Helium-neon laser 378269 221907251 2008-06-26T17:01:37Z JMiall 516097 [[WP:UNDO|Undid]] revision 221840994 by [[Special:Contributions/212.219.203.242|212.219.203.242]] ([[User talk:212.219.203.242|talk]]) - vandal deleting refs? A '''helium-neon laser''', usually called a '''HeNe laser''', is a type of small [[gas laser]]. HeNe [[laser]]s have many industrial and scientific uses, and are often used in [[laboratory]] demonstrations of [[optics]]. Its usual operation [[wavelength]] is 632.8 [[nanometre|nm]], in the [[red]] portion of the [[optical spectrum|visible spectrum]].<ref>{{GoldBookRef|title=helium–neon laser|file=H02768}}</ref> [[Image:Hene-1.png|frame|Schematic diagram of a helium-neon laser]] The [[laser construction|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 (an average 50 [[pascal (unit)|Pa]] per cm of cavity length <ref>E.F. Labuda and E.I. Gordon, J. Appl. Phys. '''35''', 1647 (1964)</ref>) in a glass envelope. The energy or pump source of the laser is provided by an [[electricity|electrical]] discharge of around 1000 [[volts]]{{Fact|date=February 2007}} through an [[anode]] and [[cathode]] at each end of the glass tube. A current of 5 to 100 mA is typical for [[Continuous wave|CW]] operation.<ref name="Verdeyen">Verdeyen, J. T., ''Laser Electronics'', Third ed., Prentice Hall series in solid state physical electronics (Prentice Hall, Upper Saddle River, 2000) pp. 326-332</ref>. 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 [[Power (physics)|power]]s ranging from 1 m[[watt|W]] 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 drift|Frequency stabilized]] versions enable the wavelength to be maintained within about 2 parts in 10<sup>12</sup> <ref> [http://jilawww.colorado.edu/YeLabs/PDFfiles/YoonApplPhysB2001.pdf Absolute frequency measurement of the iodine-stabilized He−Ne laser at 633 nm] in Applied Physics B 72, 221–226 (2001) by T.H. Yoon, J. Ye, J.L. Hall, and J.-M. Chartier </ref> <ref name="Niebauer"> Niebauer, TM: Frequency stability measurements on polarization-stabilized He-Ne lasers, Applied Optics, 27(7) p.1285 </ref> <ref> [http://www.npl.co.uk/optical_frequency_standards/npl_research/frequency_metrology/hene_measurements.html Measurements] from the [[National Physical Laboratory, UK|National Physical Laboratory]] </ref> for months and years of continuous operation. [[Image:Laser DSC09088.JPG|thumb|right|A HeNe laser demonstrated at the [[Kastler-Brossel Laboratory]] at [[Pierre and Marie Curie University|Univ. Paris 6]].]] The laser process in a HeNe laser starts with collision of [[electron]]s from the electrical discharge with the helium atoms in the gas. This excites helium from the [[ground state]] to the 2<sup>3</sup>S<sub>1</sub> and 2<sup>1</sup>S<sub>0</sub> 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 3s<sub>2</sub> level<ref name="Verdeyen"> repeat reference </ref>. This is due to a coincidence of [[energy level]]s between the helium and neon atoms. This process is given by the reaction equation: :He(2<sup>1</sup>S)* + Ne + ΔE → He(1<sup>1</sup>S) + Ne3s<sub>2</sub>* 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 [[electronvolt|eV]] or 387 cm<sup>-1</sup>, which is supplied by kinetic energy.<ref name="Verdeyen"> repeat reference </ref>. 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 emission|Spontaneous]] and [[stimulated emission]] between the 3s<sub>2</sub> and 2p<sub>4</sub> 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<ref name="Verdeyen"> repeat reference </ref>. [[Image:Helium neon laser spectrum.png|thumb|right|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]] [http://en.wikipedia.org/wiki/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 [[micrometre|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 (colour)|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 coating]]s 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<ref name="Niebauer"> ignore this text </ref><ref name=Sam's> [http://www.repairfaq.org/sam/laserhen.htm#henhlc0 Sam's Laser FAQ]</ref> lasing on a single [[longitudinal mode]]. 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<ref>[http://museum.nist.gov/object.asp?ObjID=50 Iodine Stabilized Helium-Neon Laser] at the NIST museum site </ref>. 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 R. Bennett, Jr.|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<ref>Javan, A., Bennett, W. R. and Herriott, D. R.: [http://prola.aps.org/abstract/PRL/v6/i3/p106_1 "Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture"]. ''Phys. Rev. Lett.'' '''6''' 3, 106-110 (1961).</ref>. ==See also== *[[list of lasers]] ==References== <references/> == External links == {{Commonscat|Helium-Neon lasers}} [[Category:Gas lasers]] [[Category:Helium]] [[Category:Neon]] [[ca:Làser d'heli-neó]] [[cs:He-Ne laser]] [[de:Helium-Neon-Laser]] [[hr:He-Ne laser]] [[it:Laser a elio-neon]] [[nl:Helium-neonlaser]] [[pl:Laser helowo-neonowy]] [[ru:Гелий-неоновый лазер]]