The laser is a source of coherent radiation, in the optical region of the electromagnetic spectrum (including the ultraviolet, visible and infrared). A common feature of all types of lasers is the occurrence of inversion in the active element of the laser: a non-equilibrium state in which amplification of an electromagnetic wave passing through the active element is more probable than its absorption. This situation, which cannot occur under equilibrium conditions is achieved by a supply of energy from outside the active element. This is known as pumping the laser.

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