JP2008256368A - Optical fiber current sensor, and current measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely control a polarized wavefront of light propagated through a sensor fiber in an optical fiber current sensor and a current measuring method using the optical fiber current sensor. <P>SOLUTION: This optical fiber current sensor includes a Faraday rotator 30 and a polarized wave control means 20, and the polarized wave control means 20 is constituted of the second sensor fiber 21, and a current coil 22 disposed to surround the second sensor fiber 21. Linear polarized light imparted with Faraday rotation gets incident into the first sensor fiber 11 for measuring a current to be measured I, by the Faraday rotator 30 and the polarized wave control means 20. A control current I<SB>ctrl</SB>flowing in the current coil 22 is regulated to control thereby the polarized wavefront of the linear polarized light incident into the first sensor fiber 11. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical fiber current sensor and a current measurement method for measuring current using a Faraday effect in which a polarization plane of light propagating in an optical fiber is rotated by a magnetic field.

In recent years, an optical fiber current sensor using an optical fiber as a sensor has attracted attention as a current measuring device for monitoring power facilities and the like.
In this optical fiber current sensor, the current is measured using the Faraday effect in which the polarization plane of light propagating in the magnetic medium rotates in proportion to the magnitude of the magnetic field in the propagation direction. An optical fiber is also a kind of magnetic medium. When linearly polarized light is incident on an optical fiber used as a sensor and placed near a conductor through which a current to be measured flows, that is, a magnetic field generation source, the optical fiber is polarized to linearly polarized light in the optical fiber by the Faraday effect. Wavefront rotation (Faraday rotation) is given. At this time, since a magnetic field proportional to the current is generated, the rotation angle of the plane of polarization (Faraday rotation angle) due to the Faraday effect is proportional to the magnitude of the current to be measured. Therefore, the magnitude of the current can be obtained by measuring the Faraday rotation angle. This is the principle of the optical fiber current sensor.

  Conventionally, as such an optical fiber current sensor, a transmission type (for example, see Non-Patent Document 1) and a reflection type are known. The reflection type optical fiber current sensor will be briefly described with reference to FIG.

FIG. 5 is a diagram showing a configuration of a reflection type optical fiber current sensor in a conventional example (see Patent Document 1).
In the same figure, the reflection type optical fiber current sensor has an optical circulator 19, a polarization separation element 18, a Faraday rotator 102, and a sensor fiber 11B. The sensor fiber 11B is arranged so as to go around the conductor 100 such as a power transmission line through which the current I to be measured flows. A Faraday rotator 102 is attached to one end of the sensor fiber 11B, and a reflection portion (mirror) 111 is formed at the other end by vapor deposition of a metal thin film. Further, the Faraday rotator 102 and the polarization separation element 18, and the polarization separation element 18 and the optical circulator 19 are respectively connected by optical fibers, and the optical circulator 19 is connected in a direction in which light from the light source 12 is transmitted to the sensor fiber 11B side. The

  With respect to the thus configured optical fiber current sensor, light emitted from the light source 12 is incident on the polarization separation element 18 via the light transmission fiber 71 and the optical circulator 19. This light is converted into linearly polarized light in which the vibration direction of the electric field is aligned in one direction (the principal axis direction of the polarization separation element 18) by the polarization separation element 18, and is input to the Faraday rotator 102. The Faraday rotator 102 includes a permanent magnet 104 and a ferromagnetic garnet 103 that is a ferromagnetic crystal that is magnetically saturated by the permanent magnet 104, and the light passing through the ferromagnetic garnet 103 is 22.5 degrees one way. Gives Faraday rotation. The linearly polarized light exiting the Faraday rotator 102 is input to the sensor fiber 11B, and subjected to Faraday rotation by the magnetic field generated around the current I to be measured flowing through the conductor 100 in the surrounding portion of the sensor fiber 11B. It rotates by the Faraday rotation angle proportional to the magnitude of the magnetic field.

