L&P 14 laser types

8/10/2019 L&P 14 laser types

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Important Types of Lasers


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Laser Types

There are literally more than 10,000 types of lasersdeveloped by today.

Most of them are developed only in laboratory, but some

found very broad applications.We will only consider the most common and importantones.


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Solid StateLasers


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Solid State Lasers: General propertiesSolid state lasers use high density solid media as active lasermaterials. Ions are introduced as an impurity into host materials, whichcan be crystalline or glass. Semiconductors are excluded since their

energy levels are quite different.Ions of transition elements, particularly rare earth elements (Nd, Yb,Er, Ho etc.) or transition metals (Cr, Ti etc.) are used most commonlyas dopants.

Host materials include crystals like sapphire (Al 2O3), YLF (YLiF 4), YAG(Y3Al5O12 ), LISAF (LiSrAlF 6), LICAF (LiCaAlF 6), as well as glassesfrom silicate (SiO 2) and phosphate (P 2O5) classes.

All relevant transitions occur within 4f (RE) and 3d (TM) states. Theyare typically electric dipole forbidden, so they have very long lifetimes.

Glasses are easier to fabricate, but crystals have better thermal

properties.


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Ruby LaserRuby laser is the first working laser demonstrated by Ted Maiman.

Host material is corundum (Al 2O3) crystal, dopants are Cr ions.

Lasing Wavelength is 694 nm.

Pumping is done with flashlamps and laser operates in pulsed regime.

Ruby is a three level laser, not particularly easy to operate.

In the past, ruby lasers were used for material processing applications.Now they are seldom used.


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Ruby Laser

Ted Maiman and hisruby laser.


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Neodymium (Nd) LasersNd lasers are the most popular solid state lasers.

Main emission wavelength is 1064 nm.

Host media can be YAG, YLF, vanadate (YVO 4) or glass.

Doping level is typically 1%.

Nd:YAG is one of the best lasers ever built. It is used inmany applications.


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Nd: YAG LaserNd:YAG is a four-level laser. Energy levels result from inner shell 4f-4ftransitions.

Pump bands are at 730 nm and 800 nm.Strongest laser transition is at 1064 nm. 964 nm operation is also possible.

Linewidth at 1064 nm is 126 GHz. At modelocked operation, this supports a

duration of 5 ps. However, it is most commonly operated in the Q-switchedmode with ns pulse duration.

Energy levels of Nd:YAG


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Nd:YAG LaserNd:YAG can be pump with lamps or AlGaAs diodelasers.

Xe lamps are used for pulsed and Kr lamps are used forCW operation.

Nd:YAG is typically used as rods, with 3-6 mm diametersand 5-15 cm length.

Average output powers can beup to kW levels.


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Nd:YAG ApplicationsNd:YAG lasers are used in many applications:

- Material processing: cutting, welding, drillingetc.

- Medical applications: coagulation, tissueevaporation, tissue cutting etc.

- Military applications: range finding, targetdesignation etc.

- Art cleaning

- Spectroscopy

Many other uses...


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Nd:Glass LaserNd: Glass laser is similar to Nd:YAG. However, due toinhomogeneities in glass, linewidth is much broader (about 40

times). Therefore, it is mores suitable for modelocked operation.It can support ~100 fs duration.

Main emission wavelength is shifted to 1054 nm.

Nd:Glass is used for short pulse, high energy, low repetition rateapplication.

National Ignition Facility (NIF) at LLNL will be based onNd:Glass, aiming to deliver 10 MJ pulse energies.


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A snapshot from NIF: you are looking at a laser cavity.


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Other Nd LasersNd:YLF lasers have more linewidth and better beam quality. However, therods are more fragile and difficult to handle. They are used to pumpTi:Sapphire amplifiers.

Nd:YVO4 are preferred for high CW power (>10 W) applications.

A Q-switched Nd:YAG.Find 7 differences from the NIF laser.


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Green Laser PointersGreen laser pointers are actually diode pumped (808 nm) Nd:YVO4 (1054nm) laser, frequency doubled (527 nm) by a KDP crystal.

