Picture: Schematic of a laser tube

• An electrical discharge pumps the CO2 molecules to create a large collection of excited-state species
• Some of these emit photons in a direction parallel to the tube’s axis and are reflected.
• As they pass back through the tube, they stimulate emission from another excited molecule. This photon vibrates with the same frequency and direction as the incoming photon.
• Monochromatic, coherent and directional light leaves the tube through the partial reflector.