A laser is a device that emits light through a process of optical
amplification based on the stimulated emission of electromagnetic
radiation. The term "laser" originated as an acronym for "light
amplification by stimulated emission of radiation". The first
laser was built in 1960 by
Theodore H. Maiman
Theodore H. Maiman at Hughes Research
Laboratories, based on theoretical work by
Charles Hard Townes
Charles Hard Townes and
Arthur Leonard Schawlow.
A laser differs from other sources of light in that it emits light
coherently, spatially and temporally.
Spatial coherence allows a laser
to be focused to a tight spot, enabling applications such as laser
cutting and lithography.
Spatial coherence also allows a laser beam to
stay narrow over great distances (collimation), enabling applications
such as laser pointers. Lasers can also have high temporal coherence,
which allows them to emit light with a very narrow spectrum, i.e.,
they can emit a single color of light.
Temporal coherence can be used
to produce pulses of light as short as a femtosecond.
Among their many applications, lasers are used in optical disk drives,
laser printers, and barcode scanners; DNA sequencing instruments,
fiber-optic and free-space optical communication; laser surgery and
skin treatments; cutting and welding materials; military and law
enforcement devices for marking targets and measuring range and speed;
and laser lighting displays in entertainment.
3.1 Stimulated emission
3.2 Gain medium and cavity
3.3 The light emitted
3.4 Quantum vs. classical emission processes
4 Continuous and pulsed modes of operation
Continuous wave operation
4.2 Pulsed operation
4.2.3 Pulsed pumping
5.4 Recent innovations
6 Types and operating principles
6.1 Gas lasers
6.1.1 Chemical lasers
6.2 Solid-state lasers
6.3 Fiber lasers
6.4 Photonic crystal lasers
6.5 Semiconductor lasers
6.6 Dye lasers
6.7 Free-electron lasers
6.8 Exotic media
7.1 In medicine
7.2 As weapons
7.4 Examples by power
9 See also
11 Further reading
12 External links
Modern telescopes use laser technologies to compensate for the
blurring effect of the Earth's atmosphere.
Lasers are distinguished from other light sources by their coherence.
Spatial coherence is typically expressed through the output being a
narrow beam, which is diffraction-limited.
Laser beams can be focused
to very tiny spots, achieving a very high irradiance, or they can have
very low divergence in order to concentrate their power at a great
Temporal (or longitudinal) coherence implies a polarized wave at a
single frequency whose phase is correlated over a relatively great
distance (the coherence length) along the beam. A beam produced by
a thermal or other incoherent light source has an instantaneous
amplitude and phase that vary randomly with respect to time and
position, thus having a short coherence length.
Lasers are characterized according to their wavelength in a vacuum.
Most "single wavelength" lasers actually produce radiation in several
modes having slightly differing frequencies (wavelengths), often not
in a single polarization. Although temporal coherence implies
monochromaticity, there are lasers that emit a broad spectrum of light
or emit different wavelengths of light simultaneously. There are some
lasers that are not single spatial mode and consequently have light
beams that diverge more than is required by the diffraction limit.
However, all such devices are classified as "lasers" based on their
method of producing light, i.e., stimulated emission. Lasers are
employed in applications where light of the required spatial or
temporal coherence could not be produced using simpler technologies.
Laser beams in fog, reflected on a car windshield
The word laser started as an acronym for "light amplification by
stimulated emission of radiation". In this usage, the term "light"
includes electromagnetic radiation of any frequency, not only visible
light, hence the terms infrared laser, ultraviolet laser, X-ray laser,
gamma-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 the early technical
literature, especially at Bell Telephone Laboratories, the laser was
called an optical maser; this term is now obsolete.
A laser that produces light by itself is technically an optical
oscillator rather than an optical amplifier as suggested by the
acronym. It has been humorously noted that the acronym LOSER, for
"light oscillation by stimulated emission of radiation", would have
been more correct. With the widespread use of the original acronym
as a common noun, optical amplifiers have come to be referred to as
"laser amplifiers", notwithstanding the apparent redundancy in that
The back-formed verb to lase is frequently used in the field, meaning
"to produce laser light," especially in reference to the gain
medium of a laser; when a laser is operating it is said to be
"lasing." Further use of the words laser and maser in an extended
sense, not referring to laser technology or devices, can be seen in
usages such as astrophysical maser and atom laser.
Components of a typical laser:
Laser pumping energy
Animation explaining stimulated emission and the laser principle
A laser consists of a gain medium, a mechanism to energize it, and
something to provide optical feedback. The gain medium is a
material with properties that allow it to amplify light by way of
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 in a process called pumping. The energy is typically supplied
as an electric current or as light at a different wavelength. Pump
light may be provided by a flash lamp or by another laser.
The most common type of laser uses feedback from an optical cavity—a
pair of mirrors on either end of the gain medium.
Light bounces back
and forth between the mirrors, passing through the gain 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. Depending on the design of the cavity (whether
the mirrors are flat or curved), the light coming out of the laser may
spread out or form a narrow beam. In analogy to electronic
oscillators, this device is sometimes called a laser oscillator.
Most practical lasers contain additional elements that affect
properties of the emitted light, such as the polarization, wavelength,
and shape of the beam.
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Electrons and how they interact with electromagnetic fields are
important in our understanding of chemistry and physics.
Main article: Stimulated emission
In the classical view, the energy of an electron orbiting an atomic
nucleus is larger for orbits further from the nucleus of an atom.
