Device which emits light via optical amplification
A
laser
is a device that emits
light
through a process of
optical amplification
based on the
stimulated emission
of
electromagnetic radiation
. The word
laser
is an
anacronym
that originated as an acronym for
light amplification by stimulated emission of radiation
.
[1]
[2]
The first laser was built in 1960 by
Theodore Maiman
at
Hughes Research Laboratories
, based on theoretical work by
Charles H. Townes
and
Arthur Leonard Schawlow
.
[3]
A laser differs from other sources of light in that it emits light that is
coherent
.
Spatial coherence
allows a laser to be focused to a tight spot, enabling applications such as
laser cutting
and
lithography
. It also allows a laser beam to stay narrow over great distances (
collimation
), a feature used in applications such as
laser pointers
and
lidar
(light detection and ranging). Lasers can also have high
temporal coherence
, which permits them to emit light with a very narrow
frequency spectrum
. Alternatively, temporal coherence can be used to produce
ultrashort pulses
of light with a broad spectrum but durations as short as a
femtosecond
.
Lasers are used in
optical disc drives
,
laser printers
,
barcode scanners
,
DNA sequencing instruments
,
fiber-optic
, and
free-space optical communication
, semiconducting chip manufacturing (
photolithography
),
laser surgery
and skin treatments, cutting and
welding
materials, military and
law enforcement
devices for marking targets and
measuring range
and speed, and in
laser lighting displays
for entertainment. Semiconductor lasers in the blue to
near-UV
have also been used in place of
light-emitting diodes
(LEDs) to excite
fluorescence
as a white light source; this permits a much smaller emitting area due to the much greater
radiance
of a laser and avoids the
droop
suffered by LEDs; such devices are already used in some car
headlamps
.
[4]
[5]
[6]
[7]
Terminology
The first device using amplification by stimulated emission operated at
microwave
frequencies, and was called a
maser
, for "microwave amplification by stimulated emission of radiation".
[8]
When similar
optical
devices were developed they were first known as
optical masers
, until "microwave" was replaced by "light" in the acronym, to become
laser
.
[9]
Today, all such devices operating at frequencies higher than microwaves (approximately above 300
GHz
) are called lasers (e.g.
infrared lasers
,
ultraviolet lasers
,
X-ray lasers
,
gamma-ray lasers
), whereas devices operating at
microwave
or lower
radio frequencies
are called masers.
[10]
[11]
The
back-formed
verb "
to lase
" is frequently used in the field, meaning "to give off coherent light," especially about the
gain medium
of a laser;
[12]
when a laser is operating it is said to be "
lasing
".
[13]
The terms
laser
and
maser
are also used for naturally occurring coherent emissions, as in
astrophysical maser
and
atom laser
.
[14]
[15]
A laser that produces light by itself is technically an optical oscillator rather than an
optical amplifier
as suggested by the acronym.
[16]
It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation", would have been more correct.
[15]
With the widespread use of the original acronym as a common noun, optical amplifiers have come to be referred to as
laser amplifiers
.
[17]
Fundamentals
Modern physics describes light and other forms of
electromagnetic radiation
as the group behavior of
fundamental particles
known as
photons
. Photons are released and absorbed through
electromagnetic
interactions with other fundamental particles that carry
electric charge
. A common way to release photons is to heat an object; some of the
thermal energy
being applied to the object will cause the
molecules
and
electrons
within the object to gain energy, which is then lost through
thermal radiation
, that we see as light. This is the process that causes a candle flame to give off light.
Thermal radiation is a random process, and thus the photons emitted have a range of different
wavelengths
, travel in different directions, and are released at different times. The energy within the object is not random, however: it is stored by atoms and molecules in "
excited states
", which release photons with distinct wavelengths. This gives rise to the science of
spectroscopy
, which allows materials to be determined through the specific wavelengths that they emit.
The underlying physical process creating photons in a laser is the same as in thermal radiation, but the actual emission is not the result of random thermal processes. Instead, the release of a photon is triggered by the nearby passage of another photon. This is called
stimulated emission
. For this process to work, the passing photon must be similar in energy, and thus wavelength, to the one that could be released by the atom or molecule, and the atom or molecule must be in the suitable excited state.
