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
Different from a neon light radiating in any
directions, a laser is a beam of coherent light
radiating in the same direction with high
intensity and a particular wavelength. All
lasers contain an amplifying medium, located
in a cavity between two mirrors, that can
increase the intensity of light beam passing
through it. This increased intensity will give
additional energy into the beam itself.

The energized amplifying medium has more
atoms or molecules of the medium in the
excited state to create a population inversion.
Then, an input photon with a particular
wavelength strikes an excited atom of the
medium and pushes it back to a lower energy
state to emit a photon with the same
wavelength and in phase and in the same
direction as the input photon. The process is
called stimulated emission.

The beam is repeatedly reflected and
amplified between two mirrors. One of the
mirrors reflects almost the incident beam ;
the reflection efficiency of the other mirror is
between 20% and 80%. So, the unflected
light passes through the mirror and becomes
the laser.

Lasers are applied in many fields such as
consumer electronics, medicine, surveying,
manufacturing,
construction
industry,
scientific instrumentation, and military
systems. Besides, it is also used for ignition of
a fusion reaction, laser printers, compact disc
and scan bar codes etc.

In telecommunications, laser can replace
electrical transmission over traditional copper
wires, but laser beams are easily unfavorably
interfered by external conditions, such as
rain, fog, and clouds.

The invention of optical fiber enables to
transmit voice, data, pictures and video by
light. For example, semiconductor lasers can
transmit billions of bits of information per
second by light pulses over glass fibers. By
wavelength division multiplexing technology,
several wavelengths of light can transmit
trillions of bits simultaneously over a single
optical fiber.