However, quantum mechanical effects force electrons to take on
discrete positions in orbitals. Thus, electrons are found in specific
energy levels of an atom, two of which are shown below:
When an electron absorbs energy either from light (photons) or heat
(phonons), it receives that incident quantum of energy. But
transitions are only allowed in between discrete energy levels such as
the two shown above. This leads to emission lines and absorption
When an electron is excited from a lower to a higher energy level, it
will not stay that way forever. An electron in an excited state may
decay to a lower energy state which is not occupied, according to a
particular time constant characterizing that transition. When such an
electron decays without external influence, emitting a photon, that is
called "spontaneous emission". The phase associated with the photon
that is emitted is random. A material with many atoms in such an
excited state may thus result in radiation which is very spectrally
limited (centered around one wavelength of light), but the individual
photons would have no common phase relationship and would emanate in
random directions. This is the mechanism of fluorescence and thermal
An external electromagnetic field at a frequency associated with a
transition can affect the quantum mechanical state of the atom. As the
electron in the atom makes a transition between two stationary states
(neither of which shows a dipole field), it enters a transition state
which does have a dipole field, and which acts like a small electric
dipole, and this dipole oscillates at a characteristic frequency. In
response to the external electric field at this frequency, the
probability of the atom entering this transition state is greatly
increased. Thus, the rate of transitions between two stationary states
is enhanced beyond that due to spontaneous emission. Such a transition
to the higher state is called absorption, and it destroys an incident
photon (the photon's energy goes into powering the increased energy of
the higher state). A transition from the higher to a lower energy
state, however, produces an additional photon; this is the process of
Gain medium and cavity
A helium–neon laser demonstration at the Kastler-Brossel Laboratory
at Univ. Paris 6. The pink-orange glow running through the center of
the tube is from the electric discharge which produces incoherent
light, just as in a neon tube. This glowing plasma is excited and then
acts as the gain medium through which the internal beam passes, as it
is reflected between the two mirrors.
Laser output through the front
mirror can be seen to produce a tiny (about 1 mm in diameter)
intense spot on the screen, to the right. Although it is a deep and
pure red color, spots of laser light are so intense that cameras are
typically overexposed and distort their color.
Spectrum of a helium neon laser illustrating its very high spectral
purity (limited by the measuring apparatus). The 0.002 nm
bandwidth of the lasing medium is well over 10,000 times narrower than
the spectral width of a light-emitting diode (whose spectrum is shown
here for comparison), with the bandwidth of a single longitudinal mode
being much narrower still.
The gain medium is put into an excited state by an external source of
energy. In most lasers this medium consists of a population of atoms
which have been excited into such a state by means of an outside light
source, or an electrical field which 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, liquid, solid, or plasma. The gain medium
absorbs pump energy, which raises some electrons into higher-energy
("excited") quantum states. Particles can interact with light by
either absorbing or emitting photons. Emission can be spontaneous or
stimulated. In the latter case, the photon is emitted in the same
direction 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 to light 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.
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 a
laser beam without any need for a resonant or reflective cavity (see
for example nitrogen laser). Thus, reflection in a resonant cavity
is usually required for a laser, but is not absolutely necessary.
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. The resonator typically consists of two mirrors between which a
coherent beam of light travels in both directions, reflecting back on
itself so that an average photon will pass through the gain medium
repeatedly before it is emitted from the output aperture or lost to
diffraction or absorption. If the gain (amplification) in the medium
is larger than the resonator losses, then the power of the
recirculating light can rise exponentially. But each stimulated
emission event returns an atom from its excited state to the ground
state, reducing the gain of the medium. With increasing beam power the
net gain (gain minus loss) reduces to unity and the gain medium is
said to be saturated. In a continuous wave (CW) laser, the balance of
pump power against gain saturation and cavity losses produces an
equilibrium value of the laser power inside the cavity; this
equilibrium determines the operating point of the laser. If the
applied pump power is too small, the gain will never be sufficient to
overcome the resonator losses, and laser light will not be produced.
The minimum pump power needed to begin laser action is called the
lasing threshold. The gain medium will amplify any photons passing
through it, regardless of direction; but only the photons in a spatial
mode supported by the resonator will pass more than once through the
medium and receive substantial amplification.
The light emitted
The light generated by stimulated emission is very similar to the
input signal in terms of wavelength, phase, and polarization. This
gives laser light its characteristic 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
traveling in free space (or a homogeneous medium) rather than
waveguides (as in an optical fiber laser), can be approximated as a
Gaussian beam in most lasers; such beams exhibit the minimum
divergence for a given diameter. However some high power lasers may be
multimode, with the transverse modes often approximated using
Hermite–Gaussian or Laguerre-Gaussian functions. It has been shown
that unstable laser resonators (not used in most lasers) produce
fractal shaped beams. Near the beam "waist" (or focal region) it
is highly collimated: the wavefronts are planar, normal to the
direction of propagation, with no beam divergence at that point.
However, due to diffraction, that can only remain true well within the
Rayleigh range. The beam of a single transverse mode (gaussian beam)
laser eventually diverges at an angle which varies inversely with the
beam diameter, as required by diffraction theory. Thus, the "pencil
beam" directly generated by a common helium–neon laser would spread
out to a size of perhaps 500 kilometers when shone on the Moon (from
the distance of the earth). On the other hand, the light from a
semiconductor laser typically exits the tiny crystal with a large
divergence: up to 50°. However even such a divergent beam can be
transformed into a similarly collimated beam by means of a lens
system, as is always included, for instance, in a laser pointer whose
light originates from a laser diode. That is possible due to the light
being of a single spatial mode. This unique property of laser light,
spatial coherence, cannot be replicated using standard light sources
(except by discarding most of the light) as can be appreciated by
comparing 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
emission, where energy is extracted from a transition in an atom or
molecule. This is a quantum phenomenon discovered by
derived the relationship between the A coefficient describing
spontaneous emission and the B coefficient which applies to absorption
and stimulated emission. However, in the case of the free electron
laser, atomic energy levels are not involved; it appears that the
operation of this rather exotic device can be explained without
reference to quantum mechanics.
Continuous and pulsed modes of operation
Lidar measurements of lunar topography made by Clementine mission.
