JP2001050989A - Current sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a current sensor which is of a simple structure, restrains the influence of a change in the phase of propagation light caused by the twist of a sensor fiber, and can enhance the measuring accuracy of current. SOLUTION: A sensor fiber 4 which is wound on a conductor 5, in which a current as an object to be measured flows, is provided. A quarter-wave plate 3 which converts a linearly polarized light and which obtains right-handed circularly polarized light and left-handed circularly polarized light so as to be incident on one, end of the sensor fiber 4 is provided. A photodetector 8 which detects the interference intensity of the right-handed circularly polarized light and the left-handed circularly polarized light which are propagated through the sensor fiber 4, is provided. An LPF 9, a subtracter 10 and a divider 11 by which the current value of, the current flowing in the conductor 5 is found on the basis of the interference intensity detected by the photodetector 8 are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current sensor having a sensor fiber disposed so as to surround a conductor through which a current flows, and measuring a current value of a current flowing through the conductor based on light propagating in the sensor fiber. In particular, the present invention relates to a current sensor capable of suppressing the influence of disturbance on propagation light due to pressure, vibration, and the like applied to a sensor fiber.

[0002]

2. Description of the Related Art In a current sensor for measuring a current value of a current flowing through a conductor based on light propagating in a sensor fiber wound around a conductor through which a current to be measured flows, a pressure is applied to the sensor fiber from outside. When vibration or the like is applied, the sensor fiber is twisted, and the phase of the propagating light changes due to the twist. Therefore, the current value of the current flowing through the conductor cannot be measured accurately.

In order to suppress the adverse effect caused by the twisting of the sensor fiber, Japanese Patent Laid-Open Publication No. Sho 58-11869 (hereinafter referred to as first prior art) discloses a method in which a mirror is provided at one end of a sensor fiber to reciprocate linearly polarized light. It is described that the change in the phase is canceled by performing the operation.

Japanese Unexamined Patent Publication No. Sho 56-55864 (hereinafter referred to as "second prior art") discloses that a circularly polarized light is propagated from both ends of a sensor fiber and two propagated light beams interfere with each other to reduce a phase change due to twist. It is stated to be offset.

Further, Japanese Unexamined Patent Publication No.
Japanese Patent Application Laid-Open No. H11-163873 describes that a mechanism for suppressing movement of the sensor fiber is provided in order to prevent twisting of the sensor fiber due to vibration.

[0006]

In the first prior art, the phase change is canceled by reciprocating propagation using a mirror. However, in practice, the phase change of the forward light and the phase change of the backward light are completely different. Is not equal, the phase change due to the twisting of the sensor fiber cannot be eliminated. Further, in the second prior art, circularly polarized light is propagated from both ends of the sensor fiber and two propagated lights interfere with each other to cancel a phase change due to torsion. It is difficult to make them completely identical, and in practice, the effects of twisting cannot be eliminated.

Further, in the third prior art, a mechanism for suppressing the movement of the sensor fiber must be provided in order to prevent the sensor fiber from being twisted due to vibration, and the structure of the current sensor becomes complicated.

An object of the present invention is to provide a current sensor which has a simple structure and suppresses the influence of a change in the phase of propagating light caused by twisting of a sensor fiber, thereby improving current measurement accuracy.

[0009]

A feature of the present invention to achieve the above object is that a sensor fiber wound around a conductor through which a current to be measured flows, a clockwise circularly polarized light and a counterclockwise circular light from one end of the sensor fiber. Means for injecting polarized light, a photodetector disposed at the other end of the sensor fiber, for detecting the interference intensity of the clockwise circularly polarized light and the counterclockwise circularly polarized light propagated through the sensor fiber, and the light detection Means for obtaining a current value of a current flowing through the conductor based on the interference intensity detected by the detector.

[0010] Since two circularly polarized lights, clockwise circularly polarized light and counterclockwise circularly polarized light, are incident from the same end of the sensor fiber, the two circularly polarized lights propagate on exactly the same path, and a phase generated by twisting of the sensor fiber. Since the changes are equal in the two circular polarizations, the interference of the two circular polarizations cancels the phase changes caused by the twisting of the sensor fiber. On the other hand, the phase change caused by the magnetic field generated by the current flowing through the conductor causes one of the two circularly polarized lights to advance the phase and the other to retard the phase. The flowing current can be determined accurately. Thus, according to the present invention,
With a simple structure without a mechanism for suppressing the movement of the sensor fiber, it is possible to suppress the influence of a change in the phase of the propagating light due to the twisting of the sensor fiber and improve the accuracy of current measurement.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 shows a configuration diagram of a current sensor according to a preferred embodiment of the present invention. In FIG.
The light source 1 outputs linearly polarized light, and the linearly polarized light output from the light source 1 is incident on a polarization maintaining fiber 2a (hereinafter abbreviated as PMF 2a). The PMF 2a propagates the input linearly polarized light while maintaining the plane of polarization, and makes the linearly polarized light incident on the connected PMF 2b with the main polarization axis inclined at 45 °. P
The MF 2a and the PMF 2b are connected so that the main polarization axis is inclined by 45 °, so that the linearly polarized light as shown in FIG. 2A in the PMF 2a and the two orthogonal components in the PMF 2b as shown in FIG. Of linearly polarized light.