Packaging is amazingly compact!


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Yb:YAG laserYb:YAG is a quasi-three level laser (lower level is a sublevel of ground state).It needs to be pumped by intense diodes.

Main emission is at 1030 nm, resulting from 4f transitions.

Main pumping bands are 968 and 941 nm They are pumped byInGaAs/GaAs quantum well diode lasers at 943 nm.

Yb:YAG is more suitable for short pulse,high energy operation. It has a highpump threshold, making it not asversatile as Nd:YAG.


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Fiber LasersFiber lasers are an interesting class of solid state lasers. Active media arethe core of rare-earth (Er, Yb etc.) doped fiber (typically single mode silicafiber).

Pump light can be in the core or the cladding.Fiber lasers can be very compact and rugged. They are becoming verypopular after the advent of suitable diode pumps.

Since the fiber core is very small, threshold pump power is a few orders ofmagnitude less as compared to the bulk case.


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Fiber LasersSince core-pumping requires high spatial quality lasers, diode bars andarrays cannot be used. This limits the pump power.

To resolve this problem, high power fiber lasers use pumping from claddingof the fiber.

Cladding pumped Nd and Yb doped fiber can yield ~10 W output power.


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Tunable and Mode-Locked LasersBroad emission bandwidth is desirable both for tunability and short-pulsemodelocked operation.


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Alexandrite LaserAlexandrite is Cr doped chrysoberyl (BeAl 2O4). It is an archtype of tunablesolid-state lasers. It has 100 nm bandwidth centered around 760 nm. Thebandwidth is increased due to very large number of vibrational transitions.

Alexandandrite lasers are typicallylamp pumped. Cavity engineeringis similar to Nd:YAG.


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Titanium Sapphire (Ti:Sapphire) Laser

Ti:Sapphire is the mostwidely used tunable andmodel locked solid-statelaser. It has a bandwidth ofabout 400 nm, centeredaround 800 nm.


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Titanium Sapphire

It can be pumped with a(continuous) Argon laser (~450-515 nm) or a doubled-Nd laser(~532 nm). No diodes areavailable at this wavelength.This is the main drawback ofTi:Sapphire.

Emissions result from 3dtransitions.

Ti:Sapphire lases from~700 nm to ~1000 nm.

(nm)

Pulse energy from a single oscillator is ~1 nJ, but can beamplified up to J levels.


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Cr:LiSaF and Cr:LiCaF LasersThese lasers are more recently developed. Their main advantage is that theycan be pumped by diodes (GaInP / AlGaInP QW). Hence they are compactand inexpensive.

Center wavelength is around 850 nm.


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Dye Lasers


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Dye LasersDye lasers use solutions of organic dyes as active media. Solvents can bealcohol, glycerol or water.

There are mainly three types of laser dyes:

- Polymethine dyes: laser emission in the NIR (700-1500 nm).

- Xanthene dyes: laser emission in the visible. Most common example isRhodamine 6G.

- Coumarin dyes: laser emission in the blue-green.


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Dye Lasers Cover a Very LargeSpectrum


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Dye Electron PhysicsElectronic transitions in dyes result from nearly free lectrons boundwithin the chain. The potential is similar to an infinite well.


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Dye Laser ConstructionDye lasers can be pumped with lamps or other lasers. For high outputpowers, excimer laser (UV) pumping is also used.

Since dyes are highly tunable, a grating in the cavity can be used todetermine oscillation wavelength.

In modelocked operation (achieved by saturable absorbers) ~25 fs durationhas been achived.


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Dye Laser ApplicationsDue to the large wavelength rage, dye lasers are used in many scientificspectroscopic applications.

They are also used in medicine for retinopathy and curing dermatologicaldiseases.


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SemiconductorLasers


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Semiconductor LasersSemiconductor lasers are one of the most important laser typestoday. They can be used directly in applications, as well as to

pump solid state lasers.To work as laser active media, semiconductors must have adirect bandgap (so, elements like Si and Ge would not work).

