Lasers are devices that emit light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term “laser” is an acronym for “light amplification by stimulated emission of radiation”. Lasers differ from other light sources in that they emit light coherently, meaning that the photons emitted have a constant phase relationship with each other.
In most lasers, the active gain medium is contained in a crystal or glass rod which is pumped optically so as to produce population inversion between its energy levels. The random photons present in the environment are amplified by stimulated emission to produce a beam of coherent photons travelling in a well-defined direction. This beam can be further amplified by passing it through an optical resonator consisting of mirrors which serve to confine and reflect the light back and forth through the gain medium many times before it finally exits the laser cavity.
The first working laser was demonstrated in 1960 by Townes and Schawlow using ruby as the gain medium. Since then, lasers have been built using nearly every material known including gases, liquids, solids and even plasmas. They have found applications in medicine (e.g., LASIK eye surgery), communications (e.g., fiber optics), manufacturing (e.g., cutting and welding), entertainment (e.g., laser shows) and scientific research (e.g., interferometry).