparent with a reflection coefficient of 98 %. High-frequency electric field or a high DC voltage inside the tube causing electric discharge and exciting the gas atoms.  Operation: 1. The potential difference inside the tube leads to the excitation of the helium atoms to higher levels. 2. The excited helium atoms collide with the unexcited neon atoms inelastic collisions, Thus, energy is transferred from the excited helium atoms to the neon atoms due to the close energy value of the excitation levels in both atoms, Neon atoms are thus excited. 3. The excitation level of a neon atom has a relatively long lifetime (nearly 10−3 s) such a level is called metastable state, Due to the continuous collision between the excited helium atoms and neon atoms, an accumulation of excited neon atoms occur, Hence, a population inversion occurs in neon atoms. 4. A group of neon atoms that are excited relaxes to a lower excitation state, In doing so, they emit spontaneous photons, which have energy equal to the difference in energy levels, Then, photons propagate randomly in all directions inside the tube. 5. Photons which propagate along the axis of the tube are reflected back by one of the two mirrors in its way, they bounce off inside the tube and can not get out. 6. During the propagation of these photons inside the tube between the two mirrors, they may collide with some neon atoms in the excited metastable state, which its lifetime is not yet over, Thus, they stimulate the neon atoms to emit photons of the same energy, frequency and phase as the colliding photon, Thus the number of photons moving inside the tube multiplies. 7. The new stream of photons repeats the process and thus, they are multiplied by the lasing action, This is how amplification takes place. 8. When radiation inside the tube reaches a certain level, part of it gets out through the semi-transparent mirror in the form of a laser beam, while the rest of the radiation remains trapped inside the tube, to continue the stimulated emission and the lasing action. 9. Neon atoms which have relaxed to a lower level, soon enough they lose whatever left of their energy in different forms and finally go back to the ground state, Helium atoms collide again with neon atoms and the cycle repeats. 10. The helium atoms which have lost their energy due to collision by neon atoms excited again by the electric discharge and so on. Metastable energy level is the energy level which is characterized by long lifetime relatively (10−3 s), It is necessary for laser sources during operation, the active medium reaches the inverse population state which it isn’t required in ordinary light sources, because the base of laser action is the existence of a large number of atoms in metastable state to be the stimulated emission is the dominant emission.  Laser: Characteristics & Applications By- Dr Pawan Kumar Assistant Professor School of Physics and Materials Science Shoolini University, Solan,HP  LASER L-Light A-Amplification by S-Stimulated E-Emission of R-Radiations  Characteristics of Laser Laser light has four unique characteristics that differentiate it from ordinary light: these are 1) Coherence 2) Directionality 3)Monochromatic 4) High intensity  Coherence We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition. In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase. In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light. Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.  Directionality In conventional light sources (lamp, sodium lamp and torchlight), photons travel in random direction. Therefore, these light sources emit light in all directions. On the other hand, in laser, all photons will travel in same direction. Therefore, laser emits light only in one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading. If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.  Monochromatic Monochromatic light means a light containing a single color or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths or colors. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths. On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Hence, the light waves of laser have single wavelength or color. Therefore, laser light covers a very narrow range of frequencies or wavelengths.  High Intensity You know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions. If you look at a 100 Watt lamp filament from a distance of 30 cm, the power entering your eye is less than 1/1000 of a watt. In laser, the light spreads in small region of space and in a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light. If you look directly along the beam from a laser (caution: don’t do it), then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp. Conclusion: Thus, these four properties of laser beam enable us to cut a huge block of steel by melting. They are also used for recording and reproducing large information on a compact disc (CD).  Absorption Absorption is the process in which optical energy is converted to internal energy of electrons, atoms, or molecules. When a photon is absorbed, the energy may cause an electron in an atom to go from a lower to a higher energy level, thereby changing the internal momentum of the electron and the electron's internal quantum numbers.  Spontaneous Emission Spontaneous emission is an energy conversion process in which an excited electron or molecule decays to an available lower energy level and in the process gives off a photon. This process occurs naturally and does not involve interaction of other photons. The average time for decay by spontaneous emission is called the spontaneous emission lifetime. For some excited energy levels this spontaneous decay occurs on average within nanoseconds while in other materials it occurs within a few seconds As with absorption, this process can occur in isolated atoms, ionic compounds, molecules, and other types of materials, and it can occur in solids, liquids, and gases. Energy is conserved when the electron decays to the lower level, and that energy must go somewhere. The energy may be converted to heat, mechanical vibrations, or electromagnetic photons. If it is converted to photons, the process is called spontaneous emission, and the energy of the photon produced is equal to the energy divergence between the electron energy levels involved. The emitted photon may have any direction, phase, and electromagnetic polarization.  Spontaneous emission processes may be classified based on the source of energy which excites the electrons, If the initial source of energy for spontaneous emission is supplied optically, the process is called photoluminescence. Glow in the dark materials emit light by this process. If the initial form of energy is supplied by a chemical reaction, the process is called chemiluminescence. Glow sticks produce spontaneous emission by chemiluminescence. If the initial form of energy is supplied by a voltage, the process is called electroluminescence. LEDs emit light by electroluminescence. If the initial form of energy is caused by sound waves, the process is called sonoluminescence. If the initial form of energy is due to accelerated electrons hitting a target, this process is called cathodoluminescence. If spontaneous emission occurs in a living organism, such a firefly, the process is called bioluminescence.  