  The light propagating through the sensor fiber 11 </ b> B is further reflected by the reflecting portion 111, passes through the circulation portion again, undergoes Faraday rotation, and is input to the Faraday rotator 102. By passing through the Faraday rotator 102 again, 22.5 degree Faraday rotation is given, so that the Faraday rotator 102 sets an optical bias of 45 degrees in a reciprocating manner (the optical bias will be described later). It will be. The light that has passed through the Faraday rotator 102 is guided again to the polarization separation element 18 and separated into two polarization components whose polarization directions are orthogonal to each other (the main axis direction of the polarization separation element 18 and light perpendicular thereto). One of the separated lights is received by the light receiving element 13A through the optical circulator 19 and the light receiving fiber 72A, and converted into an electric signal S1. The other light is received by the light receiving element 13B through the light receiving fiber 72B and converted into an electric signal S2.

  Since the amount of light received by the light receiving elements 13A and 13B changes according to the Faraday rotation angle given to the linearly polarized light propagating in the circular portion of the sensor fiber 11B, the electric signals S1 and S2 reflecting this change are subjected to signal processing. By applying the processing by the circuit 141, the given Faraday rotation angle can be obtained. Then, the measured current I is calculated from the obtained Faraday rotation angle. The light receiving elements 13A and 13B and the signal processing circuit 141 constitute a signal processing unit 14.

Here, when the Faraday rotation angle in the sensor fiber 11B is θ _F , and the measured current I = 0, the polarization direction of the linearly polarized light incident on the polarization separation element 18 through the sensor fiber 11B and the principal axis of the polarization separation element 18 It is known that the intensity of light output from the analyzer 16 and received by the light receiving elements 13A and 13B changes according to cos (2θ ₀ −2θ _F ), where θ ₀ is an angle formed with the direction. From this equation, by changing the angle theta _0, it can be seen that setting the operating point for measuring the Faraday rotation angle theta _F is the amount to be measured accordingly. Thus, changing and setting the angle θ ₀ to determine the operating point is referred to as setting the optical bias. Setting the optical bias appropriately is important for improving the accuracy of measurement. For example, when the Faraday rotation angle θ _F changes by a small amount, in order to maximize the light intensity change ratio (detection sensitivity) given by the above equation, θ ₀ = π / 4 is used. It will be good. From this, the linearly polarized light returned from the sensor fiber 11B using the Faraday rotator 102 so that the optical bias becomes θ ₀ = π / 4 (45 degrees), that is, when the measured current I = 0. Is set so that the plane of polarization rotates 45 degrees.

  In the above-described optical fiber current sensor, a lead glass fiber having a very small photoelastic coefficient is suitably used for the sensor fiber 11B. This is because, when the photoelastic coefficient is large, mode conversion occurs in the propagating light through the photoelastic effect under the influence of anisotropic stress in the fiber, and the output light intensity from the fiber fluctuates. This is because there is a problem.

Note that the phenomenon that the polarization of the light propagating in the fiber depends on the shape of the curve formed by the sensor fiber when the sensor fiber is installed is generally the case even when using a lead glass fiber with a small photoelastic coefficient. appear. However, in the optical fiber current sensor having the reflection type configuration described above, the light propagating through the sensor fiber 11B is reflected by the reflector 111 and reciprocates in the sensor fiber 11B. They are offset and are not affected by the above phenomenon as a whole.
Japanese Patent No. 3685906 Kurosawa et al., Compact and Flexible Optical Fiber Current Sensor, 30th Lightwave Sensing Technology Study Group, Japan Society of Applied Physics, December 2002, LST 30-19, 133-140 Fujii et al., Tb3Al5O12 single crystal growth and optical property evaluation showing Faraday effect, NEW GLASS, New Glass Forum, 2003, Vol. 18, No. 4, pp. 32-36

Incidentally, as described above, it is important to set the optical bias appropriately in order to measure the current with good sensitivity and accuracy using the optical fiber current sensor.
However, since the Faraday rotator 102 (ferromagnetic garnet 103) used for applying an optical bias in a reflective optical fiber current sensor is an industrial product, variations in characteristics of individual products are inevitable. There is a problem in that it becomes an error and affects the measurement.