The photon that is emitted by stimulated emission is identical to the photon that triggered its emission, and both photons can go on to trigger stimulated emission in other atoms, creating the possibility of a
chain reaction
. For this to happen, many of the atoms or molecules must be in the proper excited state so that the photons can trigger them. In most materials, atoms or molecules drop out of excited states fairly rapidly, making it difficult or impossible to produce a chain reaction. The materials chosen for lasers are the ones that have
metastable states
, which stay excited for a relatively long time. In
laser physics
, such a material is called an
active laser medium
. Combined with an energy source that continues to "pump" energy into the material, this makes it possible to have enough atoms or molecules in an excited state for a chain reaction to develop.
Lasers are distinguished from other light sources by their
coherence
. Spatial (or transverse) 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 a very low divergence to concentrate their power at a great distance. 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.
[18]
[
page needed
]
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 produce radiation in several
modes
with slightly different wavelengths. Although temporal coherence implies some degree of
monochromaticity
, some lasers emit a broad spectrum of light or emit different wavelengths of light simultaneously. Certain lasers are not single spatial mode and have light beams that
diverge
more than is required by the
diffraction limit
. All such devices are classified as "lasers" based on the method of producing light by stimulated emission. Lasers are employed where light of the required spatial or temporal coherence can not be produced using simpler technologies.
Design
A laser consists of a
gain medium
, a mechanism to energize it, and something to provide optical
feedback
.
[19]
The gain medium is a material with properties that allow it to
amplify
light by way of stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified (power increases). Feedback enables stimulated emission to amplify predominantly the optical frequency at the peak of the gain-frequency curve. As stimulated emission grows, eventually one frequency dominates over all others, meaning that a coherent beam has been formed.
The process of stimulated emission is analogous to that of an audio oscillator with positive feedback which can occur, for example, when the speaker in a public-address system is placed in proximity to the microphone. The screech one hears is audio oscillation at the peak of the gain-frequency curve for the amplifier.
[21]
[
page needed
]
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 the properties of the emitted light, such as the polarization, wavelength, and shape of the beam.
[
citation needed
]
Laser physics
Electrons
and how they interact with
electromagnetic fields
are important in our understanding of
chemistry
and
physics
.
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:
An electron in an atom can absorb energy from light (
photons
) or heat (
phonons
) only if there is a transition between energy levels that match the energy carried by the photon or phonon. For light, this means that any given transition will only
absorb
one particular
wavelength
of light. Photons with the correct wavelength can cause an electron to jump from the lower to the higher energy level. The photon is consumed in this process.
When an electron is
excited
from one state to that at a higher energy level with energy difference ΔE, it will not stay that way forever. Eventually, a photon will be spontaneously created from the vacuum having energy ΔE. Conserving energy, the electron transitions to a lower energy level that is not occupied, with transitions to different levels having different time constants. This process is called
spontaneous emission
. Spontaneous emission is a quantum-mechanical effect and a direct physical manifestation of the Heisenberg
uncertainty principle
. The emitted photon has a random direction, but its wavelength matches the absorption wavelength of the transition. This is the mechanism of
fluorescence
and
thermal emission
.
A photon with the correct wavelength to be absorbed by a transition can also cause an electron to drop from the higher to the lower level, emitting a new photon. The emitted photon exactly matches the original photon in wavelength, phase, and direction. This process is called stimulated emission.
Gain medium and cavity
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 that have been excited into such a state using an outside light source, or an electrical field that 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. In this state, the rate of stimulated emission is larger than the rate of absorption of light in the medium, and therefore the light is amplified. A system with this property is called an
optical amplifier
. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.
[22]
For lasing media with extremely high gain, so-called
superluminescence
, light can be sufficiently amplified in a single pass through the gain medium without requiring a resonator. Although often referred to as a laser (see for example
nitrogen laser
),
[23]
the light output from such a device lacks the spatial and temporal coherence achievable with lasers. Such a device cannot be described as an oscillator but rather as a high-gain optical amplifier that amplifies its spontaneous emission. The same mechanism describes so-called
astrophysical masers
/lasers.
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 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 cavity 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
In most lasers, lasing begins with spontaneous emission into the lasing mode. This initial light is then amplified by stimulated emission in the gain medium. Stimulated emission produces light that matches the input signal in direction, wavelength, and polarization, whereas the
phase
of the emitted light is 90 degrees in lead of the stimulating light.
