The word laser means the light

«Lase» redirects here. For uses of «Laze», see Laze.

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][3][4][5]} 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.^[6]

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. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar (light detection and ranging). 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.

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.^{[7][8][9][10]}

Fundamentals

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.^[11] 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. 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.

Terminology

The first device using amplification by stimulated emission operated at microwave frequencies, and was named «maser» («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 its acronym.^[12]

All such devices operating at frequencies higher than microwaves are called lasers (including infrared lasers, ultraviolet lasers, 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.^[13] 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,»^[14] especially about 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.

Design

A laser consists of a gain medium, a mechanism to energize it, and something to provide optical feedback.^[15] 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.^[16] 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.^[17]

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.^[18]

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),^[19] 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.^[20] 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^[21] 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.^[22] 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.^[16]

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.^[23] 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.

Continuous and pulsed 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.^[24] In 1928, Rudolf W. Ladenburg confirmed the existence of the phenomena of stimulated emission and negative absorption.^[25] In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify «short» waves.^[26] In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission.^[25] 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.^[27]

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.^[28] 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.^[29]

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.^[30] 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.^[31] 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.^[32]

External audio
“The Man, the Myth, the Laser”, Distillations Podcast, Science History Institute

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.^[33]

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.^[3][13] 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.^[34]

On May 16, 1960, Theodore H. Maiman operated the first functioning laser^[35][36] at Hughes Research Laboratories, Malibu, California, ahead of several research teams, including those of Townes, at Columbia University, Arthur L. Schawlow, at Bell Labs,^[37] 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. 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 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.^[38][39][40]

In 2017, researchers at the Delft University of Technology demonstrated an AC Josephson junction microwave laser.^[41] 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.^[42] 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.^[43]

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.^[44][45]

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 (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%.^[46] 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.^[47] 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),^[48] 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.^[49][50] 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).^[51]
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 F₂ 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: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.^[52] 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.^[53]

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 have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of long-living color centers.^{[citation needed]}

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.^{[54][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.^[55] 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.^[56] 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.^[57][58]

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,^[59] 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.^[60] 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.^[43] 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.

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.^[61][62][63] 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.^[64]

Some of the early studies were directed toward short pulses of neutrons exciting the upper isomer state in a solid so the gamma-ray transition could benefit from the line-narrowing of Mössbauer effect.^[65][66] In conjunction, several advantages were expected from two-stage pumping of a three-level system.^[67] 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.^[68][69] 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.^{[70][71][72][73][74][75][76]}

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.^[77] Dr. David Cassidy of the University of California, Riverside proposed that a single such laser could be used to ignite a nuclear fusion reaction, replacing the banks of hundreds of lasers currently employed in inertial confinement fusion experiments.^[77]

Space-based X-ray lasers pumped by a nuclear explosion have also been proposed as antimissile weapons.^[78][79] Such devices would be one-shot weapons.

Living cells have been used to produce laser light.^[80][81] 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.^[82] Mars,^[83] Venus and MWC 349 exhibit this phenomenon.

Uses

When lasers were invented in 1960, they were called «a solution looking for a problem».^[84] 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, 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^[85][86]
• 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 2004, excluding diode lasers, approximately 131,000 lasers were sold with a value of US$2.19 billion.^[87] In the same year, approximately 733 million diode lasers, valued at US$3.20 billion, were sold.^[88]

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.^[89][90]

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.^[91] 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.^[92]

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.

The continuous or average power required for some uses:

Power	Use
1–5 mW	Laser pointers
5 mW	CD-ROM drive
5–10 mW	DVD player or DVD-ROM drive
100 mW	High-speed CD-RW burner
250 mW	Consumer 16× DVD-R burner
400 mW	DVD 24× dual-layer recording[93]
1 W	Green laser in Holographic Versatile Disc prototype development
1–20 W	Output of the majority of commercially available solid-state lasers used for micro machining
30–100 W	Typical sealed CO2 surgical lasers[94]
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^[95]
• 10 PW (10×10¹⁵ W)—world’s most powerful laser as of 2019, located at the ELI-NP facility in Măgurele, Romania.^[96]

Safety

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. 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.^[97]

See also

References

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.
• 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

• Applied Physics B: Lasers and Optics (ISSN 0946-2171)
• IEEE Journal of Lightwave Technology (ISSN 0733-8724)
• IEEE Journal of Quantum Electronics (ISSN 0018-9197)
• IEEE Journal of Selected Topics in Quantum Electronics (ISSN 1077-260X)
• IEEE Photonics Technology Letters (ISSN 1041-1135)
• Journal of the Optical Society of America B: Optical Physics (ISSN 0740-3224)
• Laser Focus World (ISSN 0740-2511)
• Optics Letters (ISSN 0146-9592)
• Photonics Spectra (ISSN 0731-1230)

External links

• Encyclopedia of laser physics and technology by Dr. Rüdiger Paschotta
• A Practical Guide to Lasers for Experimenters and Hobbyists by Samuel M. Goldwasser
• Homebuilt Lasers Page by Professor Mark Csele
• 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 Prof. F. Balembois and Dr. 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
• 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

Lasers are a source of great excitement, often reminding us of science fiction films and future scientific developments. These devices seem supernatural, and have been employed by creators of blockbusters…

Lasers are a source of great excitement, often reminding us of science fiction films and future scientific developments. These devices seem supernatural, and have been employed by creators of blockbusters including X-Men and Star Wars, where jedi warriors fight each other using laser lightsabers.

However, lasers have left the realm of science fiction a long time ago and have since become a working instrument in many areas of contemporary science. These devices are extremely useful and can be found almost everywhere in our everyday lives.

What does the name mean?

The name originates from the expression: Light Amplification by Stimulated Emission of Radiation. The first letters of each word together create the abbreviation LASER.

In simple terms, lasers create highly concentrated streams of light.

Who invented lasers?

The first discoveries that eventually brought us lasers were made at the dawn of the XX century.

