Helium-neon laser

A helium-neon laser, usually called a HeNe laser, is a small gas laser of a type often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 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 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.

Energy level diagram of a HeNe laser

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 them into the 2s and 3s states. 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.

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 3s and 2s, and 3p and 3p states. Spontaneous emission between the 3s and 2p states results in emission of 632.8 nm wavelength light, the typical operating wavelength of a HeNe laser.

After this, fast radiative decay occurs from the 2p to the 1s energy levels, which then decay to the 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 size and power.

With the correct selection of cavity mirrors, other wavelengths of laser emission of the HeNe laser are possible. The 3s→3p and 2s→2p transitions give infrared operation at 3.39 μm and 1.15 μm wavelengths, and a variety of 2s→1s transitions are possible in the green (543.5 nm, the so-called GreeNe laser), the yellow (594 nm) and the orange (612 nm).

The gain bandwidth of the laser is dominated by Doppler broadening, and is quite narrow at around 1.5 GHz. This, along with the visible output and excellent beam quality possible from these lasers, makes the HeNe a useful source for holography and as a reference for spectroscopy. Other applications include use in barcode scanners.