Helium-neon laser

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A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 633 nm, in the red portion of the visible spectrum.

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, approximately in the ratio 5:1, contained at low pressure (typically ~300 Pa) in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 V through an anode and cathode at each end of the glass tube. The optical cavity of the laser typically 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.

HeNe lasers are typically small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.

The red HeNe laser wavelength is usually reported as 632nm. However, the true wavelength in air is 632.816 nm, so 633nm 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 ^[1] ^[2] ^[3] for months and years of continuous operation.

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

The laser process in a HeNe laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 2³S₁ and 2¹S₀ long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms, exciting neon electrons into the 5s level. This is due to a coincidence of energy levels between the helium and neon atoms.

This process is given by the reaction equation:

He* + Ne → He + Ne* + ΔE

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.

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 [1].

The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion between the neon 5s, 3p, and other electronic levels. Spontaneous and stimulated emission between the 5s (²P_1/2) and 3p(²P_1/2) states results in emission of 632.991 nm wavelength light, the typical operating wavelength of a HeNe laser.

After this, fast radiative decay occurs from the 3p to the 2p ground state via collisions of the neon atoms with the container walls. Because of this last required step, the bore size of the laser cannot be made very large and the HeNe laser is limited in both size and power.

With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.5 nm, the so-called GreeNe laser), a yellow (594 nm) and an orange (612 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 633nm transition^[2]^[4] lasing on a single longitudinal mode. The visible output of the HeNe laser, and it's excellent spatial quality, makes the HeNe 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^[5].

Prior to the invention of cheap, abundant diode lasers, HeNe lasers were used in barcode scanners. The HeNe laser was the first gas laser to be invented, by Ali Javan, William Bennet Jr. and Donald Herriot at Bell Labs, who in 1960 achieved continuous wave emission of the laser on the 1.15 μm wavelength line.