Verdet constant
Encyclopedia
The Verdet constant is an optical "constant" that describes the strength of the Faraday effect
for a particular material.
The Verdet constant for most materials is extremely small and is wavelength
dependent. It is strongest in substances containing paramagnetic ion
s such as terbium
. The highest Verdet constants are found in terbium doped dense flint glasses or in crystal
s of terbium gallium garnet (TGG). This material has excellent transparency properties and is very resistant to laser
damage.
The Faraday effect is chromatic (i.e. it depends on wavelength) and therefore the Verdet constant is quite a strong function of wavelength. At 632.8 nm
, the Verdet constant for TGG is reported to be -134 rad
/T
·m, whereas at 1064 nm it falls to -40 rad/T·m. This behavior means that the devices manufactured with a certain degree of rotation at one wavelength, will produce much less rotation at longer wavelengths. Many Faraday rotator
s and isolators are adjustable by varying the degree to which the active TGG rod is inserted into the magnetic field
of the device. In this way, the device can be tuned for use with a range of lasers within the design range of the device. Truly broadband
sources (such as ultra-short pulse lasers and the tunable vibronic lasers) will not see the same rotation across the whole wavelength band.
The Verdet constant is named after the French physicist Émile Verdet
.
Faraday effect
In physics, the Faraday effect or Faraday rotation is a Magneto-optical phenomenon, that is, an interaction between light and a magnetic field in a medium...
for a particular material.
The Verdet constant for most materials is extremely small and is 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...
dependent. It is strongest in substances containing paramagnetic ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s such as terbium
Terbium
Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife...
. The highest Verdet constants are found in terbium doped dense flint glasses or in crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s of terbium gallium garnet (TGG). This material has excellent transparency properties and is very resistant to 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...
damage.
The Faraday effect is chromatic (i.e. it depends on wavelength) and therefore the Verdet constant is quite a strong function of wavelength. At 632.8 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
, the Verdet constant for TGG is reported to be -134 rad
Radian
Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...
/T
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
·m, whereas at 1064 nm it falls to -40 rad/T·m. This behavior means that the devices manufactured with a certain degree of rotation at one wavelength, will produce much less rotation at longer wavelengths. Many Faraday rotator
Faraday rotator
A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magneto-optic effect....
s and isolators are adjustable by varying the degree to which the active TGG rod is inserted into the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of the device. In this way, the device can be tuned for use with a range of lasers within the design range of the device. Truly broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
sources (such as ultra-short pulse lasers and the tunable vibronic lasers) will not see the same rotation across the whole wavelength band.
The Verdet constant is named after the French physicist Émile Verdet
Émile Verdet
Émile Verdet was a French physicist. He worked in magnetism and optics, editing the works of Augustin-Jean Fresnel. Verdet did much to champion the early theory of the conservation of energy in France through his editorial supervision of the Annales de chimie et de physique.The Verdet constant is...
.