= Carbon dioxide laser
= CO2 laser
{c}
{synonym}
A \b[carbon dioxide laser], or <CO2 laser>, uses excited <carbon dioxide> molecules as its <laser gain medium>. Its characteristic emission lies in the <infrared>, with important lines near $10.6\,\mathrm{\mu m}$. The emitting transitions change the molecule's vibrational and rotational states. This connects <laser> action with <molecular vibration> and <molecular rotation>, rather than with a requirement to break the molecule into its constituent atoms.
Energy supplied to the gas creates an appropriate excited-state distribution. <Stimulated emission> amplifies light on a selected transition; an <optical cavity> returns it through the medium, and a partly transmitting output element extracts useful <radiant power>. The <laser threshold> depends on complete round-trip gain and loss. The presence of carbon dioxide alone does not establish <population inversion> or select a unique oscillating line.
A primary experimental landmark is <C. K. N. Patel>'s 1964 report from Bell Telephone Laboratories. It identified continuous-wave <laser> action on several rotational lines of two vibrational bands, with the strongest reported vacuum wavelength $10.6324\,\mathrm{\mu m}$ and continuous output around $1\,\mathrm{mW}$. The same transitions were observed under pulsed-discharge excitation, and time-dependent output was studied. These are properties of that reported experiment, not universal power or wavelength limits for every <CO2 laser>.<Continuous-Wave Laser Action on Vibrational-Rotational Transitions of CO2 by C. K. N. Patel>[Publisher byline and abstract, Physical Review136 (1964), pp.A1187–A1193]{ref}
The molecular line structure explains why a quoted wavelength is only part of the specification. Several allowed transitions can supply gain, while the <optical cavity> favours its own <normal modes>. A device need not emit every line of a <rovibrational spectrum>, nor is its line selected merely by saying that the medium contains carbon dioxide. A spectral measurement can test which transitions actually oscillate.
An original calculation connects the reported wavelength with the size of an energy quantum. Using the <Planck constant> $h$ and the <speed of light> $c$,
$$
E_\gamma=\frac{hc}{\lambda}
=\frac{(6.62607015\times10^{-34})(299792458)}{10.6324\times10^{-6}}\,\mathrm J
\simeq1.8683\times10^{-20}\,\mathrm J.
$$
For a stipulated monochromatic $1\,\mathrm{mW}$ beam at that wavelength, the rate of emitted <photons> is
$$
\dot N=\frac{P}{E_\gamma}\simeq5.35\times10^{16}\,\mathrm{s^{-1}}.
$$
The individual <photon energy> is small even when the collective beam carries measurable <power>. This bookkeeping does not establish how efficiently the electrical input becomes output radiation; that comparison requires separate input and output measurements.
The film-activation apparatus in <laser capture microdissection> supplies a distinct application: a carbon dioxide laser pulse activates a <thermoplastic> film locally to collect selected tissue. Its use there depends on the material's response and the spatial delivery of energy, rather than the name of the laser alone.<Laser capture microdissection by Michael R. Emmert-Buck and others>[Abstract: carbon-dioxide pulse and transparent-film capture]{ref}
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