Carbon-dioxide laser

Last updated
A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser. Carbon Dioxide Laser At The Laser Effects Test Facility.jpg
A test target bursts into flame upon irradiation by a continuous-wave kilowatt-level carbon-dioxide laser.

The carbon-dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed. It was invented by Kumar Patel of Bell Labs in 1964 [1] and is still one of the most useful types of laser. Carbon-dioxide lasers are the highest-power continuous-wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering on 9.6 and 10.6  micrometers (μm).

Contents

Amplification

The active laser medium (laser gain/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 (CO
2), around 10–20% nitrogen (N
2), a few percent hydrogen (H
2) and/or xenon (Xe), with the remainder being helium (He).[ citation needed ] A different mixture is used in a flow-through laser, where CO
2 is continuously pumped through it. The specific proportions vary according to the particular laser.

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 metastable and relatively long-lived. N
2{v1(1)} and CO
2{v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3 cm−1 when accounting for 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), N
2 collisionally de-excites by transferring its vibrational mode energy to the CO2 molecule, causing the carbon dioxide to excite to its {v3(1)} (asymmetric stretch) vibrational mode quantum state. The CO
2 then radiatively emits at either 10.6 μm [lower-roman 1] by dropping to the {v1(1)} (symmetric-stretch) vibrational mode, or 9.6 μm [lower-roman 1] 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 CO2 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 N
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 CO2 lasers is useful because it falls into an important 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. [3]

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

Construction

Because CO2 lasers operate in the infrared, special materials are necessary for their construction. Typically, the mirrors are 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 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 CO2 laser consists of a gas discharge (with a mix close to that specified above) with a total reflector at one end, and an output coupler (a partially reflecting mirror) at the output end. [4]

The CO2 laser can be constructed to have continuous wave (CW) powers between milliwatts (mW) and hundreds of kilowatts (kW). [5] It is also very easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers of up to gigawatts (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 the laser cavity. Prisms are not practical as tuning elements because most 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 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

A medical CO2 laser Sharplan 40C.jpg
A medical CO2 laser

Industrial (cutting and welding)

Because of the high power levels available (combined with reasonable cost for the laser), CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. [8] In selective laser sintering, CO2 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 surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). [9] CO2 lasers may be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps or podules. CO2 lasers can be used to remove vocal-fold lesions, [10] such as vocal-fold cysts. Researchers in Israel are experimenting with using CO2 lasers to weld human tissue, as an alternative to traditional sutures. [11]

The 10.6 μm CO2 laser remains the best surgical laser for the soft tissue where both cutting and hemostasis are achieved photo-thermally (radiantly). [12] [13] [14] [15] CO2 lasers can be used in place of a 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. CO2 lasers are the best suited for soft-tissue procedures in human and animal specialties, as compared to laser with other wavelengths. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others.

The CO2 laser at 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

The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 μm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. [17]

Because the atmosphere is quite transparent to infrared light, CO2 lasers are also used for military rangefinding using LIDAR techniques.

CO2 lasers are used in spectroscopy [18] and the Silex process to enrich uranium.

In semiconductor manufacturing, CO2 lasers are used for extreme ultraviolet generation.

The Soviet Polyus was designed to use a megawatt carbon-dioxide laser as an in-orbit weapon to destroy SDI satellites.

See also

Notes

  1. 1 2 The exact wavelength depends upon the isotopic composition of the CO
    2     molecule.

Related Research Articles

<span class="mw-page-title-main">Carbon dioxide</span> Chemical compound with formula CO₂

Carbon dioxide is a chemical compound with the chemical formula CO2. It is made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature, and as the source of available carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate, which causes ocean acidification as atmospheric CO2 levels increase.