Einstein

In 1917, Albert Einstein released a revolutionary paper where he described the basics of quantum mechanical working principles of lasers. It was a real breakthrough because the author predicted a new physical phenomenon: stimulated emission. Einstein’s theory states that light can be emitted and absorbed — and not only in spontaneous ways. Forced (or stimulated) emission is also possible. This means that multiple electrons can be ‘forced’ to emit light of a specified wavelength, all at once.

Maiman

This idea was only practically implemented in the 1960s. The very first laser was created by California-based physicist Theodore Maiman on May 16, 1960. The laser was based on a ruby crystal and a Fabry-Perot resonator. A flash lamp was used for pumping. The laser worked impulsively with a wavelength of 694.3 nm.

Basov, Prokhorov and Townes

In 1952, Soviet scientists Nikolai Basov and Aleksandr Prokhorov told the world that it is possible to create a new microwave laser based on ammonia. At the same time, this idea was also developed independently by American physicist Charles Townes. He showed how this laser can work in 1954. A decade later, in 1964, all three were awarded with the Nobel Prize in physics for their discoveries.

Present day

We are currently living in an era of intense development of lasers. New types of lasers (chemical, excimer, semiconductor, free electron) are introduced almost every year.

How lasers work

For a better understanding of how lasers work, let’s take a look at their structure. The average laser looks something like this: a tube with a hard crystal on the inside, usually a ruby. It is covered in mirrors on both ends: one is partially reflective, the other is fully reflective. Atoms of the crystal generate light waves under the influence of electrical coils. These waves move from one mirror to the next until they reach a level of intensity that is sufficient for them to pass through the partially reflective mirror.

How are laser beams created?

• Stage 1: turned off laser

The electrons in all atoms (the black dots on the inner circles in the image) take up the main energy level.

• Stage 2: the moment after turning on the laser.

Influenced by the energy from the flash tube, electrons move to higher energy orbits (the outer circles in the image).

• Stage 3: appearance of a laser beam.

The electrons start leaving the higher energy orbits and return to the basic level. At the same time, they start emitting light and influencing all other electrons. There is a resulting beam of light, with each source producing the same wavelength. The light of the laser increases in intensity as more new electrons return to the lower orbits.

Focus sharpness

The laser beam is made up of light waves of the same length, so the color is also the same for the entire beam. Using a lens, this light can be accurately targeted towards a single point. (See image: on the left — laser beam, on the right — natural light). When comparing a laser to natural light, you will notice that the latter does not focus as well as the former. By concentrating a large amount of energy on a narrow beam, lasers can transmit these beams across large distances, without the light getting diffused or weakened as it does when it is made up of many colors, like natural light. These properties make lasers an indispensable tool for humanity.

Physical explanation

Let’s take a closer look at the laser mechanism described above. We will find out which underlying laws of physics are responsible for its operation.

Active medium

Laser radiation requires a so-called ‘active medium’. This medium is the only environment in which the phenomenon can take place. How is an active medium created? First of all, you need a special substance, which is usually made up of ruby crystals or YAG. This substance represents the active medium. A tube or rod made out of this substance is inserted into the resonator, which is made up of two mirrors aligned in parallel to each other. The front mirror is partially transparent, while the one in the back is a regular mirror. A flash lamp is mounted next to the tube. The tube and the flash lamp are surrounded by a mirror, which is usually made out of quartz covered in a layer of metal. The mirror helps collect the light on the tube.

Atomic energy levels

One important point: the active medium is composed in such a way that each atom has at least three energy levels. In their resting state, atoms in the active medium are at the lowest energy level, E0. As soon as the flash lamp is turned on, atoms absorb the energy emitted by its light, move to the E1 level and spend a relatively long time in this activated state. This is what causes the laser impulse.

Population inversion

Population inversion is a fundamental concept in physics. This is a state of a medium when the number of particles at a higher energy level in an atom is higher than it is at the lower level. A medium is considered active if it has an inverse population in its energy levels.

Photons and light beams

Electrons are not chaotically distributed within an atom. They have their own orbits surrounding the nucleus. An atom that receives a quantum of energy has a high probability of passing into an agitated state which is characterized by electrons switching orbits — from the lowest ones (metastable / basic) to the ones with the highest level of energy. Electrons cannot stay too long on this orbit, so they are forced to return to the basic level. When this happens, each electron emits a lightwave called a photon. A single atom launches a chain reaction, with electrons in many other atoms also shifting to orbits with lower energy levels. Identical light waves are propelled by an enormous flow. The changes in these waves are consistent over time and join together to create a powerful light beam. This light beam is called a laser beam. Some lasers create beams that are strong enough to cut through stone or metal.

Laser classification

There are several types of lasers which differ from one another based on aggregate state of the active medium and the way it is activated. We will list the main types.

Solid-state lasers

This was the first type of laser invented. They had a solid active medium consisting of ruby crystals and a small amount of chromium ions. Pumping was performed using a flash lamp. The first ruby laser was built by American scientist T. Maiman in 1960. Solid-state lasers are also made from glass with an addition of Nd, yttrium aluminium garnet Y2Al5O12 with an addition of chromium and neodymium — all of these substances also make up the active medium of solid-state lasers.

Gas lasers

The active medium in gas lasers is made up of gases with extremely low pressure or some combination of these gases. The gases fill a glass tube which is inlaid with electrodes. American scientists A. Javan, W. Bennett and D. Herriott first created the gas laser in 1960. This type of laser uses electrical impulses created by high-frequency generators for pumping. Gas lasers are characterized by their continuous beams. Gas density is rather low, so there is a need for a relatively long rod for the active medium. The intensity of the beams is created by the mass of the active medium.

Gas-dynamic, chemical and excimer lasers

These three types can be generally classified as gas lasers.

