Helium–neon laser

A helium-neon laser, usually called a HeNe laser, is a type of gas laser made by passing an electrical discharge in a helium-neon gas mixture through a small bore capillary tube. The best known and must widely used HeNe operates at a wavelength of 632.8 nm in the red part of the visible spectrum. It was developed at Bell Telephone Laboratories in 1962 , a year after the pioneering demonstration at the same laboratory of the first continuous infrared HeNe gas laser. . Red HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics and the properties of lasers because of its excellent beam quality.

Schematic diagram of a helium-neon laser

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. The energy or pump source of the laser is provided by a high voltage electrical discharge passed through the gas by electrodes (anode and cathode) placed within the tube. A current of 3 to 20 mA is typically required for CW operation. The optical cavity of the laser most often consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.

Commercial HeNe lasers are usually small, compact devices with cavity lengths ranging from 15 cm up to 1.0 m, and optical output power ranging from 0.5 to 50 mW.

The red HeNe laser wavelength is usually reported as 632 nm. However, the true wavelength in air is 632.816 nm, so 633 nm is actually closer to the true value. For the purposes of calculating the photon energy, the vacuum wavelength of 632.991 nm should be used. The precise operating wavelength lies within about 0.002 nm of this value, and fluctuates within this range due to thermal expansion of the cavity. Frequency stabilized versions enable the wavelength to be maintained within about 2 parts in 10¹² for months and years of continuous operation.

A HeNe laser demonstrated at the Kastler-Brossel Laboratory at Univ. Paris 6.

The mechanism producing population inversion and light amplification in a HeNe laser plasma originates with inelastic collision of energetic electrons with ground state helium atoms in the gas mixture. As shown in the accompanying energy level diagram, these collisions excite helium atoms from the ground state to higher energy excited states, among them the 2³S₁ and 2¹S₀ long-lived metastable states. Because of a fortuitous near coincidence between the energy levels of the two He metastable states, and the 3₂ and 2₂ (Paschen notation) levels of neon, collisions between these helium metastable atoms and ground state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon. Excitation energy transfer increases the population of the neon 2s and 3s levels manyfold. When the population of these two upper levels exceeds that of the corresponding lower level neon states, 2p and 3p to which they are optically connected, population inversion is present. The medium becomes capable of amplifying light in a narrow band at 632.8 nm (corresponding to the 3s to 2p transition at 632.8 nm) and in a narrow band at 1.15 microns, (corresponding to the 2s to 2p transition). The remaining step in transforming a light amplifier into a light oscillator is to place highly reflecting feedback mirrors at each end of the amplifying medium, so as to permit suitably directed photons to make multiple passes through the medium, gaining power on each pass until self-sustaining oscillation is achieved.

This excitation energy transfer process is given by the reaction equations:

He(2¹S)* + Ne + ΔE → He(1¹S) + Ne3s₂*

and

He////////////////////////////////////////

where (*) represents an excited state, and ΔE is the small energy difference between the energy states of the two atoms, of the order of 0.05 eV or 387 cm⁻¹, which is supplied by kinetic energy.. The lower laser levels are emptied by fast radiative decay to the 1S ground state.

Spectrum of a helium neon laser showing the very high spectral purity intrinsic to most lasers. Compare with the relatively broad spectral emittance of a light-emitting diode Image:Red-YellowGreen-Blue LED spectra.gif.

Not too long after the red HeNe was demonstrated, it was found that a number of other optically connected atomic neon levels are also inverted by the excitation energy transfer process. With the careful selection of mirror spectral reflectivity, these optical transitions will also "lase" in the HeNe laser. (reference to C. S. Willet "An Introduction to Gas Lasers" Pergamon Press 1974) Today there are lasing transitions ranging from over 100 microns in the far infrared to μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.365 nm, the so-called GreeNe laser), a yellow (593.932 nm), a yellow-orange (604.613 nm), and an orange (611.802 nm) transition. The typical 633 nm wavelength red output of a HeNe laser actually has a much lower gain compared to other wavelengths such as the 1.15 μm and 3.39 μm lines, but these can be suppressed by choosing cavity mirrors with optical coatings that reflect only the desired wavelengths.

The gain bandwidth of the laser is dominated by Doppler broadening, and is quite narrow at around 1.5 GHz for the 633 nm transition. With cavities having typical lengths of 15 cm to 50 cm, this allows about 2 to 8 longitudnal modes to simultaneously lase (however single longitudnal mode units are possible for special applications). The visible output of the red HeNe laser, and its excellent spatial quality, makes this laser a useful source for holography and as a reference for spectroscopy. It is also one of the benchmark systems for the definition of the meter.

Prior to the invention of cheap, abundant diode lasers, red HeNe lasers were used in barcode scanners at supermarket checkout counters.

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See also

• List of lasers