JPH0760164B2 - Optical voltage electric field sensor - Google Patents

Description

BACKGROUND OF THE INVENTION [Field of the Industrial] This invention, Bi ₁₂ GeO ₂₀ single crystal of electro-optic effect (Pockels
The present invention relates to a photovoltage electric field sensor to which the effect) is applied, and particularly to improvement of its temperature characteristic.

[Conventional technology]

Conventionally, this type of device has been shown in FIG.
In the figure, (1) is a polarizer, (2) is an electro-optical material for detecting an electric field, (3) is a wave plate, that is, a quarter wave plate in this example, (4) is an analyzer, and (5) is AC voltage V applied to the electro-optical material (2), (6) transparent electrodes provided on both sides of the electro-optical material (2), (7) light, (8) passed through the polarizer (1) An arrow showing the polarization direction of the light (7), (9) an arrow showing the polarization direction of the light (7) passing through the analyzer (4), (10) a photodetector, and (11) an axis of the wave plate. A direction arrow, (12) is an arrow indicating the birefringence axis direction due to the electro-optic effect.

Next, the operation will be described. The light (7) that has passed through the polarizer (1) has a plane of polarization indicated by the arrow (8), and this light (7) has passed through the electro-optic material (2) and the quarter-wave plate (3). Sometimes the light (7) entering the detector (10) is adjusted so that it is perpendicular to the direction (9) of the analyzer (4).
Will be the least. Next, when an AC voltage (5) is applied to the electro-optic material (2), the polarization state of the light (7) that has passed through the quarter-wave plate (3) changes and passes through the analyzer (4). The amount of light increases, and the increased amount can be detected by the photodetector (10). At this time, when the voltage (5) applied to the electrode (6) on the electro-optical material (2) changes, the amount of light passing through the analyzer (4) changes, and the change is caused by the change in the electro-optical material (2). ) As a Bi ₁₂ GeO ₂₀ single crystal (hereinafter abbreviated as BGO)
When is used, the light quantity I is given by the following equation.

I = (1/2) I _o {1 + Γ / φ) sinφ} φ = Γ ² + (2θ1) ² } ^1/2 , I _o : Polarizer output light amount Γ: Linear birefringence due to electro-optical effect of BGO φ: BGO optical rotatory power 1: BGO optical path length λ: Working wavelength n _o : BGO Refractive index r ₄₁ : BGO Electro-optic coefficient V: Applied voltage.

[Problems to be solved by the invention]

The conventional photovoltage electric field sensor is configured as described above, and in order to eliminate the error due to the change in the light quantity I _o emitted from the polarizer and the error due to the transmission loss of the light quantity I to the photodetector (10), the DC component of the light quantity is removed. The applied AC voltage was detected by detecting I _Dc and AC component I _Ac and performing division. However,
There is a problem that the _noo ³ γ ₄₁ and the optical rotation θ that represent the electro-optical effect of BGO change with temperature, and these temperature changes cause a detection error of the photovoltage electric field sensor.

Also, the quarter wave plate (3) is a wave plate having a phase difference of 90 °, but this also changes with temperature. Further, even when the phase difference of the wave plate (3) is not 90 °, if the phase difference is Δ, the light quantity I is: I = (1/2) I _o {1-cosΔcosφ + (Γ / φ) sinΔsinφ } Therefore, there is a problem that an error is caused even by the temperature change of the wave plate (3).

The present invention has been made to solve the above problems, and the sensitivity I of the photovoltage electric field sensor is adjusted by adjusting the optical rotation angle due to the optical path length of the BGO or the phase difference of the wave plate.
_The purpose is to obtain a highly accurate photovoltage electric field sensor by reducing the temperature coefficient of _AC / I _DC .

