a laser is a device that emits light

8/3/2019 A Laser is a Device That Emits Light

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3 Laser physics 4 Continuous and pulsed modes of operation

o 4.1 Continuous wave operationo 4.2 Pulsed operation

4.2.1 Q-switching 4.2.2 Mode-locking 4.2.3 Pulsed pumping

5 Historyo 5.1 Foundationso 5.2 Masero 5.3 Lasero 5.4 Recent innovations

6 Types and operating principleso 6.1 Gas lasers

6.1.1 Chemical lasers 6.1.2 Excimer lasers

o 6.2 Solid-state laserso 6.3 Fiber laserso 6.4 Photonic crystal laserso 6.5 Semiconductor laserso 6.6 Dye laserso 6.7 Free electron laserso 6.8 Bio lasero 6.9 Exotic laser media

7 Useso 7.1 Examples by powero 7.2 Hobby uses

8 Safety 9 As weapons 10 Fictional predictions 11 See also 12 References 13 External links

Terminology
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Design

Principal components:1. Gain medium2. Laser pumping energy3. High reflector4.Output coupler5. Laser beamMain article:Laser construction

A laser consists of again mediuminside a highly reflectiveopticalcavity, as well as a means to supply energy to the gain medium. Thegain medium is a material with properties that allow it to amplify light

by stimulated emission. In its simplest form, a cavity consists of twomirrors arranged such that light bounces back and forth, each timepassing through the gain medium. Typically one of the two mirrors,theoutput coupler, is partially transparent. The output laser beam isemitted through this mirror.

Light of a specific wavelength that passes through the gain medium isamplified(increases in power); the surrounding mirrors ensure that

most of the light makes many passes through the gain medium, beingamplified repeatedly. Part of the light that is between the mirrors (thatis, within the cavity) passes through the partially transparent mirrorand escapes as abeam of light.

The process of supplying theenergyrequired for the amplification iscalledpumping. The energy is typically supplied as an electricalcurrent or as light at a different wavelength. Such light may beprovided by aflash lampor perhaps another laser. Most practical
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lasers contain additional elements that affect properties such as thewavelength of the emitted light and the shape of the beam.

Laser physics

Ahelium-neon laserdemonstration at theKastler-Brossel LaboratoryatUniv. Paris 6. The pink-orange glow running through the center ofthe tube is from the electric discharge which inadvertently producesincoherent light, just as in a neon tube. That glowing plasma howeveralso acts as thegain mediumthrough which the internal beam passes

as it is reflected in between the two mirrors. Laser radiation outputfrom the front mirror can be seen to produce a tiny (about 1mm indiameter) intense spot on the screen to the right. Although it is a deepand pure red color, spots of laser light are so intense that cameras aretypically overexposed and distort their color, often appearing morewhite.

Spectrum of a helium neon laser illustrating its very high spectralpurity (limited by the measuring apparatus). The .002 nm bandwidthof the lasing medium is well over 10,000 times narrower than thespectral width of a light-emitting diode (whose spectrum is shown
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herefor comparison), with the bandwidth of a single longitudinalmode being much narrower still.See also:Laser science

The gain medium of a laser is a material of controlled purity, size,concentration, and shape, which amplifies the beam by the process ofstimulated emission. It can be of anystate:gas,liquid,solid, orplasma. The gain medium absorbs pump energy, which raises someelectrons into higher-energy ("excited")quantum states. Particles caninteract with light by either absorbing or emitting photons. Emissioncan be spontaneous or stimulated. In the latter case, the photon isemitted in the same direction as the light that is passing by. When the

number of particles in one excited state exceeds the number ofparticles in some lower-energy state,population inversionis achievedand the amount of stimulated emission due to light that passes throughis larger than the amount of absorption. Hence, the light is amplified.By itself, this makes anoptical amplifier. When an optical amplifier isplaced inside a resonant optical cavity, one obtains a laser.

The light generated by stimulated emission is very similar to the input

signal in terms of wavelength,phase, and polarization. This giveslaser light its characteristic coherence, and allows it to maintain theuniform polarization and often monochromaticity established by theoptical cavity design.

