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UNIT – IV --LASER, LASER- Syllabus, Light Amplification by Stimulated Emission of Radiation.,  Introduction to interaction of radiation with matter,  Characteristic of the laser,  Basic requirements for production of laser beams,  The relation between Einstein's A and B coefficients,  Types of lasers (Block Diagram),  Ruby laser,  Carbon dioxide,  He-Ne laser,  Applications of laser, , 1. Characteristics of the laser:, Laser when compared with the conventional light (sunlight or tube light), laser, possesses few outstanding characteristics, they are, 1) Monocromacity: the property of exhibiting a single wavelength or single colour by, a light is called “monocromacity”., i.e., when it is sent through a prism then a single line will be appeared in the, optical spectrum. (If the frequency of radiation is 1015Hz, the width of the line, will be 1 Hz, as such laser light is considered highly monochromatic.), 2) Directionality: Laser can travel very large distance without divergence and without, loss of energy. Due to its angular spreading will be less and occupied less are where, it incident. Hence it possesses “high degree of directionality”., 3) Brightness: Due to its directionality many beams of light incident in a small area,, therefore the intensity of light is high. Hence its brightness is more. So it is used for, the welding.

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4) Coherence: the property of existing either zero (or) constant phase difference, between two or more waves is known as “coherence”., Coherence is of two types i.e., , 1) Spatial Coherence, , 2) Temporal Coherence, , Spetial Coherence: it there exists either Zero (or) Constant phase angle difference, between two points on a wave front then wave is said to have spetial coherence., Temporal Coherence: if there exists either Zero (or) Constant phase angle, difference between two light fields measured at two instants at the same point then, wave is said to have temporal coherence., , 2. Difference between ordinary light and laser light, Ordinary light, , Laser light, , 1, , In ordinary light the angular spread is In laser beam the angular spread is less, more, , 2, , They are not directional, , They are highly directional, , 3, , It is less intense, , It is highly intense, , 4, , It is not a coherent beam and it is not It is a coherent beam and it is in phase, in phase, , 5, , The radiations are polychromatic, , The radiations are monochromatic, , 6, , Ex: Sun light, Mercury, Vapor lamp, , Ex: He-Ne laser Co2 laser, , 3. Basic Definitions to produce the LASER beam:, To understand the working part of a laser system, one should have few, fundamental concepts, they are, , , , , , , , , , , Absorption (Induced absorption), Spontaneous Emission, Stimulation Emission, Life time, Metastable state, Population, Population inversion, Pumping, Lasing action

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3.1. Induced Absorption (Stimulation):,  If the atom is initially in the lower state E 1, it can be raised to E2 by absorbing a, photon of energy (E=h𝛎=E2-E1). This process is called “induced absorption”.,  The probable rate of occurrence of the absorption transition from E 1 state to E2 state,  Depends on 1) the properties of E1, E2 states, 2) is proportional to energy density u(𝛎) of the radiation of frequency, , (𝛎) incident, , on the atom, P12 α u(𝛎), P12 =B12 u(𝛎), B12= Einstein's coefficient of absorption of radiation., , 3.2. Spontaneous Emission:,  If the atom is initially in the higher energy state E2, it falls to its lower energy state, E1 by itself by emitting a photon energy (E=h𝛎=E2-E1). This process is called, “spontaneous emission”.,  The emission which takes place without external energy is called spontaneous, emission,  Depends only on the properties of E1, E2 states, P21 =A21, A21= Einstein's coefficient of Spontaneous emission

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3.3. Stimulation Emission:,  If the atom is initially in the higher energy state E2, it falls to its lower energy state, E1 by incident photon energy (E=h𝛎=E2-E1) on it. The incident photon stimulates, the emitting an additional photon of some photon energy and returns to the ground, state. Hence two photons are emitted instead of one photon on excited state. This, process is called “stimulation emission”.,  Depends on 1) the properties of E1, E2 states, 2) It is proportional to energy density u(𝛎) of the radiation of, frequency (𝛎) incident on the atom, , P12 α u(𝛎), , P21 =B21 u(𝛎), A21= Einstein's coefficient of Stimulation emission

