View source for Carbon-dioxide laser

{{Short description|Form of gas laser}}
[[Image:Carbon Dioxide Laser At The Laser Effects Test Facility.jpg|thumb|right|400px|A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser.]]
The '''carbon-dioxide laser''' ('''CO<sub>2</sub> laser''') was one of the earliest [[gas laser]]s to be developed. It was invented by [[C. Kumar N. Patel|Kumar Patel]] of [[Bell Labs]] in 1964<ref>{{cite journal |last=Patel |first=C. K. N. |author-link=C. Kumar N. Patel |year=1964 |title=Continuous-Wave Laser Action on Vibrational-Rotational Transitions of CO<sub>2</sub> |journal=[[Physical Review]] |volume=136 |issue=5A |pages=A1187–A1193 |bibcode=1964PhRv..136.1187P |doi=10.1103/physrev.136.a1187 |doi-access=free}}</ref> and is still one of the most useful types of laser. [[Carbon dioxide|Carbon-dioxide]] lasers are the highest-power [[continuous-wave laser]]s that are currently available. They are also quite efficient: the ratio of output power to [[Laser pumping|pump]] power can be as large as 20%.
The CO<sub>2</sub> laser produces a beam of [[infrared light]] with the principal [[wavelength]] bands centering on 9.6 and 10.6&nbsp;[[Micrometre|micrometers]] (μm).

==Amplification==
The [[active laser medium]] (laser gain/[[Amplifier|amplification]] medium) is a [[gas discharge]] which 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]] ({{chem|CO|2}}), around 10–20% [[nitrogen]] ({{chem|N|2}}), a few percent [[hydrogen]] ({{chem|H|2}}) and/or [[xenon]] (Xe), with  the remainder being [[helium]] (He).{{citation needed|date=December 2015}} A different mixture is used in a ''flow-through'' laser, where {{chem|CO|2}} is continuously pumped through it. The specific proportions vary according to the particular laser.

{{Too technical|date=November 2024|section}}

The [[population inversion]] in the laser is achieved by the following sequence: [[electron]] impact excites the {v1(1)} quantum [[vibrational modes]] of nitrogen. Because nitrogen is a [[homonuclear molecule]], it cannot lose this energy by [[photon]] emission, and its excited vibrational modes are therefore [[metastability|metastable]] and relatively long-lived. {{chem|N|2}}{v1(1)} and {{chem|CO|2}}{v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3&nbsp;cm<sup>−1</sup> when accounting for {{chem|N|2}} anharmonicity, centrifugal distortion and vibro-rotational interaction, which is more than made up for by the [[Maxwell speed distribution]] of translational-mode energy), {{chem|N|2}} collisionally de-excites by transferring its vibrational mode energy to the CO<sub>2</sub> molecule, causing the carbon dioxide to excite to its {v3(1)} (asymmetric stretch) vibrational mode quantum state. The {{chem|CO|2}} then radiatively emits at either 10.6&nbsp;μm{{efn-lr|name=isotopic|The exact wavelength depends upon the isotopic composition of the {{chem|CO|2}} molecule.}} by dropping to the {v1(1)} (symmetric-stretch) vibrational mode, or 9.6&nbsp;μm{{efn-lr|name=isotopic}} 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 CO<sub>2</sub> and 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 of {{chem|N|2}}, due to a near-resonant dissociation reaction with metastable He(2<sup>3</sup>S<sub>1</sub>). Substituting helium with other noble gases, such as neon or argon, does not lead to an enhancement of laser output.<ref>{{cite journal|last=Patel|first=C.K.N. |display-authors=etal |year=1965|title=CW High-Power CO<sub>2</sub>-N<sub>2</sub>-He Laser|journal=[[Applied Physics Letters]]|volume=7|issue=11|page=290|doi=10.1063/1.1754264|bibcode=1965ApPhL...7..290P |doi-access=free}}</ref>

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&nbsp;μm wavelength of CO<sub>2</sub> lasers is useful because it falls into an important [[Infrared window|window 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.<ref name="laserworld">[http://www.laserfocusworld.com/articles/print/volume-52/issue-11/features/gas-lasers-co2-lasers-progressing-from-a-varied-past-to-an-application-specific-future.html] Yong Zhang and Tim Killeen, ''Gas Lasers: CO<sub>2</sub> Lasers - progressing from a varied past to an application-specific future'', LaserFocusWorld (4 November 2016)</ref>

The laser wavelength can be tuned by altering the isotopic ratio of the carbon and oxygen atoms comprising the {{chem|CO|2}} molecules in the discharge tube.

