JPH10132866A - Optical voltage-electric field sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical voltage - electric field sensor capable of improving temperature characteristics and accurately measuring over the use temperature range by optimizing the relationship of optical parts. SOLUTION: By making the deviation direction from the reference phase difference 90 degrees of a λ/4 plate 3 constant, the position of an analyzer 5 is controlled according to the deviation direction. That is, in the case the analyzer 5 is arranged at the reference position where the modulation sensitivity becomes maximum and the error of direct current component of the output of the analyzer 5 reduces compared with the reference phase difference of 90 degrees, the analyzer 5 is arranged within the angle of ca. 30 degrees turning leftward from the upstream to the downstream of the light progression direction. On the other hand, in the case the error of the direct current component increases compared with the reference phase difference of 90 degrees, the analyzer 5 is arranged within the angle of ca. 30 degrees turning rightward from the upstream to the downstream of the light progression direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical voltage / electric field sensor for measuring the strength of a voltage or an electric field using an electro-optical element such as a Pockels element.

[0002]

2. Description of the Related Art A voltage transformer is widely used for measuring a voltage of a power system. However, there is a problem in that the voltage transformer becomes larger as the system voltage to be measured becomes higher, resulting in an increase in cost and space. In particular, in a gas insulated switchgear using an inert gas called GIS, downsizing and space saving are strongly required, and it is difficult to mount such a voltage transformer.

[0003] For this reason, an optical voltage sensor using an electro-optical element such as a Pockels element has been used conventionally. 20 and 21 show a configuration of a typical conventional optical voltage sensor.

As shown in FIG. 20, the optical voltage sensor 80 includes a sensor section 50 composed of optical components and a light emitting circuit 6.
0 and a light receiving circuit 70 as a signal processing circuit. The sensor unit 50 is disposed in connection with the switchgear, and the remaining light emitting circuit 60 and the light receiving circuit 70 are disposed in a substation building separated from the sensor unit 50. Are connected by an optical fiber (not shown).

[0005] As shown in FIG.
Is a collimator 51 that converts light from the light emitting circuit 60 introduced through an optical fiber into parallel light, a polarizer 52 that converts the random parallel light into linearly polarized light, and λ / Four plates 53 and a Pockels device 5 that performs phase modulation according to an applied voltage to convert the circularly polarized light into elliptically polarized light.
4, an analyzer 55 that extracts only a component of the elliptically polarized light from the Pockels element 54 in a predetermined polarization direction, and a collimator 56.

A transparent electrode (not shown) is provided on the surface of the Pockels element 54, and a voltage to be measured is applied in the light traveling direction. For example, the Pockels element 54 has a stray capacitance formed by a charging conductor of a power system, a floating conductor arranged in parallel with the charging conductor, and a capacitor provided between the floating conductor and a ground potential. Thus, the voltage of the charging conductor is divided and applied. If the Pockels element 54 is arranged in an electric field without providing an electrode on the surface thereof, it is possible to measure the intensity of the electric field in the traveling direction of light, and it becomes an optical electric field sensor. Here, the description will be continued as an optical voltage sensor.)

Each optical component of the sensor unit 50 is usually
It has the following positional relationship. In order to explain this positional relationship, as shown in FIG. 21, a rectangular coordinate system including an x-axis, a y-axis, and a z-axis is considered, and each crystal axis of the electro-optic crystal used as the Pockels element 54 is represented by the above-mentioned x. Axis, y
A description will be given of a case where the crystal is arranged so as to match the axis and the z-axis (in the case of a uniaxial crystal, the optical axis is matched to the z-axis).
Note that the z-axis direction is the traveling direction of light.

Generally, as shown in FIG. 1, the c-axis (optical axis) of the λ / 4 plate 53 is inclined by 45 ° with respect to the x-axis, and the polarization direction of the polarizer 52 is the c-axis. Are arranged on the x-axis inclined at 45 ° with respect to. Then, the polarization direction of the analyzer 55 is arranged in parallel with the polarization direction of the polarizer 52 (so-called parallel polarizer) as shown in FIG.
Are arranged so that the polarization direction of the polarizer 52 is orthogonal to the polarization direction of the polarizer 52 (so-called orthogonal polarizer).

The light modulated by the sensor unit 50 is shown in FIG.
As shown at 0, the light is input to the light receiving circuit 70 via an optical fiber. In the light receiving circuit 70, the light output from the sensor unit 50 is subjected to light / electric conversion by a photodiode (abbreviated PD) 71. An output from the photodiode 71 is amplified by an amplifier 72 and then a high-pass filter (HPF) 73.
And a low-pass filter (abbreviated LPF) 74. The output from the high-pass filter 73 is divided by the divider 75 with the output from the low-pass filter 74 and output.

Therefore, the output of the divider 75 is a value obtained by dividing the AC component AC extracted by the high-pass filter 73 by the DC component DC extracted by the low-pass filter 74. Eliminating the effect of light quantity loss due to changes in the light receiving sensitivity of the photodiode 71 and changes in the transmission path loss of optical fibers and the like,
Accurate measurement of system voltage can be performed.

[0011]

In the optical voltage sensor 1 configured as described above, the input / output relationship of the λ / 4 plate 53 and the Pockels element 54 can be expressed by the following equation (1).

I = (Io / 2) ｛1 ± Sin (Γv + δ)｝ (1) where Io is the amount of light after passing through the polarizer 52, Γv is a phase difference caused by the Pockels effect, δ is the amount of shift from 90 ° of the phase difference of the light decomposed when passing through the λ / 4 plate 4. I is the analyzer 5
5, which is the amount of light emitted from the
When it is a component, the term of sin (Γv + δ) becomes +, and when it is an S component which is reflected light, sin (Γv
The term + δ) becomes-(in the example of FIG. 21, transmitted light is output from the analyzer 55). Here, the sin (Γ
When (v + δ) is small (≪1), Equation 1 can be approximated as Equation 2.

I = (Io / 2) {1 ± ({v + δ)} (2) By the way, the λ / 4 plate 53 has a temperature coefficient βa, and a working temperature range (reference) of the optical voltage sensor 50. If the difference from the temperature is Ta, and the phase difference after the shift according to the temperature is δa, δa = δo (1 + βa · Ta) (3) Here, δo is the phase difference at the reference temperature.

For example, the temperature coefficient β of the λ / 4 plate 54
a is −1.014 × 10 ⁻⁴ (° / ° C.), the operating temperature width is 100 (° C.) from −20 ° C. to 80 ° C., and δo = π
/ 2 = 90 (°) and substituting these values into the above equation (3), δa = 90 {1 + (− 1.014 × 10 ⁻⁴ ) × 100}
= 89 (°), which means that a shift of δ = 1 (°) has occurred.

The phase difference in the λ / 4 plate 53 is 90 °
, The light emitted from the λ / 4 plate 53 becomes elliptically polarized light. As a result, it is not known whether the change in the light amount of the output light from the analyzer 55 is due to the Pockels effect, that is, due to Δv in the above equation (1) or due to the phase difference shift amount δ of the λ / 4 plate 53. The voltage applied to the Pockels element 54 cannot be measured accurately.

The present invention has been made in view of the above, and an object of the present invention is to improve the temperature characteristics of a sensor by optimizing the positional relationship between optical components, and to perform high-precision measurement over the operating temperature range. To provide an optical voltage / electric field sensor capable of performing the following.

[0017]

According to a first aspect of the present invention, there is provided an optical voltage / electric field sensor which includes a polarizer (for example, a polarization beam splitter), a wave plate (for example, an optimal optical bias) along a traveling direction of light. A functioning λ / 4 plate), an electro-optic crystal, and an analyzer (for example, a polarizing beam splitter) are sequentially arranged, and the following measures are taken to solve the above-mentioned problem. It is characterized by.

First, the wave plate is set such that the direction of deviation from the reference phase difference at the reference temperature is always constant within the operating temperature range. Further, the position of the analyzer is adjusted as follows. A case where the analyzer is arranged at a reference position where the modulation sensitivity determined by the positional relationship between the analyzer and the electro-optic crystal is maximized. A) The detection is performed by shifting the phase difference of the wave plate in the setting direction. When the DC component of the photon output is smaller than that at the time of the reference phase difference, the analyzer falls within an angle range of about 30 ° counterclockwise from upstream to downstream in the light traveling direction from the reference position. B) when the DC component of the analyzer output increases as compared with the reference phase difference due to the shift of the phase difference of the wave plate in the above-described setting direction, the light traveling direction from the reference position. The analyzer is arranged in a clockwise direction within an angle range of about 30 ° from upstream to downstream.

The electro-optical crystal is Bi ₄ G
a crystal having no optical rotation such as e ₃ O ₁₂ or LiNbO ₃ ;
There is a crystal having optical rotation such as ₁₂ GeO ₂₀ or Bi ₁₂ SiO _20, but a crystal having optical rotation can be used as well as a crystal having no optical rotation.

To keep the direction of the phase difference change of the wave plate constant within the operating temperature range as described above, it is _{necessary to reduce the} temperature below the minimum temperature T _{bott in} the operating temperature range T _{bott to} T _top or above the maximum temperature. Can be realized by setting the wavelength plate to have a reference phase difference under the temperature condition using the reference temperature as a reference temperature.

When the phase difference of the wave plate shifts due to the temperature change, it appears as a change in the DC component (optical bias amount) of the light output from the analyzer, and ac changes with the change in the optical bias amount. Changes in components also occur. There is a certain correlation between the error of the DC component and the AC component (the ratio error of each component in the reference phase difference) and the position where the analyzer is arranged (the rotation angle from the reference position where the modulation sensitivity is maximum). Have a relationship.

