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Carbon dioxide laser
The carbon dioxide laser (CO2 laser) was one of the earliest
lasers to be developed, and is still one of the most useful.
The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers.
Construction
Because CO2 lasers operate in the infrared, special materials are necessary for their construction. Typically, the mirrors are made of either molybdenum or gold, while windows and lenses are made of either germanium or zinc selenide. For high power applications, gold and zinc selenide are preferred.
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 (usually a semi-reflective coated zinc selenide mirror) at the output end. The reflectivity of the output coupler is typically around 5-15%.
The CO2 laser can be constructed to have powers between milliwatts (mW) and gigawatts (GW). It is also very easy to actively Q-switch a CO2 laser by means of a rotating mirror, giving rise to Q-switched peak powers 100 to 1000 times higher than the equivalent continuous wave laser of any particular design.
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. Because transmissive materials in the infrared are rather lossy, 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. 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 - 1090 cm-1. Such "line-tuneable" carbon dioxide lasers are principally of interest in research applications.
Application
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. They also find use in medicine where, for example, they are used for laser surgery and skin resurfacing laser facelifts (which essentially consist of burning the skin to promote collagen formation).
Because the atmosphere is quite transparent to infrared light, CO2 lasers are also used for military rangefinding using LIDAR techniques.
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