Laser

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This article is about . For , see Laser (disambiguation).
File:Laser sizes.jpg
The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion.

A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam. Laser light is typically near-monochromatic, i.e. consisting of a single wavelength or hue, and emitted in a narrow beam. This is in contrast to 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 understood by application of quantum mechanics and thermodynamics theory (see laser science).

The verb "to lase" means "to produce coherent light" or possibly "to cut or otherwise treat with coherent light", and is a back-formation of the term laser.

Physics

File:Usaf-laser.jpg
Laser (US Air Force)

A laser is composed of a gain medium and a resonant optical cavity.

The gain medium is a material of controlled purity, size, and shape, which uses a quantum mechanical effect called stimulated emission (discovered by Einstein while researching the photoelectric effect) to amplify the beam. For a laser to operate, the gain medium must be "pumped" by an external energy source, such as electricity or light (from a classical source such as a flash lamp, or another laser). The pump energy is absorbed by the laser medium to produce excited states in the medium. When the number of particles in one excited state exceeds the number of particles in some lower state, population inversion is achieved. In this condition, an optical beam passing through the medium produces more stimulated emission than 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 resonant cavity (see also cavity resonator) contains a coherent beam of light between reflective surfaces so that each photon passes through the gain medium multiple times before being 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. However, 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 which determines the operating point of the laser. If the pump power is chosen too small (below the "laser threshold"), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers.

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 often has a very small divergence (highly collimated), but a perfectly collimated beam cannot be created, due to the effect of diffraction. Nonetheless, a laser beam will spread much less than a beam of incoherent light. The distance over which the beam remains collimated increases with the square of the beam diameter, and the angle at which the beam eventually diverges varies inversely with the diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost immediately on exiting the aperture, at an angle that may be as high as 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens (optics). In contrast, the light from non-laser light sources cannot be collimated by optics as well or much.

File:Laser DSC09088.JPG
A HeNe 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; though it is the gain medium through which the laser passes, it is 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.

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 the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds).

A further note on the terminology is necessary. As laser stands for light amplification by stimulated emission of radiation, it should be understood that the word light is here meant in the expansive sense, as photons of any energy; not as simply photons in the visible spectrum. Hence there are X-ray lasers, IR lasers, UV lasers, etc. Devices that emit in the microwave and radio portion of the spectrum are usually called masers in modern terminology, however. There is some dispute whether 'laser' or 'maser' is the correct generic term for all devices of this type.

History

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. The theory was forgotten until after World War II.

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' maser was incapable of continous 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 for 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."

In 1957 Charles 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).

Simultanously, 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. After that meeting, Gould made notes about his ideas for a "laser" in November 1957. In 1958, Prokhorov proposed an open resonator which became an important ingredient of future lasers. The first introduction of the term "laser" to the public was in Gould's 1959 paper "The LASER, Light Amplification by Stimulated Emission of Radiation". Gould intended "aser" to be a suffix, to be used with an appropriate prefix for the spectra of light emitted by the device (e.g. X-ray laser = xaser, UltraViolet laser = uvaser). None of the other terms became popular, although "raser" is sometimes used for 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 it to Bell Labs in 1960. This sparked a legal battle that spanned three decades, 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 a patent to him for each of the optically pumped and the gas discharge laser.

The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, and Arthur L. Schawlow at Bell Labs. 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 transitions. Later in the same year the Iranian physicist Ali Javan, together with William Bennet and Donald Herriot, made the first gas laser using helium and neon. Javan later received the Albert Einstein Award.

The concept of the semiconductor laser was proposed by Basov and Javan; and the first laser diode was demonstrated by Robert N. Hall in 1962. Hall's device was constructed in the GaAs material system and produced emission 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 Hayashi and Panish of Bell Telephone Laboratories independently developed continously operating laser diodes at room temperature, using the heterojunction structure.

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.

Recent innovations

File:History of laser intensity.jpg
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

and this research continues to this day.

An unforeseen discovery in 1992, lasing without maintaining the medium excited into a population inversion, was discovered in sodium gas and again in 1995 each in sodium and rubidium gas by various international teams. Normally, electrons in the ground state absorb the pumping and emitted radiation, thwarting the laser gain by heating up the medium. So media with electron levels and transitions amenable to the driving current are desired, and generally those which involve three or four energy levels rather than two make better lasers because the electrons are kept above the ground state, excited, and optically-transparent so as not to heat up, but such media are prone to noisy beams. By using an external maser to induce "optical transparency" in the media by introducing and destructively interfering the ground electron transitions between two paths, the likelihood for the ground electrons to absorb any energy has been cancelled. Though there were initial hopes that this discovery would allow an increase in efficiency (higher than the .01 to .3 for typical media and wavelengths), the idea never panned out commercially or otherwise and remains little more than a backwater in laser research.