On the other hand, it is desired to improve the sensitivity of current measurement by an optical fiber current sensor, and as one method for that purpose, it is conceivable to shorten the wavelength of light used for measurement (details will be described later). To do). However, the above-mentioned ferromagnetic garnet, which is a ferromagnetic crystal, is not suitable for use because the light transmittance deteriorates in the wavelength band of light of about 1000 nm or less. Therefore, it is conceivable to construct a Faraday rotator using a paramagnetic material having a good light transmittance in a wavelength band of 1000 nm or less as a material to replace the ferromagnetic garnet. As such a paramagnetic substance, paramagnetic garnets such as TGG and TAG exist. For example, when TAG is used, according to the non-patent document 2, the transmittance can be used up to a wavelength of about 500 nm without being lowered.
However, the temperature dependence of the Verde constant that determines the magnitude of the Faraday effect is larger in such paramagnetic materials than in ferromagnetic crystals (for example, ferromagnetic garnet), so that optical fibers using these paramagnetic materials are used. In the current sensor, the optical bias varies depending on the environmental temperature, which affects the current measurement. Further, since the paramagnetic material is not magnetically saturated, there is a problem that the setting of the optical bias fluctuates (the operating point fluctuates) depending on the influence of the external magnetic field and the temperature characteristics of the permanent magnet to which the magnetic field is applied.

  The present invention has been made in view of the above points, and an object thereof is to accurately control the polarization plane of light propagating through a sensor fiber in an optical fiber current sensor and a current measurement method using the optical fiber current sensor. Is to make it possible to do.

The present invention has been made in order to solve the above-mentioned problems, and includes a first sensor fiber, linearly polarized light is input to the first sensor fiber, and flows through a conductor installed around the first sensor fiber. In the optical fiber current sensor that measures the current to be measured by detecting the Faraday rotation angle of the linearly polarized light induced by the magnetic field generated by the measurement current, a predetermined Faraday is applied to the linearly polarized light that passes through the first sensor fiber. A Faraday rotator for imparting rotation, a second sensor fiber connected to the first sensor fiber and receiving linearly polarized light, and a current coil surrounding the second sensor fiber, and adjusting a current flowing through the current coil. Polarization control for imparting Faraday rotation to linearly polarized light in the second sensor fiber by a magnetic field generated by the current And stage, the provided, characterized in that to provide the desired Faraday rotation by said Faraday rotator and said polarization control means to linearly polarized light that passes through the first sensor fiber.
In this invention, a magnetic field is generated in the second sensor fiber by the current flowing through the current coil, and Faraday rotation corresponding to the magnetic field is given to the linearly polarized light propagating in the second sensor fiber. The linearly polarized light subjected to the Faraday rotation and the Faraday rotation by the Faraday rotator is incident on the first sensor fiber. By adjusting the current flowing through the current coil, the total Faraday rotation angle applied to the linearly polarized light input to the first sensor fiber can be controlled. Therefore, it is possible to control the polarization plane of light with high accuracy by adjusting the current.

In the optical fiber current sensor, the influence of characteristic fluctuations of the Faraday rotator is compensated by adjusting a current flowing through the current coil.
According to the present invention, even when the Faraday rotation angle by the Faraday rotator is originally deviated from a desired value or fluctuates due to environmental changes or the like, the influence is eliminated by a simple and flexible method of current adjustment. be able to.

In the above optical fiber current sensor, the characteristic variation is a change in Faraday rotation angle depending on temperature.
According to the present invention, current measurement independent of the environmental temperature can be performed.

In the optical fiber current sensor, the Faraday rotator is a paramagnetic material or a ferromagnetic material magnetized by applying a magnetic field.
According to the present invention, the current flowing through the current coil can be reduced by selecting a paramagnetic material or a ferromagnetic material that exhibits a large Faraday effect.

In the above optical fiber current sensor, the paramagnetic material is TGG or TAG that is a paramagnetic crystal, or FR5 that is paramagnetic glass.
In the present invention, since TGG, TAG, and FR5 are transparent even in the visible wavelength band, it is possible to perform current measurement using a wavelength band that provides a highly sensitive Faraday effect.

In the optical fiber current sensor, the polarization control means includes the linear second sensor fiber, and one or a plurality of windings of the current coil installed so that the second sensor fiber penetrates the second sensor fiber. It is characterized by comprising.
In the present invention, by making the current coil into a plurality of turns, if the magnetic field applied to the second sensor fiber is the same, the current flowing through the current coil can be reduced, and the power consumption can be reduced. Further, it is possible to reduce the size of the power supply that supplies current to the current coil.

