Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
2000-03-06
2002-06-25
Sherry, Michael J. (Department: 2809)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
C324S11700H
Reexamination Certificate
active
06411077
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical-voltage sensor for measuring a voltage with high accuracy and, more particularly to an optical-voltage sensor aimed at high sensitivity using an electro-optic crystal having a natural optical rotatory power.
2. Related Background Art
There are some crystals in which a plane of polarization of linearly polarized light is rotated as the light travels when the light is transmitted through the crystals. This physical property is called as the natural optical rotatory power (hereinafter, referred to as optical rotatory power), and the angle of rotation of the linear polarization plane is called as the angle of optical rotation. An angle of optical rotation is proportional to a thickness of the crystal in a light transmission direction (hereinafter, referred to as a light path length). On the other hand, the optical-voltage sensor utilizes the electro-optical effect (Pockels effect) that the transmitted light is phase-modulated by applying a voltage to the electro-optical crystal. When the electro-optical crystal having the optical rotatory power is used for an optical-voltage sensor, handling of the optical rotatory power becomes important for the optical-voltage sensor because the both phenomena appear at a time.
However, since the measurement sensitivity of the optical-voltage sensor is decreased by the optical rotatory power, various kinds of ideas to prevent the decrease have been studied. For example, the “Optical Fiber Sensor” Information Research Committee (1985) proposes that in order to prevent the decrease in measurement sensitivity by the optical rotatory power, the optical path length of an element is set to a value less than 2 mm when a single crystal of Bi
12
GeO
20
(hereinafter, referred to as BGO) or Bi
12
SiO
20
(hereinafter, referred to as BSO) is used. This first prior art intends that the effect on the sensitivity of the optical-voltage sensor is reduced by shortening the optical path length to make the angle of optical rotation smaller.
Further, from the viewpoint that the maximum sensitive angle of the electro-optical effect is changed by the optical rotatory power, Japanese Patent Application Laid-Open No.6-258352 and Japanese Patent Application Laid-Open No.7-280852 propose a method of reducing the effect of the angle of optical rotation that the electro-optical element is rotated in a direction inverse to the angle of optical rotation by ½ of the angle of optical rotation. Moreover, in cases of improving the sensitivity by laminating the electro-optical elements, Japanese Patent Application Laid-Open No.6-258352 and Japanese Patent Application Laid-Open No.7-280852 propose a method that the electro-optical elements are laminated by rotating each of the elements step by step, and a method that electro-optical crystals cut with shifting the crystal orientation are laminated without rotating. These second prior arts are an idea that the crystal orientations of the electro-optical elements are arranged so as to maximize the sensitivity.
Among the above-mentioned prior arts, the shortening of the optical path length in the first prior art means that the thickness of the element is thinned. However, thinning of the thickness of element means increasing of the intensity of electric field in inverse proportion to the thickness of element when the applied voltage is constant. Therefore, there is a problem in that the reliability of insulation is reduced when the electric field is high.
On the other hand, the second prior arts have a problem in that the structure of the optical-voltage sensor becomes complex because the electro-optical crystals are laminated with inclining the crystal orientations. Further, the method of using the electro-optical crystals cut with shifting the crystal orientation has a problem that the fabrication accuracy is difficult to maintain and the quality control becomes complex. In general, the element having required size is formed by cutting a single crystal ingot in a direction of the crystal orientation. Furthermore, both of the proposed methods have a problem in that electric insulator is necessary to be placed between the laminated electro-optical elements and accordingly the structure becomes complex.
An object of the present invention is to make a measurement sensitivity of an electro-optical element higher and to provide an optical-voltage sensor having a simple structure capable of being formed by directly laminating the highly sensitive electro-optical elements.
SUMMARY OF THE INVENTION
In order to attain the above object, the present invention is characterized by an optical-voltage sensor comprising a polarizer for converting incident light into linearly polarized light; a ¼ wavelength plate for converting the linearly polarized light into circular polarized light; electro-optical elements to transmit the circular polarized light therethrough and to perform phase modulation of the light corresponding to a voltage applied to transparent electrodes in the both surfaces; and an analyzer for sensing light transmitted through the electro-optical elements, wherein two of the electro-optical elements are arranged adjacently to each other along a transmitting direction of the light, and a voltage to be measured is applied to each of the two electro-optical elements at a time with a polarity opposite to each other.
According to the structure described above, because the voltage to be measured is applied to the two electro-optical elements arranged adjacently to each other along the transmitting direction of the light at a time with a polarity opposite to each other, the Pockels effects are produced in the electro-optical elements with an inverse polarity and accordingly the degree of modulation can be increased even if the angle of optical rotation is increased. Thereby, the measurement sensitivity of the optical-voltage sensor can be increased.
As the method of producing the Pockels effects with an inverse polarity, in addition to the method of applying the voltage to be measured to each of the electro-optical elements with a polarity opposite to each other, as described above, there is another method that a crystal surface of one of the electro-optical elements is arranged inclining by 90 degrees with respect to that of the other of the electro-optical elements, and the voltage to be measured is applied to the both electro-optical elements with an equal polarity. That is, the present invention is characterized by an optical-voltage sensor having the structure similar to the optical-voltage sensor described above, wherein two of the electro-optical elements are arranged adjacently to each other along a transmitting direction of the light, and the two electro-optical elements are arranged so that an angle difference between crystal orientations of the electro-optical elements becomes 90 degrees in a plane perpendicular to the transmitting direction of the light, and a voltage to be measured is applied to each of the two electro-optical elements at a time with an equal polarity. In such a structure, the Pockels effects are also produced in the electro-optical elements adjacent to each other with an inverse polarity. Therefore, the measurement sensitivity of the optical-voltage sensor can be increased.
Further, the present invention is characterized by an optical-voltage sensor having the structure similar to the optical-voltage sensor described above, wherein an even number of the electro-optical elements are arranged continuously along a transmitting direction of the light, and a voltage to be measured is applied to each of the even number of electro-optical elements at a time with polarities opposite to each other with respect to adjacent electro-optical elements.
According to the structure described above, because the voltage to be measured is applied to the electro-optical elements adjacently to each other among the even number of electro-optical elements arranged continuously along a transmitting direction of the light at
Higaki Masaru
Sone Isamu
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Sherry Michael J.
Tang Minh N.
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