Laser
A laser is a device that emits light
through a process of optical amplification based on the stimulated emission of
electromagnetic radiation. The term "laser" originated as an acronym
for "light amplification by stimulated emission of radiation". Lasers
differ from other sources of light because they emit light coherently. Spatial
coherence allows a laser to be focused to a tight spot, enabling applications
like laser cutting and lithography. Spatial coherence also allows a laser beam
to stay narrow over long distances (collimation), enabling applications such as
laser pointers. Lasers can also have high temporal coherence which allows them
to have a very narrow spectrum, i.e., they only emit a single color of light.
Temporal coherence can be used to produce pulses of light—as short as a
femtosecond.
Lasers have
many important applications. They are used in common consumer devices such as
DVD players, laser printers, and barcode scanners. They are used in medicine
for laser surgery and various skin treatments, and in industry for cutting and
welding materials. They are used in military and law enforcement devices for
marking targets and measuring range and speed. Laser lighting displays use
laser light as an entertainment medium. Lasers also have many important
applications in scientific research.
Fundamentals
Lasers are
distinguished from other light sources by their coherence. Spatial coherence is
typically expressed through the output being a narrow beam which is
diffraction-limited, often a so-called "pencil beam." Laser beams can
be focused to very tiny spots, achieving a very high irradiance, or they can be
launched into beams of very low divergence in order to concentrate their power
at a large distance.
Temporal (or
longitudinal) coherence implies a polarized wave at a single frequency whose
phase is correlated over a relatively large distance (the coherence length)
along the beam. A beam produced by a thermal or other incoherent light source
has an instantaneous amplitude and phase which vary randomly with respect to
time and position, and thus a very short coherence length.
Most
so-called "single wavelength" lasers actually produce radiation in
several modes having slightly different frequencies (wavelengths), often not in
a single polarization. And although temporal coherence implies
monochromaticity, there are even lasers that emit a broad spectrum of light, or
emit different wavelengths of light simultaneously. There are some lasers which
are not single spatial mode and consequently their light beams diverge more
than required by the diffraction limit. However all such devices are classified
as "lasers" based on their method of producing that light: stimulated
emission. Lasers are employed in applications where light of the required
spatial or temporal coherence could not be produced using simpler technologies.
Terminology
The word laser started as an acronym
for "light amplification by stimulated emission of radiation"; in
modern usage "light" broadly denotes electromagnetic radiation of any
frequency, not only visible light, hence infrared laser, ultraviolet laser,
X-ray laser, and so on. Because the microwave predecessor of the laser, the
maser, was developed first, devices of this sort operating at microwave and
radio frequencies are referred to as "masers" rather than
"microwave lasers" or "radio lasers". In the early
technical literature, especially at Bell Telephone Laboratories, the laser was
called an optical maser; this term is now obsolete.
A laser
which produces light by itself is technically an optical oscillator rather than
an optical amplifier as suggested by the acronym. It has been humorously noted
that the acronym LOSER, for "light oscillation by stimulated emission of
radiation," would have been more correct. With the widespread use of the
original acronym as a common noun, actual optical amplifiers have come to be
referred to as "laser amplifiers", notwithstanding the apparent
redundancy in that designation.
The
back-formed verb to lase is frequently used in the field, meaning "to
produce laser light," especially in reference to the gain medium of a
laser; when a laser is operating it is said to be "lasing." Further
use of the words laser and maser in an extended sense, not referring to laser
technology or devices, can be seen in usages such as astrophysical maser and
atom laser.
Design
A laser
consists of a gain medium, a mechanism to supply energy to it, and something to
provide optical feedback. The gain medium is a material with properties that
allow it to amplify light by stimulated emission. Light of a specific
wavelength that passes through the gain medium is amplified (increases in
power).
For the gain
medium to amplify light, it needs to be supplied with energy. This process is
called pumping. The energy is typically supplied as an electrical current, or
as light at a different wavelength. Pump light may be provided by a flash lamp
or by another laser.
The most
common type of laser uses feedback from an optical cavity—a pair of mirrors on
either end of the gain medium. Light bounces back and forth between the
mirrors, passing through the gain medium and being amplified each time.