Stimulated Emission Stimulated emission is the process in which an excited electron or molecule interacts with a photon, decays to an available lower energy level, and in the process gives o a photon. As with the other processes, this process can occur in isolated atoms, ionic compounds, organic molecules, and other types of materials, and it can occur in solids, liquids, and gases. If an incoming photon, with energy equal to the difference between allowed energy levels, interacts with an electron in an excited state, stimulated emission can occur. The energy of the excited electron will be converted to the energy of a photon. The stimulated photon will have the same frequency, direction, phase, and electromagnetic polarization as the incoming photon which initiated the process  Components of LASER Every LASER consists of three basic components. These are – Lasing material or active medium. External energy source (pumping Source) Optical resonator (Resonance Cavity)  The active medium is excited by the external energy source(pump source) to produce the population inversion. In the gain medium that spontaneous and stimulated emission of photons takes place, leading to the phenomenon of optical gain, or amplification. Semiconductors, organic dyes, gases (He, Ne, CO2, etc), solid materials (YAG, sapphire (ruby) etc.) are usually used as lasing materials and often LASERs are named for the ingredients used as a medium. The excitation source, pump source provides energy which is needed for the population inversion and stimulated emission to the system. Pumping can be done in two ways – electrical discharge method and optical method. Examples of pump sources are electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions etc. Resonator guide basically provides the guidance about the simulated emission process. It is induced by high-speed photons. Finally, a laser beam will be generated.  Types of LASER There are many types of LASERs available for different purposes. Depending upon the sources they can be described as below. 1. Solid State LASER In this kind of LASERs solid state, materials are used as active medium. The solid state materials can be ruby, neodymium-YAG (yttrium aluminum garnet) etc. 2. Gas LASER These LASERs contain a mixture of helium and Neon. This mixture is packed up into a glass tube. It acts as active medium. We can use Argon or Krypton or Xenon as the medium. CO2 and Nitrogen LASER can also be made. 3. Dye or Liquid LASER In this kind of LASERs organic dyes like Rhodamine 6G in liquid solution or suspension used as active medium inside the glass tube. 4.Excimer LASER Excimer LASERs (the name came from excited and dimers) use reactive gases like Chlorine and fluorine mixed with inert gases like Argon or Krypton or Xenon. These LASERs produce light in the ultraviolet range. 5.Chemical LASER A chemical laser is a LASER that obtains its energy from a chemical reaction. Examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the hydrogen fluoride laser, deuterium fluoride laser etc 6.Semiconductor LASER In these lasers, junction diodes are used. The Semiconductor is doped by both the acceptors and donors. These are known as injection laser diodes. Whenever the current is passed, light can be seen at the output.  Population Inversion Population inversion, in physics, the redistribution of atomic energy levels that takes place in a system so that laser action can occur. Normally, a system of atoms is in temperature equilibrium and there are always more atoms in low energy states than in higher ones. Although absorption and emission of energy is a continuous process, the statistical distribution (population) of atoms in the various energy states is constant. When this distribution is disturbed by pumping energy into the system, a population inversion will take place in which more atoms will exist in the higher energy states than in the lower.  So,Population inversion occurs when more electrons, in a particular situation, are in a higher energy state than in a lower energy state. Population inversion can be thought of as an inversion from the standard, since electrons are typically located in lower energy states.  Pumping For maintaining a state of population inversion atoms have to be raised continuously to excited state. It requires energy to be supplied to the system. The process of supplying energy to the medium with a view to transfer it into state of population inversion is known as pumping. Commonly used pumping types are : — Optical pumping: light is used to raise the atoms to higher energy states. Chemical pumping: chemical reactions are used to raise the atoms. Electrical pumping: A strong field is applied to the atomic system with the use of high voltage power supply. The high energy electrons collide with the atoms and transfer their kinetic energy to the later. As a result, atoms rise to the higher states Applications of Laser Thank you all !  Numerical Aperture The numerical aperture (NA) of an optical system (e.g. an imaging system) is a measure for its angular acceptance for incoming light. It is defined based on geometrical considerations and is thus a theoretical parameter which is calculated from the optical design. It cannot be directly measured, except in limiting cases with rather large apertures and negligible diffraction effects. Numerical Aperture of an Optical System The numerical aperture of an optical system is defined as the product of the refractive index of the beam from which the light input is received and the sine of the maximum ray angle against the axis, for which light can be transmitted through the system based on purely geometric considerations (ray optics): For the maximum incidence angle, it is demanded that the light can get through the whole system and not only through an entrance aperture.  Numerical Aperture of an Optical Fiber or Waveguide Although an optical fiber or other kind of waveguide can be seen as a special kind of optical system, there are some special aspects of the term numerical aperture in such cases. In case of a step-index fiber, one can define the numerical aperture based on the input ray with the maximum angle for which total internal reflection is possible at the core– cladding interface: Figure 2: An incident light ray is first refracted and then undergoes total internal reflection at the core–cladding interface. However, that works only if the incidence angle is not too large.  The numerical aperture (NA) of the fiber is the sine of that maximum angle of an incident ray with respect to the fiber axis. It can be calculated from the refractive index difference between core and cladding, more precisely with the following relation: Note that the NA is independent of the refractive index of the medium around the fiber. For an input medium with higher refractive index, for example, the maximum input angle will be smaller, but the numerical aperture remains unchanged.  