• Gas-dynamic lasers are based on the same principles as jet engines. Within the laser, fuel with additional particles of active medium gases is combusted. During the fuel combustion process, gas molecules pass into an excited state, and then as a cold ultrasonic flow they start emitting a powerful coherent radiation, thus producing energy.
• In chemical lasers, radiation impulses appear as a result of a chemical reaction. The strongest laser of this type is based on a reaction between atomic fluoride and hydrogen.
• Excimer lasers operate based on special molecules that are always in an excited state.

Liquid lasers

The first liquid lasers appeared almost concurrently with solid-state lasers, in the 1960s. An active medium is created using various solutions of organic compounds. This substance has a higher density than gas, but lower than that of solids. That’s why these lasers can generate a relatively powerful radiation (up to 20 W), even though the volume of the active medium contained within them is relatively low. They can operate in impulsive and continuous modes. Flash lamps and other lasers are used for pumping.

Semiconductor lasers

The first semiconductor lasers appeared in 1962 as a result of simultaneous discoveries made by several American scientists: R. Hall, M.I. Nathan, T. Quist, together with their groups. The operation of this laser was theoretically proved earlier, in 1958, by Russian physicist N.G. Basov.

Semiconductor lasers use semiconductor crystals (such as GaAs, gallium arsenide) as an active medium. At first glance, one might be tempted to place them in one category with solid-state lasers. However, they have a distinctive differentiator: radiation transitions occur between energy zones or subzones inside the crystal instead of atomic energy levels. In such lasers, pumping is done using a constant electric current. The edges of the semiconductor crystal are carefully polished, allowing them to serve as an excellent resonator.

Natural lasers

Scientists have discovered natural lasers in our Universe. There are some giant interstellar clouds created by condensed gases. Populations are naturally inverse inside them. Pumping is enabled by light from nearby stars or other sources of cosmic radiation, with gas clouds serving as an excellent active medium with a length of several hundred million kilometres. A natural astrophysical laser is thus created, with no need for a resonator: a forced electromagnetic radiation is naturally created within the laser, as soon as the light wave passes.

Properties of laser radiation

Laser light has a range of unusual and very valuable properties setting it apart from light emitted by regular heat sources.

• Laser radiation is coherent and almost completely monochromatic. These features were previously discovered only for radio waves from well stabilized transmitters.
• The diffusion of forced radiation takes place only along the axis of the resonator. Laser beam expansion is very weak, with an almost unnoticeable divergence (several arc seconds).
• The above properties allow laser beams to focus on an extremely small point. The energy in the focal point has an extremely high density.
• Due to the monochromatic character of the radiation and the high energy density, laser radiation can reach extremely high temperatures. For example, the temperature of the radiation of an impulse laser with a power of 1 Petawatt (1015 W) is over 100 million degrees.

Laser applications

The properties of laser radiation are truly unique. This has made lasers indispensable in a variety of scientific and technical industries. Lasers are also widely used in medicine, entertainment, transportation and everyday life.

Technological lasers

• Due to their high capacity, continuous lasers are actively used for cutting, welding or soldering parts made of various materials. At high laser radiation temperatures, it is even possible to weld together materials that cannot be connected to each other using any other methods. For example, welding together metals and ceramics to create metal-ceramics, a new material with unique properties.

• Laser beams that can focus on a minuscule point with the diameter of a micron are used to create microcircuits.
• Laser beams are perfectly straight — yet another amazing quality. This allows us to use them as the most accurate ‘rulers’ in construction. Both construction and geodesy use impulse lasers to measure large distances by calculating the time it takes for a light impulse to move from one point to another.

Laser communication

With the advent of lasers, communication and information recording have reached new heights.

As radio communication developed, it eventually transitioned to ever shorter wavelengths as it was proven that high frequencies (with shorter wavelengths) offer the highest bandwidth for communication channels. The real breakthrough came with the understanding that light is also an electromagnetic wave, except it is several tens of thousands of times shorter. As a result, laser beams can be used to transmit tens of thousands of times more information than you can achieve through high-frequency radio channels. This discovery led to the advancement of various types of communication around the globe.

Laser beams are also used to record and play CDs with sounds (music) and images (photos and movies). The audio recording industry took a giant step forward with the help of this instrument.

Medical applications of lasers

Laser technologies are widely used both in surgery and general therapy.

• For example, thanks to its unique properties, laser beams can be easily injected through eye pupils to ‘weld’ a detached retina back into place, thus correcting significant defects in a hard-to-reach area in the optic disk.
• Contemporary surgery makes great use of laser scalpels in difficult operations, as it minimizes damage to live tissues.
• Low-intensity laser radiation speeds up the regeneration of damaged tissues. It also has an effect similar to acupuncture practiced in Eastern medicine, giving birth to a process called laser acupuncture.
• Multibeam and picosecond lasers are actively used in cosmetology.

Contemporary scientific research

• Due to the high density of laser energy and the high temperature of its radiation, it is possible to study chemicals in the extreme states they have in burning stellar depths.
• Contemporary scientists aim to create a thermonuclear reaction. To achieve this, they need to use laser beams to compress an ampoule containing deuterium and tritium (so-called thermonuclear synthesis).
• Lasers are irreplaceable in genetic engineering and nanotechnology (that work with objects the size of one-millionth of a millimeter — 10–9 m). Laser beams are used to overcome serious limitations: cutting, moving and connecting gene components, biological molecules and nanotechnology parts that are invisible to the naked eye.
• Laser locators are LIDAR used to study the properties of the atmosphere.

Military lasers

Lasers have a wide range of applications in the military. For example, they are often used for reconnaissance missions, specifically for target location and communication. However, lasers are primarily used to develop and manufacture new types of weapons. Chemical or excimer laser beams based on Earth or orbit are extremely powerful. They can easily destroy or disable enemy military satellites and planes in the course of military operations. There are already some ongoing developments and prototypes of laser guns which will be given to military space station crew members. This isn’t the plot of a science fiction film — these are the latest scientific developments!

Lasers in the entertainment industry

Lasers have found a wide range of applications in the entertainment industry. Many readers are familiar with laser shows: these performances often accompany festivals, concerts, various celebrations. Laser shows can be created both inside and outside. Event organizers can pick the right equipment for their goals and project any images in any range of colors.

