Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
2001-01-29
2003-01-28
Nguyen, Vinh P. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
C324S11700H
Reexamination Certificate
active
06512357
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a polarimetric sensor for the optical detection of a measured variable. In particular, the invention relates to a magnetic field sensor or an electric field sensor.
It is known to provide an optical magnetic field sensor for detecting a magnetic field, which in particular is caused by an electric current flowing in a current conductor. The Faraday effect is used in that case. Such a magnetic field sensor is also referred to as a magnet-optical current converter. The Faraday effect is understood to mean the rotation of the plane of polarization of linearly polarized light which is being propagated in a medium in the presence of a magnetic field. The angle of that rotation is proportional to a path integral of the magnetic field along a path traced by the light, with the Verdet constant as a proportionality constant. The Verdet constant is in turn dependent on the material in which the light is being propagated and on the wavelength of the light. A Faraday element in the form of an optically transparent material exhibiting the Faraday effect is disposed in the vicinity of the current conductor in order to measure the current. Linearly polarized light is coupled into the Faraday element. The magnetic field produced by the electric current has the effect of rotating the plane of polarization of the light being propagated in the Faraday element by a polarization rotation angle, which is evaluated by an evaluation unit as a measure of the strength of the magnetic field and therefore of the intensity of the electric current.
It is also known to provide an optical sensor for detecting an electric field (E-field sensor) and it is possible for the electric field to be caused, in particular, by an electric voltage. In that case, use is made of the Pockels effect, for example. Such an E-field sensor is also referred to as an electro-optical converter. The Pockels effect is understood to mean the phase shift caused by the electric field between two mutually orthogonal polarization components of light, in particular circularly polarized light, which is being propagated in a medium in the presence of an electric field. As a result, the state of circular polarization is generally converted into a state of elliptical polarization. A measured variable for the electric field to be detected may be determined from that change in the polarization.
An article entitled “Development of Optical Current Transformers and Application to Fault Location Systems for Substations” by Y. Yamagata et al., in IEEE Transactions on Power Delivery, Vol. 8, No. 3, July 1993, pages 1 to 7, discloses a magneto-optical current converter which contains a Faraday element made of a volume element formed as a bismuth silicon oxide (BSO) monocrystal. In that case, the acronym “BSO” represents an abbreviation for a crystal having the elemental composition Bi
12
SiO
20
or Bi
4
Si
3
O
12
. That Faraday element is located in an air gap in a magnetic flux concentrator which surrounds an electric conductor carrying current. As a result of the flux concentration, a homogeneous magnetic field results in the air gap, and is virtually exclusively determined by an electric current flowing in the current conductor. Polarized light radiated into the Faraday element changes its plane of polarization under the influence of the magnetic field in the air gap. That polarization modulation is converted into an intensity modulation in an analyzer directly adjacent the Faraday element. The resulting light signal is led through an optical waveguide to an evaluation unit, which calculates the current value from the intensity modulation. The path to be covered by the optical waveguide may amount to several kilometers, depending on the position of the evaluation unit. Glass-fiber optical waveguides are preferably used in the case of such large distances. That is because, due to their low attenuation, they are particularly suitable for an application with a relatively long optical transmission path.
European Patent EP 0 473 429 B1, corresponding to U.S. Pat. No. 5,355,084, discloses a magnetic field sensor which contains a Faraday element in the form of a volume element. In that case, light is led in and out through optical waveguides. In order to focus light emerging from an input optical waveguide to be fed into an output optical waveguide, in each case collimators are provided in the form of spherical or cylindrical lenses. In the case of the magnetic field sensor disclosed, they are needed in order to avoid beam widening of the emergent light and to ensure adequate coupling of light into the very small core cross section of the output optical waveguide. In general, the use of optical waveguides with small core diameters necessitates increased requirements placed upon the adjustment accuracy during production. In the case of monomode optical waveguides, the core diameter is, for example, on the order of magnitude of a few &mgr;m. In general, it is currently usual to use special glass-fiber optical waveguides with a very small core cross section for a magnetic field sensor. The components, such as the light source and light receivers, optical waveguides, polarizers and collimators, used in the prior art for the magnetic field sensor, are mostly complicated and therefore expensive special components which, in particular, are matched to the infrared wavelength range.
U.S. Pat. No. 5,475,489 discloses a polarimetric sensor in the form of a magnetic field sensor or an E-field sensor in each case in a reflex configuration. A light signal radiated into a sensor element, which is constructed as a Faraday or Pockels element, is focussed onto an output optical waveguide through the use of at least one reflection element. In that case, the reflection element is complicated to produce and has to be positioned very accurately during assembly in relation to the output optical waveguide.
Furthermore, German Published, Non-Prosecuted Patent Application DE 39 42 547 A1 discloses a magnetic field sensor based on an yttrium iron garnet (YIG), likewise in a reflex configuration. In order to increase sensitivity in the case of that magnetic field sensor, provision is made for the light signal to pass through the YIG sensor element twice. For that reason, a reflex configuration is provided, in which the yttrium iron garnet has a convex end surface that, on one hand, reflects the light signal and, on the other hand, focuses it onto the output optical waveguide. However, an yttrium iron garnet may likewise only be produced with considerable effort. In that case too, great care is necessary during the positioning of the input and output optical waveguides with regard to their configuration relative to the convex end surface of the yttrium iron garnet.
U.S. Pat. No. 4,812,767 discloses a magnetic field sensor in a reflex configuration which, in addition to the polarizer, the Faraday element and the analyzer, contains two prisms, constructed independently of each other, for beam deflection. That structure is located in a cylindrical tube, so that the magnetic field sensor becomes very large overall and also expensive to produce.
U.S. Pat. No. 4,560,932 discloses a further magnetic field sensor, in which the light signal is reflected many times within the Faraday element in order to increase the sensitivity. However, that multiple reflection is associated with beam widening of the light signal, so that after passing through the Faraday element, only a very low proportion of the intensity of the light signal is still coupled into the output optical waveguide. The measurement information may then only be reconstructed from the received light signal with an increased outlay.
U.S. Pat. No. 5,008,611 and German Published, Non-Prosecuted Patent Application 2 017 863 are documents relating to the general prior art in the field of magneto-optical metrology. In both documents, in each case a magnetic field sensor is described which contains more or less complicated beam guidance optics for the light signal.
Bosselmann Thomas
Mohr Stephan
Willsch Michael
Greenberg Laurence A.
Locher Ralph E.
Nguyen Vinh P.
Siemens Aktiengellschaft
Stemer Werner H.
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