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    Different types of laser

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    Contents ..................................................................................................................... 2

    Chemicals used ..................................................................................................... 17

    Construction .......................................................................................................... 18

    Working: ................................................................................................................ 21

    Narrow linewidth dye lasers .................................................................................. 23

    Construction:......................................................................................................... 24

    Applications ........................................................................................................... 26

    Technology ............................................................................................................ 33

    Applications ........................................................................................................... 35

    Medicine ............................................................................................................. 35

    Manufacturing .................................................................................................... 36

    Fluid dynamics ................................................................................................... 36

    Dentistry ............................................................................................................ 36

    Military and defense ........................................................................................... 37

    Cavity ring-down spectroscopy (CRDS) .............................................................. 37

    Laser-induced breakdown spectroscopy (LIBS) .................................................. 37

    Laser pumping ................................................................................................... 38

    Types of Femtosecond Lasers ............................................................................... 38

    Bulk Lasers ......................................................................................................... 38

    Fiber Lasers ........................................................................................................ 39

    Dye Lasers .......................................................................................................... 39

    Semiconductor Lasers ........................................................................................ 39

    Other Types ........................................................................................................ 40

    Important Parameters of Femtosecond Lasers ...................................................... 40


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    LASER stands forLIGHT AMPLIFICATION BY STIMULATEDEMISSION OF RADIATION.Laser is a device used to produce very intense, highly directional,coherent and monochromatic beam of light.Laser of different power and application can be produced by usingdifferent materials.


    there must be a meta stable state in the system.The system must achieve population inversion.The photons emitted must be confined in the system for a time to

    allow them further stimulated emission.


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    The principle of laser production is based on the fact that atoms of amaterial have a number of energy levels in which at least one is metastable state.

    Consider a three level atomic system having energies E1, E2 andE3 respectively.

    Let the atoms are at ground state E1. If photons interact with an atom inground state, the atom absorbs the photon and reaches the excited stateE3 . We know that the excited state is an unstable state, therefore,electron must return back to ground state E1 but such transitions are notallowed and the electron first reach the state E2. Atoms in the stateE3 which has a life time of about 10-8 sec decay spontaneously from state

    E3 to state E2 which is meta stable and has life time of 10-3

    sec . Thismeans that the atoms reach state E2 much faster than they leave stateE2. This results in an increase in number of atoms in state E2, and hencepopulation inversion is achieved.

    Lasers are divided into three main classes depending upon their origin.Solid LaserLiquid Laser

    Gas Laser

    1. Solid-state laser :

    A solid-state laseris a laserthat uses a gain medium that is a solid,

    rather than a liquid such as in dye lasers or a gas as in gas

    lasers.Semiconductor-based lasers are also in the solid state, but are

    generally considered as a separate class from solid-state lasers

    Solid-state lasers are lasers based on solid-state gain media such

    as crystals orglassesdoped with rare earth ortransition metal ions,

    orsemiconductor lasers. (Althoughsemiconductor lasers are of course

    also solid-state devices, they are often not included in the term solid-

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    state lasers.) Ion-doped solid-state lasers (also sometimes

    called doped insulator lasers) can be made in the form ofbulk

    lasers, fiber lasers, or other types ofwaveguide lasers. Solid-state

    lasers may generate output powers between a few milliwatts and(in high-power versions) many kilowatts.

    Types of solid state laser(continous):

    Ruby laser

    Semiconductor laser

    Uranium laser

    And many more.

    Ruby Laser:

    The first laser (which is the abbreviation of the words Light Amplification by

    Stimulated Emission of Radiation) was created in 1961 by Theodore

    Maiman (b.1927) at the Hughes Research Laboratories. He used a rod of

    synthetic ruby as the lasing medium. The crystalline structure of ruby is

    similar to the one of corundum, i.e. a crystal of aluminum oxide (Al2O3), in

    which the small part of atoms of aluminum (about 0,05 %) is replaced with

    ions Cr+++. Ruby rod is illuminated by intense impulse of light, which is

    generated by helical xenon discharge lamp as shown in animation. The

    ends of ruby rod are highly polished and silvered to serve as laser mirrors.The impulse of light creates the inverse population of electrons in ruby rod

    and due to the presence of mirrors the laser generation is excited. The

    duration of the laser impulse is a little bit shorter than the pump impulse of

    the flash lamp.

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    Ruby is an aluminum oxide crystal in which some of the aluminum atoms

    have been replaced with chromium atoms. Chromium gives ruby its

    characteristic red color and is responsible for the lasing behavior of the

    crystal. Chromium atoms Absorb green and blue Light and emit or reflect

    only red light. For a ruby laser, a crystal of ruby is formed into a cylinder.

    A fully reflecting mirror is placed on one end and a partially reflecting

    mirror on the other. A high-intensity lamp is spiraled around the ruby

    cylinder to provide a flash of white light that triggers the Laseraction.

    The green and blue wavelengths in the flash excite electrons in thechromium atoms to a higherEnergy level. Upon returning to their normal

    state, the electrons emit their characteristic ruby-red light. The mirrors

    reflect some of this light back and forth inside the ruby crystal,

    stimulating other excited chromium atoms to produce more red light, until

    the light pulse builds up to high power and drains the energy stored in

    the crystal. The optically pumped, solid-state laser uses sapphire as the

    host lattice and chromium as the active ion. The Emission takes place in

    the red portion of the spectrum.