Acceptance Angle in Fiber Optics The acceptance angle of an optical fiber is defined based on a purely geometrical consideration (ray optics): it is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core. The sine of that acceptable angle (assuming an incident ray in air or vacuum) is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum: Here, ncore and ncladding are the refractive indices of core and cladding, respectively, and n0 is the refractive index of the medium around the fiber, which is close to 1 in case of air. Figure 1: An incident light ray is first refracted and then undergoes total internal reflection at the core–cladding interface. However, that works only if the incidence angle is not too large.  Optical Fiber An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair.Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths (data rates) than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics. The term was coined by Indian-American physicist Narinder Singh Kapany, who is widely acknowledged as the father of fiber optic The structure of a typical single-mode fiber. 1. Core: 8 µm diameter 2. Cladding: 125 µm dia. 3. Buffer: 250 µm dia. 4. Jacket: 400 µm dia. A TOSLINK optical fiber cable with a clear jacket. These cables are used mainly for digital audio connections between devices.  Types of Fiber optics: Generally optical fiber is classified into two categories based on: the number of modes, and the refractive index. These are explained as following below. 1. On the basis of the Number of Modes: It is classified into 2 types: (a). Single-mode fiber: In single-mode fiber, only one type of ray of light can propagate through the fiber. This type of fiber has a small core diameter (5um) and high cladding diameter (70um) and the difference between the refractive index of core and cladding is very small. There is no dispersion i.e. no degradation of the signal during traveling through the fiber. The light is passed through it through a laser diode. Single-mode fiber  (b). Multi-mode fiber: Multimode fiber allows a large number of modes for the light ray traveling through it. The core diameter is generally (40um) and that of cladding is (70um). The relative refractive index difference is also greater than single mode fiber. There is signal degradation due to multimode dispersion. It is not suitable for long-distance communication due to large dispersion and attenuation of the signal.There are two categories on the basis of Multi- mode fiber i.e.Step Index Fiber and Graded Index Fiber.Basically these are categories under the types of optical fiber on the basis of Refractive Index  2. On the basis of Refractive Index: It is also classified into 2 types: (a). Step-index optical fiber: The refractive index of core is constant. The refractive index of the cladding is also constant. The rays of light propagate through it in the form of meridional rays which cross the fiber axis during every reflection at the core-cladding boundary.  (b). Graded index optical fiber: In this type of fiber, the core has a non- uniform refractive index that gradually decreases from the center towards the core- cladding interface. The cladding has a uniform refractive index. The light rays propagate through it in the form of skew rays or helical rays. it is not cross the fiber axis at any time. Types of optical fiber Use of optical fiber in a decorative lamp or nightlight  Section-2 at a Glance EM waves and dielectrics: Relationship between electric field & potential, Dielectric polarization, displacement current, Types of polarization,  Section-2 at a Glance Maxwell’s equations, Equation of EM waves in free space, Velocity of EM waves, Electromagnetic Spectrum (Basic idea)  Section-2 at a Glance Magnetic materials: Basic idea of Dia, Para, Ferro & Ferri, Ferrites, Magnetic anisotropy, Magnetostriction and its applications in production of Ultrasonic waves,.  Section-2 at a Glance Superconductivity, Superconductors as ideal diamagnetic materials, Signatures of Superconducting state, Type I & Type II superconductors  Semiconductor Laser A semiconductor laser (LD) is a device that causes laser oscillation by flowing an electric current to semiconductor. The mechanism of light emission is the same as a light-emitting diode (LED).... When the two meet at the junction, an electron drops into a hole and light is emitted at the time. Inventors:Nick Holonyak, Jun-ichi Nishizawa  Principle of Semiconductor laser When a p-n junction diode is forward biased, the electrons from n – region and the holes from the p- region cross the junction and recombine with each other. During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As  What is the function of laser diode? Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated.  What is the difference between laser and LED? The main difference between lasers and LEDs is that a laser has one single wavelength and a LED emits a Gaussian-like distribution of wavelengths as displayed in figure 1  Basic structure of semiconductor lasers The basic structure of a semiconductor laser is shown in Figure 1. The active layer (light emission layer) sandwiched between the p- and n-type clad layers (double heterostructure) is formed on an n- type substrate, and voltage is applied across the p-n junction from the electrodes. Both edges of the active layer has mirror-like surface.When forward voltage is applied, electrons conbine with holes at the p-n junction, and emitt the light. This light is not a laser yet; it is confined within the active layer because the refractive index of the clad layers are lower than that of the active layer. In addition, both ends of the active layer act as a reflecting mirror where the light reciprocates in the active layer. Then, the light is amplified by the stimulated emission process and laser oscillation is generated.  Types of semiconductor laser The center wavelength of a semiconductor laser principally depends on the band gap energy of the active layer semiconductor. However, the details of laser spectra are different depending on the LD types even though the band gap energies are same. (1) Fabry-Perot semiconductor laser This laser has the simplest structure and is used for many applications, including optical pickups for CDs, DVDs, and BDs; laser printers; and excitation of fiber lasers. It is characterized by the use of the cleavage plane of a laser crystal for reflection of the light emitted in the active layer. (2) DFB semiconductor laser The DFB laser (Distributed Feedback Laser) has a grating below or above the active layer, and oscillates at the single wavelength defined by the Bragg wavelength of the grating. Figure 3 illustrates the structure outline. It exhibits superior performances, including a narrow spectrum width and low noise, and hence is used as an optical signal source in a long- haul optical communication. (3) FBG wavelength stabilized semiconductor laser Though DFB laser has an excellent performance like a single wavelength oscillation, it is expensive for manufacturing difficulty. More economical laser to oscillate a single wavelength is a laser diode stabilizing the wavelength by using FBG.  