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United States Air Force laser experiment
LaserFrom Wikipedia, the free encyclopedia
A laseris a device that emitslight (electromagnetic radiation)
through a process of optical
amplification based on the
stimulated emission of photons.
The term "laser" originated as anacronym forLight Amplification
by Stimulated Emission of
Radiation.[1][2]The emitted laser
light is notable for its high degree
of spatial and temporal
coherence.
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Red (635 nm), green (532 nm), and blue-vio
(445 nm) lasers
Spatial coherence is typically
expressed through the output
being a narrow beam which is
diffraction-limited, often a so-
called "pencil beam." Laser
beams can be focused to very tiny
spots, achieving a very high
irradiance, or they can be
launched into beams of very low
divergence in order toconcentrate their power at a large
distance.
Temporal (or longitudinal)
coherence implies a polarized
wave at a single frequency whose
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phase is correlated over a
relatively large distance (the coherence length) along the beam.[3]A beam pr
by a thermal or other incoherent light source has an instantaneous amplitude
phase which vary randomly with respect to time and position, and thus a verycoherence length.
Most so-called "single wavelength" lasers actually produce radiation in sever
having slightly different frequencies (wavelengths), often not in a single pola
And although temporal coherence implies monochromaticity, there are even
that emit a broad spectrum of light, or emit different wavelengths of lightsimultaneously. There are some lasers which are not single spatial mode and
consequently their light beams diverge more than required by the diffraction
However all such devices are classified as "lasers" based on their method of
producing that light: stimulated emission. Lasers are employed in application
light of the required spatial or temporal coherence could not be produced usi
simpler technologies.
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Contents
1 Terminology2 Design3 Laser physics
3.1 Stimulated emission3.2 Gain medium and cavity3.3 The light emitted3.4 Quantum vs. classical emission processes
4 Continuous and pulsed modes of operation4.1 Continuous wave operation4.2 Pulsed operation
4.2.1 Q-switching
4.2.2 Mode-locking
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4.2.3 Pulsed pumping5 History
5.1 Foundations5.2 Maser
5.3 Laser5.4 Recent innovations
6 Types and operating principles6.1 Gas lasers
6.1.1 Chemical lasers6.1.2 Excimer lasers
6.2 Solid-state lasers6.3 Fiber lasers6.4 Photonic crystal lasers6.5 Semiconductor lasers6.6 Dye lasers6.7 Free electron lasers
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6.8 Bio laser6.9 Exotic laser media
7 Uses7.1 Examples by power
7.2 Hobby uses8 Safety9 As weapons10 Fictional predictions11 See also12 References
13 External links
Terminology
The word laserstarted as an acronym for "light amplification by stimulated e
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Laser beams in fog, reflectedwindshield
of radiation"; in modern usage "light" broadly
denotes electromagnetic radiation of any
frequency, not only visible light, hence
infrared laser, ultraviolet laser,X-ray laser,
and so on. Because the microwave
predecessor of the laser, the maser, was
developed first, devices of this sort operating
at microwave and radio frequencies are
referred to as "masers" rather than
"microwave lasers" or "radio lasers". In theearly technical literature, especially at Bell
Telephone Laboratories, the laser was called
an optical maser; this term is now
obsolete.[4]
A laser which produces light by itself is technically an optical oscillator rathe
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an optical amplifier as suggested by the acronym. It has been humorously no
the acronym LOSER, for "light oscillation by stimulated emission of radiatio
would have been more correct.[5]With the widespread use of the original acr
a common noun, actual optical amplifiers have come to be referred to as "lasamplifiers", notwithstanding the apparent redundancy in that designation.
The back-formed verb to laseis frequently used in the field, meaning "to pro
laser light,"[6]especially in reference to the gain medium of a laser; when a l
operating it is said to be "lasing." Further use of the words laserand maserin
extended sense, not referring to laser technology or devices, can be seen in usuch as astrophysical maserand atom laser.
Design
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Components of a typical lase
1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
Main article: Laser construction
A laser consists of a gain medium, a
mechanism to supply energy to it, and
something to provide optical feedback.[7]The
gain medium is a material with properties that
allow it to amplify light by stimulated
emission. Light of a specific wavelength that
passes through the gain medium is amplified
(increases in power).
For the gain medium to amplify light, it needs
to be supplied with energy. This process is
called pumping. The energy is typically
supplied as an electrical current, or as light at
a different wavelength. Pump light may be
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animation explaining the stimulated emission and
Laser principle
provided by a flash lamp or
by another laser.
The most common type of
laser uses feedback from anoptical cavitya pair of
mirrors on either end of the
gain medium. Light bounces
back and forth between the
mirrors, passing through thegain medium and being
amplified each time.
Typically one of the two
mirrors, the output coupler,
is partially transparent. Some of the light escapes through this mirror. Depen
the design of the cavity (whether the mirrors are flat or curved), the light com
0:00
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of the laser may spread out or form a narrow beam. This type of device is som
called a laser oscillatorin analogy to electronic oscillators, in which an elect
amplifier receives electrical feedback that causes it to produce a signal.
Most practical lasers contain additional elements that affect properties of thelight such as the polarization, the wavelength, and the shape of the beam.
Laser physics
See also: Laser science
Electrons and how they interact with electromagnetic fields are important in
understanding of chemistry and physics.
Stimulated emission
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In the classical view, the energy of an electron orbiting an atomic nucleus is
for orbits further from the nucleus of an atom. However, quantum mechanica
force electrons to take on discrete positions in orbitals. Thus, electrons are fo
specific energy levels of an atom, two of which are shown below:
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When an electron absorbs energy either from light (photons) or heat (phonon
receives that incident quantum of energy. But transitions are only allowed in
discrete energy levels such as the two shown above. This leads to emission li
absorption lines.
When an electron is excited from a lower to a higher energy level, it will not
way forever. An electron in an excited state may decay to a lower energy sta
is not occupied, according to a particular time constant characterizing that tra
When such an electron decays without external influence, emitting a photon,
called "spontaneous emission". The phase associated with the photon that is e
is random. A material with many atoms in such an excited state may thus res
radiation which is very spectrally limited (centered around one wavelength o
but the individual photons would have no common phase relationship and wo
emanate in random directions. This is the mechanism of fluorescence and the
emission.
