Lecture Lasers PDF

UNIT 1 Lasers and Optical Fibers  LASERS Light Amplification by stimulated emission of radiations Concept of Stimulated Emission given by Albert Einstein 1917 Charles H Townes utilized to build MASER 1953 Extension to optical region Theodore H. Maiman in 1960 built first Laser utilizi...

UNIT 1 Lasers and Optical Fibers  LASERS Light Amplification by stimulated emission of radiations Concept of Stimulated Emission given by Albert Einstein 1917 Charles H Townes utilized to build MASER 1953 Extension to optical region Theodore H. Maiman in 1960 built first Laser utilizing concept of stimulated emission Properties of Laser light Monochromaticity high degree of monochromaticity Coherent All the emitted photons bear a constant phase relationship with each other in both time and phase Directional A very well collimated beam which travels long distances with very little spread. Intense A very highly intense beam Interaction of Radiation with matter Absorption Probability for absorption 𝑷𝟏𝟐 = 𝑩𝟏𝟐 𝑼 𝝂 𝒅𝝂 B12 is called the Einstein coefficient of absorption Total number of upward transitions 𝑁1 𝑃12 = 𝑁1 𝑩𝟏𝟐 𝑼 𝝂 𝒅𝝂 Spontaneous Emission Probability for spontaneous emission 𝑷𝟐𝟏 = 𝑨𝟐𝟏 A21 is called the Einstein coefficient of Spontaneous Emission number of downward transitions due to spontaneous emission 𝑵𝟐 𝑷𝟐𝟏 = 𝑵𝟐 𝑨𝟐𝟏 Stimulated Emission Probability for stimulated emission 𝑷′𝟐𝟏 = 𝑩𝟐𝟏 𝑼 𝝂 𝒅𝝂 B21 is called the Einstein coefficient of stimulated emission number of downward transitions due to stimulated emission 𝑷′𝟐𝟏 = 𝑵𝟐 𝑩𝟐𝟏 𝑼 𝝂 𝒅𝝂 Relation Between Einstein Coefficients In Thermal Equilibrium : The total upward transition equals total downward transitions ′ 𝑁2 𝑃21 + 𝑁2 𝑃21 = 𝑁1 𝑃12 where N1 and N2 are the populations of E1 and E2 Substituting for Probabilities we get 𝑁2 𝐵21 𝑢 𝜈 𝑑𝜈 + 𝑁2 𝐴21 = 𝑁1 𝐵12 𝑢 𝜈 𝑑𝜈 Rearranging this we get 𝐴21 𝑢 𝜈 𝑑𝜈 = 𝐵12 𝑁1 𝐵21 −1 𝐵21 𝑁2 From Maxwell- Boltzmann law we have 𝑁1 𝐸2 − 𝐸1 = 𝑒 𝑘𝑇 𝑁2 From Planck’s radiation Law we have 8𝜋ℎ𝜈 3 1 𝑢 𝜈 = ℎ𝜈 𝑐3 𝑒 𝑘𝑇 −1 Type equation here.Comparing this with previous equation for energy density in terms of Einstein coefficients we get relation between Einstein coefficients as 𝐵12 𝐴21 8𝜋ℎ𝜈 3 =1 𝐵21 = 𝐵21 𝑐3 Principle of Laser Action The main Principle for amplifying light in a Laser system is the stimulated emission Concept of Population Inversion and the Metastable states State of the medium in which there are larger number of atoms in higher energy state as compared to the lower energy state The existence of metastable state enhances the probability of population inversion Condition for the laser operation If N1 > N2 radiation is mostly absorbed spontaneous radiation dominates. if N2 >> N1 - population inversion most electrons occupy level E2, weak absorption stimulated emission prevails light is amplified Necessary condition: population inversion Population Inversion This situation in which the number of electrons in the higher state exceed that in the lower state (N2 > N1) is known as population inversion. Methods to obtain population inversion Supply Energy from outside Optical Pumping : Solid state lasers energy bands excitation by Xenon flash lamp Ruby laser Electrical Pumping: Gaseous Lasers energy levels excitation by Electronic excitation He Ne Laser Chemical Pumping: Dye lasers energy excitation by exothermic chemical reaction Laser Pumping: excitation by using laser, specific mechanism, increased efficiency Types of Lasers Solid state Lasers : The active medium is solid crystal such as Ruby Nd:YAG (Nd:Y3Al5O12 ) etc Gaseous Lasers : The active medium is Gaseous such as He-Ne laser, Co2 lasere.t.c Dye Lasers: Macromolecules used for different laser light from single medium Semiconductor Lasers : PN junction diode used for producing LASER Generating a population inversion “Pumping”: delivery of energy to produce a population inversion Possibility of Lasing action in Different Energy level systems Lasers that maintain a population inversion indefinitely produce continuous output – termed CW (for continuous wave) lasers Lasers that have a short-lived population inversion produce pulsed output – these are pulsed lasers  Ruby Laser (Three Level Laser) Ruby (Al2O3) monocrystal, Cr doped. A flashtube, also called a flashlamp, is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. Flashtubes are made of a length of glass tubing with electrodes at either end Drawbacks of ruby laser The laser requires high pumping power. The efficiency of ruby laser is small. The laser output is not continuous. The output occurs in form of pulses of microsecond duration. Gas Lasers For continuous laser beam, gas lasers are used. The vapor metal ions are employed as active media. Main advantage is exceptionally high monochromaticity, most pure spectrum and high stability of frequency. The output of gas lasers is moderate but inferior to that of crystal lasers. He-Ne Laser First gaseous Laser to be developed Developed in 1961 by Ali Javan and associates A four Level Laser Emits wavelength of 6328Ȧ  Ruby Laser He-Ne Laser Solid state Laser Gaseous Laser Three level laser Four level laser Pulsed Laser CW Laser Optical pumping Electrical pumping Coolent required No coolent required Output power of 10kW Output power of 0.5 to 5mW Operation duration is few hours Operation duration is ~ 10000 hrs  Applications of Lasers Laser beams are very intense so are used for welding, cutting of materials. Lasers are used for eye surgery, treatment of dental decay and skin diseases. Lasers are used for barcode scanners in library and in super markets. Laser is used in printers (Laser printers). Lasers are used for Nuclear Fusion. Laser are used in CD/DVD Player Laser is used in Holography.