In carbon dioxide lasers, CO2 generates the laser beam. Excitation is first achieved by an RF glow discharge of N2 molecules. In the second step, these emit their energy through collisions with CO2 molecules. If a photon hits the excited CO2 molecule, the CO2 can release its energy by emitting another photon. The two photons are coherent (in phase).
Pleiger resonator mirrors (plane mirrors or spheres) are precision elements in such CO2 laser resonators or laser systems which are decisive for their performance: The emitted monochromatic photons show an extraordinarily high coherence. Depending on the application and the physically required property (reflectivity), the resonator mirrors are metallic or dielectric coated.