[24]
This, combined with the filtering effect of the optical resonator gives laser light its characteristic coherence, and may give it uniform polarization and monochromaticity, depending on the resonator's design. The fundamental
laser linewidth
[25]
of light emitted from the lasing resonator can be orders of magnitude narrower than the linewidth of light emitted from the passive resonator. Some lasers use a separate
injection seeder
to start the process off with a beam that is already highly coherent. This can produce beams with a narrower spectrum than would otherwise be possible.
In 1963,
Roy J. Glauber
showed that coherent states are formed from combinations of
photon number
states, for which he was awarded the
Nobel Prize in physics
.
[26]
A coherent beam of light is formed by single-frequency quantum photon states distributed according to a
Poisson distribution
. As a result, the arrival rate of photons in a laser beam is described by Poisson statistics.
Many lasers produce a beam that can be approximated as a
Gaussian beam
; such beams have the minimum divergence possible for a given beam diameter. Some lasers, particularly high-power ones, produce multimode beams, with the
transverse modes
often approximated using
Hermite
?
Gaussian
or
Laguerre
-Gaussian functions. Some high-power lasers use a flat-topped profile known as a "
tophat beam
". Unstable laser resonators (not used in most lasers) produce fractal-shaped beams.
[28]
Specialized optical systems can produce more complex beam geometries, such as
Bessel beams
and
optical vortexes
.
Near the "waist" (or
focal region
) of a laser beam, 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 that 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 employing 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.
A
laser beam profiler
is used to measure the intensity profile, width, and divergence of laser beams.
Diffuse reflection
of a laser beam from a matte surface produces a
speckle pattern
with interesting properties.
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
[
dubious
–
discuss
]
that was predicted by
Albert Einstein
, who 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
.
Modes of operation
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 to create pulses of light. When the modulation rate is on time scales much slower than the
cavity lifetime
and the 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 into 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
) laser. Many types of lasers can be made to operate in continuous-wave mode to satisfy such an application. Many of these lasers lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will 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 longer periods, with the very high-frequency power variations having little or no impact on 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 round-trip time.)
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 destroying the laser by producing excessive heat. Such lasers cannot be run in CW mode.
Pulsed operation
The pulsed operation of lasers refers to any laser not classified as a 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 many 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 between pulses. In
laser ablation
, for example, a small volume of material at the surface of a workpiece 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 to obtain
nonlinear optical
effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as
Q-switching
.
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 that can thus generate pulses of light as short as a few
femtoseconds
(10
?15
s).
Q-switching
In a 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.
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 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 duration.
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). Unlike the giant pulse of a Q-switched laser, consecutive pulses from a mode-locked laser are phase-coherent, that is, the pulses (and not just their
envelopes
) are identical and perfectly periodic. For this reason, and the extremely large peak powers attained by such short pulses, such lasers are invaluable in certain areas of research.
Pulsed pumping
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 that 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.
History
Foundations
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 coefficients (
Einstein coefficients
) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation.
[29]
In 1928,
Rudolf W. Ladenburg
confirmed the existence of the phenomena of stimulated emission and negative absorption.
[30]
[
page needed
]
In 1939,
Valentin A. Fabrikant
predicted the use of stimulated emission to amplify "short" waves.
[31]
In 1947,
Willis E. Lamb
and R.
C.
Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission.
[30]
[
page needed
]
In 1950,
Alfred Kastler
(Nobel Prize for Physics 1966) proposed the method of
optical pumping
, which was experimentally demonstrated two years later by Brossel, Kastler, and Winter.
[32]
Maser
In 1951,
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.
[33]
After this presentation,
RCA
asked Weber to give a seminar on this idea, and
Charles H. Townes
asked him for a copy of the paper.
[34]
In 1953, Charles H. 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.
[35]
Meanwhile, in the Soviet Union,
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 pumping.
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.
[36]
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".
Laser
In April 1957, Japanese engineer
Jun-ichi Nishizawa
proposed the concept of a "
semiconductor optical maser
" in a patent application.
[37]
That same year, Charles H. Townes and Arthur Leonard Schawlow, then at
Bell Labs
, began a serious study of infrared "optical masers". As ideas developed, they abandoned
infrared
radiation to instead concentrate on
visible light
. 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
, which was published in 1958.