The opticalresonatoris sometimes referred to as an "optical cavity",but this is a misnomer: lasers use open resonators as opposed to theliteral cavity that would be employed at microwave frequencies in amaser. The resonator typically consists of two mirrors between whicha coherent beam of light travels in both directions, reflecting back onitself so that an average photon will pass through the gain mediumrepeatedly before it is emitted from the output aperture or lost todiffraction or absorption. If the gain (amplification) in the medium islarger than the resonator losses, then the power of the recirculatinglight can riseexponentially. But each stimulated emission eventreturns an atom from its excited state to the ground state, reducing thegain of the medium. With increasing beam power the net gain (gain

times loss) reduces to unity and the gain medium is said to be
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saturated. In a continuous wave (CW) laser, the balance of pumppower against gain saturation and cavity losses produces anequilibrium value of the laser power inside the cavity; thisequilibrium determines the operating point of the laser. If the appliedpump power is too small, the gain will never be sufficient toovercome the resonator losses, and laser light will not be produced.The minimum pump power needed to begin laser action is called thelasing threshold. The gain medium will amplify any photons passingthrough it, regardless of direction; but only the photons in aspatialmodesupported by the resonator will pass more than once through themedium and receive substantial amplification.

The beam in the cavity and the output beam of the laser, whentravelling in free space (or a homogenous medium) rather thanwaveguides (as in anoptical fiberlaser), can be approximated as aGaussian beamin most lasers; such beams exhibit the minimumdivergence for a given diameter. However some high power lasersmay be multimode, with thetransverse modesoften approximatedusingHermite-GaussianorLaguerre-Gaussian functions. It has beenshown that unstable laser resonators (not used in most lasers) produce

fractal shaped beams.[7]

Near the beam "waist" (orfocal region) it ishighlycollimated: the wavefronts are planar, normal to the directionof propagation, with nobeam divergenceat that point. However duetodiffraction, that can only remain true well within theRayleighrange. The beam of a single transverse mode (gaussian beam) lasereventually diverges at an angle which varies inversely with the beamdiameter, as required bydiffractiontheory. Thus, the "pencil beam"directly generated by a commonhelium-neon laserwould spread out

to a size of perhaps 500 kilometers when shone on the Moon (fromthe distance of the earth). On the other hand the light from asemiconductor lasertypically exits the tiny crystal with a largedivergence: up to 50. However even such a divergent beam can betransformed into a similarly collimated beam by means of alenssystem, as is always included, for instance, in alaser pointerwhoselight originates from alaser diode. That is possible due to the lightbeing of a single spatial mode. This unique property of laser light,

spatial coherence, cannot be replicated using standard light sources(except by discarding most of the light) as can be appreciated by
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averaged over any longer time periods, with the very high frequencypower variations having little or no impact in the intended application.(However the term is not applied tomode lockedlasers, where theintention is to create very short pulses at the rate of the round-triptime).

For continuous wave operation it is required for the populationinversion of the gain medium to be continually replenished by asteady pump source. In some lasing media this is impossible. In someother lasers it would require pumping the laser at a very highcontinuous power level which would be impractical or destroy thelaser by producing excessive heat. Such lasers cannot be run in CW

mode.

Pulsed operation

Pulsed operation of lasers refers to any laser not classified ascontinuous wave, so that the optical power appears in pulses of someduration at some repetition rate. This encompasses a wide range oftechnologies addressing a number of different motivations. Somelasers are pulsed simply because they cannot be run incontinuousmode.

In other cases the application requires the production of pulses havingas large an energy as possible. Since the pulse energy is equal to theaverage power divided by the repetition rate, this goal can sometimesbe satisfied by lowering the rate of pulses so that more energy can bebuilt up in between pulses. Inlaser ablationfor example, a smallvolume of material at the surface of a work piece can be evaporated if

it is heated in a very short time, whereas supplying the energygradually would allow for the heat to be absorbed into the bulk of thepiece, never attaining a sufficiently high temperature at a particularpoint.