[Means for solving problems]

The photovoltage electric field sensor according to the present invention uses a Bi ₁₂ GeO ₂₀ single crystal as an electro-optical material, and in the photovoltage electric field sensor configured by a polarizer, the electro-optical material, a wave plate, and an analyzer, the phase difference Δ of the wave plate, the Bi ₁₂ GeO
_{20 For the} angle of rotation φ due to the optical path of a single crystal, the following condition cosΔ ＝ cosφ − or (Where α is β temperature coefficient of the birefringence of the waveplate temperature coefficient of optical rotatory power of Bi ₁₂ GeO ₂₀ single crystal) when the value of delta satisfy the Δ _comp, Δ _comp -0.35 radians ~Δ _comp +0.35 Either set it in the range of radians, or set the above-mentioned optical rotation angle φ to the above-mentioned phase difference Δ to the above formula or the value of φ that satisfies the formula.
when the _comp, is to set in the range of phi _comp -0.35 radians to [phi] _comp +0.35 radian.

[Action]

In the present invention, the conditional expression or cos Δ = cos φ − for the optical rotation angle φ due to the optical path of BGO. When the value of delta _comp retardation delta wave plates satisfying, by the phase difference delta is using the wavelength plate in the range of delta _comp -0.35 radians ~Δ _comp +0.35 rad, the sensitivity of the optical voltage electric field sensor The temperature coefficient of can be reduced.

Also, with respect to the phase difference Δ of the wave plate, when the value of the optical rotation angle φ of BGO that satisfies the above conditional expression or equation is φ _comp , by adjusting the optical path length of BGO while considering the wavelength of the light source used, , by adjusting the angle of rotation phi, by setting the value of phi in the range of phi _comp -0.35 radians to [phi] _comp +0.35 radian, can reduce the temperature coefficient of the sensor sensitivity.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. The configuration is the same as the conventional one shown in FIG. 1 except for the following points.
That is, instead of the conventional quarter wave plate (3), a wave plate (3) having an adjustable phase difference Δ is combined to form a photovoltage electric field sensor. At this time, the transmitted light amount I is I = (1/2) I _o {1-cos Δcos φ + (Γ / φ) sin Δsin φ} φ = Γ ² + (2θ1) ² } ^1/2 , I _o : Polarizer output light amount Γ: Linear birefringence due to electro-optical effect of BGO φ: BGO optical rotatory power 1: BGO optical path length λ: Working wavelength n _o : BGO Refractive index r ₄₁ : BGO Electro-optic coefficient V: Applied voltage.

Now, when the applied voltage V is constant, the temperature changes of Γ, Δ, and θ are respectively set as Δ = Δ _o (1 + αT) θ = θ _o (1 + βT) Γ = Γ _o (1 + γT). However, Δ _o , θ _o , and Γ _o are the wave plate phase difference at the use center temperature, the optical rotation power, and the linear phase difference of BGO at a constant voltage value, respectively. Further, T = t−t _o , t is the operating temperature, and t _o is the operating center temperature. Further, α, β, γ are temperature coefficients in each, and β = −
250 ppm / ° C., γ = −250 ppm / ° C., α is −116 ppm / ° C. in the case of a crystal wave plate (3).

In the photovoltage electric field sensor, light is generally transmitted to and incident on these optical system detectors through an optical fiber or the like, and emitted light is returned by the optical fiber for measurement. Therefore, since errors due to those optical transmission losses occur,
These errors are removed by separating the AC variation I _AC and the DC variation I _{DC of the} light quantity and performing division. For minute AC voltage, I _DC = (1/2) I _o (1-cosΔcosφ) I _AC = (1/2) I _o (Γ / φ) sin Δsinφ, and the sensitivity η after division is Becomes Here, substituting the temperature coefficient, η = η _o (1 + δT) φ _o = 2θ _o 1, Is. For example, in the formula, θ _o = 0.183 rad / mm = 10.5 °
/ mm, 1 = 1 mm, and α, β, γ were substituted, and the change in δ due to Δ _o was determined, and the results are shown in FIG. From this figure, when the phase difference Δ _o of the wave plate is 90 ° as in the conventional example, that is, when the quarter wave plate is used, δ becomes −80 ppm / ° C., but Δ _o is 115 °. Then, it can be seen that the temperature coefficient δ becomes zero.

Although the phase difference Δ _{o is set} to 115 ° in the above embodiment,
It can be seen from FIG. 2 that the temperature coefficient is improved in the range of more than 0 ° to 130 °.

The wave plate (3) may be a single plate type or a laminated type, and if the phase difference Δ _o is within the above range, the temperature change is similar and the characteristics are improved.

