Hilium Neon Laser

8/4/2019 Hilium Neon Laser

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oundations

In 1917, Albert Einstein established the theoretic foundations for the laser and the maserin the paperZur Quantentheorie der Strahlung(On the Quantum Theory of Radiation);via a re-derivation ofMax Plancks law of radiation, conceptually based upon probabilitycoefficients (Einstein coefficients) for the absorption, spontaneous emission, andstimulated emission of electromagnetic radiation; in 1928,Rudolf W. Ladenburgconfirmed the existences of the phenomena of stimulated emission and negativeabsorption;[8] in 1939, Valentin A. Fabrikant predicted the use of stimulated emission toamplify short waves;[9] in 1947, Willis E. Lamb and R. C. Retherford found apparentstimulated emission in hydrogen spectra and effected the first demonstration ofstimulated emission;[8] in 1950,Alfred Kastler(Nobel Prize for Physics 1966) proposedthe method ofoptical pumping, experimentally confirmed, two years later, by Brossel,

Kastler, and Winter

Helium-Neon Lasers

[Note to reader: topic sentences are in green; remaining weakness in red.]

ABSTRACT

The design of the helium-neon laser is not complex by modern standards. They consist ofonly three essential components and operate by the process of stimulated emission andlight amplification. Because of their many advantages over other types of lasers, helium-neon lasers are used for many applications in research and industry.

The typical helium-neon laser consists of three components: the laser tube, a high-voltagepower supply, and structural packaging. The laser tube consists of a sealed glass tubewhich contains the laser gas, electrodes, and mirrors. Depending on the power output ofthe laser, the tube may vary in size from one to several centimeters in diameter, and fromfive centimeters to several meters in length. The laser gas is a mixture of helium and neonin proportions of between 5:1 and 14:1, respectively. Electrodes, situated near each endof the tube, discharge electricity through the gas. Mirrors, located at each end of the tube,
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increase efficiency. The power supply provides the high voltages needed (10kV to startlaser emission and 1-2kV to maintain it.) The structural packaging consists of mounts forthe laser tube and power supply. The laser may also include safety shutters to preventrandom exposure and external optics to fine-tune the beam.

[Paragraph below this is my revision of it.]The acronym, LASER, stands for LightAmplification by the Stimulated Emission of Radiation. The processes of lightamplification and stimulated emission make the helium-neon laser work. Stimulatedemission occurs when electricity is discharged into the laser gas. Electrons in thedischarge collide with gas atoms imparting energy to them. These energized atoms areleft in an unstable state in which some of their electrons have moved to a higher energylevel. Excited atoms will quickly return to their ground state as their electrons drop totheir normal levels. Each time an electron drops in level, it will emit a photon equal inenergy to the difference between the levels. This type of emission is referred to asspontaneous emission. Stimulated emission occurs when a photon of the proper energystrikes an already excited atom, creating an identical photon. These photons will travel

through the laser gas causing even more stimulated emission. This ever-increasingreproduction of photons is called light amplification. Using this process, the laser caneffectively generate large numbers of photons from relatively few spontaneous emissions.

[Note revision, in navy incorporates the following concepts: (1) Familiar material is putat the beginning of each sentence; unfamiliar at the end. (2) In particular, each scientificterm is mentioned after it is explained, at the end of a sentence. (3) The end of onesentence tends to lead to the subject of the next sentence.]The basic processes in ahelium-neon laser are mirrored in the acronym laser, Light Amplification by StimulatedEmission of Radiation. In the laser tube, the electrodes discharge energetic electronswhich subsequently transfer energy to the laser-gas atoms. These energized atoms,

necessarily unstable, quickly return to their ground state. In the simplest case of a singleexcited level the excited atom spontaneously emits a photon equal to the energydifference between the excited and ground states; this process is called spontaneousemission. More importantly, such a spontaneously emitted photon can itself stimulateanother excited atom to emit a photon. This "stimulated emission of radiation" is thesecond half of the acronym. Moreover since the energy difference between the excitedand ground state of the atom equal the photon's energy, these photons are particularlyeffective at stimulating the emission of additional photons. As the spontaneously emittedphotons travel through the gas encountering exited atoms. they strongly stimulate theemission of photos, thus leading to an amplification in the number of photons. Thissecond process is denoted "light amplification" in the acronym laser.

In helium-neon lasers, the neon atoms are the source of laser light. Because stimulatedemission only takes place when there are excited neon atoms available, the process willquickly come to an end unless the neon atoms are replenished with energy. The heliumatoms in the laser gas carry out the process of re-energizing the neon. Helium is perfectfor this task because it has a meta-stable state (does not decay as quickly) correspondingto the energy required to re-energize the neon. Therefore, not only do the helium atoms


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have the proper energy to re-energize the neon, they can hold onto that energy longenough to transfer it.

