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CO2 Lasers

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Definition: lasers based on a gas mixture in which light is amplified by carbon dioxide molecules

German: CO2-Laser, Kohlendioxidlaser

Category: lasers

How to cite the article; suggest additional literature

The CO2 laser (carbon dioxide laser) is a laser based on a gas mixture as the gain medium, which contains carbon dioxide (CO2), helium (He), nitrogen (N2), and possibly some hydrogen (H2), water vapor and/or xenon (Xe). Such a laser is electrically pumped via a gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or in the radio frequency (RF) domain. Nitrogen molecules are excited by the discharge into a metastable vibrational level and transfer their excitation energy to the CO2 molecules when colliding with them. Helium serves to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (formed in the discharge) to carbon dioxide.

CO2 laser

Figure 1: Schematic setup of a sealed-tube carbon dioxide laser. The gas tube has Brewster windows and is water-cooled.

CO2 lasers typically emit at a wavelength of 10.6 μm, but there are other lines in the region of 9–11 μm (particularly at 9.6 μm). In most cases, average powers are between some tens of watts and many kilowatts. The power conversion efficiency can be well above 10%, i.e., it is higher than for most lamp-pumped solid-state lasers, but lower than for many diode-pumped lasers.

Laser Types

The family of CO2 lasers is very diverse:

The concepts differ mainly in the technique of heat extraction, but also in the gas pressure and electrode geometry used. In low-power sealed-tube lasers (used e.g. for laser marking), waste heat is transported to the tube walls by diffusion or a slow gas flow. The beam quality can be very high. High-power CO2 lasers utilize a fast forced gas convection, which may be in the axial direction (i.e., along the beam direction) or in the transverse direction (for the highest powers).

Applications

CO2 lasers are widely used for material processing, in particular for

Other applications include laser surgery (including ophthalmology) and range finding.

CO2 lasers used for material processing (e.g. welding and cutting of metals, or laser marking) are in competition with solid-state lasers (particularly YAG lasers and fiber lasers) operating in the 1-μm wavelength regime. These shorter wavelengths have the advantages of more efficient absorption in a metallic workpiece, and the potential for beam delivery via fiber cables. (There are no optical fibers for high-power 10-μm laser beams.) The potentially smaller beam parameter product of 1-μm lasers can also be advantageous. However, the latter potential normally cannot be realized with high-power lamp-pumped lasers, and diode-pumped lasers tend to be more expensive. For these reasons, CO2 lasers are still widely used in the cutting and welding business, particularly for parts with a thickness greater than a few millimeters, and their sales make more than 10% of all global laser sales (as of 2013). This may to some extent change in the future due to the development of high-power thin-disk lasers and advanced fiber cables in combination with techniques which exploit the high beam quality of such lasers.

Due to their high powers and high drive voltages, CO2 lasers raise serious issues of laser safety. However, their long operation wavelength makes them relatively eye-safe at low intensities.

Bibliography

[1]C. K. N. Patel, “Continuous-wave laser action on vibrational–rotational transitions of CO2”, Phys. Rev. 136 (5A), A1187 (1964)
[2]C. K. N. Patel, “Interpretation of CO2 optical maser experiments”, Phys. Rev. Lett. 12 (21), 588 (1964)
[3]A. Robinson and D. Johnson, “A carbon dioxide laser bibliography, 1964–1969”, IEEE J. Quantum Electron. 6 (10), 590 (1970)
[4]P. T. Woods et al., “Stable single-frequency carbon dioxide lasers”, J. Phys. E: Sci. Instrum. 9, 395 (1976)
[5]A. L. S. Smith and J. Mellis, “Operating efficiencies in pulsed carbon dioxide lasers”, Appl. Phys. Lett. 41, 1037 (1982)
[6]K. M. Abramski et al., “Power scaling of large-area transverse radiofrequency discharge CO2 lasers”, Appl. Phys. Lett. 54, 1833 (1989)
[7]O. Svelto, Principles of Lasers, Plenum Press, New York (1998)

(Suggest additional literature!)

See also: lasers, laser marking, high-power lasers, solid-state lasers, YAG lasers

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