Emission wavelengths are in the range 630 1600 nm.Majority of semiconductor laser use compounds of III-Velements (Al, Ga, In ... N, P, As, Sb ...). Other compounds are

rare but possible. II-IV compounds yield shorter, IV-VIwavelength yield longer wavelengths.


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Operation and PumpingIn semiconductors, allowed electron energy levels for valence andconduction bands. The transitions between the two bands can give rise toabsorption and emission.

Pumping is achieved by electrical current, where the semiconductor is usedas a diode. In order to achieve lasing action, carrier density in the conductionband should exceed a threshold (equivalent to the population inversion).


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Homojunction LasersThe easiest way of achieving the threshold inversion is to use a p-n junctionin forward bias. If the junction uses the same material on both sides, it iscalled homojunction.

When a forward bias is applied, electrons are injected to the conduction bandfrom the n region.

The problem is that electrons also diffuse into the p region.To prevent this, very low temperatures should be used.


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Double-Heterostructure LasersIn order to overcome the limitations with the homojunction lasers,heterostructures are developed. The compounds on n and p sides aredifferent. The bandgaps can be engineered to confine electrons in theactive region, even at room temperature.

Semiconductor lasers that we use today are predominantly

heterostructures.


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Applications of Semiconductor LasersAs we have seen earlier, semiconductor lasers are inherently low power, yet byusing arrays and bars very high powers can be reached.

Semiconductor lasers are very versatile and can be used in variety ofapplications, from very low to very high power.

Applications include CD players, laser printers, laser pointers, opticalcommunications, laser pumping, medical precesses, materials processing etc.


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Gas Laser

Gas lasers use low density gaseous materials as activemedia. Linewidths are much narrower as compared to solidstate, dye and semiconductor lasers. Main broadeningmechanism is the Doppler effect.

Due to narrow linewidths, optical pumping would be veryineficient. Instead, electrical pumping (continuous, RF orpulsed) is used.

The gas lasers can be made from neutral atoms (He-Ne,metal vapor etc), ions (e.g. Ar +) or molecules (e.g. CO 2).

ll


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Hellium-Neon LaserHe-Ne is the most important noble gas laser.He atoms are used for collisional excitation of Ne. Laser tansistions resultfrom electronic states of Ne. Mixture ratio is typically 5:1.

Main lasing wavelength is 633 nm.Other wavelengths (543 nm, 1150 nm,3390 nm) are also possible.

Pumping is achieved by a longitudinaldischarge.


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He-Ne ApplicationsHe-Ne lasers are low power (~mW). They are used for alignmet purposes,holography, barcode scanners (even though they are being replaced bydiodes now), interferometry etc.


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Ar-Ion LasersThe ground state in this case is singlyionized Ar ion. The ions are then excited toobtain population inversion.

Main lasing wavelengh is 515 nm (blue).Required ion temperature is very high (3000K), hence water cooling is necessary.

Ar-ion lasers are widely used inophtalmology to treat diabetic retinopathy.They are also used to pump Ti:Sapphire

laser.


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CO 2 LasersCO 2 is one of the most powerful lasers. >100 kW powers are possible.

It is used very commonly for hardcore materials processing like cutting andwelding.

Lasing action results from transitions between vibrational levels of CO 2. N2and He is also added for collisional excitation.

Main lasing wavelength is 10.6 m.


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CO 2 LasersCO 2 lasers are typically RF discharge pumped. They can operatepulsed or CW. They are mostly used in industry and medicine, wherehigh powers are needed.

E i L


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Excimer LasersThese laser are based on a very exotic class of molecules called eximers(excited dimers).

Excimers are bound molecules of excited state of a noble gas. Lasing

wavelength is in the UV.

ion

neutralatom

Some examples are KrF (248 nm), ArF(193 nm), XeF (351 nm).

Electrical pumping generates reactions ofthe form:

Excimer lasers are best known for LASIK operations. However,they are also used for precision micromaching.

L&P 14 laser types

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