HeliumNeonLaser

Helium–neon laser, or HeNe laser, is a gas laser in which a discharge through a helium–neon mixture at low pressure excites neon atoms. Energy transfer from metastable helium to neon promotes neon to higher energy levels, from which photons are emitted as neon returns to lower states. The most common lasing transition produces red light at 632.8 nanometers and defines the distinctive beam of HeNe lasers. Some tubes can emit other visible wavelengths, such as green at 543.5 nm or yellow–orange lines, but these are uncommon and require special tube designs or pumping conditions.

Construction consists of a sealed glass discharge tube with electrodes, filled with the gas mixture. The tube is placed inside an optical resonator formed by mirrors, one of which is partially transmitting to form the output port. Electrical energy from a high-voltage supply sustains the discharge. HeNe lasers are relatively simple, compact, and stable, typically delivering a few milliwatts to tens of milliwatts of output power.

Operation and characteristics: In short tubes the laser often runs in a single longitudinal mode; longer tubes can support multiple modes. The spectral linewidth is narrow and the coherence length is long. Performance depends on gas pressure, temperature, and cavity alignment.

Applications: HeNe lasers have been widely used for alignment and leveling, holography, interferometry, spectroscopy, optical calibration, and educational demonstrations. Because of the precise 632.8 nm wavelength, they have served as a standard reference in many experiments.

History: The first continuous‑wave HeNe laser was demonstrated in 1960 by Ali Javan and colleagues at Bell Telephone Laboratories, contributing to early advances in laser technology.