Further, in the optical fiber current sensor, the polarization control means includes the coil-shaped second sensor fiber, and the one or a plurality of turns of the current coil installed so as to be linked to the second sensor fiber, It is comprised from these.
In this invention, by making the second sensor fiber into a coil shape, the Faraday rotation angle imparted to the linearly polarized light propagating in the second sensor fiber can be multiplied by the number of turns of the coil. Therefore, when the measurement sensitivity (the magnitude of the measured Faraday rotation angle) is the same, the current flowing through the current coil can be further reduced, and the power consumption can be further reduced. Further, it is possible to further reduce the size of the power source that supplies current to the current coil.

In the optical fiber current sensor, the first sensor fiber has a reflecting portion at the other end opposite to the incident end to which linearly polarized light is input, and includes the Faraday rotator and the polarization control means. An optical bias setting means is installed on the incident end side of the first sensor fiber, converts light input from a light source into linearly polarized light and outputs the linearly polarized light to the optical bias setting means, and the reflecting portion of the first sensor fiber. And a polarization separation element installed on the light source side with respect to the optical bias setting means for separating the light reflected from the light into two polarization components orthogonal to each other.
According to the present invention, a reflection type optical fiber current sensor can be configured by the first sensor fiber having the reflection portion, the Faraday rotator, the polarization control means, and the polarization separation element.

In the optical fiber current sensor, the optical bias setting means sets the optical bias so that the rate of change in the intensity of the light output from the analyzer is the largest with respect to the change in the Faraday rotation angle. It is characterized by.
According to the present invention, by adjusting the current flowing through the current coil, the optical bias when the light reciprocates once through the optical bias setting means can be set to a state in which the detection sensitivity with respect to the change in light intensity is maximized. The current can be measured with high sensitivity.

In the optical fiber current sensor, at least one of the first sensor fiber and the second sensor fiber is a lead glass fiber.
According to this invention, since the photoelastic coefficient of the sensor fiber is small, fluctuations in the intensity of light propagating through the sensor fiber can be suppressed.

  According to the present invention, it is possible to realize an optical fiber current sensor and a current measurement method capable of accurately controlling the polarization plane of light propagating through a sensor fiber.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 shows a block diagram of an optical fiber current sensor according to a first embodiment of the present invention.
In the figure, the optical fiber current sensor has a first sensor fiber 11, a polarization control means 20, and a Faraday rotator 30. The polarization control means 20 includes a second sensor fiber 21 and a current coil 22 installed so as to surround the second sensor fiber 21, and the second sensor fiber 21 is connected to the first sensor fiber 11. Yes. The Faraday rotator 30 includes a Faraday element 31 and a permanent magnet 32 that applies a magnetic field to the Faraday element 31, and the Faraday element 31 is attached to the other end of the second sensor fiber 21.

Both the first sensor fiber 11 and the second sensor fiber 21 are preferably lead glass fibers, but only one of them may be a lead glass fiber. When both are lead glass fibers, the first sensor fiber 11 and the second sensor fiber 21 may be integrated (single) continuous fibers.
This lead glass fiber has a characteristic that the photoelastic coefficient is very small and the Verde constant that determines the magnitude of the Faraday effect is relatively large, and is very excellent as a sensor fiber of an optical fiber current sensor. Further, it has a high transmittance over a wavelength of about 630 nm in the visible region from the infrared 1550 nm band, and can be used for light having a wide range of wavelengths.

A control current _Ictrl whose current value is controlled by an external current control circuit (not shown) is supplied to the current coil 22 of the polarization control means 20 from an external power supply (not shown). The control current I _ctrl flowing through the current coil 22 generates a magnetic field in the direction along the fiber in the second sensor fiber 21, and the Faraday rotation is caused to the light propagating in the second sensor fiber 21 by the generated magnetic field. It has come to be given.

