It was Albert Einstein who in 1917 defined the theory of the
Stimulated Emission of Radiation, developing and expanding
on the work of Niels Bohr, who in 1913 had formulated
the Spontaneous Emission theory. Einstein described the
electrons of molecules being excited by a source of energy,
usually heat, and directed in a specific way. The excited
electron releases a spontaneously emitted photon which interacts
with a molecule of the active medium, causing those
electrons to move to a less stable, higher energy state and
producing further photons. This process exponentially increases
the number of identical photons which are focused
by mirrors at either end of the laser tube and emitted into
the delivery system. The acronym “LASER” represents “light
amplification by stimulated emission of radiation”.