The amount of radiation that the neon atoms can emit is insufficient to produce apowerful beam without using some form of amplification. Much like a light bulb, the

photons in the laser gas travel in random directions making it impossible to create afocused beam. The randomness of the photon paths also makes the laser inefficientbecause many photons may escape the tube before stimulating further emission. Thisproblem is solved by placing mirrors at either end of the laser tube. Although manyphotons continue to escape the tube without being productive, those photons that areemitted parallel to the axis between the mirrors will be reflected many times. Each timethe photons are reflected through the laser gas, they can cause more photons to be emittedin the same direction. In a short period of time, the dominant direction of emission will bealong the axis between the mirrors. In standard configurations, one of the mirrors istotally reflective while the other can transmit one percent of all incident light. The beamis formed by the photons that escape through the partially transparent mirror.

While not the most powerful or efficient laser, the helium-neon laser has manyadvantages over other types of lasers. Most lasers have an efficiency of about 1 percent,about ten times the efficiency of the typical helium-neon laser. Most lasers are capable ofdelivering power far in excess of the helium-neon laser's 75 milliwatt limit. Theadvantages of helium-neon lasers are that they can emit visible light, are affordable andhave good beam quality. While most lasers cannot efficiently emit visible light, helium-neon lasers usually emit at 632.8nm, producing a red beam. Helium-neon lasers do notrequire any consumables (sapphire rods or cryogenic gases for example), nor do theygenerate enough heat to require special cooling devices. They also have good beamquality, that is, their beams stay tightly focused even over long distances.

Helium-neon lasers are versatile devices that have many useful applications. They areoften found in integrated bar code readers (the hand-held bar code readers use redsemiconductor lasers or red LEDs.) Because they can emit visible light, helium-neonlasers are used in laser surgery to position the powerful infrared cutting beams. Surveyorstake advantage of the helium-neon laser's good beam quality to take precisemeasurements over long distances or across inaccessible terrain. Red helium-neon lasersare also used in holography.

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Chapter 4 : He-Ne laser 40
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There are several different lasers which are used in the production process for holograms.The most common lasers used in holography are Helium-neon (He-Ne), Helium-cadmium (He-Cd), Argon-ion (Ar+) and Krypton-ion (Kr+) lasers.38 Many types of CWlasers can also be operated in a pulsed mode, though so far none of them seems to besuitable for holography. Monocrystalline aluminium oxide doped with lanthanide

elements such as yttrium ( yttrium aluminium garnet, or YAG crystal ) can be used tochange the wavelength of a laser.

A semiconductor laser is a special kind of light-emitting diode. It produces a beam oflight in the near infrared with a divergence of about 15o, but the cone of emitted light iselliptical rather than circular, so that the beam appears to have originated from a linerather than a point. If the astigmatism of this beam is corrected by means of asphericaloptics, a spatially-coherent beam can be obtained, and this has been used experimentallyfor making holograms. The main attraction of semiconductor lasers is that they are cheapand very small. They also operate at comparatively low voltages and art similar powerrange to that of He-Ne lasers.

There are a number of things to be considered in the choosing of a laser. A laser used toproduce holograms needs good stability, and must be free from vibrations.37The laserbeam must be as plane as possible. A laser beam with multi modes is useless for makingholograms. We want that the laser should have a circular beam diameter without anynoise. The beam diameter is the important parameter in the calculation of the pinhole ofthe spatial filter.

The coherence length of the laser should be as large as possible. If the coherence length issmall, the requirements of the path difference between the object and reference beambecome harder to meet. This means that the path difference between these beams must be

nearly zero. The number of modes in the laser is also an important parameter. Inholography we prefer a laser with as few as possible modes. If we use a multi mode laser,we have problems with low visibility and the contrast in the hologram will be low.

4.1 The laser principle50

The laser consists mainly of three parts. The resonator, an active medium in the resonatorand an energy source for activating the medium. With these components it constitutes aself-excited oscillator.
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Figure 4-1 The basic element of the laser

Three basic interaction process of light with matter are important for the laser.40 These areabsorption, stimulated emission, and spontaneous emission. We assume that two states,of energies E1 and E2, take part in the interaction.7

Absorption is when a photon of energy hv strikes an atom of the laser medium in the stateE1 and disappears, exciting the atom to the higher state E2. The photon can only beabsorbed, if the absorption energy is hv E2 E1. When no suitable energy level is available,no absorption takes place, and the medium is transparent for photons of this energy.

We have stimulated emission when the atomic system has absorbed the energy hv andthus the upper level is occupied, a second photon of energy hv may cause this energy tobe emitted as a photon.33 Then two photons having identical properties leave the atom.Upon absorption, the atomic system starts from the state of lower energy, uponstimulated emission it starts from the state of higher energy. The transmission probability

is equal for both processes.

In spontaneous emission the atomic system in the state of higher energy, E2, decays into astate of lower energy, E1, by the emission of a photon. The word spontaneous indicatesthat the transition take place with the randomness that is characteristic for quantumprocesses.

Where the frequency is given by

(4.1)

E1 Energy level 1, also called ground level.