One of the most memorable events accompanied by a laser show was a concert given by famous musician Jean-Michel Jarre in Vorobiovy Gory in 1995. He was invited by Yury Luzhkov to celebrate Moscow’s 850th anniversary.

The musician performed in front of the MSU building while fragments from the history of the city were projected onto the building.

Today, it’s hard to surprise anyone with a laser show. In November 2012, New York became the setting for a short-term laser installation called Global Rainbows: a 35-kilometer laser beam shot into the sky. The piece was installed following the devastation of Hurricane Sandy in October 2012. The giant rainbow symbolized the resilience of life in the city despite the impact of the catastrophe.

Another interesting example of using lasers in the entertainment industry is a laser party suit created by Taiwanese designer Wei-Chieh Shih. The clothing is made up of a laser installation that colors everything around it with red light by creating beams facing different directions.

Lasers in transportation

Lasers can also be useful in transportation. Take the Netherlands for example: there are plans to implement an installation of laser emitters on train locomotives, which will remove garbage and fallen leaves from the tracks while the train is moving. All foreign objects sticking to the tracks increase the braking distance as well as the risk of an accident.

Lasers can also be used for cycling. Not all streets have special markings for cyclists. When it’s dark, some drivers might not even see the markings anyway. ‘Smart’ bikes now come equipped with an unusual feature: they can project a bike lane using a laser installation. This approach increases safety on the road: cyclists gain visibility for other road users after dark.

Another similar application of lasers was proposed by the creators of innovative street safety system Guardian. The development is aimed towards installing special emitters onto signposts near streetlights. When the red light is on for pedestrians, passage is closed with a laser. As soon as it switches to green, the red light blocks passage for cars. The system is geared towards increasing safety on the roads: it works as a psychological deterrent.

Laser gadgets

Lasers are built into certain modern gadgets. For example, the Magic Cube device can project a virtual keyboard onto a desktop or any other surface. The gadget is geared towards tablet and smartphone users.

Laser applications in sports

Nike came up with a particularly interesting application for lasers. Their development includes a mobile installation which can project soccer playing fields using laser beams. The playing field can be projected onto any flat surface: both in the city and in the country.

Conclusions

We’re not exaggerating when we say that since their invention in the middle of the XX century, lasers have played an important role in our lives, much like electricity and the radio. Lasers have entered into almost every area of activity, and their sudden disappearance would lead to a much less comfortable world. Even the text of this article that you are reading on the screen of your computer or smartphone is available thanks to semiconductor lasers, which are actively used in the latest optical means of communication. It’s impossible to imagine computers without lasers, and thus, we would miss out on an enormous part of our modern lives. Due to their interesting structure, lasers open new development prospects for modern science. Their qualities are extremely nuanced, and it’s safe to say that laser beams light the way for themselves into absolutely all areas of our lives, making us feel better and happier!

Asked by: Dr. Georgiana Champlin Jr.

Score: 4.6/5
(3 votes)

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».

What is laser in simple words?

A laser is a machine that makes an amplified, single-colour source of light. It uses special gases or crystals to make the light with only a single color. The gases are energized to make them emit light. … The word «laser» is an acronym for «light amplification by stimulated emission of radiation«.

What does the laser mean Class 11?

It stands for Light Amplification by Stimulated Emission of Radiation.

What is the scientific word for laser?

What are the 3 types of lasers?

Types of lasers

20 related questions found

Did you know that the word laser is actually an acronym?

Maybe you’ve worked with lasers for a long time, but never took the time to stop and ask yourself what the full meaning of the laser acronym is. Well, here it is…

The full meaning of the LASER acronym

The word LASER stands for Light Amplification by Stimulated Emission of Radiation.

Keep reading to learn more interesting facts about lasers.

Quick overview of the world of lasers

What makes lasers unique compared to other light sources is that they emit light that is both spatially coherent and temporally coherent. What it means is that all the photons have the same frequency and phase.

The first one was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow. At that time, the lasers were called “a solution in search of a problem”, which is often the case for innovative invention.

Nowadays, one can say that new applications for lasers see the light (no pun intended) almost every day. The most life-changing application of lasers in modern society is probably the use of optical fibers in telecommunications.

Gentec-EO’s high-accuracy laser beam measurement instruments help engineers, scientists and technicians in all sorts of laser applications from the factory to the hospital, laboratory and research center. Learn about our solutions for these measurement types:

• Laser power meters
• Laser energy meters
• Laser beam profilers
• Terahertz power meters

For example, according to an article from OSA (The Optical Society) in 2016, Ethernet technology has evolved from an original speed of 2.94 megabits per second (mbps) to 100 mbps. A team of researchers recently managed to send 560 gigabits per second of data over two kilometers in a single mode optical fiber. That kind of rate is game-changing in commercial and industrial applications.

There are 3 parameters to keep in mind in the laser world to characterize how one can help in particular application:

1. Power: high-power lasers are now widely available. Companies dealing with metal processing and defense benefit the most from double-digit kilowatt lasers, and either a larger company or a modest one can increase its yield and production with such lasers.
2. Precision: thanks to the prevalence of high-quality optics in the current market, it is possible to collimate, or at least focus down, a beam to a very small area when it reaches its target. This allows higher refinement on manufactured items, such as in laser marking and peening.
3. Choice: lasers have different functions depending on their spectral component. Just like a medical expert has many different solutions to treat different ailments, an engineer can do different things in his production line using lasers of different wavelengths.

That being said, having so many options to choose from requires you to ensure you have the right stuff: to do so, you need to confirm either the power, energy per pulse, or even the quality of the beam profile.

This is where our 45 years of experience in laser beam measurement at Gentec-EO comes into the picture. We can measure basically any kind of laser of any power or energy level. Our baseline catalog selection includes more than 500 products, so it goes without saying that we rarely come short of solutions when dealing in a new business opportunity. When it does happen though, we are not shy to go for a custom offering.

