Helium–neon laser

<h2 id="history-of-he-ne-laser-development">History of He-Ne laser development</h2>
The first He-Ne lasers emitted <a href="/facts/Infrared/44XdJNcc">infrared</a> at 1150 <a href="/facts/Nanometer/a98sjHyu">nm</a>, and were the first gas lasers and the first lasers with continuous wave output. However, a laser that operated at visible wavelengths was much more in demand. A number of other neon transitions were investigated to identify ones in which a <a href="/facts/Population_inversion/58ps0V5c">population inversion</a> could 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 He-Ne lasers. However, other visible and infrared stimulated-emission wavelengths are possible, and by using mirror coatings with their peak reflectance at these other wavelengths; He-Ne lasers could be engineered to employ those transitions, including visible lasers appearing red, orange, yellow, and green.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> Stimulated emissions are known from over 100 μm in the far infrared to 540 nm in the visible.
Because visible transitions 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 He-Ne laser (of a different intended wavelength) because the <a href="/facts/Optical_cavity/BITPG3ga">cavity</a> and mirrors are lossy at that wavelength. However, in high-power He-Ne lasers having a particularly long cavity, <a href="/facts/Amplified_spontaneous_emission/jmGtd6hy">superluminescence</a> 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 He-Ne laser operates at a wavelength of 632.8 nm, in the red part of the <a href="/facts/Visible_spectrum/frPBh8o5">visible spectrum</a>. It was developed at <a href="/facts/Bell_Telephone_Laboratories/fKP5Gk0H">Bell Telephone Laboratories</a> in 1962,<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a><a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> 18 months after the pioneering demonstration at the same laboratory of the first continuous infrared He-Ne gas laser in December 1960.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>

<h2 id="construction-and-operation">Construction and operation</h2>
The <a href="/facts/Laser_construction/5zvnBJ6y">gain medium</a> of the laser, as suggested by its name, is a mixture of <a href="/facts/Helium/yRul93TG">helium</a> and <a href="/facts/Neon/5bF1FwGL">neon</a> gases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope. The gas mixture is mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states, responsible for non-laser lines.
A neon laser with no helium can be constructed, but it is much more difficult without this means of energy coupling. Therefore, a He-Ne laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will lose its laser functionality because the pumping efficiency will be too low.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> The energy or pump source of the laser is provided by a high-voltage <a href="/facts/Electrical_discharge/06wCdIrV">electrical discharge</a> passed through the gas between electrodes (<a href="/facts/Anode/TjYt95zG">anode</a> and <a href="/facts/Cathode/tqdlGayU">cathode</a>) within the tube. A <a href="/facts/DC_power/r9MNUx84">DC</a> current of 3 to 20 mA is typically required for <a href="/facts/Continuous_wave/XdKlBpgP">CW operation</a>. The <a href="/facts/Optical_cavity/BITPG3ga">optical cavity</a> of the laser usually consists of two concave mirrors or one plane and one concave mirror: one having very high (typically 99.9%) reflectance, and the <a href="/facts/Output_coupler/PBWd1Qk4">output coupler</a> mirror allowing approximately 1% transmission.

Commercial He-Ne lasers are relatively small devices compared to other gas lasers, having cavity lengths usually ranging from 15 to 50 cm (but sometimes up to about 1 meter to achieve the highest powers), and optical output <a href="/facts/Power_(physics)/mkaEyRnn">power</a> levels ranging from 0.5 to 50 m<a href="/facts/Watt/VK8oqRrm">W</a>.
The precise wavelength of red He-Ne lasers is 632.991 nm in a vacuum, which is refracted to about 632.816 nm in air. The wavelengths of the stimulated emission modes lie within about 0.001 nm above or below this value, and the wavelengths of those modes shift within this range due to thermal expansion and contraction of the cavity. <a href="/facts/Frequency_drift/yY8tspSu">Frequency-stabilized</a> versions enable the wavelength of a single mode to be specified to within 1 part in 108 by the technique of comparing the powers of two longitudinal modes in opposite polarizations.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> Absolute stabilization of the laser's frequency (or wavelength) as fine as 2.5 parts in 1011 can be obtained through use of an iodine absorption cell.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>

The mechanism producing <a href="/facts/Population_inversion/58ps0V5c">population inversion</a> and <a href="/facts/Optical_amplifier/1Fj9Pfes">light amplification</a> in a He-Ne laser plasma<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> originates with inelastic collision of energetic electrons with ground-state helium atoms in the gas mixture. As shown in the accompanying energy-level diagram, these collisions excite helium atoms from the ground state to higher energy excited states, among them the 23S1 and 21S0 (<a href="/facts/Term_symbol/7v7IFEM4">LS, or Russell–Saunders coupling</a>, front number 2 indicates that an excited electron is n = 2 state) are long-lived metastable states. Because of a fortuitous near-coincidence between the energy levels of the two He metastable states and the 5s2 and 4s2 (<a href="/facts/Term_symbol/7v7IFEM4">Paschen notation</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>) levels of neon, collisions between these helium metastable atoms and ground-state neon atoms results in a selective and efficient transfer of excitation energy from the helium to neon. This excitation energy transfer process is given by the reaction equations

