Fabry-Perot etalons (simply called etalons) are wavelength selecting optical devices that are based on the Fabry-Perot etalon theory. The name etalon was coined by Charles Fabry and Alfred Perot to describe an optical cavity between two reflecting surfaces. Etalons are generally made of one transparent plate with two parallel reflecting mirrors or surfaces (a planar etalon). While almost all etalons are planar etalons, etalons can also be confocal (or spherical) and consist of two identical spherical mirrors with their concave sides facing each other and with the distance between the mirrors equal to each mirror’s radius of curvature.
Etalons can range in size from 1 mm square to 100 mm in diameter. Etalons need high quality, very flat optical surfaces and extreme parallelism to achieve high performance. Etalons can be made from a wide variety of materials including fused silica and silicon. Most often fused silica or diamond etalons are used for their stable mirror-to-mirror distances and the ability to keep stable frequencies even changes in temperature. Diamond is preferred for its ability to transmit greater amounts of heat and minimize expansion. One recent innovation involved making etalons that are themselves solid optical fibers. This has resulted in the elimination of mounting, alignment, and cooling issues.
Etalons have a variety of applications including lasers, radiometers, and other electro-optical devices. Fabry-Perot etalons are frequently used in telecommunications and data transmission, for example as optical filters in fiberoptic models. Etalons are also widely used in lasers, spectroscopy, and astronomy to control and measure the wavelength of light.