Lyot filter

Lyot filter

A Lyot filter, named for its inventor Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. Lyot filters are often used in astronomy, particularly for solar astronomy.

A Lyot filter is made from one or more birefringent plates (usually quartz), with (in multi-plate filters) each plate being half the thickness of the previous one. Because the plates are birefringent, the "ordinary" and "extraordinary" polarization components of a light beam experience a different refractive index and thus have a different phase velocity. Therefore, the polarization state of light with an arbitrary wavelength will in general be modified after a passage through the filter plate, and this causes a loss of optical power in a subsequent polarizer. For certain wavelengths, however, the optical path length difference is an integer multiple of the wavelength, so that the losses are very small. By rotating the plates, one can shift the wavelengths of the transmission peaks.

Each stage of a Lyot filter is 1/2 the size of the preceding stage. The largest stage sets the bandwidth and the smallest stage sets the Free Spectral Range. If you use two of the second largest crystals it will increase the contrast of the desired line. If you split the cristals in half and add a 1/2 waveplates in the middle you can increase the field of view of the filter.

If electrically tunable birefringent elements are inserted (liquid crystal or electrooptic), then we obtain an "Electrically Tunable Lyot Filter".

The separation and narrowness of the transmission peaks depends on the number, thicknesses, and orientation of the plates.

Single and multi-plate Lyot filters are often used inside the optical cavity of lasers to allow tuning of the laser. In this case, Brewster losses from the plate and other intracavity elements are usually sufficient to produce the polarizing effect and no additional polarizers are required. Although their mechanisms are different, modelocking lasers and Lyot-filter lasers both produce a comb of multiple wavelengths which can be placed on the ITU grid for Dense Wave Division Multiplexing (DWDM) or used to give each suburban home its own return-signal laser wavelength in a passive optical network (PON) used to provide FTTH (Fiber To The Home).

ee also

* Lyot stop
* Lyot depolarizer


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  • Lyot-Filter —   [ljo ; nach B. Lyot], optisches Filter mit spektral sehr schmalem Transmissionsbereich, das die Dispersion der Drehung der Polarisationsebene in doppelbrechenden Kristallen ausnutzt (Polarisationsinterferenzfilter) …   Universal-Lexikon

  • Lyot-Filter — Das nach seinem Erfinder, dem französischen Astronomen Bernard Ferdinand Lyot, benannte Lyot Filter ist ein optisches Filter, das Doppelbrechung nutzt, um einen schmalen Durchlassbereich der übertragenen Wellenlängen zu erzeugen. Der… …   Deutsch Wikipedia

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  • Lyot — may refer to: *Bernard Ferdinand Lyot, French astronomer *Lyot filter *Lyot stop *Lyot (crater) *2452 Lyot, asteroid *Bernard Lyot Telescope …   Wikipedia

  • Lyot —   [ ljo], Bernard Ferdinand, französischer Astrophysiker, * Paris 27. 2. 1897, ✝ Kairo 2. 4. 1952; arbeitete am Meudon Observatorium und am Pic du Midi Observatorium; bahnbrechende Arbeiten zur Sonnenforschung, erfand 1930 den Koronographen, 1933 …   Universal-Lexikon

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  • Lyot, Bernard — ▪ French astronomer born Feb. 27, 1897, Paris, Fr. died April 2, 1952, Cairo, Egypt       French astronomer who invented the coronagraph (1930), an instrument which allows the observation of the solar corona when the Sun is not in eclipse.… …   Universalium

  • Bernard Lyot — Observatorium von Meudon Bernard Ferdinand Lyot (* 27. Februar 1897 in Paris; † 2. April 1952 in Kairo) war ein französischer Astronom, der sich insbesondere der Erforschung der Sonne widmete. Lyot studierte Ingenieurwesen …   Deutsch Wikipedia

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