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.