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A laser (light amplification by stimulated emission of radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light. Light from a laser is often very collimated and monochromatic, but this is not true of all laser types.
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 generally 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 divergenceBeam divergence Of an electromagnetic beam, in any plane that intersects the beam axis, the increase in beam diameter with distance from the aperture from which the beam emerges. Beam divergence is usually used to characterize electromagnetic beams in the (highly collimated). A perfectly collimated beam cannot be created, due to the effect of diffractionIn physics, diffraction is a wave phenomenon: the apparent bending and spreading of waves when they meet an obstruction. Diffraction occurs with electromagnetic waves, such as light and radio waves, and also in sound waves and water waves. Diffraction als, but a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a heliumHelium is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most- neonThis page is about the chemical element. For the automobile, see Dodge Neon. For a new semiconductor technology see ARM Neon is the chemical element in the periodic table that has the symbol Ne and atomic number 10. A colorless nearly inert noble gas, neo (HeNe) laserA helium-neon laser usually called a HeNe laser is a small gas laser of a type often used in laboratory demonstrations of optics. Its usual operation wavelength is 632. 8 nm, in the red portion of the visible spectrum. Schematic diagram of a helium-neon l spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla's surface to the MoonFor other moons in the solar system see natural satellite. For other uses see Moon (disambiguation). The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is occasionally called Luna ( Latin for moon to d. Some lasers, especially semiconductor lasers due to their small size, produce very divergent beams. 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 can generally not be collimated.
A laser medium 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 of wavelength, phase and polarisation; this is particularly important in optical communications.
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, modelocking or Gain-switching. In pulsed operation, much higher peak powers can be achieved.
The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by 'pumping' the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. 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.
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 and graduate students J. P. Gordon, and H. J. Zeiger 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 word laser was coined in 1957 by Gordon Gould. Gordon also coined the words iraser, intending "aser" as the suffix and the spectra of light emitted at as the prefix (examples: X-ray laser = xaser, Ultra Violet laser = uvaser) but these terms never became popular. Gordon was also credited with lucrative patent rights for a gas-discharge laser in 1987, following a protracted 30 year legal battle.
A HeNe laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6 . The glowing ray in the middle is a 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 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. In the same year the Iranian physicist Ali Javan invented the gas laser. He later received the Albert Einstein Award.
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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