[38]
Simultaneously,
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; afterward, 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 of this idea. Meanwhile, Schawlow and Townes had decided on an open-resonator laser design ? apparently unaware of Prokhorov's publications and Gould's unpublished laser work.
At a conference in 1959, Gordon Gould first published the acronym "LASER" in the paper
The LASER, Light Amplification by Stimulated Emission of Radiation
.
[39]
[15]
Gould's intention was that different "-ASER" acronyms should be used for different parts of the spectrum: "XASER" for x-rays, "UVASER" for ultraviolet, etc. "LASER" ended up becoming the generic term for non-microwave devices, although "RASER" was briefly popular for denoting radio-frequency-emitting devices.
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
United States Patent and Trademark Office
(USPTO) 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 USPTO 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 historians.
[40]
On May 16, 1960, Theodore H. Maiman operated the first functioning laser
[41]
[42]
at
Hughes Research Laboratories
, Malibu, California, ahead of several research teams, including those of Townes, at
Columbia University
,
Arthur L. Schawlow
, at
Bell Labs
,
[43]
[
page needed
]
and Gould, at the TRG (Technical Research Group) company. Maiman's functional laser used a
flashlamp
-pumped synthetic
ruby
crystal
to produce red laser light at 694 nanometers wavelength. The device was only capable of pulsed operation, due to its three-level pumping design scheme. Later that year, the
Iranian
physicist
Ali Javan
, and
William R. Bennett Jr.
, and
Donald R. 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 World Award of Science
in 1993. In 1962,
Robert N. Hall
demonstrated the first
semiconductor laser
, which was made of
gallium arsenide
and emitted in the
near-infrared
band of the spectrum at 850 nm. 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 Labs also independently developed room-temperature, continual-operation diode lasers, using the
heterojunction
structure.
Recent innovations
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
- minimum linewidth
- maximum power efficiency
- minimum cost
and this research continues to this day.
In 2015, researchers made a white laser, whose light is modulated by a synthetic nanosheet made out of zinc, cadmium, sulfur, and selenium that can emit red, green, and blue light in varying proportions, with each wavelength spanning 191 nm.
[44]
[45]
[46]
In 2017, researchers at the
Delft University of Technology
demonstrated an
AC Josephson junction
microwave laser.
[47]
Since the laser operates in the superconducting regime, it is more stable than other semiconductor-based lasers. The device has the potential for applications in
quantum computing
.
[48]
In 2017, researchers at the
Technical University of 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.
[49]
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
millihertz.
[50]
[51]
Types and operating principles
Gas lasers
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) can operate at many 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 (CO
2
) 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 CO
2
laser is unusually high: over 30%.
[52]
Argon-ion
lasers can operate at several 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.
[53]
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
),
[54]
making them candidates for use in
fluorescence
suppressed
Raman spectroscopy
.
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.
[55]
[56]
[
page needed
]
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 canceled.
Chemical lasers
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
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 that can only exist with one atom in an
excited electronic state
. Once the molecule transfers its excitation energy to a photon, 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. Excimer lasers 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).
[57]
[
page needed
]
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 since F
2
is a stable compound.
Solid-state lasers
Solid-state lasers
use a crystalline or glass rod that 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 maintained in the dopant. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flash tube or 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:YVO
4
),
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
doubled
,
tripled
or quadrupled in frequency 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 solid-state lasers.
[58]
[
page needed
]
Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF
2
, 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 (d
n
/d
T
) 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 one kilowatt.
[59]
Fiber lasers
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 a high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce the 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, like other optical media, can suffer from the effects of
photodarkening
when they are exposed to radiation of certain wavelengths. In particular, this can lead to degradation of the material and loss in laser functionality over time. The exact causes and effects of this phenomenon vary from material to material, although it often involves the formation of
color centers
.
[60]
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.
[61]
[
clarification needed
]
Semiconductor lasers
Semiconductor lasers are
diodes
that are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal forms 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.
[62]
Low to 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 laser diodes, with power of up to 20 kW, are used in industry for cutting and welding.
[63]
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.
In 2012,
Nichia
and
OSRAM
developed and manufactured commercial high-power green laser diodes (515/520 nm), which compete with traditional diode-pumped solid-state lasers.
