JPH05119075A - Optical voltage sensor - Google Patents

Abstract

PURPOSE:To obtain an optical voltage sensor which can suppress a decline in measurement accuracy resulting from a change in optical rotary power caused by a change in ambient temperature by incorporating an electrooptic element in which both an electrooptic effect and optical rotary power coexist in the sensor. CONSTITUTION:This optical voltage sensor leads the light introduced through an optical fiber 3 to a light receiving system 13 through a route composed of a polarizer 6, electrooptic element 8 the magnitude of double refraction of which changes in corresponding to applied voltages, and analyzer 9, and optical fiber 12 and, at the same time, a phase element 7 is provided in the route between the polarizer 6 and analyzer 9. In addition, the light transmitting surface of the analyzer 9 is aligned in the direction of the principal axis of an ellipse representing the polarized state of the light when the light passes through the element 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical voltage sensor which can measure a voltage only by transmitting and receiving an optical signal.

[0002]

2. Description of the Related Art Electric voltage sensors, which are based on the transmission and reception of electric signals, are greatly limited in the places where they can be used. For example, in order to be able to use it in a high electric field, a high magnetic field, or in a flammable gas, it is necessary to have some large-scale insulation, non-induction, and safety measures.

For these reasons, a so-called optical voltage sensor has recently appeared, which can measure the voltage only by transmitting and receiving an optical signal. This optical voltage sensor is usually
Light guided through an optical fiber is passed through a path consisting of a polarizer, an electro-optical element whose birefringence changes according to the applied voltage, and an analyzer, and then to the light receiving system via the optical fiber. A structure in which a phase element is interposed in the path between the polarizer and the analyzer is adopted so as to obtain an output signal corresponding to the applied voltage from the light receiving system.

By the way, various electro-optical elements (Pockels elements) are used in the optical voltage sensor. Among these electro-optical elements, single crystals of cubic oxides such as Bi ₁₂ GeO ₂₀ and Bi ₁₂ SiO ₂₀ are known as those having excellent sensitivity and temperature characteristics.

However, the above-mentioned single crystal has a large optical rotatory power, and further has a characteristic that the optical rotatory power also changes with a change in ambient temperature. Therefore, in an optical voltage sensor in which these single crystals are incorporated as an electro-optical element, it is difficult to perform highly accurate measurement due to the effect of optical rotation.

[0006]

As described above, Bi ₁₂ G
In an optical voltage sensor incorporating an electro-optical element having an electro-optical effect and optical activity such as eO ₂₀ and Bi ₁₂ SiO ₂₀ , a change in optical activity due to a change in ambient temperature is the cause. It was difficult to measure with high accuracy.

Therefore, in the present invention, in the case where the electro-optical element in which the electro-optical effect and the optical rotatory power coexist, is incorporated, it is possible to prevent the measurement accuracy from deteriorating due to the change in the optical rotatory power accompanying the change in ambient temperature. It is intended to provide an optical voltage sensor.

[0008]

In order to achieve the above object, in the photovoltage sensor according to the present invention, the transmitting surface is arranged in the principal axis direction of the ellipse representing the polarization state when light passes through the electro-optical element. An analyzer is provided so as to match.

[0009]

When the analyzer is arranged in the above relationship, even if the optical rotation changes by Δγ _t due to the change in ambient temperature, the change in the emitted light intensity of the analyzer becomes 1-cos 2 Δγ _t , and Δγ _t
It is possible to obtain the most difficult state to be affected by.

[0010]

Embodiments will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an optical voltage sensor according to an embodiment of the present invention. In this optical voltage sensor, the drive circuit 1 causes the LED 2 to emit light. Then, the light emitted from the LED 2 is guided to the measurement optical unit 4 via the optical fiber 3.

The measurement optical section 4 converts the light guided through the optical fiber 3 into a collimator lens 5.
And a polarizer 6 for converting the light emitted from the collimator lens 5 into linearly polarized light, and a 1/4 wavelength plate 7 as a phase element for giving an initial phase to the light emitted from the polarizer 6 and converting it into circularly polarized light.
And the refractive index changes in proportion to the applied voltage,
Bi ₁₂ GeO ₂₀ or B that converts circularly polarized light emitted from
an electro-optical element 8 made of a single crystal such as i ₁₂ SiO ₂₀ ;
It is composed of an analyzer 9 for converting the elliptically polarized light transmitted through the electro-optical element 8 into a light intensity, and a lens 10 for condensing the light emitted from the analyzer 9. Transparent electrodes 11a and 11b are provided on both surfaces of the electro-optical element 8 in the light transmitting direction. The voltage Vs to be measured is applied to the electro-optical element 8 via the transparent electrodes 11a and 11b.