That’s it! Now you not only know what the word laser stands for but also have a better understanding of this light source. Want to learn more stuff about the world of lasers? Subscribe to our free newsletter below to receive our next articles written by our laser beam measurement experts at Gentec-EO.

Vice-President of Sales and Marketing at Gentec-EO

More from this author

What
Is a Laser?

The word laser is an acronym for light amplification by stimulated
emission of radiation, although common usage today is to use the word
as a noun — laser — rather than as an acronym — LASER.

A laser is a device that creates and amplifies a narrow, intense beam
of coherent light.

Atoms emit radiation. We see it every day when the «excited»
neon atoms in a neon sign emit light. Normally, they radiate their
light in random directions at random times. The result is incoherent
light — a technical term for what you would consider a jumble of
photons going in all directions.

The trick in generating coherent light — of a single or just a few
frequencies going in one precise direction — is to find the right
atoms with the right internal storage mechanisms and create an
environment in which they can all cooperate — to give up their light
at the right time and all in the same direction.

Exciting
atoms or molecules

In a laser, the atoms or molecules of a crystal, such as ruby or
garnet — or of a gas, liquid, or other substance — are excited in
what is called the laser cavity so that more of them are at
higher energy levels than are at lower energy levels. Reflective
surfaces at both ends of the cavity permit energy to reflect back and
forth, building up in each passage.

If a photon whose frequency corresponds to the energy difference
between the excited and ground states strikes an excited atom, the
atom is stimulated as it falls back to a lower energy state to emit a
second photon of the same (or a proportional) frequency, in phase
with and in the same direction as the bombarding photon.

This process is called stimulated emission. The bombarding
photon and the emitted photon may then each strike other excited
atoms, stimulating further emission of photons, all of the same
frequency and phase. This process produces a sudden burst of coherent
radiation as all the atoms discharge in a rapid chain reaction.

Wide
range of sizes and uses

First built in 1960, lasers now range in size from semiconductor
lasers as small as a grain of salt to solid-state and gas lasers as
large as a storage building. The light beam produced by most lasers
is pencil-thin and maintains its size and direction over very large
distances.

Lasers are widely used in industry for cutting and boring metals and
other materials, in medicine for surgery, and in communications,
scientific research, and holography. They are an integral part of
such familiar devices as bar-code scanners used in supermarkets,
scanners, laser printers, and compact disk players.

Why
Lasers Are Important Today

The laser has contributed to humanity as a powerful scientific tool
for expanding human knowledge and in its many applications that help
people directly. In the 40 years since Arthur L. Schawlow and Charles
H. Townes published their technical paper on the principles of the
laser in 1958, the device has been put to work in a vast range of
applications and has assumed many forms.

Their paper caused an explosion of research by scientists at Bell
Labs and at universities and industrial laboratories around the world
that is unabated today at Bell Labs and elsewhere.

In communications, engineers recognized the potential of the laser to
replace electrical transmission over copper wires, but how to
transmit the pulses presented enormous problems. In 1960, Schawlow,
D.F. Nelson, R.J. Collins and others transmitted pulses of light
between Bell Labs facilities in Murray Hill, N.J., and Crawford Hill,
N.J., a distance of 25 miles. Then called an optical maser, Townes’
preferred name for the device, the laser produced an intense and
extremely narrow beam of light that was more than a million times
brighter than the sun.

Lasers
in search of a medium

Unfortunately, laser beams could easily be adversely affected by
atmospheric conditions, such as rain, fog, low clouds, and objects in
the air, such a birds. Scientists and engineers suggested a number of
novel schemes to protect the light from interference, including
shielding it in metal tubes and using specially designed mirrors and
thermal gas lenses to navigate around bends.

It took another major innovation, the development in the early 1970s
of hair-thin strands of encased glass, called fiber optic waveguides,
before the laser could transmit telephone signals. Since then,
optical fiber has increasingly become the medium of choice for
telecommunications companies to transmit voice, data, and video.

Telecommunications, once largely electronic, today relies on photons,
as tiny semiconductor lasers routinely transmit light pulses carrying
billions of bits of information per second over glass fibers.
Wavelength division multiplexing technology uses various wavelengths,
or colors, of light to transmit trillions of bits simultaneously over
a single fiber.

A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. The term «laser» is an acronym for Light Amplification by Stimulated Emission of Radiation.^[1][2] Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum («monochromatic» light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously. The coherence of typical laser emission is distinctive. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position.

The first working laser was demonstrated on 16 May 1960 by Theodore Maiman at Hughes Research Laboratories.^[3] Since then, lasers have become a multi-billion dollar industry. By far the largest single application of lasers is in optical storage devices such as compact disc and DVD players,Template:Fact in which a semiconductor laser less than a millimeter wide scans the surface of the disc. The second-largest application is fiber-optic communication. Other common applications of lasers are bar code readers, laser printers and laser pointers.

In manufacturing, lasers are used for cutting, bending, and welding metal and other materials, and for «marking»—producing visible patterns such as letters by changing the properties of a material or by inscribing its surface. In science, lasers are used for many applications. One of the more common is laser spectroscopy, which typically takes advantage of the laser’s well-defined wavelength or the possibility of generating very short pulses of light. Lasers are used by the military for range-finding, target designation, and illumination. Lasers have also begun to be used as directed-energy weapons. Lasers are used in medicine for surgery, diagnostics, and therapeutic applications.

Design[]

A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.

The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

Terminology[]

File:Spectre.svg

The word laser originated as an acronym for light amplification by stimulated emission of radiation. The word light in this phrase is used in the broader sense, referring to electromagnetic radiation of any frequency, not just that in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.^[4]

The back-formed verb to lase means «to produce laser light» or «to apply laser light to».^[5] The word «laser» is sometimes used to describe other non-light technologies. For example, a source of atoms in a coherent state is called an «atom laser«.

Laser physics[]

File:Laser DSC09088.JPG

File:Helium neon laser spectrum.svg

Template:Seealso
The gain medium of a laser is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. It 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 both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.

