![]() There are therefore two basic designs of isolators in our range.įor the shorter wavelengths we provide our FOI 5/57 design. It is also wavelength sensitive as the verdet constant (which provides a sort of figure of merit) is strongly wavelength dependent.Īt increasing wavelengths, it is required to use especially strong magnetic fields in order to achieve a 45° rotation angle. The Faraday effect is weak, even in TGG and so a strong magnetic field is required. The input polariser will therefore reject back reflections.Īn output polariser is necessary to clean up the polarisation state for transmission back through the isolator (see diagram) and thus to ensure good isolation even if the light is de-polarized before returning through the device. Rotation is additive for reflectors passing back through the isolator giving 90° difference between the input and reflected polarisation states at the input to the isolator. To make a Faraday isolator, it is necessary to add a pair of polarizers to the rotator and to set the rotation angle to be 45°. This offers the highest Verdet constant for the visible to near infra red region and provides high power handling with low optical distortion. SF57 and special grades of glass selected for their Verdet constant) may be used, but we only use the highest quality single crystal TGG (terbium gallium garnet). The basic design of a practical device is to use a rod of optical material placed in the axial magnetic field produced by permanent magnets, usually face magnetized toroids. If the light is retro-reflected back through the same medium however, the same observer would again see the light polarization axis rotate through an additional, equal clockwise angle (whereas in an optically active medium the light would have undergone first a clockwise then an equal anti-clockwise rotation returning it to its original state).Ī device which exhibits this behaviour is called a faraday rotator. Light passing one way through the medium will be seen by an observer as having its polarization axis rotated through a clockwise angle for example. However, the effect is different to optical activity in that the sense of the rotation angle is not dependent on the direction of propagation through the medium. The effect is similar to that produced when a light ray passes through an optically active material such as quartz. Published under an exclusive license by AIP Publishing.The Faraday effect causes a rotation of the plane of light polarization when a light beam passes through a material in the presence of an axial magnetic field. This makes the NTF crystal one of the most prospective magneto-active media.ģ0, 485 (1977). According to the measurement result, with the use of crystals with normal absorption, it is possible to develop a traditional (single element) Faraday isolator operating at room temperature and ensuring the isolation ratio of about 30 dB at the laser radiation power of ∼7 kW. ![]() The investigation of Faraday isolators based on NTF crystals cut in and orientations demonstrated a significant advantage of the critical orientation. The orientation is determined by the parameter ξ therefore, its accurate measurement is of particular importance. The negative value of ξ indicates that the depolarization resulting from stress-induced birefringence in this material is strongly dependent on orientation and ensures the presence of a critical orientation, with the use of which the magnitude of thermally induced depolarization in the Faraday isolator may be significantly reduced. The value of the optical anisotropy parameter ξ NTF = −0.26 ± 0.02 was refined in model experiments on samples with increased absorption, and the value of the thermo-optical constant Q characterizing thermally induced depolarization was measured to be Q NTF = (3.44 ± 0.4)♱0 −6 K −1. The characteristics of a magneto-optical material promising for the development of Faraday isolators for high-power lasers-the Na 0.37Tb 0.63F 2.26 (NTF) solid solution crystal with a negative value of optical anisotropy parameter-were investigated.
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