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An external electromagnetic field at a frequency associated with a transition
affect the quantum mechanical state of the atom. As the electron in the atom
transition between two stationary states (neither of which shows a dipole fiel
enters a transition state which does have a dipole field, and which acts like a
electric dipole, and this dipole oscillates at a characteristic frequency. In respthe external electric field at this frequency, the probability of the atom enteri
transition state is greatly increased. Thus, the rate of transitions between two
stationary states is enhanced beyond that due to spontaneous emission. Such
transition to the higher state is called absorption, and it destroys an incident p
(the photon's energy goes into powering the increased energy of the higher sttransition from the higher to a lower energy state, however, produces an addi
photon; this is the process of stimulated emission.
Gain medium and cavity
The gain medium is excited by an external source of energy into an excited s
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most lasers this medium consists
of population of atoms which
have been excited into such a
state by means of an outside light
source, or an electrical fieldwhich supplies energy for atoms
to absorb and be transformed into
their excited states.
The gain medium of a laser is
normally a material of controlled
purity, size, concentration, and
shape, which amplifies the beam
by the process of stimulated
emission described above. This
material can be of any state: gas,
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A helium-neon laser demonstration at the K
Brossel Laboratory at Univ. Paris 6. The pin
orange glow running through the center of t
is from the electric discharge which produc
incoherent light, just as in a neon tube. This
plasma is excited and then acts as the gain m
through which the internal beam passes, as
reflected between the two mirrors. Laser rad
output through the front mirror can be seen
produce a tiny (about 1mm in diameter) inteon the screen, to the right. Although it is a d
pure red color, spots of laser light are so int
cameras are typically overexposed and disto
color.
liquid, solid, or plasma. The gain
medium absorbs pump energy,
which raises some electrons into
higher-energy ("excited")
quantum states. Particles caninteract with light by either
absorbing or emitting photons.
Emission can be spontaneous or
stimulated. In the latter case, the
photon is emitted in the samedirection as the light that is
passing by. When the number of
particles in one excited state
exceeds the number of particles
in some lower-energy state,
population inversion is achieved and the amount of stimulated emission due
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Spectrum of a helium neon la
illustrating its very high specpurity (limited by the measur
apparatus). The .002 nm band
the lasing medium is well ove
times narrower than the spect
of a light-emitting diode (who
spectrum is shown herefor
that passes through is larger than the amount
of absorption. Hence, the light is amplified.
By itself, this makes an optical amplifier.
When an optical amplifier is placed inside a
resonant optical cavity, one obtains a laser
oscillator.[8]
In a few situations it is possible to obtain
lasing with only a single pass of EM radiation
through the gain medium, and this produces alaser beam without any need for a resonant or
reflective cavity (see for example nitrogen
laser).[9]Thus, reflection in a resonant cavity
is usually required for a laser, but is not
absolutely necessary.
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comparison), with the bandw
single longitudinal mode bein
narrower still.
The optical resonator is sometimes referred to
as an "optical cavity", but this is a misnomer:
lasers use open resonators as opposed to the
literal cavity that would be employed at
microwave frequencies in a maser. Theresonator typically consists of two mirrors between which a coherent beam o
travels in both directions, reflecting back on itself so that an average photon
through the gain medium repeatedly before it is emitted from the output aper
lost to diffraction or absorption. If the gain (amplification) in the medium is l
than the resonator losses, then the power of the recirculating light can riseexponentially. But each stimulated emission event returns an atom from its e
state to the ground state, reducing the gain of the medium. With increasing b
power the net gain (gain minus loss) reduces to unity and the gain medium is
be saturated. In a continuous wave (CW) laser, the balance of pump power a
gain saturation and cavity losses produces an equilibrium value of the laser p
inside the cavity; this equilibrium determines the operating point of the laser
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applied pump power is too small, the gain will never be sufficient to overcom
resonator losses, and laser light will not be produced. The minimum pump po
needed to begin laser action is called the lasing threshold. The gain medium
amplify any photons passing through it, regardless of direction; but only the
in a spatial mode supported by the resonator will pass more than once througmedium and receive substantial amplification.
The light emitted
The light generated by stimulated emission is very similar to the input signalof wavelength, phase, and polarization. This gives laser light its characteristi
coherence, and allows it to maintain the uniform polarization and often
monochromaticity established by the optical cavity design.
The beam in the cavity and the output beam of the laser, when travelling in fr
(or a homogenous medium) rather than waveguides (as in an optical fiber las
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be approximated as a Gaussian beam in most lasers; such beams exhibit the m
divergence for a given diameter. However some high power lasers may be
multimode, with the transverse modes often approximated using Hermite-Ga
Laguerre-Gaussian functions. It has been shown that unstable laser resonator
used in most lasers) produce fractal shaped beams.[10]Near the beam "waist"
focal region) it is highly collimated: the wavefronts are planar, normal to the
direction of propagation, with no beam divergence at that point. However du
diffraction, that can only remain true well within the Rayleigh range. The be
single transverse mode (gaussian beam) laser eventually diverges at an angle
varies inversely with the beam diameter, as required by diffraction theory. Th"pencil beam" directly generated by a common helium-neon laser would spre
to a size of perhaps 500 kilometers when shone on the Moon (from the distan
the earth). On the other hand the light from a semiconductor laser typically e
tiny crystal with a large divergence: up to 50. However even such a diverge
can be transformed into a similarly collimated beam by means of a lens syste
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always included, for instance, in a laser pointer whose light originates from a
diode. That is possible due to the light being of a single spatial mode. This un
property of laser light, spatial coherence, cannot be replicated using standard
sources (except by discarding most of the light) as can be appreciated by com
the beam from a flashlight (torch) or spotlight to that of almost any laser.