Laserlink point to point optical wireless network
Laser Altimeter (MLA) of the
A laser can be classified as operating in either continuous or pulsed
mode, depending on whether the power output is essentially continuous
over time or whether its output takes the form of pulses of light on
one or another time scale. Of course even a laser whose output is
normally continuous can be intentionally turned on and off at some
rate in order to create pulses of light. When the modulation rate is
on time scales much slower than the cavity lifetime and the time
period over which energy can be stored in the lasing medium or pumping
mechanism, then it is still classified as a "modulated" or "pulsed"
continuous wave laser. Most laser diodes used in communication systems
fall in that category.
Continuous wave operation
Some applications of lasers depend on a beam whose output power is
constant over time. Such a laser is known as continuous wave (CW).
Many types of lasers can be made to operate in continuous wave mode to
satisfy such an application. Many of these lasers actually lase in
several longitudinal modes at the same time, and beats between the
slightly different optical frequencies of those oscillations will in
fact produce amplitude variations on time scales shorter than the
round-trip time (the reciprocal of the frequency spacing between
modes), typically a few nanoseconds or less. In most cases these
lasers are still termed "continuous wave" as their output power is
steady when averaged over any longer time periods, with the very high
frequency power variations having little or no impact in the intended
application. (However the term is not applied to mode-locked lasers,
where the intention is to create very short pulses at the rate of the
For continuous wave operation, it is required for the population
inversion of the gain medium to be continually replenished by a steady
pump source. In some lasing media this is impossible. In some other
lasers it would require pumping the laser at a very high continuous
power level which would be impractical or destroy the laser by
producing excessive heat. Such lasers cannot be run in CW mode.
Pulsed operation of lasers refers to any laser not classified as
continuous wave, so that the optical power appears in pulses of some
duration at some repetition rate. This encompasses a wide range of
technologies addressing a number of different motivations. Some lasers
are pulsed simply because they cannot be run in continuous mode.
In other cases, the application requires the production of pulses
having as large an energy as possible. Since the pulse energy is equal
to the average power divided by the repetition rate, this goal can
sometimes be satisfied by lowering the rate of pulses so that more
energy can be built up in between pulses. In laser ablation, for
example, a small volume of material at the surface of a work piece can
be evaporated if it is heated in a very short time, while supplying
the energy gradually would allow for the heat to be absorbed into the
bulk of the piece, never attaining a sufficiently high temperature at
a particular point.
Other applications rely on the peak pulse power (rather than the
energy in the pulse), especially in order to obtain nonlinear optical
effects. For a given pulse energy, this requires creating pulses of
the shortest possible duration utilizing techniques such as
The optical bandwidth of a pulse cannot be narrower than the
reciprocal of the pulse width. In the case of extremely short pulses,
that implies lasing over a considerable bandwidth, quite contrary to
the very narrow bandwidths typical of CW lasers. The lasing medium in
some dye lasers and vibronic solid-state lasers produces optical gain
over a wide bandwidth, making a laser possible which can thus generate
pulses of light as short as a few femtoseconds (10−15 s).
Main article: Q-switching
Q-switched laser, the population inversion is allowed to build up
by introducing loss inside the resonator which exceeds the gain of the
medium; this can also be described as a reduction of the quality
factor or 'Q' of the cavity. Then, after the pump energy stored in the
laser medium has approached the maximum possible level, the introduced
loss mechanism (often an electro- or acousto-optical element) is
rapidly removed (or that occurs by itself in a passive device),
allowing lasing to begin which rapidly obtains the stored energy in
the gain medium. This results in a short pulse incorporating that
energy, and thus a high peak power.
Main article: Mode-locking
A mode-locked laser is capable of emitting extremely short pulses on
the order of tens of picoseconds down to less than 10 femtoseconds.
These pulses will repeat at the round trip time, that is, the time
that it takes light to complete one round trip between the mirrors
comprising the resonator. Due to the Fourier limit (also known as
energy-time uncertainty), a pulse of such short temporal length has a
spectrum spread over a considerable bandwidth. Thus such a gain medium
must have a gain bandwidth sufficiently broad to amplify those
frequencies. An example of a suitable material is titanium-doped,
artificially grown sapphire (Ti:sapphire) which has a very wide gain
bandwidth and can thus produce pulses of only a few femtoseconds
Such mode-locked lasers are a most versatile tool for researching
processes occurring on extremely short time scales (known as
femtosecond physics, femtosecond chemistry and ultrafast science), for
maximizing the effect of nonlinearity in optical materials (e.g. in
second-harmonic generation, parametric down-conversion, optical
parametric oscillators and the like). Due to the large peak power and
the ability to generate phase-stabilized trains of ultrafast laser
pulses, mode-locking ultrafast lasers underpin precision metrology and
Another method of achieving pulsed laser operation is to pump the
laser material with a source that is itself pulsed, either through
electronic charging in the case of flash lamps, or another laser which
is already pulsed. Pulsed pumping was historically used with dye
lasers where the inverted population lifetime of a dye 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 switched
to discharge through flashlamps, producing an intense flash. Pulsed
pumping is also required for three-level lasers in which the lower
energy level rapidly becomes highly populated preventing further
lasing until those atoms relax to the ground state. These lasers, such
as the excimer laser and the copper vapor laser, can never be operated
in CW mode.
In 1917, Albert
Einstein established the theoretical foundations for
the laser and the maser in the paper Zur Quantentheorie der Strahlung
(On the Quantum Theory of Radiation) via a re-derivation of Max
Planck's law of radiation, conceptually based upon probability
Einstein coefficients) for the absorption, spontaneous
emission, and stimulated emission of electromagnetic radiation. In
Rudolf W. Ladenburg
Rudolf W. Ladenburg confirmed the existence of the phenomena of
stimulated emission and negative absorption. In 1939, Valentin A.