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4. Pumping methods:, o Optical pumping,, o Electric discharge method,, o Inelastic atom-atom collision, o Direct conversion,, o Chemical reactions

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5. Differences between Spontaneous emission and Stimulated emission, , Spontaneous Emission, , Stimulated Emission, , 1, , Emission takes place without any Emission takes place with the help of, stimulates energy, stimulates energy, , 2, , In-coherent radiation, , Coherent radiation, , 3, , Low intense and less directional, , High intense and more directional, , 4, , Polychromatic radiation, , Monochromatic radiation, , 5, , This emission is postulated by Bohr, , This emission is postulated by, Einstein, , 6, , Ex: light from ordinary light, sodium Ex: light from Ruby, He-Ne laser, vapour lamp and mercury lamp, etc……

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6. Types of Laser Systems:, On the basis of active medium used in the laser system, lasers are classified into several, types. Most popular laser systems are:, 1) Solid state laser (Ruby laser), 2) Gaseous laser (He-Ne laser, Co2 laser), 3) Liquid laser (Europium laser), 4) Dye laser (Coumasia Dye laser), 5) Diode laser (Semiconductor laser), Block Diagram of laser beam, , Active Media: it is the material in which the laser action takes place., Source (Energy source): Energy source pumps the active centers from ground state to, excited state to achieve population inversion., Resonant Cavity: Resonance cavity consists of active medium enclosed between two, mirrors one is highly reflective mirror (fully reflective mirror) and the other is partially, reflective mirror.

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6.1. RUBY LASER (SOLID STATE LASER), , Construction:, 1. Active medium: A ruby rod crystal in the form of cylindrical shape with 3 to 5 mm, in diameter and 4 to 30 cm long., 2. Source: Helical xenon flash tube with required power supply., 3. Resonant cavity: Made of fully reflected plate at the left of ruby crystal and a, partially reflected plate at the right of ruby crystal. Both the plates are optically, plane and exactly parallel to each other., 4. It is a solid state laser with three level laser system., 5. Each pulse will come out I duration of 10 nanosec., 6. The Ruby rod is crystal of Aluminum oxide (Al2O3) doped with 0.05% Chromium, oxide (Cr2O3). So that some of the “Al” atoms are replaced by Cr +3 ions., 7. Its colour is “Pink”. Pink rod of 4cm length and 0.5cm in diameter is generally used., 8. The end of the faces of the rod are made optically flat and accurately parallel. One, end is fully silvered (fully reflected end face) and the other end is only partially, silvered (partially reflected end face)., 9. The rod is surrounded by a glass tube. The glass tube surrounded by a “helical, Xenon flash tube” which provide the pumping light to raise the “Cr3+ ions” to, higher energy level., 10. The flash of the xenon tube lasts several milliseconds and, , the tube consumes, , several thousand joules of energy., 11. Only a part of this energy is used in pumping the “Cr +3 ions” while the rest heats up, the apparatus. For this purpose a cooling arrangement with water in glass tube is, used.

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Working method:, 1) As shown in figure E1, E2, E3 represent the energy levels of “Cr3+ ion”. The Cr+3, ions are in lower state (E1); these ions are excited from E1 to E3 energy level by the, absorption of light of wavelength 5600Å, from the “Xenon flash tube” in optical, pumping method., 2) In figure E2 state is metastabel state, this energy level life time is greater than, excited state life time. The metastable state width is 14Å and excited state width is, 800Å, 3) The few excited Cr atoms are returns to ground state E1 and other goes to second, state E2., 4) The transition R3->2 are non-Radiative transition. In this case the no. of atoms in, metastabel (E2 state) is greater than the no. of atoms in ground state (E 1 state), (N2>N1), this is known as “population inversion”., 5) In this position which are emitted from spontaneous transition are reaction with, metastabel state atoms and these from as induced or stimulated emission, so it fall, into the E1 state by emitting 2 photons.

