A sealed CO2 laser consists of two slab electrodes inside a sealed tube. The tube is filled with a gas mixture including carbon dioxide, nitrogen and helium.
The electrodes are water cooled.
A mirror is placed at each end, forming an optical resonator. One of the mirrors is partially reflective, allowing light to exit the resonator.
RF power is applied to the electrodes, exciting a gas discharge between them.
As the gas molecules lose energy, photons are emitted. The photons travel back and forth within the resonator, stimulating the emission of more photons as they collide with the excited molecules.
As the photons all travel in the same direction, in phase and with the same wavelength, a laser beam is produced.