Laser

Physics

To understand the fundamentals of how lasers work and what makes their emissions so special requires a knowledge of the interaction of electromagnetic radiation and matter (see the "introduction to quantum mechanics" article).

See also: Laser construction

A laser is composed of an 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 process of optical amplifiers.

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 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 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 or much.

The output of a laser may be a continuous constant-amplitude output (known as CW 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).

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.

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 X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.

Because the 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]

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 Max Planck's law of radiation based on the concepts of probability coefficients (later to be termed 'Einstein coefficients') for the absorption, spontaneous, and stimulated emission.

In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission and negative absorption.^[5]

In 1939, Valentin A. Fabrikant (USSR) predicted the use of stimulated emission to amplify "short" waves.^[6]

In 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and made the first demonstration of stimulated emission.^[7]

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.^[8]

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 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[2].

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).

  At the same time 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".^{[9] [10]} 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 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^[11] at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Townes at Columbia University, Arthur L. Schawlow at Bell Labs,^[12] and Gould at a company called TRG (Technical Research Group). Maiman used a solid-state flashlamp-pumped synthetic 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 R. Bennett and Donald Herriot, made the first neon. Javan later received the Albert Einstein Award in 1993.

The concept of the semiconductor liquid nitrogen temperatures (77 K).

In 1970, 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 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 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 chirped pulse amplification, or CPA, discovered by Gérard Mourou. These high intensity pulses can produce filament propagation in the atmosphere.

Continuous wave and pulsed lasing

A laser may either be built to emit a continuous beam or a train of short pulses. This makes fundamental differences in construction, usable laser media, and applications.

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

Main article: Q-switching

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 favorable 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

Main article: Modelocking

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 Ti:sapphire).

The modelocked laser is a most versatile tool for researching processes happening at extremely fast time scales (femtosecond physics and femtosecond chemistry, also called 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.

Types and operating principles

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

Gas lasers

gases have been built and used for many purposes. They are one of the oldest types of laser.

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^[13] 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.^[14]

Metal ion lasers are gas lasers that generate Raman spectroscopy.

Chemical lasers

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

noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).^[16]

Solid-state lasers

corundum).

UV) light when those wavelengths are needed.

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.

ultrashort pulse laser.

Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and disk lasers have been shown to produce up to kilowatt levels of power.^[17]

Fiber-hosted lasers

Solid state lasers also include glass or optical fiber hosted lasers, for example, with erbium or ytterbium ions as the active species. These allow extremely long gain regions and can support very high output powers because the fiber's high surface area to volume ratio allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. Quite often, the fiber is designed as a double-clad glass 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. 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.

Semiconductor lasers

Commercial 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.

Vertical cavity surface-emitting lasers (quantum wells.

The development of 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

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.

Nuclear reaction lasers

In September 2007, the BBC News reported that there was speculation about the possibility of using gamma ray laser. ^[19] This laser is believed to be powerful enough to jump-start a nuclear reaction, with a single gamma ray laser, rather than the hundreds of conventional lasers involved in current experiments.

Uses

Main article: Laser applications

When lasers were invented in 1960, they were called "a solution looking for a problem".^{[citation needed]} 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: Bleedless surgery, laser healing, survical 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), RADAR alternative
• Research: Spectroscopy, laser ablation, Laser annealing, laser scattering, laser interferometry, LIDAR
• Product development/Commercial: Laser Printers, CDs, Barcode scanners, laser pointers, Holograms)

In 2004, excluding diode lasers, approximately 131,000 lasers were sold world-wide, with a value of US$2.19 billion.^[20] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.^[21]

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.^[22]
• 500 mW - output power of Sony SLD1332V red laser diode, used in consumer DVD-R burner.^[23]
• 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 laser cutting.
• 1 kW - Output power expected to be achieved by "a single 1 cm diode laser bar"^[24]
• 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.^[25] 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).

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.^[26] 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.^[27]

Laser safety

Main articles: Lasers and aviation safety

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.

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 wavelengths can burn the cornea, causing severe eye damage.

Related terminology

In analogy with optical lasers, a device which produces any particles or electromagnetic radiation.

The back-formed verb lase means "to produce laser light" or "to apply laser light to".^[28]

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 cause scattering, except if there were a significant amount of fine shrapnel and other organic particles in that region.

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 thermal blooming" is used to describe these self-induced thermal distortions. 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". One approach for overcoming thermal distortion is to use a short-duration laser pulse.

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 demonstrated 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 1964 James Bond film Goldfinger. 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 oxyacetylene torch. Goldfinger's laser makes a whirring electronic sound, while a real laser would have produced a fairly heat-free and silent cut.^[29]

In addition to movies and popular culture, laser misconceptions are present in some popular science publications or simple introductory explanations. 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 mode locked lasers are designed to operate with thousands or millions of frequencies locked together to form a short pulse.

Fictional predictions

For lasers in fiction, see also 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: 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.)

See also

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)

Notes and references