The applicant of the present invention adjusts the position of the analyzer according to the shift direction of the wave plate if the shift direction of the phase difference of the wave plate is fixed as described above, thereby obtaining the error of the AC component. And the difference between the DC component error and the DC component error can be reduced. In the signal processing of the sensor output, the value obtained by dividing the AC component by the DC component (AC / D
C) is used, but the error of this value is approximated as the difference between the error of the AC component and the error of the DC component. Therefore, by adjusting the position of the analyzer according to the shift direction of the phase difference, As a result, the temperature characteristics can be improved.

There are a plurality of analyzer reference positions at which the modulation sensitivity can be maximized. That is, if the modulation sensitivity is maximized by placing the analyzer at a certain position, ±
Even at a position where the analyzer is rotated by 90 ° × n (n is an integer) around the light traveling direction, the modulation sensitivity also becomes maximum.

When the phase difference of the wave plate is changed (increased or decreased) in a certain direction at a certain reference position of the analyzer, a positive or negative offset occurs in the ratio error of the DC component. As in a) above, when the DC component of the analyzer output is reduced as compared with the reference phase difference due to the shift of the phase difference of the wave plate in the set direction (that is, the ratio error of the DC component is −
), When the analyzer is rotated counterclockwise from the upstream to the downstream in the light traveling direction from the reference position,
The AC / DC ratio error is reduced. On the other hand, as in b) above, when the DC component of the analyzer output when the phase difference of the wave plate is shifted in the above-described setting direction increases as compared with the reference phase difference (that is, the DC component of the When the ratio error is +), when the analyzer is rotated clockwise from the reference position toward the downstream from the upstream in the traveling direction of the light, AC
/ DC ratio error is reduced. Considering that the required sensitivity cannot be obtained if the position deviates too much from the position of the maximum sensitivity, it is necessary to consider that the position is approximately 30
It is preferable that the rotation be made within the range of °.

As described above, since the temperature characteristics of the sensor can be improved by optimizing the positional relationship between the optical components, highly accurate measurement can be performed over the operating temperature range without increasing new components. It can be carried out.

According to a second aspect of the present invention, there is provided an optical voltage / electric field sensor according to the first aspect of the present invention, wherein the electro-optic crystal has an optical rotation, and the reference position of the analyzer is the same as that of the analyzer. The polarization direction of the electro-optic crystal was tilted with respect to a crystal axis orthogonal to the light traveling direction in the same direction as the direction in which the polarization plane rotates due to the optical rotation of the crystal by an angle of approximately half the optical rotation angle. It is characterized by the position.

When an electro-optic crystal having optical rotation is used, the above-described reference position (the position where sensitivity is maximum) is different from a crystal having no optical rotation. When considering this reference position, it is necessary to consider at what angle the characteristic axis of the polarized light (that is, the main axis of the refractive index ellipsoid) at the light emission surface of the electro-optic crystal exists.

When light passes through the electro-optic crystal, the light is decomposed into linearly polarized light that oscillates in two orthogonal directions. The characteristic axis of the above-mentioned polarized light coincides with the decomposition direction of the light.
This axis corresponds to two main axes orthogonal to the traveling direction of light in the index ellipsoid. In an electro-optic crystal having optical rotation, compared to an electro-optic crystal having no optical rotation, the polarization of the polarization at the light exit surface is substantially the same as the direction in which the polarization plane rotates due to the optical rotation, and is approximately half the angle of the optical rotation. It is considered that the unique axis is rotating (details are described in the section of the embodiment).

Here, if the analyzer (polarization direction) is arranged so as to be parallel to the intrinsic axis of the polarized light on the light exit surface of the electro-optic crystal, the sensitivity becomes minimum (almost 0). If the analyzer is arranged at a position rotated about the light traveling direction by an angle of 45 ° ± 90 ° × n (n is an integer) from the eigen axis of, the modulation sensitivity becomes maximum.

Therefore, each crystal axis of the electro-optic crystal is defined as x
Considering the case where light is incident in the z-axis direction when the axis, the y-axis, and the z-axis (each axis is orthogonal to each other), when the electro-optic crystal having no optical rotation is used, the polarization direction of the analyzer is changed to the electro-optic crystal. When the electro-optic crystal having optical rotation is used, the polarization direction of the analyzer is changed as described above because the modulation sensitivity is maximized when the crystal axis (x-axis or y-axis) is matched with the crystal axis of A crystal axis (x) orthogonal to the light traveling direction in the optical crystal
(Or the y-axis), the modulation sensitivity is maximized if the crystal is tilted by the optical rotation of the crystal in the same direction as the direction of rotation of the polarization plane by approximately half the angle of rotation.

An electro-optic crystal having optical rotation is used by determining the reference position of the analyzer as described above and adjusting the position of the analyzer according to the shift direction of the phase difference. In this case, the temperature characteristics of the sensor can be improved.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1] An embodiment of the present invention will be described below with reference to FIGS.

In the present embodiment, an optical voltage sensor using a Pockels element as an electro-optical effect element will be described as an example.

As shown in FIG. 2, the optical voltage sensor 10
A sensor unit 11 including optical components, a light emitting circuit 12, and a light receiving circuit 13 as a signal processing circuit are provided. The sensor unit 11 includes a collimator 1, a polarizer 2, a λ / 4 plate 3 (wave plate), a Pockels element 4, an analyzer 5, and a collimator 6, which are arranged in the casing 7 in order from the light incident side.
It has. Incidentally, the polarizer 2 and the analyzer 5 can be constituted by a polarizing beam splitter, and any of the transmitted light or the reflected light may be used. When the reflected light is used,
The collimator 1 and the collimator 6 are provided on the upper or lower part of the casing 7.

An optical fiber 8a for guiding light from the light emitting circuit 12 into the casing 7 is connected to the collimator 1. The incident light guided to the optical fiber 8a becomes parallel light by passing through the collimator 1. Although the direction of polarization of the parallel light is random, it becomes linearly polarized light by the polarizer 2. This linearly polarized light is the optical axis of the λ / 4 plate 3 (c
Incident at an angle of 45 ° to the
The light passing through the / 4 plate 3 becomes circularly polarized light. Then, this circularly polarized light enters the Pockels element 4.

The Pockels element 4 has a pair of transparent electrodes (not shown) that do not hinder the transmission of light on the light incident surface and the light exit surface of the electro-optic crystal. A pair of transparent electrodes of the Pockels element 4 are connected to a pair of electrodes 9 provided on an outer wall surface of the casing 7 through lead wires 9.
a. 9a, and by applying a voltage to be detected to the electrodes 9a.
An electric field is applied in the light traveling direction. Then, the Pockels element 4 performs phase modulation according to the applied voltage (causing a phase difference according to the applied voltage to linearly polarized light whose vibration planes propagating in the crystal are orthogonal to each other), and converts the circularly polarized light to elliptically polarized light. To

The elliptically polarized light having an ellipticity (ratio between the major axis and the minor axis) corresponding to the applied voltage and phase-modulated by the Pockels element 4 enters the analyzer 5. The analyzer 5 extracts only a component in a predetermined polarization direction and outputs linearly polarized light having a light intensity according to the ellipticity of the elliptically polarized light (that is, according to the voltage applied to the Pockels element 4). Then, the output light of the analyzer 5 is input to the light receiving circuit 13 via the collimator 6 and the optical fiber 8b.

In the light receiving circuit 13, the output light of the sensor section 11 is subjected to light / electric conversion by the photodiode 14. The output from the photodiode 14 is amplified by an amplifier 15 and then output to a high-pass filter 16 and a low-pass filter 1.
7 are commonly input. The output from the high-pass filter 16 is supplied to a divider 18 by a low-pass filter 1.
7 and output. Therefore, the output of the divider 18 is a value (AC / DC) obtained by dividing the AC component AC extracted by the high-pass filter 16 by the DC component DC extracted by the low-pass filter 17.
In the AC / DC method, it is possible to eliminate the influence of light amount loss due to a change in the amount of light emitted from the light emitting circuit 12, a change in light receiving sensitivity of the photodiode 14, and a change in transmission line loss of the optical fibers 8a and 8b.

In the sensor section 11 having the above-described configuration, the modulation sensitivity of the optical voltage sensor 10 changes depending on the positional relationship between the Pockels element 4 and the analyzer 5 as described later, and these have a certain fixed positional relationship. Sometimes the modulation sensitivity is at a maximum.

The electro-optic crystal used as the Pockels device 4 includes a crystal having no optical rotation (Bi ₄ Ge ₃
O _12, and LiNbO _3, etc.), there is a crystal having a spontaneous optical rotation (Bi ₁₂ GeO ₂₀ or Bi ₁₂ SiO _20, etc.), the positional relationship between the Pockels element 4 modulation sensitivity is maximized and the analyzer 5, It depends on the presence or absence of the optical rotation of the Pockels element 4.

However, regardless of whether the Pockels element 4 has optical rotation or not, the analyzer 5 is moved from the position where the modulation sensitivity is maximized to a predetermined direction centered on the traveling direction of the light by a predetermined angle. By rotating the optical voltage sensor 10 only by this amount, the temperature characteristics of the optical voltage sensor 10 can be improved.

Here, the Pockels device 4 having no optical rotation will be described first.

For example, a Bi ₄ Ge ₃ O ₁₂ crystal is a crystal of the crystal point group T _d (43 m of international symbol) and belongs to an optically isotropic equiaxed crystal system. The LiNbO ₃ crystal belongs to the trigonal system of the crystal point group C _3v (international symbol 3 m),
It is a uniaxial crystal. These electro-optical crystals forming the Pockels element 4 are centered so that the crystal surface and the crystal axis are parallel.