    Pumping Levels for Ruby Laser

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    The ruby laser is a three level solid state laser. The active laser

    medium (laser gain/amplification medium) is a synthetic ruby rod that is

    energized through optical pumping, typically by a xenon flashtube. Ruby

    has very broad and powerful absorption bands in the visual spectrum, at

    400 and 550 nm, and a very long fluorescence lifetime of 3 milliseconds.

    This allows for very high energy pumping, since the pulse duration can be

    much longer than with other materials. While ruby has a very wide

    absorption profile, its conversion efficiency is much lower than other


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    In early examples, the rod's ends had to be polished with great precision,

    such that the ends of the rod were flat to within a quarter of a wavelength of

    the output light, and parallel to each other within a few seconds of arc. The

    finely polished ends of the rod were silvered; one end completely, the other

    only partially. The rod, with its reflective ends, then acts as a FabryProt

    etalon (or a Gires-Tournois etalon). Modern lasers often use rods

    with antireflection coatings, or with the ends cut and polished at Brewster's

    angle instead. This eliminates the reflections from the ends of the rod.External dielectric mirrors then are used to form the optical cavity.Curved

    mirrors are typically used to relax the alignment tolerances and to form a

    stable resonator, often compensating for thermal lensing of the rod.

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    Transmittance of ruby in optical and near-IR spectra. Note the two broad

    blue and green absorption bands and the narrow absorption band at 694

    nm, which is the wavelength of the ruby laser.

    Ruby also absorbs some of the light at its lasing wavelength. To overcome

    this absorption, the entire length of the rod needs to be pumped, leaving no

    shaded areas near the mountings. The active part of the ruby is the dopant,

    which consists ofchromium ions suspended in a sapphirecrystal. The

    dopant often comprises around 0.05% of the crystal, and is responsible for

    all of the absorption and emission of radiation. Depending on theconcentration of the dopant, synthetic ruby usually comes in either pink or



    Ruby is an aluminum oxide crystal in which some of the aluminum atoms

    have been replaced with chromium atoms.The crystalline structure of ruby

    is similar to the one of corundum, i.e. a crystal of aluminum oxide (Al2O3), in

    which the small part of atoms of aluminum (about 0,05 %) is replaced with

    ions Cr+++. Ruby rod is illuminated by intense impulse of light, which is

    generated by helical xenon discharge lamp


    a ruby laser consist of the ruby rod.ruby is the combination of the al2o3 and

    cr2o3.when the al rod is doped .5% with the cr3+ the color of the ruby rod

    become pink.the length of ruby rod is 4cm and width is .5cm.The apparatus

    is fitted in the glass .rod .Two electrodes are also fitted in the glass rod and

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    connected to the high voltage.the ends of the ruby rod is ground and then

    polished in such a way that one end become partially and other end

    become fully reflecting. The partially is placed at the right and the fully

    reflecting mirror is placed at left side of the rod. The partially reflectingmirror is used to eject the laser beam. The spiral binding is placed all

    around the ruby.

    Ruby Laser and Flash Tube


    External dielectric mirrors then are used to form the optical cavity

    WORKINGLet the electrons are raised from ground state E1 to Excited

    state E3 which has a life time 10-8 sec. The atoms from the state

    E3 make transition to state E2. Since E2 is meta-stable state

    having life time equal to 10-3 sec. This means that the atoms

    reach state E2 much faster than they leave state E2. This results

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    in an increase in the number of atoms in state E2 and hence

    population inversion is achieved.

    In this process few Cr atoms make spontaneous transition from

    E2 to E1 and emitted photons stimulate further transition. In this

    way we obtain an intense, coherent, monochromatic beam of

    red laser.

    1. High-voltage electricity causes the

    quartz flash tube to emit an intense burst oflight, exciting some of the atoms in the rubycrystal to higher energy levels.

    2. At a specific energy level, some atomsemit particles of light called photons. At firstthe photons are emitted in all directions.

    Photons from one atom stimulate emissionof photons from other atoms and the lightintensity is rapidly amplified.

    3. Mirrors at each end reflect the photonsback and forth, continuing this process ofstimulated emission and amplification.


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    4. The photons leave through the partiallysilvered mirror at one end. This is laserlight.


    Ruby lasers have declined in use with the discovery of better lasing

    media. They are still used in a number of applications where short

    pulses of red light are required. Holographers around the worldproduce holographic portraits with ruby lasers, in sizes up to a metre


    Many non-destructive testing labs use ruby lasers to create

    holograms of large objects such as aircraft tires to look for

    weaknesses in the lining.

    Ruby lasers were used extensively in tattoo and hair removal.

    the ruby laser is still used, mainly as a light source for medical and

    cosmetic procedures.

    in high speed photography and pulsed holography.

    Semiconductor Lasers

    Semiconductor lasers are said to be "the laser of the future". The reasons

    are: they are compact, they have the potential of mass production, they can

    be easily integrated, their properties are in rapid improvement, they are

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    becoming more and more powerful and efficient and they have found a

    widespread use as pumps for solidstate lasers.


