Introduction to Lasers
The laser is a light
source that exhibits unique properties and a wide variety of applications.
Lasers are used in welding, surveying, medicine, communication, national
defense, and as tools in many areas of scientific research. Many types
of lasers are commercially available today, ranging in size from devices
that can rest on a fingertip to those that fill large buildings. All these
lasers have certain basic characteristic properties in common.
A laser is a coherent
and highly directional radiation source. LASER stands
for Light Amplification by Stimulated Emission of Radiation.
A laser consists of at least three components:
- a gain medium that
can amplify light that passes through it
- an energy pump source
to create a population inversion in the gain medium
- two mirrors that
form a resonator cavity
The gain medium can
be solid, liquid, or gas and the pump source can be an electrical discharge,
a flashlamp, or another laser. The specific components of a laser vary
depending on the gain medium and whether the laser is operated continuously
(cw) or pulsed.
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If light of the right color hits an atom, it will bump an electron up
to a higher energy level. And later the electron falls back down, giving
off light of the same color in some random direction.
But when a photon hits an atom that is already excited, the atom releases
a new photon that is completely identical to the incoming photon; same
color, going in the same direction. We call this process "stimulated
emission". This is basic of all lasers.
 
The light emitted by
lasers is different from that produced by more common light sources such
as incandescent bulbs, fluorescent lamps, and high-intensity arc lamps.
An understanding of the unique properties of laser light may be achieved
by contrasting it with the light produced by other, less unique sources.
Laser beam is more
directional than any other source of light.
But, perfectly parallel
beams of directional light—which we refer to as collimated light—cannot
be produced. All light beams eventually spread (diverge) as they move
through space. But laser light is more highly collimated, that is, it
is far more directional than the light from any conventional source and
thus less divergent. In some applications, optical systems are employed
with lasers to improve the directionality of the output beam. One system
of this type can produce a spot of laser light only one-half mile in diameter
on the moon (a distance of 250,000 miles).
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