Fabrikant predicted the use of stimulated emission to amplify "short"
waves. In 1947,
Willis E. Lamb
Willis E. Lamb and R. C. Retherford found apparent
stimulated emission in hydrogen spectra and effected the first
demonstration of stimulated emission. In 1950, Alfred Kastler
(Nobel Prize for
Physics 1966) proposed the method of optical pumping,
experimentally confirmed, two years later, by Brossel, Kastler, and
Main article: Maser
Joseph Weber submitted a paper on using stimulated emissions
to make a microwave amplifier to the June 1952 Institute of Radio
Engineers Vacuum Tube Research Conference at Ottawa, Ontario,
Canada. After this presentation,
RCA asked Weber to give a seminar
on this idea, and
Charles Hard Townes
Charles Hard Townes asked him for a copy of the
Charles Hard Townes
Charles Hard Townes and graduate students James P. Gordon and
Herbert J. Zeiger produced the first microwave amplifier, a device
operating on similar principles to the laser, but amplifying microwave
radiation rather than infrared or visible radiation. Townes's maser
was incapable of continuous output. Meanwhile, in the
Nikolay Basov and Aleksandr Prokhorov were independently
working on the quantum oscillator and solved the problem of
continuous-output systems by using more than two energy levels. These
gain media could release stimulated emissions between an excited state
and a lower excited state, not the ground state, facilitating the
maintenance of a population inversion. In 1955, 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
Townes reports that several eminent physicists—among them Niels
Bohr, John von Neumann, and Llewellyn Thomas—argued the maser
violated Heisenberg's uncertainty principle and hence could not work.
Others such as
Isidor Rabi and
Polykarp Kusch expected that it would
be impractical and not worth the effort. In 1964 Charles H.
Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize
in Physics, "for fundamental work in the field of quantum electronics,
which has led to the construction of oscillators and amplifiers based
on the maser–laser principle".
Charles Hard Townes
Charles Hard Townes and Arthur Leonard Schawlow, then at Bell
Labs, began a serious study of the infrared laser. As ideas developed,
they abandoned infrared radiation to instead concentrate upon visible
light. The concept originally was called an "optical maser". In 1958,
Bell Labs filed a patent application for their proposed optical maser;
and Schawlow and Townes submitted a manuscript of their theoretical
calculations to the Physical Review, published that year in Volume
112, Issue No. 6.
LASER notebook: First page of the notebook wherein
Gordon Gould coined
the LASER acronym, and described the elements for constructing the
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 a general 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 (in the USSR) of this idea. Elsewhere, in the U.S.,
Schawlow and Townes had agreed to an open-resonator laser design –
apparently unaware of Prokhorov's publications and Gould's unpublished
At a conference in 1959,
Gordon Gould published the term LASER in the
paper The LASER,
Light Amplification by Stimulated Emission of
Radiation. Gould's linguistic intention was using the "-aser"
word particle as a suffix – to accurately denote the spectrum of the
light emitted by the LASER device; thus x-rays: xaser, ultraviolet:
uvaser, et cetera; none established itself as a discrete term,
although "raser" was briefly popular for denoting
Gould's notes included possible applications for a laser, such as
spectrometry, interferometry, radar, and nuclear fusion. He continued
developing the idea, and filed a patent application in April 1959. The
U.S. Patent Office denied his application, and awarded a patent to
Bell Labs, in 1960. That provoked a twenty-eight-year lawsuit,
featuring scientific prestige and money as the stakes. Gould won his
first minor patent in 1977, yet it was not until 1987 that he won the
first significant patent lawsuit victory, when a Federal judge ordered
the U.S. Patent Office to issue patents to Gould for the optically
pumped and the gas discharge laser devices. The question of just how
to assign credit for inventing the laser remains unresolved by
On May 16, 1960,
Theodore H. Maiman
Theodore H. Maiman operated the first functioning
laser  at Hughes Research Laboratories, Malibu, California,
ahead of several research teams, including those of Townes, at
Columbia University, Arthur Schawlow, at Bell Labs, and Gould, at
the TRG (Technical Research Group) company. Maiman's functional laser
used a solid-state flashlamp-pumped synthetic ruby crystal to produce
red laser light, at 694 nanometers wavelength; however, the device
only was capable of pulsed operation, because of its three-level
pumping design scheme. Later that year, the Iranian physicist Ali
Javan, and William R. Bennett, and Donald Herriott, constructed the
first gas laser, using helium and neon that was capable of continuous
operation in the infrared (U.S. Patent 3,149,290); later, Javan
received the Albert
Einstein Award in 1993. Basov and Javan proposed
the semiconductor laser diode concept. In 1962, Robert N. Hall
demonstrated the first laser diode device, made of gallium arsenide
and emitting at 850 nm in the near-infrared band of the spectrum.
Later that year, Nick Holonyak, Jr. demonstrated the first
semiconductor laser with a visible emission. This first semiconductor
laser could only be used in pulsed-beam operation, and when cooled to
liquid nitrogen temperatures (77 K). In 1970, Zhores Alferov, in
the USSR, and Izuo Hayashi and Morton Panish of Bell Telephone
Laboratories also independently developed room-temperature,
continual-operation diode lasers, using the heterojunction structure.
Graph showing the history of maximum laser pulse intensity throughout
the past 40 years.
Since the early period of laser history, laser research has produced a
variety of improved and specialized laser types, optimized for
different performance goals, including:
new wavelength bands
maximum average output power
maximum peak pulse energy
maximum peak pulse power
minimum output pulse duration
maximum power efficiency
and this research continues to this day.