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6) The probability of spontaneous transitions at any moment is very high. The, spontaneous transition may cause an induced transition which produces a photon., This photon knocks out a second photon from neighboring atom; the atom from, which the first photon was emitted is now brought to ground level. The two photons, will knock out two more photons and their total will be four photon, so on., 7) Now the stimulated radiation along the axis starts dominating due to multiple, reflections., 8) These stimulate the emission of similar other photons chain reaction quickly, develops a beam of photons all moving parallel to the rod, which is monochromatic, and coherent. When a beam develops sufficient intensity, it emerges through the, partially silvered end., 9) Once all the Cr+3 ions in the metastable level have returned to ground state, the, laser action stops. It is seen necessary to send one more flash of pumping radiation, through the rod., 10) Thus the ruby laser operates only pulses means it produced the discontinuous, pulsed laser beam.

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6.2. He – Ne Laser: (Gases laser),  He-Ne laser is a gaseous laser system and it produced a continuous laser.,  It is highly directional, monochromatic, Coherence and stable, but the output power, is moderated.,  It is very useful in making holograms and interferometric experiments., Source of energy, , : R.F.Oscillator, , Active media, , : He-Ne gas mixture, , Optical cavity, , : Arrangement of reflector., , Construction:, 1) The He-Ne laser consists of Quartz tube, which has electrodes to connect, R.F Oscillator., 2) This tube is filled with a mixture of Ne under a pressure of 0.1mm of Hg and He, under a pressure of 1mm of Hg., 3) There is a majority are He atoms and minority is Ne atoms., 4) At one end of the tube a perfect reflector is arranged a while on the other end a, partial reflector.

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Working method:, 1. The schematic energy diagram He-Ne mixture is shown in fig., 2. In He atom three active energy levels are presented, they are He 1, He2 and He3., 3. In Ne atom six active energy levels, they are named as Ne 1, Ne2, Ne3, Ne4, Ne5 and, Ne6., 4. The Ne4 and He2, Ne6 and He3 have same energy and life time., 5. When the discharge is passed through the He-Ne gas mixture, then e- s are, accelerated towards positive (+Ve) electrode., 6. During their passage they collide with “He” and “Ne” atoms, but one He atoms are, suitable to excite to the upper states labeled “He2“and “He3“. these are metastable, states in “He” atoms., 7. Now these He atoms interact with Ne atoms, which are in the ground state. The, interaction excites the Ne atoms to their metastable states i.e., Ne 4 and Ne6 which, the He atoms return to their ground state., 8. As the energy exchange continue between “He” and “Ne” atoms, the population of, Ne atoms in the excited states Ne4 and Ne6 increase more and more., 9. At this state population inversion will be achieved in the metastable state Ne 4 and, Ne6., , (After achieving population inversion), 10. Few Ne atoms de-excited from Ne6 to Ne5 with wavelength of 3.39μm (3390 Å), 11. Many other Ne atoms de-excited from Ne6 to Ne3 with wavelength of 6328 Å. This, is the important and major wavelength in this laser system., 12. The de-excited from Ne4 to Ne3 with wavelength of 1150 Å, 13. After reaching all the Ne atoms to Ne3, spontaneously those will de-excited to Ne2., with wavelength of 6000Å (Spontaneous light) is emitted., 14. Finally Ne atoms take non radiative transitions by making collisions with walls of, the tube from Ne2 to Ne1 with producing an laser beam with wavelength 6328 Å.

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6.3. Carbon dioxide laser (Co2 laser):,  It is one of the most powerful and efficient lasers and we used this laser in an, industrial application., •, , Principle: The active medium is a gas mixture of CO2, N2 and He. The laser, transition takes place between the vibrational states of CO2 molecules., , Construction:, 1. The schematic diagram of a Co 2 laser is shown in fig., 2. In this construction along with Co 2, N2 and He gases are consider at ratio of, pressure of Co2: N2: He= 1:4:5., 3. The active material in active medium is Co 2 gas molecule., 4. As “He” is used for excitation of “Ne” atoms in He-Ne laser, in Co 2 laser for, efficient excitation of Co 2 molecules, N2 molecules are used. Addition of “He” the, gas mixture enhances the efficiency., 5. It consists of a quartz tube 5 m long and 2.5 cm in the diameter. This discharge tube, is filled with gaseous mixture of CO2(active medium), helium and nitrogen with, suitable partial pressures., 6. The terminals of the discharge tubes are connected to a D.C power supply. The, ends of the discharge tube are fitted with NaCl Brewster windows so that the laser, light generated will be polarized.

