The word LASER is an acronym for “Light Amplification by Stimulated Emission of Radiation”. As the name suggests, in lasers, the light is amplified with the help of a process called “Stimulated emission” Thus, laser is based on the principle of stimulated emission.
In other words, its is a device to produce a monochromatic beam, highly intense and coherent beam of light. The theoretical basis for the development of laser was provided great scientist Albert Einstein in 1917, when he predicted the possibility of stimulated emission of radiations.
Therefore laser has following properties:
1. Highly monochromatic
2. Highly directional
Coherence is again of two types;
1. Temporal coherence (also known as longitudinal coherence)
2. Spatial coherence (also known as transverse coherence)
The lasers can be a solid laser or gas laser or dye laser or even semiconductor laser. Ruby laser was the first laser to be operated successfully. If you want to understand how a laser works, then you can go through the articles related to construction of ruby laser and working of ruby laser.
If You have any doubt regarding the laser, please post in the comment section.