Laser


Laser is an acronym for "Light Amplification by Stimulated Emission of
Radiation". It uses a quantum mechanical effect, stimulated emission, to
generate a very collimated, monochromatic and coherent beam of light.

Common light sources, such as the electric light bulb emit photons in all
directions, usually over a wide spectrum of wavelengths. Most light sources
are also incoherent, i.e., there is no fixed phase relationship between the
photons emitted by the light source.

By contrast, a laser emits photons in a narrow, well-defined beam of light.
The light is often near-monochromatic, consisting of a single wavelength or
color, is highly coherent and is often polarised. Some types of laser, 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).

Laser light can be highly intense — able to cut steel and other
metals. The beam emitted by a laser often has a very small divergence (i.e.
it is highly collimated). The beam will eventually spread due to the effect
of diffraction but much less so than a beam of light generated by other
means. A beam generated by a small laboratory laser such as a helium-neon
(HeNe) laser spreads to approximately 1 mile in diameter if shone from the
Earth's surface to the Moon.

A laser can also function as an optical amplifier when seeded with light
from another source. The amplified signal can be very similar to the input
signal in terms wavelength, phase and polarisation; this is particularly
important in optical communications.

Even low power lasers can be hazardous to a person's eyesight. 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. Certain wavelengths of laser light can
cause cataracts or even boiling of the vitreous humor, the fluid in the
eyeball. Infrared and ultraviolet lasers are particularly dangerous, since
the body's "blink reflex", which can protect an eye from excessively bright
light, works only if the light is visible. Lasers are classified by
wavelength and maximum output power into safety classes, from class I
(inherently safe; no possibility of eye damage even from hours of direct
exposure) to class IV (highly dangerous; even non-direct scattering of light
from the beam can blind). Users of class III lasers and above must usually
wear appropriate eye protection when operating the laser.

The output of a laser may be a continuous, constant-amplitude output (known
as c.w. or continuous wave), or pulsed, by using the techniques of
Q-switching or modelocking.

The basic physics of lasers centres around the idea of producing a
population inversion in a laser medium. The medium may then amplify light by
the process of stimulated emission, which if the light is fed back through
the medium by means of a cavity resonator, will continue to be amplified
into a high-intensity beam. A great deal of quantum mechanics and
thermodynamics theory can be applied to laser action (see laser science),
though in fact many laser types were discovered by trial and error.

Population inversion is also the concept behind the maser, which is similar
in principle to a laser but works with microwaves. The first maser was built
by Charles H. Townes in 1953. Townes later worked with Arthur L. Schawlow to
describe the theory of the laser, or optical maser as it was then known.

The first maser, developed by Townes, was incapable of continuous output.
Nikolai Basov and Alexander Prokhorov of the USSR 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. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov
shared a Nobel Prize in Physics "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 first working laser was made by Theodore H. Maiman in 1960 at Hughes
Research Laboratories in Malibu, California, beating several research teams
including those of Townes at Columbia University, and Schawlow at Bell
laboratories. Maiman used a solid-state flashlamp-pumped ruby crystal to
produce red laser light at 694 nanometeres wavelength.

Types of lasers include:

   * Ruby lasers, producing red (694 nm) light. The first type invented,
     often used for medical purposes including the removal of tattoos and
     birthmarks.
   * Semiconductor lasers, used in laser pointers, laser printers, and
     CD/DVD players;
   * Dye lasers
   * Carbon dioxide lasers - used in industry for cutting and welding
   * Excimer lasers, producing ultraviolet light, used in semiconductor
     manufacturing;
   * Neodymium-doped YAG lasers (Nd:YAG), a high-power laser operating in
     the infrared, used for cutting, welding and marking of metals and other
     materials;
   * Holmium-doped glass, a high-power laser operating in the infrared, it
     is explosively absorbed by water-bearing tissues in sections less than
     a millimeter thick. It is usually operated in a pulsed mode, and passed
     through optic fiber surgical devices to resurface joints, remove rot
     from teeth, vaporize cancers, and to 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.
   * Random laser, different with conventional lasers, the necessary
     feedback is from strong scattering inside disordered media instead of
     end mirrors.

See also Laser applications, Laser construction, Laser science, LADAR,
active laser medium, laser diode, Ring laser gyroscope

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

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Laser is a resin produced by the silphium plant, also called laserwort.