Further, according to the formula, it is found that when φ _o = 2θ _o 1 is determined, there are two Δ _o values at which the temperature coefficient becomes zero. here,
φ _o = 2θ _o 1 depends on the optical path length 1 of the electro-optic material (2) and the optical rotatory power θ _o , and the optical rotatory power changes depending on the wavelength of the used light. FIG. 3 shows a contour line of the temperature coefficient in which the horizontal axis represents the optical rotation angle 2θ _o 1 and the vertical axis represents the wave plate phase difference Δ _o . In the equation, since γ-β happens to be 0, the condition of zero temperature coefficient is cos Δ _o = cos (2θ _o 1) − or Is.

Here, the conditional expression with a temperature coefficient of zero is established regardless of the material of the wave plate (3). Also, the conditional expression with zero temperature coefficient is However, φ _o = 2θ _o 1, and the birefringence temperature coefficient α of the wave plate (3) and Bi ₁₂ Ge
The optical activity of the O ₂₀ single crystal depends on the temperature coefficient β of θ _o . In this case, β is −250 ppm / deg. When quartz is used as the wave plate (3), α is −116 ppm / deg, so β / α is
It becomes 2.198. When using an Al ₂ O ₃ single crystal or a MgF ₂ single crystal as the wave plate (3), α is −137 ppm / deg and −49 ppm / deg, and β / α is 1.825 and 5.102, respectively. .

From FIG. 3, the phase difference Δ _o of the wave plate is less than ± 80 ppm / deg even if it is changed within the range of ± 0.35 radian ≈ 20 ° under the condition of zero temperature coefficient. You can see that it is practically acceptable. Although FIG. 3 shows the case where the wave plate (3) is quartz, Al ₂ O ₃
The same applies to single crystals and MgF ₂ single crystals. Also, the third
From the figure, the value of the optical rotation angle φ is ± 0.
Even if the temperature is changed within the range of about 35 radians ≒ 20 °, the temperature coefficient becomes ± 80 ppm / deg or less, which means that it is practically acceptable. The optical rotation angle φ is adjusted by adjusting the optical path length of the BGO while considering the wavelength of the light source used.

In addition, when the Bi ₁₂ GeO ₂₀ single crystal (2) is divided and arranged, or when an optical system of a system in which a reflecting layer, a prism or the like is provided to reflect at least once, Bi ₁₂
Set the total optical path length of GeO ₂₀ single crystal as 2θ _o 1 and Δ
The same effect can be obtained by obtaining _o .

Furthermore, when the phase difference Δ _o is π / 2, that is, when a quarter-wave plate is used, the above temperature coefficient is 0 at φ = π / 2 from the conditional expression, and the optical rotation angle φ is π / 2-0.35. Radian ~
The temperature coefficient can be reduced by setting it in the range of π / 2 + 0.35 radians. As a wave plate, phase difference
With a 90 ° quarter-wave plate, the phase difference Δ _o is Δ _o = N × 360
For those with ± 90 ° (N: integer), even if the value of N is different, it has the same function as a wave plate, but the temperature characteristics are different. In the temperature compensation conditional expression, the actual phase difference is substituted.

〔The invention's effect〕

As described above, according to the present invention, a Bi ₁₂ GeO ₂₀ single crystal is used as an electro-optical material, and a polarizer, the electro-optical material, a wave plate, and an optical voltage electric field sensor configured by an analyzer, The phase difference Δ of the wave plate is calculated as the Bi ₁₂ GeO ₂₀
For the angle of rotation φ due to the optical path of the single crystal, the following condition cosΔ ＝ cosφ − or (Where α is β temperature coefficient of the birefringence of the waveplate temperature coefficient of optical rotatory power of Bi ₁₂ GeO ₂₀ single crystal) when the value of delta satisfy the Δ _comp, Δ _comp -0.35 radians ~Δ _comp +0.35 Either set it in the range of radians or set the optical rotation angle φ to the above formula or the value of φ satisfying the above formula with respect to the above phase difference Δ.
when the _comp, since set in the range of phi _comp -0.35 radians to [phi] _comp +0.35 rad, the temperature coefficient of the sensor sensitivity can be substantially zero, and optical voltage field sensor of high accuracy is obtained effective.