Other applications rely on the peak pulse power (rather than theenergy in the pulse), especially in order to obtainnonlinear opticaleffects. For a given pulse energy, this requires creating pulses of the

shortest possible duration utilizing techniques such asQ-switching.
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The optical bandwidth of a pulse cannot be narrower than thereciprocal of the pulse width. In the case of extremely short pulses,that implies lasing over a considerable bandwidth, quite contrary tothe very narrow bandwidths typical of CW lasers. The lasing mediumin some dye lasers and vibronic solid-state lasers produces opticalgain over a wide bandwidth, making a laser possible which can thusgenerate pulses of light as short as a fewfemtoseconds(1015 s).

Q-switching

Main article:Q-switching

In a Q-switched laser, the population inversion is allowed to build upby introducing loss inside the resonator which exceeds the gain of themedium; this can also be described as a reduction of the quality factoror 'Q' of the cavity. Then, after the pump energy stored in the lasermedium has approached the maximum possible level, the introducedloss mechanism (often an electro- or acousto-optical element) israpidly removed (or that occurs by itself in a passive device),allowing lasing to begin which rapidly obtains the stored energy in thegain medium. This results in a short pulse incorporating that energy,and thus a high peak power.

Mode-locking

Main article:Modelocking

A mode-locked laser is capable of emitting extremely short pulses onthe order of tens ofpicosecondsdown to less than 10femtoseconds.

These pulses will repeat at the round trip time, that is, the time that ittakes light to complete one round trip between the mirrors comprisingthe resonator. Due to theFourier limit(also known as energy-timeuncertainty), a pulse of such short temporal length has a spectrumspread over a considerable bandwidth. Thus such a gain medium musthave a gain bandwidth sufficiently broad to amplify thosefrequencies. An example of a suitable material istitanium-doped,artificially grownsapphire(Ti:sapphire) which has a very wide gain

bandwidth and can thus produce pulses of only a few femtosecondsduration.
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Such mode-locked lasers are a most versatile tool for researchingprocesses occurring on extremely short time scales (known asfemtosecond physics,femtosecond chemistryandultrafast science),for maximizing the effect ofnonlinearityin optical materials (e.g. insecond-harmonic generation,parametric down-conversion,opticalparametric oscillatorsand the like) due to the large peak power, and inablation applications.

[citation needed] Again, because of the extremelyshort pulse duration, such a laser will produce pulses which achievean extremely high peak power.

Pulsed pumping

Another method of achieving pulsed laser operation is to pump thelaser material with a source that is itself pulsed, either throughelectronic charging in the case of flash lamps, or another laser whichis already pulsed. Pulsed pumping was historically used with dyelasers where the inverted population lifetime of a dye molecule was soshort that a high energy, fast pump was needed. The way to overcomethis problem was to charge up largecapacitorswhich are thenswitched to discharge through flashlamps, producing an intense flash.

Pulsed pumping is also required for three-level lasers in which thelower energy level rapidly becomes highly populated preventingfurther lasing until those atoms relax to the ground state. These lasers,such as the excimer laser and the copper vapor laser, can never beoperated in CW mode.

History

Foundations

In 1917,Albert Einsteinestablished the theoretic foundations for thelaser and the maser in the paperZur Quantentheorie der Strahlung(On the Quantum Theory of Radiation); via a re-derivation ofMaxPlancks law of radiation, conceptually based upon probabilitycoefficients (Einstein coefficients) for the absorption, spontaneousemission, and stimulated emission of electromagnetic radiation; in1928,Rudolf W. Ladenburgconfirmed the existences of the

phenomena of stimulated emission and negative absorption;[8]

in1939, Valentin A. Fabrikant predicted the use of stimulated emission
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to amplify short waves;[9]

in 1947,Willis E. Lamband R. C.Retherford found apparent stimulated emission in hydrogen spectraand effected the first demonstration of stimulated emission;[8]in 1950,Alfred Kastler(Nobel Prize for Physics 1966) proposed the method ofoptical pumping, experimentally confirmed, two years later, byBrossel, Kastler, and Winter.