Here, the electro-optical element 8 is arranged such that the principal axis of its index ellipsoid is rotated by 45 degrees with respect to the principal axis of the polarizer 6. As shown in FIG. 2, the analyzer 9 has a direction of the principal axis of the polarizer 6 such that the direction I of the transmission surface thereof coincides with the principal axis direction of the ellipse representing the polarization state of the light transmitted through the electro-optical element 8. It is arranged to be rotated by an angle φ with respect to x.

The light emitted from the collimator lens 10 is guided to a photodiode 13 via an optical fiber 12 and is converted into an electric signal by the photodiode 13.
This electric signal is amplified by the amplifier 14 and then introduced into one input end of the differential amplifier 15 and also introduced into the other input end of the differential amplifier 15 via the low-pass filter 16. The differential amplifier 15 outputs an output proportional to the applied voltage V from the terminal 17, as described later. Then, the drive circuit 1 sets the LE so that the output of the low-pass filter 16 becomes constant.
Drive D2.

In the optical voltage sensor constructed as described above, the intensity P of the light incident on the photodiode 13 is
Assuming that the intensity of light when no voltage is applied to the electro-optical element 8 is P ₀ , P = P ₀ (1 + sin αV) (1) In the equation (1), V is an applied voltage and α is a proportional constant. When αV is sufficiently smaller than 1, sin α
Since V can be regarded as αV, the equation (1) is approximately P = P ₀ (1 + αV) (2)

[0015] P ₀ by the low-pass filter 16 is extracted, the P ₀ is provided to the other input terminal of the differential amplifier 15. Therefore, at the output terminal 17 of the differential amplifier 15,
An output of αV appears, and the level of the applied voltage V can be known.

Actually, in order to compensate the output change of the LED 2 and the loss change in the entire optical path including the optical fiber,
The injection current into the LED 2 is adjusted by the drive circuit 1 so that P ₀ becomes constant.

By the way, in this embodiment, the electro-optical element 8 is used.
The transmission surface of the analyzer 9 is aligned with the direction of the principal axis of the ellipse that represents the polarization state of the light that passes through. Therefore, even if the optical rotatory power of the electro-optical element 8 changes with the change of the ambient temperature, it is possible to minimize the influence of the change of the optical rotatory power on the output. The reason for this will be described below.

In the measurement optical section 4 of this optical voltage sensor, as shown in FIG. 3, assuming that the light passing through the polarizer 6 is the incident light H ₁ , this incident light H ₁ passes through the electro-optical element 8. It becomes emitted light H ₂ , and this emitted light H ₂ passes through the analyzer 9 and becomes emitted light H ₃ . Note that Bi _{12 is used} as the electro-optical element 8.
When a GeO ₂₀ single crystal is used, this single crystal is arranged so that its main axis is the main axis of the polarizer 6, that is, the x ′ and y ′ directions rotated by 45 ° with respect to the x axis in FIG.

The inventor of the present invention has found that the electro-optical element 8 having both the electro-optical effect and the optical rotatory power, such as the Bi ₁₂ GeO ₂₀ single crystal.
When H is used, the emitted light H emitted from this element
₂ and a program for calculating the polarization state of the emitted light H ₃ emitted from the analyzer 9 have been developed.

First, a Bi ₁₂ GeO ₂₀ single crystal is divided into N in the optical path direction, and a thin element having a thickness ΔL (= L / N, where L is the length of the element in the optical path direction) is considered. After a phase delay of Δδ (= δ / N) due to the electro-optic effect, the polarization plane is changed to Δγ (= γ / N) due to the optical rotatory power.
It is considered to rotate by an angle. δ is a phase delay when the length is L. The phase delay is the phase difference between the electric field component of light in the x ′ direction and the electric field component of light in the y ′ direction.

The polarization state of light is represented by a Jones vector, and is represented by a matrix showing a phase delay element consisting of a thin element of ΔL and a matrix showing an optical rotatory power. For information on how to display these, see D.Cl.
arke, JFGrainger “Polrized Light and Optical Mea
surement ”Pergamon Press. B
The Jones vector of the incident light H _{1 on} the i ₁₂ GeO ₂₀ single crystal is

[0022]

[Equation 1] Is expressed as The Jones vector of the emitted light H ₂ from the Bi ₁₂ GeO ₂₀ single crystal is

[0023]

[Equation 2] Is expressed as On the other hand, the Jones vector of the emitted light H ₃ from the analyzer 9 is

[0024]

[Equation 3] Is expressed as The phase delay element is a coordinate system of principal axes x'and y ',

[0025]

[Equation 4] Is expressed as

[0026]

[Equation 5] Is expressed as The matrix for converting from the xy coordinate system to the x'-y 'coordinate system of the main axis is

[0027]

[Equation 6] Therefore, the emitted light H ₂ of the Bi ₁₂ GeO ₂₀ single crystal is

[0028]

[Equation 7] Becomes The analyzer 9 has a coordinate system in which the direction of the transmitting surface is x _A and the direction perpendicular to this is y _A.