[Brief description of drawings]

FIG. 1 is a block diagram showing a general photovoltage electric field sensor according to an embodiment of the present invention and a conventional example, and FIG. 2 shows a relationship between a phase difference Δ _o of a crystal wave plate and a temperature coefficient of sensor sensitivity. FIG. 3 is a characteristic diagram showing the contour lines of the temperature coefficient with respect to the optical rotation angle 2θ _o 1 of the Bi ₁₂ GeO ₂₀ single crystal and the phase difference Δ _o of the wave plate. In the figure, (1) is a polarizer, (2) is a Bi ₁₂ GeO ₂₀ single crystal, (3) is a wave plate, (4) is an analyzer, (5) is an applied AC voltage, (6) is an electrode, ( 7) is the optical path, (8) is the polarization direction of the polarizer, (9) is the polarization direction of the analyzer, (10) is the photodetector, (11) is the axial direction of the wave plate, and (12) is the electro-optic effect. Is the direction of the birefringence axis.

Claims (8)

[Claims] 1. A photovoltage electric field sensor comprising a Bi ₁₂ GeO ₂₀ single crystal as an electro-optical material and comprising a polarizer, the electro-optical material, a wave plate and an analyzer, wherein the phase difference of the wave plate is delta and the Bi ₁₂ GeO ₂₀ single crystal cos = cos [phi relative angle of rotation φ by the optical path - with a value delta _comp of meeting the delta, set in the range of delta _comp -0.35 radians ~Δ _comp +0.35 rad An optical voltage electric field sensor characterized by: 2. A photovoltage electric field sensor comprising a Bi ₁₂ GeO ₂₀ single crystal as an electro-optical material, and comprising a polarizer, the electro-optical material, a wave plate and an analyzer, wherein the phase difference of the wave plate is Δ for the optical rotation angle φ due to the optical path of the Bi ₁₂ GeO ₂₀ single crystal (Where α is β temperature coefficient of the birefringence of the waveplate temperature coefficient of optical rotatory power of Bi ₁₂ GeO ₂₀ single crystal) by using the value delta _comp of delta meet, delta _comp -0.35 radians ~Δ _comp +0.35 rad A photovoltage electric field sensor characterized by being set in the range of. Using 3. A Bi ₁₂ GeO ₂₀ single crystal as the electro-optical material, a polarizer, and the electro-optical material, a wavelength plate, an optical voltage field sensor constructed in accordance with an analyzer, the Bi ₁₂
The optical rotation angle phi due to the optical path of GeO ₂₀ single crystal, cos = cos [phi with respect to the phase difference Δ of the wave plate - using the value phi _comp of phi meet, the phi _comp -0.35 radians to [phi] _comp +0.35 rad An optical voltage electric field sensor characterized by being set in a range. Used wherein Bi ₁₂ GeO ₂₀ single crystal as the electro-optical material, a polarizer, and the electro-optical material, a wavelength plate, an optical voltage field sensor constructed in accordance with an analyzer, the Bi ₁₂
The optical rotation angle φ due to the optical path of the GeO ₂₀ single crystal was compared with the phase difference Δ of the wave plate. (Where α is the temperature coefficient of the birefringence of the waveplate β temperature coefficient of optical rotatory power of Bi ₁₂ GeO ₂₀ single crystal) by using the value phi _comp of phi meet, phi _comp -0.35 radians to [phi] _comp +0.35 rad A photovoltage electric field sensor characterized by being set in the range of. 5. If the wave plate is crystal, β / α is 2.198.
5. The photovoltage electric field sensor according to claim 2 or 4. 6. When the wave plate is an Al ₂ O ₃ single crystal, β / α
Is 1.825, The photovoltage electric field sensor according to claim 2 or 4. 7. When the wave plate is a MgF ₂ single crystal, β / α is
5. The photovoltage electric field sensor according to claim 2 or 4, which is 5.102. 8. When the phase difference Δ of the wave plate is π / 2, the optical rotation angle φ is The scope of claim 3 is characterized in that
The optical voltage electric field sensor according to the item.