Quantum vs. classical emission processes
The mechanism of producing radiation in a laser relies on stimulated emissio
energy is extracted from a transition in an atom or molecule. This is a quantuphenomenon discovered by Einstein who derived the relationship between th
coefficient describing spontaneous emission and the B coefficient which app
absorption and stimulated emission. However in the case of the free electron
atomic energy levels are not involved; it appears that the operation of this rat
exotic device can be explained without reference to quantum mechanics.
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Continuous and pulsed modes of operation
A laser can be classified as operating in either continuous or pulsed mode, de
on whether the power output is essentially continuous over time or whether itakes the form of pulses of light on one or another time scale. Of course even
whose output is normally continuous can be intentionally turned on and off a
rate in order to create pulses of light. When the modulation rate is on time sc
much slower than the cavity lifetime and the time period over which energy
stored in the lasing medium or pumping mechanism, then it is still classified
"modulated" or "pulsed" continuous wave laser. Most laser diodes used incommunication systems fall in that category.
Continuous wave operation
Some applications of lasers depend on a beam whose output power is constan
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time. Such a laser is known as continuous wave(CW). Many types of lasers c
made to operate in continuous wave mode to satisfy such an application. Ma
these lasers actually lase in several longitudinal modes at the same time, and
between the slightly different optical frequencies of those oscillations will in
produce amplitude variations on time scales shorter than the round-trip time reciprocal of the frequency spacing between modes), typically a few nanosec
less. In most cases these lasers are still termed "continuous wave" as their ou
power is steady when averaged over any longer time periods, with the very h
frequency power variations having little or no impact in the intended applica
(However the term is not applied to mode-locked lasers, where theintention
create very short pulses at the rate of the round-trip time).
For continuous wave operation it is required for the population inversion of t
medium to be continually replenished by a steady pump source. In some lasi
this is impossible. In some other lasers it would require pumping the laser at
high continuous power level which would be impractical or destroy the laser
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producing excessive heat. Such lasers cannot be run in CW mode.
Pulsed operation
Pulsed operation of lasers refers to any laser not classified as continuous wavthat the optical power appears in pulses of some duration at some repetition r
encompasses a wide range of technologies addressing a number of different
motivations. Some lasers are pulsed simply because they cannot be run in co
mode.
In other cases the application requires the production of pulses having as larg
energy as possible. Since the pulse energy is equal to the average power divi
the repetition rate, this goal can sometimes be satisfied by lowering the rate o
so that more energy can be built up in between pulses. In laser ablation for ex
a small volume of material at the surface of a work piece can be evaporated i
heated in a very short time, whereas supplying the energy gradually would al
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the heat to be absorbed into the bulk of the piece, never attaining a sufficient
temperature at a particular point.
Other applications rely on the peak pulse power (rather than the energy in the
especially in order to obtain nonlinear optical effects. For a given pulse energrequires creating pulses of the shortest possible duration utilizing techniques
Q-switching.
The optical bandwidth of a pulse cannot be narrower than the reciprocal of th
width. In the case of extremely short pulses, that implies lasing over a consid
bandwidth, quite contrary to the very narrow bandwidths typical of CW laserlasing medium in some dye lasersand vibronic solid-state lasersproduces op
gain over a wide bandwidth, making a laser possible which can thus generate
of light as short as a few femtoseconds (10!15s).
Q-switching
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Main article: Q-switching
In a Q-switched laser, the population inversion is allowed to build up by intro
loss inside the resonator which exceeds the gain of the medium; this can also
described as a reduction of the quality factor or 'Q' of the cavity. Then, after energy stored in the laser medium has approached the maximum possible lev
introduced loss mechanism (often an electro- or acousto-optical element) is r
removed (or that occurs by itself in a passive device), allowing lasing to begi
rapidly obtains the stored energy in the gain medium. This results in a short p
incorporating that energy, and thus a high peak power.
Mode-locking
Main article: Mode-locking
A mode-locked laser is capable of emitting extremely short pulses on the ord
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tens of picoseconds down to less than 10 femtoseconds. These pulses will rep
the round trip time, that is, the time that it takes light to complete one round t
between the mirrors comprising the resonator. Due to the Fourier limit (also
as energy-time uncertainty), a pulse of such short temporal length has a spec
spread over a considerable bandwidth. Thus such a gain medium must have abandwidth sufficiently broad to amplify those frequencies. An example of a
material is titanium-doped, artificially grown sapphire (Ti:sapphire) which h
wide gain bandwidth and can thus produce pulses of only a few femtosecond
duration.
Such mode-locked lasers are a most versatile tool for researching processes oon extremely short time scales (known as femtosecond physics, femtosecond
chemistry and ultrafast science), for maximizing the effect of nonlinearity in
materials (e.g. in second-harmonic generation, parametric down-conversion,
parametric oscillators and the like) due to the large peak power, and in ablati
applications.[citation needed]
Again, because of the extremely short pulse durat
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a laser will produce pulses which achieve an extremely high peak power.
Pulsed pumping
Another method of achieving pulsed laser operation is to pump the laser matwith a source that is itself pulsed, either through electronic charging in the ca
flash lamps, or another laser which is already pulsed. Pulsed pumping was
historically used with dye lasers where the inverted population lifetime of a d
molecule was so short that a high energy, fast pump was needed. The way to
overcome this problem was to charge up large capacitors which are then switdischarge through flashlamps, producing an intense flash. Pulsed pumping is
required for three-level lasers in which the lower energy level rapidly becom
populated preventing further lasing until those atoms relax to the ground stat
lasers, such as the excimer laser and the copper vapor laser, can never be ope
CW mode.
