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Carbon-dioxide laser

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A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser.

Thecarbon-dioxide laser(CO2laser) was one of the earliestgas lasersto be developed. It was invented byKumar PatelofBell Labsin 1964[1]and is still one of the most useful types of laser.Carbon-dioxidelasers are the highest-powercontinuous-wave lasersthat are currently available. They are also quite efficient: the ratio of output power topumppower can be as large as 20%. The CO2laser produces a beam ofinfrared lightwith the principalwavelengthbands centering on 9.6 and 10.6micrometers(μm).

Amplification

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Theactive laser medium(laser gain/amplificationmedium) is agas dischargewhich is air- or water-cooled, depending on the power being applied. The filling gas within a sealed discharge tube consists of around 10–20%carbon dioxide(CO
2), around 10–20%nitrogen(N
2), a few percenthydrogen(H
2) and/orxenon(Xe), with the remainder beinghelium(He).[citation needed]A different mixture is used in aflow-throughlaser, whereCO
2is continuously pumped through it. The specific proportions vary according to the particular laser.

Thepopulation inversionin the laser is achieved by the following sequence:electronimpact excites the {v1(1)} quantumvibrational modesof nitrogen. Because nitrogen is ahomonuclear molecule,it cannot lose this energy byphotonemission, and its excited vibrational modes are thereforemetastableand relatively long-lived.N
2{v1(1)} andCO
2{v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3 cm−1when accounting forN
2anharmonicity, centrifugal distortion and vibro-rotational interaction, which is more than made up for by theMaxwell speed distributionof translational-mode energy),N
2collisionally de-excites by transferring its vibrational mode energy to the CO2molecule, causing the carbon dioxide to excite to its {v3(1)} (asymmetric stretch) vibrational mode quantum state. TheCO
2then radiatively emits at either 10.6 μm[i]by dropping to the {v1(1)} (symmetric-stretch) vibrational mode, or 9.6 μm[i]by dropping to the {v20(2)} (bending) vibrational mode. The carbon dioxide molecules then transition to their {v20(0)} vibrational mode ground state from {v1(1)} or {v20(2)} by collision with cold helium atoms, thus maintaining population inversion. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the laser discharge tube. In flow-through lasers, a continuous stream of CO2and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.

The addition of helium also plays a role in the initial vibrational excitation ofN
2,due to a near-resonant dissociation reaction with metastable He(23S1). Substituting helium with other noble gases, such as neon or argon, does not lead to an enhancement of laser output.[2]

Because the excitation energy of molecular vibrational and rotational mode quantum states are low, the photons emitted due to transition between these quantum states have comparatively lower energy, and longer wavelength, than visible and near-infrared light. The 9–12 μm wavelength of CO2lasers is useful because it falls into an importantwindow for atmospheric transmission(up to 80% atmospheric transmission at this wavelength), and because many natural and synthetic materials have strong characteristic absorption in this range.[3]

The laser wavelength can be tuned by altering the isotopic ratio of the carbon and oxygen atoms comprising theCO
2molecules in the discharge tube.

Construction

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See also:Laser construction

Because CO2lasers operate in the infrared, special materials are necessary for their construction. Typically, themirrorsaresilvered,while windows and lenses are made of eithergermaniumorzinc selenide.For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. There are alsodiamondwindows and lenses in use. Diamond windows are extremely expensive, but their highthermal conductivityand hardness make them useful in high-power applications and in dirty environments. Optical elements made of diamond can even besand blastedwithout losing their optical properties. Historically, lenses and windows were made out of salt (eithersodium chlorideorpotassium chloride). While the material was inexpensive, the lenses and windows degraded slowly with exposure to atmospheric moisture.

The most basic form of a CO2laser consists of a gas discharge (with a mix close to that specified above) with a totalreflectorat one end, and anoutput coupler(a partially reflecting mirror) at the output end.[4]

The CO2laser can be constructed to have continuous wave (CW) powers betweenmilliwatts(mW) and hundreds ofkilowatts(kW).[5]It is also very easy to activelyQ-switcha CO2laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers of up togigawatts(GW).[6]

Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule, the rotational structure of the P and R bands can be selected by a tuning element in thelaser cavity.Prismsare not practical as tuning elements because mostmediathat transmit in themid-infraredabsorb or scatter some of the light, so thefrequencytuning element is almost always adiffraction grating.By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected. The finest frequency selection may also be obtained through the use of anetalon.In practice, together withisotopic substitution,this means that a continuous comb of frequencies separated by around 1 cm−1(30 GHz) can be used that extend from 880 to 1090 cm−1.Such "line-tuneable" carbon-dioxide lasers[7]are principally of interest in research applications. The laser's output wavelength is affected by the particular isotopes contained in the carbon dioxide molecule, with heavier isotopes causing longer wavelength emission.[3]

Applications

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A medical CO2laser

Industrial (cutting and welding)

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Because of the high power levels available (combined with reasonable cost for the laser), CO2lasers are frequently used in industrial applications forcuttingandwelding,while lower power level lasers are used for engraving.[8]Inselective laser sintering,CO2lasers are used to fuse particles of plastic powder into parts.

