Definition of etalon

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Photonics Dictionary

etalon: An etalon is an optical device that consists of two parallel reflecting surfaces separated by a precise and known distance. It operates based on the principle of optical interference and is used to control or measure the wavelength of light with high precision. Etalons are commonly employed in various optical instruments and applications, including spectroscopy, laser systems, telecommunications, and metrology.

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Key features and characteristics of etalons include:

Parallel reflecting surfaces: Etalons consist of two highly reflective surfaces, typically made of glass or other transparent materials, that are positioned parallel to each other with a precise separation distance. The quality and flatness of these surfaces are crucial for achieving accurate optical interference effects.

Optical interference: When light passes through an etalon, it undergoes multiple reflections between the parallel surfaces. These reflections result in interference patterns that depend on the wavelength of the incident light and the thickness of the etalon. The interference effects cause certain wavelengths of light to be selectively transmitted or reflected, while others are suppressed or attenuated.

Free spectral range (FSR): The free spectral range of an etalon refers to the range of wavelengths over which interference effects occur without overlap or ambiguity. It is determined by the thickness of the etalon and the refractive index of the material. The FSR sets the resolution and spectral range of the etalon for wavelength measurement or filtering applications.

Finesse: The finesse of an etalon quantifies its ability to discriminate between adjacent wavelengths or spectral lines. It is defined as the ratio of the free spectral range to the width of the spectral peaks in the interference pattern. Higher finesse values indicate better spectral resolution and narrower linewidths.

Tuning mechanisms: Some etalons are designed to be tunable, allowing for precise control and adjustment of the separation distance between the reflecting surfaces. Tuning mechanisms may involve mechanical, thermal, or electro-optic means to vary the etalon's optical properties and resonance conditions.

Applications: Etalons are used in a wide range of applications, including wavelength measurement, spectral analysis, laser stabilization, optical filtering, frequency locking, and mode locking of lasers. They are essential components in spectrometers, laser cavities, optical resonators, Fabry-Pérot interferometers, and telecommunications devices.