The PMF 2b propagates linearly polarized light of two orthogonal components while maintaining the plane of polarization, and transmits the propagated linearly polarized light to the λ / 4 plate 3.
Incident on. The λ / 4 plate 3 converts the incident linearly polarized light
Right-handed circularly polarized light (hereinafter abbreviated as RHCP) shown in FIG. 2C and left-handed circularly polarized light (hereinafter abbreviated as LHCP) shown in FIG.
Into two circularly polarized lights. The two circularly polarized lights obtained in the λ / 4 plate 3 enter the sensor fiber 4 which is a single mode fiber, and propagate around the conductor 5 by the sensor fiber 4 wound around the conductor 5 through which current flows. Is done.

Since a current flows through the conductor 5, the conductor 5
A magnetic field is generated around. Due to the Faraday effect caused by this magnetic field, the light propagates through the sensor fiber 4.
A change occurs in the phase of the two circularly polarized lights. As described above, since the two circularly polarized lights are clockwise circularly polarized light and counterclockwise circularly polarized light,
Due to the Faraday effect, the phase of one circularly polarized light is advanced, and the phase of the other circularly polarized light is delayed. Here, when the phase change of the circularly polarized light by the Faraday effect and theta _I, between the two circular polarization phase difference of 2 [Theta] _I is produced.

As described above, the sensor fiber 4 is a single mode fiber, and the two circularly polarized lights propagate through exactly the same path. Therefore, the phase change (θ _R ) caused by the twist of the sensor fiber 4 is 2. It is exactly the same for two circularly polarized lights. The phases of the two circularly polarized lights that have propagated through the sensor fiber 4 are expressed by (Equation 1) and (Equation 2).

[0016]

[ _Equation 1] φ _RHCP = θ _R + θ _I ( _Equation 1)

[0017]

Φ _LHCP = θ _R −θ _I ( _Equation 2) where φ _RHCP is the phase of right-handed circularly polarized light, and φ _LHCP is the phase of left-handed circularly polarized light.

When the two circularly polarized lights interfere with each other, the interference intensity (light intensity) I _P is represented by (Equation 3).

[0019]

Equation 3] _{^{I P = A 2 + B 2}} + 2ABcos (φ RHCP -φ LHCP) = A 2 + B 2 + 2ABcos2θ I ... ( Equation 3) In equation (3), A and B are optical complex amplitudes of two circularly polarized light is there. As will be described later, interference for the intensity I _P is which may be detected by the light detector 8, based on the detected interference intensity I _P can be determined the phase change theta _I, the conductor from the phase change theta _I obtained 5 can be obtained.

However, as shown by (Equation 3), since the interference intensity I _P is proportional to cos 2θ _I , the sensitivity is low when θ _I is small, and it becomes difficult to calculate the current flowing through the conductor 5.

Therefore, in this embodiment, the sensor fiber 4
And a phase difference of π / 2 is further given to the two circularly polarized lights output after propagating in the sensor fiber 4. By providing a phase difference of [pi / 2 into two circularly polarized light by the Faraday element 6, the interference intensity I _P changes as shown in equation (4).

[0022]

(Equation 4)

As shown in (Equation 4), the Faraday element 6
Interference intensity I _P of the two circularly polarized light after passing through the in, sin @ 2
proportional to theta _I, theta _I sensitivity is maximized when there is a small, it is possible to obtain a current flowing in the conductor 5 easily.

The two circularly polarized lights having a phase difference given by the Faraday element 6 are input to a photodetector 8 via a fiber 7. In the optical detector 8, the interference intensity I _P is detected. The detected interference intensity _IP is equal to the value of the low-pass filter 9.
(Hereinafter abbreviated as LPF 9) is input to the subtractor 10.
Incidentally, interference intensity I _P which is detected by the photodetector 8, an AC signal. LPF9 cuts the AC component of the input interference intensity I _P, and extracts the DC component I _DC. The DC component I _DC extracted by the LPF 9 is input to a subtractor 10 and a divider 11. DC component I _DC from the detected interference intensity I _P in the photodetector 8, the subtracter 10 is subtracted,
Alternating current component I _AC of the interference intensity I _P are extracted. The extracted AC component I _AC is input to a divider 11, which converts the _AC component I _AC into a DC component I _DC as shown in (Equation 5).
To obtain an output S.

