Editing Laser (section)

== Fundamentals ==
[[File:Green Laser.jpg|thumb|right|A laser can produce a very narrow beam of light of a single wavelength, in this case, green.]]
[[Photons]], the quanta of [[electromagnetic radiation]], are released and absorbed from energy levels in atoms and molecules. In a lightbulb or a star, the energy is emitted from many different levels giving photons with a broad range of energies. This process is called [[thermal radiation]].<ref name=Hecht>{{Cite book |last=Hecht |first=Eugene |title=Optics |date=1998 |publisher=Addison-Wesley |isbn=978-0-201-83887-9 |edition=3 |location=Reading, Mass}}</ref>{{rp|575}}

The underlying physical process creating photons in a laser is the same as in thermal radiation, but the actual emission is not the result of random thermal processes. Instead, the release of a photon is triggered by the nearby passage of another photon. This is called [[stimulated emission]]. For this process to work, the passing photon must be similar in energy, and thus wavelength, to the one that could be released by the atom or molecule, and the atom or molecule must be in the suitable excited state.<ref name=Hecht/>{{rp|580}}

The photon that is emitted by stimulated emission is identical to the photon that triggered its emission, and both photons can go on to trigger stimulated emission in other atoms, creating the possibility of a [[chain reaction]]. For this to happen, many of the atoms or molecules must be in the proper excited state so that the photons can trigger them. In most materials, atoms or molecules drop out of excited states fairly rapidly, making it difficult or impossible to produce a chain reaction. The materials chosen for lasers are the ones that have [[Metastability|metastable states]], which stay excited for a relatively long time. In [[Laser science|laser physics]], such a material is called an [[active laser medium]]. Combined with an energy source that continues to "pump" energy into the material, it is possible to have enough atoms or molecules in an excited state for a chain reaction to develop.

Lasers are distinguished from other light sources by their [[coherence (physics)|coherence]]. Spatial (or transverse) coherence is typically expressed through the output being a narrow beam, which is [[Gaussian beam|diffraction-limited]]. Laser beams can be focused to very tiny spots, achieving a very high [[irradiance]], or they can have a very low divergence to concentrate their power at a great distance. Temporal (or longitudinal) coherence implies a [[Polarization (waves)|polarized]] wave at a single frequency, whose phase is correlated over a relatively great distance (the [[coherence length]]) along the beam.<ref>''Conceptual physics'', Paul Hewitt, 2002</ref>{{Page missing|date=January 2024}} A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and [[phase (waves)|phase]] that vary randomly with respect to time and position, thus having a short coherence length.

Lasers are characterized according to their [[wavelength]] in a [[vacuum]]. Most "single wavelength" lasers produce radiation in several ''modes'' with slightly different wavelengths. Although temporal coherence implies some degree of [[Monochromatic radiation|monochromaticity]], some lasers emit a broad spectrum of light or emit different wavelengths of light simultaneously. Certain lasers are not single spatial mode and have light beams that [[Beam divergence|diverge]] more than is required by the [[diffraction limit]]. All such devices are classified as "lasers" based on the method of producing light by stimulated emission. Lasers are employed where light of the required spatial or temporal coherence can not be produced using simpler technologies.