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History
Foundations
In 1917, Albert Einstein established the theoretical foundations for the laser
maser in the paperZur Quantentheorie der Strahlung(On the Quantum Theo
Radiation); via a re-derivation of Max Plancks law of radiation, conceptuall
upon probability coefficients (Einstein coefficients) for the absorption, spont
emission, and stimulated emission of electromagnetic radiation; in 1928, Rud
Ladenburg confirmed the existences of the phenomena of stimulated emissionegative absorption;[11]in 1939, Valentin A. Fabrikant predicted the use of
stimulated emission to amplify short waves;[12]in 1947, Willis E. Lamb an
Retherford found apparent stimulated emission in hydrogen spectra and effec
first demonstration of stimulated emission;[11]in 1950, Alfred Kastler (Nobe
for Physics 1966) proposed the method of optical pumping, experimentally
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confirmed, two years later, by Brossel, Kastler, and Winter.[13]
Maser
Main article: Maser
In 1953, Charles Hard Townes and graduate students James P. Gordon and H
Zeiger produced the first microwave amplifier, a device operating on similar
principles to the laser, but amplifying microwave radiation rather than infrare
visible radiation. Townes's maser was incapable of continuous output.
[citation
Meanwhile, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov w
independently working on the quantum oscillator and solved the problem of
continuous-output systems by using more than two energy levels. These gain
could release stimulated emissions between an excited state and a lower exci
not the ground state, facilitating the maintenance of a population inversion. I
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Aleksandr Prokhoro
Prokhorov and Basov suggested optical pumping of a
multi-level system as a method for obtaining the
population inversion, later a main method of laser
pumping.
Townes reports that several eminent physicists
among them Niels Bohr, John von Neumann, Isidor
Rabi, Polykarp Kusch, and Llewellyn Thomas
argued the maser violated Heisenberg's uncertainty
principle and hence could not work.[1]
(http://books.google.com/books?
id=VrbD41GGeJYC&pg=PA69&lpg=PA69&dq=%22niels+bohr%22+rabi+
http://books.google.com/books?id=VrbD41GGeJYC&pg=PA69&lpg=PA69&dq=%22niels+bohr%22+rabi+kusch+von+neumann+laser&source=web&ots=0_A7OuramT&sig=4R4yTmk6SmJTN8mZaiOMzgg-LO4http://en.wikipedia.org/wiki/Uncertainty_principlehttp://en.wikipedia.org/wiki/Llewellyn_Thomashttp://en.wikipedia.org/wiki/Polykarp_Kuschhttp://en.wikipedia.org/wiki/Isidor_Rabihttp://en.wikipedia.org/wiki/John_von_Neumannhttp://en.wikipedia.org/wiki/Niels_Bohrhttp://en.wikipedia.org/wiki/File:Aleksandr_Prokhorov.jpg8/11/2019 Laser - Wikipedia 3x5 L - Benjamin Riegel
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on+neumann+laser&source=web&ots=0_A7OuramT&sig=4R4yTmk6SmJT
OMzgg-LO4) In 1964 Charles H. Townes, Nikolay Basov, and Aleksandr Pr
shared the Nobel Prize in Physics, for fundamental work in the field of quan
electronics, which has led to the construction of oscillators and amplifiers ba
the maserlaser principle.
Laser
In 1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell L
began a serious study of the infrared laser. As ideas developed, they abandon
infrared radiation to instead concentrate upon visible light. The concept origi
was called an "optical maser". In 1958, Bell Labs filed a patent application fo
proposed optical maser; and Schawlow and Townes submitted a manuscript
theoretical calculations to the Physical Review, published that year in Volum
Issue No. 6.
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Simultaneously, at Columbia University,
graduate student Gordon Gould was working
on a doctoral thesis about the energy levels of
excited thallium. When Gould and Townes
met, they spoke of radiation emission, as ageneral subject; afterwards, in November
1957, Gould noted his ideas for a laser,
including using an open resonator (later an
essential laser-device component). Moreover,
in 1958, Prokhorov independently proposed
using an open resonator, the first published
appearance (the USSR) of this idea.
Elsewhere, in the U.S., Schawlow and
Townes had agreed to an open-resonator laser
design apparently unaware of Prokhorovs
publications and Goulds unpublished laser
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LASER notebook:First pag
notebook wherein Gordon Go
coined the LASER acronym,
described the technologic ele
for constructing the device.
work.
At a conference in 1959, Gordon Gould
published the term LASER in the paper The
LASER, Light Amplification by Stimulatedmission of Radiation.[1][5]Goulds linguistic
intention was using the -aser word particle
as a suffix to accurately denote the spectrum of the light emitted by the LA
device; thus x-rays:xaser, ultraviolet: uvaser, et cetera; none established itse
discrete term, although raser was briefly popular for denoting radio-freque
emitting devices.
Goulds notes included possible applications for a laser, such as spectrometry
interferometry, radar, and nuclear fusion. He continued developing the idea,
a patent application in April 1959. The U.S. Patent Office denied his applicat
awarded a patent to Bell Labs, in 1960. That provoked a twenty-eight-year la
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featuring scientific prestige and money as the stakes. Gould won his first min
patent in 1977, yet it was not until 1987 that he won the first significant paten
lawsuit victory, when a Federal judge ordered the U.S. Patent Office to issue
to Gould for the optically pumped and the gas discharge laser devices. The q
of just how to assign credit for inventing the laser remains unresolved byhistorians.[14]
On May 16, 1960, Theodore H. Maiman operated the first functioning laser,[
Hughes Research Laboratories, Malibu, California, ahead of several research
including those of Townes, at Columbia University, Arthur Schawlow, at Be
Labs,[17]and Gould, at the TRG (Technical Research Group) company. Maim
functional laser used a solid-state flashlamp-pumped synthetic ruby crystal to
produce red laser light, at 694 nanometres wavelength; however, the device o
capable of pulsed operation, because of its three-level pumping design schem
in 1960, the Iranian physicist Ali Javan, and William R. Bennett, and Donald
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Herriott, constructed the first gas laser, using helium and neon that was capab
continuous operation in the infrared (U.S. Patent 3,149,290); later, Javan rece
Albert Einstein Award in 1993. Basov and Javan proposed the semiconducto
diode concept. In 1962, Robert N. Hall demonstrated the first laser diodedev
made of gallium arsenide and emitted at 850 nm the near-infrared band of thespectrum. Later, in 1962, Nick Holonyak, Jr. demonstrated the first semicond
laser with a visible emission. This first semiconductor laser could only be us
pulsed-beam operation, and when cooled to liquid nitrogen temperatures (77
1970, Zhores Alferov, in the USSR, and Izuo Hayashi and Morton Panish of
Telephone Laboratories also independently developed room-temperature, con
operation diode lasers, using the heterojunction structure.
