Carbon-dioxide laser

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).

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.

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 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 N2, due to a near-resonant dissociation reaction with metastable He(23S1).

[3] The laser wavelength can be tuned by altering the isotopic ratio of the carbon and oxygen atoms comprising the CO2 molecules in the discharge tube.

Diamond windows are extremely expensive, but their high thermal conductivity and hardness make them useful in high-power applications and in dirty environments.

[4] The CO2 laser can be constructed to have continuous wave (CW) powers between milliwatts (mW) and hundreds of kilowatts (kW).

[16] 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.