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Helium–neon laser
A helium–neon laser or HeNe laser, is a type of gas laser whose gain medium consists of a mixture of helium and neon(10:1) inside of a small bore capillary tube, usually excited by a DC electrical discharge. The best-known and most widely used HeNe laser operates at a wavelength of 632.8 nm in the red part of the visible spectrum.
== History of HeNe laser development==
The first HeNe lasers emitted light at 1.15 μm, in the infrared spectrum, and were the first gas lasers. However, a laser that operated at visible wavelengths was much more in demand, and a number of other neon transitions were investigated to identify ones in which a population inversion can be achieved. The 633 nm line was found to have the highest gain in the visible spectrum, making this the wavelength of choice for most HeNe lasers. However other visible as well as infrared stimulated emission wavelengths are possible, and by using mirror coatings with their peak reflectance at these other wavelengths, HeNe lasers could be engineered to employ those transitions; this includes visible lasers appearing red, orange, yellow, and green.〔C. S. Willet "An Introduction to Gas Lasers" Pergamon Press 1974, pages 407–411〕 Stimulated emissions are known from over 100 μm in the far infrared to 540 nm in the visible. Since visible transitions at wavelengths other than 633 nm have somewhat lower gain, these lasers generally have lower output efficiencies and are more costly. The 3.39 μm transition has a very high gain but is prevented from use in an ordinary HeNe laser (of a different intended wavelength) since the cavity and mirrors are lossy at that wavelength. However in high power HeNe lasers having a particularly long cavity, superluminescence at 3.39 μm can become a nuisance, robbing power from the stimulated emission medium, often requiring additional suppression.
The best-known and most widely used HeNe laser 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.D. White and J.D. Rigden, "Continuous Gas Maser Operation in the Visible". Proc IRE vol. 50, p1697: July 1962.〕〔A. D. White, "Recollections of the First Continuous Visible Laser". Optics and Photonics News vol. 22, p34-39: October 2011.〕 18 months after the pioneering demonstration at the same laboratory of the first continuous infrared HeNe gas laser in December 1960.〔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. 63, 106–110 (1961).〕
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