Carbon dioxide laser - Definition

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.

Amplification

The active laser medium (laser gain/amplification medium) is a gas discharge, which is usually water cooled. The filling gas within the discharge tube consists primarily of:

• Carbon dioxide (CO₂) (around 10-20 %)
• Nitrogen (N₂) (around 10-20%)
• Hydrogen (H₂) (a few percent)
• 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.

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.

Applications

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.

External links

• Coherent, Inc., a typical vendor of CO₂ lasers (http://www.coherentlasers.com/)