lasers

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LASERS

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L ight A mplification by S timulated E mission of R adiation

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A laser is a device that transforms light of various frequencies into A chromatic radiation in the visible, infrared and ultraviolet regions With all the waves in phase capable of mobilizing immense heat and power when focused at close range

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Light beam is composed of packets of energy known as photons

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Spontaneous emission spontaneous emission is the process by which an atom in an excited state undergoes a transition to ground state and emits a photon by itself without the help of external radiation.

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Stimulated emission Stimulated emission is the process by which an atom in an excited state undergoes a transition to ground state and emits a photon with the help of external radiation.

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Population inversion A population inversion occurs when a system (such as a group of atoms or molecules ) exists in state with more members in an excited state than in lower energy states.

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Einstein’s coefficients B12 = Einstein coefficient of stimulated absorption B21 = Einstein coefficient of stimulated emission A21 = Einstein coefficient of spontaneous emission

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Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms

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Characteristics of laser Directionality Mono chromaticity Coherence Brightness

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Laser light travels without much divergence

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Amplification is the process that occurs inside the laser An optical cavity is at the centre of the laser device and the core is comprised of chemical elements, molecules or compounds – called as active medium Lasers are generally named after the material of the active medium

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Laser consists of a lasing medium contained with an optical cavity, with an external energy source to maintain population inversion so that stimulated emission of a specific wavelength can occur , producing monochromatic, collimated and coherent beam of light

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RUBY LASER

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Ruby laser is a 3 level, pulsed laser

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Ruby laser was constructed by T. Maiman in 1960 by optical pumping

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The active medium in Ruby laser is Cr3+ ions Wavelength of light given out by ruby laser is 694.3 nanometers

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Working of Ruby laser The chromium ions absorb strongly in the blue and green portions of the visible spectrum emitted by the flash lamp. The energy absorbed corresponding to the two absorption bands 4F2 and 4F1 cause Cr3+ ions to be excited to these levels. The ions then undergo nonradiative transitions to two lower lying metastable energy states. The two corresponding emission lines have wavelengths 694.3 nm and 692.7 though 694.3 dominates.

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Applications of ruby laser Distance measurement using ‘pulse echo’ technique Pulsed holography Used in atmospheric ranging, scattering studies and lidar measurments Used for trimming resistors and integrated circuit masks Used for drilling, high quality holes In millitary , used as target designers and range finders Used in plasma production and fluorescence spectroscopy

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Helium-neon laser

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In the He-Ne laser the active medium is a mixture of ten parts of helium to one part of neon. The neon atoms provide the energy levels for the laser transitions. He-Ne laser is excited by a d.c discharge or radio frequency discharge created by applying a high voltage across the gas filled tube of narrow diameter. During electric discharge, the electron impact with He atoms excite them to one of the two meta stable states designated 21S and 23S Two of the higher states of Ne have almost the same energy as two of the meta stable helium states Collision of excited He atoms with ground state neon atoms results in resonant transfer of energy. Population inversion is achieved

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Transition from 3S to 2P gives 632.8 nm wavelength Transition from 2S to 2P gives 1.15 micrometers wavelength Transition from 3S to 3P gives 3.39 micrometers wavelength