Here, in a rectangular coordinate system composed of an x-axis, a y-axis, and a z-axis, it is assumed that the crystal axes of the Pockels element 4 match the directions of the x-axis, the y-axis, and the z-axis of the rectangular coordinate system, respectively. For example, each crystal axis of Bi ₄ Ge ₃ O ₁₂ crystal <100
>, <010>, and <001> are each represented by x in the rectangular coordinate system.
Align with the axis, y-axis, and z-axis.

The surface of the electro-optic crystal has z
Light from a direction parallel to the axis is perpendicularly incident, and an electric field E (0, 0, E _z ) is applied to the crystal in the traveling direction of the light according to the applied voltage of the object to be measured.

The λ / 4 plate 3 is arranged such that its c-axis (optical axis) and a-axis are parallel to the surface of the wave plate crystal, and the c-axis is inclined by 45 ° with respect to the x-axis. . The polarizer 2 is arranged so that its polarization direction is parallel to the x-axis direction, whereby the linearly polarized light that has passed through the polarizer 2 is aligned with the optical axis (c-axis) of the λ / 4 plate 3. Incident at an angle of 45 °.

The polarization direction of the analyzer 5 is parallel to the x-axis as shown in FIG. 3 (when the polarization direction of the polarizer 2 is also parallel to the x-axis, a so-called parallel polarizer) or not shown. By setting the polarization direction parallel to the y-axis (a so-called orthogonal polarizer when the polarization direction of the polarizer 2 is parallel to the x-axis), the modulation sensitivity is maximized. That is, when the Pockels element 4 having no optical rotation is used, the modulation sensitivity is maximized by setting the positional relationship such that the crystal axis of the Pockels element 4 and the polarization direction of the analyzer 5 match.

As shown in FIG. 1, when the phase difference 90 ° at the reference temperature in the λ / 4 plate 3 decreases (eg, 90 ° → 89 °) due to a temperature change, the analyzer 5
Is rotated by an angle α in a predetermined direction, which will be described later, from a position where the above-described modulation sensitivity is maximum, whereby the temperature characteristics of the optical voltage sensor 10 can be improved. Further, when the phase difference 90 ° at the reference temperature in the λ / 4 plate 3 increases due to the temperature change (for example, 90 ° → 91 °),
The temperature characteristic of the optical voltage sensor 10 can be improved by rotating the analyzer 5 from the position where the modulation sensitivity becomes the maximum by the angle α in the opposite direction to the above. The angle α is preferably in a range of about 30 ° or less as described later, more preferably in a range of about 20 ° ± 10 °, and most preferably in a range of about 10 ° to about 20 °. .

Here, when the direction of rotation is indicated, it indicates the direction when viewed from upstream to downstream in the light traveling direction. Further, rightward (clockwise) rotation is described as positive (+), and leftward (counterclockwise) rotation is described as negative (−).

The reason why the temperature characteristics are improved by adjusting the position of the analyzer 5 as described above will be described below.

In the sensor section 11 having the configuration of the parallel polarizer as shown in FIG. 3, it is assumed that a change of ± Δ occurs in the phase difference 90 ° at the reference temperature of the λ / 4 plate 3 due to the temperature change. Consider what the AC component and the DC component of the light emitted from the unit 11 will be like. As described above, the incident light I _{0 that} has passed through the polarizer 2 is separated into an I _c component that vibrates in the _c- axis direction and an I _a component that vibrates in the a-axis direction when passing through the λ / 4 plate 3. When the light exits the λ / 4 plate 3, a phase difference of 90 ° is given, and the emitted light should be circularly polarized light (see a1 in FIG. 4).

However, when the phase difference at the reference temperature of the λ / 4 plate 3 shifts from 90 ° by ± Δ due to the temperature change, as shown by reference numerals a2 and a3 in FIG. Is elliptically polarized light having a major axis or a minor axis on the x-axis. When a voltage is applied to the Pockels element 4 in this state, amplitude modulation is performed so that the amplitude of light emitted from the analyzer 5 becomes maximum at a position where the analyzer 5 is placed on the x-axis (or on the y-axis).

In the above description, the deviation Δ from the phase difference of 90 ° at the reference temperature of the λ / 4 plate 3 is set to 1 °, and the angle between the polarization direction of the analyzer 5 and the x-axis is variously changed ( + 30 °, + 20 °, + 15 °, 0 °, -15 °,-
20 °, -30 °), and A
We calculated what the C and DC components would look like. When the analyzer 5 is rotated by ± 45 ° from the position where the sensitivity is maximum, the sensitivity becomes substantially 0 as described later. Therefore, here, the rotation angle of the analyzer 5 is set in a range of −45 ° to + 45 °. Think.

Table 1 shows the above calculation results. Note that the values in the table are calculation results when the modulation factor m = 2 (%) is fixed. The position of the analyzer 5 is 0 ° and λ
When the phase difference obtained by combining the phase difference of the た plate 3 and the phase difference due to the Pockels effect of the Pockels element 4 is 90 °, the amount of light emitted from the analyzer 5 is set to 1. The AC component is the difference between the maximum value and the minimum value of the amplitude of the emitted light amount,
The C component is obtained by adding the maximum value and the minimum value of the amplitude to 2
Divided by. The ratio error ε is the AC component, the DC component, and the AC / D when the phase difference at the reference temperature is 90 °.
Assuming that C is a reference value a and those values when the phase difference changes are b, ε = {(ab) / a} × 100 (4)

[0055]

[Table 1]

Incidentally, the temperature change is caused by the Pockels element 4
The change in the phase difference of the λ / 4 plate 3 is extremely slow as compared with the change in the voltage applied to the λ / 4 plate, and basically appears as a change in the DC component. However, as the phase difference (optical bias amount) of the λ / 4 plate 3 changes, the AC component also changes as shown in Table 1 above. Then, the amount of change of the AC component (that is, the ratio error) also changes depending on the position (angle) of the analyzer 5.

As an example, the phase difference of the λ / 4 plate 3 is 90 °
Looking at the case where the angle of the analyzer 5 changes from 91 ° to 90 °, as shown in Table 1 above, when the analyzer 5 is at a position of 0 °, the ratio error of the AC component is about + 0.9%. Is set to −15 °, the ratio error of the AC component is very small at about −0.3% even with the same phase difference change of 1 °.

Based on the results in Table 1 above, the phase difference at the reference temperature of the λ / 4 plate 3 is changed from 90 ° to 8
FIG. 5 is a graph showing the relationship between the angle of the analyzer 5 and the ratio error when the angle is changed to 9 °. FIG. 6 shows a case where the phase difference changes from 90 ° to 91 °.

As is apparent from FIGS. 5 and 6, the ratio error of the AC component has a slope rising to the right irrespective of the direction in which the phase difference of the λ / 4 plate 3 changes. Further, as shown in FIG.
The phase difference of the / 4 plate 3 changes in the minus direction (90 ° → 89
In the case of (°), the ratio error of the AC component shows a negative value at the position where the analyzer 5 is at 0 ° (the position where the modulation sensitivity is maximum), and then the analyzer 5 is moved at a predetermined angle (here, about +
By rotating by 15 °), the ratio error of the AC component can be made substantially zero.

On the other hand, as shown in FIG. 6, when the phase difference of the λ / 4 plate 3 changes in the plus direction (90 ° → 91 °), the ratio error of the AC component is the position where the analyzer 5 is at the 0 ° position. (Position of maximum modulation sensitivity) indicates a positive value, from which the analyzer 5 is moved in a negative direction by a predetermined angle (here, -10 ° to -15 °).
(°), the AC component ratio error can be made substantially zero.

Therefore, the AC of the light emitted from the sensor 11
In the case of the method of obtaining the measured value of the applied voltage from the components, the analyzer 5 is set in accordance with the characteristics of the λ / 4 plate 3 (characteristics in which the phase difference changes in the plus or minus direction) as described above. By rotating the λ / 4 plate 3 in a predetermined direction from the position where the modulation sensitivity is maximum, the influence of the change in the phase difference of the λ / 4 plate 3 can be suppressed, and the temperature characteristics of the sensor can be improved.

Further, in the case of the AC / DC system (see FIG. 2) which can suppress the fluctuation of the light quantity loss on the transmission path,
It is necessary to consider the ratio error of the DC component.

First, considering the AC / DC ratio error, the ratio error ε is given by the above equation (4), and the sensor output is A
When processed by C / DC, the variation of the AC component is Δ
a, if the variation of the DC component is Δd, ε = [｛(AC + Δa) / (DC + Δd)} − (AC / DC)] / (AC / DC) = ｛(Δa / AC) − (Δd / DC) {/ {1+ (Δd / DC)}} (5) If 1+ (Δd / DC) ≒ 1, then ε (Δa / AC) − (Δd / DC) (6) That is, ratio error of AC component−ratio error of DC componentＤＣratio error of AC / DC. Therefore, AC / D
In the case of the C system, if the ratio error of the AC component and the ratio error of the DC component are made substantially the same, both errors are canceled and the λ / 4 plate 3
Can be suppressed. 5 and 6 from this viewpoint.

As is clear from FIGS. 5 and 6, with respect to the ratio error of the DC component, when the phase difference of the λ / 4 plate 3 changes in the minus direction (decreases from 90 ° to 89 °), a positive offset occurs. The phase difference changes in the positive direction (90 °
When it increases to → 91 °), a negative offset occurs. The ratio error of the AC component is as described above. Therefore, as shown in FIG. 5, when the phase difference of the λ / 4 plate 3 changes in the negative direction (90 ° → 89 °), the analyzer 5 Is rotated in the plus direction from the position of 0 ° (the position of the maximum modulation sensitivity), the difference between the ratio error of the AC component and the ratio error of the DC component decreases, and the ratio error of AC / DC can be reduced. it can.

