Carbon dioxide laser

Amplification

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.

Because the energy levels of molecular vibration and rotation are similar, the photons emitted due to transition between these levels have comparatively low energy, and longer wavelength, than visible and near-infrared light. The 9-12 μm wavelength of CO₂ lasers is useful because it falls into an important window for atmospheric transmission (up to 80% atmospheric transmission at this wavelength), and because many natural and synthetic materials have strong characteristic absorptions in this range.

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 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 (a partially reflecting mirror) at the output end.

The CO₂ laser can be constructed to have continuous wave (CW) powers between milliwatts (mW) and hundreds of kilowatts (kW). 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 of up to gigawatts (GW).

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. Prisms are not practical as tuning elements because most media that transmit in the mid-infrared absorb or scatter some of the light, so 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. The finest frequency selection may also be obtained through the use of an etalon. 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 to 1090 cm⁻¹. Such "line-tuneable" carbon dioxide lasers are principally of interest in research applications.

One facet of output wavelength is due to the particular isotopes contained in the carbon dioxide molecule. Since various combinations of isotopes can produce wavelengths from 8.98 to 10.2 μm, precision laser fabricators must take this factor into account when selecting the gas for their products. The table below illustrates the output range from nine possible isotope combinations:

Industrial (cutting and welding)

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, while lower power level lasers are used for engraving.

Medical (soft-tissue surgery)

Carbon dioxide lasers have become useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). CO₂ lasers may be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing bumps or podules. CO₂ lasers can be used to remove vocal fold lesions, such as vocal fold cysts. Researchers in Israel are experimenting with using CO₂ lasers to weld human tissue, as an alternative to traditional sutures.

The CO₂ laser remains the best surgical laser for the soft tissue where both cutting and hemostasis are achieved photo-thermally (radiantly). CO₂ lasers can be used in place of a scalpel for most procedures, and are even used in places a scalpel would not be used, in delicate areas where mechanical trauma could damage the surgical site. CO₂ lasers are the best suited for soft tissue procedures in human and animal specialties, as compared to other laser wavelengths. Advantages include less bleeding, shorter surgery time, less risk of infection, and less post-op swelling. Applications include gynecology, dentistry, oral and maxillofacial surgery, and many others.

Other

The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO₂ lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers.

Because the atmosphere is quite transparent to infrared light, CO₂ lasers are also used for military rangefinding using LIDAR techniques.

CO₂ lasers are used in spectroscopy and the Silex process to enrich uranium.

The Soviet Polyus was designed to use a megawatt carbon-dioxide laser as an in-orbit weapon to destroy SDI satellites.