    The majority of semiconductor materials are based on a combination of

    elements in the third group of the Periodic Table (such as Al, Ga, In) and

    the fifth group (such as N, P, As, Sb) hence referred to as the III-V

    compounds. Examples include GaAs, AlGaAs, InGaAs and InGaAsP

    alloys. The cw laser emission wavelengths are normally within 630~1600

    nm, but recently InGaN semiconductor lasers were found to generate cw410 nm blue light at room temperature. The semiconductor lasers that can

    generate blue-green light uses materials which are the combination of

    elements of the second group (such as Cd and Zn) and the sixth group (S,



    The principle of semiconductor laser is very different from CO2 and Nd:YAG

    lasers. It is based on "Recombination Radiation". We can explain this

    principle by referring to the following figure.

    Principles of semiconductor lasers


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    The semiconductor materials have valence band V and conduction band C,

    the energy level of conduction band is Eg (Eg>0) higher than that of

    valence band. To make things simple, we start our analysis supposing the

    temperature to be 0 K. It can be proved that the conclusions we draw under0 K applies to normal temperatures.

    Under this assumption for nondegenerate semiconductor, initially the

    conduction band is completely empty and the valence band is completely

    filled. Now we excite some electrons from valence band to conduction

    band, after about 1 ps, electrons in the conduction band drop to the lowest

    unoccupied levels of this band, we name the upper boundary of the

    electron energy levels in the conduction band the quasi-Fermi level Efc.

    Meanwhile holes appear in the valence band and electrons near the top of

    the valence band drop to the lowest energy levels of the unoccupied

    valence energy levels, leave on the top of the valence band an empty part.

    We call the new upper boundary energy level of the valence band quasi-

    Fermi level Efv. When electrons in the conduction band run into the valenceband, they will combine with the holes, in the same time they emit photons.

    This is the recombination radiation. Our task is to make this recombination

    radiation to lase. Then several conditions must be met.

    First for the radiation to be amplified, the light energy hn must satisfy:

    Efc - Efv hn Eg

    From this relation we have Efc - Efv Eg. This decides the critical condition.

    The value of Efc and Efv is influenced by the pumping process, i.e., by the

    intensity (N) of the electrons being raised to the conduction band. When N

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    is increased, Efc increases and Efv decreases. The N satisfies Efc - Efv = Eg is

    named Ntr. We inject carriers into the semiconductor material to make the

    free electron intensity to be larger than N th, then the semiconductor exhibits

    a net gain. We put this active medium in a suitable cavity, laser actionoccurs when this net gain overcome losses. The pumping of semiconductor

    lasers can be realized by the beam of another laser, or by an electron

    beam, but the most convenient way is by using electrical current that flows

    through the semiconductor junctions. This uses the semiconductor laser in

    the form of diode.

    The early semiconductor lasers are Homojunction Lasers, which can

    operate cw only at cryogenic temperatures (like T=77K). Homojunction

    means the laser devices use the same material for both the p and n sides

    of the junction. The homojunction lasers were replaced by double-

    heterostructure (DH) lasers which can operate at room temperatures.

    The active medium of DH laser is sandwiched between p and n materials,

    the p and n materials differ from the active material.

    The dimension of semiconductor laser is very small, a typical value is

    100m m *200m m *50 m m. The power conversion efficiency is a few

    percent for the low power units and can reach 30% for laser arrays. The

    output power increases with the volume of the active layers, linear or

    stacked diode laser arrays can generate up to 20W cw and peak power up

    to 100W in quasi-CW operation. A problem with the semiconductor laser is

    its relatively large divergence angle (typical value 1~30 degrees), but its

    defects are being improved quickly. Lower power diode laser systems, of a

    few mW, are used in CD players, optical storage systems, laser printers

    and communications. Diode lasers with Power 0.5W/diode are available,

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    when they are packed into arrays, they can generate power of several kW.

    As we mentioned before, semiconductor lasers are developing quickly, the

    cost will be greatly reduced when they can be mass produced. A very

    important application of diode lasers is for pumping other high energy lasersystems such as Nd:YAG lasers. We will discuss this technique in

    advanced level.

    Typical Characteristics and Applications

    Some typical aspects of semiconductor lasers are:

    Electrical pumping with moderate voltages and high efficiency ispossible particularly for high-power diode lasers, and allows their use

    e.g. as pump sources for highly efficient solid-state lasers ( diode-

    pumped lasers).

    A wide range of wavelengths are accessible with different devices,

    covering much of the visible, near-infrared and mid-infrared spectral

    region. Some devices also allow forwavelength tuning.

    Small laser diodes allow fast switching and modulation of the optical

    power, allowing their use e.g. in transmitters offiber-optic links.

    Such characteristics have made semiconductor lasers the technologically

    most important type of lasers. Theirapplications are extremely widespread,

    including areas as diverse as optical data transmission, optical data

    storage, metrology, spectroscopy, material processing, pumping solid-state lasers ( diode-pumped lasers), and various kinds of medical


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    A dye laseris a laserwhich uses an organicdye as the lasing medium,

    usually as a liquidsolution. Compared to gases and most solid state lasingmedia, a dye can usually be used for a much wider range ofwavelengths.The wide bandwidth makes them particularly suitable fortunable lasers andpulsed lasers. Moreover, the dye can be replaced by another type in orderto generate different wavelengths with the same laser, although this usuallyrequires replacing other optical components in the laser as well.

    Dye lasers were independently discovered by P. P. Sorokin and F. P.Schfer(and colleagues) in 1966.

    In addition to the usual liquid state, dye lasers are also available as solidstate dye lasers (SSDL). SSDL use dye-doped organic matrices as gainmedium.

