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).
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 a sealed discharge tube consists of around 10–20% carbon dioxide (), around 10–20% nitrogen (), a few percent hydrogen () and/or xenon (Xe), with the remainder being helium (He). A different mixture is used in a flow-through laser, where 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. { v1(1)} and { v3(1)} being nearly perfectly resonant (total molecular energy differential is within 3 cm−1 when accounting for anharmonicity, centrifugal distortion and vibro-rotational interaction, which is more than made up for by the Maxwell speed distribution of translational-mode energy), 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 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. The carbon dioxide molecules then transition to their { v20(0)} vibrational mode ground state from { v1(1)} or { v20(2)} by collision with cold helium atoms, thus maintaining population inversion. 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 laser discharge tube. 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.
The addition of helium also plays a role in the initial vibrational excitation of , due to a near-resonant dissociation reaction with metastable He(23S1). Substituting helium with other noble gases, such as neon or argon, does not lead to an enhancement of laser output.
Because the excitation energy of molecular vibrational and rotational mode quantum states are low, the photons emitted due to transition between these quantum states have comparatively lower 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 absorption in this range.
The laser wavelength can be tuned by altering the isotopic ratio of the carbon and oxygen atoms comprising the molecules in the discharge tube.
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.