Laser
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For other uses, see Laser (disambiguation).
A laser (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits light in a coherent beam. The back-formed verb lase means "to produce laser light" or "to apply laser light to".[1]

In analogy with optical lasers, a device which produces any particles or electromagnetic radiation in a coherent state is also called a "laser", usually with indication of type of particle as prefix (for example, atom laser.) In most cases, "laser" refers to a source of coherent photons, i.e. light or other electromagnetic radiation.

Laser light is typically near-monochromatic, i.e., consisting of a very small band of wavelengths or a very specific color, and emitted in a narrow beam. This contrasts with common light sources, such as the incandescent light bulb, which emit incoherent photons in almost all directions, usually over a wide spectrum of wavelengths.

Laser action is explained by the theories of quantum mechanics and thermodynamics. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to the invention of many types of lasers with different characteristics suitable for different applications.

The laser was proposed as a variation of the maser principle in the late 1950s, and the first laser was demonstrated in July 1960 by Theodore Maiman at Hughes Research Laboratories. Since that time, laser manufacture has become a multi-billion dollar industry, and the laser has found applications in fields including science, the defense industry, medicine, and consumer electronics.


Experiment using a (likely argon) laser. (US military)For lasers in fiction, see also raygun.

Contents [hide]
1 Physics
2 History
2.1 Foundations: Einstein
2.2 The maser
2.3 The laser
2.4 Recent innovations
3 Uses
3.1 Example uses by typical output power
4 FASOR
5 Laser safety
6 Types and operating principles
6.1 Gas lasers
6.2 Chemical lasers
6.2.1 Excimer lasers
6.3 Solid-state lasers
7 Popular misconceptions
8 Fictional predictions
9 Hobby uses



[edit] Physics

Principal components:
1. Active laser medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam
A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the gain medium through which the laser passes, not the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.See also: Laser science and Laser construction
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the quantum mechanical process of stimulated emission, discovered by Albert Einstein while researching the photoelectric effect. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ("excited") quantum states. 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 condition, an optical beam passing through the medium produces more stimulated emission than the stimulated absorption, so the beam is amplified. An excited laser medium can also function as an optical amplifier.

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 monochromaticity established by the optical cavity design.

The optical cavity, an example of a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that each photon 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 intracavity laser power; 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 often Gaussian beams. If the beam is not a pure Gaussian shape, the transverse modes of the beam may be analyzed as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams. The beam may be highly collimated, that is, having a very small beam divergence, but a perfectly collimated beam cannot be created, due to diffraction. But a laser beam will spread much less than a beam of incoherent light. 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 kilometres (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 or much.

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 femtosecond (10-15 s).

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

It is understood that the word light in the acronym Light Amplification by Stimulated Emission of Radiation is typically used in the expansive sense, as photons of any energy; it is not limited to photons in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.

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. [2]


[edit] History

[edit] Foundations: Einstein
In 1916, Albert Einstein 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 spontaneous and induced emission.

In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption. [3]

In 1939, Valentin A. Fabrikant (USSR) predicted the use of stimulated emission to amplify "short" waves.[4]

In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission. [5]

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.[6]


[edit] The maser
In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first maser, a device operating on similar principles to the laser, but producing microwave rather than optical 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. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion.

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".


[edit] The laser
In 1957 Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared maser. 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).


The first page of Gordon Gould's laser notebook, in which he coined the acronym LASER and described the essential elements for constructing one.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"[7]. Gould intended "-aser" to be a suffix, to be used with an appropriate prefix for the spectra of light emitted by the device (x-ray laser = xaser, ultraviolet laser = 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[8] at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur L. Schawlow at Bell Labs,[9] 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 energy level pumping scheme.

Later in 1960 the Iranian physicist Ali Javan, working with William Bennet 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.


[edit] Recent innovations
This section is a stub. You can help by expanding it.


Graph showing the history of maximum laser pulse intensity throughout the past 40 years.Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including:-

new wavelength bands
maximum average output power
maximum peak 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.



[edit] Uses

Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.Main article: Laser applications
When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military.

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.

In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion.[10] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[11]


[edit] Example uses by typical output power
Different uses need lasers with different output powers. Many lasers are designed for a higher peak output with an extremely short pulse, and this requires different technology from a continuous wave (constant output) lasers, as are used in communication, or cutting. Output power is always less than the input power needed to generate the beam.

The peak power required for some uses:

5 mW - CD-ROM drive
5-10 mW - DVD player
100 mW - CD-R drive
250 mW - output power of Sony SLD253VL red laser diode, used in consumer 48-52 speed CD-R burner.[12]
1 W - green laser in current Holographic Versatile Disc prototype development.
100 to 3000 W (peak output 1.5 kW) - typical sealed CO2 lasers used in industrial Beam Laser Machines (laser cutting). These are usually compact, extremely reliable, inexpensive to run and can provide over 20,000 hours of cutting before requiring service.[13]
1 kW - Output power expected to be achieved by "a single 1 cm diode laser bar"[14]
700 terawatts (TW) - 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.[15] The system is expected to be completed in April of 2009.
1.25 petawatts (PW) - world's most powerful laser (claimed on 23 May 1996 by Lawrence Livermore Laboratory).