Applications Their applications are extremely widespread and include optical telecommunications, optical data storage, metrology, spectroscopy, material processing, pumping of other lasers, and medical treatments.  fiber optics (optical fiber) Fiber optics, or optical fiber, refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic strand or fiber. Fiber optics is used long-distance and high-performance data networking. Fiber optics are also commonly used in telecommunication services such as internet, television and telephones. As an example, companies such as Verizon and Google use fiber optics in their Verizon FIOS and Google Fiber services, providing gigabit internet speeds to users.  Fiber optic cables are used since they hold a number of advantages over copper cables, such as higher bandwidth and transmit speeds A fiber optic cable can contain a varying number of these glass fibers -- from a few up to a couple hundred. Surrounding the glass fiber core is another glass layer called cladding. A layer known as a buffer tube protects the cladding, and a jacket layer acts as the final protective layer for the individual strand.  How fiber optics works Fiber optics transmit data in the form of light particles -- or photons -- that pulse through a fiber optic cable. The glass fiber core and the cladding each have a different refractive index that bends incoming light at a certain angle. When light signals are sent through the fiber optic cable, they reflect off the core and cladding in a series of zig-zag bounces, adhering to a process called total internal reflection. The light signals do not travel at the speed of light because of the denser glass layers, instead traveling about 30% slower than the speed of light. To renew, or boost, the signal throughout its journey, fiber optics transmission sometimes requires repeaters at distant intervals to regenerate the optical signal by converting it to an electrical signal, processing that electrical signal and retransmitting the optical signal. Fiber optic cables are moving toward supporting up to 10-Gbps signals. Typically, as the bandwidth capacity of a fiber optic cable increases, the more expensive it becomes.  Electric charge Electric charge, basic property of matter carried by some elementary particles that governs how the particles are affected by an electric or magnetic field. Electric charge, which can be positive or negative, occurs in discrete natural units and is neither created nor destroyed. Electric charges are of two general types: positive and negative. Two objects that have an excess of one type of charge exert a force of repulsion on each other when relatively close together. Two objects that have excess opposite charges, one positively charged and the other negatively charged, attract each other when relatively near. The unit of electric charge in the metre–kilogram–second and SI systems is the coulomb and is defined as the amount of electric charge that flows through a cross section of a conductor in an electric circuit during each second when the current has a value of one ampere. One coulomb consists of 6.24 × 1018 natural units of electric charge, such as individual electrons or protons. From the definition of the ampere, the electron itself has a negative charge of 1.602176634 × 10−19 coulomb.  Electrical Force Electrical Force: The repulsive or attractive interaction between any two charged bodies is called as an electric force.  What is Coulomb’s Law? Coloumb’s law is an experimental law that quantifies the amount of force between two stationary electrically charged particles. The electric force between stationary charged body is conventionally known as the electrostatic force or Coloumb’s force. Coulomb’s law describes the amount of electrostatic force between stationary charges. Coulomb’s law states that: The value of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the charges and inversely proportional to the square of the distance among them.  What is the formula of electric force? Electric force formula can be obtained from Coulomb’s law as follows: F= [K q1q2/r2] r^ →F is the electric force directed between two charged bodies K is the constant of proportionality q0 and q1 are the amounts of charge on each body r is the distance between the charged bodies r^ is the variable unit vector  Electrical Force Examples The examples of electric force are as mentioned below: The charge in a bulb. Electric circuits. Static friction between cloth when rubbed by a dryer. The shock that is felt after touching a doorknob. The electric force can also be viewed through current electricity like copper wiring that carries power to the whole building. The electrostatic force exhibits electric energy through static charges like cathode-ray tubes in TVs and electrostatic spray painting.  What is an Electric Field? An electric field is a field or space around an electrically charged object where any other electrically charged object will experience a force. An electric field is measured by a term known as electric field intensity. If we place a positive unit charge near a positively charged object, the positive unit charge will experience a repulsive force. Due to this force, the positive unit charge will move away from the said charged object. The imaginary line through which the unit positive charge moves, is known as line of force.  Similarly, if we place a positive unit in the field of a negatively charged object, the unit positive charge will experience an attractive force. Due to this force, the unit positive charge will come closer to the said negatively charged object. In that case, line through which the positive unit charge moves, is called line of force.  Electromagnetic waves Definition: Electromagnetic waves or EM waves are waves that are created as a result of vibrations between an electric field and a magnetic field. In other words, EM waves are composed of oscillating magnetic and electric fields.... They are also perpendicular to the direction of the EM wave.  What are the 7 types of EM waves? The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.  How EM waves are produced? Electromagnetic waves are created by oscillating charges (which radiate whenever accelerated) and have the same frequency as the oscillation. Since the electric and magnetic fields in most electromagnetic waves are perpendicular to the direction in which the wave moves, it is ordinarily a transverse wave.  Are EM waves harmful to humans? Despite extensive research, to date there is no evidence to conclude that exposure to low level electromagnetic fields is harmful to human health. The focus of international research is the investigation of possible links between cancer and electromagnetic fields, at power line and radiofrequencies.  What are properties of EM waves? Every form of electromagnetic radiation, including visible light, oscillates in a periodic fashion with peaks and valleys, and displaying a characteristic amplitude, wavelength, and frequency that defines the direction, energy, and intensity of the radiation.  