Akash Shukla : Akash Shukla LASER Contents : Contents Introduction Characteristics of LASER Interaction of Radiation Einstein's Coefficient Component of laser Resonator Support Pumping Mechanism Population inversion Active Medium Action of laser Types of laser Ruby laser He-Ne Laser Semiconductor laser Application of laser Introduction: Introduction L ight A mplification by S timulated E mission of R adiation Characteristics of LASER: Characteristics of LASER Monochromaticity Laser light is concentrated in a narrow range of wavelengths Coherence All the emitted photons bear a constant phase relationship with each other in both time and phase Directionality laser light is usually low in divergence High Irradiance light possesses high radiant power per unit area Interaction of Radiation : Interaction of Radiation Stimulated Absorption E1 E2 E3 Stimulated Emission 10 -8 Sec Spontaneous Emission E1 E2 E3-E1=h E2-E1=h 2h E2-E1=h Stimulated Emission: Stimulated Emission It is pointed out by Einstein that: Atoms in an excited state can be stimulated to jump to a lower energy level when they are struck by a photon of incident light. Whose energy is the same as the energy-level difference involved in the jump. The electron thus emits a photon of the same wavelength as the incident photon. The incident and emitted photons travel away from the atom in phase. This process is called Stimulated Emission . Cont….: Cont…. In order to obtain the coherent light from stimulated emission, two conditions must be satisfied The atoms must be excited to the higher state. That is, an inverted population is needed, in which more atoms are in the upper state than in the lower, so that emission of photons will dominate over absorption. The higher state must be a metastable state – a state in which the electrons remain longer than usual so that the transition to the lower state occurs by stimulated emission rather than spontaneously. Einstein's Coefficient : Einstein's Coefficient Relation between spontaneous rate and stimulated emission rate Consider upward transition by Stimulated Absorption Consider downward transition by two process Spontaneous emission and Stimulated emission Cont….: Cont…. Under Thermal Equilibrium Now according to Boltzmann distribution law no of atoms N 1 and N 2 in energy state E 1 and E 2 in thermal equilibrium at temperature T at given by Boltzmann distribution law Cont….: Cont…. Comparing the above relation with the plank’s radiation law Conclusion : Conclusion Probability of stimulated absorption is equal to the probability of stimulated transition. The ratio of the probability of spontaneous to stimulated light emission depends directly on the frequency of emission or inversely to the wavelength. Component of laser : Component of laser Active Medium Excitation Mechanism Population Inversion Excitation Mechanism Active Medium Resonator Support 100% Reflecting Mirror 95% Reflecting Mirror Population Inversion: Population Inversion A population inversion exists whenever more atoms are in an excited state than in some lower energy state. The lower state may be the ground state, but in most cases it is an excited state of lower energy. E1 E2 E3 Boltzmann distribution E1 E2 E3 Population inversion Active Medium: Active Medium Active medium is the heart of the laser system and is responsible for producing gain and subsequent generation of laser. It can be a crystal, solid, liquid, semiconductor or gas medium. The material should be of controlled purity, size and shape and should have the suitable energy levels to support population inversion. In other words, it must have a metastable state to support stimulated emission Excitation Mechanism : Excitation Mechanism In gas lasers and semiconductor lasers , the excitation mechanism usually consists of an electrical-current flow through the active medium. In Solid and liquid lasers most often employ optical pumps. The Excitation Mechanism is a source of energy that excites, or "pumps," the atoms in the active medium from a lower to a higher energy state in order to create a population inversion. Resonator Support : Resonator Support Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously and do not contribute to the overall output. A positive feedback mechanism that will cause the majority of the atoms in the population to contribute to the coherent output. This is the resonator, a system of mirrors that reflects undesirable photons out of the system and reflects the desirable photons back into the excited population where they can continue to be amplified. Action of Laser: Action of Laser Three level laser E 1 E 3 E 2 Fast transition Laser action 1 3 pumping spontaneous emission 3 2. state 2 is a metastable state population inversion between states 2 and 1. stimulated emission from 2 to 1 . 10 -8 sec 10 -3 sec Types of Laser: Types of Laser Dye lasers Solid State Lasers Gas lasers Excimer lasers Semiconductor or Diode lasers : Ruby Laser: Ruby Laser Lasing medium: Matrix of Aluminum oxide doped with Chromium ions. Energy levels of the chromium ions take part in lasing action A Three level laser system A Pulse laser Cont….: Cont…. Working: Ruby is pumped optically by an intense flash lamp Chromium ions to be excited by absorption of Radiation around 0.55 µm and 0.40µm Chromium ions are excited to levels E1 and E2 Excited ions decay non- radiatively to the level M( metastable ) M- metastable level with a lifetime of ~ 3ms Laser emission occurs between level M and ground state G at an output wavelength of 0.6943 µm Transition Process: Transition Process Output lies in the visible region M Fast transition / Non- radiative 10 -8 sec E 3 E 2 E 1 E2-E1=h Laser action λ = 694.3nm Metastable state E3-E1=h He-Ne Laser : He-Ne Laser Construction Mixture of He Ne gas is filled in a tube of diameter 1.5 cm and length 80 cm. Mixture of Helium and Neon gases in the ratio 5:1 to 20:1. Medium excited by large electric discharge, through high power pump. A Four level laser system. A Continuous wave laser. Actual lasing atoms are the Neon atoms(Active Medium) Pumping action: Electric discharge is passed through the gas around 1000V through anode and cathode. Cont….: Cont…. Working: Electrons are accelerated, collide with He atom and excite them to higher energy levels Helium atom accumulates at levels F2 and F3 Levels E4 and E6 of neon atoms have almost same energy as F2 and F3 Excited Helium ions collide with Neon atoms and excite them to E4 and E6 Transitions: Transition between E6 and E3 produce 632.8 nm line output From E3 to E2 spontaneous emission takes place – 6000 A E2 – metastable state – tends to collect atoms E2 atoms relax back to ground level (after collision with wall) Transition Process : Transition Process F3 F2 F1 E1 E2 E6 E4 E3 E5 20.65 eV 19.81 eV Excitation by electric Discharge (Accelerated electron) Helium Atom Neon Atom Energy transfer by collisions by atoms Decay to Ground state (collision with walls) Fast transitions (spontaneous emission) 1.115 μ m 632.8 nm 3.391 μ m Application of Laser : Application of Laser M icro wave communication- Due to narrow band width The laser beam has become a way of communication between earth and moon or other satellites- N arrow angular spared Laser has wide industrial application i.e. cutting of material, drilling a very fine hole, surface roughness, welding etc. Laser may serve as a war weapon in military. As chemical application laser are used to initiate or sustain a chemical reaction. High power laser can be use for nuclear fusion reaction. Laser can be very useful in biological science in the treatment of censor, glaucoma, gynecological problems etc.