How does a helium-neon laser produce a red beam?

A helium-neon laser produces a red beam through a specific process involving energy transfer between gas atoms within a resonant cavity.12

The laser medium consists of a mixture containing a small amount of neon and a significantly larger amount of helium. The process functions as follows:2

• Energy Pumping: An electric current is passed through the gas mixture, which excites the helium atoms by transferring energy from passing electrons.12
• Energy Transfer: These excited helium atoms collide with neon atoms, transferring their stored energy to them.2
• Stimulated Emission: This energy transfer creates a population inversion, a state where more neon atoms exist in higher energy levels than lower ones. As these neon atoms transition back to lower energy states, they emit light.32
• Resonance: The light is reflected back and forth between mirrors in an optical resonator, amplifying the intensity through stimulated emission until a coherent red laser beam emerges from the cavity.12

While the most common helium-neon lasers emit red light, they can be configured to produce other colors, such as yellow, orange, or green.2