On the other hand, as shown in FIG. 6, when the phase difference of the λ / 4 plate 3 changes in the plus direction (90 ° → 91 °),
If the analyzer 5 is rotated in the minus direction from the position of 0 ° (the position of the maximum modulation sensitivity), the difference between the ratio error of the AC component and the ratio error of the DC component is reduced, and the ratio error of the AC / DC is reduced. can do.

When the analyzer 5 is rotated in a direction opposite to the above, the difference between the ratio error of the AC component and the ratio error of the DC component increases, and the temperature characteristics deteriorate.
The setting of the direction of rotation is very important.

Regarding the angle of rotation of the analyzer 5, considering that the error due to the temperature change is suppressed and that the required sensitivity cannot be obtained if the position deviates too much from the position of the maximum sensitivity, the maximum sensitivity is obtained. It is preferable to rotate within a range of about 30 ° from the position. In addition, the position with the maximum sensitivity (0 °) and the position with the minimum sensitivity (+ 45 ° or −45 °).
(°), the effect of improving the temperature characteristics is high at a position (approximately 20 °).
It is more preferable to rotate the analyzer 5 within the range of 0 °. When the analyzer 5 is rotated in the range of about 10 ° to about 20 °, the sensitivity is relatively small and the effect of improving the temperature characteristics is high. Therefore, it is most preferable to set the angle in this range.

Of course, the rotation direction and the rotation angle of the analyzer 5 are not limited to the case where the deviation of the phase difference of the λ / 4 plate 3 is ± 1 °. The same can be said if the direction of change of the phase difference of No. 3 is the same as above.

Although the above description is based on the position (0 °) where the analyzer 5 is placed on the x-axis, the case where the analyzer 5 is placed on the y-axis rotated ± 90 ° therefrom (orthogonal polarization) In this case, the modulation sensitivity also becomes maximum. Then, next, in the sensor unit 11 having the configuration of the orthogonal polarizer, the phase difference at the reference temperature of the λ / 4 plate 3 is 90 °
Assuming a case where a change of ± Δ occurs, what is the AC component and DC component of the light emitted from the sensor 11 will be considered. Similarly to the above, the shift amount Δ is set to 1 °, and the angle between the polarization direction of the analyzer 5 and the y-axis is variously changed (+30).
°, + 20 °, + 15 °, 0 °, -15 °, -20 °,
−30 °), and the AC component of the light emitted from the sensor unit 11,
The DC component and the AC / DC ratio error were calculated in the same manner as described above. Table 2 shows the calculation results.

[0070]

[Table 2]

Based on the results in Table 2 above, the phase difference at the reference temperature of the λ / 4 plate 3 from 90 ° to 8
FIG. 7 is a graph showing the relationship between the angle of the analyzer 5 and the ratio error when the angle is changed to 9 °. FIG. 8 shows the case where the phase difference changes from 90 ° to 91 °.

When FIG. 7 based on the orthogonal polarizer is compared with FIG. 6 based on the parallel polarizer, one of the directions of change in the phase difference is a decreasing direction and the other is an increasing direction. It can be seen that the characteristics are substantially the same. This can be easily understood from the following description.

That is, the phase difference of the λ / 4 plate 3 is 90 ° +
When it increases to Δ, the major axis of the elliptically polarized light emitted from the λ / 4 plate 3 and when the phase difference decreases to 90 ° -Δ, λ /
The major axis of the elliptically polarized light emitted from the four plates 3 differs by 90 ° as shown in FIG. When the optical bias amount changes in the direction in which the phase difference of the λ / 4 plate 3 increases as shown in FIG. 6, elliptically polarized light having a long axis on the x-axis is output from the λ / 4 plate 3 as shown in FIG. Then, in the case of a parallel polarizer, the analyzer 5 (its polarization direction) is arranged on the major axis of the elliptically polarized light (reference elliptically polarized light that enters the analyzer 5 when the applied voltage is 0 V). Become. On the other hand, when the optical bias amount changes in the direction in which the phase difference of the λ / 4 plate 3 decreases as shown in FIG. 7, the elliptically polarized light having a short axis on the x-axis and a long axis on the y-axis becomes 3
However, in the case of the orthogonal polarizer, since the analyzer 5 (its polarization direction) is on the y-axis, the analyzer 5 is also arranged on the long axis of the reference elliptically polarized light. Become. Therefore, the characteristics of FIG. 6 and FIG. 7 are substantially similar.

When FIG. 8 based on the orthogonal polarizer is compared with FIG. 5 based on the parallel polarizer, the phase difference changes in the opposite directions, but shows substantially the same characteristics. ing.
This can be easily understood for the same reason as described above.

To summarize the above description, in order to improve the temperature characteristics, the rotation direction of the analyzer 5 from the position where the modulation sensitivity is maximum depends on the direction of the change in the phase difference of the λ / 4 plate 3. It is necessary to determine the position of the analyzer 5 at which the modulation sensitivity is maximized.
0 °, 270 °), 0 ° or 180 ° with respect to the x-axis
Or the position of 90 ° or 270 ° as a reference, the rotation direction of the analyzer 5 from the position of the maximum modulation sensitivity as the reference is determined.

More specifically, this is a case where the analyzer 5 is placed at the position (reference position) where the modulation sensitivity is maximum, and the DC of the output of the analyzer 5 is changed due to the shift of the phase difference of the λ / 4 plate 3 due to the temperature change.
When the ratio error of the components is negative, when the analyzer is rotated in the minus direction (counterclockwise) from the reference position, A
The C / DC ratio error is reduced, and the temperature characteristics can be improved. Further, when the ratio error of the DC component of the output of the analyzer 5 becomes + due to the shift of the phase difference of the λ / 4 plate 3 due to the temperature change, the detection is performed in the plus direction (clockwise direction) from the reference position. Rotating the photons reduces the AC / DC ratio error and improves temperature characteristics.

By the way, as a characteristic of the λ / 4 plate 3, the direction deviating from the phase difference at the reference temperature must always be constant, but this can be achieved by designing the λ / 4 plate 3 as follows. Become.

That is, the phase difference δa of the λ / 4 plate 3 is calculated by using the temperature coefficient βa as shown in the above equation (3), δa = δo
It is represented by (1 + βa · Ta). The temperature coefficient βa is λ / 4
A value unique to the plate 3, lambda / 4 plate 3 used as a crystal positive crystal, namely a-axis direction of the refractive index n _a (= ordinary refractive index n ₀₎ to the c-axis direction of the refractive index n _c ( = if the relationship extraordinary refractive index n _e) of crystals of n _a> n _c, the temperature coefficient βa takes a negative value. When a positive crystal (n _a <n _c ) is used as the λ / 4 plate 3, the temperature coefficient βa takes a positive value.

If the temperature coefficient βa of the λ / 4 plate 3 is a negative value, the phase difference δa decreases as the temperature rises. Therefore, if the operating temperature range of the sensor unit 11 is T _{bott to} T _top , the lowest temperature If the phase difference δo is set to 90 ° under the temperature condition using a temperature _{equal to} or lower than T _bott as a reference temperature, this λ
The 板 plate 3 always has a phase difference of 90 ° or less in the operating temperature range. Further, if the phase difference δo is set to 90 ° under the temperature condition with the temperature equal to or higher than the maximum temperature T _top in the operating temperature range as the reference temperature, the λ / 4 plate 3 is always 90 ° or higher in the operating temperature range. Has a phase difference of

If the temperature coefficient βa of the λ / 4 plate 3 is a positive value, the phase difference δa increases as the temperature rises. Contrary to the above, the phase difference δa is _lower than the minimum temperature T _bott. If δo is set to 90 °, a phase difference of 90 ° or more is always maintained in the operating temperature range, and if the phase difference δo is set to 90 ° at a temperature equal to or higher than the maximum temperature T _top , the operating temperature becomes Λ / 4 plate 3 always having a phase difference of 90 ° or less in the range
Becomes In this manner, the λ / 4 plate 3 in which the direction changing from the reference phase difference of 90 ° is constant can be created.

The calculations in Tables 1 and 2 are based on the assumption that the modulation factor m is constant (m = 2%), and λ / 4
It is a simulation in which only the effect when the phase difference of the plate 3 changes is examined. Therefore, the actual output of the sensor unit 11 includes an error (mainly, a change in an AC component caused by a change in the electro-optic effect) caused by the Pockels element 4 due to a change in temperature. Is relatively large, and the rotation of the analyzer 5 causes this λ
Reducing the influence of the error of the / 4 plate 3 is very effective for improving the temperature characteristics. In order to verify this, a Bi ₄ Ge ₃ O ₁₂ crystal was used as the Pockels element 4, and a temperature characteristic test was performed with an operating temperature range of −20 ° to 60 °. The AC / DC ratio error was actually measured when the analyzer 5 was placed at 0 ° (the position at the maximum sensitivity) and when the analyzer 5 was rotated 10 ° in a direction in which the above-described temperature characteristics could be improved. At the position of 0 °, the absolute value of the maximum ratio error was 2.58, while at the position rotated 10 ° therefrom, the absolute value of the maximum ratio error was sufficiently small, 0.71.

[Embodiment 2] Another embodiment of the present invention will be described below mainly with reference to FIGS. 9 to 19. For convenience of explanation, members having the same functions as the members described in the above embodiment are denoted by the same reference numerals, and description thereof will be omitted.

In the present embodiment, as the Pockels element 4, a crystal having natural optical activity (Bi ₁₂ GeO ₂₀ or Bi ₁₂ S
The case where iO ₂₀ is used will be described.

In the case of the Pockels device 4 having optical rotation, similarly to the case without optical rotation, the analyzer 5 is moved from the position of maximum modulation sensitivity to the characteristic of the λ / 4 plate 3 (the direction in which the phase difference changes). If rotated by the angle α in the corresponding direction, the temperature characteristics can be improved. Only the position of the analyzer 5 where the modulation sensitivity is maximized differs depending on the presence or absence of the optical rotation.

