Laser

For other uses, see Laser (disambiguation).

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

Laser (US Air Force)

A laser is composed of an active laser 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. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much.

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).

It should be understood that the word light in the acronym LASER is meant in the expansive sense, as photons of any energy; and is not limited to photons in the visible spectrum. Hence there are X-ray lasers, infrared lasers, ultraviolet 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^[1].

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 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."

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^[2] 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 in 1993.

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 continuously 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

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.

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.

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.

In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion (Kincade and Anderson 2005). In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold (Steele 2005).

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.

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 and laser induced breakdown 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. Laser weapon systems under development include the airborne laser, the airborne tactical laser, the Tactical High Energy Laser, the High Energy Liquid Laser Area Defense System, and the MIRACL, or Mid-Infrared Advanced Chemical Laser.

Popular misconceptions

The representation of lasers in popular culture, especially in science fiction and action movies, is generally very misleading. For instance, contrary to their portrayal 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 other particles in its path and scatter producing a glowing "ray", in much the same way that a sunbeam glows in dusty air. This effect can be intensified to make the beam more visible 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, 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 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 would almost certainly not use visible light.

Laser safety

Main article: 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.

Common laser types

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

Spectral output of several types of lasers.

• Gas lasers
  • HeNe (543 nm and 633 nm)
  • Argon-Ion (458 nm, 488 nm or 514.5 nm)
  • Carbon dioxide lasers (9.6 µm and 10.6 µm) used in industry for cutting and welding, up to 100 kW possible
  • TEA Laser (UV Light, 337.1 nm)
  • Nitrogen laser
  • Carbon monoxide lasers, must be cooled, but extremely powerful, up to 500 kW possible
• Chemical lasers
  • Chemical oxygen iodine laser (1315 nm)
  • Hydrogen fluoride laser (2700-2900 nm)
  • Deuterium fluoride laser (3800 nm)
• Excimer gas lasers, producing ultraviolet light, used in semiconductor manufacturing and in LASIK eye surgery; F₂ (157 nm), ArF (193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), XeF (351 nm)
• Semiconductor lasers
  • Laser diodes produce wavelengths from 405 nm to 1550 nm. Low power laser diodes are used in laser pointers, 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, 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.
  • VCSELs are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices can achieve better beam quality and higher output power than conventional laser diodes, and potentially could be much cheaper to manufacture. The technology is not (as of 2005) as well developed, however.
  • 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.
• Solid-state lasers
  • Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in the infrared spectrum at 1064nm, used for cutting, welding and marking of metals and other materials also used in spectroscopy and for pumping dye lasers. Can be frequency doubled from 1064nm to 532nm to produce a green laser.
  • Ytterbium-doped lasers with crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, or Yb-doped glasses (e.g. fibers); typically operating around 1020-1050 nm; potentially very high efficiency and high powers due to a small quantum defect; extremely high powers in ultrashort pulses can be achieved with Yb:YAG
  • Erbium-doped YAG, 1645 nm, 2940 nm
  • Thulium-doped YAG, 2015 nm
  • Holmium-doped YAG, 2097 nm; an efficient laser operating in the infrared spectrum, it is strongly absorbed by water-bearing tissues in sections less than a millimeter thick. It 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) lasers, a highly tunable infrared laser, used for spectroscopy
  • Erbium-doped fiber lasers, a type of laser formed from a specially made optical fiber, which is used as an amplifier for optical communications.
• Dye lasers
• Hollow cathode sputtering metal ion lasers, generating deep ultraviolet wavelengths, of which there are two examples; Helium-Silver (HeAg) 224 nm and Neon-Copper (NeCu) 248 nm. These lasers have particularly narrow oscillation linewidths of less than 0.01 cm^-1 making them good candidates for use in fluorescence suppressed Raman spectroscopy.

See also

• International Laser Display Association
• Laser acronyms
• Laser construction
• Laser science
• Laser pointer
• Optical amplifier

References

1. ^ Bell Laboratories: Invention of the laser http://www.bell-labs.com/about/history/laser/
2. ^ Maiman, T.H. (1960): "Stimulated Optical Radiation in Ruby". Nature, 187 4736, pp. 493-494.
3. ^ Kincade, Kathy and Stephen Anderson (2005) "Laser Marketplace 2005: Consumer applications boost laser sales 10%", Laser Focus World, vol. 41, no. 1. (online)
4. ^  Steele, Robert V. (2005) "Diode-laser market grows at a slower rate", Laser Focus World, vol. 41, no. 2. (online)

Publications about lasers

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Books

• 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 0-935-70211-3
• Sifvast, William T. (1996). Laser Fundamentals, Cambridge University Press. ISBN 0-521-55617-1
• Svelto, Orasio (1989). Principles of Lasers, 3rd ed. (trans. David Hanna), Plenum Press. ISBN 0-306-42967-5
• Yariv, Amnon (1989). Quantum Electronics, 3rd ed., Wiley. ISBN 0-471-60997-8

Journals

• 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
• Optics Letters (ISSN 0146-9592)
• Photonics Spectra (ISSN 0731-1230)

External links

• Sam's Laser FAQ by Samuel M. Goldwasser
• Encyclopedia of laser physics and technology by Dr. Rüdiger Paschotta
• Liquid Light by Phil Schewe, James Riordon, and Ben Stein
• Light turns into glowing liquid by Eugenie Samuel
• Experiments Detail How Powerful Ultrashort Laser Pulses Propagate through Air
• Filamentation and Propagation of Ultra-Short, Intense Laser Pulses in Air
• Lasing Activity without Population Inversion by Phillip F. Schewe and Ben Stein
• Lasing without Inversion by Phillip F. Schewe and Ben Stein