JP2000258467A - Optical current transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical current transformer having a complete reflection film constructed of a simply-manufactured double-layer dielectric layer and preventing the variation of polarization between before and after reflection in the reflection film for dispensing with compensation of a polarization change. SOLUTION: An optical current transformer measures a current flowing through a conductor on the basis of a rotation angle based on Faraday effect. In this case, the optical current transformer is provided with a main body 1 formed of a medium exhibiting Faraday effect, an opening part 2 arranged in the main body 1 for inserting a conductor to be measured into the inside, and total reflection parts 3a-3c arranged in the main body 1 for emitting the light incident on the main body 1 to the outside after turning it around the circumference of the opening part 2. The total reflection film is constructed of a double-layer dielectric film maintaining polarization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical current transformer for measuring a current using the Faraday effect.

[0002]

2. Description of the Related Art For example, the following are disclosed as conventional techniques of this kind.

Japanese Patent Application Laid-Open No. 58-153174 discloses an optical current transformer for measuring a current using the Faraday effect, which has at least two total reflection surfaces (a main body having a square shape and three (A body having a total reflection surface, a body having a triangular shape and two total reflection surfaces) are disclosed. In this case, since one total reflection is performed for one optical path change,
The linearly polarized light changes to elliptically polarized light, the amount of rotation of the Faraday rotation angle changes, and the sensitivity decreases. In order to improve this, this publication discloses an example in which a total reflection surface is formed of two adjacent reflection surfaces.

Japanese Patent Application Laid-Open No. 64-66464 discloses the use of a Faraday element as a reciprocating optical path using a total reflection prism at an end. In addition, a corner is provided with a total reflection surface and a reflective substance for totally reflecting light and bending the light 90 ° without phase change. Assuming that the refractive index of the Faraday element is n ₁ and the refractive index of the reflective material is n ₂ , it is necessary to satisfy n ₂ = n ₁ · sin 45 °. MgF ₂ and Al ₂ O ₃ are exemplified as reflective materials.

Japanese Patent Application Laid-Open No. Hei 5-196653 discloses that a multilayer dielectric film is formed using two types of dielectric films as a unit so that polarized light is maintained on a reflection surface.

[0006]

Conventionally, in a current measuring method utilizing the Faraday effect of a polyhedral solid, since total reflection is used for internal reflection, it is necessary to correct a change in polarization generated at the time of total reflection. there were. Therefore, Japanese Patent Laid-Open No.
As in 153174, for each internal reflection,
A method has been studied in which two orthogonally arranged total reflection surfaces are provided, and a change in polarization generated by one total reflection is apparently corrected. However, in this method, since the polarized light is not linearly polarized between two orthogonally arranged total reflection surfaces, there is a problem that a measurement error occurs when a magnetic field parallel to the optical path exists in that portion. . Further, there has been a problem that the configuration becomes complicated, a large amount of material is required, and the production efficiency is poor.

On the other hand, the technology disclosed in Japanese Patent Application Laid-Open No. 64-66464 is complicated, and
Although the technology disclosed in Japanese Patent Application Laid-Open No. 58-153174 and Japanese Patent Application Laid-Open No. 64-66564 does not have the problem, it requires a multilayer (for example, 12 pairs) dielectric film, and the manufacturing process is complicated. Problem.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical current transformer having a reflection surface which can be manufactured in a simple manufacturing process and which maintains polarized light and which can perform accurate measurement.

[0009]

SUMMARY OF THE INVENTION The present invention relates to an optical current transformer for measuring a current flowing through a conductor based on a rotation angle due to the Faraday effect. An opening provided in the main body to perform light incident on the main body from outside,
After having circulated around the periphery of the opening portion, the device further comprises at least two total reflection portions provided on the main body to emit light to the outside, and the perfect reflection film is a multilayer film of a dielectric film that stores polarized light. Including.

The present invention realizes a linear polarization state in all optical paths by coating a solid reflective surface with a dielectric multilayer film and realizing a film configuration in which no change in polarization occurs during total reflection.

Preferably, the dielectric constant of the multilayer film of the dielectric film constituting the perfect reflection film and the angle of incidence of light on the dielectric film are such that the polarization does not change before and after the reflection and the reflectance is one. It is determined to be.

Preferably, of the dielectric films forming the perfect reflection film, total reflection is performed in an outermost layer, and total reflection is not performed in other layers.

