Carbon dioxide laser

A 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 $10.6\mu \mathrm{m}$. 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 $10.6324\mu \mathrm{m}$ 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.^[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 $c$,For a stipulated monochromatic $1\mathrm{mW}$ beam at that wavelength, the rate of emitted photons isThe 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.^[ref]