How fast do EM waves travel? Generally speaking, we say that light travels in waves, and all electromagnetic radiation travels at the same speed which is about 3.0 * 108 meters per second through a vacuum.  He-Ne Laser Discovery- Dr Ali and his Collegues at Bell Laboratory in 1961. Definition- It is a gas laser in which a mixture of He and Ne gas is used as active medium or laser medium.  Laser medium is mixture of Helium and Neon gases in the ratio 10:1 Medium excited by large electric discharge, flash pump or continuous high power pump In gas, atoms characterized by sharp energy levels compared to solids Actual lasing atoms are the Neon atoms  Pumping action: Electric discharge is passed through the gas Electrons are accelerated, collide with He and He atoms and excite them to higher energy levels  Electromagnetic waves Definition: Electromagnetic waves or EM waves are waves that are created as a result of vibrations between an electric field and a magnetic field. In other words, EM waves are composed of oscillating magnetic and electric fields.... They are also perpendicular to the direction of the EM wave.  What are the 7 types of EM waves? The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.  How EM waves are produced? Electromagnetic waves are created by oscillating charges (which radiate whenever accelerated) and have the same frequency as the oscillation. Since the electric and magnetic fields in most electromagnetic waves are perpendicular to the direction in which the wave moves, it is ordinarily a transverse wave.  Are EM waves harmful to humans? Despite extensive research, to date there is no evidence to conclude that exposure to low level electromagnetic fields is harmful to human health. The focus of international research is the investigation of possible links between cancer and electromagnetic fields, at power line and radiofrequencies.  What are properties of EM waves? Every form of electromagnetic radiation, including visible light, oscillates in a periodic fashion with peaks and valleys, and displaying a characteristic amplitude, wavelength, and frequency that defines the direction, energy, and intensity of the radiation.  How fast do EM waves travel? Generally speaking, we say that light travels in waves, and all electromagnetic radiation travels at the same speed which is about 3.0 * 108 meters per second through a vacuum.  FSU-030 Maxwell’s Equations and EM Wave The Electromagnetic Spectrum F(x)  Static wave F(x) = FP sin (kx + ) k = 2   k = wavenumber x  = wavelength F(x,t)  Moving wave F(x, t) = FP sin (kx - t + ) v  = 2  f  = angular frequency x f = frequency v=/k  Plane Electromagnetic Waves Ey Bz c x  Plane Electromagnetic Waves Ey E(y, t) = EP sin (ky-t) ĵ B(z, t) = BP sin (kz-t) ẑ Bz Notes: Waves are in Phase, c but fields oriented at 900. k=2. x Speed of wave is c=/k (= f) c  1 / 00  3  108 m / s At all times E=cB.  Beautiful theory Ey Bz A changing magnetic flux produces an Electric c field x A changing electric flux produces a Magnetic field Poynting vector ( P )  Poynting vector represent the rate of energy flow per unit area in a plane electromagnetic wave. 1 P  E B  E  H 0 The direction of (P ) gives the direction in which the energy is transferred. Unit: W/m2 7 Representation of Poynting vector Y Ey P Hz X Z 8  Significance of P The Vector P = E X H has interpreted as representing the amount of P  Eenergy field  H passing through the unit area of surface in unit time normally to the direction of flow of energy. This statement is termed as Poynting’s theorem and the vector P is called Poynting Vector. The direction of flow of energy is perpendicular to vectors E and H E X H i.e., in the direction of the vector E  H 9  Maxwell’s Equations Maxwell's Equations are a set of 4 complicated equations that describe the world of electromagnetics. These equations describe how electric and magnetic fields propagate, interact, and how they are influenced by objects.  Maxwell’s Equation James Clerk Maxwell [1831-1879] was an Einstein/Newton-level genius who took a set of known experimental laws (Faraday's Law, Ampere's Law) and unified them into a symmetric coherent set of Equations known as Maxwell's Equations. Maxwell was one of the first to determine the speed of propagation of electromagnetic (EM) waves was the same as the speed of light - and hence to conclude that EM waves and visible light were really the same thing.  Maxwell’s Equations of Electromagnetism q Gauss’ Law for Electrostatics  E  dA  0 Gauss’ Law for Magnetism  B  dA  0 d Faraday’s Law of Induction  E  dl   dt B d E Ampere’s Law  B  dl  0 I  00 dt  The Equations of Electromagnetism Gauss’s Laws..monopole.. q 1  E  dA  0 2  B  dA  0 ?...there’s no magnetic monopole....!!  The Equations of Electromagnetism Faraday’s Law.. if you change a d  magnetic field you 3  E  dl   dt B induce an electric field......... Ampere’s Law 4  B  dl  0 I.......is the reverse true..?  Maxwell’s Equations of Electromagnetism in Vacuum (no charges, no masses) Consider these equations in a vacuum...........no mass, no charges. no currents..... q  E  dA   0  E  dA  0  B  dA  0  B  dA  0 d B d B  E  dl   dt  E  dl   dt    d d E  B  dl  0 I  0 0 dt E  B  dl  0 0 dt Maxwell’s Equations of Electromagnetism in Vacuum (no charges, no masses)  E  dA  0  B  dA  0 d B  E  dl   dt d E  B  dl  0 0 dt 32.5: Maxwell’s Equations: Maxwell’s Equations: Differential form re Ñ·E = e0 Ñ· B = 0 ¶B Ñ´E = - ¶t ¶E Ñ ´ B = m0e0 + m0 J ¶t  Classes of Magnetic Materials The origin of magnetism lies in the orbital and spin motions of electrons and how the electrons interact with one another. The best way to introduce the different types of magnetism is to describe how materials respond to magnetic fields. This may be surprising to some, but all matter is magnetic. It's just that some materials are much more magnetic than others. The main distinction is that in some materials there is no collective interaction of atomic magnetic moments, whereas in other materials there is a very strong interaction between atomic moments. The magnetic behavior of materials can be classified into the following five major groups:---  1. Diamagnetic materials The materials which are repelled by a magnet such as zinc. mercury, lead, sulfur, copper, silver, bismuth, wood etc., are known as diamagnetic materials. Their permeability is slightly less than one. For example the relative permeability of bismuth is 0.00083, copper is 0.000005 and wood is 0.9999995. They are slightly magnetized when placed in a very string magnetic field and act in the direction opposite to that of applied magnetic field. In diamagnetic materials , the two relatively weak magnetic fields caused due to the orbital revolution and and axial rotation of electrons around nucleus are in opposite directions and cancel each other. Permanent magnetic dipoles are absent in them, Diamagnetic materials have very little to no applications in electrical engineering. Diamagnetism is a fundamental property of all