The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design.

The optical cavity, a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. 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 chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. 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 aligned with the cavity manage to pass more than once through the medium and so have significant amplification.

The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, the transverse modes of the beam can be described as a superposition of Hermite—Gaussian or Laguerre-Gaussian beams (for stable-cavity lasers). Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.^[6] The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well.

Although the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than other light sources. The operation of a free electron laser can be explained without reference to quantum mechanics.

Modes of operation[]

The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.

Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10^-15 s).

Continuous wave operation[]

In the continuous wave (CW) mode of operation, the output of a laser is relatively consistent with respect to time. The population inversion required for lasing is continually maintained by a steady pump source.

Pulsed operation[]

In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating ‘on’ and ‘off’ periods. In many applications one aims to deposit as much energy as possible at a given place in as short time as possible. In laser ablation for example, a small volume of material at the surface of a work piece might evaporate if it gets the energy required to heat it up far enough in very short time. If, however, the same energy is spread over a longer time, the heat may have time to disperse into the bulk of the piece, and less material evaporates. There are a number of methods to achieve this.

Q-switching[]

Template:Main

In a Q-switched laser, the population inversion (usually produced in the same way as CW operation) is allowed to build up by making the cavity conditions (the ‘Q’) unfavorable for lasing. Then, when the pump energy stored in the laser medium is at the desired level, the ‘Q’ is adjusted (electro- or acousto-optically) to favourable conditions, releasing the pulse. This results in high peak powers as the average power of the laser (were it running in CW mode) is packed into a shorter time frame.

Modelocking[]

Template:Main
A modelocked laser emits extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses are typically separated by the time that a pulse takes to complete one round trip in the resonator cavity. Due to the Fourier limit (also known as energy-time uncertainty), a pulse of such short temporal length has a spectrum which contains a wide range of wavelengths. Because of this, the laser medium must have a broad enough gain profile to amplify them all. An example of a suitable material is titanium-doped, artificially grown sapphire (Ti:sapphire).

The modelocked laser is a most versatile tool for researching processes happening at extremely fast time scales also 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), and in ablation applications. Again, because of the short timescales involved, these lasers can achieve extremely high powers.

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 flashlamps, 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 a broad spectrum pump flash. Pulsed pumping is also required for lasers which disrupt the gain medium so much during the laser process that lasing has to cease for a short period. These lasers, such as the excimer laser and the copper vapour laser, can never be operated in CW mode.

History[]

Foundations[]

In 1917 Albert Einstein, in his paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation), laid the foundation for the invention of the laser and its predecessor, the maser, in a ground-breaking rederivation of Max Planck‘s law of radiation based on the concepts of probability coefficients (later to be termed ‘Einstein coefficients‘) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation.

In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption.^[7] In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify «short» waves.^[8]

In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission.^[9]

In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, which was experimentally confirmed by Brossel, Kastler and Winter two years later.^[10]

Maser[]

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 rather than infrared or visible radiation. Townes’s maser was incapable of continuous output. Nikolay Basov and Aleksandr Prokhorov of the Soviet Union worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels and produced the first maser. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1955 Prokhorov and Basov suggested an optical pumping of multilevel system as a method for obtaining the population inversion, which later became one of the main methods of laser pumping.

Townes reports that he encountered opposition from a number of eminent colleagues who thought the maser was theoretically impossible — including Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch, and Llewellyn H. Thomas[1].

Townes, Basov, and Prokhorov shared the Nobel Prize in Physics in 1964 «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 1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared laser. As ideas were developed, infrared frequencies were abandoned with focus on visible light instead. The concept was originally known as an «optical maser». Bell Labs filed a patent application for their proposed optical maser a year later. Schawlow and Townes sent a manuscript of their theoretical calculations to Physical Review, which published their paper that year (Volume 112, Issue 6).

File:Gould notebook 001.jpg

At the same time Gordon Gould, a graduate student at Columbia University, was working on a doctoral thesis on the energy levels of excited thallium. Gould and Townes met and had conversations on the general subject of radiation emission. Afterwards Gould made notes about his ideas for a «laser» in November 1957, including suggesting using an open resonator, which became an important ingredient of future lasers.

In 1958, Prokhorov independently proposed using an open resonator, the first published appearance of this idea. Schawlow and Townes also settled on an open resonator design, apparently unaware of both the published work of Prokhorov and the unpublished work of Gould.

The term «laser» was first introduced to the public in Gould’s 1959 conference paper «The LASER, Light Amplification by Stimulated Emission of Radiation».^[1][11] Gould intended «-aser» to be a suffix, to be used with an appropriate prefix for the spectrum of light emitted by the device (x-rays: xaser, ultraviolet: uvaser, etc.). None of the other terms became popular, although «raser» was used for a short time to describe radio-frequency emitting devices.

Gould’s notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued working on his 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. This sparked a legal battle that ran 28 years, with scientific prestige and much money at stake. Gould won his first minor patent in 1977, but it was not until 1987 that he could claim his first significant patent victory when a Federal judge ordered the government to issue patents to him for the optically-pumped laser and the gas discharge laser.

The first working laser was made by Theodore H. Maiman in 1960^[12] at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur Schawlow at Bell Labs,^[13] and Gould at a company called TRG (Technical Research Group). Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometres wavelength. Maiman’s laser, however, was only capable of pulsed operation due to its three-level pumping scheme.

Later in 1960 the Iranian physicist Ali Javan, working with William R. Bennett and Donald Herriot, made the first gas laser using helium and neon. Javan later received the Albert Einstein Award in 1993.

The concept of the semiconductor laser diode was proposed by Basov and Javan. The first laser diode was demonstrated by Robert N. Hall in 1962. Hall’s device was made of gallium arsenide and emitted at 850 nm in the near-infrared region of the spectrum. The first semiconductor laser with visible emission was demonstrated later the same year by Nick Holonyak, Jr. As with the first gas lasers, these early semiconductor lasers could be used only in pulsed operation, and indeed only when cooled to liquid nitrogen temperatures (77 K).