Preferably, the thickness of the dielectric film constituting the perfect reflection film is determined based on a phase difference between two light beams having different polarization directions.

Preferably, to prevent light loss,
A prism smaller than the main body is provided at a light entrance and exit part.

The reflection film according to the present invention is generally designed by the following procedure. (1) The polarization does not change before and after reflection, that is, s
(⊥) Rs for polarized light and Rp for p (‖) polarized light are equal,
In addition, the dielectric constant of the dielectric film and the angle of incidence of light on the dielectric film are determined so that the reflectance becomes 1. That is, (i) n
₀ → n ₁ , n ₁ → n ₂ boundary (body 1 and dielectric film 3-1)
And between the dielectric films 3-1 and 3-2). (Ii) The boundary of n ₂ → n ₃ (dielectric film 3-
Total reflection between 2 and the outside). (2) The thickness of the dielectric film is determined based on a calculation formula of retardation (phase difference between two light beams having different polarization directions). That is, (i) parameters δ1 and δ2 at which the phase difference 入射 between the incident light and the reflected light becomes 0 are obtained. (Ii) _{formula: d i = λδ i / 4πn} i cosθ i (i =
The film thicknesses d ₁ and d ₂ on the respective reflecting surfaces are obtained by (1) and (2).

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an optical current transformer according to the present invention. An opening 2 having a predetermined size is provided at the center of a polygonal (square in the example of FIG. 1) main body block 1 made of a uniform and isotropic Faraday medium. A conductor through which a current to be measured flows is inserted into the opening 2. The collimated light enters the input / output surface of the sensor, is reflected by each of the reflection surfaces 3a to 3c provided on the main body 1, returns to the input / output surface again, and is emitted to the detector. Therefore, the lights L1 to L4 inside the main body 1 form a closed light path. The micro prism 4 is provided so that the light from the light source enters and exits the main body 1 at an appropriate angle without being reflected.

Next, a current measuring method using the Faraday effect will be described with reference to FIG. By utilizing the Faraday effect, a circuit can measure a current essentially without contact. As shown in FIG. 2, when a magnetic field H is generated on the paper surface of FIG. 2 by a current flowing inside the conductor A, when light circles inside a polygonal body made of a Faraday medium as shown by an arrow, the Faraday effect is generated. Rotates the polarization angle of light. Here, since the Faraday effect in a uniform and isotropic medium is proportional to the magnetic field, the total Faraday rotation angle 電流 along the closed optical path around the current can be calculated by using Ampere's law as ψ = V · ∫ H · ds = VI. V is a Verdet constant. Therefore, by measuring the entire Faraday rotation angle ψ, the current I passing through the closed optical path in the cell can be obtained.

In FIG. 1 and FIG. 2, when light is reflected in the Faraday medium and its polarization angle changes, a measurement error occurs. Therefore, it is desirable that the polarization does not change before and after the reflection. At the same time, the reflectivity needs to be 1 since the power of the light is required to be constant in the optical path for the measurement. In order to satisfy these conditions, FIG.
In addition, a total reflection film composed of two dielectric films is provided on the main body of the Faraday medium shown in FIG. Hereinafter, the total reflection film will be described in detail.

In order to eliminate the birefringence due to reflection in the sensor cell as shown in FIGS. 1 and 2 and maintain the state of polarization in this sensor cell, the amplitude reflection coefficient of the beam in the cell, that is, s (⊥ ) Rs, p in polarized light
(‖) Rp for polarized light must be equal. Also,
The magnitude of the amplitude reflection coefficient is required to be 1 so that there is no beam attenuation. That is, Rs = Rp = ± 1 Since a single reflective film cannot satisfy the condition of the above formula, a reflective film that satisfies the condition of the above formula is formed by a sufficiently large number of dielectric thin films whose reflective medium has an appropriate thickness. Must be composed of layers. However, as the number of layers increases, the manufacturing process becomes complicated. According to the present invention, it is possible to form a film with a minimum of two layers by causing total reflection, thereby simplifying the manufacturing process.