First, the positional relationship between the Pockels element 4 and the analyzer 5 that maximizes the modulation sensitivity will be described.

Here, a Bi ₁₂ GeO ₂₀ crystal will be described as an example of the Pockels device 4 having optical rotatory power. Bi ₁₂ GeO ₂₀ crystal has a crystal point group T (international symbol 2).
3) The crystal belongs to the equiaxed system. The Bi ₁₂ GeO ₂₀ crystal forming the Pockels device 4 has a crystal surface and Bi ₁₂ GeO
The axis is set so that the crystal axis of O ₂₀ is parallel.

As shown in FIG. 9, the crystal axes <100> and <010 of the Bi ₁₂ GeO ₂₀ crystal forming the Pockels element 4
> And <001> are defined as x in a rectangular coordinate system, respectively.
Axis, y axis, and z axis. Bi ₁₂ G belonging to the equiaxed system
When no voltage is applied to the eO ₂₀ crystal,
The refractive index ellipsoid of the Bi ₁₂ GeO ₂₀ crystal is spherical. Principal axes of the index ellipsoid at the midpoint of the z direction of the Bi ₁₂ GeO ₂₀ crystal (X axis, Y axis, Z axis), each crystal axis parallel to the x-axis of Bi ₁₂ GeO ₂₀ crystal, y-axis, coincides with the z axis.

[0088] On the surface of the Bi ₁₂ GeO ₂₀ crystal, the light from a direction parallel to the z axis is incident perpendicularly, the applied voltage to be measured in the traveling direction of light in Bi ₁₂ GeO ₂₀ crystal A corresponding electric field E (0, 0, E _z ) is applied.

[0089] The thickness of the Bi ₁₂ GeO ₂₀ traveling direction of the crystal optical (z axis direction) b (mm), when the rotatory power of Bi ₁₂ GeO ₂₀ crystal and θ (° / mm), the Bi ₁₂ GeO ₂₀ The vibration plane of light passing through the crystal has an angle φ given by θ × b.
Rotate by (°). Hereinafter, the above φ is referred to as an optical rotation angle. The optical rotation power θ is a value specific to each crystal, and Bi ₁₂
In the case of a GeO ₂₀ crystal, the wavelength of the incident light is 800 nm to 90 nm.
At 0 nm, θ ／ 10.5 ° / mm. In this case, if the thickness b of the Bi ₁₂ GeO ₂₀ crystal in the z-axis direction is, for example, about 3 mm, the optical rotation angle φ 角 30 °, and if the thickness b is about 5.7 mm, the optical rotation angle φ ≒ 60 °. In the case of Bi ₁₂ GeO ₂₀ crystal, the direction is positive (rightward when viewed from upstream to downstream in the light traveling direction).
The light vibrating surface rotates.

The analyzer 5 is located at a position inclined with respect to the x-axis direction by about half the angle of rotation (約 φ / 2) in the same direction as the direction of rotation of the polarization plane due to the optical rotation of the crystal. (Its polarization direction) provides the maximum sensitivity. As an example, the thickness b is about 3 mm as described above.
In the case of the Bi ₁₂ GeO ₂₀ crystal, since the optical rotation angle φ ≒ + 30 °, the analyzer 5 (its polarization direction) may be inclined to the right by about 15 ° with respect to the x-axis. In addition, Bi of thickness b ≒ 5.7 mm
_In the case of a ₁₂ GeO ₂₀ crystal, the rotation angle φ ≒ +
Since it is 60 °, the analyzer 5 (its polarization direction) with respect to the x-axis
Tilt about 30 ° to the right. As described above, the modulation sensitivity can be maximized by determining the positional relationship between the Pockels element 4 and the analyzer 5 according to the optical rotation angle of the Pockels element 4.

Further, even if the positions of the polarizer 2 and the λ / 4 plate 3 with respect to the Pockels element 4 change (in other words, the polarization direction of the polarizer 2 and the c-axis of the λ / 4 plate 3 and the x-axis). ), The modulation sensitivity becomes maximum at a position where the analyzer 5 is inclined by about φ / 2 with respect to the x-axis as described above. That is, in order to maximize the modulation sensitivity, the positional relationship between the Pockels element 4 and the analyzer 5 is important.
To confirm this, the following experiment was performed.

As shown in FIG. 9, a collimator 1, a polarizer 2, a λ / 4 plate 3, a Pockels element 4, an analyzer 5, and a collimator 6 are arranged in this order, and a light having a wavelength of about 850 nm is output from a stabilized light source. Thickness b ≒ 3 for Pockels element 4
A stable AC voltage of 50 V is applied in the light traveling direction of the Pockels element 4 using a Bi ₁₂ GeO ₂₀ mm crystal. Then, after the light guided to the optical fiber 8b through the analyzer 5 and the collimator 6 is subjected to optical / electrical conversion (O / E conversion), the AC component and the DC component are measured.

The relationship between the c-axis of the λ / 4 plate 3 and the crystal axis (x-axis) of the Pockels element 4 is, as shown in FIG.
The axis is kept inclined at 45 ° to the x-axis, and the angle between the polarization direction of the polarizer 2 and the x-axis is −60 °, −45 °.
°, −30 °, 0 °, + 30 °, + 45 °, + 60 ° to change the position of the polarizer 2 (rotate the polarizer 2 about the z-axis). When the polarizer 2 is arranged at each of the above positions, the position of the analyzer 5 at which the AC value of the sensor output is maximized (the polarization direction of the analyzer 5) while rotating the analyzer 5 about the z-axis. Angle between the axis and the x-axis)
Find out. The results of this experiment are shown in Table 3 below.

[0094]

[Table 3]

For reference, the AC value of the sensor output was measured by setting the polarization directions of the polarizer 2 and the analyzer 5 to 0 ° with respect to the x-axis (so-called parallel polarizer and conventional configuration). Its value was 87 mV. In order to obtain the maximum sensitivity when the Pockels device 4 having optical rotation is used,
Intuitively, the analyzer 5 is made of Bi ₁₂ GeO ₂₀
It seems that it suffices to incline (+ 30 °) as much as the optical rotation angle φ of the crystal. However, the AC value of the sensor output when the analyzer 5 was tilted by + 30 ° was 49 mV as shown in Table 3. The AC value in the case of the above parallel polarizer and when the analyzer 5 is tilted by the same angle as the optical rotation angle φ is such that the polarization direction of the analyzer 5 is + 15 ° with respect to the x-axis (Bi ₁₂ G having a thickness b ≒ 3 mm).
AC value when tilted (half the optical rotation angle of eO ₂₀ crystal) (99
mV).

Further, as shown in Table 1, even when the position of the polarizer 2 is changed, the maximum AC value (amplitude of the AC component) of the sensor output (that is, the maximum modulation sensitivity) is determined by the analyzer. + 5 ° with respect to the x-axis (thickness b ≒ 3
mm of Bi ₁₂ GeO ₂₀ crystal (half the optical rotation angle). Note that, also when the analyzer 5 is arranged at a position rotated by ± 90 ° × n from the position of the analyzer 5 where the modulation sensitivity becomes maximum, the modulation sensitivity also becomes maximum similarly.

On the other hand, if the analyzer 5 is arranged at a position rotated by ± 45 ° from the above-mentioned position where the AC value becomes the maximum, the AC value always changes even when the position (angle) of the polarizer 2 is changed. It was also confirmed that it was almost zero.

That is, the optical rotation angle of the Pockels element 4 is φ
(°), the modulation sensitivity becomes maximum if the polarization direction of the analyzer 5 is inclined by (φ / 2) ± 90 ° × n (n is an integer) with respect to the x-axis. Is inclined by (φ / 2) ± 45 ° ± 90 ° × n (n is an integer) with respect to the x-axis, the modulation sensitivity becomes minimum.

In the above experiment, the polarizer 2
0 ° with respect to the x-axis (0 ° ± 90 ° × n
However, the same is applied), linearly polarized light is incident on the λ / 4 plate 3 at an angle of 45 ° with respect to its c-axis.
The output of the plate 3 is circularly polarized light. In this case, the modulation sensitivity is better than that of disposing the polarizer 2 at another position (angle) (AC value (max) in Table 1 is 99 mV).

When the polarization direction of the polarizer 2 is arranged at ± 45 ° with respect to the x-axis (the same applies to 45 ° ± 90 ° × n), the λ / 4 plate 3 is parallel to its c-axis. Alternatively, since the linearly polarized light is vertically incident, the light passing through the λ / 4 plate 3 is not decomposed into an extraordinary ray and an ordinary ray (that is, there is no phase difference), and the λ / 4 plate 3 Is still linearly polarized, and in this case, the AC value is almost zero.

When the polarization direction of the polarizer 2 is inclined at an angle other than 0 ° and ± 45 ° with respect to the x-axis, the output of the λ / 4 plate 3 is an elliptically polarized light having an ellipticity corresponding to the angle. (The ellipticity decreases as the angle approaches 0 °). λ
If the / 4 plate 3 produces a phase difference of exactly 90 °, the major and short axes of the elliptically polarized light output from the λ / 4 plate 3 are parallel or orthogonal to the c-axis of the λ / 4 plate 3. However, the ellipticity of the elliptically polarized light increases and the AC value decreases.

Therefore, in the relationship between the polarizer 2 and the λ / 4 plate 3, the polarization direction of the polarizer 2 is inclined by 45 ° with respect to the c-axis of the λ / 4 plate 3, and the output of the λ / 4 plate 3 is circular. It can be said that it is desirable to use polarized light for improving the modulation sensitivity.

