Editing Optical cavity (section)

{{Short description|Arrangement of mirrors}}
[[File:Glass nanoparticle suspended in an optical cavity.png|thumb|A glass nanoparticle is suspended in an optical cavity]]
An '''optical cavity''', '''resonating cavity''' or  '''optical resonator''' is an arrangement of [[mirror]]s or other optical elements that confines [[light wave]]s similarly to how a [[cavity resonator]] confines microwaves. Optical cavities are a major component of [[laser]]s, surrounding the [[gain medium]] and providing [[feedback]] of the laser light. They are also used in [[optical parametric oscillator]]s and some [[interferometer]]s. Light confined in the cavity reflects multiple times, producing [[Mode (electromagnetism)|modes]] with certain [[resonance|resonance frequencies]]. Modes can be decomposed into [[longitudinal mode]]s that differ only in frequency and [[transverse mode]]s that have different intensity patterns across the cross section of the beam.  Many types of optical cavities produce [[standing wave]] modes.

Different resonator types are distinguished by the focal lengths of the two mirrors and the distance between them. Flat mirrors are not often used because of the difficulty of aligning them to the needed precision.  The geometry (resonator type) must be chosen so that the beam remains stable, i.e. the size of the beam does not continually grow with multiple reflections. Resonator types are also designed to meet other criteria such as a minimum beam waist or having no focal point (and therefore no intense light at a single point) inside the cavity.

Optical cavities are designed to have a large [[Q factor]], meaning a beam undergoes many oscillation cycles with little [[attenuation]].<ref>{{cite encyclopedia  |title=''Q'' Factor |url=https://www.rp-photonics.com/q_factor.html |encyclopedia=Encyclopedia of Laser Physics and Technology |publisher=RP Photonics |first=Rüdiger |last=Paschotta}}</ref> In the regime of high Q values, this is equivalent to the frequency [[line width]] being small compared to the resonant frequency of the cavity.