[0029]

[Equation 8] It can be expressed as. On the other hand, from the x-y coordinate system, x _A -y
The transformation matrix to the _A coordinate system is

[0030]

[Equation 9] Therefore, the output light H ₃ of the analyzer 9 is

[0031]

[Equation 10] Becomes The intensity E of the emitted light H _{3 from} the analyzer 9 is E
_xA ² ＋ E _yA ² Will be proportional to.

According to the above calculation method, when the optical rotatory power is kept constant at 50 ° and the phase difference δ occurs at the applied voltage V, Bi ₁₂
When the angle from the x axis of the elliptical main axis direction of the polarized light by the GeO ₂₀ single crystal was calculated, the results shown in the table below were obtained.

[0033]

[Table 1]

As can be seen from this table, the direction of the principal axis of the polarization ellipse in the Bi ₁₂ GeO ₂₀ single crystal is in the range of 25.0 ° to 25.1 ° even if the applied voltage V changes and δ changes.
It doesn't change much.

On the other hand, when δ = 10 ° and γ = 50 °, the direction of the transmitting surface of the analyzer 9 is changed within the range of 0 to 90 ° with respect to the X axis, and the emitted light H ₃ of the analyzer 9 is changed. When the position where the strength is maximized was obtained, it was 25 ° as shown in FIG.
It was found that they coincided with the principal axis of the polarization ellipse in the i ₁₂ GeO ₂₀ single crystal.

Next, assuming that the optical rotatory power changes with a change in ambient temperature, γ in the range of γ = 50 ° to γ = 55 °
When the fluctuation of the sensor output was examined by changing the value of, the results shown in FIG. 5 were obtained. In the case of Bi ₁₂ GeO ₂₀ single crystal, when the ambient temperature changes from 25 ° C. to 70 ° C., there is a change (Δγ _t ) in optical rotation of about 2 °. Therefore, when the ambient temperature changes from 25 ° C. to 70 ° C., the intensity of the light emitted from the analyzer 9, that is, the change in the sensor output is about 0.15%.

Since the change in γ appears as a change in the principal axis of the polarization ellipse of the outgoing light H ₂ , cos is obtained under the condition that the principal axis coincides with the transmitting surface of the analyzer 9, that is, the condition of this embodiment.
The intensity of the emitted light H ₃ in proportion to 2 [Delta] [gamma] _t is changed. Then, when Δγ _t is sufficiently smaller than 1, the intensity change becomes the smallest.

[0038]

As described above, according to the present invention,
It is possible to reduce the influence of the change in the optical rotatory power due to the change in the ambient temperature without reducing the measurement sensitivity.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an optical voltage sensor according to an embodiment of the present invention,

FIG. 2 is a diagram for explaining a direction of a transmission surface of an analyzer in the sensor,

FIG. 3 is a diagram for explaining a calculation method which is the basis of the present invention;

FIG. 4 is a diagram showing a calculation result of sensor output fluctuation with respect to a transmission surface angle of an analyzer;

FIG. 5 is a diagram showing calculation results of sensor output fluctuations with respect to changes in optical rotation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Driving circuit, 2 ... LED, 3 ... Optical fiber, 4 ... Measurement optical system, 6 ... Polarizer, 7 ... 1/4 wavelength plate, 8 ... Electro-optical element, 9 ... Analyzer, 11a, 11b ... Transparent electrode , 12 ... Optical fiber, 13 ... Photodiode, 14 ... Amplifier, 15 ... Differential amplifier, 16 ... Low-pass filter.

Claims (1)

[Claims] 1. The optical fiber is passed through a path consisting of a polarizer, an electro-optical element whose birefringence magnitude changes in response to an applied voltage, and an analyzer after the light guided through the optical fiber is passed through the optical fiber. In an optical voltage sensor having a phase element interposed in the path between the polarizer and the analyzer while being guided to a light receiving system via an ellipse indicating a polarization state when the light passes through the electro-optical element. The optical voltage sensor, wherein the analyzer is provided such that its transmission surfaces are aligned with each other in the main axis direction.