In 2017, researchers at TU Delft demonstrated an AC Josephson junction
microwave laser. Since the laser operates in the superconducting
regime, it is more stable than other semiconductor-based lasers. The
device has potential for applications in quantum computing. In
2017, researchers at TU Munich demonstrated the smallest mode locking
laser capable of emitting pairs of phase-locked picosecond laser
pulses with a repetition frequency up to 200 GHz.
In 2017, researchers from the Physikalisch-Technische Bundesanstalt
(PTB), together with US researchers from JILA, a joint institute of
the National Institute of Standards and Technology (NIST) and the
University of Colorado Boulder, established a new world record by
developing an erbium-doped fiber laser with a linewidth of only 10
Types and operating principles
For a more complete list of laser types see this list of laser types.
Wavelengths of commercially available lasers.
Laser types with
distinct laser lines are shown above the wavelength bar, while below
are shown lasers that can emit in a wavelength range. The color
codifies the type of laser material (see the figure description for
Main article: Gas laser
Following the invention of the HeNe gas laser, many other gas
discharges have been found to amplify light coherently. Gas lasers
using many different gases have been built and used for many purposes.
The helium–neon laser (HeNe) is able to operate at a number of
different wavelengths, however the vast majority are engineered to
lase at 633 nm; these relatively low cost but highly coherent
lasers are extremely common in optical research and educational
laboratories. Commercial carbon dioxide (CO2) 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 µm; such lasers are regularly used in industry for cutting
and welding. The efficiency of a CO2 laser is unusually high: over
30%. Argon-ion lasers can operate at a number of lasing
transitions between 351 and 528.7 nm. Depending on the optical
design one or more of these transitions can be lasing simultaneously;
the most commonly used lines are 458 nm, 488 nm and
514.5 nm. A nitrogen transverse electrical discharge in gas at
atmospheric pressure (TEA) laser is an inexpensive gas laser, often
home-built by hobbyists, which produces rather incoherent UV light at
337.1 nm. Metal ion lasers are gas lasers that generate deep
ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and
neon-copper (NeCu) 248 nm are two examples. Like all low-pressure
gas lasers, the gain media of these lasers have quite narrow
oscillation linewidths, less than 3
GHz (0.5 picometers), making
them candidates for use in fluorescence suppressed Raman spectroscopy.
Chemical lasers are powered by a chemical reaction permitting a large
amount of energy to be released quickly. Such very high power lasers
are especially of interest to the military, however continuous wave
chemical lasers at very high power levels, fed by streams of gasses,
have been developed and have some industrial applications. As
examples, in the hydrogen fluoride laser (2700–2900 nm) and the
deuterium fluoride laser (3800 nm) the reaction is the
combination of hydrogen or deuterium gas with combustion products of
ethylene in nitrogen trifluoride.
Excimer lasers are a special sort of gas laser powered by an electric
discharge in which the lasing medium is an excimer, or more precisely
an exciplex in existing designs. These are molecules which can only
exist with one atom in an excited 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 disintegrates. This
drastically reduces the population of the lower energy state thus
greatly facilitating a population inversion. Excimers currently used
are all noble gas compounds; noble gasses are chemically inert and can
only form compounds while in an excited state.
typically operate at ultraviolet wavelengths with major applications
including semiconductor photolithography and
LASIK eye surgery.
Commonly used excimer molecules include ArF (emission at 193 nm),
KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF
(351 nm). The molecular fluorine laser, emitting at
157 nm in the vacuum ultraviolet is sometimes referred to as an
excimer laser, however this appears to be a misnomer inasmuch as F2 is
a stable compound.
A 50 W FASOR, based on a Nd:YAG laser, used at the Starfire Optical
Solid-state lasers use a crystalline or glass rod which is "doped"
with ions that provide the required energy states. For example, the
first working laser was a ruby laser, made from ruby (chromium-doped
corundum). The population inversion is actually maintained in the
dopant. These materials are pumped optically using a shorter
wavelength than the lasing wavelength, often from a flashtube or from
another laser. The usage of the term "solid-state" in laser physics is
narrower than in typical use.
Semiconductor lasers (laser diodes) are
typically not referred to as solid-state lasers.
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 can
produce high powers in the infrared spectrum at 1064 nm. They are
used for cutting, welding and marking of metals and other materials,
and also in spectroscopy and for pumping dye lasers. These lasers are
also commonly frequency doubled, tripled or quadrupled to produce
532 nm (green, visible), 355 nm and 266 nm (UV) beams,
respectively. Frequency-doubled diode-pumped solid-state (DPSS) lasers
are used to make bright green laser pointers.
Ytterbium, holmium, thulium, and erbium are other common "dopants" in
Ytterbium is used in crystals such as Yb:YAG,
Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around
1020–1050 nm. They are potentially very efficient and high
powered due to a small quantum defect. Extremely high powers in
ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG
crystals emit at 2097 nm and form an efficient laser operating at
infrared wavelengths strongly absorbed by water-bearing tissues. The
Ho-YAG is usually operated in a pulsed mode, and passed through
optical fiber surgical devices to resurface joints, remove rot from
teeth, vaporize cancers, and pulverize kidney and gall stones.
Titanium-doped sapphire (Ti:sapphire) produces a highly tunable
infrared laser, commonly used for spectroscopy. It is also notable for
use as a mode-locked laser producing ultrashort pulses of extremely
high peak power.
Thermal limitations in solid-state lasers arise from unconverted pump
power that heats the medium. This heat, when coupled with a high
thermo-optic coefficient (dn/dT) can cause thermal lensing and reduce
the quantum efficiency. Diode-pumped thin disk lasers overcome these
issues by having a gain medium that is much thinner than the diameter
of the pump beam. This allows for a more uniform temperature in the
material. Thin disk lasers have been shown to produce beams of up to
Main article: Fiber laser
Solid-state lasers or laser amplifiers where the light is guided due
to the total internal reflection in a single mode optical fiber are
instead called fiber lasers. Guiding of light allows extremely long
gain regions providing good cooling conditions; fibers have high
surface area to volume ratio which allows efficient cooling. In
addition, the fiber's waveguiding properties tend to reduce thermal
distortion of the beam.