Here, as shown in FIG. 3, two types of dielectrics are used.
The reflecting surface of the two-layer film structure using the thin films 3-1 and 3-2 is
Coat body 1. Deflection of Faraday medium of main body 1
Folding rate n₀And the refractive index of the dielectric thin films 3-1 and 3-2
Each n₁, N₂And The outermost layer has a refractive index n₃Invitation
An electric body (for example, air). Determine the angle of incidence of light on each medium.
Each θ₀, Θ₁, Θ₂Then, according to Snell's law,
, N₀sinθ₀= N₁sinθ₁= N₂sinθ₂  Becomes

The two conditions mentioned above, namely the reflection
The polarization does not change before and after, and the reflectance is 1.
In order to satisfy, the following conditions are required in FIG.
You. (1) n₀→ n₁, N₁→ n₂Boundary (body 1 and dielectric
Between the film 3-1 and between the dielectric films 3-1 and 3-2)
That is not totally reflected at₀<N₁/ N₀  sinθ₁<N₂/ N₁  (2) n₂→ n₃Boundary (between dielectric film 3-2 and outside)
Total reflection at sinθ₂> Sin θc = 1 / n₂  In the embodiment of the present invention, in order to satisfy these conditions,
The following dielectric film was used. Faraday medium for body 1
Refractive index n as quality₀Uses 1.85 lead glass (SF)
The dielectric thin film 3-1 has a refractive index n₁Is 2.17
Lium (Ta₂SO₅) To the dielectric thin film 3-2.
Folding ratio n₂Is 1.45 silicon oxide (SiO 2)_Two)
Was. Also, the refractive index n of the outermost layer₃Is 1.00 air
And the light source was an SLD having a wavelength λ of 840 nm.

From these conditions, it can be seen that the incident angle θ ₀ to the dielectric thin film should satisfy the condition of 32.72 ° <θ ₀ <51.61 °. Accordingly, in embodiments of the present invention was the incident angle theta ₀ and 45 °.

Next, the thickness of the dielectric film is determined. In the dielectric films 3-1 and 3-2, when the phase difference 入射 between the incident light and the outgoing light is 0 and the incident angle θ ₀ is 45 °, the film thicknesses d1 and d2 of each layer are calculated by calculation described later. (Refer to "Calculation of retardation"). 5 to 1
The graph of 0 is a graph of Γ with respect to the change of δ2 with δ1 as a parameter. In FIG. 8A, Γ = 0 is not obtained, but in FIG. 8B, δ2 is obtained between δ2: 180 ° and 270 °. At this time, δ1 is between 70 ° and 80 °. Explaining the outline of the calculation procedure, first, δ1 and δ2 at which the phase difference 入射 between the incident light and the reflected light becomes 0 are obtained.
By calculation, δ1 = 73.60 °, 338.35 ° δ2 = 240.38 °, 198.71 °. Then, the film thickness _d 1, _{d 2} of each reflecting surface, by solving the equation _{d i = λδ i / 4πn i} cosθ i (i = 1,2), d 1 = 49.59 (nm), 227.99 (nm) d ₂ = 448.35 (nm) and 370.62 (nm). The cell this time has d1 = 49.59 (nm), d
2 = 448.35 (nm) was adopted.

As described above, by setting the incident angle of light, the dielectric constant of the two dielectric films, and the thickness thereof, a total reflection film in which polarized light is maintained before and after reflection can be obtained. By using this total reflection film, it is possible to provide an optical current transformer that can be manufactured by a simple manufacturing process and that can perform accurate measurement.

Next, the calculation of the retardation will be described.

What is retardation?
The phase difference between two light beams having different polarization directions.

