CO2 lasers

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, and is still one of the most useful. 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 9.4 and 10.6 micrometers (μm).

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 the 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) (usually only used in a sealed tube), and the remainder of the gas mixture helium (He). The specific proportions vary according to the particular laser.

The population inversion in the laser is achieved by the following sequence: electron impact excites vibrational motion of the nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational levels are therefore metastable and relatively long-lived. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation. The nitrogen molecules are left in a lower excited state. Their transition to ground state takes place by collision with cold helium atoms. 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 container. 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.

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