Erbium 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 fiber consists of a fiber core, an inner cladding and an outer
cladding. The index of the three concentric layers is chosen so that
the fiber core acts as a single-mode fiber for the laser emission
while the outer cladding acts as a highly multimode core for the pump
laser. This lets the pump propagate a large amount of power into and
through the active inner core region, while still having a high
numerical aperture (NA) to have easy launching conditions.
Pump light can be used more efficiently by creating a fiber disk
laser, or a stack of such lasers.
Fiber lasers have a fundamental limit in that the intensity of the
light in the fiber cannot be so high that optical nonlinearities
induced by the local electric field strength can become dominant and
prevent laser operation and/or lead to the material destruction of the
fiber. This effect is called photodarkening. In bulk laser materials,
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 that the photodarkening can be attributed
to the formation of long-living color centers.
Photonic crystal lasers
Photonic crystal lasers are lasers based on nano-structures that
provide the mode confinement and the density of optical states (DOS)
structure required for the feedback to take place.[clarification
needed] They are typical micrometer-sized[dubious – discuss] and
tunable on the bands of the photonic crystals.[clarification
Main article: Semiconductor lasers
A 5.6 mm 'closed can' commercial laser diode, probably from a CD or
Semiconductor lasers are diodes which are electrically pumped.
Recombination of electrons and holes created by the applied current
introduces optical gain. Reflection from the ends of the crystal form
an optical resonator, although the resonator can be external to the
semiconductor in some designs.
Commercial laser diodes emit at wavelengths from 375 nm to
3500 nm. Low to medium power laser diodes are used in laser
pointers, laser printers and CD/
Laser diodes are also
frequently used to optically pump other lasers with high efficiency.
The highest power industrial laser diodes, with power up to 10 kW
(70 dBm), 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 with good beam quality, wavelength-tunable
narrow-linewidth radiation, or ultrashort laser pulses.
OSRAM developed and manufactured commercial
high-power green laser diodes (515/520 nm), which compete with
traditional diode-pumped solid-state lasers.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor
lasers whose emission direction is perpendicular to the surface of the
VCSEL devices typically have a more circular output beam than
conventional laser diodes. As of 2005, only 850 nm VCSELs are
widely available, with 1300 nm VCSELs beginning to be
commercialized, and 1550 nm devices an area of research.
VECSELs are external-cavity VCSELs. Quantum cascade lasers are
semiconductor lasers that have an active transition between energy
sub-bands of an electron in a structure containing several quantum
The development of a silicon laser is important in the field of
Silicon is the material of choice for integrated
circuits, and so electronic and silicon photonic components (such as
optical interconnects) could be fabricated on the same chip.
Unfortunately, silicon is a difficult lasing material to deal with,
since it has certain properties which block lasing. However, recently
teams have produced silicon lasers through methods such as fabricating
the lasing material from silicon and other semiconductor materials,
such as indium(III) phosphide or gallium(III) arsenide, materials
which allow coherent light to be produced from silicon. These are
called hybrid silicon laser. Recent developments have also shown the
use of monolithically integrated nanowire lasers directly on silicon
for optical interconnects, paving the way for chip level
applications. These heterostructure nanowire lasers capable of
optical interconnects in silicon are also capable of emitting pairs of
phase-locked picosecond pulses with a repetition frequency up to
200 GHz, allowing for on-chip optical signal processing. Another
type is a Raman laser, which takes advantage of
Raman scattering to
produce a laser from materials such as silicon.
Lasing without maintaining the medium excited into a population
inversion was demonstrated in 1992 in sodium gas and again in 1995 in
rubidium gas by various international teams. This was
accomplished by using an external maser to induce "optical
transparency" in the medium by introducing and destructively
interfering the ground electron transitions between two paths, so that
the likelihood for the ground electrons to absorb any energy has been
Close-up of a table-top dye laser based on Rhodamine 6G
Dye lasers use an organic dye as the gain medium. The wide gain
spectrum of available dyes, or mixtures of dyes, allows these lasers
to be highly tunable, or to produce very short-duration pulses (on the
order of a few femtoseconds). Although these tunable lasers are mainly
known in their liquid form, researchers have also demonstrated
narrow-linewidth tunable emission in dispersive oscillator
configurations incorporating solid-state dye gain media. In their
most prevalent form these solid state dye lasers use dye-doped
polymers as laser media.
The free-electron laser FELIX at the FOM Institute for Plasma Physics
Free-electron lasers, or FELs, generate coherent, high power radiation
that is widely tunable, currently ranging in wavelength from
microwaves through terahertz radiation and infrared to the visible
spectrum, to soft X-rays. They have the widest frequency range of any
laser type. While FEL beams share the same optical traits as other
lasers, such as coherent radiation, FEL operation is quite different.
Unlike gas, liquid, or solid-state lasers, which rely on bound atomic
or molecular states, FELs use a relativistic electron beam as the
lasing medium, hence the term free-electron.
