Lyot filter
Encyclopedia
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 wavelength
s. 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 crystals 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 laser
s 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).
Bernard Lyot
Bernard Ferdinand Lyot was a French astronomer.His interest in astronomy started in 1914. He soon acquired a telescope and soon upgraded to a . From graduation in 1918 until 1929, he worked as a demonstrator at the Ecole Polytechnique...
, is a type of optical filter
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
that uses birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
to produce a narrow passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
of transmitted wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s. Lyot filters are often used in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, particularly for solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
astronomy.
A Lyot filter is made from one or more birefringent plates (usually quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
and thus have a different phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the 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
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
. For certain wavelengths, however, the optical path length
Optical path length
In optics, optical path length or optical distance is the product of the geometric length of the path light follows through the system, and the index of refraction of the medium through which it propagates. A difference in optical path length between two paths is often called the optical path...
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 crystals 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
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
of laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s to allow tuning of the laser. In this case, Brewster
Brewster's angle
Brewster's angle is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly...
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
Modelocking
Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds or femtoseconds ....
lasers and Lyot-filter lasers both produce a comb of multiple wavelengths which can be placed on the ITU
Itu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
grid for Dense Wave Division Multiplexing (DWDM)
Wavelength-division multiplexing
In fiber-optic communications, wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light...
or used to give each suburban home its own return-signal laser wavelength in a passive optical network
Passive optical network
A passive optical network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 16-128. A PON consists of an optical line terminal at the service provider's central...
(PON) used to provide FTTH (Fiber To The Home).