For the Faraday element 31, a ferromagnetic material or a paramagnetic material can be appropriately selected and used according to the application of the optical fiber current sensor and the wavelength used. For example, when a ferromagnetic garnet that is a ferromagnetic material is used, the wavelength used is limited to approximately 1000 nm or more. In addition, TAG (terbium aluminum garnet; Tb ₃ Al ₅ O ₁₂ ), TGG (terbium gallium garnet; Tb ₃ Ga ₅ O ₁₂ ) which is a paramagnetic crystal, FR5 which is paramagnetic glass, or the like is used. In this case, it is possible to select a wavelength in the visible region as the wavelength used, and the Verde constant of each of these substances increases by shortening the wavelength of the light used for measurement, so increase the Faraday rotation angle. Can do.

In the optical fiber current sensor having the above configuration, light undergoes Faraday rotation by the polarization control means 20 and the Faraday rotator 30 and is incident on the first sensor fiber 11.
That is, when linearly polarized light P1 having a polarization plane as indicated by an arrow from the left in the figure is input to the Faraday element 31 of the Faraday rotator 30, this linearly polarized light P1 is subjected to Faraday rotation by the Faraday element 31, and the polarization plane is rotated. Then, it is output as linearly polarized light P2 having different polarization planes. This linearly polarized light P2 is input to the second sensor fiber 21 of the polarization control means 20, and further undergoes Faraday rotation in the direction and size according to the control current _Ictrl while propagating through the second sensor fiber 21. As a result, the plane of polarization is rotated, and the linearly polarized light P3 having a different plane of polarization is output from the second sensor fiber 21.

The Faraday rotation angle given by the Faraday rotator 30 varies depending on the material of the Faraday element 31, the magnetic field to be applied (in the case of a paramagnetic material), the wavelength used, etc., but cannot be changed at any time and is a fixed value. On the other hand, in the polarization control means 20, the polarization direction of the linearly polarized light P3 to be output can be arbitrarily controlled by adjusting the magnitude and direction of the control current _Ictrl . Thus, the Faraday rotator 30 and the polarization control means 20 give a desired Faraday rotation to the linearly polarized light input to the first sensor fiber 11. For example, Faraday rotation close to a desired Faraday rotation angle can be imparted by the Faraday rotator 30 and adjustment to the desired Faraday rotation angle can be performed by the polarization control means 20. In this case, even if the Faraday rotator 30 has a characteristic deviation due to individual differences or temperature fluctuations, it is possible to perform highly accurate measurement by correctly controlling the polarization plane with the polarization control means 20.
Note that the optical fiber current sensor may be configured such that the polarization control means 20 is arranged at the front stage and the Faraday rotator 30 is arranged at the rear stage.

Now, the linearly polarized light P3 output from the polarization control means 20 is input to the first sensor fiber 11 through the fiber. The first sensor fiber 11 is installed in the vicinity of a conductor 100 (shown as being perpendicular to the paper surface) through which the current I to be measured flows. In this first sensor fiber 11, a magnetic field H is generated in the direction along the fiber by the current I to be measured, and this magnetic field H causes the Faraday rotation angle to the linearly polarized light P <b> 3 propagating in the first sensor fiber 11. Faraday rotation is given by theta _F. Thus, the linearly polarized light P4 whose polarization plane is rotated reflecting the magnitude of the current I to be measured is output from the first sensor fiber 11. The linearly polarized light P4 is incident on the analyzer 16, the main axis of the analyzer 16 (in the figure indicated by arrows) by detecting the intensity of the linearly polarized light P4 directional component, the Faraday rotation angle theta _F is determined, the determined the size of the measured current I is determined from the Faraday rotation angle theta _F that is.

FIG. 2 is a block diagram showing a modification of the polarization control means 20 in the present embodiment.
In FIG. 3A, the polarization control means 20 includes a linear (for example, linear) second sensor fiber 21A and a plurality of windings (5 in the figure) installed so that the second sensor fiber 21A penetrates the inside. And a current coil 22A. By doing so, when the magnitude of the control current _Ictrl is made constant, the magnitude of the generated magnetic field increases, so that a larger Faraday rotation can be given to the linearly polarized light P2 propagating through the second sensor fiber 21A. it can. In addition, when the Faraday rotation angle to be applied is constant, the required control current _Ictrl can be small, so that the scale of the power source that supplies the control current _Ictrl can be reduced.