E2 Energy level 2, also called excited level.

h Planck constant

v frequency

The helium-neon laser, usually abbreviated to He-Ne, is the most common type of gaslaser. The tube contains helium gas at a pressure of about 1 torr and neon pressure of

about 0.1 torr. (a torr is a unit of pressure equivalent to 1/760 of an atmosphere). Themain purpose of the helium is to act as a continuos reservoir of energy (supplied withelectrical discharge) for the neon. This laser is the one that is best suited to general-purpose holography.
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Figure 4-2 Internal design of a modern helium-neon laser

He-Ne lasers as used for holography operate at a wavelength of 632.8 nm, with a powerranging from 0.5 mW to 100 mW. The randomly-polarised type are unsuitable for seriousholography, as the direction of polarisation27 is an important factor for obtaining optimumimage quality. A laser with Brewster angle windows has a somewhat lower output thanits randomly-polarised equivalent, but it has a completely stable plane of polarisation. Inthis thesis work has the choice of laser fell on red He-Ne lasers. In the beginning of theexperimental work there was used a 12 mW red He-Ne laser. During the experimentalwork this laser was changed to a new and more powerful red He-Ne laser with an outputpower of 24 mW. The reason for the choice of this type of laser is the He-Ne laser'sadvantage in laser beam stability, laser modes, beam diameter, coherence length, output

power and price. Another reason is that most of the literature recommends the use of He-Ne laser in the production of holograms.
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Figure 4-3 Energy levels of He and Ne involved in the He-Ne laser.

4.2 Measurements of laser beam stability37

During the recording process for holographic multi-stereograms, it is important that eachof the 70 part holograms are evenly exposed. If the exposure of the film is varied, therewill be areas of the hologram that are brighter than other and the quality of the hologramwill not be as good as desired. This can also happen if some of the part holograms areunder-exposed. The power stability of the laser beam is not decisive for the visibility ofthe hologram, because the ratio between the reference and the object beam will still beconstant. For each part of the hologram the exposure time is constant, and it is thenimportant that the laser's output power is constant to get the same exposure.

For measuring the power stability of the 24 mW He-Ne laser the following set-up wasarranged on the optical table.

Figure 4-4 Optical set-up for measuring of laser power stability.
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The neutral density filter was used to reduce the laser's output power with 50 %, to areadable value for the laser power meter. To detect the power of the laser, the laser powermeter reads the data continuously. This data is then logged in the PC28 by the datalogging software programPICO ADC-1236. The data is logged for two different samplingrates and time lags. The values fromADC-12 are then converted to LOTUS 1-2-3 to

make it possible to present the data in a suitable way.

The first measurement is a short time logging made with sample pr. 100 ms in 10seconds. The other measurement is a long time logging made with 1 sample pr. second in30 minutes. The idea behind two different measures is to see how the laser works duringholographic recordings (short time) and how stable the lasers output power is over time.

Figure 4-5 Laser beam stability for 24 mW He-Ne laser with sample each 100 ms in 10

seconds.

Laser output power data from sample rate at 100 ms in 10 seconds (short time).

Average value : 680.6Standard deviation : 13.1The laser output power stability for this measurement is about 1.9 %.

From Melles Griot product catalog34the laser output power stability is given by 2.5 %.
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Figure 4-6 Laser beam stability for 24 mW He-Ne laser with sample each second in 30minutes.

Laser output data from sample rate at 1 second in 30 minutes (long time).

Average value : 673Standard deviation : 2.8The output power stability for this measurement is about 0.4 %

During the production of a holographic transmission multi-stereogram, where 70different part holograms are exposed onto the film, each exposure is about 10 seconds

and the entire recording process takes about 30 minutes. From figure 4-5 can we see thatthe laser power stability for one part exposure of the film is good, and the measurementagrees with the data from the manufacturer, Melles Griot34. In practice, the spikesmeasured in the short time of measurement should not reduce the hologram's visibility.

From figure 4-6 can we see that the output power from the laser is quite stable over thewhole recording process of 30 minutes. This means, that each of the part holograms onthe multi-stereogram are evenly exposed on the film. The possibility of getting goodresults in the holographic multi-stereogram production with the use of this laser is good.

The laser was turned on at least 3 hours before the measurement was taken. It is very

important that the laser is heated and becomes stable before the recording of holographyis started.

4.3 Laser modes

Laser resonators have two distinct types of modes, transverse and longitudinal.Transverse modes manifest themselves in the cross-sectional profile of the beam, that is,in its intensity pattern. Longitudinal modes correspond to different resonance's along the


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length of the laser cavity which occur at different frequencies or wavelengths within thegain bandwidth of the laser. A single transverse mode laser that oscillates in a singlelongitudinal mode is oscillating at only a single frequency.

Transverse modes are classified according to the number of noughts that appear across

the beam cross section in two directions. The lowest-order, or fundamental mode, whereintensity peaks at the centre, is known as TEM00. The mode with a single nought alongone axis and no nought in the perpendicular direction is TEM01 or TEM10, depending onorientation. A sampling of these modes, which is produced by stable resonators, is shownin figure 4-7.50

Figure 4-7 Lower-order laser modes that can be produced by a stable resonator.

For most applications for example like holography, the TEM00 mode is considered mostdesirable, but multi-mode beams can often deliver more power in a poorer-quality beam,and thus are acceptable for some uses.