Medical (soft-tissue surgery)

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Carbon-dioxide lasers have become useful in surgical procedures because water (which makes up mostbiological tissue) absorbs this frequency of light very well. Some examples of medical uses arelaser surgeryandskin resurfacing( "laserfacelifts",which essentially consist of vaporizing the skin to promote collagen formation).[9]CO2lasers may be used to treat certain skin conditions such ashirsuties papillaris genitalisby removing bumps or podules. CO2lasers can be used to remove vocal-fold lesions,[10]such asvocal-fold cysts.Researchers in Israel are experimenting with using CO2lasers to weld human tissue, as an alternative to traditionalsutures.[11]

The 10.6 μm CO2laser remains the bestsurgical laserfor the soft tissue where both cutting andhemostasisare achieved photo-thermally (radiantly).[12][13][14][15]CO2lasers can be used in place of ascalpelfor most procedures and are even used in places a scalpel would not be used, in delicate areas where mechanical trauma could damage the surgical site. CO2lasers are the best suited forsoft-tissueprocedures in human and animal specialties, as compared to laser with otherwavelengths.Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications includegynecology,dentistry,oral and maxillofacial surgery,and many others.

A CO2dental laserat the 9.25–9.6 μm wavelength is sometimes used in dentistry for hard-tissue ablation. The hard-tissue is ablated at temperatures as high as 5,000 °C, producing bright thermal radiation.[16]

Other

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The common plasticpoly (methyl methacrylate)(PMMA) absorbs IR light in the 2.8–25 μm wavelength band, so CO2lasers have been used in recent years for fabricatingmicrofluidic devicesfrom it, with channel widths of a few hundred micrometers.[17]

Because theatmosphereis quite transparent to infrared light, CO2lasers are also used for militaryrangefindingusingLIDARtechniques.

CO2lasers are used inspectroscopy[18]and theSilex processtoenrichuranium.

In semiconductor manufacturing, CO2lasers are used forextreme ultravioletgeneration.

The SovietPolyuswas designed to use a megawatt carbon-dioxide laser as an in-orbit weapon to destroySDI satellites.

See also

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Notes

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  1. ^abThe exact wavelength depends upon the isotopic composition of theCO
    2    molecule.

References

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  1. ^Patel, C. K. N.(1964)."Continuous-Wave Laser Action on Vibrational-Rotational Transitions of CO2".Physical Review.136(5A): A1187–A1193.Bibcode:1964PhRv..136.1187P.doi:10.1103/physrev.136.a1187.
  2. ^Patel, C.K.N.; et al. (1965)."CW High-Power CO2-N2-He Laser ".Applied Physics Letters.7(11): 290.Bibcode:1965ApPhL...7..290P.doi:10.1063/1.1754264.
  3. ^ab[1]Yong Zhang and Tim Killeen,Gas Lasers: CO2Lasers - progressing from a varied past to an application-specific future,LaserFocusWorld (4 November 2016)
  4. ^"Output Couplers".ophiropt.Ophir Optronics Solutions Ltd.Retrieved17 February2014.
  5. ^"Carbon-Based Curtain Absorbs Stray Laser Light".Tech Briefs Media Labs. 30 November 2007.Retrieved17 February2014.
  6. ^Carbon Dioxide AmplifieratBrookhaven National Lab.
  7. ^F. J. Duarte(ed.),Tunable Lasers Handbook(Academic, New York, 1995) Chapter 4.
  8. ^Andreeta, M. R. B.; et al. (2011). "Bidimensional codes recorded on an oxide glass surface using a continuous wave CO2laser ".Journal of Micromechanics and Microengineering.21(2): 025004.Bibcode:2011JMiMi..21b5004A.doi:10.1088/0960-1317/21/2/025004.S2CID137296053.
  9. ^Barton, Fritz (2014). "Skin Resurfacing". In Charles Thorne (ed.).Grabb and Smith's Plastic Surgery(7 ed.). Philadelphia: Lippincott Williams & Wilkins. p. 455.ISBN978-1-4511-0955-9.For practical purposes, there are three methods of resurfacing: mechanical sanding (dermabrasion), chemical burn (chemical peels), and photodynamic treatments (laser ablation or coagulation).
  10. ^Benninger, Michael S. (2000). "Microdissection or Microspot CO2Laser for Limited Vocal Fold Benign Lesions: A Prospective Randomized Trial ".The Laryngoscope.110(S92): 1–17.doi:10.1097/00005537-200002001-00001.ISSN1531-4995.PMID10678578.S2CID46081244.
  11. ^"Israeli researchers pioneer laser treatment for sealing wounds".Israel21c.16 November 2008. Archived fromthe originalon 28 July 2009.Retrieved8 March2009.
  12. ^Vogel, A.; Venugopalan, V. (2003)."Mechanisms of pulsed laser ablation of biological tissues".Chem. Rev.103(2): 577–644.doi:10.1021/cr010379n.PMID12580643.
  13. ^Vitruk, Peter (2014)."Oral soft tissue laser ablative and coagulative efficiencies spectra".Implant Practice US.6(7): 22–27.Retrieved15 May2015.
  14. ^Fisher, J. C. (1993). "Qualitative and quantitative tissue effects of light from important surgical lasers".Laser Surgery in Gynecology: A Clinical Guide:58–81.
  15. ^Fantarella, D.; Kotlow, L. (2014)."The 9.3 μm CO2Dental Laser "(PDF).Scientific Review. J Laser Dent.1(22): 10–27.
  16. ^"Laser Surgery Basics".American Laser Study Club.Retrieved4 May2018.
  17. ^Klank, Henning; Kutter, Jörg P.; Geschke, Oliver (2002)."CO2-laser micromachining and back-end processing for rapid production of PMMA-based microfluidic systems ".Lab on a Chip.2(4): 242–246.doi:10.1039/B206409J.PMID15100818.Retrieved21 October2009.
  18. ^C. P. Bewick, A. B. Duval, andB. J. Orr,Rotationally selective mode-to-mode vibrational energy transfer in D2CO/D2CO and D2CO/Ar collisions,J. Chem Phys.82,3470 (1985).
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