One of the main features of an optical etalon is its very precisely parallel reflecting surfaces. For highest resolution, our portfolio of high-performance etalons features very high-quality, flat and level surfaces with low roughness and extreme parallelism.
We provide a broad range of etalons, including solid etalons made from a wide variety of materials such as fused silica, germanium, silicon, zinc selenide or YAG. If your application calls for wavelength monitoring, our series of solid fused silica metalons (etalons that are broadband metallic coated) can make a difference.
A series of air spaced etalons is available, made of fused silica as the standard material, but Zerodur may also be specified. A series of tunable air spaced etalons manufactured from fused silica can be ordered with single- or larger triple-piezo tubes allowing to tune the etalon quickly by changing the size of the air gap.
We also carry a series of VIPA etalons (Virtually Imaged Phase Arrays) manufactured from fused silica or calcium fluoride that convert a single input beam to a series of parallel output beams of gradually decreasing intensity.
Sometimes you need something special – if you are looking for a customized etalon that exactly meets your specific requirements, please get in touch with the AMS Technologies etalon experts. Our partner is extremely experienced in specifying, designing and manufacturing custom etalons – we are looking forward to discussing your customized etalon solution!
Complementing our etalon offering, AMS Technologies carries a portfolio of complementary precision optics such as optical prisms, polarization optics, optical beamsplitters, optical mirrors, optical filters, optical lenses, or optical scanners and deflectors.
Additionally, a broad range of optomechanics and motion control products is available such as optical mounts, rotary and translation stages, optical tables, breadboards and platforms as well as a variety of optical test and measurement equipment – including several ready-to-use spectrometers, spectrographs and spectroscopy cameras.
Optical etalons are optically transparent, flat components with very precisely parallel reflecting surfaces. Solid etalons consist of solid plates of optical material, while air spaced etalons are formed by two optical mirrors with an air gap between them.
Fabry Pérot Etalons
Two plane, parallel and semi-transparent mirrors separated by a fixed distance form a Fabry Pérot etalon or Fabry Pérot interferometer. Light entering the etalon is subject to multiple reflections, and the interference of the light leaving the etalon during each reflection causes a modulation of both the transmitted and reflected beam. During one reflectance, the phase changes by a certain amount. Depending on the beam angle, the optical thickness of the etalon and the wavelength, constructive and destructive interference happens.
In an etalon’s transmission spectrum, a series of peaks can be seen where constructive interference is occurring. The distance between these peaks is the "free spectral range" (FSR). The reflectivity of the mirrors has no effect on this FSR, but it influences the number of reflections and improves the modulation quality. With higher reflectivity of the mirrors, the modulation peaks become sharper and narrower. An etalon’s bandwidth is defined by the full width at half maximum (FWHM) of the peak, and the ratio of the line width to the distance between peaks (the FSR) is called finesse (F, a dimensionless parameter) – so Bandwidth = FSR/F and F = FSR/Bandwidth.
Solid etalons are comparatively simple, robust, yet very parallel optical components. Solid etalons show instability to temperature changes, which can be unacceptable in certain applications. Air spaced etalons reduce this temperature dependence by using air as the etalon medium, but air spaced etalons are also more complex.
The term "tunable etalon" usually means the piezoelectric tuning of an air spaced etalon. With this technique, the etalon can be tuned quickly by changing the size of the air gap. VIPAs (Virtually Imaged Phase Arrays) are etalons with a built-in transmission window, so the beam can pass the first reflector, and the multiple reflections as well as the constructive and destructive interference do effectively start inside the etalon. Using this technology, VIPAs can feature almost 100% transmission and are used to convert a single input beam to a series of parallel output beams of gradually decreasing intensity.
Besides the material, etalons are generally specified by the parameters wavelength range, finesse, free spectral range (FSR), clear aperture and transmission – all measurable and functional specifications. The reflectivity of the mirrors, on the other hand, is usually not specified because this reflectivity does not guarantee performance. Due to scatter, surface error and tilt, the values for finesse and transmission can be significantly lower than the values that would be expected based on a given reflectivity.
Limits to Finesse
While in theory a perfect etalon with no losses or imperfections would have a 100% peak transmission, in reality there are several factors having effects on and therefore also limiting the transmission and finesse such as surface irregularity, tilt or wedge, spherical error or coating scatter. Each of these factors contributes to limiting the finesse, and a supplier combines all these contributions to come up with the expected finesse and transmission for an etalon.
Surface Figure: The surface figure is defined as the rms deviation of a surface from absolute flatness. Spherical error is not included in this surface figure and is measured separately. Usually, the surface figure is measured at the wavelength of a Helium-Neon laser (633 nm) and is given as fractions of this wavelength.
Tilt or Wedge: Not exactly parallel end mirrors cause a change in the phase of the beam passing an etalon. This reduces the finesse, because not all of the beam emerges "in phase" (creating a bright fringe) or "out of phase" (creating a dark fringe). The result is mixing of dark and bright fringes leading to low contrast.
Spherical Error: The same applies to the spherical error – due to the surface curvature, the phase of the light beam varies across the etalon surface. Both the wedge error and the sphere error are predictable and can be calculated in a certain way for predicting the expected performance.
Scatter and Material Losses: Due to scatter, light leaks out of the etalon – and materials will absorb light every time it passes through it. As long as the right coatings and materials are used, these losses are usually insignificant, unless the finesse is very high (above about 200).
Etalons are used in spectroscopy as an interferometer and for wavelength measurement or fine-structural investigation of spectral lines. High-resolution spectrometers are often based on etalons as the main optical component for splitting a light beam into its spectral components. AMS Technologies provides a broad range of free space as well as fiber-input spectrometers for various wavelength ranges from UV through visible to infrared.
Alternative Terms: Optical Etalon; Solid Etalon; Air Spaced Etalon; Tunable Etalon; VIPA; Fabry-Pérot Interferometer