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.

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).