[10]

Maser

Main article:Maser

In 1953,Charles Hard Townesand graduate students James P.

Gordon and Herbert J. Zeiger produced the first microwave amplifier,a device operating on similar principles to the laser, but amplifyingmicrowaveradiation rather than infrared or visible radiation.Townes's maser was incapable of continuous output.[citation needed]Meanwhile, in theSoviet Union,Nikolay BasovandAleksandrProkhorovwere independently working on the quantumoscillatorandsolved the problem of continuous-output systems by using more thantwo energy levels. These gain media could releasestimulatedemissionsbetween an excited state and a lower excited state, not theground state, facilitating the maintenance of apopulation inversion. In1955, Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, latera main method of laser pumping.

Townes reports that several eminent physicistsamong themNielsBohr,John von Neumann,Isidor Rabi,Polykarp Kusch, andLlewellyn Thomasargued the maser violated Heisenberg's

uncertainty principleand hence could not work.[1]In 1964 Charles H.Townes, Nikolay Basov, and Aleksandr Prokhorov shared theNobelPrize in Physics, for fundamental work in the field of quantumelectronics, which has led to the construction of oscillators andamplifiers based on the maserlaser principle.

Laser

In 1957,Charles Hard TownesandArthur Leonard Schawlow, then atBell Labs, began a serious study of the infrared laser. As ideas
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At a conference in 1959, Gordon Gould published the term LASER inthe paper The LASER, Light Amplification by Stimulated Emission ofRadiation.

[1][5]Goulds linguistic intention was using the -aser wordparticle as a suffixto accurately denote the spectrum of the lightemitted by the LASER device; thus x-rays:xaser, ultraviolet: uvaser,et cetera; none established itself as a discrete term, although raserwas briefly popular for denoting radio-frequency-emitting devices.

Goulds notes included possible applications for a laser, such asspectrometry,interferometry,radar, andnuclear fusion. He continueddeveloping the idea, and filed apatent applicationin April 1959. TheU.S. Patent Officedenied his application, and awarded a patent to

Bell Labs, in 1960. That provoked a twenty-eight-yearlawsuit,featuring scientific prestige and money as the stakes. Gould won hisfirst minor patent in 1977, yet it was not until 1987 that he won thefirst significant patent lawsuit victory, when a Federal judge orderedthe U.S. Patent Office to issue patents to Gould for the opticallypumped and thegas dischargelaser devices. The question of just howto assign credit for inventing the laser remains unresolved byhistorians.[11]

On May 16, 1960,Theodore H. Maimanoperated the first functioninglaser,

[12][13]atHughes Research Laboratories, Malibu, California,

ahead of several research teams, including those ofTownes, atColumbia University,Arthur Schawlow, atBell Labs,

[14]and Gould,

at the TRG (Technical Research Group) company. Maimansfunctional laser used a solid-stateflashlamp-pumped syntheticrubycrystalto produce red laser light, at 694 nanometres wavelength;

however, the device only was capable of pulsed operation, because ofits three-level pumping design scheme. Later in 1960, theIranianphysicistAli Javan, andWilliam R. Bennett, andDonald Herriott,constructed the firstgas laser, usingheliumandneonthat was capableof continuous operation in the infrared (U.S. Patent 3,149,290); later,Javan received theAlbert Einstein Awardin 1993. Basov and Javanproposed the semiconductorlaser diodeconcept. In 1962,Robert N.Halldemonstrated the first laser diode device, made ofgallium