The angle of incidence of light on each medium is θ₀, Θ
₁, Θ₂Then, according to Snell's law, n₀sinθ₀= N₁sinθ₁= N₂sinθ₂ (1) Amplitude between Faraday medium 1 and dielectric film 3-1
The reflectance R is Rs for each of the polarization directions s and p.₀₁= −sin (θ₀−θ₁) / Sin (θ₀+ Θ₁) Rp₀₁= Tan (θ₀−θ₁) / Tan (θ₀+ Θ₁(2) Similarly, amplitude reflection between the dielectric films 3-1 and 3-2
The rates Rs and Rp are Rs₁₂= −sin (θ₁−θ₂) / Sin (θ₁+ Θ₂) Rp₁₂= Tan (θ₁−θ₂) / Tan (θ₁+ Θ₂(3) Δs and Δp are tan (Δs / 2) = − (sin²θ₂− (1 / n₂)²)^1/2/ Cosθ₂  tan (Δp / 2) = − n₂ ²(Sin²θ₂− (1 / n₂)²)^1/2/ Cosθ ₂ (4) φ_{s, 12}, Φ_{p, 12}Is tan (φ_{s, 12}/ 2) = -Rs₁₂sin (δ₂+ Δs) / (1 + Rs)₁₂cos (δ₂+ Δs)) tan (φ_{p, 12}/ 2) = -Rp₁₂sin (δ₂+ Δp) / (1 + Rp₁₂cos (δ₂+ Δp)) (5) φ_{s, 01}, Φ_{p, 01}Is tan (φ_{s, 01}/ 2) = -Rs₀₁sin (δ₁+ Δ₂+ Δs + φ_{s, 12}) / (1 + Rs)₀₁cos (δ₁ + Δ₂+ Δs + φ_{s, 12})) Tan (φ_{p, 01}/ 2) = -Rp₀₁sin (δ₁+ Δ₂+ Δp + φ_{p, 12}) / (1 + Rp)₀₁cos (δ₁ + Δ₂+ Δp + φ_{p, 12})) (6) X = Δs−Δp (7) Y = φ_{s, 12}−φ_{p, 12} (8) Z = φ_{s, 01}−φ_{p, 01} (9) Γ = X + Y + Z (10) The calculation of Γ is performed using the above equations (1) to (10). Concrete
Typically, n₀= 1.85, n₁= 2.25, n₂= 1.
45, λ = 840 nm, θ₀= 45 °, δ₁,
δ₂Is changed in the range of 0 ° to 360 ° to obtain each value.
You. That is, from equation (1), θ₁, Θ₂From equation (2)
Rs₀₁, Rp₀₁From equation (3)_{1 2}, R
p₁₂ From equation (4), Δs, Δp, from equation (5)
_{s, 12}, Φ_{p, 12}From equation (6)_{s, 01}, Φ
_{p, 01}From formulas (7) to (10), Γ is δ₁, Δ₂Noseki
It can be calculated as a number.

[0029]

According to the present invention, since the complete reflection film can be constituted by at least two dielectric layers, the production process can be simplified. In addition, since the polarization does not change before and after reflection in the reflection film, a configuration for compensating for the change in polarization is not required, and a highly accurate optical current transformer can be provided with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a front view and a side view showing a structure of an optical current transformer according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining the operation principle of the optical current transformer.

FIG. 3 is a cross-sectional view showing a structure of a multilayer film used in the optical current transformer according to the embodiment of the present invention.

FIG. 4 is a specific example of the structure of a multilayer film used in the optical current transformer according to the embodiment of the present invention.

FIG. 5 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ = 1);
Graph of Γ−δ ₂ when 0 ° and 20 °).

FIG. 6 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ = 3).
Graph of Γ-δ ₂ when 0 ° and 40 °).

FIG. 7 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ = 5).
Graph of Γ−δ ₂ when 0 ° and 60 °).

FIG. 8 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ = 7);
0 °, when the 80 °, the graph of the Γ-δ _2).

FIG. 9 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ = 9).
0 °, when the 100 °, the graph of the Γ-δ _2).

FIG. 10 is a graph for explaining a procedure for determining the thickness of the multilayer film according to the embodiment of the present invention (δ ₁ =
Graph of Γ−δ ₂ when 110 ° and 120 °).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Main body block 2 Opening 3 Dielectric multilayer film 4 Micro prism

Claims (5)

[Claims] 1. An optical current transformer for measuring a current flowing through a conductor based on a rotation angle due to the Faraday effect, wherein the main body is formed of a medium exhibiting the Faraday effect, and is provided on the main body for interpolating the conductor to be measured. An opening, and light that has entered the main body from the outside, after circling the periphery of the opening, is provided with at least two completely reflecting portions provided in the main body for emitting to the outside, The reflective film is
An optical current transformer comprising a multilayer film of a dielectric film for preserving polarized light. 2. The dielectric constant of the multilayer film of the dielectric film constituting the perfect reflection film and the angle of incidence of light on the dielectric film are such that the polarization does not change before and after reflection and the reflectance is 1 The optical current transformer according to claim 1, wherein the optical current transformer is determined as follows. 3. The optical current transformer according to claim 2, wherein, among the dielectric films constituting the perfect reflection film, total reflection is performed in an outermost layer and total reflection is not performed in other layers. 4. The optical transformer according to claim 1, wherein the thickness of the dielectric film constituting the perfect reflection film is determined based on a phase difference between two light beams having different polarization directions. vessel. 5. An optical current transformer according to claim 1, further comprising an input / output prism smaller than said main body at a light entrance and exit part. .