chapter  3
Concept of Laser
Pages 64

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.