[0025]

(Equation 5)

[0026] In this equation (5), if equal A and B is the optical complex amplitudes of two circularly polarized light, the output S of the divider 11 is expressed by S = sin2θ _I. Therefore, (Equation 5)
The phase change θ _I is obtained based on the output S (actually measured value) of the divider 11. Further, the relationship of (Equation 6) holds between the phase change θ _I and the current I flowing through the conductor 5.

[0027]

Θ _I = nV _E I (Equation 6) Here, V _E is a Verdet constant, and n is the number of turns of the sensor fiber 4 with respect to the conductor 5, and both are constants.
The current value of the current flowing through the conductor 5 is obtained based on (Equation 6).

As described above, according to this embodiment,
By simultaneously entering clockwise circularly polarized light and counterclockwise circularly polarized light having the same optical complex amplitude from one end of the sensor fiber 4,
Since the two circularly polarized lights propagate through exactly the same path, the phase change θ _R caused by the torsion of the sensor fiber 4 becomes equal between the two circularly polarized lights, and the two circularly polarized lights interfere with each other to reduce the phase change θ _R due to the torsion. Can be completely offset. Therefore, the phase change θ _I generated between the two circularly polarized lights by the magnetic field generated by the current flowing through the conductor 5 can be accurately obtained, and the measurement accuracy of the current flowing through the conductor 5 can be improved.

In this embodiment, the detection sensitivity of the photodetector 8 when the phase change θ _I is small can be improved by giving a phase difference of π / 2 between two circularly polarized lights by the Faraday element 6. it can.

In this embodiment, a phase difference of π / 2 is given to the two circularly polarized lights that have propagated through the sensor fiber 4. However, as shown in FIG. 4 and a phase difference of π / 2 between the two circularly polarized lights by the Faraday element 6.
A similar result can be obtained even if two circularly polarized lights enter the sensor fiber 4.

(Embodiment 2) FIG. 4 shows a configuration of a current sensor according to another embodiment of the present invention. In this embodiment, a mirror is provided at the other end of the sensor fiber to cause light to reciprocate,
The difference from the first embodiment is that conversion from linearly polarized light to circularly polarized light is performed using a fiber. Hereinafter, the present embodiment will be described focusing on the differences from the first embodiment.

In FIG. 4, light output from the light source 1 is transmitted through a fiber coupler 12 to a fiber polarizer 13.
Is incident on. In addition, the fiber coupler 12 is a light source 1
The light output from is input to the fiber polarizer 13 and the light reflected and returned by a mirror 15 described later is input to the photodetector 8. The fiber polarizer 13
The PMF is wound in a coil shape, and converts incident light into linearly polarized light.

The linearly polarized light obtained by the fiber polarizer 13 has a main polarization axis of 45 ° with respect to the fiber polarizer 13.
The light is incident on the PMF 2b that is connected by being inclined. PMF
The linearly polarized light incident on 2b is a linearly polarized light of two orthogonal components. The other end of the PMF 2b is connected to a fiber 14 whose main polarization axis is inclined by 45 °. Fiber 14 is 10π
It has birefringence of [rad / m], and propagates 5 [cm] of linearly polarized light incident from the PMF 2b. Accordingly, a phase difference of π / 2 can be given to the two linearly polarized lights, and P
The two linearly polarized lights are converted into clockwise circularly polarized light (RHCP) and counterclockwise circularly polarized light (LHCP) because the main polarization axis is inclined by 45 ° with respect to MF2b. That is, the fiber 14 of this embodiment has a function of converting linearly polarized light of two orthogonal components into two circularly polarized lights, similarly to the λ / 4 plate 3 of the first embodiment.

The two circularly polarized lights obtained by the fiber 14 are input to the Faraday element 6, and the two circularly polarized lights are given a phase difference of π / 4. The two circularly polarized lights that have passed through the Faraday element 6 propagate through the sensor fiber 4, are reflected by a mirror 15 provided at the other end of the sensor fiber 4, and propagate through the sensor fiber 4 again. Incident on. In the Faraday element 6, since a phase difference of π / 4 is again given to the two circularly polarized lights, a phase difference of π / 2 is given to the two circularly polarized lights including the forward path and the return path.

Thereafter, the two circularly polarized lights are converted into fibers 14,
The light is input to the photodetector 8 via the PMF 2b, the fiber polarizer 13, and the fiber coupler 12. In the optical detector 8, is detected similarly interference intensity I _P as in Example 1, the current flowing in the conductor 5 is measured based on the detected interference intensity I _P. Since the point of obtaining the current value of the current flowing through the conductor from the interference intensity I _P is the same as in Example 1,
Description is omitted.