Recent innovations
Since the early period of laser history, laser research has produced a variety o
improved and specialized laser types, optimized for different performance go
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Graph showing the history of maxi
laser pulse intensity throughout the
years.
including:
new wavelength bandsmaximum average output power
maximum peak pulse energymaximum peak pulse powerminimum output pulse durationmaximum power efficiencyminimum cost
and this research continues to this day.
Lasing without maintaining the medium
excited into a population inversion was
discovered in 1992 in sodium gas and
again in 1995 in rubidium gas by various international teams.[citation needed]T
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accomplished by using an external maser to induce "optical transparency" in
medium by introducing and destructively interfering the ground electron tran
between two paths, so that the likelihood for the ground electrons to absorb a
energy has been cancelled.
Types and operating principles
For a more complete list of laser types see this list of laser types.
Gas lasers
Main article: Gas laser
Following the invention of the HeNe gas laser, many other gas discharges ha
found to amplify light coherently. Gas lasers using many different gases have
l d d f
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Wavelengths of commercially available lasers. Laser types w
distinct laser lines are shown above the wavelength bar, while
are shown lasers that can emit in a wavelength range. The col
codifies the type of laser material (see the figure description f
details).
built and used for
many purposes.
The helium-neon
laser (HeNe) is
able to operate ata number of
different
wavelengths,
however the vast
majority are
engineered to laseat 633 nm; these
relatively low
cost but highly
coherent lasers
are extremely
i
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common in
optical research and educational laboratories. Commercial carbon dioxide (C
lasers can emit many hundreds of watts in a single spatial mode which can be
concentrated into a tiny spot. This emission is in the thermal infrared at 10.6
such lasers are regularly used in industry for cutting and welding. The efficieCO2laser is unusually high: over 10%. Argon-ion lasers can operate at a num
lasing transitions between 351 and 528.7 nm. Depending on the optical desig
more of these transitions can be lasing simultaneously; the most commonly u
are 458 nm, 488 nm and 514.5 nm. A nitrogen transverse electrical discharge
at atmospheric pressure (TEA) laser is an inexpensive gas laser, often home-
hobbyists, which produces rather incoherent UV light at 337.1 nm.[18]Metal
lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silve
224 nm and neon-copper (NeCu) 248 nm are two examples. Like all low-pre
lasers, the gain media of these lasers have quite narrow oscillation linewidths
than 3 GHz (0.5 picometers),[19]making them candidates for use in fluoresce
d R t
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suppressed Raman spectroscopy.
Chemical lasers
Chemical lasers are powered by a chemical reaction permitting a large amouenergy to be released quickly. Such very high power lasers are especially of
to the military, however continuous wave chemical lasers at very high power
fed by streams of gasses, have been developed and have some industrial appl
As examples, in the Hydrogen fluoride laser (27002900 nm) and the Deuter
fluoride laser (3800 nm) the reaction is the combination of hydrogen or deute
gas with combustion products of ethylene in nitrogen trifluoride.
Excimer lasers
Excimer lasers are a special sort of gas laser powered by an electric discharg
hi h th l i di i i i l i l i i
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which the lasing medium is an excimer, or more precisely an exciplex in exis
designs. These are molecules which can only exist with one atom in an excite
electronic state. Once the molecule transfers its excitation energy to a photon
therefore, its atoms are no longer bound to each other and the molecule disin
This drastically reduces the population of the lower energy state thus greatlyfacilitating a population inversion. Excimers currently used are all noble gas
compounds; noble gasses are chemically inert and can only form compounds
an excited state. Excimer lasers typically operate at ultraviolet wavelengths w
major applicatons including semiconductor photolithography and LASIK eye
surgery. Commonly used excimer molecules include ArF (emission at 193 nm
(222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm).[20]The molecu
fluorine laser, emitting at 157 nm in the vacuum ultraviolet is sometimes refe
as an excimer laser, however this appears to be a misnomer inasmuch as F2i
compound.
S lid t t l
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A frequency-doubled green la
pointer, showing internal
construction. Two AAA cells
electronics power the laser m
(lower diagram) This contain
Solid-state lasers
Solid-state lasers use a crystalline or glass rod
which is "doped" with ions that provide the
required energy states. For example, the firstworking laser was a ruby laser, made from
ruby (chromium-doped corundum). The
population inversion is actually maintained in
the "dopant", such as chromium or
neodymium. These materials are pumped
optically using a shorter wavelength than the
lasing wavelength, often from a flashtube or
from another laser.
It should be noted that "solid-state" in this
sense refers to a crystal or glass, but this
powerful 808 nm IR diode lausage is distinct from the designation of
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powerful 808 nm IR diode la
optically pumps a Nd:YVO4
inside a laser cavity. That las
produces 1064 nm (infrared)
which is mainly confined insiresonator. Also inside the lase
however, is a non-linear KTP
which causes frequency doub
resulting in green light at 532
front mirror is transparent to
visible wavelength which is t
expanded and collimated usin
lenses (in this particular desig
usage is distinct from the designation of
"solid-state electronics" in referring to
semiconductors. Semiconductor lasers (laser
diodes) are pumped electrically and are thus
notreferred to as solid-state lasers. The classof solid-state lasers would, however, properly
include fiber lasers in which dopants in the
glass lase under optical pumping. But in
practice these are simply referred to as "fiber
lasers" with "solid-state" reserved for lasers
using a solid rod of such a material.
Neodymium is a common "dopant" in various
solid-state laser crystals, including yttrium
orthovanadate (Nd:YVO4), yttrium lithium
fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers
produce high powers in the infrared spectrum at 1064 nm They are used for
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produce high powers in the infrared spectrum at 1064 nm. They are used for
welding and marking of metals and other materials, and also in spectroscopy
pumping dye lasers.
