The theory of stimulated emission was put forwarded by A.
Einstein (1917), which predicted that as light passed through a
substance it could stimulate the emission of more light. In 1954,
this phenomenonwas used by Charles Townes andArthur Schawlow
in the construction of a microwave ampliﬁcation by stimulated
emission of radiation (maser) using ammonia gas and microwave
radiation. Later, this operating principle of maser was put forth to
the optical frequencies by them from the Bell Telephone laboratories
in 1958, which led to the realization of the device, known as light
ampliﬁcation by stimulated emission of radiation (laser; Townes and
Schawlow, 1958). A laser is a quantum optical device that produces
a nearly monochromatic (single wavelength), and coherent beam of
light by exciting atoms to a higher energy level and causing them to
radiate their energy in phase.