Etalons

An optical etalon (also called Fabry–Pérot etalon) was originally a Fabry–Pérot interferometer in the form of a transparent plate (often made of fused silica) with parallel reflecting surfaces (solid etalon). However, the term is often also used for Fabry–Pérots consisting of two mirrors with some air gap in between (air-spaced etalon).

The reflectivities of the etalon's surfaces may simply result from the refractive index discontinuity between the etalon material and air (Fresnel reflection) or may be modified with dielectric coatings. By increasing the reflectivities, it is possible to increase the finesse, i.e., to sharpen the resonances without reducing the free spectral range.

When inserted into a laser beam, an etalon acts as an optical resonator (cavity), where the transmissivity varies approximately periodically with the optical frequency. (Some deviations from perfect periodicity result from chromatic dispersion.) In resonance, the reflections from the two surfaces cancel each other via destructive interference. The highest reflection losses and thus the lowest transmissivity occur in anti-resonance. The transmissivity versus frequency can be described with an Airy function, which approximately fits a simple sinusoidal function for not too high surface reflectivities.

The resonance effects occur even with some tilt (Figure 1), provided that the tilt angle is so small that the overlap of counterpropagating waves is not significantly reduced. (That works well for a small etalon thickness and a large beam radius.) The tilt angle can then be used to control the resonance frequencies. An etalon can therefore be used as an adjustable optical filter, e.g. for tuning the wavelength of a laser.

The effective finesse of an etalon may not reach the value which could be expected based on the surface reflectivities: It can be reduced, for example, if the reflecting surfaces are not perfectly parallel and flat. For high surface reflectivities, one may require an extremely high surface quality (low roughness) to realize the theoretically possible finesse. A reduced finesse can also result from using a too small or not properly collimated beam, a too large tilt angle, or a reduced beam quality.

When operated away from resonance or anti-resonance, an etalon provides chromatic dispersion. This is exploited in some dispersion compensation modules for optical fiber communications.

While etalons are often tuned by tilting them relative to the beam, they can also be tuned by varying their temperature. The resonance frequencies shift due to the temperature dependence of the refractive index ($\textrm{d}n/\textrm{d}T$) and the thermal expansion of the material. This method allows for tuning without mechanical movement and is particularly useful for fine-tuning or stabilizing the wavelength of single-frequency lasers (e.g., in laser diodes or solid-state lasers).

A special type of etalon is the Gires–Tournois interferometer (GTI) or Gires–Tournois etalon. In this configuration, one of the surfaces has a reflectivity near 100%, while the other is partially reflective. Such a device is used in reflection rather than transmission. Assuming zero losses, the modulus of the complex amplitude reflectivity is unity for all wavelengths (i.e., it works as a perfect mirror in terms of power), but the optical phase shift upon reflection varies strongly with wavelength. This property allows GTIs to be used for introducing significant chromatic dispersion into a system, typically for pulse compression in ultrafast lasers or for dispersion compensation.