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Types of Lasers

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Dr Raj Kumar CSIR - Central Scientific Instruments Organisation, Chandigarh Overview Introduction to basics of laser physics • Working principle of a Laser • Main components of a Laser • Lasers based on number of energy levels • Lasers modes • Main properties of a Laser • Types of Lasers Solid State Lasers • Ruby Laser: the first laser • Nd: YAG & Nd: Glass Lasers • Tunable Solid State Lasers • Alexandrite Laser • Ti: Sapphire Laser • Colour Center Lasers • Fiber Lasers Applications of Solid State Lasers What is a Laser ? Light Amplification by Stimulated Emission of Radiation Spontaneous emission Stimulated emission Working principle of a Laser E2 E2 h h h h =E2-E1 E1 E1 Absorption Spontaneous Stimulated emission emission Working principle of a Laser Let n1 be number of atoms in E1 state and n2 be number of atoms in E2 then If n > n 1 2 E2 • radiation is mostly absorbed • spontaneous radiation dominates E1 If n2 >> n1 • most atoms occupy level E2, weak absorption • stimulated emission dominates • light is amplified Necessary condition: population inversion For stimulated emission to dominate, there must be more atoms in excited states than in ground state. Such a configuration of atoms is called a population inversion. Main components of a Laser All the lasers comprise of three basic components • Active medium, • Excitation source/pump • Reflecting mirrors/ resonator . Lasers differ only in terms of Active medium or Excitation process. Lasers based on number of energy levels Three-level laser • No lasing action in two level system : no population inversion • Three level system: lasing possible but require high pump energy than four level system • Example: Ruby Laser (three level) Lasers based on number of energy levels Four-level laser • Number of thermally excited ions in the lower laser level is small • Easy to achieve population inversion even by pumping a relatively small number of ions into the upper laser level • Lower threshold compared to a three-level system • Example: Nd: YAG Laser Lasers modes Longitudinal mode frequency separation • Laser oscillates in a number of transverse and longitudinal modes • Transverse mode is selected by using mechanical apertures in the cavity to allow only selected mode and suppress other oscillating modes • Longitudinal mode is selected by using Fabry-Perot Etalon in the cavity • TEM00 is preferred for most of the applications Main properties of a Laser Coherence: from phase correlation Directionality High intensity: results from directionality Monochromaticity: results in high temporal coherence Short pulse duration Types of Lasers Several ways to classify lasers . Mode of operation : Continuous Wave (CW) or Pulsed . Active medium: - Solid lasers - Gas lasers - Liquid lasers - Semiconductor lasers Classification may be done on basis of other parameters . Gain of the laser medium . Power delivered by laser . Efficiency or . Applications Solid State Laser • For historical reasons, solid-state lasers are lasers in which active ions in crystal or glass host materials are optically pumped to create a population inversion • Other types of lasers that employ solid-state gain media are semiconductor lasers and optical fiber lasers and amplifiers. Since these lasers employ very specialized technologies and design principles, they are usually treated separately from conventional bulk solid-state lasers • Semiconductor or diode lasers are mostly electrically pumped (though in principle, optical pumping may be possible with some) Solid State Laser Are versatile and provide a large range of average and peak power, pulse width, pulse repetition rate, and wavelength The flexibility of solid-state lasers stems from the fact that: • The size and shape of the active material can be chosen to achieve a particular performance • Different active materials can be selected with different gain, energy storage, and wavelength properties • Output energy can be increased by adding amplifiers • A large number of passive and active components are available to shape the spectral, temporal and spatial profile of the output beam Solid State Laser: basics Active centers are fixed /doped (~ 1%) in a dielectric crystal or glassy material Electrically non-conducting also called Doped-insulator lasers. • Crystal atoms act as host lattice to active centers • Crystal usually shaped as rod • Pumping: Flash lamp or diode laser • Active centers are from the rare earth, transition metals, or actinides • Water cooled Solid State Laser: schematic Mirrors on both sides of laser rod form a resonant cavity Solid State Laser: requirements Requirements for Host material : • Should not absorb light at laser wavelength • Must possess sharp fluorescent lines, strong absorption