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7. Two concave mirrors one fully reflecting and the other partially form an optical, resonator., 8. In a molecular gas laser, laser action is achieved by transitions between vibrational, and rotational levels of molecules. The output of this laser is continuous., 9. In CO2 molecular gas laser, transition takes place between the vibrational states of, Carbon dioxide molecules., 10. It is a four level laser and it operates at 10.6 μm in the far IR region. It is a very, efficient laser., , Energy states of CO2 molecules:, A carbon dioxide molecule has a carbon atom at the center with two oxygen atoms, attached, one at both sides. Such a molecule exhibits three independent modes of, vibrations. They are, a), , Symmetric stretching mode., , b), , Bending mode, , c), , Asymmetric stretching mode.

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a) Symmetric stretching mode, In this mode of vibration, carbon atoms are at rest and both oxygen atoms vibrate, simultaneously along the axis of the molecule departing or approaching the fixed, carbon atoms., b) Bending mode:, In this mode of vibration, oxygen atoms and carbon atoms vibrate perpendicular to, molecular axis., c) Asymmetric stretching mode:, In this mode of vibration, oxygen atoms and carbon atoms vibrate asymmetrically,, i.e., oxygen atoms move in one direction while carbon atoms in the other direction., Working of CO2 Laser system:, 1. Energy levels of nitrogen and carbon dioxide molecules shown in fig., 2. When an electric discharge occurs in the gas, the electrons collide with nitrogen, molecules and they are raised to excited states. This process is represented by the, equation, N2 + e* = N2* + e, N2 = Nitrogen molecule in ground state e* = electron with kinetic energy, N2* = nitrogen molecule in excited state e= same electron with lesser energy, 3. Now N2 molecules in the excited state collide with CO2 atoms in ground state and, excite to higher electronic, vibrational and rotational levels., 4. This process is represented by the equation N2* + CO2 = CO2* + N2, N2* = Nitrogen molecule in excited state. CO2 = Carbon dioxide atoms in ground, state CO2* = Carbon dioxide atoms in excited state N2 = Nitrogen molecule in, ground state., 5. Since the excited level of nitrogen is very close to the E 5 level of CO2 atom,, population in E5 level increases., 6. As soon as population inversion is reached, any of the spontaneously emitted photon, will trigger laser action in the tube. There are two types of laser transition possible.

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 Transition E5 to E4 : This will produce a laser beam of wavelength 10.6μm,  Transition E5 to E3: This transition will produce a laser beam of wavelength, 9.6μm. Normally 10.6μm transition is more intense than 9.6μm transition. The, power output from this laser is 10kW., , Characteristics of CO2 laser:, 1. Type: It is a molecular gas laser., 2. Active medium: A mixture of CO2 , N2 and helium or water vapour is used, , as, , active medium, 3. Pumping method: Electrical discharge method is used for Pumping action, 4. Optical resonator: Two concave mirrors form a resonant cavity, 5. Power output: The power output from this laser is about 10kW., 6. Nature of output: The nature of output may be continuous wave or pulsed wave., 7. Wavelength of output: The wavelength of output is 9.6μm and 10.6μm., Applications CO2 laser:, 1. High power CO2 laser finds applications in material processing, welding, drilling,, cutting soldering etc., 2. The low atmospheric attenuation 10.6μm makes CO2 laser suitable for open air, communication., 3. It is used for remote sensing