arsenideand emitted at 850 nm the near-infraredband of thespectrum. Later, in 1962,Nick Holonyak, Jr.demonstrated the first
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byhttp://en.wikipedia.org/wiki/Flashlamphttp://en.wikipedia.org/wiki/Laser#cite_note-13http://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/wiki/Arthur_L._Schawlowhttp://en.wikipedia.org/wiki/Columbia_Universityhttp://en.wikipedia.org/wiki/Charles_H._Towneshttp://en.wikipedia.org/wiki/Hughes_Research_Laboratorieshttp://en.wikipedia.org/wiki/Laser#cite_note-11http://en.wikipedia.org/wiki/Laser#cite_note-11http://en.wikipedia.org/wiki/Theodore_Maimanhttp://en.wikipedia.org/wiki/Laser#cite_note-10http://en.wikipedia.org/wiki/Gas_dischargehttp://en.wikipedia.org/wiki/Lawsuithttp://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/wiki/United_States_Patent_and_Trademark_Officehttp://en.wikipedia.org/wiki/Patent_applicationhttp://en.wikipedia.org/wiki/Nuclear_fusionhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Interferometryhttp://en.wikipedia.org/wiki/Spectroscopyhttp://en.wikipedia.org/wiki/Laser#cite_note-Gould1959-0http://en.wikipedia.org/wiki/Laser#cite_note-Gould1959-0


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semiconductor laser with a visible emission. This first semiconductorlaser could only be used in pulsed-beam operation, and when cooledtoliquid nitrogentemperatures (77 K). In 1970,Zhores Alferov, inthe USSR, and Izuo Hayashi and Morton Panish ofBell TelephoneLaboratoriesalso independently developed room-temperature,continual-operation diode lasers, using theheterojunctionstructure.

Recent innovations

Graph showing the history of maximum laser pulse intensitythroughout the past 40 years.

Since the early period of laser history, laser research has produced avariety of improved and specialized laser types, optimized fordifferent performance goals, including:

new wavelength bands maximum average output power maximum peak pulse energy maximum peak pulse power minimum output pulse duration maximum power efficiency minimum costand this research continues to this day.

Lasing without maintaining the medium excited into a populationinversion

[dubiousdiscuss] was discovered in 1992 insodiumgas andagain in 1995 inrubidiumgas by various international teams.

[citation

needed]

This was accomplished by using an external maser to induce"optical transparency" in the medium by introducing and destructively
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interfering the ground electron transitions between two paths, so thatthe likelihood for the ground electrons to absorb any energy has beencancelled.

Types and operating principles

For a more complete list of laser types see thislist of laser

types.

Wavelengths of commercially available lasers. Laser types withdistinct laser lines are shown above the wavelength bar, while

below are shown lasers that can emit in a wavelength range. Thecolor codifies the type of laser material (see the figure descriptionfor more details).

Gas lasers

Main article:Gas laser

Following the invention of the HeNe gas laser, many other gas

discharges have been found to amplify light coherently. Gas lasersusing many differentgaseshave been built and used for manypurposes. Thehelium-neon laser(HeNe) is able to operate at anumber of different wavelengths, however the vast majority areengineered to lase at 633 nm; these relatively low cost but highlycoherent lasers are extremely common in optical research andeducational laboratories. Commercialcarbon dioxide (CO2) laserscan emit many hundreds of watts in a single spatial mode which

can be concentrated into a tiny spot. This emission is in the thermalinfrared at 10.6 m; such lasers are regularly used in industry for
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cutting and welding. The efficiency of a CO2 laser is unusuallyhigh: over 10%.Argon-ionlasers can operate at a number of lasingtransitions between 351 and 528.7 nm. Depending on the opticaldesign one or more of these transitions can be lasingsimultaneously; the most commonly used lines are 458 nm,488 nm and 514.5 nm. A nitrogentransverse electrical discharge ingas at atmospheric pressure(TEA) laser is an inexpensive gaslaser, often home-built by hobbyists, which produces ratherincoherent UV light at 337.1 nm.

[15]Metal ion lasers are gas lasers

that generatedeep ultravioletwavelengths.Helium-silver(HeAg)224 nm andneon-copper(NeCu) 248 nm are two examples. Likeall low-pressure gas lasers, the gain media of these lasers have

quite narrow oscillationlinewidths, less than 3GHz(0.5picometers),[16]making them candidates for use influorescencesuppressedRaman spectroscopy.