In FIG. 2B, the polarization control means 20 is installed so as to be linked to the second sensor fiber 21B wound in a coil shape (twice in the figure) and the second sensor fiber 21B. The current coil 22B is composed of a plurality of windings (5 windings in the figure). In this configuration, the ring formed by the second sensor fiber 21B and the ring formed by the current coil 22B intersect with each other in a chain shape by penetrating the inside of each ring. By doing so, when the magnetic field generated by the control current _Ictrl is made constant, the distance that the linearly polarized light P2 propagating in the second sensor fiber 21B passes through the magnetic field becomes long, so that the linearly polarized light P2 is larger. Faraday rotation can be given. In addition, when the Faraday rotation angle to be applied is constant, the required control current _Ictrl is smaller than the configuration shown in FIG. 2A, so the power supply for supplying the control current _Ictrl can be further reduced in scale. can do.

(Second Embodiment)
FIG. 3 shows a configuration diagram of a reflective optical fiber current sensor according to the second embodiment of the present invention.
In the figure, the optical fiber current sensor has an optical circulator 19, a polarization separation element 18, a Faraday rotator 30, a polarization control means 20, and a sensor fiber 11B. The sensor fiber 11B is arranged so as to go around the conductor 100 such as a power transmission line through which the current I to be measured flows. The polarization control means 20 and the Faraday rotator 30 are the same as the configuration of FIG. 1 or FIG. 2 described as the first embodiment. One end of the sensor fiber 11B is connected to one end of the second sensor fiber 21 of the polarization control means 20, and a reflecting portion (mirror) 111 is formed at the other end by vapor deposition of a metal thin film. A Faraday element 31 of a Faraday rotator 30 is attached to the other end of the second sensor fiber 21, and the Faraday element 31 is further connected to the optical circulator 19 via the polarization separation element 18. The optical circulator 19 is connected in a direction in which light from the light source 12 is transmitted to the sensor fiber 11B side.

With respect to the thus configured optical fiber current sensor, light emitted from the light source 12 is incident on the polarization separation element 18 via the light transmission fiber 71 and the optical circulator 19. This light is converted into linearly polarized light in which the vibration direction of the electric field is aligned in one direction (the principal axis direction of the polarization separation element 18) by the polarization separation element 18, and is input to the Faraday rotator 30. The linearly polarized light input to the Faraday rotator 30 is output after being subjected to Faraday rotation of a predetermined magnitude and input to the polarization control means 20. The linearly polarized light input to the polarization control means 20 is subjected to Faraday rotation having a direction and magnitude corresponding to the control current _Ictrl , and its polarization plane is rotated, and is introduced into the first sensor fiber 11B. In this embodiment, this control current _Ictrl is set so that the Faraday rotation angle given by the Faraday rotator 30 and the polarization control means 20 is 22.5 degrees. The linearly polarized light that is introduced into the sensor fiber 11B and propagates through the fiber is further subjected to Faraday rotation by the magnetic field generated around the current I to be measured flowing through the conductor 100 in the circumferential portion of the fiber, and the plane of polarization is the magnitude of the magnetic field. Rotate by Faraday rotation angle proportional to.

  The light propagating through the sensor fiber 11B is further reflected by the reflecting portion 111, passes through the circular portion again, undergoes Faraday rotation, and sequentially passes through the polarization control means 20 and the Faraday rotator 30. By passing through the polarization control means 20 and the Faraday rotator 30 again, a 22.5 degree Faraday rotation is given, so that the polarization control means 20 and the Faraday rotator 30 reciprocate a 45 degree optical bias. Is set. The light exiting the Faraday rotator 30 is guided again to the polarization separation element 18 and separated into two polarization components whose polarization directions are orthogonal to each other (the main axis direction of the polarization separation element 18 and light perpendicular thereto). One of the separated lights is received by the light receiving element 13A through the optical circulator 19 and the light receiving fiber 72A, and converted into an electric signal S1. The other light is received by the light receiving element 13B through the light receiving fiber 72B and converted into an electric signal S2.