==Construction==
{{see also|Laser construction}}
Because CO<sub>2</sub> lasers operate in the infrared, special materials are necessary for their construction. Typically, the [[mirror]]s are [[silvering|silvered]], while windows and lenses are made of either [[germanium]] or [[zinc selenide]]. For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. There are also [[diamond]] windows and lenses in use. Diamond windows are extremely expensive, but their high [[thermal conductivity]] and hardness make them useful in high-power applications and in dirty environments. Optical elements made of diamond can even be [[sand blasting|sand blasted]] without losing their optical properties. Historically, lenses and windows were made out of salt (either [[sodium chloride]] or [[potassium chloride]]). While the material was inexpensive, the lenses and windows degraded slowly with exposure to atmospheric moisture.

The most basic form of a CO<sub>2</sub> laser consists of a gas discharge (with a mix close to that specified above) with a total [[Reflection (physics)|reflector]] at one end, and an [[output coupler]] (a partially reflecting mirror) at the output end.<ref>{{cite web |title=Output Couplers |url=http://www.ophiropt.com/co2-lasers-optics/cavity-optics/partial |work=ophiropt.com |publisher=Ophir Optronics Solutions Ltd. |access-date=17 February 2014}}</ref>

The CO<sub>2</sub> laser can be constructed to have continuous wave (CW) powers between [[milliwatt]]s (mW) and hundreds of [[kilowatt]]s (kW).<ref>{{cite web |title=Carbon-Based Curtain Absorbs Stray Laser Light |url=http://www.defensetechbriefs.com/component/content/article/940-dtb/features/tech-transfer-reports/4738 |publisher=Tech Briefs Media Labs |access-date=17 February 2014 |date=30 November 2007}}</ref> It is also very easy to actively [[Q-switching|Q-switch]] a CO<sub>2</sub> laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers of up to [[gigawatt]]s (GW).<ref>[https://web.archive.org/web/20061001020521/http://www.bnl.gov/atf/core_capabilities/co2amp.asp Carbon Dioxide Amplifier] at [[Brookhaven National Lab]].</ref>

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 the [[laser cavity]]. [[Dispersive prism|Prism]]s are not practical as tuning elements because most [[medium (optics)|media]] that transmit in the [[mid-infrared]] absorb or scatter some of the light, so the [[frequency]] tuning element is almost always a [[diffraction 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 an [[etalon]]. In practice, together with [[isotopic substitution]], this means that a continuous comb of frequencies separated by around 1&nbsp;cm<sup>−1</sup> (30&nbsp;GHz) can be used that extend from 880 to 1090&nbsp;cm<sup>−1</sup>. Such "line-tuneable" carbon-dioxide lasers<ref>[[F. J. Duarte]] (ed.), ''Tunable Lasers Handbook'' (Academic, New York, 1995) Chapter&nbsp;4.</ref> 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.<ref name=laserworld/>

==Applications==
[[Image:Sharplan 40C.jpg|thumb|right|A medical CO<sub>2</sub> laser]]

===Industrial (cutting and welding)===
Because of the high power levels available (combined with reasonable cost for the laser), CO<sub>2</sub> lasers are frequently used in industrial applications for [[laser cutting|cutting]] and [[laser beam welding|welding]], while lower power level lasers are used for engraving.<ref>{{cite journal|last=Andreeta|first=M. R. B. |display-authors=etal |year=2011|title=Bidimensional codes recorded on an oxide glass surface using a continuous wave CO<sub>2</sub> laser|journal=[[Journal of Micromechanics and Microengineering]]|volume=21|issue=2|article-number=025004|doi=10.1088/0960-1317/21/2/025004|bibcode=2011JMiMi..21b5004A|s2cid=137296053 }}</ref> In [[selective laser sintering]], CO<sub>2</sub> lasers are used to fuse particles of plastic powder into parts.

