Optical resonators are often called cavities. This term has been taken over from microwave technology, where resonators really look like closed cavities, whereas optical resonators normally have an “open” kind of setup, with reflecting surface only at few locations. That difference in geometry is related to the fact that optical resonators are usually very large compared with the optical wavelength, whereas microwave cavities are often not much longer than a wavelength, so that diffraction effects are much stronger.
Only so-called optical microcavities have dimensions in the micrometer or even sub-micrometer regime, and can tightly enclose a light field in all directions. Microcavities can be realized e.g. with tiny semiconductor or glass structures, such as microtoroids, or as defect structures in photonic crystals.
There are also spherical resonators, supporting the so-called whispering gallery modes; for such resonators, the term cavities is also quite appropriate.
Even though the term “cavity” is often not completely appropriate, as explained above, many related terms are based on it. Examples are laser cavities (laser resonators), cavity modes (resonator modes), cavity dumping and cavity design (resonator design).
The two basic types of optical cavities are:
- standing-wave (or linear) cavities, where light bounces back and forth between two end mirrors
- ring cavities (ring resonators), where the light can do round trips in two different directions
The article on optical resonators contains more details.
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