Laser
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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 acronym for "light
amplification by stimulated emission of radiation". The first laser was
built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on
theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.
A laser differs from other sources of light in that it emits
light which is coherent. 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 and lidar. Lasers
can also have high temporal coherence, which allows them to emit light with a
very narrow spectrum. Alternatively, temporal coherence can be used to produce
ultrashort pulses of light with a broad spectrum but durations as short as a femtosecond.
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Lasers are used in optical disc drives, laser printers,
barcode scanners, DNA sequencing instruments, fiber-optic, semiconducting chip
manufacturing (photolithography), 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 in
laser lighting displays for entertainment. Semiconductor lasers in the blue to
near-UV have also been used in place of light-emitting diodes (LED's) 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
LED's; such devices are already used in some car headlamps.
Contents
- 1: Fundamentals
- 1.1: Terminology
- 2: Design
- 3: Laser physics
- 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
- 4.1: Continuous wave operation
- 4.2: Pulsed operation
- 4.2.1: Q-switching
- 4.2.2: Mode-locking
- 4.2.3: Pulsed pumping
- 5: History
- 5.1: Foundations
- 5.2: Maser
- 5.3: Laser
- 5.4: Recent innovations
- 6: Types and operating principles
- 6.1: Gas lasers
- 6.1.1: Chemical lasers
- 6.1.2: Excimer 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
- 6.9: Natural lasers
- 7: Uses
- 7.1: In medicine
- 7.2: As weapons
- 7.3: Hobbies
- 7.4: Examples by power
- 8: Safety
Fundamentals
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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
very low divergence in order 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. 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 with slightly different wavelengths. Although temporal coherence
implies some degree of monochromaticity, there are lasers that emit a broad
spectrum of light or emit different wavelengths of light simultaneously. Some
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.
The first device using amplification by stimulated emission
operated at microwave frequencies, and was named "maser", an acronym
for "microwave amplification by stimulated emission of radiation".
When similar optical devices were developed they were first known as
"optical masers", until "microwave" was replaced by
"light" in the acronym.
All such devices operating at frequencies higher than
microwaves are called lasers (including infrared laser, ultraviolet laser,
X-ray laser and gamma-ray laser). All devices operating at microwave or lower
radio frequencies are called masers.
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".
The back-formed verb to lase is frequently used in the
field, meaning "to give off coherent light,"especially in reference
to the gain medium of a laser; when a laser is operating it is said to be
"lasing". The words laser and maser are also used in cases where there
is a coherent state unconnected with any manufactured device, as in
astrophysical maser and atom laser.
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 stimulated emission.
Light of a specific wavelength that passes through the gain medium is amplified
(increases in power). 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.
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.
An electron in an atom can absorb energy from light (photons) or heat (phonons) only if there is a transition between energy levels that matches 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 which 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 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.
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. 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.
For lasing media with extremely high gain, so-called
superluminescence, it is possible for light to 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), 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 is a high
gain optical amplifier which amplifies its own 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 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 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.
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. 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.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 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.
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.
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.
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 round-trip time.)
Terminology
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Design
 Components of a typical laser: 1.
Gain medium 2.
Laser pumping energy 3.
High reflector 4.
Output coupler 5. Laser beam |
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 matches 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.
Gain medium and cavity
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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 emitted light is 90 degrees in lead of the stimulating light. 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 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.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 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.Continuous and pulsed modes of operation
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Continuous wave operation
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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.
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 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 which can thus generate pulses of light as short as a few
femtoseconds (10−15 s).
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.
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. After
this presentation, RCA asked Weber to give a seminar on this idea, and Charles
Hard Townes asked him for a copy of the paper.
In 1953, 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 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. 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".
That same year, Charles Hard 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.
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 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. 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 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 historians.
On May 16, 1960, 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 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, 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, 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 Telephone Laboratories also independently developed room-temperature,
continual-operation diode lasers, using the heterojunction structure.
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:
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.
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 millihertz.
Pulsed operation
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.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 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 duration.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 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.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. In 1928, 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 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 Winter.Maser
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Laser
In April 1957, Japanese engineer Jun-ichi Nishizawa proposed the concept of a "semiconductor optical maser" in a patent application.
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Recent innovations
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- 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
Types and operating principles
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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) 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.
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 cancelled.
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 solid-state lasers. 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 one kilowatt.
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 which 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). 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.Solid-state lasers
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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 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.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. They are typical micrometer-sized and tunable on the bands of the photonic crystals.
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Dye lasers
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Free-electron lasers
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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. 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.Natural lasers
Like astrophysical masers, irradiated planetary or stellar gases may amplify light producing a natural laser. Mars, Venus and MWC 349 exhibit this phenomenon.Uses
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- 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, 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
- 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
In medicine
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.As weapons
A laser weapon is a laser that is used as a directed-energy weapon.Hobbies
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Examples by power
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The continuous or average power required for some uses: |
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Power |
Use |
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1–5 mW |
Laser pointers |
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5 mW |
CD-ROM drive |
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5–10 mW |
DVD player or DVD-ROM drive |
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100 mW |
High-speed CD-RW burner |
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250 mW |
Consumer 16× DVD-R burner |
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400 mW |
DVD 24× dual-layer recording |
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1 W |
Green laser in Holographic Versatile Disc prototype
development |
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1–20 W |
Output of the majority of commercially available
solid-state lasers used for micro machining |
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30–100 W |
Typical sealed CO2 surgical lasers |
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100–3000 W |
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 chamber
- 10 PW (10×1015 W) – world's most powerful laser as of 2019, located at the ELI-NP facility in Măgurele, Romania.
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 |
- 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 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.
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Laser