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4. It is used for treatment of liver and lung diseases., 5. It is mostly used in neuro surgery and general surgery., 6. It is used to perform microsurgery and bloodless operations., Advantages CO2 laser:, 1., , The construction of CO2 laser is simple, , 2., , The output of this laser is continuous., , 3., , It has high efficiency, , 4., , It has very high output power., , 5. The output power can be increased by extending the length of, the gas tube., Disadvantages CO2 laser:, 1. The contamination of oxygen by carbon monoxide will have some effect on laser, action, 2. The operating temperature plays an important role in determining the output power, of laser., 3. The corrosion may occur at the reflecting plates., 4. Accidental exposure may damage our eyes, since it is invisible (infra red region) to, our eyes., , 7. Applications of Lasers:, Laser light is different from an ordinary light. It has various unique properties such, as coherence, monochromacity, directionality, and high intensity. Because of these, unique properties, lasers are used in various applications., The most significant applications of lasers include:, •, , Lasers in medicine, , •, , Lasers in communications, , •, , Lasers in industries, , •, , Lasers in science and technology, , •, , Lasers in military

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Lasers in Medicine:, 1. Lasers are used for bloodless surgery., 2. Lasers are used to destroy kidney stones., 3. Lasers are used in cancer diagnosis and therapy., 4. Lasers are used for eye lens curvature corrections., 5. Lasers are used in fiber-optic endoscope to detect ulcers in the intestines., 6. The liver and lung diseases could be treated by using lasers., 7. Lasers are used to study the internal structure of microorganisms and cells., 8. Lasers are used to produce chemical reactions., 9. Lasers are used to create plasma., 10. Lasers are used to remove tumors successfully., 11. Lasers are used to remove the caries or decayed portion of the teeth., 12. Lasers are used in cosmetic treatments such as acne treatment, cellulite and hair, removal., , Lasers in Medicine:, Argon Laser – Neuro surgery, Ophthalmology, General surgery, Gynecology,, Dermatology., He-Ne Laser – Diagnostic application, Laser holography, Ruby Laser – Ophthalmology and Dermatology, Nd-YAD Laser – Neuro surgery, Dermatology, Gynecology, Krypton Laser - Ophthalmology, Lasers in Communications, 1. Laser light is used in optical fiber communications to send information over large, distances with low loss., 2. Laser light is used in underwater communication networks., 3. Lasers are used in space communication, radars and satellites., 4. More channels can be simultaneously transmitted., 5. Lasers are used in high speed photo copiers and printers.

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Lasers in Industries, 1. Lasers are used to cut glass and quartz., 2. Lasers are used in electronic industries for trimming the components of Integrated, Circuits (ICs)., 3. Lasers are used for heat treatment in the automotive industry., 4. Laser light is used to collect the information about the prefixed prices of various, products in shops and business establishments from the bar code printed on the, product., 5. Ultraviolet lasers are used in the semiconductor industries for photolithography., Photolithography is the method used for manufacturing printed circuit board (PCB), and microprocessor by using ultraviolet light., 6. High power laser is used to weld or melt any material., Lasers in Science and Technology, 1. A laser helps in studying the Brownian motion of particles., 2. With the help of a helium-neon laser, it was proved that the velocity of light is same, in all directions., 3. With the help of a laser, it is possible to count the number of atoms in a substance., 4. Lasers are used in computers to retrieve stored information from a Compact Disc, (CD)., 5. Lasers are used to store large amount of information or data in CD-ROM., 6. Lasers are used to measure the pollutant gases and other contaminants of the, atmosphere., 7. Laser helps in determining the rate of rotation of the earth accurately., 8. Lasers are used in computer printers., 9. Lasers are used for producing three-dimensional pictures in space without the use, of lens., 10. Lasers are used for detecting earthquakes and underwater nuclear blasts.

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Lasers in Military, 1. Laser range finders are used to determine the distance to an object., 2. The ring laser gyroscope is used for sensing and measuring very small angle of, rotation of the moving objects., 3. Lasers can be used as a secretive illuminators for reconnaissance during night with, high precision., 4. Lasers are used to dispose the energy of a warhead by damaging the missile., 5. Laser light is used in LIDAR’s to accurately measure the distance to an object., , 8. Relation between Einstein Co-efficient “A” AND “B” :, Derive the derivation from your written notes, , --------%%%%% %%%%%%%%%%%%----------------------