Helium–neon Lasers

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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.

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 in 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 the 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.

More to Learn

Encyclopedia articles:

• gas lasers
• red lasers
• visible lasers

Bibliography

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Suppliers

The RP Photonics Buyer's Guide contains 26 suppliers for helium–neon lasers. Among them:

RPMC Lasers

Serving North America, RPMC Lasers offers a selection of HeNe laser tubes and modules, featuring a robust mechanical design, excellent beam quality, and a variety of configurations, with wavelength options at 543, 594, and 632.8 nm and up to 20mW CW output power. Our stabilized HeNe lasers boast an excellent TEM00 beam and a long service life of up to 30,000 hours. Standard and customized models are available with a host of options including single-mode or multimode, random or linear polarization, Brewster window tubes, fiber-coupling, and frequency stability. All models can be provided with either a laboratory HeNe power supply, or one of three OEM power supply versions. Let RPMC help you find the right laser today!

Edmund Optics

Edmund Optics offers helium–neon lasers from Lumentum with up to 22.5 mW output power and improved stability. They are used in interferometers and metrology, for example.

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