Chemical lasers

Chemical lasersare powered by a chemical reaction permitting alarge amount of energy to be released quickly. Such very high

power lasers are especially of interest to the military, howevercontinuous wave chemical lasers at very high power levels, fed bystreams of gasses, have been developed and have some industrialapplications. As examples, in theHydrogen fluoride laser(2700-2900 nm) and theDeuterium fluoride laser(3800 nm) the reactionis the combination of hydrogen or deuterium gas with combustionproducts ofethyleneinnitrogen trifluoride.

Excimer lasers

Excimer lasersare a special sort of gas laser powered by an electricdischarge in which the lasing medium is anexcimer, or moreprecisely anexciplexin existing designs. These are moleculeswhich can only exist with one atom in anexcited electronic state.Once the molecule transfers its excitation energy to a photon,therefore, its atoms are no longer bound to each other and themolecule disintegrates. This drastically reduces the population of

the lower energy state thus greatly facilitating a population
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inversion. Excimers currently used are allnoble gas compounds;noble gasses are chemically inert and can only form compoundswhile in an excited state. Excimer lasers typically operate atultravioletwavelengths with major applicatons includingsemiconductorphotolithographyandLASIKeye surgery.Commonly used excimer molecules include ArF (emission at193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF(351 nm).

[17]The molecularfluorinelaser, emitting at 157 nm in

the vacuum ultraviolet is sometimes referred to as an excimerlaser, however this appears to be a misnomer inasmuch as F2 is astable compound.

Solid-state lasers

A frequency-doubled greenlaser pointer, showing internalconstruction. Two AAA cells and electronics power the lasermodule (lower diagram) This contains a powerful 808 nm IR diodelaser that optically pumps a Nd:YVO4 crystal inside a laser cavity.That laser produces 1064 nm (infrared) light which is mainlyconfined inside the resonator. Also inside the laser cavity,however, is a non-linear KTP crystal which causes frequency

doubling, resulting in green light at 532 nm. The front mirror istransparent to this visible wavelength which is then expanded andcollimated using two lenses (in this particular design).

Solid-state lasersuse a crystalline or glass rod which is "doped"with ions that provide the required energy states. For example, thefirst working laser was aruby laser, made fromruby(chromium-dopedcorundum). Thepopulation inversionis actually maintainedin the "dopant", such aschromiumorneodymium. These materials
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are pumped optically using a shorter wavelength than the lasingwavelength, often from a flashtube or from another laser.

It should be noted that "solid-state" in this sense refers to a crystal

or glass, but this usage is distinct from the designation of "solid-state electronics" in referring to semiconductors. Semiconductorlasers (laser diodes) are pumped electrically and are thus notreferred to as solid-state lasers. The class of solid-state laserswould, however, properly includefiber lasersin which dopants inthe glass lase under optical pumping. But in practice these aresimply referred to as "fiber lasers" with "solid-state" reserved forlasers using a solid rod of such a material.

Laser spots (650, 532, 405 nm)

Neodymiumis a common "dopant" in various solid-state lasercrystals, includingyttrium orthovanadate(Nd:YVO4),yttriumlithium fluoride(Nd:YLF) andyttrium aluminium garnet(Nd:YAG). All these lasers can produce high powers in theinfraredspectrum at 1064 nm. They are used for cutting, weldingand marking of metals and other materials, and also in

spectroscopyand for pumpingdye lasers.

These lasers are also commonlyfrequency doubled, tripledorquadrupled, in so-called "diode pumped solid state" or DPSSlasers. Under second, third, or fourth harmonic generation theseproduce 532 nm (green, visible), 355 nm and 266 nm (UV) beams.This is the technology behind the brightlaser pointersparticularlyat green (532 nm) and other short visible wavelengths.
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Ytterbium,holmium,thulium, anderbiumare other common"dopants" in solid-state lasers. Ytterbium is used in crystals such asYb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2,typically operating around 1020-1050 nm. They are potentiallyvery efficient and high powered due to a small quantum defect.Extremely high powers in ultrashort pulses can be achieved withYb:YAG.Holmium-doped YAG crystals emit at 2097 nm andform an efficient laser operating atinfraredwavelengths stronglyabsorbed by water-bearing tissues. The Ho-YAG is usuallyoperated in a pulsed mode, and passed through optical fibersurgical devices to resurface joints, remove rot from teeth,vaporize cancers, and pulverize kidney and gall stones.