Typically one of the two mirrors, the output coupler, is partially transparent.
Some of the light escapes through this mirror. Depending on the design of the
cavity (whether the mirrors are flat or curved), the light coming out of the
laser may spread out or form a narrow beam. This type of device is sometimes
called a laser oscillator in analogy to electronic oscillators, in which an
electronic amplifier receives electrical feedback that causes it to produce a
signal.
Most
practical lasers contain additional elements that affect properties of the
emitted light such as the polarization, the wavelength, and the shape of the
beam.
Laser
physics
Electrons
and how they interact with electromagnetic fields are important in our
understanding of chemistry and physics.
Stimulated
emission
In the
classical view, the energy of an electron orbiting an atomic nucleus is larger
for orbits further from the nucleus of an atom. However, quantum mechanical
effects force electrons to take on discrete positions in orbitals. Thus,
electrons are found in specific energy levels of an atom, two of which are
shown below:
When an
electron absorbs energy either from light (photons) or heat (phonons), it
receives that incident quantum of energy. But transitions are only allowed in
between discrete energy levels such as the two shown above. This leads to
emission lines and absorption lines.
When an
electron is excited from a lower to a higher energy level, it will not stay
that way forever. An electron in an excited state may decay to a lower energy
state which is not occupied, according to a particular time constant
characterizing that transition. When such an electron decays without external
influence, emitting a photon, that is called "spontaneous emission".
The phase associated with the photon that is emitted is random. A material with
many atoms in such an excited state may thus result in radiation which is very
spectrally limited (centered around one wavelength of light), but the
individual photons would have no common phase relationship and would emanate in
random directions. This is the mechanism of fluorescence and thermal emission.
An external
electromagnetic field at a frequency associated with a transition can affect
the quantum mechanical state of the atom. As the electron in the atom makes a
transition between two stationary states (neither of which shows a dipole
field), it enters a transition state which does have a dipole field, and which
acts like a small electric dipole, and this dipole oscillates at a
characteristic frequency. In response to the external electric field at this
frequency, the probability of the atom entering this transition state is
greatly increased. Thus, the rate of transitions between two stationary states
is enhanced beyond that due to spontaneous emission. Such a transition to the
higher state is called absorption, and it destroys an incident photon (the photon's
energy goes into powering the increased energy of the higher state). A
transition from the higher to a lower energy state, however, produces an
additional photon; this is the process of stimulated emission.
Gain
medium and cavity
The gain
medium is excited by an external source of energy into an excited state. In
most lasers this medium consists of population of atoms which have been excited
into such a state by means of an outside light source, or an electrical field
which supplies energy for atoms to absorb and be transformed into their excited
states.
The gain
medium of a laser is normally a material of controlled purity, size,
concentration, and shape, which amplifies the beam by the process of stimulated
emission described above. This material can be of any state: gas, liquid,
solid, or plasma. The gain medium absorbs pump energy, which raises some
electrons into higher-energy ("excited") quantum states. Particles
can interact with light by either absorbing or emitting photons. Emission can
be spontaneous or stimulated. In the latter case, the photon is emitted in the
same direction as the light that is passing by. When the number of particles in
one excited state exceeds the number of particles in some lower-energy state,
population inversion is achieved and the amount of stimulated emission due to
light that passes through is larger than the amount of absorption. Hence, the
light is amplified. By itself, this makes an optical amplifier. When an optical
amplifier is placed inside a resonant optical cavity, one obtains a laser
oscillator.
In a few
situations it is possible to obtain lasing with only a single pass of EM
radiation through the gain medium, and this produces a laser beam without any
need for a resonant or reflective cavity (see for example nitrogen laser).
Thus, reflection in a resonant cavity is usually required for a laser, but is
not absolutely necessary.
