Optical voltage sensor

Details Optical voltage sensor Optical voltage sensor

2000-07-28

2002-03-05

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Having particular chemical composition or structure

C359S245000, C359S246000, C324S072000, C324S096000

Reexamination Certificate

active

06353494

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical voltage sensor for detecting voltages of power transmission lines or of distribution lines, or power-supply voltages to drive motors, or the like.
2. Description of the Background Art
FIG. 21
is a perspective front view showing a conventional optical voltage sensor simplified in structure. The optical voltage sensor includes a sensor part, a light emission part, a light reception part, and signal processing circuits provided each on the light emission and reception sides (both not shown). The sensor part has a polarizer
1
, a quarter wavelength plate (or “&lgr;/4 plate”)
2
, an electro-optic crystal
3
, and an analyzer
4
arranged in order of light incidence on an optical axis. The light emission part includes an E/O circuit including a light emitting element such as LED (Light Emitting Diode) used as a light source. The light emission part is also provided with an input side optical system including an optical fiber
32
a
, a ferrule
38
a
, a GRIN lens
33
a
, and a holder
28
a
, all of which are arranged on an optical axis. As to these optical components, optical axis planes adjacent to each other are bonded by an adhesive. The light reception part is structured by an output side optical system in the same manner as with the input side optical system, and an O/E circuit in which an optical signal emitted from the output side optical system is converted into an electrical signal with a converting element.
As described above, as to the optical components arranged on the optical axis in the sensor part of the above-described optical voltage sensor, i.e., the polarizer
1
, the &lgr;/4 plate
2
, the electro-optic crystal
3
, and the analyzer
4
, the optical axis planes adjacent to each other are bonded by the adhesive. The optical axis planes herein mean planes perpendicular to the respective optical axes, and each optical component has two planes where light comes in and goes out (the same is applicable to the below). Onto the electro-optic crystal
3
, a pair of electrodes
35
are evaporated. The electrodes
35
are electrically connected to a pair of electrode terminals
24
, respectively, via lead wires
34
. A voltage is applied to between the electrode terminals
24
and is measured by the optical voltage sensor.
The signal processing circuits each provided on the light emission and reception sides are connected to the sensor part via the light emission and reception parts, respectively. The input-side optical axis plane of the polarizer
1
in the sensor part is fixed by the adhesive to the optical axis plane of the GRIN lens
33
a
, while the output-side optical axis plane of the analyzer
4
in the sensor part to the optical axis plane of a GRIN lens
33
b
. The sensor part, the input side optical system in the light emission part, and the output side optical system in the light reception part all fixed by the adhesive are mechanically fixed in a case (not shown). As the adhesive for the optical components is used an resin selected from among epoxy resins, urethane base resins, and the like.
In the optical voltage sensor, as the electro-optic crystal
3
is used a crystal such as Bi
12
SiO
20
(BSO), KDP, or LiNbO
3
or LiTaO
3
having natural birefringence property.
Next, the operational principle of the optical voltage sensor will be described by referring to FIG.
1
. When an LED having a center wavelength of 0.85 &mgr;m is exemplarily used as a light source in the light emission part, an unpolarized light
7
of the LED emitted from a light emission part
10
becomes linearly polarized after having passed through the polarizer
1
in the sensor part. The linearly polarized light becomes circularly polarized after having passed through the &lgr;/4 plate
2
. The circularly polarized light then becomes elliptical according to a voltage Vm applied to the electro-optic crystal
3
after having passed therethrough. That is, the polarization state of the light passed through the electro-optic crystal
3
is changed by the applied voltage Vm. Such elliptically polarized light goes through the analyzer
4
, and then reaches a light reception part
12
as an emitted light
8
. The emitted light
8
varies in strength according to the polarization state of the light after having passed the electro-optic crystal
3
. Since the polarization state is affected by the voltage Vm, it is possible to measure the voltage Vm outputted from the analyzer
4
by monitoring any change in strength in an optical receiver via the optical fiber
32
b
and then calculating modulation depth of optical power (strength). The modulation depth of optical power herein means a ratio of AC component to DC component of optical power.
Since the optical voltage sensor is usually used outdoors under severe conditions, a high performance in temperature dependence is required, more specifically, a change of modulation depth of ±1.0% or less is preferable at a temperature within a range of −20° C. to 80° C. Factors for the temperature dependence in the conventional optical voltage sensor include, for example, a change of refractive index due to stress applied to the bonded portions of the &lgr;/4 plate
2
or the electro-optic crystal
3
, and a temperature dependence due to birefringence property of the &lgr;/4 plate. In the case that the LiNbO
3
crystal having natural birefringence property is used as the electro-optic crystal, the temperature dependence due to angular deviation of the incident light may also be included.
For betterment of such factors, the following methods are currently in use.
1. A method for reducing the temperature dependence of the electro-optic crystal by easing the stress applied thereto.
In this method, the electro-optic crystal is fixed without any adhesive so that the stress applied thereto is eased. This method is disclosed in Japanese Patent Laying-Open No. 9-145745 (97-145745)(U.S. Pat. No. 5,748,392).
2. A method for reducing the temperature dependence due to birefringence property of the &lgr;/4 plate.
In this method, a zero-order single plate having temperature dependence lowered in degree is used as the &lgr;/4 plate.
3. A method for reducing the temperature dependence due to natural birefringence property in the electro-optic crystal by reducing an angular deviation of the incident light.
In this method, the angular deviation of the light coming into the electro-optic crystal is reduced to 0.2° or less with a plane-making accuracy of the optical components, and whereby the temperature dependence due to the angular deviation of the incident light can be reduced. This method is disclosed in Japanese Patent Laying-Open No. 3-44562 (91-44562).
The first method is effective with ease of use. As to the second method, the zero-order single plate used as the &lgr;/4 plate is expensive and fragile, and thereby requiring experience for handling to a considerable extent. Therefore, a low cost multilayered plate with ease of handling has been commonly used as the &lgr;/4 plate. As shown in
FIG. 22
, its sensitivity converted as the modulation depth shows a relative change (relative sensitivity change with reference to the sensitivity at 25° C.) of ±1.5% or less at a temperature within a range of −20° C. to 80° C. The third method is easy to use, but reducing the angular deviation is not sufficient to stabilize characteristics. This is because the angular deviation may affect not only the angular deviation from the optical axis but also an angular deviation on the optical axis plane. As such, the temperature dependence of the conventional optical voltage sensor resulted from the stress is reduced, but not yet the temperature dependence about 1.5% of the &lgr;/4 plate. Further, the temperature dependence of the optical voltage sensor remains unstable due to the angularly-deviated light coming into the electro-optic crystal.
As described above, the &lgr;/4 plate having temperature dependence of about 1.5% is used in the conventional optical voltage sen

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