Titanium-dopedsapphire(Ti:sapphire) produces a highlytunableinfraredlaser, commonly used forspectroscopy. It is also notablefor use as a mode-locked laser producingultrashort pulsesofextremely high peak power.

Thermal limitations in solid-state lasers arise from unconvertedpump power that manifests itself as heat. This heat, when coupled

with a high thermo-optic coefficient (dn/dT) can give rise tothermal lensing as well as reduced quantum efficiency. These typesof issues can be overcome by another novel diode-pumped solid-state laser, the diode-pumped thindisk laser. The thermallimitations in this laser type are mitigated by using a laser mediumgeometry in which the thickness is much smaller than the diameterof the pump beam. This allows for a more even thermal gradient inthe material. Thindisk lasershave been shown to produce up to

kilowatt levels of power.

[18]

Fiber lasers

Main article:Fiber laser

Solid-state lasers or laser amplifiers where the light is guided dueto thetotal internal reflectionin a single modeoptical fiberareinstead calledfiber lasers. Guiding of light allows extremely long

gain regions providing good cooling conditions; fibers have highsurface area to volume ratio which allows efficient cooling. In
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addition, the fiber's waveguiding properties tend to reduce thermaldistortion of the beam.Erbiumandytterbiumions are commonactive species in such lasers.

Quite often, the fiber laser is designed as adouble-clad fiber. Thistype of fiber consists of a fiber core, an inner cladding and an outercladding. The index of the three concentric layers is chosen so thatthe fiber core acts as a single-mode fiber for the laser emissionwhile the outer cladding acts as a highly multimode core for thepump laser. This lets the pump propagate a large amount of powerinto and through the active inner core region, while still having ahigh numerical aperture (NA) to have easy launching conditions.

Pump light can be used more efficiently by creating afiber disklaser, or a stack of such lasers.

Fiber lasers have a fundamental limit in that the intensity of thelight in the fiber cannot be so high that optical nonlinearitiesinduced by the local electric field strength can become dominantand prevent laser operation and/or lead to the material destructionof the fiber. This effect is calledphotodarkening. In bulk lasermaterials, the cooling is not so efficient, and it is difficult toseparate the effects of photodarkening from the thermal effects, butthe experiments in fibers show that the photodarkening can beattributed to the formation of long-livingcolor centers.[citation needed]

Photonic crystal lasers

Photonic crystal lasers are lasers based on nano-structures that

provide the mode confinement and thedensity of optical states(DOS) structure required for the feedback to take place.[clarificationneeded] They are typical micrometre-sized[dubiousdiscuss] and tunableon the bands of the photonic crystals.[19][clarification needed]

Semiconductor lasers
http://en.wikipedia.org/wiki/Erbiumhttp://en.wikipedia.org/wiki/Erbiumhttp://en.wikipedia.org/wiki/Erbiumhttp://en.wikipedia.org/wiki/Ytterbiumhttp://en.wikipedia.org/wiki/Ytterbiumhttp://en.wikipedia.org/wiki/Ytterbiumhttp://en.wikipedia.org/wiki/Double-clad_fiberhttp://en.wikipedia.org/wiki/Double-clad_fiberhttp://en.wikipedia.org/wiki/Double-clad_fiberhttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Photodarkeninghttp://en.wikipedia.org/wiki/Photodarkeninghttp://en.wikipedia.org/wiki/Photodarkeninghttp://en.wikipedia.org/wiki/Color_centerhttp://en.wikipedia.org/wiki/Color_centerhttp://en.wikipedia.org/wiki/Color_centerhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Density_of_stateshttp://en.wikipedia.org/wiki/Density_of_stateshttp://en.wikipedia.org/wiki/Density_of_stateshttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Disputed_statementhttp://en.wikipedia.org/wiki/Wikipedia:Disputed_statementhttp://en.wikipedia.org/wiki/Wikipedia:Disputed_statementhttp://en.wikipedia.org/wiki/Talk:Laser#Dubioushttp://en.wikipedia.org/wiki/Talk:Laser#Dubioushttp://en.wikipedia.org/wiki/Talk:Laser#Dubioushttp://en.wikipedia.org/wiki/Laser#cite_note-18http://en.wikipedia.org/wiki/Laser#cite_note-18http://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Laser#cite_note-18http://en.wikipedia.org/wiki/Talk:Laser#Dubioushttp://en.wikipedia.org/wiki/Wikipedia:Disputed_statementhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarifyhttp://en.wikipedia.org/wiki/Density_of_stateshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Color_centerhttp://en.wikipedia.org/wiki/Photodarkeninghttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Fiber_disk_laserhttp://en.wikipedia.org/wiki/Double-clad_fiberhttp://en.wikipedia.org/wiki/Ytterbiumhttp://en.wikipedia.org/wiki/Erbium