    Chemicals used

    Rhodamine 6G Chloride powder; mixed with methanol; emitting yellow lightunder the influence of a green laser

    Some of the laser

    dyes are rhodamine, fluorescein, coumarin, stilbene, umbelliferone, tetrace

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    ne,malachite green, and others. While some dyes are actually used in food

    coloring, most dyes are very toxic, and often carcinogenic. Many dyes,

    such as rhodamine 6G, (in its chloride form), can be very corrosive to all

    metals except stainless steel.

    A wide variety of solvents can be used, although some dyes will dissolve

    better in some solvents than in others. Some of the solvents used

    are water, glycol, ethanol, methanol, hexane, cyclohexane,cyclodextrin,

    and many others. Solvents are often highly toxic, and can sometimes be

    absorbed directly through the skin, or through inhaled vapors. Many

    solvents are also extremely flammable.

    Adamantane is added to some dyes to prolong their life.

    Cycloheptatriene and cyclooctatetraene (COT) can be added

    as triplet quenchers for rhodamine G, increasing the laser output power.

    Output power of 1.4 kilowatt at 585 nm was achieved using Rhodamine 6G

    with COT in methanol-water solution.


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    A dielectric mirror used in a dye laser.

    A dye laser consists of an organic dye mixed with a solvent, which may be

    circulated through a dye cell, or streamed through open air using a dye jet.

    A high energy source of light is needed to "pump"the liquid beyond

    its lasing threshold. A fast discharge flashlamp or an external laser is

    usually used for this purpose. Mirrors are also needed to oscillate the light

    produced by the dyes fluorescence, which is amplified with each pass

    through the liquid. The output mirror is normally around 80% reflective,

    while all other mirrors are usually more than 99% reflective. The dye

    solution is usually circulated at high speeds, to help avoid triplet absorption

    and to decrease degradation of the dye. Aprism ordiffraction grating is

    usually mounted in the beam path, to allow tuning of the beam.

    Because the liquid medium of a dye laser can fit any shape, there are a

    multitude of different configurations that can be used. A FabryProt laser

    cavity is usually used for flashlamp pumped lasers, which consists of two

    mirrors, which may be flat or curved, mounted parallel to each other withthe laser medium in between. The dye cell is usually side-pumped, with

    one or more flashlamps running parallel to the dye cell in a reflector cavity.

    The reflector cavity is often water cooled, to prevent thermal shock in the

    dye caused by the large amounts of near-infrared radiation which the

    flashlamp produces. Axial pumped lasers have a hollow, annular-shaped

    flashlamp that surrounds the dye cell, which has lowerinductance for ashorter flash, and improved transfer efficiency. Coaxial pumped lasers have

    an annular dye cell that surrounds the flash lamp, for even better transfer

    efficiency, but have a lower gain due to diffraction losses. Flash pumped

    lasers can only be used for pulsed output.

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    A ring dye laser. P-pump laser beam; G-gain dye jet; A-saturable absorber

    dye jet; M0, M1, M2-planar mirrors; OCoutput coupler; CM1 to CM4-

    curved mirrors.

    A ring laser design is often chosen for continuous operation, although a

    FabryProt design is sometimes used. In a ring laser, the mirrors of the

    laser are positioned to allow the beam to travel in a circular path. The dye

    cell, or cuvette, is usually very small. Sometimes a dye jet is used to help

    avoid reflection losses. The dye is usually pumped with an external laser,

    such as a nitrogen,excimer, orfrequency doubledNd:YAG laser. The liquid

    is circulated at very high speeds, to prevent triplet absorption from cutting

    off the beam. Unlike FabryProt cavities, a ring laser does not

    generate standing waves which cause spatial hole burning, a phenomenon

    where energy becomes trapped in unused portions of the medium between

    the crests of the wave. This leads to a better gain from the lasing medium.

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    A dielectric mirror used in a dye laser as a cavity. There are many other

    types of cavities and resonators which include gratings, prisms, multiple-

    prism grating arrangements, andetalons.


    The dyes used in these lasers contain rather large organic molecules which

    fluoresce when exposed to the proper frequency of light. The incoming light

    excite the dye molecules, which will emit stimulated radiation as long as the

    molecules remain in their initially-formed singlet state. In this state, the

    molecules emit light viafluorescence, and the dye is quite clear to the lasing

    wavelength. Within a microsecond, or less, the molecules will change to

    theirtriplet state. In the triplet state, light is emitted via phosphorescence,

    and the molecules begin to absorb the lasing wavelength, making the dye

    opaque. Liquid dyes also have an extremely high lasing threshold.Flashlamp pumped lasers need a flash with an extremely short duration, to

    deliver the large amounts of energy necessary to bring the dye past

    threshold before triplet absorption overcomes singlet emission. Dye lasers

    with an external pump laser can direct enough energy of the proper

    wavelength into the dye with a relatively small amount of input energy, but

    the dye must be circulated at high speeds to keep the triplet molecules out

    of the beam path.

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    A cuvette used in a dye laser

    Since organic dyes tend to degrade under the influence of light, the dye

    solution is normally circulated from a large reservoir. The dye solution can

    be flowing through a cuvette, i.e., a glass container, or be as a dye jet, i.e.,

    as a sheet-like stream in open air from a specially-shapednozzle. With a

    dye jet, one avoids reflection losses from the glass surfaces and

    contamination of the walls of the cuvette. These advantages come at the

    cost of a more-complicated alignment.