The Pockels device 4 having an optical rotation angle φ different from that described above (Bi ₁₂ GeO ₂₀ crystal having a thickness b ≒ 5.7 mm)
And the same experiment as above was performed. In this case as well, even when the position of the polarizer 2 is changed, the AC value of the sensor output becomes the maximum when the polarization direction of the analyzer 5 is x.
Half of the optical rotation angle φ with respect to the axis (in this case, + 60 ° / 2 = +
30 °).

In the above experiment, the relationship between the c axis of the λ / 4 plate 3 and the crystal axis (x axis) of the Pockels element 4 was constant (45 °). Therefore, in the following experiment, λ /
The angle between the c-axis and the x-axis of the four plates 3 was changed, and conditions for obtaining the maximum sensitivity were examined.

(1) First, as shown in FIG.
The c axis of the plate 3 is inclined by 45 ° with respect to the x axis, and the polarization direction of the polarizer 2 is inclined by −10 ° with respect to the x axis.

(2) The Pockels element 4 is removed from the optical path so that only the λ / 4 plate 3 exists between the polarizer 2 and the analyzer 5.

(3) The position of the analyzer 5 is 0 ° to -170 °
The DC value at that time is read, and the state of elliptically polarized light output from the λ / 4 plate 3 is observed.

The state of the elliptically polarized light obtained by the above operation is shown by a dashed line in FIG. in this case,
Since the polarizer 2 is inclined more toward the a-axis than the c-axis of the λ / 4 plate 3, it becomes elliptically polarized light having a long axis on the a-axis.

(4) Next, in the above state (1),
Pockels element 4 (Bi ₁₂ GeO with thickness b ≒ 5.7 mm)
_{(Using 20} crystals) between the λ / 4 plate 3 and the analyzer 5. In this case, as shown in FIG. 9, each crystal axis of the Pockels element 4 is made to coincide with the x-axis, the y-axis, and the z-axis. Then, in this state, the position of the analyzer 5 is set to 1 in the same manner as in the above (3).
The value is changed at 0 ° intervals, the DC value at that time is read, and the state of the elliptically polarized light output from the Pockels element 4 is observed.
Here, no voltage is applied to the Pockels element 4.

The state of the elliptically polarized light obtained by the above operation is shown by a solid line in FIG. This elliptically polarized light has substantially the same ellipticity as the elliptically polarized light in the case of only the λ / 4 plate 3 obtained in (3) above (the shape of the elliptically polarized light is preserved), and its major axis is the Pockels element 4. Optical rotation angle φ ≒ + 60
It is rotated by °.

(5) Next, a stable AC voltage of 50 V is applied to the Pockels element 4 in the traveling direction of light, and while the analyzer 5 is rotated about the z axis, the A
The position of the analyzer 5 where the C value is maximized (the angle between the polarization direction of the analyzer 5 and the x-axis) is examined.

As a result of the above operation, as shown in FIG.
The position of the analyzer 5 where the AC value is maximum is such that the angle between the polarization direction and the x axis is + 30 ° (that is, half of the optical rotation angle φ) ± 90 ° × n, and the AC value (max) is 83 mV. Met.

(6) Next, as shown in FIG.
The c-axis of the plate 3 is inclined by 35 ° with respect to the x-axis, and the polarization direction of the polarizer 2 is inclined by −20 ° with respect to the x-axis (polarizer 2
Has an angle of 55 ° with the c-axis of the λ / 4 plate 3 as in the above (1)).

(7) In the above state, the above (3) to (3)
Perform the same operation as (5).

FIG. 13 shows the result of the above operation.
Compared with the experiments (1) to (5) above, the λ / 4 plate 3
Angle of incidence on Pockels element 4 due to rotation by 0 ° (angle between the major axis of elliptically polarized light and the x-axis of Pockels element 4)
Is changed, but the rotation angle due to the optical rotation power is + 60 ° and does not change. Also in this case, the position of the analyzer 5 at which the AC value becomes maximum is determined by the angle between the polarization direction and the x-axis being +
30 ° (that is, half of the optical rotation angle φ) ± 90 ° × n, and the AC value (max) was 82 mV. from this result,
Even if the angle between the c-axis of the λ / 4 plate 3 and the x-axis of the Pockels element 4 changes, the polarization direction of the analyzer 5 is changed to Bi ₁₂ GeO.
_It can be seen that the modulation sensitivity is always maximized if it is inclined by half the optical rotation angle φ of the Pockels device 4 with respect to the x-axis of the ₂₀ crystals.

(8) Next, as shown in FIG.
The c axis of the plate 3 is inclined by 35 ° with respect to the x axis, and the polarization direction of the polarizer 2 is inclined by −10 ° with respect to the x axis (polarizer 2
And the c-axis of the λ / 4 plate 3 has an angle of 45 °).

(9) In the above state, the above (3) to (3)
Perform the same operation as (5).

FIG. 15 shows the result of the above operation.
The output of the λ / 4 plate 3 is supposed to be circularly polarized light, but this is slightly elliptically polarized light because the polarizer 2 or λ /
This is probably due to slight displacement of the four plates 3. Also in this case, the position of the analyzer 5 where the AC value is the maximum is also the angle between the polarization direction and the x-axis being + 30 ° (that is, the angle is + 30 °).
(Half of the optical rotation angle φ) ± 90 ° × n, and the AC value (max) was 89 mV.

From the above experimental results, even if the angle formed by the polarization direction of the polarizer 2 and the c-axis of the λ / 4 plate 3 with the crystal axis of the Pockels device 4 (the above-mentioned x-axis) is changed, the detection is always performed. It can be seen that the modulation sensitivity is maximized at a position where the photon 5 is inclined by about half the optical rotation angle φ with respect to the x-axis. It is considered that the modulation sensitivity becomes maximum due to the positional relationship between the Pockels element 4 and the analyzer 5 for the following reason.

First, as shown in FIG. 3, an electro-optical crystal having no optical rotation (such as Bi ₄ Ge ₃ O ₁₂ or LiNbO ₃ )
Let us consider a configuration of a parallel polarizer using a Pockels element 4 composed of The main axes (X-axis, Y-axis, Z-axis) of the refractive index ellipsoid (index ellipsoid when no voltage is applied) of the electro-optic crystal used as the Pockels element 4 are shown in FIG. These are coincident with the coordinate axes x, y, and z.

When a voltage is applied to the Pockels element 4 in the light traveling direction (z-axis direction), the refractive index ellipsoid of the electro-optic crystal is deformed according to the applied voltage. Specifically, when the electro-optic crystal is a uniaxial crystal such as LiNbO ₃ , the crystal becomes a biaxial crystal by applying a voltage.
Then, as shown in FIG. 3, the two principal axes (X axis and Y axis) of the refractive index ellipsoid when no voltage is applied are rotated by 45 ° with respect to the x and y axes by applying a voltage. Are the main axes (X ′ axis, Y ′ axis) of the refractive index ellipsoid.

FIG. 16 shows a cross section of the refractive index ellipsoid when no voltage is applied (the cross section including the origin perpendicular to the light traveling direction).
FIG. 16A shows a cross section of the refractive index ellipsoid when a voltage is applied.
(B) shows each. As shown in FIG. 16A, when no electric field is applied and the electric field in the z-axis direction is 0, the main axes (X-axis and Y-axis) of the refractive index ellipsoid coincide with the x-axis and the y-axis, and the main refractive index n
_X and n _Y are equal. As shown in FIG. 10B, when an electric field is applied in the z-axis direction by applying a voltage, the main axes (X ′ axis, Y ′ axis) of the refractive index ellipsoid form an angle of 45 ° with the x axis and y axis. None, the main refractive indices n _{X ′} and n _{Y ′} are n _X ≠ n _{Y ′} .

Since the main axis of the refractive index ellipsoid is the characteristic axis of the polarized light, the light incident on the Pockels element 4 to which the voltage has been applied is divided into two straight lines vibrating in the X'-axis direction and the Y'-axis direction orthogonal to each other. Decomposed into polarized light. The main refractive index n _{X ′} in the X′-axis direction and the main refractive index n _{Y ′} in the Y′-axis direction change in proportion to the voltage applied to the Pockels element 4, and the main refractive index n _{X ′} in the X′-axis direction and Y ′
The two linearly polarized lights that vibrate in the axial direction generate a phase difference according to the applied voltage and are output from the Pockels element 4 (that is, phase modulation is performed).

The electro-optical crystal of the Pockels element 4 is
Since the crystal has no optical rotation, the two linearly polarized lights output from the crystal are also light that vibrate in the X′-axis direction and the Y′-axis direction. The two linearly polarized light beams that have passed through the Pockels element 4 and whose vibration directions are orthogonal to each other are combined into elliptically polarized light according to the applied voltage. Here, for example, when the analyzer 5 is tilted ± 45 ° with respect to the x-axis to make the polarization direction of the analyzer 5 coincide with the X′-axis direction or the Y′-axis direction, the sensitivity of the sensor becomes minimum. This is because the output light of the Pockels element 4 is a vector sum of light oscillating in the X′-axis direction and the Y′-axis direction. Therefore, when the analyzer 5 is arranged in parallel with the X′-axis or the Y′-axis, This is because the light quantity does not change. on the other hand,
When the polarization direction of the analyzer 5 is inclined by 45 ° with respect to the X′-axis direction or the Y′-axis direction, that is, when the analyzer 5 is aligned with the x-axis or the y-axis as shown in FIG. The sensitivity is at its maximum.

That is, when the Pockels element 4 having no optical rotation is used, two principal axes (X ′ ′) orthogonal to the light traveling direction (z-axis direction) in the refractive index ellipsoid when a voltage is applied.
The maximum sensitivity can be obtained by arranging the analyzer 5 at an angle of 45 ° to the axis (Y ′ axis).

