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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]

Q-switching stores excitation in a pumped laser gain medium while preventing oscillation, then rapidly changes the optical cavity from high loss, or low quality factor, to low loss, or high quality factor. The initially suppressed stimulated field can then grow rapidly from a seed and deplete a large population inversion in a short pulse. The resulting peak power is much higher than for continuous operation at the same average pumping rate. This is energy storage and release, rather than the phase locking of many longitudinal modes used in mode locking.

An active implementation uses a Pockels cell and a polarizer. An applied voltage makes the cell change the intracavity polarization, causing the polarizer to introduce sufficient loss to prevent lasing while pumping builds the inversion. Rapidly changing the voltage removes that loss and opens the high-Q state. The switching time should be short relative to the pulse build-up and depletion times. A saturable absorber provides an alternative passive Q switch: its absorption decreases as the intracavity intensity rises.

A ring laser is a laser whose optical cavity follows a closed circulating path through a laser gain medium. Mirrors can fold the path into a triangle or another polygon; guided structures can implement the same circulating feedback principle. A travelling optical wave can return to its starting position and direction after one full circuit. The laser threshold requires round-trip gain to balance the complete losses; a closed path alone is not enough to produce laser oscillation.^[ref]

The phase condition explains the allowed normal modes. Neglect dispersion and take a constant round-trip reflection phase ${\varphi }_r$. If the circuit's optical path length is $L_{\mathrm{opt}}$, resonance requiresSubtracting the equations for consecutive modes gives the free spectral rangeMore generally the spacing is approximately the inverse round-trip group delay. A vacuum circuit of full geometric perimeter $P=0.90\mathrm{m}$ has a round-trip time about $3.00\mathrm{ns}$ and mode spacing about $333\mathrm{MHz}$. There is no additional factor of two: $P$ already counts the whole returning route.^[ref]

Gain and phase impose different conditions. In an original startup model let the three mirror intensity reflectivities be $0.99$, $0.99$ and $0.90$, and let the intensity gain accumulated during one complete circuit be $G=1.20$, with other losses neglected. The round-trip intensity multiplier isThe modeled field can grow from a seed. Reducing $G$ to $1.05$ gives $\mathcal{M}=0.9261945<1$, so it decays. These are intensity quantities, not amplitude reflectivities. Saturation and competition between normal modes must determine real steady operation.

A reciprocal ideal ring supports possible clockwise and counterclockwise waves. Their simultaneous stable emission is a dynamical question because both can interact with the same laser gain medium. A ring laser gyroscope uses rotation-dependent differences between the two circulation directions through the Sagnac effect; ring geometry by itself does not guarantee a useful rotation measurement.

A different motivating experiment used the directional dynamics as a nonlinear sensor. In 1988 Bruce McNamara, Kurt Wiesenfeld and Rajarshi Roy periodically modulated the asymmetry between counter-rotating lasing modes and added controlled external noise. Their measured signal-to-noise ratio improved for injected noise relative to no external noise, demonstrating stochastic resonance. This observation concerns a particular modulated dynamical response, not a claim that adding noise improves every optical measurement.^[ref]

A semiconductor laser uses a semiconductor as its laser gain medium. Pumping produces carrier populations that permit stimulated emission between suitable electronic states. An optical resonator supplies feedback, and oscillation begins when the gain compensates round-trip losses.

A laser diode supplies pumping by electrical injection; other semiconductor lasers can be optically pumped. The band gap, confinement and resonator determine the possible emission range. A measurement of optical output versus pump strength helps locate the laser threshold, while an emission spectrum tests the modes involved. Increasing output alone does not establish that the device has entered coherent laser oscillation.