    Liquid dyes are very high gain laser mediums. The beam only needs to

    make a few passes through the liquid for high gains in power, and hence,

    the high transmittance of the output coupler. This high gain nature also

    leads to very high losses, as any reflections generated by the dye cell

    walls, or flashlamp reflector, will dramatically reduce the amount of energy

    available to the beam. Pumping cavities are often coated, anodized, or

    otherwise made of a material that will absorb the lasing wavelength while

    effectively reflecting the pumping energy.

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    Narrow linewidth dye lasers

    Multiple prisms are often used to tune the output of a dye laser


    Dye lasers emission is inherently broad. However, tunable narrow linewidth

    emission has been central to the success of the dye laser. In order to

    produce narrow bandwidth tuning these lasers use many types of cavities

    and resonators which include gratings, prisms, multiple-prism grating

    arrangements, andetalons.

    The first narrow linewidth dye laser used a Galilean telescope as beamexpanderto illuminate the diffraction grating. Next were the grazing-

    incidence grating designs and the multiple-prism grating configurations.

    The various resonators and oscillator designs developed for dye lasers

    have been successfully adapted to other laser types such as the diode


    3. GAS LASER:


    The carbon dioxide laser(CO2 laser) was one of the earliest gas lasers to

    be developed (invented by Kumar Patel ofBell Labs in 1964), and is still

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    one of the most useful. Carbon dioxide lasers are the highest-power

    continuous wave lasers that are currently available. They are also quite

    efficient: the ratio of output power topump power can be as large as 20%.

    The CO2 laser produces a beam ofinfrared light with the

    principalwavelength bands centering around 9.4 and 10.6 micrometers.


    The active laser medium (laser gain/amplification medium) is a gas

    discharge which is air cooled (water cooled in higher power applications).

    The filling gas within the discharge tube consists primarily of:

    Carbon dioxide (CO2) (around 1020%)

    Nitrogen (N2) (around 1020%)

    Hydrogen (H2) and/orxenon (Xe) (a few percent; usually only used in

    a sealed tube.)

    Helium (He) (The remainder of the gas mixture)

    The specific proportions vary according to the particular laser.


    Because CO2 lasers operate in the infrared, special materials are

    necessary for their construction. Typically, the mirrors are silvered, while

    windows and lenses are made of eithergermanium orzinc selenide. For

    high power applications, gold mirrors and zinc selenide windows and

    lenses are preferred. There are also diamond windows and even lenses

    in use. Diamond windows are extremely expensive, but their high thermal

    conductivity and hardness make them useful in high-power applications

    and in dirty environments. Optical elements made of diamond can even

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    be sand blasted without losing their optical properties. Historically, lenses

    and windows were made out of salt (eithersodium chloride orpotassium

    chloride). While the material was inexpensive, the lenses and windows

    degraded slowly with exposure to atmospheric moisture.

    The most basic form of a CO2 laser consists of a gas discharge (with a mix

    close to that specified above) with a total reflectorat one end, and an

    output coupler (usually a semi-reflective coated zinc selenide mirror) at the

    output end. The reflectivity of the output coupleris typically around 5-15%.

    The laser output may also be edge-coupled in higher power systems to

    reduce optical heating problems.

    The CO2 laser can be constructed to have CW powers

    between milliwatts (mW) and hundreds ofkilowatts (kW).It is also very easy

    to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-

    optic switch, giving rise to Q-switched peak powers up to gigawatts(GW) of

    peak power.

    Because the laser transitions are actually on vibration-rotation bands of a

    linear triatomic molecule, the rotational structure of the P and R bands can

    be selected by a tuning element in the laser cavity. Because transmissive

    materials in the infrared are rather lossy, the frequency tuning element is

    almost always a diffraction grating. By rotating the diffraction grating, a

    particular rotational line of the vibrational transition can be selected. The

    finest frequency selection may also be obtained through the use ofan etalon. In practice, together with isotopic substitution, this means that a

    continuous comb of frequencies separated by around 1 cm1 (30 GHz) can

    be used that extend from 880 to 1090 cm1. Such "line-tuneable" carbon

    dioxide lasers are principally of interest in research applications.

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    The population inversion in the laser is achieved by the following sequence:

    1. Electron impact excites vibrational motion of the nitrogen.

    Because nitrogen is a homonuclear molecule, it cannot lose this

    energy byphoton emission, and its excited vibrational levels are

    therefore metastable and live for a long time.

    2. Collisional energy transfer between the nitrogen and the carbon

    dioxide molecule causes vibrational excitation of the carbon dioxide,with sufficient efficiency to lead to the desired population inversion

    necessary for laser operation.

    3. The nitrogen molecules are left in a lower excited state. Their

    transition to ground state takes place by collision with cold helium

    atoms. The resulting hot helium atoms must be cooled in order to

    sustain the ability to produce a population inversion in the carbon

    dioxide molecules. In sealed lasers, this takes place as the helium

    atoms strike the walls of the container. In flow-through lasers, a

    continuous stream of CO2 and nitrogen is excited by the plasma

    discharge and the hot gas mixture is exhausted from the resonator

    by pumps.


