Helium–neon laser

The 633 nm line was found to have the highest gain in the visible spectrum, making this the wavelength of choice for most He-Ne lasers.

However, in high-power He-Ne lasers having a particularly long cavity, superluminescence at 3.39 μm can become a nuisance, robbing power from the stimulated emission medium, often requiring additional suppression.

The best-known and most widely used He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum.

[5] The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope.

[6] The energy or pump source of the laser is provided by a high-voltage electrical discharge passed through the gas between electrodes (anode and cathode) within the tube.

[7] Absolute stabilization of the laser's frequency (or wavelength) as fine as 2.5 parts in 1011 can be obtained through use of an iodine absorption cell.

[8] The mechanism producing population inversion and light amplification in a He-Ne laser plasma[5] originates with inelastic collision of energetic electrons with ground-state helium atoms in the gas mixture.

The remaining step in utilizing optical amplification to create an optical oscillator is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular spatial mode will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction.

[7][10] With cavities having typical lengths of 15 to 50 cm, this allows about 2 to 8 longitudinal modes to oscillate simultaneously (however, single-longitudinal-mode units are available for special applications).

They are widely used in laboratory demonstrations in the field of optics because of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence (a single-mode Gaussian beam) and long coherence length (however, since about 1990 semiconductor lasers have offered a lower-cost alternative for many such applications).

Helium–neon laser at the University of Chemnitz, Germany
Schematic diagram of a typical 2-3 mW red (633 nm) helium–neon laser tube
Energy levels in a He-Ne Laser
Ring He-Ne Laser
Spectrum of a helium–neon laser illustrating its very high spectral purity (limited by the measuring apparatus). The 0.002 nm bandwidth of the stimulated emission medium is well over 10 000 times narrower than the spectral width of a light-emitting diode (see its spectrum for comparison), with the bandwidth of a single longitudinal mode being much narrower still.
Solid glass-ceramic block core of the Honeywell GG1320 Ring Laser Gyro used for primary navigation in many commercial aircraft and elsewhere.