These lasers are also commonly frequency doubled, tripled or quadrupled, incalled "diode pumped solid state" or DPSS lasers. Under second, third, or fou
harmonic generation these produce 532 nm (green, visible), 355 nm and 266
(UV) beams. This is the technology behind the bright laser pointers particula
green (532 nm) and other short visible wavelengths.
Ytterbium, holmium, thulium, and erbium are other common "dopants" in solasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW,
Yb:BOYS, Yb:CaF2, typically operating around 10201050 nm. They are po
very efficient and high powered due to a small quantum defect. Extremely hi
powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped
crystals emit at 2097 nm and form an efficient laser operating at infrared wav
strongly absorbed by water bearing tissues The Ho YAG is usually operated
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strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated
pulsed mode, and passed through optical fiber surgical devices to resurface jo
remove rot from teeth, vaporize cancers, and pulverize kidney and gall stone
Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared lascommonly used for spectroscopy. It is also notable for use as a mode-locked
producing ultrashort pulses of extremely high peak power.
Thermal limitations in solid-state lasers arise from unconverted pump power
manifests itself as heat. This heat, when coupled with a high thermo-optic co
(dn/dT) can give rise to thermal lensing as well as reduced quantum efficienctypes of issues can be overcome by another novel diode-pumped solid-state l
diode-pumped thin disk laser. The thermal limitations in this laser type are m
by using a laser medium geometry in which the thickness is much smaller th
diameter of the pump beam. This allows for a more even thermal gradient in
material. Thin disk lasers have been shown to produce up to kilowatt levels o
[21]
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power.[21]
Fiber lasers
Main article: Fiber laser
Solid-state lasers or laser amplifiers where the light is guided due to the total
reflection in a single mode optical fiber are instead called fiber lasers. Guidin
light allows extremely long gain regions providing good cooling conditions;
have high surface area to volume ratio which allows efficient cooling. In add
fiber's waveguiding properties tend to reduce thermal distortion of the beam.and ytterbium ions are common active species in such lasers.
Quite often, the fiber laser is designed as a double-clad fiber. This type of fib
consists of a fiber core, an inner cladding and an outer cladding. The index o
three concentric layers is chosen so that the fiber core acts as a single-mode f
the laser emission while the outer cladding acts as a highly multimode core f
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the laser emission while the outer cladding acts as a highly multimode core f
pump laser. This lets the pump propagate a large amount of power into and t
the active inner core region, while still having a high numerical aperture (NA
have easy launching conditions.
Pump light can be used more efficiently by creating a fiber disk laser, or a st
such lasers.
Fiber lasers have a fundamental limit in that the intensity of the light in the fi
cannot be so high that optical nonlinearities induced by the local electric fiel
strength can become dominant and prevent laser operation and/or lead to thedestruction of the fiber. This effect is called photodarkening. In bulk laser ma
the cooling is not so efficient, and it is difficult to separate the effects of
photodarkening from the thermal effects, but the experiments in fibers show
photodarkening can be attributed to the formation of long-living color
centers.[citation needed]
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medium power laser diodes are used in laser
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A 5.6 mm 'closed can' comme
laser diode, probably from a C
DVD player
medium power laser diodes are used in laser
pointers, laser printers and CD/DVD players.
Laser diodes are also frequently used to
optically pump other lasers with high
efficiency. The highest power industrial laserdiodes, with power up to 10 kW
(70dBm)[citation needed], are used in industry
for cutting and welding. External-cavity
semiconductor lasers have a semiconductor
active medium in a larger cavity. These
devices can generate high power outputs withgood beam quality, wavelength-tunable
narrow-linewidth radiation, or ultrashort laser pulses.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers w
emission direction is perpendicular to the surface of the wafer. VCSEL devic
typically have a more circular output beam than conventional laser diodes, an
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yp y o o p o o o ,
potentially could be much cheaper to manufacture. As of 2005, only 850 nm
are widely available, with 1300 nm VCSELs beginning to be commercialize
1550 nm devices an area of research. VECSELs are external-cavity VCSELs
Quantum cascade lasers are semiconductor lasers that have an active transitiobetween energy sub-bandsof an electron in a structure containing several qu
wells.
The development of a silicon laser is important in the field of optical comput
Silicon is the material of choice for integrated circuits, and so electronic and
photonic components (such as optical interconnects) could be fabricated on tchip. Unfortunately, silicon is a difficult lasing material to deal with, since it
certain properties which block lasing. However, recently teams have produce
lasers through methods such as fabricating the lasing material from silicon an
semiconductor materials, such as indium(III) phosphide or gallium(III) arsen
materials which allow coherent light to be produced from silicon. These are c
hybrid silicon laser. Another type is a Raman laser, which takes advantage o
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y yp , g
scattering to produce a laser from materials such as silicon.
Dye lasers
Dye lasers use an organic dye as the gain medium. The wide gain spectrum o
available dyes, or mixtures of dyes, allows these lasers to be highly tunable,
produce very short-duration pulses (on the order of a few femtoseconds). Alt
these tunable lasers are mainly known in their liquid form, researchers have a
demonstrated narrow-linewidth tunable emission in dispersive oscillator
configurations incorporating solid-state dye gain media.[24]In their most pre
form these solid state dye lasers use dye-doped polymers as laser media.
Free electron lasers
Free electron lasers, or FELs, generate coherent, high power radiation, that is
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, , g , g p ,
tunable, currently ranging in wavelength from microwaves, through terahertz
radiation and infrared, to the visible spectrum, to soft X-rays. They have the
frequency range of any laser type. While FEL beams share the same optical t
other lasers, such as coherent radiation, FEL operation is quite different. Unlliquid, or solid-state lasers, which rely on bound atomic or molecular states,
a relativistic electron beam as the lasing medium, hence the termfree electro
Bio laser
Living cells can be genetically engineered to produce Green fluorescent prot(GFP). The GFP is used as the laser's "gain medium", where light amplificati
place. The cells are then placed between two tiny mirrors, just 20 millionths
metre across, which acted as the "laser cavity" in which light could bounce m
times through the cell. Upon bathing the cell with blue light, it could be seen
directed and intense green laser light.[25][26]
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directed and intense green laser light.