matter, although it is usually very weak. It is due to the non-cooperative behavior of orbiting electrons when exposed to an applied magnetic field. Diamagnetic substances are composed of atoms which have no net magnetic moments (ie., all the orbital shells are filled and there are no unpaired electrons). However, when exposed to a field, a negative magnetization is produced and thus the susceptibility is negative. Diagram  1. Paramagnetic materials The materials which are not strongly attracted to a magnet are known as paramagnetic material. For example: aluminium, tin magnesium etc. Their relative permeability is small but positive. For example: the permeability of aluminium is: 1.00000065. Such materials are magnetized only when placed on a super strong magnetic field and act in the direction of the magnetic field. Paramagnetic materials have individual atomic dipoles oriented in a random fashion as shown below:  The resultant magnetic force is therefore zero. When a strong external magnetic field is applied , the permanent magnetic dipoles orient them self parallel to the applied magnetic field and give rise to a positive magnetization. Since, the orientation of the dipoles parallel to the applied magnetic field is not complete , the magnetization is very small.  3. Ferromagnetic materials The materials which are strongly attracted by a magnetic field or magnet is known as ferromagnetic material for eg: iron, steel , nickel, cobalt etc. The permeability off these materials is very very high ( ranging up to several hundred or thousand). The opposite magnetic effects of electron orbital motion and electron spin do not eliminate each other in an atom of such a material. There is a relatively large contribution from each atom which aids in the establishment of an internal magnetic field, so that when the material is placed in a magnetic field, it’s value is increased many times thee value that was present in the free space before the material was placed there.  For the purpose of electrical engineering it will suffice to classify the materials as simply ferromagnetic and and non- ferromagnetic materials. The latter includes material of relative permeability practically equal to unity while the former have relative permeability many times greater than unity. Paramagnetic and diamagnetic material falls in the non-ferromagnetic materials.  4. Ferrimagnetism Ferrimagnetism is only observed in compounds, which have more complex crystal structures than pure elements. Within these materials the exchange interactions lead to parallel alignment of atoms in some of the crystal sites and anti-parallel alignment of others. The material breaks down into magnetic domains, just like a ferromagnetic material and the magnetic behaviour is also very similar, although ferrimagnetic materials usually have lower saturation magnetisations. For example in Barium ferrite (BaO.6Fe2O3) the unit cell contains 64 ions of which the barium and oxygen ions have no magnetic moment, 16 Fe3+ ions have moments aligned parallel and 8 Fe3+ aligned antiparallel giving a net magnetisation parallel to the applied field, but with a relatively low magnitude as only ⅛ of the ions contribute to the magnetisation of the material. Magnetite is a well known ferrimagnetic material. Indeed, magnetite was considered a ferromagnet until Néel in the 1940's, provided the theoretical framework for understanding ferrimagnetism.  5. Antiferromagnetism In the periodic table the only element exhibiting antiferromagnetism at room temperature is chromium. Antiferromagnetic materials are very similar to ferromagnetic materials but the exchange interaction between neighbouring atoms leads to the anti-parallel alignment of the atomic magnetic moments. Therefore, the magnetic field cancels out and the material appears to behave in the same way as a paramagnetic material. Like ferromagnetic materials these materials become paramagnetic above a transition temperature, known as the Néel temperature, TN. (Cr: TN=37ºC). Examples:-Transition metals Mn, Cr & many of their compound, e.g. MnO, CoO, NiO, Cr2O3, MnS, MnSe, CuCl2  Absorption Absorption is the process in which optical energy is converted to internal energy of electrons, atoms, or molecules. When a photon is absorbed, the energy may cause an electron in an atom to go from a lower to a higher energy level, thereby changing the internal momentum of the electron and the electron's internal quantum numbers.  Spontaneous Emission Spontaneous emission is an energy conversion process in which an excited electron or molecule decays to an available lower energy level and in the process gives off a photon. This process occurs naturally and does not involve interaction of other photons. The average time for decay by spontaneous emission is called the spontaneous emission lifetime. For some excited energy levels this spontaneous decay occurs on average within nanoseconds while in other materials it occurs within a few seconds As with absorption, this process can occur in isolated atoms, ionic compounds, molecules, and other types of materials, and it can occur in solids, liquids, and gases. Energy is conserved when the electron decays to the lower level, and that energy must go somewhere. The energy may be converted to heat, mechanical vibrations, or electromagnetic photons. If it is converted to photons, the process is called spontaneous emission, and the energy of the photon produced is equal to the energy divergence between the electron energy levels involved. The emitted photon may have any direction, phase, and electromagnetic polarization.  Spontaneous emission processes may be classified based on the source of energy which excites the electrons, If the initial source of energy for spontaneous emission is supplied optically, the process is called photoluminescence. Glow in the dark materials emit light by this process. If the initial form of energy is supplied by a chemical reaction, the process is called chemiluminescence. Glow sticks produce spontaneous emission by chemiluminescence. If the initial form of energy is supplied by a voltage, the process is called electroluminescence. LEDs emit light by electroluminescence. If the initial form of energy is caused by sound waves, the process is called sonoluminescence. If the initial form of energy is due to accelerated electrons hitting a target, this process is called cathodoluminescence. If spontaneous emission occurs in a living organism, such a firefly, the process is called bioluminescence.  