LASER PHY110 Unit II PDF

PHY 110 ( LASERS AND APPLICATIONS) UNIT-II Light Amplification by Stimulated Emission of Radiation 1  Unit-II (Lasers and Applications) What we got here? Principle of Production of Laser. Absorption and emission of Light. ...

PHY 110 ( LASERS AND APPLICATIONS) UNIT-II Light Amplification by Stimulated Emission of Radiation 1  Unit-II (Lasers and Applications) What we got here? Principle of Production of Laser. Absorption and emission of Light. Concept of spontaneous and stimulated emission. Population Inversion and Resonant Cavity. ND-YAG, He-Ne and semiconductor Laser. Applications of Laser. Holography and applications of hologram. Radiation interaction with matter : Absorption and emission, stimulated emission, Einstien’s A and B coefficients 3  Absorption of Radiation https://www.youtube.com/watch?v=WgzynezPiyc  Spontaneous Emission Statistical process (random phase) – emission by an isolated atom or molecule. The excited atom/electron naturally comes down to lower level by emitting a photon.  Stimulated Emission E2 A Photon stimulates an atom/electron to come down to it’s lower energy level and thus emitting a photon. But now, we got two photons, the 2hn original one and the other one due to emission hn process. Both photons are alike, completely in same phase. This is the beauty of stimulated E1 emission. It is an artificially designed process and never happens naturally. Same phase as “stimulating” optical field Same polarization https://www.youtube.com/watch?v=YHmGNDMV1cY Same direction of propagation  Putting it all together… Assume that we have a two state system in equilibrium with a blackbody radiation field. After Stimulated emission Photon assisting E2 Stimulated emission After spontaneous emission Spontaneous emission Absorption Photon causing E1 absorption Stimulated emission  Einstein Coefficients of radiation For two energy levels 1 (lower) and 2 (upper) we have 2 A21: Einstein’s coefficient of spontaneous emission. B21: Einstein’s coefficient of stimulated emission. 1 B12 : Einstein’s coefficient of absorption. Bij is the coefficient for stimulated emission or absorption between states i and j. Einstein Coefficients 2 1 9  Relation between Einstein Coefficients 2 1 At thermal equilibrium, the emission and absorption probabilities must be equal, 10  Relation between Einstein Coefficients Therefore, ------------(A) 11  Relation between Einstein Coefficients According to Boltzmann distribution law number of atoms N1 and N2 in energy states E1 and E2 in thermal equilibrium at temperature T are given by 2 1 = Using the above expression in equation (A), we got the following: 12  Relation between Einstein Coefficients From Planck’s law of radiation, we know that Comparing this expression with B, we can write, = 𝐀𝟐𝟏 𝟖𝛑𝐡𝛎𝟑 𝐀𝟐𝟏 𝐓𝐡𝐞𝐫𝐞𝐟𝐨𝐫𝐞, = 𝟑 = 𝐁𝟏𝟐 𝐜 𝐁𝟐𝟏 This is the relationship between Einstein’s A and B coefficients. 13  Characteristics of LASER Lights  Coherence.  Directionality.  Monochromaticity.  High intensity Coherence: All the photons emitted in LASER transition got same phase in time and space. Is it possible from any natural light sources? No, Not possible. One can only generate coherent light by artificial techniques (stimulated emission) and thus, LASER is coherent. In ordinary light sources (lamp, sodium lamp and flash light etc), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. 14 The photons emitted from ordinary light sources have different energies, frequencies and they are not in 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 (due to stimulated emission). Hence, the light waves of laser light have single wavelength or color and they are in phase. 1. https://www.youtube.com/watch?v=bWTxf5dSUBE 2. https://www.youtube.com/watch?v=0aE02BAPlRk 15 Directionality: In conventional light sources (lamp, sodium lamp and flashlight), photons travel in random direction. Therefore, these light sources emit light in all directions. On the other hand, in laser, all photons 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 much. 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 may spread to a diameter less than 2 cm. The highly directional characteristics of LASER is associated with the way it is produced. https://www.youtube.com/watch?v=BKVMw4jpDZw 16 Monochromaticity: 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 Because electrons transit from different energy levels very close to each other). On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Because, all the electrons transit from same energy level all the time. 17 High intensity: 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. In laser, the light spreads in small region of space. 