The optical
resonator is sometimes referred to as an "optical cavity", but this
is a misnomer: lasers use open resonators as opposed to the literal cavity that
would be employed at microwave frequencies in a maser. The resonator typically
consists of two mirrors between which a coherent beam of light travels in both
directions, reflecting back on itself so that an average photon will pass
through the gain medium repeatedly before it is emitted from the output
aperture or lost to diffraction or absorption. If the gain (amplification) in
the medium is larger than the resonator losses, then the power of the
recirculating light can rise exponentially. But each stimulated emission event
returns an atom from its excited state to the ground state, reducing the gain
of the medium. With increasing beam power the net gain (gain minus loss)
reduces to unity and the gain medium is said to be saturated. In a continuous
wave (CW) laser, the balance of pump power against gain saturation and cavity
losses produces an equilibrium value of the laser power inside the cavity; this
equilibrium determines the operating point of the laser. If the applied pump power
is too small, the gain will never be sufficient to overcome the resonator
losses, and laser light will not be produced. 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
in a spatial mode supported by the resonator will pass more than once through
the medium and receive substantial amplification.
The light
emitted
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 beam in
the cavity and the output beam of the laser, when travelling in free space (or
a homogenous medium) rather than waveguides (as in an optical fiber laser), can
be approximated as a Gaussian beam in most lasers; such beams exhibit the
minimum divergence for a given diameter. However some high power lasers may be
multimode, with the transverse modes often approximated using Hermite-Gaussian
or Laguerre-Gaussian functions. It has been shown that unstable laser
resonators (not used in most lasers) produce fractal shaped beams. Near the
beam "waist" (or focal region) it is highly collimated: the
wavefronts are planar, normal to the direction of propagation, with no beam
divergence at that point. However due to diffraction, that can only remain true
well within the Rayleigh range. The beam of a single transverse mode (gaussian
beam) laser eventually diverges at an angle which varies inversely with the
beam diameter, as required by diffraction theory. Thus, the "pencil
beam" directly generated by a common helium-neon laser would spread out to
a size of perhaps 500 kilometers when shone on the Moon (from the distance of
the earth). On the other hand the light from a semiconductor laser typically
exits the tiny crystal with a large divergence: up to 50°. However even such a
divergent beam can be transformed into a similarly collimated beam by means of
a lens system, as is always included, for instance, in a laser pointer whose
light originates from a laser diode. That is possible due to the light being of
a single spatial mode. This unique property of laser light, spatial coherence,
cannot be replicated using standard light sources (except by discarding most of
the light) as can be appreciated by comparing the beam from a flashlight
(torch) or spotlight to that of almost any laser.
Quantum
vs. classical emission processes
The
mechanism of producing radiation in a laser relies on stimulated emission,
where energy is extracted from a transition in an atom or molecule. This is a
quantum phenomenon discovered by Einstein who derived the relationship between
the A coefficient describing spontaneous emission and the B coefficient which
applies to absorption and stimulated emission. However in the case of the free
electron laser, atomic energy levels are not involved; it appears that the
operation of this rather exotic device can be explained without reference to
quantum mechanics.
Continuous
and pulsed modes of operation
A laser can
be classified as operating in either continuous or pulsed mode, depending on
whether the power output is essentially continuous over time or whether its
output takes the form of pulses of light on one or another time scale. Of
course even a laser whose output is normally continuous can be intentionally
turned on and off at some rate in order to create pulses of light. When the
modulation rate is on time scales much slower than the cavity lifetime and the
time period over which energy can be stored in the lasing medium or pumping
mechanism, then it is still classified as a "modulated" or
"pulsed" continuous wave laser. Most laser diodes used in
communication systems fall in that category.
Continuous
wave operation
Some
applications of lasers depend on a beam whose output power is constant over
time. Such a laser is known as continuous wave (CW). Many types of lasers can
be made to operate in continuous wave mode to satisfy such an application. Many
of these lasers actually lase in several longitudinal modes at the same time,
and beats between the slightly different optical frequencies of those
oscillations will in fact produce amplitude variations on time scales shorter
than the round-trip time (the reciprocal of the frequency spacing between
modes), typically a few nanoseconds or less. In most cases these lasers are
still termed "continuous wave" as their output power is steady when
averaged over any longer time periods, with the very high frequency power
variations having little or no impact in the intended application. (However the
term is not applied to mode-locked lasers, where the intention is to create
very short pulses at the rate of the round-trip time).
For
continuous wave operation it is required for the population inversion of the
gain medium to be continually replenished by a steady pump source. In some
lasing media this is impossible. In some other lasers it would require pumping
the laser at a very high continuous power level which would be impractical or
destroy the laser by producing excessive heat. Such lasers cannot be run in CW
mode.