[edit] FASOR

A 50W FASOR used at the Starfire Optical RangeFor other meanings of "FASOR" or "fasor", see FASOR.
FASOR is Frequency Addition Source of Optical Radiation. The example in this image is used for laser guide star experiments. It is tuned to D2A hyperfine component of the sodium D line and used to excite sodium atoms in the mesospheric upper atmosphere. It consists of 1064nm and 1319nm Nd:YAG single frequency injection locked lasers that are both resonant in a cavity containing a Lithium Triborate (LBO) crystal which sums the frequencies yielding 589.158nm light. (1/1064 + 1/1319 = 1/588.93).


[edit] Laser safety
Main articles: Laser safety and Lasers and aviation safety
Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as 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.

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 classified into safety classes numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin damage). Laser products available for consumers, such as CD players and laser pointers are usually in class I, II, or III. 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 long wavelengths will burn the cornea, causing severe eye damage.

This effect is used in a surgical treatment for detached retina: the resulting pinpoint damage heals with scarring, which "spot-welds" the retina onto its backing to try to stop more detachment.


[edit] Types and operating principles
For a more complete list of laser types see this list of laser types.

Spectral output of several types of lasers.
[edit] Gas lasers
Gas lasers using many gases have been built and used for many purposes. They are one of the oldest types of laser.


Spectrum of a helium neon laser showing the very high spectral purity intrinsic to nearly all lasers. Compare with the relatively broad spectral emittance of a light emitting diode.The helium-neon laser (HeNe) emits 543 nm and 633 nm and is very common in education because of its low cost.

Carbon dioxide lasers emit up to 100 kW at 9.6 µm and 10.6 µm, and are used in industry for cutting and welding.

Argon-ion lasers emit 458 nm, 488 nm or 514.5 nm.

Carbon monoxide lasers must be cooled but can produce up to 500 kW.

A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV Light at 337.1 nm.

Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. 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.


[edit] 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.


[edit] 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)).


[edit] Solid-state lasers
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, or (chromium-doped sapphire).

Another common type is made from Neodymium-doped yttrium aluminium garnet (YAG), known as Nd:YAG. Nd:YAG 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. Nd:YAG lasers are also commonly frequency doubled to produce 532 nm when a visible (green) coherent source is required.

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.[17]





[edit] Popular misconceptions
The representation of lasers in popular culture, especially in science fiction and action movies, is often misleading. Contrary to their portrayal in many science fiction movies, a laser beam would not be visible (at least to the naked eye) in the near vacuum of space as there would be insufficient matter to scatter off.

In air, however, moderate intensity (tens of mW/cm²) laser beams of shorter green and blue wavelengths and high intensity beams of longer orange and red wavelengths can be visible due to Rayleigh scattering. With even higher intensity pulsed beams, the air can be heated to the point where it becomes a plasma, which is also visible. This causes rapid heating and explosive expansion of the surrounding air, which makes a popping noise analogous to the thunder which accompanies lightning. This phenomenon can cause retro-reflection of the laser beam back into the laser source, possibly damaging its optics. When this phenomenon occurs in certain scientific experiments it is referred to as a "plasma mirror" or "plasma shutter".

Some action movies depict security systems using lasers of visible light (and their foiling by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some dust in the air. It is far easier and cheaper to build infrared laser diodes rather than visible light laser diodes, and such systems almost never use visible light lasers. Additionally, putting enough dust in the air to make the beam visible is likely to be enough to "break" the beam and trigger the alarm (as proven on an episode of Mythbusters on the Discovery Channel).

Science fiction films special effects often depict laser beams propagating at only a few metres per second—slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light and would seem to appear instantly to the naked eye from start to end.

Several of these misconceptions can be found in the James Bond film Goldfinger, the first film to feature a laser. In one of the most famous scenes in the Bond films, Bond, played by Sean Connery, faces a laser beam approaching his groin while melting the solid gold table to which he is strapped. The director Guy Hamilton found that a real laser beam would not show up on camera so it was added as an optical effect. In fact the table was precut up the center, and then painted over. Then as the laser appears to cut right through it, it is melted away by the man underneath with an oxyacetylene torch, while a real laser would have produced a fairly heat-free and silent cut. [18]

In addition to movies and popular culture, laser misconceptions are present in some popular science publications or simple introductory explanations.[citation needed] For example, laser light is not perfectly parallel as is sometimes claimed; all laser beams spread out to some degree as they propagate due to diffraction. In addition, no laser is perfectly monochromatic (i.e. coherent); most operate at several closely spaced frequencies (colors) and even those that nominally operate a single frequency still exhibit some variation in frequency. Furthermore, mode locked lasers are designed to operate with thousands or millions of frequencies locked together to form a short pulse.


[edit] Fictional predictions
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: see Raygun#In specific scenarios (scroll down to alphabetical order 'H' in the left column).
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.)
The descriptions of laser-like devices in the early sci-fi novels are realistic (scientific) in comparison with "lasers" as they appear in later confusing Hollywood movies typical for years 1950 to 2000. See raygun.


[edit] 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.[19] However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved.

Popping balloons, lighting matches, and doing laser shows appear to be popular among laser enthusiasts. Laser show enthusiasts often use Electronica music, such as techno and trance.

Due to the cost of lasers, some hobbyists use cheaper means to obtain lasers, such as extracting diodes from DVD burners. [1]

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