[64]
[65]
Vertical cavity surface-emitting lasers (
VCSELs
) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. 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,
[66]
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 wells
.
The development of a
silicon
laser is important in the field of
optical computing
. 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 that 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.
[67]
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.
[49]
Another type is a
Raman laser
, which takes advantage of
Raman 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 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.
Bubble lasers
are dye lasers that use a
bubble
as the optical resonator.
Whispering gallery modes
in the bubble produce an output spectrum composed of hundreds of evenly spaced peaks; a
frequency comb
. The spacing of the whispering gallery modes is directly related to the bubble circumference, allowing bubble lasers to be used as highly sensitive pressure sensors.
[68]
Free-electron lasers
Free-electron lasers
(FEL) 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
.
Exotic media
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.
[69]
[70]
[71]
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 be realized.
[72]
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
Mossbauer effect
.
[73]
[
page needed
]
[74]
In conjunction, several advantages were expected from two-stage pumping of a three-level system.
[75]
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.
[76]
[77]
Furthermore, the 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 frequency.
[78]
[79]
[80]
[81]
[82]
[83]
[84]
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.
[85]
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
experiments.
[85]
Space-based
X-ray lasers
pumped by a nuclear explosion have also been proposed as antimissile weapons.
[86]
[87]
Such devices would be one-shot weapons.
Living cells have been used to produce laser light.
[88]
[89]
The cells were genetically engineered to produce
green fluorescent protein
, which served as the laser's gain medium. The cells were then placed between two 20-micrometer-wide mirrors, which acted as the laser cavity. When the cell was illuminated with blue light, it emitted intensely directed green laser light.
Natural lasers
Like
astrophysical masers
, irradiated planetary or stellar gases may amplify light producing a natural laser.
[90]
Mars
,
[91]
Venus
and
MWC 349
exhibit this phenomenon.
Uses
When lasers were invented in 1960, they were called "a solution looking for a problem".
[92]
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 widely noticeable use of lasers 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 space
- Medicine: see
below
- Industry:
cutting
including
converting
thin materials,
welding
, material
heat treatment
,
marking parts
(
engraving
and
bonding
),
additive manufacturing
or
3D printing
processes such as
selective laser sintering
and
selective laser melting
,
laser metal deposition
, and non-contact measurement of parts and
3D scanning
, and
laser cleaning
.
- Military: marking targets, guiding
munitions
,
missile defense
,
electro-optical countermeasures (EOCM)
,
lidar
, blinding troops,
firearms sight
. See
below
- Law enforcement
:
LIDAR traffic enforcement
. Lasers are used for latent
fingerprint
detection in the
forensic identification
field
[93]
[94]
- Research:
spectroscopy
,
laser ablation
, laser
annealing
, laser
scattering
, laser
interferometry
,
lidar
,
laser capture microdissection
,
fluorescence microscopy
,
metrology
,
laser cooling
- Commercial products:
laser printers
,
barcode scanners
,
thermometers
,
laser pointers
,
holograms
,
bubblegrams
- Entertainment:
optical discs
,
laser lighting displays
,
laser turntables
.
- Informational markings: Laser lighting display technology can be used to project informational markings onto surfaces such as playing fields, roads, runways, or warehouse floors.
[95]
[96]
[97]
In 2004, excluding diode lasers, approximately 131,000 lasers were sold with a value of
US$2.19
billion.
[98]
In the same year, approximately 733 million diode lasers, valued at
US$3.20
billion, were sold.
[99]
In medicine
Lasers have many uses in medicine, including
laser surgery
(particularly
eye surgery
), laser healing (photobiomodulation therapy),
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 cancer cells. Lasers are more precise than traditional surgery methods and cause less damage, pain,
bleeding
, swelling, and scarring. A disadvantage is that surgeons must acquire specialized training and thus it will likely be more expensive than other treatments.
[100]
[101]
As weapons
A
laser weapon
is a laser that is used as a
directed-energy weapon
.
Hobbies
In recent years, some hobbyists have taken an interest in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb
(see
§ Safety
)
, although some have made their own class IV types.
[102]
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),
Blu-ray
players (violet), or even higher power laser diodes from CD or
DVD burners
.
[103]
Hobbyists have also used surplus lasers taken from retired military applications and modified them for
holography
. Pulsed ruby and YAG lasers work well for this application.
