The electrical insulation of the field winding must isolate the conductors from the rotor body which is at ground potential, must separate adjacent turns within a coil, and must be designed to do this while successfully handling a severe mechanical duty, particularly in large generators. Harrington, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IX.C.2.b Insulation If the temperature is not very low as compared to T c, current “leaks” at sub-gap voltages due to the smeared Fermi distribution in the normal electrode with tail extending up to energies above the top of the energy gap in the superconductor.ĭean B. Above the threshold voltage of ∣eV∣ = Δ the current increases rapidly and soon resumes the ohmic value determined by the normal state tunnel resistance of the junction, R T. Therefore, at low voltages, ∣eV∣ < Δ, and at low temperatures, k B T ≪ Δ or T ≪ T c, virtually no current flows through the junction. If we make the typically very well justified approximation of low environmental impedance of the conductors, tunneling represents a horizontal crossing between S and N through the insulator I from an occupied state into an empty one, but not into the forbidden gap region. The bias voltage V lifts the Fermi level of the superconductor with respect to the normal electrode by the amount eV. Therefore, in the backward direction of a two-stage isolator the light is blocked twice by the polarizers, resulting in higher isolation.įigure 9 is the “semiconductor” energy diagram of an NIS tunnel junction, with occupied states shaded at a nonzero temperature. Since the polarization direction of this beam is 90 degrees to the direction of the input polarizer, this beam is further blocked by the input polarizer. The polarization of this small amount of light is further rotated by the first Faraday rotator and becomes a horizontally polarized light. Since the polarization direction of this beam is 90 degrees to the direction of the first analyzer, this beam is blocked by the first analyzer and only a small amount of light can pass through due to the limited extinction ratio of the analyzer and the rotation angle error caused by the imperfection of the Faraday rotator. The polarization of this beam is then rotated −45 degrees again by the Faraday rotator and becomes a 45-degree polarized beam. 8b), when a light is launched into the isolator, most of the light is blocked by the output analyzer, except for the light with a horizontal polarization direction, which will pass through the analyzer. In the backward direction operation ( Fig. Operating principles of a two-stage, polarization-dependent optical isolator: (a) operation in the forward direction and (b) operation in the backward direction. To construct a two-stage optical isolator, the required minimum materials are three polarizers (birefringent crystals) and two Faraday rotators, and they can be configured by inserting the Faraday rotators between the polarizers (birefringent crystals).įIGURE 8. A multistage isolator can be constructed by simply cascading multiple single-stage isolators or by optimizing configurations that use fewer materials. For example, the isolation of a two-stage isolator generally is twice that of a single-stage isolator.
#Alternative to optical isolator series
Optical isolators can be used in series (cascaded) to increase the isolation. In some applications, a much higher isolation is desired. 7, the peak isolation of a typical single-stage isolator is about 40 dB, and the overall isolation is less than 30 dB with a rotation angle error of 2 degrees, which can be easily introduced due to environmental changes. Isolators described above are so-called single-stage isolators that have a basic configuration of a Faraday rotator sandwiched by two polarizing elements. Yale Cheng, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 I.E Multistage Optical Isolators
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