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A 5.6 mm 'closed can' commercial laser diode, probably from aCDorDVD player

Laser beams (red, green, violet)

Semiconductor lasers arediodeswhich are electrically pumped.

Recombination of electrons and holes created by the appliedcurrent introduces optical gain. Reflection from the ends of thecrystal form an optical resonator, although the resonator can beexternal to the semiconductor in some designs.

Commerciallaser diodesemit at wavelengths from 375 nm to1800 nm, and wavelengths of over 3 m have been demonstrated.Low to medium power laser diodes are used inlaser printersandCD/DVD players. Laser diodes are also frequently used to

opticallypumpother lasers with high efficiency. The highestpower industrial laser diodes, with power up to 10 kW(70dBm)

[citation needed], are used in industry for cutting and welding.External-cavity semiconductor lasers have a semiconductor activemedium in a larger cavity. These devices can generate high poweroutputs with good beam quality, wavelength-tunable narrow-linewidthradiation, or ultrashort laser pulses.
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Vertical cavity surface-emitting lasers (VCSELs) aresemiconductor lasers whose emission direction is perpendicular tothe surface of the wafer. VCSEL devices typically have a morecircular output beam than conventional laser diodes, andpotentially could be much cheaper to manufacture. As of 2005,only 850 nm VCSELs are widely available, with 1300 nmVCSELs beginning to be commercialized,

[20]and 1550 nm devices

an area of research.VECSELsare external-cavity VCSELs.Quantum cascade lasersare semiconductor lasers that have anactive transition between energy sub-bands of an electron in astructure containing severalquantum wells.

The development of asiliconlaser is important in the field ofoptical computing. Silicon is the material of choice forintegratedcircuits, and so electronic andsilicon photoniccomponents (suchasoptical interconnects) could be fabricated on the same chip.Unfortunately, silicon is a difficult lasing material to deal with,since it has certain properties which block lasing. However,recently teams have produced silicon lasers through methods suchas fabricating the lasing material from silicon and other

semiconductor materials, such asindium(III) phosphideorgallium(III) arsenide, materials which allow coherent light to beproduced from silicon. These are calledhybrid silicon laser.Another type is aRaman laser, which takes advantage ofRamanscatteringto produce a laser from materials such as silicon.

Dye lasers

Dye lasersuse an organic dye as the gain medium. The wide gainspectrum of available dyes, or mixtures of dyes, allows these lasersto be highly tunable, or to produce very short-duration pulses (onthe order ofa fewfemtoseconds). Although thesetunable lasersaremainly known in their liquid form, researchers have alsodemonstrated narrow-linewidth tunable emission in dispersiveoscillator configurations incorporating solid-state dye gainmedia.

[21]In their most prevalent form thesesolid state dye lasers

use dye-doped polymers as laser media.
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Free electron lasers

Free electron lasers, or FELs, generate coherent, high powerradiation, that is widely tunable, currently ranging in wavelength

from microwaves, throughterahertz radiationand infrared, to thevisible spectrum, to soft X-rays. They have the widest frequencyrange o

a laser is a device that emits light

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