Editing Laser (section)

=== Gain medium and cavity ===
[[File:Laser DSC09088.JPG|thumb|A [[helium–neon laser]] demonstration. The glow running through the center of the tube is an electric discharge. This glowing plasma is the [[active laser medium|gain medium]] for the laser. The laser produces a tiny, intense spot on the screen to the right. The center of the spot appears white because the image is [[overexposure|overexposed]] there.]]
[[File:Helium neon laser spectrum.svg|thumb|Spectrum of a helium–neon laser. The actual bandwidth is much narrower than shown; the spectrum is limited by the measuring apparatus.]]

The gain medium is put into an [[excited state]] by an external source of energy. In most lasers, this medium consists of a population of atoms that have been excited into such a state using an outside light source, or an electrical field that supplies energy for atoms to absorb and be transformed into their excited states.

The gain medium of a laser is normally a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission described above. This material can be of any [[state of matter|state]]: gas, liquid, solid, or [[plasma (physics)|plasma]]. The gain medium absorbs pump energy, which raises some electrons into higher energy ("[[excited state|excited]]") [[quantum state]]s. Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, [[population inversion]] is achieved. In this state, the rate of stimulated emission is larger than the rate of absorption of light in the medium, and therefore the light is amplified. A system with this property is called an [[optical amplifier]]. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.<ref>{{cite book |first = Anthony E. |last=Siegman |year=1986 |title=Lasers |url = https://archive.org/details/lasers0000sieg |url-access = registration |publisher=University Science Books |isbn= 978-0-935702-11-8 |page=[https://archive.org/details/lasers0000sieg/page/4 4]}}</ref>

For lasing media with extremely high gain, so-called [[superluminescence]], light can be sufficiently amplified in a single pass through the gain medium without requiring a resonator. Although often referred to as a laser (see, for example, [[nitrogen laser]]),<ref name="StrongLight">{{cite book
  |title=Light and Its Uses: Making and using lasers, holograms, interferometers, and instruments of dispersion
  |chapter=Nitrogen Laser
  |date=June 1974
  |isbn=978-0-7167-1185-8
  |pages=40–43
  |url=https://archive.org/details/lightitsusesmaki0000unse
  |url-access=registration
  |last1=Strong
  |first1=C.L.
  |publisher=W. H. Freeman
 }}</ref> the light output from such a device lacks the spatial and temporal coherence achievable with lasers. Such a device cannot be described as an oscillator but rather as a high-gain optical amplifier that amplifies its spontaneous emission. The same mechanism describes so-called [[astrophysical maser]]s/lasers.

The optical [[resonator]] is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a [[maser]].
The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption.
If the gain (amplification) in the medium is larger than the resonator losses, then the power of the recirculating light can rise [[exponential growth|exponentially]]. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power, the net gain (gain minus loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the applied pump power is too small, the gain will never be sufficient to overcome the cavity losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the ''[[lasing threshold]]''. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons in a [[spatial mode]] supported by the resonator will pass more than once through the medium and receive substantial amplification.