    Because of the high power levels available (combined with reasonable cost

    for the laser), CO2 lasers are frequently used in industrial applications

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    forcutting and welding, while lower power level lasers are used for

    engraving.They are also very useful in surgical procedures because water

    (which makes up most biological tissue) absorbs this frequency of light very

    well. Some examples of medical uses are laser surgery, skinresurfacing ("laserfacelifts") (which essentially consist of burning the skin

    to promote collagen formation), and dermabrasion. Also, it could be used to

    treat certain skin conditions such as hirsuties papillaris genitalis by

    removing embarrassing or annoying bumps, podules, etc. Researchers in

    Israel are experimenting with using CO2 lasers to weld human tissue, as an

    alternative to traditional sutures.

    The common plastic Poly (methyl methacrylate) (PMMA) absorbs IR light in

    the 2.825 m wavelength band, so CO2 lasers have been used in recent

    years for fabricating microfluidic devices from it, with channel widths of a

    few hundred micrometers.

    Because the atmosphere is quite transparent to infrared light, CO2 lasers

    are also used for military rangefinding using LIDAR techniques.

    Helium-Neon Laser

    How the Helium-Neon Laser Works?

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    There are three principal elements of a laser, which are (1) an energy

    pump, (2) an optical gain medium, and (3) an optical resonator. These

    three elements are described in detail below 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 a glass tube containing an optimal mixture (typically 5:1 to 7:1) of

    helium and neon gas, as shown in Fig. 1 and indicated in the diagram

    ofFig. 2. The discharge current is limited to about 5 mA by a 91 k ballast

    resistor. Energetic electrons accelerating from the cathode to the anode

    collide with He and Ne atoms in the laser tube, producing a large number of

    neutral He and Ne atoms in excited states. He and Ne atoms in excited

    states can deexcite and return to their ground states by spontaneously

    emitting light. This light makes up the bright pink-red glow of the plasma

    that is seen even in the absence of laser action.

    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 a discharge. In other types of lasers, pumping is

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    achieved by light from a bright flashlamp or by chemical reactions.

    Common to all lasers is the need for some process to prepare an ensemble

    of atoms, ions or molecules in appropriate excited states so that a desired

    type of light emission can occur.

    (2) Optical gain medium.

    To achieve laser action it is necessary to have a large number of atoms in

    excited states and to establish what is termed a population inversion. To

    understand the significance of a population inversion to HeNe laser action,

    it is useful to consider the processes leading to excitation of He and Neatoms in the discharge, using the simplified diagram of atomic He 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 means that the He atom is in an excited state.

    (b) The excited He*(21S0) atom collides with an unexcited Ne atom and the

    atoms exchange internal energy, with an unexcited He atom and excited

    Ne atom, written Ne*(3S2), resulting. This energy exchange process occurs

    with high probability only because of the accidental near equality of the two

    excitation energies of the two levels in these atoms.

    (c) The 3S2 level of Ne is an example of a metastable atomic state,

    meaning that it is only after a relatively long period of time - on atomic time

    scales - that the Ne*(3S2) atom deexcites to the 2P4 level by emitting a

    photon of wavelength 6328 . It is this emission of 6328 light by Ne

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    atoms that, in the presence of a suitable optical configuration, leads to

    lasing action.

    (d) The excited Ne*(2P4) atom rapidly deexcites to its ground state by

    emitting additional photons or by collisions with the plasma tube walls.

    Because of the extreme quickness of the deexcitation process, at any

    moment in the HeNe plasma, there are more Ne atoms in the 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 levelsof the Ne atoms in the discharge, the discharge can act as an optical gain

    or amplification medium for light of wavelength 6328 . This is because a

    photon incident on the gas discharge will have a greater probability of being

    replicated in a 3S2-->2P4stimulated emission process (discussed below)

    than of being destroyed in the complementary 2P4-->3S2 absorption


    (3) Optical resonator or cavity.

    As mentioned in 2(c) above, Ne atoms in the 3S2 metastable state decay

    spontaneously to the 2P4 level after a relatively long period of time under

    normal circumstances; however, a novel circumstance arises if, as shown

    in Fig. 2, a HeNe discharge is placed between two highly reflecting mirrors

    that form an optical cavity or resonator along the axis of the discharge.

    When a resonator structure is in place, photons from the Ne* 3S2--

    >2P4 transition that are emitted along the axis of the cavity can be reflected

    hundreds of times between the two highly reflecting end mirrors of the

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    cavity. These reflecting photons can interact with other excited Ne*(3S2)

    atoms and cause them to emit 6328 light in a process known as

    stimulated emission. The new photon produced in stimulated emission has

    the same wavelength and polarization, and is emitted in the same direction,as the stimulating photon. It is sometimes useful for purposes of analogy to

    think of the stimulated emission process as a "cloning" process for photons,

    as depicted in Fig. 4. The stimulated emission process should be

    contrasted with spontaneous emission processes that, because they are

    not caused by any preceding event, produce photons that are emitted

    isotropically, with random polarization, and over a broader range of


    As stimulated emission processes occur along the axis of the resonator a

    situation develops in which essentially all 3S2-->2P4 Ne* decays contribute

    deexcitation photons to the photon stream reflecting between the two

    mirrors. This photon multiplication (light amplification) process produces a

    very large number of photons of the same wavelength and polarization thattravel back and forth between the two cavity mirrors. To extract a light

    beam from the resonator, it is only necessary to have one of the two

    resonator mirrors, usually called the output coupler, have a reflectivity of

    only 99% so that 1% of the 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 small diameter, narrow

    bandwidth, and strong polarization of the HeNe laser beam are determined

    by the properties of the resonator mirrors and other optical components

    that lie along the axis of the optical resonator.