Stimulated Emission Stimulated emission is the process in which an excited electron or molecule interacts with a photon, decays to an available lower energy level, and in the process gives o a photon. As with the other processes, this process can occur in isolated atoms, ionic compounds, organic molecules, and other types of materials, and it can occur in solids, liquids, and gases. If an incoming photon, with energy equal to the difference between allowed energy levels, interacts with an electron in an excited state, stimulated emission can occur. The energy of the excited electron will be converted to the energy of a photon. The stimulated photon will have the same frequency, direction, phase, and electromagnetic polarization as the incoming photon which initiated the process  Ruby Laser Ruby laser definition:A ruby laser is a solid-state laser that uses the synthetic ruby crystal as its laser medium. Ruby laser is the first successful laser developed by Maiman in 1960. Ruby laser is one of the few solid-state lasers that produce visible light. It emits deep red light of wavelength 694.3 nm. Construction of ruby laser:A ruby laser consists of three important elements: laser medium, the pump source, and the optical resonator. Laser medium or gain medium in ruby laser In a ruby laser, a single crystal of ruby (Al2O3 : Cr3+) in the form of cylinder acts as a laser medium or active medium. The laser medium (ruby) in the ruby laser is made of the host of sapphire (Al2O3) which is doped with small amounts of chromium ions (Cr3+). The ruby has good thermal properties. Pump source or energy source in ruby laser The pump source is the element of a ruby laser system that provides energy to the laser medium. In a ruby laser, population inversion is required to achieve laser emission. Population inversion is the process of achieving the greater population of higher energy state than the lower energy state. In order to achieve population inversion, we need to supply energy to the laser medium (ruby). In a ruby laser, we use flashtube as the energy source or pump source. The flashtube supplies energy to the laser medium (ruby). When lower energy state electrons in the laser medium gain sufficient energy from the flashtube, they jump into the higher energy state or excited state. Optical resonator The ends of the cylindrical ruby rod are flat and parallel. The cylindrical ruby rod is placed between two mirrors. The optical coating is applied to both the mirrors. The process of depositing thin layers of metals on glass substrates to make mirror surfaces is called silvering. Each mirror is coated or silvered differently. At one end of the rod, the mirror is fully silvered whereas, at another end, the mirror is partially silvered. The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows a small portion of light through it to produce output laser light.  Working of ruby laser The ruby laser is a three level solid-state laser. In a ruby laser, optical pumping technique is used to supply energy to the laser medium. Optical pumping is a technique in which light is used as energy source to raise electrons from lower energy level to the higher energy level. Consider a ruby laser medium consisting of three energy levels E1, E2, E3 with N number of electrons. We assume that the energy levels will be E1 < E2 < E3. The energy level E1 is known as ground state or lower energy state, the energy level E2 is known as metastable state, and the energy level E3 is known as pump state. Let us assume that initially most of the electrons are in the lower energy state (E1) and only a tiny number of electrons are in the excited states (E2 and E3)  When light energy is supplied to the laser medium (ruby), the electrons in the lower energy state or ground state (E1) gains enough energy and jumps into the pump state (E3). The lifetime of pump state E3 is very small (10-8 sec) so the electrons in the pump state do not stay for long period. After a short period, they fall into the metastable state E2 by releasing radiationless energy. The lifetime of metastable state E2 is 10-3 sec which is much greater than the lifetime of pump state E3. Therefore, the electrons reach E2 much faster than they leave E2. This results in an increase in the number of electrons in the metastable state E2 and hence population inversion is achieved. After some period, the electrons in the metastable state E2 falls into the lower energy state E1 by releasing energy in the form of photons. This is called spontaneous emission of radiation. When the emitted photon interacts with the electron in the metastable state, it forcefully makes that electron fall into the ground state E1. As a result, two photons are emitted. This is called stimulated emission of radiation.  When these emitted photons again interacted with the metastable state electrons, then 4 photons are produced. Because of this continuous interaction with the electrons, millions of photons are produced. In an active medium (ruby), a process called spontaneous emission produces light. The light produced within the laser medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the ground state by releasing light energy. This is called stimulated emission. Likewise, millions of electrons are stimulated to emit light. Thus, the light gain is achieved. The amplified light escapes through the partially reflecting mirror to produce laser light.  Applications of the ruby laser Rangefinding is one of the first applications of the ruby laser. It was initially used to optically pump tunable dye lasers. It is rarely used in industry due to its low repetition rates and low efficiency. Some typical applications of ruby laser include the following: Laser metal working systems for drilling holes in hard materials High-power systems for frequency doubling into the UV spectrum High-brightness holographic camera systems with long coherent length Medical laser systems for tattoo removal and cosmetic dermatology High-power Q-switched system.  CO2 Lasers The CO2 laser (carbon dioxide laser) is a molecular gas laser based on a gas mixture as the gain medium, which contains carbon dioxide (CO2), helium (He), nitrogen (N2), and possibly some hydrogen (H2), water vapor and/or xenon (Xe). Such a laser is electrically pumped via an electrical gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or in the radio frequency (RF) domain. Nitrogen molecules are excited by the electric discharge into a metastable vibrational level and transfer their excitation energy to the CO2 molecules when colliding with them. The exited CO2 molecules then largely participate in the laser transition. Helium serves to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (formed in the discharge) to carbon dioxide.  CO2 lasers typically emit at a wavelength of 10.6 μm, but there are other spectral lines in the region of 9–11 μm (particularly at 9.6 μm). In most cases, average output powers are between some tens of watts and many kilowatts. The power conversion efficiency can be well above 10%, i.e., it is higher than for most gas lasers (due to a particularly favorable excitation pathway), also higher than for lamp-pumped solid-state lasers, but lower than for many diode-pumped lasers. Due to their high output powers and long emission wavelengths, CO2 lasers require high-quality infrared optics, often made of materials like zinc selenide (ZnSe) or zinc sulfide (ZnS).  