The multiple longitudinal mode structure gives rise to a power fluctuation phenomenontermed mode sweeping. All unstabilized helium neon lasers exhibit this effect, which isdue to thermal instability causing variation in the cavity length. As the cavity lengthchanges, there is a small change in mode spacing which is typically 10 kHz or less undernormal conditions.


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However, the absolute wavelength of each cavity mode is also changed by variation intube length. This is typically 2.5 10-3 nm/C; i.e., 2103 MHz/C, depending on the glass typeused for the tube. In effect, the comb of longitudinal modes drifts with respect to theDoppler broadened line centre, repeating its initial relative position in less than 1K.Because of the non-flat, Gaussian profile of the gain curve, the overall power output

changes. If the mode spacing is very small, as with a long laser tube, these changes maybe very small. On the other hand, a short laser tube may have only one or two cavitymodes under the Doppler profile, and the sum of their position on the Gaussisan gaincurve.

This effect is almost identical for all unstabilized commercial TEM00 tubes and is afunction of cavity length. The overall amplitude fluctuations are typical a few percent.

In the production of holographic multi-stereograms, where the recording process can belong, it is very important that the laser is thermal stable. If there is thermal instability andthe output power is changing, the hologram can be unevenly exposed.

4.4 Coherence and visibility629

Ordinary light is disorganised, not capable of producing interference. Such light is calledincoherent. Light from a laser is highly organised, and easily produces interference. Suchlight is called coherent.

Some electromagnetic radiation such as microwaves, radio waves as well as soundwaves, water waves and other mechanical waves can be generated as an infinite numberof waves, one after another. Light wave cannot, because light waves always come inwave trains. The wave trains are of finite length, and each train containing only a limited

number of waves. The length of a wavetrain is called the coherence length.

Figure 4-8 Wavetrain from a laser

Coherence length can be expressed as the product of the number of waves, N, containedin the train and their wave length, .

The formula for coherence length is then given by

s = N (4.2)
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Since the velocity is the distance travelled per unit of time, it takes a wave train of lengths a certain length of time, t, to pass a given point and we get therefore

(4.3)

where c is the velocity of light, and the length of time t is called the coherence time.

In holography it is important that the path difference between the reference and object-beam is zero, or very small.

If the path different between these waves is too long, as long as the coherence length, thecontrast of the image will be very weak and it is impossible to see the image.

4.4.1 Measurements of coherence length

To find the coherence length of the laser, we have to know how many modes the laserhas.

That can be done with help of Michelson interferometer,741 and plotting the visibility as afunction of the path difference between these waves.

With the knowledge of the coherence length and the visibility plot shown in figure 4-10 itis possible to find the difference of the laser beam distance between the reference andobject beam, which reduces the holograms contrast.

Figure 4-9 The Michelson interferometer.
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The light from the laser is divided into two beams by the cube splitter ( 50 : 50 splittingratio). One beam is reflected back onto itself by a fixed mirror, the other one is alsoreflected back by a mirror, but one that can be shifted along the beam. Both reflectedbeams are divided again into two by the beam splitter, whereby one beam from eachmirror propagates to a screen. On

this screen the light intensity is measured by a laser power meter. When the position ofthe adjustable mirror is changed, the interference fringes on the screen also change. Thelight intensity from the laser is measured for several different positions of the adjustablemirror.

The light intensity from the laser is measured for 30 different path lengths of the laser-interferometer arm. The adjustable mirror on Michelson interferometer is changed fromzero path difference to a total of 150 cm path difference, at a step rate of 5 cm. The datafrom the measurements is logged with the help of a software program calledPicolo.36Thedata was logged for one sample for every 100 ms, and a total of 2000 samples. From this

data it is possible to find the coherence length of the laser.

The visibility of the fringes is defined as

(4.4)

Because of the light from the background, this must be corrected.

(4.5)The theoretical visibility for a laser with 3 modes is given by

(4.6)

The visibility data from the Michelson interferometer visibility measurement and thetheoretical visibility is plotted in figure 4-10.
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Figure 4-10 Visibility plot for theoretical and measured data.

From figure 4-10 can we see that the experimental data correspond quite well with thetheoretical visibility of a 3-mode laser. This means that the 24 mW He-Ne laser used inthis thesis has 3 modes. The practical definition of coherence length is the distancetravelled by the laser beam where the visibility is reduced to 1/e2, measured withMichelson interferometer.

The plot of measured data shows the coherence length is around 30 cm.

The visibility maximum occur when the path difference is 0 and 115 cm. It means that(2L=115cm).

From the technical data for the laser, the longitudinal mode spacing is given as 257 MHz.

The formula for the distance between two longitudinal modes is given by

, and . (4.7)

The measured value for the distance between two longitudinal modes fits the value forthe fabrication data of the laser.

The coherence length for a 3-mode laser is given by

Lk 0.596L (4.8)

From the measured data of visibility plot we know that 2L = 115 cm.


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Thus the He-Ne laser has a coherence length of 34 cm.

