heliumneon

Helium–neon, often abbreviated He–Ne, refers to a gas mixture used in helium–neon lasers and to the family of devices based on that mixture. The gas inside a sealed discharge tube is typically roughly 85–90% helium and 10–15% neon, at a low total pressure (on the order of a few Torr). When an electrical discharge is applied, helium atoms are excited and can transfer energy to neon atoms through collisions. Neon atoms then reach metastable states from which photons are emitted as they relax, and the optical cavity surrounding the tube amplifies this light to produce a coherent laser beam. The principal emission is a red line at 632.8 nanometers.

The He–Ne laser is valued for its stable, continuous-wave output and good beam quality, with modest power levels measured in milliwatts to tens of milliwatts in common laboratory tubes. Its design is relatively simple and robust, making it a long-standing workhorse in optics labs and educational settings. In addition to the standard red line, some specialized configurations can access other lines in the visible or infrared, but the 632.8 nm line is by far the most well known.

Applications include optical alignment, interferometry, holography, spectroscopy, and general teaching demonstrations. He–Ne lasers were among the first gas lasers to be developed and remain widespread as accessible, inexpensive light sources for precision optics work. Safety considerations focus on visible-laser exposure to the eyes and the hazards associated with high-voltage power supplies and sealed gas tubes.