Exotic laser media
In September 2007, the BBC News reported that there was speculation aboutpossibility of using positronium annihilation to drive a very powerful gamma
laser.[27]Dr. David Cassidy of the University of California, Riverside propos
single such laser could be used to ignite a nuclear fusion reaction, replacing t
of hundreds of lasers currently employed in inertial confinement fusion
experiments.[27]
Space-based X-ray lasers pumped by a nuclear explosion have also been pro
antimissile weapons.[28][29]Such devices would be one-shot weapons.
Uses
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Main article: List of applications for lasers
When lasers were invented in 1960, they were called "a solution looking for
problem".[30]
Since then, they have become ubiquitous, finding utility in thohighly varied applications in every section of modern society, including cons
electronics, information technology, science, medicine, industry, law enforce
entertainment, and the military.
The first use of lasers in the daily lives of the general population was the sup
barcode scanner, introduced in 1974. The laserdisc player, introduced in 197the first successful consumer product to include a laser but the compact disc
was the first laser-equipped device to become common, beginning in 1982 fo
shortly by laser printers.
Some other uses are:
Medicine: Bloodless surgery, laser
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healing, surgical treatment, kidneystone treatment, eye treatment, dentistryIndustry: Cutting, welding, materialheat treatment, marking parts, non-contact measurement of partsMilitary: Marking targets, guidingmunitions, missile defence, electro-optical countermeasures (EOCM),alternative to radar, blinding troops.
Law enforcement: used for latentfingerprint detection in the forensic
identification field[31][32]
Research: Spectroscopy, laser ablation,laser annealing, laser scattering, laserinterferometry, LIDAR, laser capture
Lasers range in size from micmicrodissection, fluorescence
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g
diode lasers (top) with numer
applications, to football field
neodymium glass lasers (bott
for inertial confinement fusionuclear weapons research and
high energy density physics
experiments.
microscopyProduct development/commercial: laserprinters, optical discs (e.g. CDs and thelike), barcode scanners, thermometers,laser pointers, holograms, bubblegrams.Laser lighting displays: Laser lightshowsCosmetic skin treatments: acnetreatment, cellulite and striae reduction,and hair removal.
In 2004, excluding diode lasers, approximately 131,000 lasers were sold with
of US$2.19 billion.[33]In the same year, approximately 733 million diode las
valued at $3.20 billion, were sold.[34]
Examples by power
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p y p
Different applications need lasers with different output powers. Lasers that p
continuous beam or a series of short pulses can be compared on the basis of t
average power. Lasers that produce pulses can also be characterized based oneakpower of each pulse. The peak power of a pulsed laser is many orders o
magnitude greater than its average power. The average output power is alwa
than the power consumed.
The continuous or average power required for some uses:
Power Use
15 mW Laser pointers
5 mW CD-ROM drive
510 mW DVD player or DVD-ROM drive
- -
250 mW Consumer 16" DVD R burner
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Laser application in
astronomical adaptiv
250 mW Consumer 16"DVD-R burner
400 mWBurning through a jewel case including disk within 4 seconds
DVD 24"dual-layer recording.[36]
1 WGreen laser in current Holographic Versatile Disc prototypedevelopment
120 WOutput of the majority of commercially available solid-state laused for micro machining
30100 W Typical sealed CO2surgical lasers[37]
1003000 W Typical sealed CO2lasers used in industrial laser cutting
100 kW Claimed output of a CO2laser being developed by Northrop
Grumman for militar (wea on) a lications
imagingExamples of pulsed systems with high peak power:
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700 TW (700"1012W) National IgnitionFacility, a 192-beam, 1.8-megajoule laser system adjoining a 10-meter
diameter target chamber.[38]
1.3 PW (1.3"1015W) world's most powerful laser as of 1998, locate
Lawrence Livermore Laboratory[39]
Hobby uses
In recent years, some hobbyists have taken interests in lasers. Lasers used byhobbyists are generally of class IIIa or IIIb (see Safety), although some have
their own class IV types.[40]However, compared to other hobbyists, laser ho
are far less common, due to the cost and potential dangers involved. Due to t
of lasers, some hobbyists use inexpensive means to obtain lasers, such as salv
laser diodes from broken DVD players (red), Blu-ray players (violet), or even[41]
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power laser diodes from CD or DVD burners.[41]
Hobbyists also have been taking surplus pulsed lasers from retired military
applications and modifying them for pulsed holography. Pulsed Ruby and puYAG lasers have been used.
Safety
Main article: Laser safety
Even the first laser was recognized as being potentially dangerous. Theodore
characterized the first laser as having a power of one "Gillette" as it could bu
through one Gillette razor blade. Today, it is accepted that even low-power la
with only a few milliwatts of output power can be hazardous to human eyesi
when the beam from such a laser hits the eye
di tl ft fl ti f hi f
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Warning symbol for laser
directly or after reflection from a shiny surface.
At wavelengths which the cornea and the lens
can focus well, the coherence and low
divergence of laser light means that it can befocused by the eye into an extremely small spot
on the retina, resulting in localized burning and
permanent damage in seconds or even less time.
Lasers are usually labeled with a safety class
number, which identifies how dangerous thelaser is:
Class 1 is inherently safe, usually because the light is contained in anenclosure, for example in CD players.Class 2 is safe during normal use; the blink reflex of the eye will preve
damage. Usually up to 1 mW power, forexample laser pointers
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Laser warning label
example laser pointers.Class 3R (formerly IIIa) lasers are usuallyup to 5 mW and involve a small risk of eyedamage within the time of the blink reflex.Staring into such a beam for severalseconds is likely to cause damage to a spoton the retina.Class 3B can cause immediate eye damageupon exposure.Class 4 lasers can burn skin, and in somecases, even scattered light can cause eyeand/or skin damage. Many industrial andscientific lasers are in this class.