As shown in FIG. 3, the λ / 4 plate 3
When the c-axis (optical axis) is parallel to the X′-axis of the Pockels element 14 and the polarizer 2 is inclined by 45 ° with respect to the c-axis of the λ / 4 plate 3, the Pockels element 4 having no optical rotation is provided. Is elliptically polarized light having a major axis and a minor axis on the x-axis or the y-axis. Considering this fact and the fact that the maximum sensitivity can be obtained by disposing the analyzer 5 in parallel with the x-axis, at first glance, the major axis and the minor axis of the elliptically polarized light input to the analyzer 5 and the polarization of the analyzer 5 are considered. It seems that the maximum sensitivity is always obtained when the directions are matched, but it is not. The maximum sensitivity is obtained only when the analyzer 5 is arranged so as to form an angle of 45 ° with respect to the X ′ axis and the Y ′ axis of the Pockels element 4.

Next, as shown in FIG. 9, a configuration using a Pockels element 4 made of an electro-optical crystal having optical rotation (such as Bi ₁₂ GeO ₂₀ or Bi ₁₂ SiO ₂₀ ) will be considered.

As described above, Bi ₁₂ GeO ₂₀ crystal or Bi ₁₂ GeO _{20
The 12} SiO ₂₀ crystal belongs to the equiaxed system, and its crystal axis coincides with the coordinate axes x, y, and z. In addition, since the crystal is an equiaxed crystal, the refractive index ellipsoid of the Bi ₁₂ GeO ₂₀ crystal before voltage application is spherical. The principal axes (X-axis, Y-axis, and Z-axis) of the refractive index ellipsoid at the midpoint in the z-axis direction of the Bi ₁₂ GeO ₂₀ crystal
Is considered to coincide with the coordinate axes x, y, and z axes.

When a voltage is applied to the Pockels element 4 in the light traveling direction (z-axis direction), the refractive index ellipsoid of the Bi ₁₂ GeO ₂₀ crystal is deformed according to the applied voltage. FIG.
As shown in FIG. 7, in the case of the Pockels element 4 having optical rotation, two principal axes (X ′ axis and Y ′ axis) orthogonal to the light traveling direction (z axis direction) in the refractive index ellipsoid are Bi _12. Ge
It is considered that the angle in the plane perpendicular to the z-axis changes depending on the position of the O ₂₀ crystal in the light traveling direction.

More specifically, at the midpoint in the z-axis direction of the Bi ₁₂ GeO ₂₀ crystal, the two main axes (X-axis and Y-axis) of the refractive index ellipsoid when no voltage is applied are changed by the voltage application to the x-axis and the y-axis. Is rotated by 45 ° with respect to. The cut surface of the refractive index ellipsoid at this intermediate point (the cut surface including the origin perpendicular to the light traveling direction) is considered to be as shown in FIG. 18B, and the main axis ( X '(mid) axis, Y' (mid)
Axis) is at a 45 ° angle to the x, y axes.

Further, at the position of the light incident surface of the Bi ₁₂ GeO ₂₀ crystal, the principal axes (X ′ (in) axis and Y ′ (in) axis) of the refractive index ellipsoid at the time of applying a voltage are at the above-mentioned intermediate point. Is located at a position rotated about the z-axis by half (−φ / 2) of the optical rotation angle φ in the direction opposite to the optical rotation direction than the main axes (X ′ (mid) axis and Y ′ (mid) axis) in Can be The cut surface of the refractive index ellipsoid at the position of the incident surface is considered to be as shown in FIG.

At the position of the light emitting surface of the Bi ₁₂ GeO ₂₀ crystal, the principal axes (X ′ (out) axis and Y ′ (out) axis) of the refractive index ellipsoid at the time of applying a voltage are the above-mentioned intermediate points. It is conceivable that it is located at a position rotated about the z-axis by half (+ φ / 2) of the optical rotation angle φ in the same direction as the optical rotation direction than the main axes (X ′ (mid) axis and Y ′ (mid) axis) in FIG. The cut surface of the refractive index ellipsoid at the position of the exit surface is considered to be as shown in FIG.

Considering the above, the principal axes (X ′ axis, Y ′ axis) of the refractive index ellipsoid when voltage is applied between the light incident surface and the light emitting surface of the Bi ₁₂ GeO ₂₀ crystal are optical rotation. It rotates by the angle φ. Also, the main axis (X ′ (out) of the refractive index ellipsoid at the position of the light emission surface of the Bi ₁₂ GeO ₂₀ crystal
And the Y ′ (out) axis), an optical rotation angle φ with respect to the main axes (X ′ axis, Y ′ axis) of the refractive index ellipsoid when a voltage is applied to the electro-optic crystal having no optical rotation. Are rotated by half the angle of FIG.
(C)).

The two linearly polarized lights that have passed through the Pockels element 4 are lights that vibrate in the directions of the principal axes (X ′ (out) axis and Y ′ (out) axis) of the refractive index ellipsoid at the position of the exit surface. is there. These two linearly polarized lights that oscillate in directions orthogonal to each other are combined after emitting the Pockels element 4 and become elliptically polarized light having an ellipticity according to the applied voltage.

Here, for example, when the polarization direction of the analyzer 5 is made to coincide with the X ′ (out) axis direction or the Y ′ (out) axis direction, the sensitivity of the sensor becomes minimum. This is because the output light of the Pockels element 4 is the vector sum of the light oscillating in the X ′ (out) axis direction and the Y ′ (out) axis direction. This is because the light amount does not change when they are arranged parallel to the (out) axis. On the other hand, when the polarization direction of the analyzer 5 is inclined by 45 ° with respect to the X ′ (out) axis or the Y ′ (out) axis, the sensitivity of the sensor becomes maximum.

As shown in FIG. 19, the X ′ (out) axis is inclined at an angle of − {45 ° − (φ / 2)} with respect to the x axis. Since the position of the analyzer 5 at which the maximum sensitivity is obtained is shifted by ± 45 ° ± 90 ° × n from the X ′ (out) axis, it is shifted by φ / 2 ± 90 ° × n with respect to the x axis. Position.

In the case of Bi ₁₂ GeO ₂₀ crystal having optical rotation, the two main axes (X ′ axis and Y ′ axis) of the refractive index ellipsoid change as shown in FIG. 17 depending on the position in the light traveling direction. This was confirmed by the following experiment.

(I) Only the Pockels element 4 having optical rotation is arranged between the polarizer 2 and the analyzer 5 without using the λ / 4 plate 3. As the electro-optic crystal of the Pockels device 4,
Bi ₁₂ GeO ₂₀ crystal with an optical rotation angle φ ≒ + 30 ° (thickness b ≒ 3
mm) and a Bi ₁₂ GeO ₂₀ crystal (thickness b ≒ 5.7 mm) having an optical rotation angle φ ≒ + 60 °. As shown in FIG. 9, each crystal axis of the Pockels device 4 is an x-axis,
It matches the y-axis and the z-axis.

(Ii) Next, a stable AC voltage of about 240 V (as high as possible) is applied to the Pockels element 4 in the traveling direction of light.

(Iii) While rotating the polarizer 2 and the analyzer 5 around the z-axis, search for a position where the AC value of the sensor output becomes maximum (confirm with an oscilloscope waveform).

(Iv) After fixing the analyzer 5 at the position of the maximum AC value found by the above operation (iii), only the polarizer 2 is moved to z.
While rotating about the axis, the position of the polarizer 2 at which the AC value of the sensor output becomes minimum (almost 0) (the angle between the polarization direction of the polarizer 2 and the x-axis) is examined.

As a result of the above operation, the rotation angle φ was about 30 °,
In any case of about 60 °, the AC value of the sensor output became almost 0 when the polarizer 2 was inclined by about (−φ / 2) ± 90 ° × n with respect to the x-axis. In addition, when the polarizer 2 was arranged at the above position, the AC value of the sensor output remained almost 0 regardless of the position of the analyzer 5.

This is because even if linearly polarized light inclined at an angle of about (−φ / 2) ± 90 ° × n with respect to the x-axis is incident on the light incident surface of the Pockels element 4, the linearly polarized light is When passing through the Pockels element 4, it is not decomposed into two lights that vibrate in directions orthogonal to each other (therefore, there is no phase difference).
It is considered that the linearly polarized light was output from the Pockels element 4 as it was. From this, at the position of the incident surface of the Pockels element 4, as shown in FIG. 17, the phase difference is set at an angle of about (−φ / 2) ± 90 ° × n with respect to the x-axis. It can be seen that there are principal axes (X '(in) axis and Y' (in) axis) of the refractive index ellipsoid, which are the intrinsic axes of the polarized light that is not generated.

(V) Next, after fixing the polarizer 2 at the position of the maximum AC value found by the above operation (iii), while rotating only the analyzer 5 around the z-axis, the sensor output A
The position of the analyzer 5 where the C value becomes minimum (almost 0) (analyzer 5
(The angle between the polarization direction and the x-axis) is examined.

As a result of the above operation, the angle of rotation φ was about 30 °,
In any case of about 60 °, the AC value of the sensor output became almost 0 when the analyzer 5 was inclined by about (+ φ / 2) ± 90 ° × n with respect to the x-axis. In addition, when the analyzer 5 was arranged at the above position, the AC value of the sensor output remained almost 0 regardless of the position of the polarizer 2.

This is because the two linearly polarized light beams vibrating in the directions orthogonal to each other decomposed in the Pockels element 4 emit about (+ φ / 2) ± with respect to the x-axis when exiting from the exit surface of the Pockels element 4. This is considered to be due to the vibration in the direction inclined by 90 ° × n. From this, at the position of the emission surface of the Pockels element 4, as shown in FIG. 17, the position of the polarization axis is about (+ φ / 2) ± 90 ° × n with respect to the x-axis. It can be seen that there are principal axes (X ′ (out) axis and Y ′ (out) axis) of the refractive index ellipsoid.