<span class="mw-page-title-main">Infrared spectroscopy</span> Measurement of infrared radiations interaction with matter

Infrared spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. It can be used to characterize new materials or identify and verify known and unknown samples. The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer which produces an infrared spectrum. An IR spectrum can be visualized in a graph of infrared light absorbance on the vertical axis vs. frequency, wavenumber or wavelength on the horizontal axis. Typical units of wavenumber used in IR spectra are reciprocal centimeters, with the symbol cm−1. Units of IR wavelength are commonly given in micrometers, symbol μm, which are related to the wavenumber in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. Two-dimensional IR is also possible as discussed below.

<span class="mw-page-title-main">Laser</span> Device which emits light via optical amplification

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word laser is an anacronym that originated as an acronym for light amplification by stimulated emission of radiation. The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow.

<span class="mw-page-title-main">Laser construction</span>

A laser is constructed from three principal parts:

<span class="mw-page-title-main">Raman spectroscopy</span> Spectroscopic technique

Raman spectroscopy is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.

<span class="mw-page-title-main">Helium–neon laser</span> Type of gas laser

A helium–neon laser or He-Ne laser is a type of gas laser whose high energetic medium gain medium consists of a mixture of ratio of helium and neon at a total pressure of approximately 101325/760 pascals inside a small electrical discharge. The best-known and most widely used He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum.

The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics and photons. Photons play a central role in information technologies, such as fiber optics, the way electrons do in electronics.

<span class="mw-page-title-main">Raman scattering</span> Inelastic scattering of photons

Raman scattering or the Raman effect is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. This is called normal Stokes Raman scattering. The effect is exploited by chemists and physicists to gain information about materials for a variety of purposes by performing various forms of Raman spectroscopy. Many other variants of Raman spectroscopy allow rotational energy to be examined and electronic energy levels may be examined if an X-ray source is used in addition to other possibilities. More complex techniques involving pulsed lasers, multiple laser beams and so on are known.

<span class="mw-page-title-main">Gas laser</span> Laser in which electricity is discharged through gas

A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate on the principle of converting electrical energy to a laser light output. The first gas laser, the Helium–neon laser (HeNe), was co-invented by Iranian engineer and scientist Ali Javan and American physicist William R. Bennett, Jr., in 1960. It produced a coherent light beam in the infrared region of the spectrum at 1.15 micrometres.

<span class="mw-page-title-main">Nitrogen laser</span>

A nitrogen laser is a gas laser operating in the ultraviolet range using molecular nitrogen as its gain medium, pumped by an electrical discharge.

Molecular laser isotope separation (MLIS) is a method of isotope separation, where specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage over AVLIS is low energy consumption and use of uranium hexafluoride instead of vaporized uranium.

A chemical laser is a laser that obtains its energy from a chemical reaction. Chemical lasers can reach continuous wave output with power reaching to megawatt levels. They are used in industry for cutting and drilling.

Far-infrared laser or terahertz laser is a laser with output wavelength in between 30-1000 µm, in the far infrared or terahertz frequency band of the electromagnetic spectrum.

<span class="mw-page-title-main">Laser pumping</span> Powering mechanism for lasers

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. When for a period of time the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier. The pump power must be higher than the lasing threshold of the laser.

A TEA laser is a gas laser energized by a high voltage electrical discharge in a gas mixture generally at or above atmospheric pressure. The most common types are carbon dioxide lasers and excimer lasers, both used extensively in industry and research; less common are nitrogen lasers. The acronym "TEA" stands for Transversely Excited Atmospheric.

Separation of isotopes by laser excitation (SILEX) is a process under development to enrich uranium on an industrial scale for nuclear reactors. It is strongly suspected that it utilizes laser condensation repression to excite the uranium-235 isotope in uranium hexafluoride (UF6), allowing this lighter molecule to move more rapidly to the outer rim of a gaseous jet and resist condensing compared to the heavier, unexcited 238UF6. This differs greatly from previous methods of laser enrichment explored for their commercial prospects: one using atomic uranium (Atomic Vapor Laser Isotope Separation (AVLIS)) and another molecular method that uses lasers to dissociate a fluorine atom from 235UF6 (Molecular Laser Isotope Separation (MLIS)), allowing the enriched product to precipitate out as a solid.

