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Faraday Crystals TGG and Glasses MOS-4 and MOS-10
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Strontium-Barium Niobate
The Faraday or Magneto-Optic Effect
   In 1845 Michael Faraday discovered that when a block of glass is subjected to a strong magnetic field, it becomes optically active. When plane-polarized light is sent through glass in a direction parallel to the applied magnetic field, the plane of vibration is rotated. Since Faraday's early discovery the phenomenon has been observed in many solids, liquids, and gases. The amount of rotation observed for any given substance is found by experiment to be proportional to the field strength and to the distance the light travels through the medium.
   The constant, called the Verdet constant, is defined as the rotation per unit path per unit field strength. In gases the density must also be specified.
Unlike the electro-optic effect, the magneto-optic effect causes a true rotation of the plane of polarization for any input polarization angle. In a simple electro-optic device, only pure rotations are available; all other intermediate voltages produce different degrees of elliptical polarization states from a linear input state. A Faraday rotator however will truly rotate the plane of input polarization through any angle (providing you can provide a strong enough magnetic field).
   The verdet constant for most materials is extremely small and is wavelength dependent. The effect is at its strongest in those substances containing paramagnetic ions such as terbium. The highest verdet constants are in fact found in terbium doped glasses. Although expensive, this material has significant benefits and other substrates, notably excellent transparency, high optical quality, big size and high resistance to laser damage.
   Although the Faraday effect is not itself chromatic, the verdet constant itself is quite strongly a function of wavelength. At 632.8 nm, the verdet constant for Faraday Rotator Glass is 0.329 - 0.37 whereas at 1064 nm, it has fallen to 0.108. This behavior means that the devices manufactured with a certain degree of rotation at one wavelength, will produce much less rotation at longer wavelengths.
   Faraday Isolator. The most common application for a Faraday rotator is when coupled with input and output polarizers to form an isolator. At high power optical feedback, it can damage or disrupt the operation of femtosecond laser systems. To reduce this feedback an optical isolator based on the Faraday Effect is inserted into the system. Faraday Isolators are passive unidirectional, non reciprocal devices that utilize the phenomenon of Magneto-Optic Rotation to isolate the source from reflections in an optical system. The isolator protects the laser oscillator from optical feedback making Faraday Isolators a key component in many of today's laser systems.
   Faraday Rotators are also used for example in ring laser systems to introduce a loss mechanism (in conjunction with some other intra-cavity polarization selective element) which is greater for one direction of propagation than for the other.
   The magneto-optical glasses MOS-4 and MOS-10 are designed for laser light flux output control in the magnetic field in visible and near-IR spectral range. It is used in manufacture of modulator's optical shutters based on Faraday rotation of the light polarization plane. They feature the increased Verdet constant.

Main properties:
Material
MOS-4
MOS-10
Density, g/cm3
4.3
4.83
Mohs hardness
4 - 5
4 - 5
Refractive index, at 587.5 nm
n = 1.6889
n = 1.7350
Non-linear coefficient, esu
2.65 x 10-13
3.07 x 10-13
Thermal expansion coefficient
96.4 x 10-7 x K-1
63.2 x 10-7 x K-1
Verdet constant, rad x T-1 x m-1
» = 633 nm
73
87
» = 1060 nm
21
26
Optical losses at 633 and 1060 nm, cm-1
0.001
0.001
Optical quality (” n), cm-1
< 0.5 x 10-6
< 0.5 x 10-6
Elements maximum dimensions, mm
dia. 150 x 80
dia. 150 x 100

   The magneto-optical crystal Terbium Gallium Garnet TGG is an optimum material for Faraday devices (Rotator and Isolator) in the range from 400 nm-1100 nm, excluding 470-500 nm. TGG has a combination of excellent properties such as large Verdet constant, low light loss, high thermal conductance and high light damage threshold which makes it a unique material for Faraday devices particularly suitable for YAG lasers and Ti: sapphire tunable lasers, ring lasers and seed injected lasers.

Main properties:
Chemical Formula
Tb3Ga5O12
Lattice Parameter
a=12.355Å
Density
7.13g/cm3
Mohs Hardness
8.0
Melting Point
1725°C
Refractive Index
1.954 at 1064 nm
Thermal Conductivity
7.4 W cm-1 K-1
Verdet constant, rad x T-1 x m-1
» = 632 nm
134
» = 1064 nm
40
Nonlinear Index, n
8.0
Figure of Merit, V/a
27
Figure of Merit, V/n2
5

We supply elements, made from mentonied crystals with different cross-sections and dimensions as well as Faraday Rotators / Isolators according to custumors' specifications.

I N Q U I R Y    F O R M :

Material:
Quantity of elements:
Application:

polished sites:

BLOCK    or  BLOCK
 a-b b-c a-c
or d  
Orientation:
(if applicated)
Dimensions/Tolerances:
For example:
10.0 +/- 0.1
AR-coating or electrodes:
a = mm;
c = mm
b = mm or
dia. = mm;
This field is for additional information such as application,
comments, add. requirements etc.

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