In 1970, Zhores Alferov in the Soviet Union and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories independently developed laser diodes continuously operating at room temperature, using the heterojunction structure.

Recent innovations[]

File:History of laser intensity.svg

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 output power
• minimum output pulse duration
• maximum power efficiency
• maximum charging
• maximum firing

and this research continues to this day.

Lasing without maintaining the medium excited into a population inversion, was discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams. 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.

In 1985 at the University of Rochester‘s Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification, or CPA, discovered by Gérard Mourou. These high intensity pulses can produce filament propagation in the atmosphere.

Types and operating principles[]

For a more complete list of laser types see this list of laser types.

File:Laser spectral lines.svg

Gas lasers[]

Gas lasers using many gases have been built and used for many purposes.

The helium-neon laser (HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education because of its low cost.

Carbon dioxide lasers can emit hundreds of kilowatts^[14] at 9.6 µm and 10.6 µm, and are often used in industry for cutting and welding. The efficiency of a CO₂ laser is over 10%.

Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser tube a different number of lines is usable but 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 producing UV Light at 337.1 nm.^[15]

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. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),^[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.

Chemical lasers[]

Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, 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. They were invented by George C. Pimentel.

Excimer lasers[]

Excimer lasers are powered by a chemical reaction involving an excited dimer, or excimer, which is a short-lived dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. They typically produce ultraviolet light, and are used in semiconductor photolithography and in LASIK eye surgery. Commonly used excimer molecules include F₂ (fluorine, emitting at 157 nm), and noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).^[17]

Solid-state lasers[]

File:Starfield Optical Range — sodium laser.jpg

Solid-state laser materials are commonly made by «doping» a crystalline solid host 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», such as chromium or neodymium. Formally, the class of solid-state lasers includes also fiber laser, as the active medium (fiber) is in the solid state. Practically, in the scientific literature, solid-state laser usually means a laser with bulk active medium, while wave-guide lasers are caller fiber lasers.

«Semiconductor lasers» are also solid-state lasers, but in the customary laser terminology, «solid-state laser» excludes semiconductor lasers, which have their own name.

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 (UV) and 266 nm (UV) light when those wavelengths are needed.

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 as well as the most common ultrashort pulse laser.

Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid-state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by utilizing a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kilowatt levels of power.^[18]

Fiber-hosted lasers[]

Solid-state lasers where the light is guided due to the total internal reflection in an optical fiber are called fiber lasers. Guiding of light allows extremely long gain regions providing good cooling conditions; fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber’s waveguiding properties tend to reduce thermal distortion of the beam. Erbium and ytterbium ions are common active species in such lasers.

Quite often, the fiber laser is designed as a double-clad fiber. This type of fiber consists of a fiber core, an inner cladding and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a single-mode fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region, while still having a high numerical aperture (NA) to have easy launching conditions.

Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers.

Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of long-living color centers.Template:Fact

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 placeTemplate:Huh?. They are typical micrometre-sized and tunable on the bands of the photonic crystals. [2]Template:Huh?

Semiconductor lasers[]

Semiconductor lasers are actually solid-state lasers, too, but because semiconductor lasers have a different mode of laser operation, they have a different name. This can be confusing to someone who knows the term solid-state electronics.

Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths of over 3 µm have been demonstrated. Low power laser diodes are used in laser printers and CD/DVD players. More powerful laser diodes are frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm), are used in industry for cutting and welding. External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.

File:Diode laser.jpg

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, and potentially could be much cheaper to manufacture. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,^[19] 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, since it means that if silicon, the chief ingredient of computer chips, were able to produce lasers, it would allow the light to be manipulated like electrons are in normal integrated circuits. Thus, photons would replace electrons in the circuits, which dramatically increases the speed of the computer. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However, recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are called hybrid silicon laser. 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 allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds)

Free electron lasers[]

Free electron lasers, or FELs, generate coherent, high power radiation, that is widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free electron.

Exotic laser media[]

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.^[20] Dr. David Cassidy of the University of California, Riverside proposed that a single such laser could be used to ignite a nuclear fusion reaction, replacing the hundreds of lasers used in typical inertial confinement fusion experiments.^[20]

Space-based X-ray lasers pumped by a nuclear explosion have also been proposed as antimissile weapons.^[21][22] Such devices would be one-shot weapons.

Uses[]

File:Laser sizes.jpg

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When lasers were invented in 1960, they were called «a solution looking for a problem».^[23] 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.

The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers’ homes, beginning in 1982, followed shortly by laser printers.

Some of the other applications include:

• Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry
• Industry: Cutting, welding, material heat treatment, marking parts
• Defense: Marking targets, guiding munitions, missile defence, electro-optical countermeasures (EOCM), alternative to radar
• Research: Spectroscopy, laser ablation, Laser annealing, laser scattering, laser interferometry, LIDAR, Laser capture microdissection
• Product development/commercial: laser printers, CDs, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.
• Laser lighting displays: Laser light shows
• Laser skin procedures such as acne treatment, cellulite reduction, and hair removal.

In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion.^[24] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.^[25]

Examples by power[]

Different uses need lasers with different output powers. Lasers that produce a continuous beam or a series of short pulses can be compared on the basis of their average power. Lasers that produce pulses can also be characterized based on the peak power of each pulse. The peak power of a pulsed laser is many orders of magnitude greater than its average power. The average output power is always less than the power consumed.

