Fabry–Pérot interferometer

An optical resonator made from two parallel reflecting surfaces. Repeated reflections cause light to interfere, producing sharp transmission peaks at wavelengths satisfying a resonance condition.

Optical resonance: The enhancement of an optical field when its frequency matches a system’s natural frequency. Transmission peaks occur when round-trip phase makes successive reflections reinforce one another.
Optical path length: The distance light would travel in vacuum while accumulating the same phase as it does along a given path. The cavity’s optical path sets the round-trip phase condition for resonance.
Fabry–Pérot etalon: A two-surface optical element with a fixed spacing that produces periodic spectral transmission peaks. Etalons use the same resonant filtering principle as the interferometer.
Charles Fabry: A French physicist known for optical spectroscopy and for developing the Fabry–Pérot interferometer with Alfred Pérot. Fabry’s collaboration gave the instrument its name and established its precision-spectroscopy use.
Multiple-beam interference: Interference among several coherent waves that combine at the same location. Each transmitted beam includes light that has made a different number of cavity round trips.
Phase (waves): The position of a point within a repeating wave cycle, often expressed as an angle. Constructive interference requires the round-trip phase to be an integer multiple of 2π.
Laser resonator: An optical cavity that provides feedback and selects modes in a laser. Fabry–Pérot cavities can form the feedback resonator of a laser.
Alfred Pérot: A French physicist who developed the Fabry–Pérot interferometer with Charles Fabry. Pérot co-designed the instrument and helped demonstrate its use in spectroscopy.
Free spectral range: The frequency or wavelength spacing between adjacent resonances of a periodic spectrum. It sets the spacing between neighboring transmission peaks.
Reflectance: The fraction of incident radiant power reflected by a surface or optical component. Higher mirror reflectance generally produces narrower transmission resonances.

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