Etalons

Etalon (plural etalons) is a term used in optics and metrology to denote two related concepts: a high-finesse optical resonator used as a wavelength-selective element, and a standard or reference device used for calibration. The word derives from the French étalon, meaning standard or reference.

In optics, an etalon is typically formed by two parallel, partially reflecting surfaces separated by a fixed gap. The cavity can be air-spaced or filled with a dielectric material. Light undergoing multiple reflections within the cavity interferes constructively at certain wavelengths, producing transmission peaks. For normal incidence, constructive interference occurs when 2 n L equals m times the wavelength, where n is the refractive index, L is the cavity length, and m is an integer. The transmitted spectrum shows evenly spaced maxima whose separation is called the free spectral range. The sharpness of the peaks is described by the finesse, which depends on the mirror reflectivity. Etalons are used as narrow-band filters, in spectroscopy, in laser stabilization, and in astronomical instrumentation, among other applications. They can be tuned by changing the cavity length, the angle of incidence, or the temperature, but require careful alignment and mirror parallelism to maintain spectral purity.

In metrology, an etalon also refers to a standard or reference device used for calibration and traceability. Such etalons provide known reference values (for thickness, spacing, or optical path length) that calibrate instruments like interferometers and spectrometers. The term is common in European usage and is closely related to the general concept of an étalon as a standard.