The functioning of CO2 Laser The nitrogen molecules which exist in the gas mixture gain energy after stimulating through an electric current. In other words, you can say that the nitrogen molecules get excited. Thus, we use nitrogen in this process as it can hold this state of excitement for a longer time. Moreover, during this state, nitrogen does not discharge the energy in form of light or photons. Furthermore, nitrogen produces high-energy vibrations which thus excite the carbon dioxide molecules. A state called the population inversion is achieved by the laser at this stage. This stage is one where the system consists of more excited particles than non-excited ones. Therefore, the atoms of nitrogen have to lose their state of excitement through the release of energy in the form of photons so that they can produce a beam of light. This happens when the atoms of nitrogen which are in the state of excitement get in touch with the extremely cold atoms of helium, causing the nitrogen to produce light.  Applications of CO2 Lasers CO2 lasers are widely used for laser material processing, in particular for cutting plastic materials, wood, die boards, etc., exhibiting high absorption at 10.6 μm, and requiring moderate power levels of 20–200 W cutting and welding metals such as stainless steel, aluminum or copper, applying multi-kilowatt powers laser marking of various materials Other applications include laser surgery (including ophthalmology) and range finding.  Ferrite Ferrite is a ceramic-like material with magnetic properties, which is used in many types of electronic devices. Ferrite is used in: 1 Permanent magnets 2 Ferrite cores for transformers and toroidal inductors 3 Computer memory elements 4 Solid-state devices Ferrites are composed of iron oxide and one or more other metals in chemical combination, and their properties include: Hard Brittle Iron-containing Polycrystalline Generally gray or black Ferrite is also known as ferrate  Properties of ferrites Properties of ferrites include: Significant saturation magnetization High electrical resistivity Low electrical losses Very good chemical stability  Types of Ferrites Ferrites are often classified as "soft" or "hard" in terms of their magnetic properties: 1. Soft ferrites - used in transformer or electromagnetic cores. They have a low coercivity (manganese-zinc ferrite, nickel-zinc ferrite). 2. Hard ferrites- have a high coercivity. They are cheap, and are widely used in household products such as refrigerator magnets (strontium ferrite, barium ferrite). **Soft ferrite does not retain significant magnetization, whereas hard ferrite magnetization is considered permanent.  Magnetic Anisotropy The dependence of magnetic properties on a preferred direction is called magnetic anisotropy. There are different types of anisotropy: Type - depends on 1. magnetocrystalline- crystal structure 2. shape- grain shape 3. stress- applied or residual stresses Magnetic anisotropy strongly affects the shape of hysteresis loops and controls the coercivity and remanence. Anisotropy is also of considerable practical importance because it is exploited in the design of most magnetic materials of commercial importance.  Magnetostriction Magnetostriction is a property of ferromagnetic materials which causes them to expand or contract in response to a magnetic field. This effect allows magnetostrictive materials to convert electromagnetic energy into mechanical energy. As a magnetic field is applied to the material, its molecular dipoles and magnetic field boundaries rotate to align with the field. Figure 11: Molecular dipole rotation during magnetostriction.  As the applied magnetic field increases in intensity, the magnetostrictive strain on the material increases. Ferromagnetic materials that are isotropic and have few impurities are most effective in magnetostriction because these properties allow their molecular dipoles to rotate easily Magnetostriction was first measured by James Prescott Joule (1818-1889) who was able to magnetize an iron sample and measure its the change in length. The opposite effect, in which an applied stress caused the material to create a magnetic field, was discovered by E. Villari (1836-1904). Gustav Wiedemann then discovered that a ferromagnetic rod would oscillate torsionally when exposed to a longitudinal and circular magnetic field.  Superconductivity Superconductivity is defined as the complete disappearance of electrical resistance in various solids when they are cooled below a characteristic temperature. This temperature, called the transition temperature, varies for different materials but generally is below 20 K (−253 °C). Such type of solid is called Superconductor. Discovery-Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes; he was awarded the Nobel Prize for Physics in 1913 for his low-temperature research. Kamerlingh Onnes found that the electrical resistivity of a mercury wire disappears suddenly when it is cooled below a temperature of about 4 K (−269 °C); absolute zero is 0 K, the temperature at which all matter loses its disorder. He soon discovered that a superconducting material can be returned to the normal (i.e., nonsuperconducting) state either by passing a sufficiently large current through it or by applying a sufficiently strong magnetic field to it. 1  TYPES OF SUPERCONDUCTORS Type I or soft superconductors Type II or Hard Superconductors 2  Type I or soft superconductors These are usually made of pure metal. When it is cooled below its critical temperature it exhibits zero resistivity and displays perfect diamagnetism. This means that the magnetic fields cannot penetrate it while it is in the superconducting state. They strictly obey Meissner Effect. These are called soft superconductors because they give away their nature at very low field strength. They do not have any useful technical applications. Example: Pure specimens of Al,Pb,Hg,Indium etc. 3  Type II or Hard Superconductors These superconductors are usually alloys or transition metals with high values of electrical resistivity. Their diamagnetism is more complex. They have two critical magnetic fields Hc1 and Hc2. These allow the magnetic field to penetrate it. Thus they do not obey Meissner effect. These are called Hard superconductors because relatively large fields are required to bring them back to normal state. These are used for strong field superconducting metals and hence are technically more useful than soft superconductors. These can carry large currents. Example: transition metals and alloys consisting of niobium, Al,Si and vanadium etc. 4 Diagrams 5 APPLICATIONS OF SUPERCONDUCTORS Used in cables :For electric power transmission without any loss. For Producing very strong magnetic fields of about 20-30 tesla which are used in power generators and in medical diagnostic equipments. To fabricate high field magnets used in NMR spectrometers. Magnetic energy can be stored in large superconductors to counter voltage fluctuations. They are used to perform logic and storage functions in computers. These are used to produce electromagnetic shields. 6 APPLICATIONS OF SUPERCONDUCTORS Used in the fabrication of small IC chips of electronic devices and computers. Used as magnetic separators in ore refining. Superconducting film can be used as phonon detector or nuclear radiation detector. In magnetic levitation trains. 7 8