In holography this is an important value because the visibility plot gives us an idea of thecontrast of the hologram. From the visibility plot of the laser, we can see that the contrastwill

fall to 0.6 if the different between the object and reference beam is 10 cm. The best resultis obtained when the difference between the beams is 0 or 115 cm, when the visibility(contrast) is maximum.

How the Helium-Neon Laser Works?


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Fig.1

Fig.2
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Fig.3

Fig.4

There are three principal elements of a laser, which are (1) an energy pump, (2) an opticalgain medium, and (3) an optical resonator. These three elements are described in detailbelow for the case of the HeNe laser .

(1) Energy pump.

A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in aglass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas, asshown in Fig. 1 and indicated in the diagram ofFig. 2. The discharge current is limited toabout 5 mA by a 91 k ballast resistor. Energetic electrons accelerating from thecathode to the anode collide with He and Ne atoms in the laser tube, producing a largenumber of neutral He and Ne atoms in excited states. He and Ne atoms in excited statescan deexcite and return to their ground states by spontaneously emitting light. This lightmakes up the bright pink-red glow of the plasma that is seen even in the absence of laseraction.

The process of producing He and Ne in specific excited states is known as pumping and

in the HeNe laser this pumping process occurs through electron-atom collisions in adischarge. In other types of lasers, pumping is achieved by light from a bright flashlampor by chemical reactions. Common to all lasers is the need for some process to prepare anensemble of atoms, ions or molecules in appropriate excited states so that a desired typeof light emission can occur.

(2) Optical gain medium.
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To achieve laser action it is necessary to have a large number of atoms in excited statesand to establish what is termed a population inversion. To understand the significance ofa population inversion to HeNe laser action, it is useful to consider the processes leadingto excitation of He and Ne atoms in the discharge, using the simplified diagram of atomicHe and Ne energy levels given in Fig. 3. A description of the rather complex HeNe

excitation process can be given in terms of the following four steps.

(a) An energetic electron collisionally excites a He atom to the state labeled 21S0 in Fig.3. A He atom in this excited state is often written He*(21S0), where the asterisk meansthat the He atom is in an excited state.

(b) The excited He*(21S0) atom collides with an unexcited Ne atom and the atomsexchange internal energy, with an unexcited He atom and excited Ne atom, writtenNe*(3S2), resulting. This energy exchange process occurs with high probability onlybecause of the accidental near equality of the two excitation energies of the two levels inthese atoms.

(c) The 3S2 level of Ne is an example of a metastable atomic state, meaning that it is onlyafter a relatively long period of time - on atomic time scales - that the Ne*(3S2) atomdeexcites to the 2P4 level by emitting a photon of wavelength 6328 . It is this emissionof 6328 light by Ne atoms that, in the presence of a suitable optical configuration, leadsto lasing action.

(d) The excited Ne*(2P4) atom rapidly deexcites to its ground state by emitting additionalphotons or by collisions with the plasma tube walls. Because of the extreme quickness ofthe deexcitation process, at any moment in the HeNe plasma, there are more Ne atoms inthe 3S2 state than there are in the 2P4 state, and a population inversion is said to be

established between these two levels.

When a population inversion is established between the 3S2 and 2P4 levels of the Neatoms in the discharge, the discharge can act as an optical gain or amplification mediumfor light of wavelength 6328 . This is because a photon incident on the gas dischargewill have a greater probability of being replicated in a 3S2-->2P4 stimulated emissionprocess (discussed below) than of being destroyed in the complementary 2P4-->3S2absorption process.

(3) Optical resonator or cavity.

As mentioned in 2(c) above, Ne atoms in the 3S2 metastable state decay spontaneously tothe 2P4 level after a relatively long period of time under normal circumstances; however,a novel circumstance arises if, as shown inFig. 2, a HeNe discharge is placed betweentwo highly reflecting mirrors that form an optical cavity or resonator along the axis of thedischarge. When a resonator structure is in place, photons from the Ne* 3S2-->2P4transition that are emitted along the axis of the cavity can be reflected hundreds of timesbetween the two highly reflecting end mirrors of the cavity. These reflecting photons caninteract with other excited Ne*(3S2) atoms and cause them to emit 6328 light in a
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process known as stimulated emission. The new photon produced in stimulated emissionhas the same wavelength and polarization, and is emitted in the same direction, as thestimulating photon. It is sometimes useful for purposes of analogy to think of thestimulated emission process as a "cloning" process for photons, as depicted in Fig. 4. Thestimulated emission process should be contrasted with spontaneous emission processes

that, because they are not caused by any preceding event, produce photons that areemitted isotropically, with random polarization, and over a broader range of wavelengths.

As stimulated emission processes occur along the axis of the resonator a situationdevelops in which essentially all 3S2-->2P4 Ne* decays contribute deexcitation photons tothe photon stream reflecting between the two mirrors. This photon multiplication (lightamplification) process produces a very large number of photons of the same wavelengthand polarization that travel back and forth between the two cavity mirrors. To extract alight beam from the resonator, it is only necessary to have one of the two resonatormirrors, usually called the output coupler, have a reflectivity of only 99% so that 1% ofthe photons incident on it travel out of the resonator to produce an external laser beam.