<span class="mw-page-title-main">C. Kumar N. Patel</span> Indian electrical engineer

Chandra Kumar Naranbhai Patel (born 2 July 1938) is an electrical engineer. He developed the carbon dioxide laser in 1963; it is now widely used in industry for cutting and engraving a wide range of materials like plastic and wood. Because the atmosphere is quite transparent to infrared light, CO2 lasers are also used for military rangefinding using LIDAR techniques.

<span class="mw-page-title-main">Nondispersive infrared sensor</span> Infrared detector without diffracting element dispersing the IR wave lengths

A nondispersive infrared sensor is a simple spectroscopic sensor often used as a gas detector. It is non-dispersive in the fact that no dispersive element is used to separate out the broadband light into a narrow spectrum suitable for gas sensing. The majority of NDIR sensors use a broadband lamp source and an optical filter to select a narrow band spectral region that overlaps with the absorption region of the gas of interest. In this context narrow may be 50-300nm bandwidth. Modern NDIR sensors may use Microelectromechanical systems (MEMs) or mid IR LED sources, with or without an optical filter.

A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 1013 Hz to approximately 1014 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm−1 and wavelengths of approximately 30 to 3 µm.

<span class="mw-page-title-main">Electromagnetic absorption by water</span>

The absorption of electromagnetic radiation by water depends on the state of the water.

References

  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. 1 2 Yong Zhang and Tim Killeen, Gas Lasers: CO2 Lasers - progressing from a varied past to an application-specific future, LaserFocusWorld (4 November 2016)
  4. ↑ "Output Couplers". ophiropt.com. Ophir Optronics Solutions Ltd. Retrieved 17 February 2014.
  5. ↑ "Carbon-Based Curtain Absorbs Stray Laser Light". Tech Briefs Media Labs. 30 November 2007. Retrieved 17 February 2014.
  6. ↑ Carbon Dioxide Amplifier at Brookhaven 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 CO2 laser". Journal of Micromechanics and Microengineering . 21 (2): 025004. Bibcode:2011JMiMi..21b5004A. doi:10.1088/0960-1317/21/2/025004. S2CID   137296053.
  9. ↑ Barton, Fritz (2014). "Skin Resurfacing". In Charles Thorne (ed.). Grabb and Smith's Plastic Surgery (7 ed.). Philadelphia: Lippincott Williams & Wilkins. p. 455. ISBN   978-1-4511-0955-9. 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 CO2 Laser for Limited Vocal Fold Benign Lesions: A Prospective Randomized Trial". The Laryngoscope. 110 (S92): 1–17. doi:10.1097/00005537-200002001-00001. ISSN   1531-4995. PMID   10678578. S2CID   46081244.
  11. ↑ "Israeli researchers pioneer laser treatment for sealing wounds". Israel21c. 16 November 2008. Archived from the original on 28 July 2009. Retrieved 8 March 2009.
  12. ↑ Vogel, A.; Venugopalan, V. (2003). "Mechanisms of pulsed laser ablation of biological tissues". Chem. Rev. 103 (2): 577–644. doi:10.1021/cr010379n. PMID   12580643.
  13. ↑ Vitruk, Peter (2014). "Oral soft tissue laser ablative and coagulative efficiencies spectra". Implant Practice US. 6 (7): 22–27. Retrieved 15 May 2015.
  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 CO2 Dental Laser" (PDF). Scientific Review. J Laser Dent. 1 (22): 10–27.
  16. ↑ "Laser Surgery Basics". American Laser Study Club. Retrieved 4 May 2018.
  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. PMID   15100818 . Retrieved 21 October 2009.
  18. ↑ C. P. Bewick, A. B. Duval, and B. J. Orr, Rotationally selective mode-to-mode vibrational energy transfer in D2CO/D2CO and D2CO/Ar collisions, J. Chem Phys.82, 3470 (1985).

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.