===Medical (soft-tissue surgery)===
Carbon-dioxide lasers have become useful in surgical procedures because water (which makes up most [[biological tissue]]) absorbs this frequency of light very well. Some examples of medical uses are [[Laser applications#Medical|laser surgery]] and [[Laser resurfacing|skin resurfacing]] ("laser [[facelift]]s", which essentially consist of vaporizing the skin to promote collagen formation).<ref>{{cite book |last=Barton |first=Fritz |title=Grabb and Smith's Plastic Surgery |date=2014 |publisher=Lippincott Williams & Wilkins |location=Philadelphia |isbn=978-1-4511-0955-9 |page=455 |edition=7 |editor=Charles Thorne |chapter=Skin Resurfacing |quote=For practical purposes, there are three methods of resurfacing: mechanical sanding (dermabrasion), chemical burn (chemical peels), and photodynamic treatments (laser ablation or coagulation).}}</ref> CO<sub>2</sub> lasers may be used to treat certain skin conditions such as [[hirsuties papillaris genitalis]] by removing bumps or podules. CO<sub>2</sub> lasers can be used to remove vocal-fold lesions,<ref>{{Cite journal |last=Benninger |first=Michael S. |date=2000 |title=Microdissection or Microspot CO<sub>2</sub> Laser for Limited Vocal Fold Benign Lesions: A Prospective Randomized Trial |journal=The Laryngoscope |volume=110 |issue=S92 |pages=1–17 |doi=10.1097/00005537-200002001-00001 |pmid=10678578 |s2cid=46081244 |issn=1531-4995 |doi-access=}}</ref> such as [[vocal-fold cyst]]s. Researchers in Israel are experimenting with using CO<sub>2</sub> lasers to weld human tissue, as an alternative to traditional [[Surgical suture|sutures]].<ref>{{cite web |url=http://www.israel21c.org/bin/en.jsp?enDispWho=Articles%5El2338&enPage=BlankPage&enDisplay=view&enDispWhat=object&enVersion=0&enZone=Health |title=Israeli researchers pioneer laser treatment for sealing wounds |work=Israel21c |date=16 November 2008 |access-date=8 March 2009 |archive-url=https://web.archive.org/web/20090728021643/http://www.israel21c.org/bin/en.jsp?enDispWho=Articles%5El2338&enPage=BlankPage&enDisplay=view&enDispWhat=object&enVersion=0&enZone=Health |archive-date=28 July 2009 }}</ref>

The 10.6&nbsp;μm CO<sub>2</sub> laser remains the best [[Laser surgery|surgical laser]] for the soft tissue where both cutting and [[hemostasis]] are achieved photo-thermally (radiantly).<ref>{{Cite journal |last1=Vogel |first1=A. |last2=Venugopalan |first2=V. |date=2003 |title=Mechanisms of pulsed laser ablation of biological tissues |pmid=12580643 |journal=Chem. Rev. |volume=103 |issue=2 |pages=577–644 |doi=10.1021/cr010379n |url=http://www.escholarship.org/uc/item/5qb7v6qb}}</ref><ref>{{Cite journal |last=Vitruk |first=Peter |date=2014 |title=Oral soft tissue laser ablative and coagulative efficiencies spectra |url=https://lightscalpel.com/publications/oral-soft-tissue-laser-ablative-and-coagulative-efficiencies-spectra/ |journal=Implant Practice US |volume=6 |issue=7 |pages=22–27 |access-date=15 May 2015}}</ref><ref>{{Cite journal |last=Fisher |first=J. C. |date=1993 |title=Qualitative and quantitative tissue effects of light from important surgical lasers |journal=Laser Surgery in Gynecology: A Clinical Guide |pages=58–81}}</ref><ref>{{Cite journal |last1=Fantarella |first1=D. |last2=Kotlow |first2=L. |date=2014 |title=The 9.3&nbsp;μm CO<sub>2</sub> Dental Laser |url=https://www.laserdentistry.org/uploads/files/members/jld/JLD_22_1/JLD_22_1_PG_10-27_ScientificReview.pdf |journal=Scientific Review. J Laser Dent. |volume=1 |issue=22 |pages=10–27 |archive-date=5 February 2015 |access-date=18 March 2016 |archive-url=https://web.archive.org/web/20150205114345/https://www.laserdentistry.org/uploads/files/members/jld/JLD_22_1/JLD_22_1_PG_10-27_ScientificReview.pdf |url-status=dead }}</ref> CO<sub>2</sub> lasers can be used in place of a [[Laser scalpel|scalpel]] for 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. CO<sub>2</sub> lasers are the best suited for [[soft-tissue]] procedures in human and animal specialties, as compared to laser with other [[wavelength]]s. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include [[Gynaecology|gynecology]], [[dentistry]], [[oral and maxillofacial surgery]], and many others.

