Everything about Fabry-p Rot Interferometer totally explained
In
optics, a
Fabry-Pérot interferometer or
etalon is typically made of a transparent plate with two
reflecting surfaces, or two parallel highly reflecting mirrors. (Technically the former is an etalon and the latter is an
interferometer, but the terminology is often used inconsistently.) Its transmission
spectrum as a function of
wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. It is named after
Charles Fabry and
Alfred Perot. "Etalon" is from the French
étalon, meaning "measuring gauge" or "standard".
The resonance effect of the Fabry-Pérot interferometer is identical to that used in a
dichroic filter. That is, dichroic filters are very thin sequential arrays of Fabry-Pérot interferometers, and are therefore characterised and designed using the same mathematics.
Etalons are widely used in
telecommunications,
lasers and
spectroscopy to control and measure the
wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Pérot interferometers.
Theory
The varying transmission function of an etalon is caused by
interference between the multiple reflections of light between the two reflecting surfaces. Constructive interference occurs if the transmitted beams are in
phase, and this corresponds to a high-transmission peak of the etalon. If the transmitted beams are out-of-phase, destructive interference occurs and this corresponds to a transmission minimum. Whether the multiply-reflected beams are in-phase or not depends on the wavelength (λ) of the light (in vacuum), the angle the light travels through the etalon (θ), the thickness of the etalon (
l) and the
refractive index of the material between the reflecting surfaces (
n).
The phase difference between each succeeding reflection is given by δ:
»
Applications
The most important common applications are as dichroic filters, in which a series of etalonic layers are deposited on an optical surface by vapor deposition. These optical filters usually have more exact reflective and pass bands than absorptive filters. When properly designed, they run cooler than absorptive filters because they can reflect unwanted wavelengths. Dichroic filters are widely used in optical equipment such as light sources, cameras and astronomical equipment.
Telecommunications networks employing wavelength division multiplexing have add-drop multiplexers with banks of miniature tuned fused silica or diamond etalons. These are small iridescent cubes about 2 mm on a side, mounted in small high-precision racks. The materials are chosen to maintain stable mirror-to-mirror distances, and to keep stable frequencies even when the temperature varies. Diamond is preferred because it has greater heat conduction and still has a low coefficient of expansion. In 2005, some telecommunications equipment companies began using solid etalons that are themselves optical fibers. This eliminates most mounting, alignment and cooling difficulties.
A wavemeter is a combination of up to five Fabry-Pérot interferometers with a factor of ten difference in Δλ between any two of them. The beam is made divergent by a cylindrical lens and the distance between two bright lines is recorded by means of a CCD camera.
Laser resonators are often described as Fabry-Pérot resonators, although for many types of laser the reflectivity of one mirror is close to 100%, making it more similar to a Gires-Tournois interferometer. Semiconductor diode lasers sometimes use a true Fabry-Pérot geometry, due to the difficulty of coating the end facets of the chip.
Etalons are used to construct single-mode lasers. Without an etalon, a laser will generally produce light over a wavelength range corresponding to a number of cavity modes, which are similar to Fabry-Pérot modes. Inserting an etalon into the laser cavity, with well-chosen finesse and free-spectral range, can suppress all cavity modes except for one, thus changing the operation of the laser from multi-mode to single-mode.
Fabry-Pérot etalons can be used to prolong the interaction length in laser absorption spectrometry techniques.
A Fabry-Pérot etalon can be used to make a spectrometer capable of observing the Zeeman effect, where the spectral lines are far too close together to distinguish with a normal spectrometer.
In astronomy an etalon is used to select a single frequency of an atomic transition for imaging. The most common is the H-alpha line of the sun. Etalons are available to visual astronomers with bandwidths (FSR) from 0.2 nm to 1 nm. The CaK line from the sun is also commonly imaged with etalons in the 1 nm to 2 nm range.Further Information
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