87176-20041-helium-neon_laser-1

8/7/2019 87176-20041-Helium-neon_laser-1

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HELIUMHELIUM--NEON LASERNEON LASER


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Historical factsHistorical facts

TheThe HeliumHelium--NeonNeon laserlaser waswas thethe firstfirst continuouscontinuous laserlaser.. ItIt waswas inventedinvented byby JavanJavan etet.. alal.. inin 19611961.. ButBut howhow diddid JavanJavan managemanage toto

dodo this?this? ThisThis showsshows thatthat itit isis nono coincidencecoincidence thatthat Javan'sJavan's firstfirst HeHe--NeNe laserlaser

oscillatedoscillated atat aa WavelengthWavelength ofof 11..55m,m, sincesince thethe amplificationamplification atat thisthis wavelengthwavelength isisconsiderablyconsiderably

HigherHigher thanthan thethe 632632 nmnm lineline whichwhich isis reachedreached atat whatwhat isis nownow commonlycommonlyknownknown asas thethe redred line,line, whichwhich waswas mademade toto oscillateoscillate onlyonly oneone yearyear laterlaterbyby WhiteWhite andand RidgenRidgen..

TheThe similaritysimilarity betweenbetween thethe manufacturingmanufacturing techniquestechniques ofof HeHe--NeNe laserslasersandand electronelectron valvesvalves helpedhelped inin thethe massmass productionproduction andand distributiondistribution ofofHeHe--NeNe laserslasers....

It is now clear that HeIt is now clear that He--Ne lasers will have to increasingly compete withNe lasers will have to increasingly compete withlaser diodes in the future. But Helaser diodes in the future. But He--Ne lasers are still unequalled as far asNe lasers are still unequalled as far asbeam geometry and the purity of the modes are concerned. Laserbeam geometry and the purity of the modes are concerned. Laserdiodes will have to be improve to a great extent before they pose adiodes will have to be improve to a great extent before they pose aserious threat to heliumserious threat to helium--neon laserneon laser


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IntroductionIntroduction

A heliumA helium--neon laser, usually called a Heneon laser, usually called a He--Ne laser, is a type of small gasNe laser, is a type of small gaslaser. HeNe lasers have many industrial and scientific uses, and arelaser. HeNe lasers have many industrial and scientific uses, and areoften used in laboratory demonstrations of optics.often used in laboratory demonstrations of optics.

HeHe--Ne laser is a fourNe laser is a four--level laser.level laser.

Its usual operation wavelength is 632.8 nm, in the red portion of theIts usual operation wavelength is 632.8 nm, in the red portion of thevisible spectrum.visible spectrum. It operates in Continuous Working (CW) mode.It operates in Continuous Working (CW) mode.


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Construction of HeConstruction of He--Ne laserNe laser The setup consists of a discharge tube of length 80 cm and boreThe setup consists of a discharge tube of length 80 cm and bore

diameter of 1.5cm.diameter of 1.5cm.

The gain medium of the laser, as suggested by its name, is a mixture ofThe gain medium of the laser, as suggested by its name, is a mixture ofhelium and neon gases, in a 5:1 to 20:1 ratio, contained at lowhelium and neon gases, in a 5:1 to 20:1 ratio, contained at lowpressure (an average 50 Pa per cm of cavity length ) in a glasspressure (an average 50 Pa per cm of cavity length ) in a glassenvelope.envelope.

The energy or pump source of the laser is provided by an electricalThe energy or pump source of the laser is provided by an electricaldischarge of around 1000 volts through an anode and cathode at eachdischarge of around 1000 volts through an anode and cathode at eachend of the glass tube. A current of 5 to 100 mA is typical for CWend of the glass tube. A current of 5 to 100 mA is typical for CWoperation.operation.

The optical cavity of the laser typically consists of a plane, highThe optical cavity of the laser typically consists of a plane, high--reflecting mirror at one end of the laser tube, and a concave outputreflecting mirror at one end of the laser tube, and a concave outputcoupler mirror of approximately 1% transmission at the other end.coupler mirror of approximately 1% transmission at the other end.

HeNe lasers are normally small, with cavity lengths of around 15 cm upHeNe lasers are normally small, with cavity lengths of around 15 cm upto 0.5 m, and optical output powers ranging from 1 mW to 100 mW.to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.


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HeHe--Ne Energy level diagramNe Energy level diagram The left side of the representation shows the lower levels of the heliumThe left side of the representation shows the lower levels of the heliumatoms.The energy scale is interrupted and that there is a largeratoms.The energy scale is interrupted and that there is a larger

difference in energy in the recombination process than is evident in thedifference in energy in the recombination process than is evident in thediagram.diagram.