The pursuit of a high-quantum-energy laser using transitions between
isomeric states of an atomic nucleus has been the subject of
wide-ranging academic research since the early 1970s. Much of this is
summarized in three review articles. This research has
been international in scope, but mainly based in the former Soviet
Union and the United States. While many scientists remain optimistic
that a breakthrough is near, an operational gamma-ray laser is yet to
Some of the early studies were directed toward short pulses of
neutrons exciting the upper isomer state in a solid so the gamma-ray
transition could benefit from the line-narrowing of Mössbauer
effect. In conjunction, several advantages were expected from
two-stage pumping of a three-level system. It was conjectured that
the nucleus of an atom, embedded in the near field of a laser-driven
coherently-oscillating electron cloud would experience a larger dipole
field than that of the driving laser. Furthermore,
nonlinearity of the oscillating cloud would produce both spatial and
temporal harmonics, so nuclear transitions of higher multipolarity
could also be driven at multiples of the laser
In September 2007, the
BBC News reported that there was speculation
about the possibility of using positronium annihilation to drive a
very powerful gamma ray laser. Dr. David Cassidy of the University
of California, Riverside proposed that a single such laser could be
used to ignite a nuclear fusion reaction, replacing the banks of
hundreds of lasers currently employed in inertial confinement fusion
Space-based X-ray lasers pumped by a nuclear explosion have also been
proposed as antimissile weapons. Such devices would be
Living cells have been used to produce laser light. The cells
were genetically engineered to produce green fluorescent protein
(GFP). The GFP is used as the laser's "gain medium", where light
amplification takes place. The cells were then placed between two tiny
mirrors, just 20 millionths of a meter across, which acted as the
"laser cavity" in which light could bounce many times through the
cell. Upon bathing the cell with blue light, it could be seen to emit
directed and intense green laser light.
Lasers range in size from microscopic diode lasers (top) with numerous
applications, to football field sized neodymium glass lasers (bottom)
used for inertial confinement fusion, nuclear weapons research and
other high energy density physics experiments.
Main article: List of applications for lasers
When lasers were invented in 1960, they were called "a solution
looking for a problem". Since then, they have become ubiquitous,
finding utility in thousands of highly varied applications in every
section of modern society, including consumer electronics, information
technology, science, medicine, industry, law enforcement,
entertainment, and the military.
Fiber-optic communication using
lasers is a key technology in modern communications, allowing services
such as the Internet.
The first use of lasers in the daily lives of the general population
was the supermarket barcode scanner, introduced in 1974. The laserdisc
player, introduced in 1978, was the first successful consumer product
to include a laser but the compact disc player was the first
laser-equipped device to become common, beginning in 1982 followed
shortly by laser printers.
Some other uses are:
Communications: besides fiber-optic communication, lasers are used for
free-space optical communication, including laser communication in
Medicine: see below.
Industry: cutting, welding, material heat treatment, marking parts,
non-contact measurement of parts.
Military: marking targets, guiding munitions, missile defense,
electro-optical countermeasures (EOCM), lidar, blinding troops. See
Law enforcement: LIDAR traffic enforcement. Lasers are used for latent
fingerprint detection in the forensic identification field
Research: spectroscopy, laser ablation, laser annealing, laser
scattering, laser interferometry, lidar, laser capture
microdissection, fluorescence microscopy, metrology.
Commercial products: laser printers, barcode scanners, thermometers,
laser pointers, holograms, bubblegrams.
Entertainment: optical discs, laser lighting displays
In 2004, excluding diode lasers, approximately 131,000 lasers were
sold with a value of US$2.19 billion. In the same year,
approximately 733 million diode lasers, valued at $3.20 billion, were
Laser medicine and Lasers in cancer treatment
Lasers have many uses in medicine, including laser surgery
(particularly eye surgery), laser healing, kidney stone treatment,
ophthalmoscopy, and cosmetic skin treatments such as acne treatment,
cellulite and striae reduction, and hair removal.
Lasers are used to treat cancer by shrinking or destroying tumors or
precancerous growths. They are most commonly used to treat superficial
cancers that are on the surface of the body or the lining of internal
organs. They are used to treat basal cell skin cancer and the very
early stages of others like cervical, penile, vaginal, vulvar, and
non-small cell lung cancer.
Laser therapy is often combined with other
treatments, such as surgery, chemotherapy, or radiation therapy.
Laser-induced interstitial thermotherapy (LITT), or interstitial laser
photocoagulation, uses lasers to treat some cancers using
hyperthermia, which uses heat to shrink tumors by damaging or killing
Laser are more precise than surgery and cause less
damage, pain, bleeding, swelling, and scarring. A disadvantage is that
surgeons must have specialized training. It may be more expensive than
The US-Israeli Tactical High
Energy weapon has been used to shoot down
rockets and artillery shells.
Lasers of all but the lowest powers can potentially be used as
incapacitating weapons, through their ability to produce temporary or
permanent vision loss in varying 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 laser, the
wavelength(s), the collimation of the beam, the exact orientation of
the beam, and the duration of exposure. Lasers of even a fraction of a
watt in power can produce immediate, permanent vision loss under
certain conditions, making such lasers potential non-lethal but
incapacitating weapons. The extreme handicap that laser-induced
blindness represents makes the use of lasers even as non-lethal
weapons morally controversial, and weapons designed to cause blindness
have been banned by the Protocol on Blinding
Laser Weapons. Incidents
of pilots being exposed to lasers while flying have prompted aviation
authorities to implement special procedures to deal with such
Laser weapons capable of directly damaging or destroying a target in
combat are still in the experimental stage. The general idea of
laser-beam weaponry is to hit a target with a train of brief pulses of
light. The rapid evaporation and expansion of the surface causes
shockwaves that damage the target. The power needed
to project a high-powered laser beam of this kind is beyond the limit
of current mobile power technology, thus favoring chemically powered
gas dynamic lasers. Example experimental systems include
the Tactical High
Boeing YAL-1. The laser system is mounted in a turret attached to the
Throughout the 2000s, the
United States Air Force
United States Air Force worked on the Boeing
YAL-1, an airborne laser mounted in a Boeing 747. It was intended to
be used to shoot down incoming ballistic missiles over enemy
territory. In March 2009,
Northrop Grumman claimed that its engineers
in Redondo Beach had successfully built and tested an electrically
powered solid state laser capable of producing a 100-kilowatt beam,
powerful enough to destroy an airplane. According to Brian Strickland,
manager for the United States Army's Joint High Power Solid State
Laser program, an electrically powered laser is capable of being
mounted in an aircraft, ship, or other vehicle because it requires
much less space for its supporting equipment than a chemical
laser. However, the source of such a large electrical power in a
mobile application remained unclear. Ultimately, the project was
deemed to be infeasible, and was cancelled in December
2011, with the
Boeing YAL-1 prototype being stored and eventually
United States Navy
United States Navy is developing a laser weapon referred to as the
Laser Weapon System
Laser Weapon System or LaWS.