According to this embodiment described above, Embodiment 1
In addition to the function and effect of the above, the light is reciprocated by using the mirror 15, so that the sensor fiber 4 is affected twice by the magnetic field caused by the current flowing through the conductor 5, and as a result, the measurement accuracy of the current is improved. be able to.

In this embodiment, the fiber polarizer 13 for producing linearly polarized light and the fiber 14 for converting linearly polarized light to circularly polarized light are used. And can be easily assembled and adjusted.

(Embodiment 3) FIG. 5 shows the configuration of a current sensor according to another embodiment of the present invention. This embodiment is different from the second embodiment in that a phase modulator 16 is provided instead of the Faraday element 6.
And different. Hereinafter, points different from the second embodiment will be described.

In FIG. 5, the phase modulator 16 has a PMF2
b, and based on the output of the oscillator 17, the PMF 2b
Phase modulation is applied to linearly polarized light propagating inside. The phase modulator 16 is configured by winding a fiber around a piezoelectric element, and changes the fiber length by using the expansion of the piezoelectric element by applying a voltage to apply phase modulation to light.
The light phase-modulated by the phase modulator 16 is transmitted to the mirror 1.
Assuming that the propagation time from the reflection at 5 to the return to the phase modulator 16 is T, the interference intensity I _P detected by the photodetector 8 is represented by (Equation 7) and (Equation 8).

[0040]

I _p = A ² {1 + cos (4θ _I + φ _m (t))} (Equation 7)

[0041]

Equation 8] φ _m (t) = φ amplitude of 1 times the modulation frequency component included in the I _P of {coswt-cos (wt-WT )} ... ( 8) (7) is proportional to Sin4shita _I to order to detect the amplitude of the first time the modulation frequency component included in the I _P based on the output of the oscillator 17 in synchronism detector 18 can determine the current flowing in the conductor 5 on the basis of the detected amplitude.

According to this embodiment, the same functions and effects as those of the second embodiment can be obtained.

[0043]

According to the present invention, the influence of the change in the phase of the propagating light due to the twisting of the sensor fiber can be suppressed with a simple structure, and the current measurement accuracy can be improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a current sensor according to a preferred embodiment of the present invention.

FIG. 2 is a diagram showing linearly polarized light in the polarization-maintaining fibers 2a and 2b in FIG. 1 and circularly polarized light obtained by a λ / 4 plate 3;

FIG. 3 is an example in which a Faraday element 6 is arranged between a λ / 4 plate 3 and a sensor fiber 4;

FIG. 4 is a configuration diagram of a current sensor according to another embodiment of the present invention.

FIG. 5 is a configuration diagram of a current sensor according to another embodiment of the present invention.

[Explanation of symbols]

1. Light source, 2a, 2b ... Polarization maintaining fiber (PM
F), 3 ... λ / 4 plate, 4 ... sensor fiber, 5 ... conductor,
6: Faraday element, 7: Fiber, 8: Photodetector, 9
... Low-pass filter (LPF), 10 ... Subtractor, 11 ...
Divider.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Isamu Sone 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi, Ltd. Electric Power and Electrical Development Laboratory 2G025 AA00 AA17 AB10

Claims (4)

[Claims] 1. A sensor fiber wound around a conductor through which a current to be measured flows, means for injecting right-handed circularly polarized light and left-handed circularly polarized light from one end of the sensor fiber, and the other end of the sensor fiber A photodetector that is disposed and detects the interference intensity of the right-handed circularly polarized light and the left-handed circularly polarized light that has propagated through the sensor fiber, and a current that flows through the conductor based on the interference intensity detected by the photodetector Means for determining the current value of the current sensor. 2. A sensor fiber wound around a conductor through which a current to be measured flows, means for injecting clockwise circularly polarized light and counterclockwise circularly polarized light from one end of the sensor fiber, and the other end of the sensor fiber. A mirror disposed to reflect the clockwise circularly polarized light and the counterclockwise circularly polarized light that has propagated through the sensor fiber, and disposed at one end of the sensor fiber where the clockwise circularly polarized light and the counterclockwise circularly polarized light are incident; And a photodetector that detects the interference intensity of the clockwise circularly polarized light and the counterclockwise circularly polarized light that have propagated through the sensor fiber after being reflected by the mirror, based on the interference intensity detected by the photodetector. Means for calculating a current value of a current flowing through the conductor. 3. A current sensor according to claim 1, further comprising means for giving a phase difference of π / 2 to said clockwise circularly polarized light and said counterclockwise circularly polarized light. 4. The means for injecting the right-handed circularly polarized light and the left-handed circularly polarized light includes a converting means for converting two-component orthogonally polarized light into right-handed circularly polarized light and left-handed circularly polarized light. Is a fiber that is connected to the fiber that propagates the orthogonal two-component linearly polarized light while the main polarization axis is inclined at 45 ° and has birefringence.
The current sensor according to any one of the above.