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Definition: gas lasers based on a helium–neon mixture
Helium–neon (He–Ne) lasers are a frequently used type of continuously operating gas lasers, most often emitting 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 helium and neon gas in a glass tube, which normally has a length of the order of 15–50 cm.
A DC current, which is applied via two electrodes with a voltage of the order of 1 kV, maintains an electric glow discharge with a moderate current density. In the simplest case, a ballast resistor stabilizes the electric current. The current is e.g. 10 mA, 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 can not be exchanged. Brewster windows are then not required.
In the gas discharge, helium atoms are excited into a metastable state. During collisions, the helium atoms can efficiently transfer energy to neon atoms, which have an excited state with similar excitation energy. 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 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.
Due to the narrow gain bandwidth, He–Ne lasers typically exhibit stable single-frequency operation, even though mode hopping is possible in some temperature ranges where two longitudinal resonator modes have similar gain.
Helium–neon lasers, particularly the standard devices emitting at 632.8 nm, are often used for alignment and in interferometers. They compete with laser diodes, which are more compact and efficient, but have less convenient beam profiles.
|||A. Javan, W. R. Bennett Jr., and D. R. Herriott, “Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture”, Phys. Rev. Lett. 6 (3), 106 (1961)|
|||W. R. Bennett, “Background of an inversion: the first gas laser”, IEEE J. Sel. Top. Quantum Electron. 6 (6), 869 (2000)|