Cavities
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: resonators for light (or for microwaves)
Alternative term: resonators
Categories:
Related: optical resonators resonator design resonator modes stability zones of optical resonators reference cavities free spectral range finesse bandwidth Q-factor
Page views in 12 months: 1961
DOI: 10.61835/5yt Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn
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What is an Optical Cavity?
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 a 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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is the difference between an optical cavity and an optical resonator?
The terms are mostly used synonymously. The term 'cavity' was adopted from microwave technology, where resonators are enclosed spaces. While many optical resonators are 'open' structures, the term 'cavity' is still widely used.
When is the term 'optical cavity' most appropriate?
The term is most fitting for optical microcavities or spherical resonators supporting whispering gallery modes, as these structures tightly confine the light field in all directions, similar to a microwave cavity. On the other hand, the term is less appropriate for a bulk laser resonator, for example.
What are the basic kinds of optical cavities?
The two fundamental types are standing-wave (or linear) cavities, formed by two end mirrors, and ring cavities, where light circulates in a closed loop.
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