bands, and high quantum efficiency • Crystal should have good thermal conductivity Problems with Host material : o Most of excitation energy ends up as heat rather than light o Excess heat damages the laser crystal . Active centres are ions from: Chromium (Cr), Neodymium (Nd), Titanium (Ti), Cerium (Ce), Erbium (Er), Holmium (Ho) and Cobalt (Co) Chromium is active centre in Ruby and Alexandrite lasers Neodymium is active centre in commonly used Nd: YAG laser Representative Solid State Laser • Ruby Laser • Nd:YAG Laser • Nd:Glass Laser . Tunable Solid State Lasers • Alexandrite Laser • Titanium-Sapphire Laser • Colour-Centre Laser . Fiber Lasers • Erbium in a Glass host Ruby Laser: the first laser . First Laser developed in 1960 (TH Maiman) Ruby laser rod: 3+ . A synthetic pink Ruby crystal (Al2O3 doped with Cr ions) . Cr3+ ions concentration: 0.05%, Approx 1.61025 ions per cubic meter. • Ruby crystal as cylindrical • Active Centres (Cr3+ ions) have a set of three energy rod (4cm length 0.5 cm in diameter) • Aluminum & Oxygen • Helical photographic flash ions are inert lamp filled with Xenon. The Al2O3 (sapphire) host is hard, with high thermal conductivity, and transition metals can readily be incorporated substitutionally for the Al Ruby Laser: the first laser A typical Ruby laser (a) with internal mirrors (b) with external mirrors Ruby Laser: commercial . End faces grounded and polished . Mostly silvered faces (100% & 90 % reflection) Febry-Perot Resonator • System is cooled with the help of a coolant circulating around the ruby rod . In practical lasers flash lamps of helical design no longer used . Most commonly used are linear lamps Ruby Laser : energy levels Energy levels of chromium ions is Ruby laser Ruby Laser : working principle . A Three level laser system . E2 - metastable state (3ms) • Ruby rod pumped with an intense Xenon flash lamp • Ground state of Cr3+ ions absorb light at pump bands 550nm 400nm • Non-radiative transitions to E2 • Population Inversion at E2 Radiative transitions from E2 to E1 Red wavelength at 694.3 nm Under intense excitation: Pumping > Critical threshold A spontaneous fluorescent photon (red) acts as input and trigger Stimulated emission; SYSTEM LASES Ruby Laser: output Laser Output: Pulsed with low repetition rate (1 to 2 per sec) Ruby laser light pulses • Series of irregular spikes stretching over the duration of pump pulse • Q-switching concentrates output into a single pulse Ruby Laser: output • Stimulated transitions faster than rate at which population inversion is maintained • Once stimulated emission commence, the metastable state E2, depopulate very rapidly • At the end of each pulse, population at E2 falls below the threshold value required for sustaining emission of light • Lasing ceases & Laser becomes inactive Next pulse will arrive only after P.I. is restored High energy storage capability due to long upper laser level lifetime Pulse energy upto 100J Relatively inefficient; 0.1 to 1% Variety of applications: Plasma diagnostics; Holography. Nd: YAG Laser • Yttrium Aluminum Garnet (YAG) Y3Al5O12 best choice of a host for neodymium ions (Nd) • YAG offers low threshold and high gain • YAG is a very hard, isotropic crystal • good thermal and mechanical properties • can be grown and fabricated in rods of high optical quality • Operation: CW and pulsed mode (high repetition rate) • Efficiency about 10 times as compared to ruby • Drastic weight reduction • Replaced ruby in military Rangefinders, other applications • Used in the semiconductor industry for resistor trimming, silicon scribing, and marking For continuous or very high repetition-rate operation, crystalline materials provide higher gain and greater thermal conductivity Nd: YAG Laser . Active center: Neodymium (Nd) ion- a rare earth metallic ion . Host: YAG . Emission at 1.064m • In Nd:YAG laser, Nd 3+ ions take place of yttrium ions • Doping conc. ; 0.72% by weight corresponds to 1.41026 atoms/m3 • Rod: 10cm in length, 12mm in diameter . Nd: YAG rod & a linear flash lamp housed in an elliptical cavity . In practice, external mirrors (100% , 99% reflectivity) used . System cooled by water circulation Nd: YAG Laser Nd: YAG Laser lifetime 230 μs Energy levels of Nd –ions in a crystal Nd: YAG Laser . A Four level laser system: Require lower pump energy • Terminal laser level sufficiently far from ground state • E3 – metastable level (lifetime 230 μs) • Two pump bands: 700 nm & 800nm • Pump: intense Xenon flash lamp 3+ • Nd ions level E4, decays to upper laser level at E3 • Population inversion easily achieved between E3 and E2 levels. • Stimulated to emit 1064 nm laser transition. 3+ From E2 level, Nd ions quickly drop to E1 by transferring energy to crystal Nd: YAG Laser . Many other transitions in near IR region; all weaker than 1064
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