Helium-Neon Laser

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. The best known form operates at a wavelength of 632.8 nm, in the red portion of the visible spectrum.[1]

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 a 5:1 to 20:1 ratio, contained at low pressure (He at 1 torr and Ne at 0.1 torr; an average 50 Pa per cm of cavity length[2]) in a glass envelope. The energy or pump source of the laser is provided by a high voltage electrical discharge through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation.[3]. 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 normally 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 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[4] 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 3s2 level[3]. This is due to a coincidence of energy levels between the helium and neon atoms.

This process is given by the reaction equation:

He(21S)* + Ne + ΔE → He(11S) + Ne3s2*

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.[3]. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of 632.82 nm wavelength light, the typical operating wavelength of a HeNe laser. After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper level saturates with higher current and the lower level varies linearly with current, the HeNe laser is restricted to low power operation to maintain population inversion[3].
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.

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 (543.365 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[4][5]. 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 HeNe laser, and its 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[6].

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 Bennett Jr. and Donald Herriott at Bell Labs, who in 1960 achieved continuous wave emission of the laser on the 1.15 μm wavelength line[7].

References

1. ^ International Union of Pure and Applied Chemistry. "helium–neon laser". Compendium of Chemical Terminology Internet edition.
2. ^ E.F. Labuda and E.I. Gordon, J. Appl. Phys. 35, 1647 (1964)
3. ^ a b c d Verdeyen, J. T., Laser Electronics, Third ed., Prentice Hall series in solid state physical electronics (Prentice Hall, Upper Saddle River, 2000) pp. 326-332
4. ^ a b Niebauer, TM: Frequency stability measurements on polarization-stabilized He-Ne lasers, Applied Optics, 27(7) p.1285
5. ^ Sam's Laser FAQ
6. ^ Iodine Stabilized Helium-Neon Laser at the NIST museum site
7. ^ Javan, A., Bennett, W. R. and Herriott, D. R.: "Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture". Phys. Rev. Lett. 6 3, 106-110 (1961).