The indicated powers are for visible-light, continuous-wave lasers. For pulse
and invisible wavelengths, other power limits apply. People working with cla
and class 4 lasers can protect their eyes with safety goggles which are design
b b li ht f ti l l th
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absorb light of a particular wavelength.
Infrared lasers with wavelengths beyond about 1.4 micrometres are often ref
as "eye-safe", because the cornea strongly absorbs light at these wavelengths
protecting the retina from damage. The label "eye-safe" can be misleading, h
as it only applies to relatively low power continuous wave beams; a high pow
switched laser at these wavelengths can burn the cornea, causing severe eye
and even moderate power lasers can injure the eye.
As weapons
Lasers of all but the lowest powers can potentially be used as incapacitating
through their ability to produce temporary or permanent vision loss in varyin
degrees when aimed at the eyes. The degree, character, and duration of vision
impairment caused by eye exposure to laser light varies with the power of the
the wavelength(s) the collimation of the beam the exact orientation of the b
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the wavelength(s), the collimation of the beam, the exact orientation of the b
the duration of exposure. Lasers of even a fraction of a watt in power can pro
immediate, permanent vision loss under certain conditions, making such lase
potential non-lethal but incapacitating weapons. The extreme handicap that linduced blindness represents makes the use of lasers even as non-lethal weap
morally controversial, and weapons designed to cause blindness have been b
the Protocol on Blinding Laser Weapons. Incidents of pilots being exposed to
while flying have prompted aviation authorities to implement special procedu
deal with such hazards.[42]
Laser weapons capable of directly damaging or destroying a target in comba
in the experimental stage. The general idea of laser-beam weaponry is to hit
with a train of brief pulses of light. The rapid evaporation and expansion of t
surface causes shockwaves that damage the target.[citation needed]The power n
to project a high-powered laser beam of this kind is beyond the limit of curre
mobile power technology thus favoring chemically powered gas dynamic la
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mobile power technology, thus favoring chemically powered gas dynamic la
Example experimental systems include MIRACL and the Tactical High Ener
Laser.
The U.S. Air Force is working on the Boeing YAL-1, an airborne laser moun
Boeing 747. It is intended to be used to shoot down incoming ballistic missil
enemy territory. On March 18, 2009 Northrop Grumman claimed that its eng
Redondo Beach had successfully built and tested an electrically powered soli
laser capable of producing a 100-kilowatt beam, powerful enough to destroy
airplane. According to Brian Strickland, manager for the United States ArmyHigh Power Solid State Laser program, an electrically powered laser is capab
being mounted in an aircraft, ship, or other vehicle because it requires much
space for its supporting equipment than a chemical laser.[43]However the so
such a large electrical power in a mobile application remains unclear.
Fictional predictions
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See also: Raygun
Several novelists described devices similar to lasers, prior to the discovery ofstimulated emission:
A very early example is the Heat-Ray featured in H. G. Wells' novel T
of the Worlds(1898).[44]
A laser-like device was described in Alexey Tolstoy's science fiction n
Hyperboloid of Engineer Garinin 1927.Mikhail Bulgakov exaggerated the biological effect (laser bio stimulatintense red light in his science fiction novel Fatal Eggs(1925), withoureasonable description of the source of this red light. (In that novel, thelight first appears occasionally from the illuminating system of an advmicroscope; then the protagonist Prof. Persikov arranges a special set-
generation of the red light.)
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See also
Bessel beamCoherent perfectabsorberdazzler (weapon)Free-space optical
communicationHomogeneousbroadeningInduced gammaemissionInjection seeder
Laser beamprofilerLaser bondingLaser convertingLaser cooling
Laser engravingLaser medicineLaser scalpel3D scannerLaser turntableLaser beam
List of laserarticlesList of lightsourcesMercury lase
NanolaserReference beRytov numbeSoundAmplificationStimulated
International LaserDisplay
welding Emission ofRadiation SA
http://en.wikipedia.org/wiki/Sound_Amplification_by_Stimulated_Emission_of_Radiationhttp://en.wikipedia.org/wiki/Rytov_numberhttp://en.wikipedia.org/wiki/Reference_beamhttp://en.wikipedia.org/wiki/Nanolaserhttp://en.wikipedia.org/wiki/Mercury_laserhttp://en.wikipedia.org/wiki/List_of_light_sourceshttp://en.wikipedia.org/wiki/List_of_laser_articleshttp://en.wikipedia.org/wiki/Laser_beam_weldinghttp://en.wikipedia.org/wiki/Laser_turntablehttp://en.wikipedia.org/wiki/3D_scannerhttp://en.wikipedia.org/wiki/Laser_scalpelhttp://en.wikipedia.org/wiki/Laser_medicinehttp://en.wikipedia.org/wiki/Laser_engravinghttp://en.wikipedia.org/wiki/Laser_coolinghttp://en.wikipedia.org/wiki/Laser_convertinghttp://en.wikipedia.org/wiki/Laser_bondinghttp://en.wikipedia.org/wiki/Laser_beam_profilerhttp://en.wikipedia.org/wiki/Injection_seederhttp://en.wikipedia.org/wiki/Induced_gamma_emissionhttp://en.wikipedia.org/wiki/Homogeneous_broadeninghttp://en.wikipedia.org/wiki/Free-space_optical_communicationhttp://en.wikipedia.org/wiki/Dazzler_(weapon)http://en.wikipedia.org/wiki/Coherent_perfect_absorberhttp://en.wikipedia.org/wiki/Bessel_beamhttp://en.wikipedia.org/wiki/Sound_Amplification_by_Stimulated_Emission_of_Radiationhttp://en.wikipedia.org/wiki/Laser_beam_weldinghttp://en.wikipedia.org/wiki/International_Laser_Display_Association8/11/2019 Laser - Wikipedia 3x5 L - Benjamin Riegel
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DisplayAssociationLaseraccelerometer
Lasers andaviation safety
Radiation SASelective lasesinteringSpaser
Speckle patteTophat beam
References
Notes
1. ^ abGould, R. Gordon (1959). "The LASER, Light Amplification by Stimul