The continuous or average power required for some uses:

• 5 mW – CD-ROM drive
• 5–10 mW – DVD player or DVD-ROM drive
• 100 mW – High-speed CD-RW burner
• 250 mW – Consumer DVD-R burner
• 1 W – green laser in current Holographic Versatile Disc prototype development
• 1–20 W – output of the majority of commercially available solid-state lasers used for micro machining
• 30–100 W – typical sealed CO₂ surgical lasers^[26]
• 100–3000 W (peak output 1.5 kW) – typical sealed CO₂ lasers used in industrial laser cutting
• 1 kW – Output power expected to be achieved by a prototype 1 cm diode laser bar^[27]

Examples of pulsed systems with high peak power:

• 700 TW (700×10¹² W) – The National Ignition Facility is working on a system that, when complete, will contain a 192-beam, 1.8-megajoule laser system adjoining a 10-meter-diameter target chamber.^[28] The system is expected to be completed in April 2009.
• 1.3 PW (1.3×10¹⁵ W) – world’s most powerful laser as of 1998, located at the Lawrence Livermore Laboratory^[29]

Hobby uses[]

In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types.^[30] 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 extracting diodes from DVD burners.^[31]

Hobbyists also have been taking surplus pulsed lasers from retired military applications and modifying them for pulsed holography. Pulsed Ruby and Pulsed YAG lasers have been used.

Laser safety[]

File:DIN 4844-2 Warnung vor Laserstrahl D-W010.svg

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Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as having a power of one «Gillette«; as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight, when the beam from such a laser 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 I/1 is inherently safe, usually because the light is contained in an enclosure, for example in cd players.
• Class II/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 IIIa/3R 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 (minor) eye damage.
• Class IIIb/3B can cause immediate severe eye damage upon exposure. Usually lasers up to 500 mW, such as those in cd and dvd burners.
• Class IV/4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class.

The indicated powers are for visible-light, continuous-wave lasers. For pulsed lasers and invisible wavelengths, other power limits apply. People working with class 3B and class 4 lasers can protect their eyes with safety goggles which are designed to absorb light of a particular wavelength.

Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being «eye-safe». This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye’s cornea that no light remains to be focused by the lens onto the retina. The label «eye-safe» can be misleading, however, as it only applies to relatively low power continuous wave beams and any high power or Q-switched laser at these wavelengths can burn the cornea, causing severe eye damage.

Lasers as weapons[]

Though laser beams are perhaps most famously employed as weapon systems in science fiction, the first scientific demonstration of laser technology was in 1960. The general idea of laser-beam weaponry is to hit a target object with a short burst/beam of light typically burning the surface layer and the interior of the target.

The power needed to project a high-powered laser beam of this kind surpasses current mobile power technology, thus such weapons are not anticipated to be produced in any near time.

Lasers of all but the lowest powers can potentially be used as incapacitating weapons, through their ability to produce temporary or permanent vision loss in varying degrees when aimed at the eyes. The degree, character, and duration of vision impairment caused by eye exposure to laser light varies with the power of the laser, the wavelength(s), the collimation of the beam, the exact orientation of the beam, and the duration of exposure. Lasers of even a fraction of a watt in power can produce immediate, permanent vision loss under certain conditions, making such lasers potential non-lethal but incapacitating weapons. The extreme handicap that laser-induced blindness represents makes the use of lasers even as non-lethal weapons morally controversial.

In the field of aviation, the hazards of exposure to ground-based lasers deliberately aimed at pilots have grown to the extent that aviation authorities have special procedures to deal with such hazards.

Fictional predictions[]

For lasers in fiction, see also the raygun.

Before stimulated emission was discovered, novelists used to describe machines that we can identify as «lasers».

• The first fictional device similar to a military CO₂ laser (see Heat-Ray) appears in the sci-fi novel The War of the Worlds by H. G. Wells in 1898.
• A laser-like device was described in Alexey Tolstoy‘s sci-fi novel The Hyperboloid of Engineer Garin in 1927.
• Mikhail Bulgakov exaggerated the biological effect (laser biostimulation) of intensive red light in his sci-fi novel Fatal Eggs (1925), without any reasonable description of the source of this red light. (In that novel, the red light first appears occasionally from the illuminating system of an advanced microscope; then the protagonist Prof. Persikov arranges the special set-up for generation of the red light.)

See also[]

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• Laser acronyms
• Laser applications
• Laser beam profiler
• Laser capture microdissection
• Laser construction
• Laser converting
• Laser cutting
• Laser dazzler
• Laser engraving
• Laser bonding
• Laser ablation
• Laser scalpel
• Laser scanning

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• Laser accelerometer
• Laser science
• Laser cooling
• Laser welding
• Bessel beam
• Laser lighting display
• Laser pointer
• Laser turntable
• Holography
• Induced gamma emission
• Injection seeder

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• International Laser Display Association
• List of light sources
• Maser
• Optical amplifier
• Raygun
• Reference beam
• Selective laser sintering
• Speckle pattern
• Tophat beam
• Homogeneous broadening
• US Air Force’s YAL-1 Airborne Laser

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Notes and references[]

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Further reading[]

Books

• Bertolotti, Mario (1999, trans. 2004). The History of the Laser, Institute of Physics. ISBN 0-750-30911-3
• 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 0-684-83515-0.
• 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

• Applied Physics B: Lasers and Optics (ISSN 0946-2171)
• IEEE Journal of Lightwave Technology (ISSN 0733-8724)
• IEEE Journal of Quantum Electronics (ISSN 0018-9197)
• IEEE Journal of Selected Topics in Quantum Electronics (ISSN 1077-260X)
• IEEE Photonics Technology Letters
• Journal of the Optical Society of America B: Optical Physics (ISSN 0740-3224)
• Laser Focus World (ISSN 0740-2511)
• Optics Letters (ISSN 0146-9592)
• Photonics Spectra (ISSN 0731-1230)

External links[]

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• Encyclopedia of laser physics and technology by Dr. Rüdiger Paschotta
• A Practical Guide to Lasers for Experimenters and Hobbyists by Samuel M. Goldwasser
• Homebuilt Lasers Page by Professor Mark Csele
• 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 (New Scientist, 9 April 2008)
• Template:HSW
• Homemade laser project by Kip Kedersha
• «The Laser: basic principles» an online course by Prof. F. Balembois and Dr. S. Forget. Instrumentation for Optics, 2008
• Laserati, a community website for laser people.