A characteristic of helium is that its first states to be excited, 21S1 andA characteristic of helium is that its first states to be excited, 21S1 and21S0 are metastable, i.e. optical transitions to the ground state 11S021S0 are metastable, i.e. optical transitions to the ground state 11S0

are not allowed, because this would violate the selection rules forare not allowed, because this would violate the selection rules foroptical transitions. As a result of gas discharge, these states areoptical transitions. As a result of gas discharge, these states arepopulated by electron collisionspopulated by electron collisions

A collision is called a collision of the second type if one of the collidingA collision is called a collision of the second type if one of the collidingbodies transfers energy to the other so that a transition from thebodies transfers energy to the other so that a transition from the

previous energy state to the next higher or lower takes place. Apartprevious energy state to the next higher or lower takes place. Apartfrom the electron collision of the second type there is also the atomicfrom the electron collision of the second type there is also the atomiccollision of the second type. In the latter, an excited helium atomcollision of the second type. In the latter, an excited helium atomreaches the initial state because its energy has been used in thereaches the initial state because its energy has been used in theexcitation of a Ne atom. Both these processes form the basis for theexcitation of a Ne atom. Both these processes form the basis for theproduction of a population inversion in the Ne system.production of a population inversion in the Ne system.


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Working of HeWorking of He--Ne laserNe laser

A description of the rather complex HeNe excitation process can beA description of the rather complex HeNe excitation process can begiven in terms of the following four steps.given in terms of the following four steps.

(a)When the power is switched on, An energetic electron collisionally(a)When the power is switched on, An energetic electron collisionallyexcites a He atom to the state labeled 21So . A He atom in this excitedexcites a He atom to the state labeled 21So . A He atom in this excitedstate is often written He*(21So), where the asterisk means that the Hestate is often written He*(21So), where the asterisk means that the Heatom is in an excited state.atom is in an excited state.

(b) The excited He*(21So) atom collides with an unexcited Ne atom(b) The excited He*(21So) atom collides with an unexcited Ne atomand the atoms exchange internal energy, with an unexcited He atomand the atoms exchange internal energy, with an unexcited He atom

and excited Ne atom, written Ne*(3s2), resulting. This energy exchangeand excited Ne atom, written Ne*(3s2), resulting. This energy exchangeprocess occurs with high probability only because of the accidental nearprocess occurs with high probability only because of the accidental nearequality of the two excitation energies of the two levels in these atoms.equality of the two excitation energies of the two levels in these atoms.Thus, the purpose of population inversion is fulfilled.Thus, the purpose of population inversion is fulfilled.


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When the excited Ne atom passes from metastable state(3s) to lowerWhen the excited Ne atom passes from metastable state(3s) to lowerlevel(2p), it emits photon of wavelength 632 nm.level(2p), it emits photon of wavelength 632 nm.

This photon travels through the gas mixture parallel to the axis of tube,This photon travels through the gas mixture parallel to the axis of tube,it is reflected back and forth by the mirror ends until it stimulates anit is reflected back and forth by the mirror ends until it stimulates anexcited Ne atom and causes it to emit a photon of 632nm with theexcited Ne atom and causes it to emit a photon of 632nm with thestimulating photon.stimulating photon.

The stimulated transition from (3s) level to (2p) level is laser transition.The stimulated transition from (3s) level to (2p) level is laser transition.

This process is continued and when a beam of coherent radiationThis process is continued and when a beam of coherent radiationbecomes sufficiently strong, a portion of it escape through partiallybecomes sufficiently strong, a portion of it escape through partiallysilvered end.silvered end.

The Ne atom passes to lower level 1s emitting spontaneous emission.The Ne atom passes to lower level 1s emitting spontaneous emission.and finally the Ne atom comes to ground state through collision withand finally the Ne atom comes to ground state through collision with

tube wall and undergoes radiationless transition.tube wall and undergoes radiationless transition.


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Applications of HeApplications of He--Ne laserNe laser

The Narrow red beam of HeThe Narrow red beam of He--Ne laser is used in supermarkets to readNe laser is used in supermarkets to readbar codes.bar codes.

The HeThe He-- Ne Laser is used in Holography in producing the 3D images ofNe Laser is used in Holography in producing the 3D images of

objects.objects. HeHe--Ne lasers have many industrial and scientific uses, and are oftenNe lasers have many industrial and scientific uses, and are often

used in laboratory demonstrations of optics.used in laboratory demonstrations of optics.

87176-20041-helium-neon_laser-1

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