When this effect becomes strong, the gain is said to be saturated. 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. 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. 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. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and monochromaticity established by the optical cavity design. The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. An excited laser medium can also function as an optical amplifier. In this condition, an optical beam passing through the medium produces more stimulated emission than the stimulated absorption, so the beam is amplified. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ('excited') quantum states. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The gain medium is energized, or pumped, by an external energy source. 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 transfers external energy into the laser beam. Since that time, laser manufacture has become a multi-billion dollar industry, and the laser has found applications in fields including science, the defense industry, industry, medicine, and consumer electronics.ДЄ laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The laser was proposed as a variation of the maser principle in the late 1950s, and the first laser was demonstrated in 1960.

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. Laser action is explained by the theories of quantum mechanics and thermodynamics.

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 light is typically near-monochromatic, i.e., consisting of a single wavelength or color, and emitted in a narrow beam.

light or other electromagnetic radiation.

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. The back-formed verb lase means 'to produce laser light' or 'to apply laser light to'. The term has since entered the English language as a standard word, laser, losing the capitalization in the process. A LASER (acronym for Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam.

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