Examples by power
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.
Examples of pulsed systems with high peak power:
Safety
Left: European laser warning symbol required for Class 2 lasers and higher. Right: US laser warning label, in this case for a Class 3B laser
Even the first laser was recognized as being potentially dangerous.
Theodore Maiman
characterized the first laser as having the power of one "Gillette" as it could burn through one
Gillette
razor
blade.
[108]
[109]
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 less time.
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 pointers.
- 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 lasers (5?499 mW) can cause immediate eye damage upon exposure.
- Class 4 lasers (≥ 500 mW) can burn skin, and in some cases, even scattered light from these lasers 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 wavelength.
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.
Lasers can be a hazard to both civil and military aviation, due to the potential to temporarily distract or blind pilots. See
Lasers and aviation safety
for more on this topic.
Cameras based on
charge-coupled devices
may be more sensitive to laser damage than biological eyes.
[110]
See also
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Further reading
Books
- Bertolotti, Mario (1999, trans. 2004).
The History of the Laser
. Institute of Physics.
ISBN
0-7503-0911-3
.
- Bromberg, Joan Lisa (1991).
The Laser in America, 1950?1970
. MIT Press.
ISBN
978-0-262-02318-4
.
- Csele, Mark (2004).
Fundamentals of Light Sources and Lasers
. Wiley.
ISBN
0-471-47660-9
.
- Koechner, Walter (1992).
Solid-State Laser Engineering
. 3rd ed. Springer-Verlag.
ISBN
0-387-53756-2
.
- Siegman, Anthony E. (1986).
Lasers
. University Science Books.
ISBN
0-935702-11-3
.
- Silfvast, William T.
(1996).
Laser Fundamentals
. Cambridge University Press.
ISBN
0-521-55617-1
.
- Svelto, Orazio (1998).
Principles of Lasers
. 4th ed. Trans. David Hanna. Springer.
ISBN
0-306-45748-2
.
- Taylor, Nick (2000).
LASER: The inventor, the Nobel laureate, and the thirty-year patent war
. New York: Simon & Schuster.
ISBN
978-0-684-83515-0
.
- Pearsall, Thomas (2020).
Quantum Photonics, 2nd edition
. Graduate Texts in Physics. Springer.
doi
:
10.1007/978-3-030-47325-9
.
ISBN
978-3-030-47324-2
.
S2CID
240934073
.
Archived
from the original on February 25, 2021
. Retrieved
February 23,
2021
.
- Wilson, J. & Hawkes, J.F.B. (1987).
Lasers: Principles and Applications
. Prentice Hall International Series in Optoelectronics,
Prentice Hall
.
ISBN
0-13-523697-5
.
- Yariv, Amnon (1989).
Quantum Electronics
. 3rd ed. Wiley.
ISBN
0-471-60997-8
.
Periodicals
External links
Wikimedia Commons has media related to
Lasers
.
- Encyclopedia of laser physics and technology by Rudiger Paschotta
- A Practical Guide to Lasers for Experimenters and Hobbyists by Samuel M. Goldwasser
- Homebuilt Lasers Page by Professor Mark Csele
Archived
June 1, 2009, at the
Wayback Machine
- Powerful laser is 'brightest light in the universe'
?The world's most powerful laser as of 2008 might create supernova-like shock waves and possibly even antimatter
- "
Laser Fundamentals
" an online course by F. Balembois and S. Forget.
- Northrop Grumman's Press Release on the Firestrike 15 kW tactical laser product
- Website on Lasers 50th anniversary by APS, OSA, SPIE
- Advancing the Laser anniversary site by SPIE: Video interviews, open-access articles, posters, DVDs
Archived
April 23, 2021, at the
Wayback Machine
- Bright Idea: The First Lasers
Archived
October 3, 2012, at the
Wayback Machine
history of the invention, with audio interview clips.
- Free software for Simulation of random laser dynamics
- Video Demonstrations in Lasers and Optics
Produced by the Massachusetts Institute of Technology (MIT). Real-time effects are demonstrated in a way that would be difficult to see in a classroom setting.
- MIT Video Lecture: Understanding Lasers and Fiberoptics
- Virtual Museum of Laser History, from the touring exhibit by SPIE
- website with animations, applications and research about laser and other quantum based phenomena
Universite Paris Sud