Etalons

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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).

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 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.

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.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 31 suppliers for etalons. Among them:

ALPHALAS

ALPHALAS offers etalons made of fused silica for wavelength tuning and single-frequency operation of lasers. Etalons with diameter 8 mm and thicknesses 0.2 mm, 0.9 mm or 1.3 mm, coated with different reflectivities in the near infrared region, are available from stock.

Upgraded to resonant reflectors (stack of very high precision fused quartz flats with spacers and different number of elements), these have very high periodic spectral selectivity and very high damage threshold.

Customer-specific designs are also available.

AMS Technologies

For highest resolution, our portfolio of high-performance etalons features very high-quality, flat and level surfaces with low roughness and extreme parallelism:

• solid etalons made from a wide variety of materials such as fused silica, germanium, silicon, zinc selenide, or YAG
• broadband metallic coated solid fused silica metalons can make a difference for wavelength monitoring
• fixed or tunable air spaced etalons
• VIPA etalons (Virtually Imaged Phase Arrays)
• customized etalons that exactly meet your specific requirements

LightMachinery

LightMachinery manufactures the world's finest solid and air spaced etalons. Our fluid jet polishing systems allow us to routinely create surfaces that are better than λ/100 peak to valley.

Solid etalons, air spaced etalons, piezo tunable etalons, Gires–Tournois etalons – LightMachinery has extensive expertise in the manufacturing and testing of all kinds of Fabry–Pérot etalons from 1 mm square to 100 mm in diameter. These devices require high quality, very flat optical surfaces and extreme parallelism to achieve high performance, making them a good match for the polishing and metrology at LightMachinery.

Bibliography

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