As described above, when the Pockels element 4 having optical rotation is used, the optical rotation angle φ is reduced in the same direction as the direction in which the polarization plane rotates due to the optical rotation of the crystal, as compared with the case without the optical rotation. A position inclined by a half angle (≒ φ / 2) is set as a reference position at which the modulation sensitivity is maximized, and the analyzer 5 is moved from there to a direction corresponding to the characteristics of the λ / 4 plate 3 (the direction in which the phase difference changes). If rotated by the angle α, the temperature characteristics can be improved.

In order to verify this, the temperature when the temperature was changed from about 50 ° (reference temperature) to about 30 ° using a Bi ₁₂ GeO ₂₀ crystal having a thickness of b ≒ 3 mm as the Pockels element 4 was used. A characteristic test was performed. When the analyzer 5 is placed at the position of the maximum sensitivity (+ 15 ° with respect to the x-axis) and when the analyzer 5 is rotated by 22 and 5 ° in a direction in which the above-mentioned temperature characteristics can be improved, the AC / DC I actually measured the ratio error,
The ratio error was +0.17 at the position where the sensitivity was maximum, while the ratio error was sufficiently reduced to +0.05 at a position rotated by 22.5 ° therefrom.

In the present embodiment, Bi ₁₂ GeO ₂₀ crystal and Bi ₁₂ SiO ₂₀ crystal belonging to the isotropic point group T (international symbol 23) have been exemplified as the electro-optical crystal having optical rotation. However, the present invention is not limited to this. For example, it is possible to use other electro-optical crystals having optical rotation, such as trigonal crystals belonging to the point group C ₃ (international symbol 3) or the point group D ₃ (international symbol 32) which are uniaxial crystals. it can.

When a uniaxial crystal is used as the Pockels element 4, its optic axis may be arranged in parallel with the traveling direction of light. More specifically, the refractive index ellipsoid 3 when no voltage is applied at an intermediate point in the light traveling direction of the uniaxial electro-optic crystal.
FIG. 9 shows three main axes, an X axis, a Y axis, and a Z axis (the Z axis is an optical axis).
May be made to coincide with the orthogonal coordinate axes x, y, z.

In the first and second embodiments, the optical voltage sensor has been described. However, if the Pockels element 4 is disposed in an electric field without providing an electrode on the surface of the element, the light traveling direction can be improved. The electric field can be measured, and a high-sensitivity optical electric field sensor can be configured with the same configuration as described above.

[0152]

As described above, in the optical voltage / electric field sensor according to the first aspect of the present invention, the direction of deviation from the reference phase difference at the reference temperature of the wave plate is always constant within the operating temperature range. It is set in such a manner that the analyzer is arranged at the reference position where the modulation sensitivity determined by the positional relationship between the electro-optic crystal and the electro-optic crystal is maximized, and the phase difference of the wave plate is set in the setting direction. When the DC component of the analyzer output decreases as compared with the reference phase difference due to the shift of the angle, the angle within about 30 ° counterclockwise from the reference position toward the downstream from the upstream in the light traveling direction to the downstream. While placed within the range,
When the DC component of the analyzer output is increased as compared with the reference phase difference due to the shift of the phase difference of the wave plate in the setting direction, from the reference position toward the downstream in the light traveling direction from the reference position. And is arranged clockwise within an angle range of about 30 ° or less.

Therefore, by optimizing the positional relationship between the optical components, the temperature characteristics of the sensor can be improved without increasing the number of new components, and highly accurate measurement can be performed over the operating temperature range. This has the effect that it can be performed.

As described above, in the optical voltage / electric field sensor according to the second aspect of the present invention, in the configuration of the first aspect, the electro-optic crystal has optical rotation, and the reference position of the analyzer is: The direction of polarization of the analyzer is substantially half the angle of rotation in the same direction as the direction in which the polarization plane rotates due to the optical rotation of the crystal with respect to a crystal axis orthogonal to the direction of light propagation in the electro-optic crystal. This is a configuration in which the position is inclined.

Therefore, by determining the reference position of the analyzer and optimizing the position of the analyzer in this manner, the temperature characteristic of the sensor can be improved even when an electro-optic crystal is used. .

[Brief description of the drawings]

FIG. 1, showing an embodiment of the present invention, is a perspective view illustrating a main configuration of a sensor unit of an optical voltage sensor.

FIG. 2 is a schematic block diagram of the optical voltage sensor.

FIG. 3 is a perspective view illustrating a configuration of a parallel polarizer of a sensor unit of the optical voltage sensor.

FIG. 4 is a diagram for explaining a change in polarization shape due to a shift of a phase difference generated in a λ / 4 plate.

FIG. 5 is a graph showing a relationship between an angle of an analyzer (an angle formed with an x-axis) and a ratio error when a phase difference of a λ / 4 plate changes from 90 ° to 89 ° due to a temperature change.

FIG. 6 is a graph showing a relationship between an analyzer angle (an angle formed with an x-axis) and a ratio error when a phase difference of a λ / 4 plate changes from 90 ° to 91 ° due to a temperature change.

FIG. 7 is a graph showing the relationship between the angle of the analyzer (the angle formed with the y axis) and the ratio error when the phase difference of the λ / 4 plate changes from 90 ° to 89 ° due to a temperature change.

FIG. 8 is a graph showing the relationship between the angle of the analyzer (the angle formed with the y-axis) and the ratio error when the phase difference of the λ / 4 plate changes from 90 ° to 91 ° due to a temperature change.

FIG. 9 illustrates another embodiment of the present invention, and is a perspective view illustrating a main configuration of a sensor unit of an optical voltage sensor.

FIG. 10 is an explanatory diagram showing a polarization direction of a polarizer and directions of a and c axes of a λ / 4 plate in a rectangular coordinate system.

11 is an explanatory diagram showing a state of light emitted from a λ / 4 plate, a state of light emitted from a Pockels element, and a polarization direction of an analyzer having a maximum modulation sensitivity in the arrangement of the optical members of FIG. 10; .

FIG. 12 is an explanatory diagram showing a polarization direction of a polarizer and directions of a and c axes of a λ / 4 plate in a rectangular coordinate system.

13 is an explanatory diagram showing a state of light emitted from a λ / 4 plate, a state of light emitted from a Pockels element, and a polarization direction of an analyzer having a maximum modulation sensitivity in the arrangement of the optical members of FIG. 12; .

FIG. 14 is an explanatory diagram showing a polarization direction of a polarizer and directions of a and c axes of a λ / 4 plate in a rectangular coordinate system.

15 is an explanatory diagram showing a state of light emitted from a λ / 4 plate, a state of light emitted from a Pockels element, and a polarization direction of an analyzer having a maximum modulation sensitivity in the arrangement of the optical members of FIG. 14; .

16 is an explanatory view showing a cut surface of a refractive index ellipsoid of a Pockels element having no optical rotation; FIG. 16 (a) when no voltage is applied, and FIG. 16 (b) when a voltage is applied; 2 shows a cut surface of a refractive index ellipsoid of FIG.

FIG. 17 is an explanatory diagram showing a state in which the principal axis of the refractive index ellipsoid changes depending on the position in the traveling direction of light in the Pockels element having optical rotation.

18 is an explanatory view showing a cut surface of a refractive index ellipsoid of the Pockels element of FIG. 17; FIG. 18 (a) is a light incident surface, and FIG. 18 (b) is a light traveling direction; The middle point, (c) in the figure, shows the cut surface of the refractive index ellipsoid at the time of voltage application on the light emission surface.

FIG. 19 is an explanatory diagram showing a polarization direction of an analyzer at which a maximum sensitivity is obtained in the sensor section of the optical voltage sensor of FIG. 9;

FIG. 20 is a schematic block diagram of a conventional optical voltage sensor.

FIG. 21 is a perspective view illustrating a main configuration of a sensor unit of a conventional optical voltage sensor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 collimator 2 polarizer 3 λ / 4 plate (wave plate) 4 Pockels element (electro-optic crystal) 5 analyzer 6 collimator 10 optical voltage sensor 11 sensor unit 12 light emitting circuit 13 light receiving circuit

Claims (2)

[Claims] 1. A photovoltaic / electric field sensor in which a polarizer, a wave plate, an electro-optic crystal, and an analyzer are sequentially arranged along a traveling direction of light, wherein the wave plate has a difference from a reference phase difference at a reference temperature. The direction of the shift is set to be always constant within the operating temperature range, and the analyzer is disposed at a reference position where the modulation sensitivity determined by the positional relationship with the electro-optic crystal is maximized. In the case, when the DC component of the analyzer output is reduced as compared with the reference phase difference due to the shift of the phase difference of the wave plate in the setting direction, the reference position is shifted from the reference position in the light traveling direction. When the DC component of the analyzer output is the reference phase difference due to the shift of the phase difference of the wave plate in the above-mentioned setting direction while being arranged in the angle range of about 30 ° counterclockwise from the upstream to the downstream. If it increases, the reference position Further, the optical voltage / electric field sensor is disposed within an angle range of about 30 ° clockwise from upstream to downstream in the light traveling direction. 2. The electro-optic crystal according to claim 1, wherein the electro-optic crystal has optical rotation, and the reference position of the analyzer is such that the polarization direction of the analyzer is a crystal axis perpendicular to a light traveling direction in the electro-optic crystal. 2. The optical voltage / electric field sensor according to claim 1, wherein the position is tilted by substantially half the optical rotation angle in the same direction as the direction in which the polarization plane rotates due to the optical rotation of the crystal.