Carbon Dioxide Laser - Amplification

Amplification

The active laser medium (laser gain/amplification medium) is a gas discharge which is air-cooled (water-cooled in higher power applications). The filling gas within the discharge tube consists primarily of:

• Carbon dioxide (CO₂) (around 10–20%)
• Nitrogen (N₂) (around 10–20%)
• Hydrogen (H₂) and/or xenon (Xe) (a few percent; usually only used in a sealed tube.)
• Helium (He) (The remainder of the gas mixture)

The specific proportions vary according to the particular laser.

The population inversion in the laser is achieved by the following sequence:

1. Electron impact excites vibrational motion of the nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational levels are therefore metastable and live for a long time.
2. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation.
3. The nitrogen molecules are left in a lower excited state. Their transition to ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO₂ and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.