Pulsed
operation
Pulsed
operation of lasers refers to any laser not classified as continuous wave, so
that the optical power appears in pulses of some duration at some repetition
rate. This encompasses a wide range of technologies addressing a number of
different motivations. Some lasers are pulsed simply because they cannot be run
in continuous mode.
In other
cases the application requires the production of pulses having as large an
energy as possible. Since the pulse energy is equal to the average power
divided by the repetition rate, this goal can sometimes be satisfied by
lowering the rate of pulses so that more energy can be built up in between
pulses. In laser ablation for example, a small volume of material at the
surface of a work piece can be evaporated if it is heated in a very short time,
whereas supplying the energy gradually would allow for the heat to be absorbed
into the bulk of the piece, never attaining a sufficiently high temperature at
a particular point.
Other
applications rely on the peak pulse power (rather than the energy in the
pulse), especially in order to obtain nonlinear optical effects. For a given
pulse energy, this requires creating pulses of the shortest possible duration
utilizing techniques such as Q-switching.
The optical
bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width.
In the case of extremely short pulses, that implies lasing over a considerable
bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers.
The lasing medium in some dye lasers and vibronic solid-state lasers produces
optical gain over a wide bandwidth, making a laser possible which can thus
generate pulses of light as short as a few femtoseconds (10−15 s).
Q-switching
In a
Q-switched laser, the population inversion is allowed to build up by introducing
loss inside the resonator which exceeds the gain of the medium; this can also
be described as a reduction of the quality factor or 'Q' of the cavity. Then,
after the pump energy stored in the laser medium has approached the maximum
possible level, the introduced loss mechanism (often an electro- or
acousto-optical element) is rapidly removed (or that occurs by itself in a
passive device), allowing lasing to begin which rapidly obtains the stored
energy in the gain medium. This results in a short pulse incorporating that
energy, and thus a high peak power.
Mode-locking
A
mode-locked laser is capable of emitting extremely short pulses on the order of
tens of picoseconds down to less than 10 femtoseconds. These pulses will repeat
at the round trip time, that is, the time that it takes light to complete one
round trip between the mirrors comprising the resonator. Due to the Fourier
limit (also known as energy-time uncertainty), a pulse of such short temporal
length has a spectrum spread over a considerable bandwidth. Thus such a gain
medium must have a gain bandwidth sufficiently broad to amplify those
frequencies. An example of a suitable material is titanium-doped, artificially
grown sapphire (Ti:sapphire) which has a very wide gain bandwidth and can thus
produce pulses of only a few femtoseconds duration.
Such
mode-locked lasers are a most versatile tool for researching processes
occurring on extremely short time scales (known as femtosecond physics,
femtosecond chemistry and ultrafast science), for maximizing the effect of
nonlinearity in optical materials (e.g. in second-harmonic generation,
parametric down-conversion, optical parametric oscillators and the like) due to
the large peak power, and in ablation applications.[citation needed] Again,
because of the extremely short pulse duration, such a laser will produce pulses
which achieve an extremely high peak power.
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
flash lamps, 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 an intense flash. Pulsed pumping is
also required for three-level lasers in which the lower energy level rapidly
becomes highly populated preventing further lasing until those atoms relax to
the ground state. These lasers, such as the excimer laser and the copper vapor
laser, can never be operated in CW mode.
History
Foundations
In 1917,
Albert Einstein established the theoretical foundations for the laser and the
maser in the paper Zur Quantentheorie der Strahlung (On the Quantum Theory of
Radiation); via a re-derivation of Max Planck's law of radiation, conceptually
based upon probability coefficients (Einstein coefficients) for the absorption,
spontaneous emission, and stimulated emission of electromagnetic radiation; in
1928, Rudolf W. Ladenburg confirmed the existences of the phenomena of
stimulated emission and negative absorption; in 1939, Valentin A. Fabrikant
predicted the use of stimulated emission to amplify "short" waves; in
1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in
hydrogen spectra and effected the first demonstration of stimulated emission;
in 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of
optical pumping, experimentally confirmed, two years later, by Brossel,
Kastler, and Winter.
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