The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed

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C
– laser 
1. Introduction 
The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed 
(invented by Kumar Patel of Bell Labs in 1964[1]), 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 around 9.4 and 10.6 micrometers. 
2. Amplification 
The active laser medium is a gas discharge which is air-cooled (water-cooled in higher 
power applications). The filling gas within the discharge tube consists primarily of: 
o
Carbon dioxide (CO2) (around 10–20%) 
o
Nitrogen (N2) (around 10–20%) 
o
Hydrogen (H2) and/or xenon (Xe) (a few percent; usually only used in a sealed 
tube.) 
o
Helium (He) (The remainder of the gas mixture) 
The specific proportions vary according to the particular laser. 
The population inversion in the laser is achieved by the following sequence: 
1) 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 live for a long time. 
2) 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. 
3) 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. 
3. 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 even 
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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 (usually a semi-
reflective coated zinc selenide mirror) at the output end. The reflectivity of the output 
coupler is typically around 5–15%. The laser output may also be edge-coupled in higher 
power systems to reduce optical heating problems. 
The CO2 laser can be constructed to have CW powers between milliwatts (mW) and 
hundreds of kilowatts (kW).[2] 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 up to gigawatts (GW) of peak power. 
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.