Laser
A laser is an electronic-optical device that produces coherent radiation. The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. A typical laser emits light in a narrow, low-divergence beam and with a well-defined wavelength (i.e., monochromatic) (corresponding to a particular color if the laser is operating in the visible spectrum). This is in contrast to a light source such as the incandescent light bulb, which emits into a large solid angle and over a wide spectrum of wavelength.
Principle
A laser consists of a gain medium inside an optical cavity, with a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium. Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The process of supplying the energy required for the amplification is called pumping and the energy is typically supplied as an electrical current or as light at a different wavelength. In the latter case, the light source can be a flash lamp or another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
Uses
Lasers are ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, military and personal use.
