helium–neon lasers (original) (raw)

Definition: gas lasers based on a helium–neon mixture

Alternative terms: He-Ne lasers, HeNe lasers

Category: laser devices and laser physics

• lasers
  • gas lasers
    * ion lasers
    * molecular lasers
    * helium–neon lasers
    * helium–cadmium lasers
    * argon ion lasers
    * excimer lasers
    * hydrogen lasers
    * metal vapor lasers

Related: gas lasersred lasersvisible lasers

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Contents

What are Helium–Neon Lasers?

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.

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 10 mA, for example, 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 cannot 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.

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 increasingly replaced by 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a helium–neon laser?

A helium–neon (He–Ne) laser is a type of gas laser, typically operating continuously. Its gain medium is a mixture of helium and neon gas held in a glass tube and is pumped by an electrical glow discharge.

How does a helium–neon laser work?

In a He–Ne laser, an electric discharge excites helium atoms, which then transfer their energy to neon atoms through collisions. The excited neon atoms form a population inversion, leading to laser emission as they transition to lower energy levels.

What colors can helium–neon lasers produce?

While most He–Ne lasers emit red light at 632.8 nm, other laser transitions allow for operation in the infrared (e.g., 3.39 μm), green (543.5 nm), yellow (594 nm), and orange (612 nm) by using specific laser mirrors.

What are the primary applications of He–Ne lasers?

Why are laser diodes often preferred over He–Ne lasers?

Laser diodes are increasingly replacing He–Ne lasers because they are much cheaper, more compact, and more power-efficient. However, He–Ne lasers can still be superior in terms of beam quality and narrow emission linewidth.

What limits the output power of a He–Ne laser?

Power scaling is limited because a lower laser level must be depopulated through collisions of neon atoms with the tube walls. This requires a narrow tube diameter, which prevents simply increasing the power by using a larger gain volume.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 27 suppliers for helium–neon lasers. Among them:

⚙ hardware

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.

⚙ hardware

Serving North America, RPMC Lasers offers HeNe laser tubes and modules in 543 nm (green), 594 nm (yellow), and 633 nm (red), with excellent TEM00 beam quality, low-noise output, and powers up to 20 mW for diverse scientific, industrial, and medical needs.

Robust and long-lasting, these lasers boast up to 30,000 hours of service life, with customizable options like single- or multimode beams, random or linear polarization, fiber-coupling, Brewster window tubes, and frequency stability for tailored solutions.

Ideal for laser scanning microscopy, spectroscopy, digital imaging, metrology, industrial measurement, medical diagnostics, and education, they come with OEM or lab power supplies, meeting CDRH, IEC, CSA, CE, TÜV, and UL standards.

Let RPMC help you find the right HeNe laser today!

Bibliography

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