A CO<sub>2</sub> [[dental laser]] at the 9.25–9.6&nbsp;μm wavelength is sometimes used in dentistry for hard-tissue ablation. The hard-tissue is ablated at temperatures as high as 5,000&nbsp;°C, producing bright thermal radiation.<ref>{{Cite news |url=https://www.americanlaserstudyclub.org/laser-surgery-education/ |title=Laser Surgery Basics |work=American Laser Study Club |access-date=2018-05-04 |language=en-US}}</ref>

===Other===
The common plastic [[poly (methyl methacrylate)]] (PMMA) absorbs IR light in the 2.8–25&nbsp;μm wavelength band, so CO<sub>2</sub> lasers have been used in recent years for fabricating [[microfluidic device]]s from it, with channel widths of a few hundred micrometers.<ref>{{cite journal|url=http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b206409j|title=CO<sub>2</sub>-laser micromachining and back-end processing for rapid production of PMMA-based microfluidic systems|year=2002|doi=10.1039/B206409J|access-date=21 October 2009|last1=Klank|first1=Henning|last2=Kutter|first2=Jörg P.|last3=Geschke|first3=Oliver|journal=Lab on a Chip|volume=2|issue=4|pages=242–246|pmid=15100818|url-access=subscription}}</ref>

Because the [[Earth's atmosphere|atmosphere]] is quite transparent to infrared light, CO<sub>2</sub> lasers are also used for military [[Rangefinder|rangefinding]] using [[LIDAR]] techniques.

CO<sub>2</sub> lasers are used in [[spectroscopy]]<ref>C. P. Bewick, A. B. Duval, and [[Brian Orr|B. J. Orr]], Rotationally selective mode-to-mode vibrational energy transfer in D<sub>2</sub>CO/D<sub>2</sub>CO and D<sub>2</sub>CO/Ar collisions, ''J. Chem Phys.'' '''82''', 3470 (1985).</ref> and the [[Silex process]] to [[Enrichment of uranium|enrich]] uranium.

In semiconductor manufacturing, CO<sub>2</sub> lasers are used for [[extreme ultraviolet]] generation.

The Soviet [[Polyus (spacecraft)|Polyus]] was designed to use a megawatt carbon-dioxide laser as an in-orbit weapon to destroy [[Strategic Defense Initiative|SDI satellites]].

==See also==
* [[Beam homogenizer]]
* [[Laser beam profiler]]
* [[TEA laser]]

==Notes==
{{notelist-lr}}

==References==
{{reflist}}

==External links==
* [http://www.repairfaq.org/sam/lasercc2.htm Home-built Carbon dioxide laser]
{{use dmy dates|date=December 2016}}
{{Gas lasers}}

{{DEFAULTSORT:Carbon Dioxide Laser}}
[[Category:American inventions]]
[[Category:Gas lasers]]
[[Category:Dental lasers]]
[[Category:Applications of carbon dioxide]]