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Helium–neon Lasers

Definition: gas lasers based on a helium–neon mixture

More general term: gas lasers

German: Helium-Neon-Laser

Category: lasers

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Helium–neon (He–Ne) lasers are a frequently used type of continuously operating gas lasers, which is also the first demonstrated gas laser (already in 1961 [1]).

Most often, He–Ne lasers emit red light at 632.8 nm at a power level of a few milliwatts and with excellent beam quality. The gain medium is a mixture of mostly helium and some neon gas in a glass tube, which normally has a length of the order of 15–50 cm.

helium–neon laser
Figure 1: Setup of a helium–neon laser.

A DC current, which is applied via two electrodes with a voltage of the order of 1 kV (but higher during ignition), maintains an electric glow discharge with a moderate current density. In the simplest case, a ballast resistor stabilizes the electric current. The current is e.g. 10 mA, leading to an electrical power of the order of 10 W. The glass tube as shown in Figure 1 has Brewster windows, and the laser mirrors must form a laser resonator with a small round-trip loss of typically below 1%. Due to the polarization-dependent loss at the Brewster windows, a stable linear polarization is obtained.

Some He–Ne lasers have a tube with internal resonator mirrors, which can not be exchanged. Brewster windows are then not required.

In the gas discharge, helium atoms are excited into metastable states (23S1 and 21S0). During collisions, the helium atoms can efficiently transfer energy to neon atoms, which have excited states with quite similar excitation energies (4s2 and 5s2). Neon atoms have a number of energy levels below that pump level, so that there are several possible laser transitions. The transition at 632.8 nm (5s2 → 3p) is the most common, but other transitions allow the operation of such lasers at 1.15 μm, 543.5 nm (green), 594 nm (yellow), 612 nm (orange), or 3.39 μm. The emission wavelength is selected by using resonator mirrors which introduce high enough losses at the wavelengths of all competing transitions.

The 3.39-μm transition involves the same upper laser level manifold as the 632.8-nm transition and exhibits a rather high laser gain, while the 632.8-nm transition. Therefore, 632.8-nm operation is only possible if parasitic lasing on the 3.39-μm line is suppressed by introducing high power losses at that wavelength.

The lower laser level for 632.8-nm operation is still highly excited and is partially depopulated by spontaneous emission. Therefore, one obtains some fluorescence at wavelengths between 0.54 μm and 0.73 μm. This leads to the metastable 3s state; depopulation of that can be made fast enough by using a small diameter laser tube, so that the neon atoms can dissipate energy in collisions with the tube walls. (One also often uses a smaller laser bore tube within a larger glass envelope.) That requirement prohibits simple power scaling via the tube diameter.

The quantum defect is quite high, contributing to a relatively low power conversion efficiency.

The polarization of the laser output is stably linear when Brewster windows are used.

Due to the narrow gain bandwidth (≈1.5 GHz, determined by Doppler broadening), He–Ne lasers typically exhibit few-mode oscillation, or for short laser tubes even stable single-frequency operation, even though mode hopping is possible in some temperature ranges where two longitudinal resonator modes have similar gain.

The lifetime of a helium–neon laser can be far beyond 10,000 hours, since the glow discharge with quite moderate current density is associated with quite moderate operation conditions, e.g. with little erosion of the electrodes. A limiting factor can be leakage of helium. However, the tube may break when being subject to mechanical shock.

Applications of Helium–neon Lasers

Helium–neon lasers, particularly the standard devices emitting at 632.8 nm, are still used for alignment and in interferometers. However, they are more and more replaced with laser diodes, which are much cheaper, more compact and efficient. Remaining advantages of the He–Ne laser can be the smaller emission linewidth (particularly in case of single-mode emission), which is associated with a long coherence length, and the high beam quality.

Some He–Ne lasers are serving in optical frequency standards. For example, there are methane-stabilized 3.39-μm He–Ne lasers, and 633-nm iodine-stabilized versions.

Suppliers

The RP Photonics Buyer's Guide contains 28 suppliers for helium--neon lasers. Among them:

Questions and Comments from Users

2020-04-26

In a He-Ne gas laser, can we use any other gas in place of neon?

Answer from the author:

It is quite possible that the same tube would work with other gas mixtures, if you also exchange the resonator mirrors according to the used laser transition.

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Bibliography

[1]A. Javan, W. R. Bennett Jr. and D. R. Herriott, “Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture”, Phys. Rev. Lett. 6 (3), 106 (1961), doi:10.1103/PhysRevLett.6.106
[2]A. D. White, E. I. Gordon and J. D. Rigden, “Output power of the 6328 Å gas maser”, Appl. Phys. Lett. 2 (5), 91 (1963), doi:10.1063/1.1753793
[3]W. R. Bennett, “Background of an inversion: the first gas laser”, J. Sel. Top. Quantum Electron. 6 (6), 869 (2000), doi:10.1109/2944.902136

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See also: gas lasers, red lasers, visible lasers
and other articles in the category lasers

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