The other mirror, called the high reflector, should be as reflective as possible. The smalldiameter, narrow bandwidth, and strong polarization of the HeNe laser beam aredetermined by the properties of the resonator mirrors and other optical components thatlie along the axis of the optical resonator.

Helium-Neon Laser

The most common and inexpensive gas laser, the helium-neon laseris usuallyconstructed to operate in the red at 632.8 nm. It can also be constructed toproduce laser action in the green at 543.5 nm and in the infrared at 1523 nm.

The collimation of the beam is accomplished by mirrors on each end of theevacuated glass tube which contains about 85% helium and 15% neon gas at1/300 atmospheres pressure (Metrologic). These mirrors could be both flat, butthis requires great precision in alignment, so the common laboratory He-Ne

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Laserconcepts

Laser

types
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lasers are manufactured with the semiconfocal mirror arrangement shown.

The helium gas in the laser tube provides the pumping medium to attain thenecessarypopulation inversion for laser action.

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Heliumneon laserFrom Wikipedia, the free encyclopedia

A heliumneon laser orHeNe laser, is a type ofgas laserwhose gain medium consistsof a mixture of helium and neon inside of a small bore capillary tube, usually excited by aDC electrical discharge.

[edit] History of HeNe laser development

The first HeNe laser emitted at 1.15 m in the infrared and was the first gas laser.However a laser that operated at visible wavelengths was much more in demand, and anumber of other neon transitions were investigated to identify ones in which apopulationinversion can be achieved. The 633 nm line was found to have the highest gain in thevisible spectrum, making this the wavelength of choice for most HeNe lasers evermanufactured. However other visible as well as infrared lasing wavelengths are possible,and by using mirror coatings with their peak reflectance at these other wavelengths,
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HeNe lasers could be engineered to employ those transitions; this includes visible lasersappearing red, orange, yellow, and green.[1] Lasing transitions are known from over 100m in the far infrared to 540 nm in the visible. Since visible transitions at wavelengthsother than 633 nm have somewhat lower gain, these lasers generally have lower outputpowers and are more costly. The 3.39 m transition has a very high gain but is prevented

from lasing in an ordinary HeNe laser (of a different intended wavelength) since thecavity and mirrors are lossy at that wavelength. However in high power HeNe lasershaving a particularly long cavity, superluminescenceat 3.39 m can become a nuisance,robbing power from the lasing medium, often requiring additional suppression. The bestknown and most widely used HeNe laser operates at a wavelength of 632.8 nmin the redpart of the visible spectrum. It was developed at Bell Telephone Laboratories in 1962,[2]

18 months after the pioneering demonstration at the same laboratory of the firstcontinuous infrared HeNe gas laser in December 1960.[3]

[edit] Construction and operation

The gain medium of the laser, as suggested by its name, is a mixture ofhelium and neongases, in approximately a 10:1 ratio, contained at low pressure in a glass envelope. Theenergy or pump source of the laser is provided by a high voltage electricaldischargepassed through the gas between electrodes (anodeandcathode) within the tube. A DCcurrent of 3 to 20 mA is typically required forCW operation. The optical cavity of thelaser usually consists of two concave mirrors or one plane and one concave mirror, onehaving very high (typically 99.9%) reflectance and the output couplermirror allowingapproximately 1% transmission.

Schematic diagram of a heliumneon laser

Commercial HeNe lasers are relatively small devices, among gas lasers, having cavitylengths usually ranging from 15 cm to 50 cm (but sometimes up to about 1 meter to

achieve the highest powers), and optical outputpowerlevels ranging from 0.5 to 50 mW.

The red HeNe laser wavelength of 633 nm has an actual vacuum wavelength of632.991 nm, or about 632.816 nm in air. The wavelength of the lasing modes lie withinabout 0.001 nm above or below this value, and the wavelengths of those modes shiftwithin this range due to thermal expansion and contraction of the cavity. Frequency-stabilized versions enable the wavelength of a single mode to be specified to within 1 partin 108 by the technique of comparing the powers of two longitudinal modes in opposite
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polarizations.[4] Absolute stabilization of the laser's frequency (or wavelength) as fine as2.5 parts in 1011 can be obtained through use of an iodine absorption cell.[5]

Energy level diagram of a HeNe laser

The mechanism producingpopulation inversion and light amplificationin a HeNe laserplasma [6] originates with inelastic collision of energetic electrons with ground statehelium 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 long-lived metastable states. Because of a fortuitous near

coincidence between the energy levels of the two He metastable states, and the 3s2 and2s2 (Paschen notation[7]) levels of neon, collisions between these helium metastable atomsand ground state neon atoms results in a selective and efficient transfer of excitationenergy from the helium to neon. This excitation energy transfer process is given by thereaction equations:

He*(23S1) + Ne1S0 He(1S0) + Ne*2s2 + E

and

He*(21S) + Ne1S0 + E He(1S0) + Ne*3s2

where (*) represents an excited state, and E is the small energy difference between theenergy states of the two atoms, of the order of 0.05eV or 387 cm1, which is supplied bykinetic energy. Excitation energy transfer increases the population of the neon 2s2 and 3s2levels manyfold. When the population of these two upper levels exceeds that of thecorresponding lower level neon state, 2p4 to which they are optically connected,population inversion is present. The medium becomes capable of amplifying light in anarrow band at 1.15 m (corresponding to the 2s2 to 2p4 transition) and in a narrow band
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at 632.8 nm (corresponding to the 3s2 to 2p4 transition at 632.8 nm). The 2p4 level isefficiently emptied by fast radiative decay to the 1s state, eventually reaching the groundstate.