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    Nd:YAG laser

    (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) is

    acrystal that is used as a lasing medium forsolid-state lasers. The dopant,

    triply ionized neodymium, typically replaces yttrium in the crystal structure

    of the yttrium aluminium garnet (YAG), since they are of similar size.

    Generally the crystalline host is doped with around 1% neodymium by

    atomic percent.

    Laser operation of Nd:YAG was first demonstrated by Geusic et al. at Bell

    Laboratories in 1964.


    Neodymium ions in various types of ionic crystals, and also in glasses, act

    as a laser gain medium, typically emitting 1064 nm light from a particular

    atomic transition in the neodymium ion, after being "pumped" into excitation

    from an external source


    Nd:YAG lasers are optically pumped using a flashtube orlaser diodes.

    These are one of the most common types of laser, and are used for many

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    different applications. Nd:YAG lasers typically emit light with

    awavelength of 1064 nm, in the infrared. However, there are also

    transitions near 940, 1120, 1320, and 1440 nm. Nd:YAG lasers operate in

    both pulsed and continuous mode. Pulsed Nd:YAG lasers are typicallyoperated in the so called Q-switching mode: An optical switch is inserted in

    the laser cavity waiting for a maximum population inversion in the

    neodymium ions before it opens. Then the light wave can run through the

    cavity, depopulating the excited laser medium at maximum population

    inversion. In this Q-switched mode, output powers of 250 megawatts and

    pulse durations of 10 to 25 nanoseconds have been achieved. The high-

    intensity pulses may be efficiently frequency doubled to generate laser light

    at 532 nm, or higher harmonics at 355 and 266 nm.

    Nd:YAG absorbs mostly in the bands between 730760 nm and 790

    820 nm. At low current densitieskrypton flashlamps have higher output in

    those bands than do the more common xenon lamps, which produce more

    light at around 900 nm. The former are therefore more efficient for pumping

    Nd:YAG lasers.

    The amount of the neodymium dopant in the material varies according to its

    use. Forcontinuous wave output, the doping is significantly lower than for

    pulsed lasers. The lightly doped CW rods can be optically distinguished by

    being less colored, almost white, while higher-doped rods are pink-purplish.

    Other common host materials for neodymium are: YLF (yttrium lithiumfluoride, 1047 and 1053 nm), YVO4 (yttrium orthovanadate, 1064 nm),

    and glass. A particular host material is chosen in order to obtain a desired

    combination of optical, mechanical, and thermal properties. Nd:YAG lasers

    and variants arepumped either by flashtubes, continuous gas discharge

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    lamps, or near-infrared laser diodes (DPSS lasers). Prestabilized

    laser(PSL) types of Nd:YAG lasers have proved to be particularly useful in

    providing the main beams forgravitational waveinterferometers such

    as LIGO, VIRGO,GEO600 and TAMA.



    Slit lamp photo of Posterior capsular opacification visible few months after

    implantation of intraocular lens in eye, seen on retroillumination

    Nd:YAG lasers are used in ophthalmology to correct posterior capsular

    opacification, a condition that may occur aftercataract surgery, and for

    peripheral iridotomy in patients withacute angle-closure glaucoma, where it

    has superseded surgical iridectomy. Frequency-doubled Nd:YAG lasers

    (wavelength 532 nm) are used for pan-retinal photocoagulation in patients

    with diabetic retinopathy.

    In oncology, Nd:YAG lasers can be used to remove skin cancers.

    These lasers are also used extensively in the field of cosmetic medicine

    forlaser hair removaland the treatment of minorvasculardefects such

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    as spider veins on the face and legs. Recently used for dissecting cellulitis,

    a rare skin disease usually occurring on the scalp.

    Using hysteroscopy the Nd:YAG laser has been used for removal ofuterine

    septa within the inside of the uterus.

    In podiatry, the Nd:YAG laser is being used to treat onychomycosis, which

    is fungus infection of the toenail. The merits of laser treatment of these

    infections are not yet clear, and research is being done to establish



    Nd:YAG lasers are also used in manufacturing for engraving, etching, or

    marking a variety of metals and plastics. They are extensively used in

    manufacturing forcutting and welding steel, semiconductors and various

    alloys. For automotive applications (cutting and welding steel) the power

    levels are typically 1-5 kW. Super alloy drilling (for gas turbine parts)

    typically uses pulsed Nd:YAG lasers (millisecond pulses, not Q-switched).

    Nd:YAG lasers are also employed to make subsurface markings in

    transparent materials such as glass oracrylic glass. Lasers of up to 400 W

    are used for selective laser melting of metals in additive layered


    Fluid dynamics

    Nd:YAG lasers can also be used for flow visualization techniques in fluid

    dynamics (for example particle image velocimetry or induced fluorescence).


    Nd:YAG lasers are used forsoft tissuesurgeries in the oral cavity, such

    as gingivectomy, periodontal sulcular

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    debridement, LANAP,frenectomy, biopsy, and coagulation of graft donor


    Military and defense

    Military surplus Nd:YAG laser rangefinder firing. The laser fires through a

    collimator, focusing the beam, which blasts a hole through a rubber block,

    releasing a burst of plasma.

    The Nd:YAG laser is the most common laser used in laser

    designators and laser rangefinders.