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. Later, in 1994, it was discovered by Mourou and his team at University of Michigan that the balance between the self-focusing refraction (see Kerr effect) and self-attenuating diffraction by ionization and rarefaction of a laser beam of terawatt intensities in the atmosphere creates "filaments" which act as waveguides for the beam thus preventing divergence. If a light filament drops below the intensity needed for this dynamic balance, called modulation instability, it can merge with another filament and continue propagating without broadening as with all earlier means of sending light. The filaments, having made a plasma, though turn the narrowband laser pulse into a broadband pulse having a wholly new set of applications.

Uses of lasers

Main article: Laser applications

At the time of their invention in 1960, lasers 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 and the military. They have been widely regarded as one of the most influential technological achievements of the 20th century.

The benefits of lasers in various applications stems from their properties such as coherency, high monochromaticity, capability for reaching extremely high powers. For instance, a highly coherent laser beam can be focused down to its diffraction limit, which at visible wavelengths corresponds to only a few hundred nanometers. This property allows a laser to record gigabytes of information in the microscopic pits of a DVD. It also allows a laser of modest power to be focused to very high intensities and used for cutting, burning or even vaporizing materials. For example, a frequency doubled neodymium yttrium aluminum garnet (Nd:YAG) laser emitting 532 nanometer (green) light at 10 watts output power is theoretically capable of achieving an intensity of megawatts per square centimeter. In reality however, perfect focusing of a beam to its diffraction limit is very difficult.

File:Classical spectacular laser effects.jpg
Lasers used for visual effects during musical performance.

In consumer electronics, telecommunications, and data communications, lasers are used as the transmitters in optical communications over optical fiber and free space. They are used to store and retrieve data from compact discs and DVDs, as well as magneto-optical discs. Laser lighting displays (pictured) accompany many music concerts.

In science, lasers are employed in a wide variety of interferometric techniques, and for Raman spectroscopy. Other uses include atmospheric remote sensing, and investigation of nonlinear optics phenomena. Holographic techniques employing lasers also contribute to a number of measurement techniques. Lasers have also been used aboard scientific spacecraft.

In medicine, the laser scalpel is used for laser vision correction and other surgical techniques. Lasers are also used for dermatological procedures including removal of tattoos, birthmarks, and hair; laser types used in dermatology include ruby (694 nm), alexandrite (755 nm), pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm).

In industry, laser cutting is used to cut steel and other metals. Laser line levels are used in surveying and construction. Lasers are also used for guidance for aircraft. Lasers are used in certain types of thermonuclear fusion reactors.

In law enforcement the most widely known use of lasers is for lidar to detect the speed of vehicles. Military uses of lasers include use as target designators for other weapons; their use as directed-energy weapons is currently under research.

Popular misconceptions

The representation of lasers in popular culture, especially science fiction or other action movies, as well as their criticism are generally very misleading. For instance, contrary to what appears in movies such as Star Wars, a laser beam is never visible in the vacuum of space. In air the ray can hit dust and any other obstacles in its path and scatter the light giving the appearance of it glowing, in much the same way that a sunbeam glows in a dusty atmosphere. This effect can be intensified to make the beam more visible, for the sake of making a photograph etc, by increasing the amount of suspended particles in the air.

Very high intensity beams can be visible in air due to Rayleigh scattering or Raman scattering. With even higher intensity beams, focused to a tight spot, the air can heat up to the point where it becomes a plasma, which would be visible. This would however cause a loud explosion, and will cause a reflection of the ray back into the laser, probably damaging it (depending on the laser design).

Furthermore, science-fiction film special effects often depict weapon laser beams propagating at only a few metres per second—i.e., 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 be instantly visible along its entire length.

Some action movies depict security systems using red lasers (and being foiled by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some white dust in the air. It is actually easier and cheaper to build infrared laser diodes rather than visible light laser diodes; therefore such systems have no reason to work in visible light.

Laser safety

Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's 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 localised burning and permanent damage in seconds or even faster. 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. See also laser safety.

Common laser types

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

File:Laser spectral lines.png
Spectral output of several types of lasers.
Color Wavelength interval Frequency interval
red ~ 625 to 740 nm ~ 480 to 405 THz
orange ~ 590 to 625 nm ~ 510 to 480 THz
yellow ~ 565 to 590 nm ~ 530 to 510 THz
green ~ 520 to 565 nm ~ 580 to 530 THz
cyan ~ 500 to 520 nm ~ 600 to 580 THz
blue ~ 430 to 500 nm ~ 700 to 600 THz
violet ~ 380 to 430 nm ~ 790 to 700 THz

See also

External links

af:Laser ar:ليزر bg:Лазер ca:Làser cs:Laser da:Laser de:Laser es:Láser eo:Lasero fa:لیزر fr:Laser gl:Láser ko:레이저 id:Laser it:Laser he:לייזר lt:Lazeris hu:Lézer nl:Laser ja:レーザー no:Laser pl:Laser (źródło światła) pt:Laser ro:Laseri ru:Лазер sr:Ласер fi:Laser sv:Laser vi:Laser zh:激光