A light amplifier usually used to produce monochromatic coherent radiation in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum.
Nonlaser light sources emit radiation in all directions as a result of the spontaneous emission of photons by thermally excited solids (filament lamps) or electronically excited atoms, ions, or molecules (fluorescent lamps, etc.). The emission accompanies the spontaneous return of the excited species to the ground state and occurs randomly, i.e. the radiation is not coherent. In a laser, the atoms, ions, or molecules are first ‘pumped’ to an excited state and then stimulated to emit photons by collision of a photon of the same energy. This is called stimulated emission . In order to use it, it is first necessary to create a condition in the amplifying medium, called population inversion , in which the majority of the relevant entities are excited. Random emission from one entity can then trigger coherent emission from the others that it passes. In this way amplification is achieved.
The laser amplifier is converted to an oscillator by enclosing the amplifying medium within a resonator. Radiation then introduced along the axis of the resonator is reflected back and forth along its path by a mirror at one end and by a partially transmitting mirror at the other end. Between the mirrors the waves are amplified by stimulated emission. The radiation emerges through the semitransparent mirror at one end as a powerful coherent monochromatic parallel beam of light. The emitted beam is uniquely parallel because waves that do not bounce back and forth between the mirrors quickly escape through the sides of the oscillating medium without amplification.
Some lasers are solid, others are liquid or gas devices. Population inversion can be achieved by optical pumping with flashlights or with other lasers. It can also be achieved by such methods as chemical reactions, discharges in gases, and recombination emission in semiconducting materials (see recombination process).
Lasers have found many uses since their invention in 1960, including laser welding, surgery, holography, printing, optical communications, and the reading of digital information.
A simple ruby laser
- boson
- bosonization
- bound state
- boundary layer
- Bourdon gauge
- Boyle's law
- Boyle, Robert
- Brackett series
- Bragg's law
- Bragg, Sir William Henry
- brass
- Brattain, Walter
- Braun, Karl Ferdinand
- breakdown
- breaking stress
- breeder reactor
- Bremsstrahlung
- Brewster's law
- bridge rectifier
- Brinell hardness
- Britannia metal
- British thermal unit
- broken symmetry
- bronze
- brown dwarf