He*(23S1) + Ne1S0 → He(1S0) + Ne*4s2 + ΔE,
He*(21S) + Ne1S0 + ΔE → He(1S0) + Ne*5s2,
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 <a href="/facts/Electronvolt/YMT1fqX9">eV</a>, or 387 cm−1, which is supplied by kinetic energy. Excitation-energy transfer increases the population of the neon 4s2 and 5s2 levels manyfold. When the population of these two upper levels exceeds that of the corresponding lower level, 3p4, to which they are optically connected, population inversion is present. The medium becomes capable of amplifying light in a narrow band at 1.15 μm (corresponding to the 4s2 to 3p4 transition) and in a narrow band at 632.8 nm (corresponding to the 5s2 to 3p4 transition). The 3p4 level is efficiently emptied by fast radiative decay to the 3s state, eventually reaching the ground state.
The remaining step in utilizing <a href="/facts/Optical_amplifier/1Fj9Pfes">optical amplification</a> to create an <a href="/facts/Laser/jk6PTvvT">optical oscillator</a> is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular <a href="/facts/Transverse_mode/nCIl5D6a">spatial mode</a> will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction. When these conditions are met for one or more <a href="/facts/Longitudinal_mode/e0jPlP6r">longitudinal modes</a>, then radiation in those modes will rapidly build up until <a href="/facts/Optical_amplifier/1Fj9Pfes">gain saturation</a> occurs, resulting in a stable continuous laser-beam output through the front (typically 99% reflecting) mirror.

The gain bandwidth of the He-Ne laser is dominated by <a href="/facts/Doppler_broadening/7DW5hCNQ">Doppler broadening</a> rather than <a href="/facts/Pressure_broadening/HuI4Bj5f">pressure broadening</a> due to the low gas pressure and is thus quite narrow: only about 1.5 GHz full width for the 633 nm transition.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a><a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> With cavities having typical lengths of 15 to 50 cm, this allows about 2 to 8 <a href="/facts/Longitudinal_mode/e0jPlP6r">longitudinal modes</a> to oscillate simultaneously (however, single-longitudinal-mode units are available for special applications). The visible output of the red He-Ne laser, long <a href="/facts/Coherence_length/A845Dc1z">coherence length</a>, and its excellent spatial quality, makes this laser a useful source for <a href="/facts/Holography/4J1gW0M6">holography</a> and as a wavelength reference for <a href="/facts/Spectroscopy/mj71FuNK">spectroscopy</a>. A stabilized He-Ne laser is also one of the benchmark systems for the definition of the meter.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>
Prior to the invention of cheap, abundant <a href="/facts/Diode_lasers/TjHzJteV">diode lasers</a>, red He-Ne lasers were widely used in <a href="/facts/Barcode_scanner/k6L6FUlY">barcode scanners</a> at supermarket checkout counters. He-Ne lasers are generally present in educational and research optical laboratories. They are also unsurpassed for use in nano-positioning in applications such as <a href="/facts/Semiconductor_device_fabrication/Jd23Mu1z">semiconductor device fabrication</a>. High precision <a href="/facts/Ring_laser_gyroscope/RJh19vVm">laser gyroscopes</a> have employed He-Ne lasers operating at 633 nm in a <a href="/facts/Ring_laser/pwToTIP0">ring laser</a> configuration.