    Cavity ring-down spectroscopy (CRDS)

    The Nd:YAG may be used in the application ofcavity ring-down

    spectroscopy, which is used to measure the concentration of some light-

    absorbing substance.

    Laser-induced breakdown spectroscopy (LIBS)

    A range of Nd:YAG lasers are used in analysis of elements in the periodic

    table. Though the application by itself is fairly new with respect toconventional methods such as XRF or ICP, it has proven to be less time

    consuming and a cheaper option to test element concentrations. A high-

    power Nd:YAG laser is focused onto the sample surface to

    produce plasma. Light from the plasma is captured by spectrometers and

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    the characteristic spectra of each element can be identified, allowing

    concentrations of elements in the sample to be measured.

    Laser pumping

    Nd:YAG lasers, mainly via their second and third harmonics, are widely

    used to excite dye lasers either in the liquid orsolid state. They are also

    used as pump sources for vibronically broadened solid-state lasers such

    as Cr4+:YAG or via the second harmonic for pumpingTi:sapphire lasers.

    Femtosecond Lasers

    A femtosecond laser is a laserwhich emits optical pulses with

    a duration well below 1 ps ( ultrashort pulses), i.e., in the domain of

    femtoseconds (1 fs = 1015s). It thus also belongs to the category

    ofultrafast lasers orultrashort pulse lasers. The generation of such short

    pulses is nearly always achieved with the technique ofpassive mode


    Types of Femtosecond Lasers

    Bulk Lasers

    Passively mode-lockedsolid-statebulk laserscan emit high-quality

    ultrashort pulses with typical durations between 30 fs and 30 ps.

    Various diode-pumped lasers, e.g. based on neodymium-

    doped orytterbium-doped gain media, operate in this regime, with typical

    average output powers between 100 mW and 1 W. Titaniumsapphire

    lasers with advanced dispersion compensation are even suitable for pulse

    durations below 10 fs, in extreme cases down to approximately 5 fs.

    The pulse repetition rate is in most cases between 50 MHz and 500 MHz,

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    even though there are low repetition rate versions with a few megahertz for

    higher pulse energies, and also miniature lasers with tens of gigahertz.

    Fiber Lasers

    Various types ofultrafast fiber lasers, which are also in most cases

    passively mode-locked, typically offer pulse durations between 50 and

    500 fs, repetition rates between 10 and 100 MHz, and average powers of a

    few milliwatts. Substantially higher average powers andpulse energies are

    possible, e.g. with stretched-pulse fiber lasers or with similariton lasers, or

    in combination with a fiber amplifier. All-fiber solutions can be fairly cost-

    effective in mass production, although the effort required fordevelopment

    of a product with high performance and reliable operation can be

    substantial due to various technical challenges.

    Dye Lasers

    Dye lasers dominated the field of ultrashort pulse generation before the

    advent oftitaniumsapphire lasers. Theirgain bandwidth allows for pulse

    durations of the order of 10 fs, and different laser dyes are suitable for

    emission at various wavelengths, often in the visible spectral range. Mainly

    due to the disadvantages associated with handling a laser dye,

    femtosecond dye lasers are no longer frequently used.

    Semiconductor Lasers

    Some mode-locked diode lasers can generate pulses with femtosecond

    durations. Directly at the laser output, the pulses durations are usually at

    least several hundred femtoseconds, but with external pulse compression,

    much shorter pulse durations can be achieved.

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    It is also possible to passively mode-lock vertical external-cavity surface-

    emitting lasers(VECSELs); these are interesting particularly because they

    can deliver a combination of short pulse durations, high pulse repetition

    rates, and sometimes high average output power, whereas they are notsuitable for high pulse energies.

    Other Types

    More exotic types of femtosecond lasers are color center lasers and free

    electron lasers. The latter can be made to emit femtosecond pulses even in

    the form ofX-rays.

    Important Parameters of Femtosecond Lasers

    The key performance figures of femtosecond lasers are the following:

    the pulse duration (which is in some cases tunable in a certain range)

    the pulse repetition rate (which is in most cases fixed, or tunable only

    within a small range)

    the average output power and pulse energy

    There are, however, various additional aspects which can be important:

    The timebandwidth product (TBP) shows whether the spectral width

    is larger than necessary for the given pulse duration. Thepulse

    qualityincludes additional aspects such as details of the temporal

    and spectral pulse shape, such as the presence of temporal or

    spectral side lobes.

    Some femtosecond lasers offer a stable linearpolarization of the

    output, whereas others emit with an undefined polarization state.

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    The noise properties can differ strongly between different types and

    models of femtosecond lasers. This includes noise of the pulse timing

    ( timing jitter), the pulse energy ( intensity noise), and various

    types ofphase noise. It may also be important to check the stability ofpulse parameters, including the sensitivity of external influences such

    as mechanical vibrations or optical feedback.

    Some lasers have built-in means for stabilizing the pulse repetition

    rate to an external reference, or fortuning the output wavelength.

    The laser output can be delivered into free space e.g. through some

    glass window in the housing, or via a fiber connector.

    Built-in features for monitoring the output power, wavelength, or pulse

    duration, can be convenient.

    Other aspects of potential interest are the size of the housing, the

    electrical power consumption, the cooling requirements, and

    interfaces for synchronization or computer control.

    Apart from these aspects of the laser itself, the quality of the documentation

    material, such as product specifications, user manual, etc., can be of




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