The remaining step in utilizing optical amplification to create an optical oscillatoris to

place highly reflecting mirrors at each end of the amplifying medium so that a wave in aparticularspatial mode will reflect back upon itself, gaining more power in each passthan is lost due to transmission through the mirrors and diffraction. When theseconditions are met for one or morelongitudinal modes then radiation in those modes willrapidly build up until gain saturation occurs, resulting in a stable continuous laser beamoutput through the front (typically 99% reflecting) mirror.

Spectrum of a helium neon laser illustrating its very high spectral purity (limited by themeasuring apparatus). The .002 nm bandwidth of the lasing medium is well over 10,000times narrower than the spectral width of a light-emitting diode (whose spectrum isshownhere for comparison), with the bandwidth of a single longitudinal mode beingmuch narrower still.

The gain bandwidth of the HeNe laser is dominated by Doppler broadeningrather thanpressure broadening due to the low gas pressure, and is thus quite narrow: only about1.5 GHz full width for the 633 nm transition.[4][8]With cavities having typical lengths of15 cm to 50 cm, this allows about 2 to 8 longitudinal modes to oscillate simultaneously(however single longitudinal mode units are available for special applications). Thevisible output of the red HeNe laser, longcoherence length, and its excellent spatialquality, makes this laser a useful source forholography and as a wavelength reference forspectroscopy. A stabilized HeNe laser is also one of the benchmark systems for thedefinition of the meter.[5]

Prior to the invention of cheap, abundant diode lasers, red HeNe lasers were widely usedinbarcode scanners at supermarket checkout counters.Laser gyroscopeshave employedHeNe lasers operating at 0.633 m in a ring laserconfiguration. HeNe lasers aregenerally present in educational and research optical laboratories.
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[edit] Applications

Red HeNe lasers have many industrial and scientific uses. They are widely used inlaboratory demonstrations in the field ofopticsin view of their relatively low cost andease of operation compared to other visible lasers producing beams of similar quality in

terms of spatial coherence (a single modegaussian beam) and long coherence length(however since about 1990 semiconductor lasers have offered a lower cost alternative formany such applications). A consumer application of the Red HeNe laser is the LaserDiscplayer, made by Pioneer. The laser is used in the device to read the optical disk

The acronymlaserstands for "light amplification by stimulated emission of radiation."Lasers work as a result of resonant effects. The output of a laser is a coherentelectromagnetic field. In a coherent beam of electromagnetic energy, all the waves havethe same frequencyandphase.

In a basic laser, a chamber called a cavity is designed to internally reflect infrared (IR),visible-light, or ultraviolet (UV) waves so they reinforce each other. The cavity cancontain gases, liquids, or solids. The choice of cavity material determines the wavelengthof the output.

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At each end of the cavity, there is a mirror. One mirror is totally reflective, allowing none

of the energy to pass through; the other mirror is partially reflective, allowingapproximately 5 percent of the energy to pass through. Energy is introduced into thecavity from an external source; this is calledpumping.

As a result of pumping, an electromagnetic field appears inside the laser cavity at thenatural (resonant) frequency of the atoms of the material that fills the cavity. The wavesreflect back and forth between the mirrors. The length of the cavity is such that thereflected and re-reflected wave fronts reinforce each other in phase at the naturalfrequency of the cavity substance. Electromagnetic waves at this resonant frequencyemerge from the end of the cavity having the partially-reflective mirror. The output mayappear as a continuous beam, or as a series of brief, intense pulses.

The ruby laser, a simple and common type, has a rod-shaped cavity made of a mixture ofsolid aluminum oxide and chromium. The output is in pulses that last approximately 500microseconds each. Pumping is done by means of a helical flash tube wrapped around therod. The output is in the red visible range.

The helium-neon laseris another popular type, favored by electronics hobbyists becauseof its moderate cost. As its name implies, it has a cavity filled with helium and neon
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gases. The output of the device is bright crimson. Other gases can be used instead ofhelium and neon, producing beams of different wavelengths. Argon produces a laser withblue visible output. A mixture of nitrogen, carbon dioxide, and helium produces IRoutput.

Lasers are one of the most significant inventions developed during the20th century. Theyhave found a tremendous variety of uses in electronics, computer hardware, medicine,and experimental science.

A helium-neon laser beam has a wavelength in air of 633nm. It takes 1.38 ns for the lightto travel through 30cm of an unknown liquid.What is the wavelength of the laser beam in the liquid?