<h2 id="applications">Applications</h2>

Red He-Ne <a href="/facts/Laser/jk6PTvvT">lasers</a> have an enormous number of industrial and scientific uses. They are widely used in laboratory demonstrations in the field of <a href="/facts/Optics/p0vq4S1T">optics</a> because of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence (a <a href="/facts/Transverse_mode/nCIl5D6a">single-mode</a> <a href="/facts/Gaussian_beam/Hvo1wL29">Gaussian beam</a>) and long <a href="/facts/Coherence_length/A845Dc1z">coherence length</a> (however, since about 1990 semiconductor lasers have offered a lower-cost alternative for many such applications).
Starting in 1978, HeNe tube lasers (manufactured by <a href="/facts/Toshiba/QHRaw8Tr">Toshiba</a> and <a href="/facts/NEC/YnlUsMBA">NEC</a>) were used in <a href="/facts/LaserDisc/nU4WJhDM">LaserDisc</a> players from <a href="/facts/Pioneer_Corporation/wf4lT8IS">Pioneer</a>. This continued until the 1984 model lineup, which contained infrared <a href="/facts/Laser_diode/TjHzJteV">laser diodes</a> instead. Pioneer continued to use laser diodes in all LaserDisc players afterwards until LaserDisc was discontinued in 2009.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/List_of_laser_types/wGEmLDVD">List of laser types</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">NIST Atomic Spectral Database <a href="https://physics.nist.gov/PhysRefData/ASD/lines_form.html" target="_blank">https://physics.nist.gov/PhysRefData/ASD/lines_form.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Willet, C.S. (1974). An Introduction to Gas Lasers. Pergamon Press. pp. 407–411. <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">White, A.D.; Rigden, J.D. (1962). "Correspondence: Continuous gas maser operation in the visible". Proceedings of the IRE. 50 (7). Institute of Electrical and Electronics Engineers (IEEE): 1697. doi:10.1109/jrproc.1962.288157. ISSN 0096-8390. <a href="https://doi.org/10.1109%2Fjrproc.1962.288157" target="_blank">https://doi.org/10.1109%2Fjrproc.1962.288157</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">White, A.D. (October 2011). "Recollections of the first continuous visible laser". Optics and Photonics News. Vol. 22, no. 10. pp. 34–39. <a href="https://www.optica-opn.org/home/articles/volume_22/issue_10/features/recollections_of_the_first_continuous_visible_lase/" target="_blank">https://www.optica-opn.org/home/articles/volume_22/issue_10/features/recollections_of_the_first_continuous_visible_lase/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Javan, A.; Bennett, W.R.; Herriott, D.R. (1 February 1961). "Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture". Physical Review Letters. 6 (3). American Physical Society (APS): 106–110. Bibcode:1961PhRvL...6..106J. doi:10.1103/physrevlett.6.106. ISSN 0031-9007. <a href="https://doi.org/10.1103%2Fphysrevlett.6.106" target="_blank">https://doi.org/10.1103%2Fphysrevlett.6.106</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"Sam's Laser FAQ – Helium-Ne Lasers". K3PGP.org. <a href="http://www.k3pgp.org/Notebook/Lasersam/laserhen.htm#hentoo0" target="_blank">http://www.k3pgp.org/Notebook/Lasersam/laserhen.htm#hentoo0</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Niebauer, T.M.; Faller, James E.; Godwin, H.M.; Hall, John L.; Barger, R.L. (1988-04-01). "Frequency stability measurements on polarization-stabilized He–Ne lasers". Applied Optics. 27 (7). The Optical Society: 1285–1289. Bibcode:1988ApOpt..27.1285N. doi:10.1364/ao.27.001285. ISSN 0003-6935. PMID 20531556. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Iodine-stabilized helium–neon laser. National Institute of Standards and Technology (NIST). NIST Museum (Report). U.S. Department of Commerce. Archived from the original on 21 July 2006. <a href="https://web.archive.org/web/20060721065251/http://museum.nist.gov/object.asp?ObjID=50" target="_blank">https://web.archive.org/web/20060721065251/http://museum.nist.gov/object.asp?ObjID=50</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Javan, A.; Bennett, W.R.; Herriott, D.R. (1 February 1961). "Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture". Physical Review Letters. 6 (3). American Physical Society (APS): 106–110. Bibcode:1961PhRvL...6..106J. doi:10.1103/physrevlett.6.106. ISSN 0031-9007. <a href="https://doi.org/10.1103%2Fphysrevlett.6.106" target="_blank">https://doi.org/10.1103%2Fphysrevlett.6.106</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">"Notes on the Paschen notation". Archived from the original on 2012-06-18. <a href="https://web.archive.org/web/20120618234059/http://technology.niagarac.on.ca/lasers/Chapter3.html" target="_blank">https://web.archive.org/web/20120618234059/http://technology.niagarac.on.ca/lasers/Chapter3.html</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Niebauer, T.M.; Faller, James E.; Godwin, H.M.; Hall, John L.; Barger, R.L. (1988-04-01). "Frequency stability measurements on polarization-stabilized He–Ne lasers". Applied Optics. 27 (7). The Optical Society: 1285–1289. Bibcode:1988ApOpt..27.1285N. doi:10.1364/ao.27.001285. ISSN 0003-6935. PMID 20531556. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">"Sam's Laser FAQ". RepairFAQ. <a href="http://www.repairfaq.org/sam/laserhen.htm#henhlc0" target="_blank">http://www.repairfaq.org/sam/laserhen.htm#henhlc0</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Iodine-stabilized helium–neon laser. National Institute of Standards and Technology (NIST). NIST Museum (Report). U.S. Department of Commerce. Archived from the original on 21 July 2006. <a href="https://web.archive.org/web/20060721065251/http://museum.nist.gov/object.asp?ObjID=50" target="_blank">https://web.archive.org/web/20060721065251/http://museum.nist.gov/object.asp?ObjID=50</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
</ol>

Helium–neon laser open-in-new

Helium–neon laser