The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser is one of the outstanding inventions of the second half of the last century. Laser has becomes a valuable tool in a variety of fields starting with medicine to communications. Laser is a light source but it is very much different from many traditional light sources. Laser is not used for illumination purpose. A laser differs from other sources of light because it emits light coherently. Spatial coherence allows a laser to be focused to a tight spot, enabling applications like laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over long distances (collimation), enabling applications such as laser pointers. Lasers can also have high temporal coherence which allows them to have a very narrow spectrum, i.e., they only emit a single color of light. Temporal coherence can be used to produce pulses of light—as short as a femto second.