Laser Applications

Medical applications Welding and Cutting Surveying

Garment industry Laser nuclear fusion Communication

Laser printing CDs and optical discs Spectroscopy

Heat treatment Barcode scanners Laser cooling

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Laserconcepts

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Medical Uses of Lasers

The highly collimated beamof alasercan be further focused to a microscopicdot of extremely high energy density. This makes it useful as a cutting andcauterizing instrument. Lasers are used for photocoagulation of the retina tohalt retinal hemorrhaging and for the tacking of retinal tears. Higher powerlasers are used after cataract surgery if the supportive membrane surroundingthe implanted lens becomes milky. Photodisruption of the membrane oftencan cause it to draw back like a shade, almost instantly restoring vision. Afocused laser can act as an extremely sharp scalpel for delicate surgery,

cauterizing as it cuts. ("Cauterizing" refers to long-standing medical practicesof using a hot instrument or a high frequency electrical probe to singe thetissue around an incision, sealing off tiny blood vessels to stop bleeding.) Thecauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver.

Lasers have been used to make incisions half a micron wide, compared toabout 80 microns for the diameter of a human hair.

Laser applications

Index

Laserconcepts

ReferenceBoraiko

HyperPhysics***** Quantum Physics ***** Optics R Nave Go Back
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Welding and Cutting

The highly collimated beamof alasercan be further focused to a microscopicdot of extremely high energy density for welding and cutting.

The automobile industry makes extensive use ofcarbon dioxide lasers withpowers up to several kilowatts for computer controlled welding on autoassembly lines.

Garmirepoints out an interesting application of CO2 lasers to the welding ofstainless steel handles on copper cooking pots. A nearly impossible task forconventional welding because of the great difference in thermal conductivitiesbetween stainless steel and copper, it is done so quickly by the laser that thethermal conductivities are irrelevant.

Laser applications

Index

Laserconcepts

ReferenceOhanianEssay X


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Surveying and Ranging

Helium-neon and semiconductor lasers have become standard parts of thefield surveyor's equipment. A fast laser pulse is sent to a corner reflector atthe point to be measured and the time of reflection is measured to get thedistance.

Some such surveying is long distance! The Apollo 11 and Apollo 14astronauts put corner reflectors on the surface of the Moon for determinationof the Earth-Moon distance. A powerful laser pulse from the MacDonaldObservatory in Texas had spread to about a 3 km radius by the time it got tothe Moon, but the reflection was strong enough to be detected. We now knowthe range from the Moon to Texas within about 15 cm, a nine significant digitmeasurement. A pulsed ruby laserwas used for this measurement.

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Lasers in the Garment Industry

Laser cutters are credited with keeping the U.S. garment industry competitivein the world market. Computer controlled laser garment cutters can beprogrammed to cut out 400 size 6 and then 700 size 9 garments - and thatmight involve just a few cuts. The programmed cutter can cut dozens tohundreds of thicknesses of cloth, and can cut out every piece of the garmentin a single run.

The usefulness of thelaserfor such cutting operations comes from the factthat the beam is highly collimatedand can be further focused to a microscopicdot of extremely high energy density for cutting.

Laser applications

Index

Laserconcepts

ReferenceOhanian

Essay X


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Lasers in Communication

Fiber optic cables are a major mode of communication partly becausemultiple signals can be sent with high quality and low loss by lightpropagating along the fibers. The light signals can be modulated with theinformation to be sent by either light emitting diodes or lasers. The lasershave significant advantages because they are more nearly monochromatic andthis allows the pulse shape to be maintained better over long distances. If a

better pulse shape can be maintained, then the communication can be sent athigher rates without overlap of the pulses. Ohanian quotes a factor of 10advantage for the laser modulators.

Telephone fiber drivers may be solid state lasers the size of a grain of sandand consume a power of only half a milliwatt. Yet they can sent 50 millionpulses per second into an attached telephone fiber and encode over 600

Index

Laserconcepts

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simultaneous telephone conversations (Ohanian).

Laser applications


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Heat TreatmentHeat treatments for hardening or annealing have been long practiced inmetallurgy. But lasers offer some new possibilities for selective heattreatments of metal parts. For example, lasers can provide localized heattreatments such as the hardening of the surfaces of automobile camshafts.These shafts are manufactured to high precision, and if the entire camshaft isheat treated, some warping will inevitably occur. But the working surfaces ofthe cams can be heated quickly with a carbon dioxide laser and hardenedwithout appreciably affecting the remainder of the shaft, preserving theprecision of manufacture.

Laser applications

Index

Laserconcepts

ReferenceGarmire


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Barcode Scanners

Supermarket scanners typically use helium-neon lasers to scan the universalbarcodes to identify products. The laser beam bounces off a rotating mirrorand scans the code, sending a modulated beam to a light detector and then to acomputer which has the product information stored. Semiconductor lasers canalso be used for this purpose.

Laser applications

Index

Laserconcepts

ReferenceGarmire


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Hilium Neon Laser