In recent years, some hobbyists have taken interests in lasers. Lasers
used by hobbyists are generally of class IIIa or IIIb (see Safety),
although some have made their own class IV types. However,
compared to other hobbyists, laser hobbyists are far less common, due
to the cost and potential dangers involved. Due to the cost of lasers,
some hobbyists use inexpensive means to obtain lasers, such as
salvaging laser diodes from broken
DVD players (red),
(violet), or even higher power laser diodes from CD or DVD
Hobbyists also have been taking surplus pulsed lasers from retired
military applications and modifying them for pulsed holography. Pulsed
Ruby and pulsed YAG lasers have been used.
Examples by power
Laser application in astronomical adaptive optics imaging
Different applications need lasers with different output powers.
Lasers that produce a continuous beam or a series of short pulses can
be compared on the basis of their average power. Lasers that produce
pulses can also be characterized based on the peak power of each
pulse. The peak power of a pulsed laser is many orders of magnitude
greater than its average power. The average output power is always
less than the power consumed.
The continuous or average power required for some uses:
DVD player or DVD-ROM drive
Burning through a jewel case including disc within 4 seconds
DVD 24× dual-layer recording.
Green laser in current
Holographic Versatile Disc
Holographic Versatile Disc prototype
Output of the majority of commercially available solid-state lasers
used for micro machining
Typical sealed CO2 surgical lasers
Typical sealed CO2 lasers used in industrial laser cutting
Examples of pulsed systems with high peak power:
700 TW (700×1012 W) – National Ignition Facility, a 192-beam,
1.8-megajoule laser system adjoining a 10-meter-diameter target
1.3 PW (1.3×1015 W) – world's most powerful laser as of 1998,
located at the Lawrence Livermore Laboratory
Left: European laser warning symbol required for Class 2 lasers and
higher. Right: US laser warning label, in this case for a Class 3B
Even the first laser was recognized as being potentially dangerous.
Theodore Maiman characterized the first laser as having a power of one
"Gillette" as it could burn through one Gillette razor blade. Today,
it is accepted that even low-power lasers with only a few milliwatts
of output power can be hazardous to human eyesight when the beam hits
the eye 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 be
focused by the eye into an extremely small spot on the retina,
resulting in localized burning and permanent damage in seconds or even
Lasers are usually labeled with a safety class number, which
identifies how dangerous the laser is:
Class 1 is inherently safe, usually because the light is contained in
an enclosure, for example in CD players.
Class 2 is safe during normal use; the blink reflex of the eye will
prevent damage. Usually up to 1 mW power, for example laser
Class 3R (formerly IIIa) lasers are usually up to 5 mW and
involve a small risk of eye damage within the time of the blink
reflex. Staring into such a beam for several seconds is likely to
cause damage to a spot on the retina.
Class 3B can cause immediate eye damage upon exposure.
Class 4 lasers can burn skin, and in some cases, even scattered light
can cause eye and/or skin damage. Many industrial and scientific
lasers are in this class.
The indicated powers are for visible-light, continuous-wave lasers.
For pulsed lasers and invisible wavelengths, other power limits apply.
People working with class 3B and class 4 lasers can protect their eyes
with safety goggles which are designed to absorb light of a particular
Infrared lasers with wavelengths longer than about 1.4 micrometers are
often referred to as "eye-safe", because the cornea tends to absorb
light at these wavelengths, protecting the retina from damage. The
label "eye-safe" can be misleading, however, as it applies only to
relatively low power continuous wave beams; a high power or Q-switched
laser at these wavelengths can burn the cornea, causing severe eye
damage, and even moderate power lasers can injure the eye.
Coherent perfect absorber
Induced gamma emission
Laser Display Association
Lasers and aviation safety
Laser beam profiler
Laser beam welding
List of laser articles
List of light sources
Sound amplification by stimulated emission of radiation
Selective laser sintering
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List of laser articles
List of laser types
List of laser applications
Laser types: Solid-state
Active laser medium
Amplified spontaneous emission
Resolved sideband cooling
Chirped pulse amplification
Laser beam profiler
Multiple-prism grating laser oscillator
Multiphoton intrapulse interference phase scan
Cavity ring-down spectroscopy
Confocal laser scanning microscopy
Laser-based angle-resolved photoemission spectroscopy
Laser diffraction analysis
Laser-induced breakdown spectroscopy
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
Second-harmonic imaging microscopy
Terahertz time-domain spectroscopy
Tunable diode laser absorption spectroscopy
Two-photon excitation microscopy
Ultrafast laser spectroscopy
Above threshold ionization
Atmospheric-pressure laser ionization
Matrix-assisted laser desorption/ionization
Resonance-enhanced multiphoton ionization
Soft laser desorption
Surface-assisted laser desorption/ionization
Surface-enhanced laser desorption/ionization
Laser beam welding
Laser cutting bridge
Pulsed laser deposition
Selective laser melting
Selective laser sintering
Computed tomography laser mammography
Laser capture microdissection
Laser hair removal
Laser thermal keratoplasty
Low level laser therapy
Optical coherence tomography
Soft-tissue laser surgery
Laser integration line
Long path laser
National Ignition Facility
3D laser scanner
Laser lighting display
Advanced Tactical Laser
Laser warning receiver
Laser Engagement System
Laser Ordnance Neutralization System)