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 without a recommended safety glass), 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. https://www.youtube.com/watch?v=oC0sqqzq-BM 18  Population Inversion Normally the population of the lower energy levels is larger than that of the higher levels. The processes of stimulated radiation/absorption and spontaneous emission are going on in the same time. The process of creating (artificially) more population of atoms/electrons in higher energy levels than that of lower energy level, is known as population inversion. This is a mechanism by which we can add more atoms to the metastable state and keep them there long enough, thereby allowing the population of metastable state higher than that of lower energy state. To do this, we pump atoms into the metastable level at a rate that exceeds the rate at which they leave. A large number of atoms are therefore excited to and held in this level, leaving an almost empty level at ground state. The atoms stay in this metastable level without de- exciting quickly while the population builds up, giving rise to a population inversion. In practise laser action cannot be achieved for only two levels, as described above. Three and four level systems work. 19 Life-time of a state: The maximum amount of time an excited atom/electron can stay in a state (energy level) without jumping back to lower energy state/ground state. Metastable state: It is an intermediate state between two energy levels, where atoms/electrons can stay longer than they could at the highest energy state. The life-time of a metastable state is higher than that of it’s immediate higher energy state. Metastable State Lifetime Population of metastable state more than that of lowest energy state. 20  Methods of Achieving Population Inversion In order to achieve population inversion, we need to supply energy to the laser medium. The process of supplying energy to the laser medium is called pumping. The source that supplies energy to the laser medium is called pump source. The type of pump source used depends on the laser medium. Different pump sources are used for different laser mediums to achieve population inversion. Some of the most commonly used pump sources are as follows: Optical pumping: As the name suggests, in this method, light is used to supply energy to the laser medium. An external light source like xenon flash lamp is used to produce more electrons (a high population) in the higher energy level of the laser medium. Electric discharge or excitation by electrons: Electric discharge refers to flow of electrons or electric current through a gas, liquid or solid. In this method of pumping, electric discharge acts as the pump source or energy source. A high voltage electric discharge (flow of electrons, electric charge, or electric current) is passed through the laser medium or gas. The intense electric field accelerates the electrons to high speeds and they collide with neutral atoms in the gas. As a result, the electrons in the lower energy state gains sufficient energy from external electrons and jumps into the higher energy state. This method of pumping is used in 21 gas lasers such as argon lasers. Inelastic atom-atom collisions: Like the electric discharge method, here also a high voltage electric discharge acts as a pump source. However, in this method, a combination of two types of gases, say X and Y are used. The excited state of gas X is represented as X+ whereas gas Y is represented as Y+. Both X and Y gases have the same excited states (X+ and Y+), same energy levels. Initially, during electric discharge, the lower energy state electrons in gas X or atom X gets excited to X+ due to continuous collision with electrons. The excited state electrons in gas X+ now collide with the lower energy state electrons in gas Y. As a result, the lower energy state electrons in gas Y gains sufficient energy and jump into the excited state Y+. This method is used in the Helium–Neon (He-Ne) laser. Thermal pumping: Sometime we can achieve population inversion by heating the laser medium. In thermal pumping, heat acts as the pump source or energy source. In this method, population inversion is achieved by supplying heat into the laser medium. The process of achieving population inversion in thermal pumping is almost similar to the optical pumping or electric discharge method, except that in this method heat is used as pump source instead of light or electric discharge. 22 Chemical Reactions: If an atom or a molecule is produced through some chemical reaction and remains in an excited state at the time of production, then it can be used for pumping. The hydrogen fluoride molecule is produced in an excited state when hydrogen and fluorine gas chemically combine. The number of produced excited atoms or molecules is greater than the number of normal state atoms or molecules. Thus, population inversion is achieved. For example, H2 + F2 → 2HF, in this chemical reaction, hydrogen (H2) and fluorine (F2) molecules are chemically combined to produce hydrogen fluoride molecule (2HF) in an excited state. LASER Construction and production of LASER A typical LASER or laser system consists of three important components: a pump source, laser medium and optical resonator. The pump source or energy source is the part of a laser system that provides energy to the laser medium. To get laser emission, first we need to produce population inversion. Detailed about several pumping mechanism have already been discussed. 23 The laser medium is a medium or material where spontaneous and stimulated emission of radiation takes place. It is also known as active medium. The laser medium is surrounded by two parallel mirrors which provides feedback of the light or reflects the light. One mirror is fully reflective (100 % reflective) whereas another one is partially reflective (