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 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). 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 (CO2), around 10–20% nitrogen (N2), a few percent hydrogen (H2) and/or xenon (Xe), with the remainder being helium (He). A different mixture is used in a flow-through laser, where CO2 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. N2{v1(1)} and CO2{v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3 cm−1 when accounting for N2 anharmonicity, centrifugal distortion and vibro-rotational interaction, which is more than made up for by the Maxwell speed distribution of translational-mode energy), N2 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 CO2 then radiatively emits at either 10.6 μm by dropping to the {v1(1)} (symmetric-stretch) vibrational mode, or 9.6 μm by dropping to the {v20(2)} (bending) vibrational mode.