- Introduction
- The maser
- The laser
- Uses
- Applications by output power
- Popular misconceptions
- Laser safety
- Categories by type
A LASER (from the acronym of Light
Amplification by Stimulated Emission of Radiation) is an optical source that
emits photons in a coherent beam. The term has since entered the English language
as a standard word, laser, losing the capitalization in the process. The back-formed
verb to lase means "to produce laser light" or possibly "to apply
laser light to".
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 single
wavelength or colour, 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 1960. Since that time, laser manufacture
has become a multi-billion dollar industry, and the laser has found applications
in fields including science, Defense industry, industry, medicine, and consumer
electronics.
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, 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, putting 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).
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; 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.
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 should be 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 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.
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.
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.
In 1928, Rudolph W. Landenburg confirmed the existence of stimulated emission
and the negative absorption.
In 1939, Valentin A. Fabrikant (URSS) predicted the use of the induced emission
to amplify "short" waves.
In 1947, Willis E. Lamb and R. C. Retherford found induced emission suspect
in hydrogen spectra and made the first demonstration of stimulated emission.
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.
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".
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).
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
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, but "raser"
was used for a short time to describe radio-frequency emitting devices (and
in speech can be confused with "razor").
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 at Hughes Research
Laboratories in Malibu, California, beating several research teams including
those of Townes at Columbia University, Arthur L. Schawlow at Bell Labs, 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.
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
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, 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. In the same year, approximately 733 million
diode lasers, valued at $3.20 billion, were sold.
Applications by output power
Different applications require lasers with different output power. Many lasers
are designed for a high peak output with an extremely short pulse, and this
requires different technology from a continuous wave (constant output) laser
such as is used in communication, or cutting. Also, the output power is less
than the input power needed to generate the laser beam.
The peak power required for some applications:
- 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.
- 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.
- The National Ignition Facility is working on a system that, when complete,
will contain a 192-beam, 1.8-megajoule, 700-terawatt laser system adjoining
a 10-meter-diameter target chamber. The system is expected to be completed
in April of 2009.
- 1 kW - Output power expected to be achieved by "a single 1 cm diode
laser bar"
- 1.25 PW - world's most powerful laser (claimed on 23 May 1996 by Lawrence
Livermore Laboratory).
Popular misconceptions
Due to science fiction
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 would also be visible. This would also cause rapid heating and explosive
expansion of the surrounding air, which would produce a popping noise analogous
to the thunder which accompanies lightning. This phenomenon is also capable
of causing a 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.
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. The melting effect on the table was achieved by
a man underneath the table holding an oxyacetylene torch, while a real laser
would have produced a fairly heat-free and silent cut.
Due to popular science
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 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.
Laser 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 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 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.
Categories by type
Gas lasers
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.
The 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), making them candidates for use in fluorescence suppressed
Raman spectroscopy.
Chemical lasers
Chemical lasers are powered by a chemical reaction, and can achieve high powers
in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900
nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination
of hydrogen or deuterium gas with combustion products of ethylene in nitrogen
trifluoride.
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)).
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,
used for spectroscopy.
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, and its waveguiding
properties reduce thermal distortion of the beam.
Semiconductor lasers
Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths
of over 3 µm have been demonstrated. Low power laser diodes are used in
laser 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.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers
whose emission direction is perpendicular to the surface of the wafer. VCSEL
devices typically have a more circular output beam than conventional laser diodes,
and potentially could be much cheaper to manufacture. As of 2005, only 850 nm
VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,
and 1550 nm devices an area of research. VECSELs are external-cavity VCSELs.
Quantum cascade lasers are semiconductor lasers that have an active transition
between energy sub-bands of an electron in a structure containing several quantum
wells.
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).
