Editing Laser construction (section)

==Gain medium / Laser medium==
The ''gain medium'' is the major determining factor of the wavelength of operation, and other properties, of the laser. ''Gain media'' in different materials have linear spectra or wide spectra. ''Gain media'' with  wide spectra allow tuning of the laser frequency. There are hundreds if not thousands of different gain media in which laser operation has been achieved (see [[list of laser types]] for a list of the most important ones).  The gain medium is excited by the pump source to produce a [[population inversion]], and it is in the gain medium where spontaneous and [[stimulated emission]] of photons takes place, leading to the phenomenon of optical gain, or amplification.

Examples of different gain media include:
*Liquids, such as dye lasers. These are usually [[organic chemistry|organic]] chemical [[solvents]], such as [[methanol]], [[ethanol]] or [[ethylene glycol]], to which are added chemical dyes such as [[coumarin]], [[rhodamine]], and [[fluorescein]]. The exact chemical configuration of the dye molecules determines the operation wavelength of the [[dye laser]].
*Gases, such as [[carbon dioxide]], [[argon]], [[krypton]] and mixtures such as [[helium]]&ndash;[[neon]]. These lasers are often pumped by electrical discharge.
*Solids, such as [[crystal]]s and [[glass]]es. The solid ''host'' materials are usually doped with an impurity such as [[chromium]], [[neodymium]], [[erbium]] or [[titanium]] [[ion]]s. Typical hosts include [[Yttrium aluminium garnet|YAG]] ([[yttrium]] [[aluminium]] [[garnet]]), YLF (yttrium [[lithium]] [[fluorine|fluoride]]), [[sapphire]] (aluminium oxide) and various glasses. Examples of solid-state laser media include Nd:YAG, [[Ti-sapphire laser|Ti:sapphire]], Cr:sapphire (usually known as [[ruby]]), Cr:LiSAF (chromium-doped lithium [[strontium]] aluminium fluoride), Er:YLF, Nd:glass, and Er:glass. Solid-state lasers are usually pumped by flashlamps or light from another laser.
*[[Semiconductor]]s, a type of solid, crystal with uniform dopant distribution or material with differing dopant levels  in which the movement of [[electron]]s  can cause laser action. Semiconductor lasers are typically very small, and can be pumped with a simple electric current, enabling them to be used in consumer devices such as [[compact disc]] players. See [[laser diode]].