Editing Optical cavity (section)

==Resonator types==


The most common types of optical cavities consist of two facing plane (flat) or spherical mirrors. The simplest of these is the plane-parallel or [[Fabry–Pérot interferometer|Fabry–Pérot]] cavity, consisting of two opposing flat mirrors.<ref>{{cite journal |first1=A.G. |last1=Fox |first2=T. |last2=Li |url=http://www.nature.com/nature/journal/v402/n6758/abs/402138a0.html |title=Resonant modes in a maser interferometer |journal=Bell Syst. Tech. J. |volume=40 |issue=2 |pages=453–488 |year=1961|doi=10.1002/j.1538-7305.1961.tb01625.x |url-access=subscription }}</ref><ref name=IsmailPollnau2016>{{cite journal | last1 = Ismail | first1 = N. | last2 = Kores | first2 = C. C. | last3 = Geskus | first3 = D. | last4 = Pollnau | first4 = M. | year = 2016 | title = Fabry-Pérot resonator: spectral line shapes, generic and related Airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity | journal = Optics Express | volume = 24 | issue = 15| pages = 16366–16389 | doi = 10.1364/OE.24.016366 | pmid = 27464090 | bibcode = 2016OExpr..2416366I | url = http://kth.diva-portal.org/smash/get/diva2:948682/FULLTEXT01 | doi-access = free }}</ref><ref name="Lotsch_1969_65">{{cite journal |first=H.K.V. |last=Lotsch |title=The Fabry-Perot resonator Part I |journal=Optik |volume=28 |year=1968 |pages=65–75}}</ref><ref name="Lotsch_1969_328">{{cite journal |first=H.K.V. |last=Lotsch |title=The Fabry-Perot resonator Part II |journal=Optik |volume=28 |year=1969 |pages=328–345}}</ref><ref name="Lotsch_1969_555">{{cite journal |first=H.K.V. |last=Lotsch |title=The Fabry-Perot resonator. Part III |journal=Optik |volume=28 |year=1969 |pages=555–574}}</ref><ref name="Lotsch_1969_130">{{cite journal |first=H.K.V. |last=Lotsch |title=The Fabry-Perot resonator. Part IV |journal=Optik |volume=29 |year=1969 |pages=130–145}}</ref><ref>{{cite journal |first=H.K.V. |last=Lotsch |title=The Fabry-Perot resonator Part V |journal=Optik |volume=29 |year=1969 |pages=622–623}}</ref> While simple, this arrangement is rarely used in large-scale lasers due to the difficulty of alignment; the mirrors must be aligned parallel within a few [[arcseconds|seconds of arc]], or "walkoff" of the intracavity beam will result in it spilling out of the sides of the cavity. However, this problem is much reduced for very short cavities with a small mirror separation distance (''L'' < 1&nbsp;cm). Plane-parallel resonators are therefore commonly used in microchip and [[optical microcavity|microcavity]] lasers and [[semiconductor laser]]s. In these cases, rather than using separate mirrors, a reflective [[optical coating]] may be directly applied to the laser medium itself. The plane-parallel resonator is also the basis of the [[Fabry–Pérot interferometer]].

For a resonator with two mirrors with radii of curvature ''R''<sub>1</sub> and ''R''<sub>2</sub>, there are a number of common cavity configurations. If the two radii are equal to half the cavity length (''R''<sub>1</sub> = ''R''<sub>2</sub> = ''L''&nbsp;/<small>&nbsp;</small>2), a concentric or spherical resonator results. This type of cavity produces a [[Diffraction-limited system|diffraction-limited]] beam waist in the centre of the cavity, with large beam diameters at the mirrors, filling the whole mirror aperture. Similar to this is the hemispherical cavity, with one plane mirror and one mirror of radius equal to the cavity length.

A common and important design is the confocal resonator, with mirrors of equal radii to the cavity length (''R''<sub>1</sub> = ''R''<sub>2</sub> = ''L'').<ref>{{cite journal |first1=G.D. |last1=Boyd |first2=J.P. |last2=Gordon |title=Confocal multimode resonator for millimeter through optical wavelength masers |journal=Bell Syst. Tech. J. |volume=40 |issue=2 |pages=489–508 |year=1961|doi=10.1002/j.1538-7305.1961.tb01626.x }}</ref><ref>{{cite journal |first1=G.D. |last1=Boyd |first2=H. |last2=Kogelnik |title=Generalized confocal resonator theory |journal=Bell Syst. Tech. J. |volume=41 |issue=4 |pages=1347–1369 |year=1962|doi=10.1002/j.1538-7305.1962.tb03281.x }}</ref><ref name="Lotsch_1969_1">
{{cite journal |first=H.K.V. |last=Lotsch |title=The confocal resonator system I |journal=Optik |volume=30 |year=1969 |pages=1–14}}</ref><ref name="Lotsch_1969_181">
{{cite journal |first=H.K.V. |last=Lotsch |title=The confocal resonator system II |journal=Optik |volume=30 |year=1969 |pages=181–201}}</ref><ref name="Lotsch_1970_217">
{{cite journal |first=H.K.V. |last=Lotsch |title=The confocal resonator system III |journal=Optik |volume=30 |year=1970 |pages=217–233}}</ref><ref name="Lotsch_1970_563">
{{cite journal |first=H.K.V. |last=Lotsch |title=The confocal resonator system IV |journal=Optik |volume=30 |issue=6 |year=1970 |pages=563–576}}</ref> This design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, and is often used in lasers where the purity of the transverse mode pattern is important.

A concave-convex cavity has one convex mirror with a negative radius of curvature. This design produces no intracavity focus of the beam, and is thus useful in very high-power lasers where the intensity of the light might be damaging to the intracavity medium if brought to a focus.

Less common resonator types include [[optical ring resonators]] and [[whispering-gallery mode]] resonators, in which a resonance is formed by waves moving in a closed loop rather than reflecting between two mirrors.