Then, by processing the electric signals S1 and S2 by the signal processing circuit 141, the Faraday rotation angle given by the measured current I is obtained, and the measured current I is calculated from the Faraday rotation angle. Further, the signal processing circuit 141 calculates a deviation from a desired optical bias (here, the optimum value of 45 degrees) using the electrical signals S1 and S2, and sends the result to the current control unit 60 as a control signal. . The current control unit 60 constantly maintains a desired optical bias by supplying a control current _Ictrl whose direction and magnitude are adjusted from the DC power supply 61 to the current coil 22 in accordance with this control signal. As a result, even if the Faraday rotation angle in the Faraday rotator 30 varies due to, for example, the influence of a magnetic field generated externally or the environmental temperature changes, the Faraday rotator 30 When viewed as a whole of the polarization control means 20, a constant Faraday rotation is always applied, and the accuracy and stability of current measurement increase.
The light receiving elements 13A and 13B and the signal processing circuit 141 constitute a signal processing unit 14.

  In the present embodiment, the optical bias is set to 45 degrees as described above and is maintained, so that the light receiving elements 13A and 13B when the Faraday rotation angle due to the current I to be measured changes by a small amount. The light intensity at 1 changes with the highest sensitivity, and the measurement accuracy is maintained in a good state.

In the present invention, the method for obtaining the control signal is not limited. For example, the control current _Ictrl is oscillated at a frequency different from that of the current I to be measured, and the control signals I1 and S2 are passed through the bandpass filter. _A method of extracting a component corresponding to _ctrl is applicable.

(Third embodiment)
FIG. 4 shows a configuration diagram of a reflective optical fiber current sensor according to the third embodiment of the present invention.
This embodiment is different from the second embodiment described above only in the method of controlling the control current _Ictrl , so only that portion will be described and the description of the other portions will be omitted.

The optical fiber current sensor of the present embodiment is configured such that a temperature sensor 62 is attached to the Faraday element 31 and the temperature sensor 62 detects the temperature of the Faraday element 31 and notifies the current control unit 60 of the temperature. Since the Faraday element 31 exhibits a certain temperature dependency on the Faraday rotation angle imparted to the light passing therethrough, the actual Faraday rotation angle can be obtained from this temperature dependency and the detected temperature. The current control unit 60 controls the control current _Ictrl supplied from the DC power supply 61 using this principle.

(Example)
Next, an actual optical fiber current sensor to which the present invention is applied will be described.
Referring to FIG. 2B, here, the polarization control means 20 is configured with 1000 turns of the current coil 22B and 5 turns of the second sensor fiber 21B. At this time, if the current coil 22B is made of a copper wire having a diameter of 1 mm, the diameter of the cross section of the current coil 22B is approximately 32 mm. When the second sensor fiber 21B is wound around the current coil 22B having a cross-sectional diameter of 32 mm for five turns, the length of the second sensor fiber 21B is approximately 50 cm.

  On the other hand, in this embodiment, a He—Ne (helium-neon) laser having a wavelength of 633 nm is used as light used for measurement. In the past, measurements were often made using light in the 1550 nm band, but the Verde constant, which determines the magnitude of the Faraday effect, is known to be proportional to the square of the wavelength, shortening the wavelength. By doing so, it can be expected that the Faraday rotation angle is increased even if the magnitude of the magnetic field is the same, and the sensitivity of current measurement is improved. In the case of using the wavelength of 633 nm, the Verde constant is about 6 times that in the case of using the wavelength of 1550 nm.

  In the present embodiment, a lead glass fiber is used for the second sensor fiber 21B. It is known that the Verde constant of a lead glass fiber at a wavelength of 1550 nm is approximately 45 degrees / 200 kA (A represents a current flowing through the current coil in amperes). This Verde constant indicates that when light having a wavelength of 1550 nm is used for measurement, a current required for obtaining a Faraday rotation angle of 45 degrees is 200 kA when both the current coil and the second sensor fiber are wound.

  In this embodiment, the temperature dependence of the Faraday rotator 30 is eliminated by using the polarization control means 20 having such a configuration. The Faraday element 31 of the Faraday rotator 30 is made of FR5 that is paramagnetic glass. It is known that the temperature dependence of the Verde constant in FR5 is about 0.34 degrees / ° C. On the other hand, the optical fiber current sensor needs to compensate the measurement accuracy within a temperature range of about 100 ° C. Therefore, the polarization control means 20 is required to be capable of controlling the Faraday rotation angle by 3.4 degrees (= 0.34 × 100).

In this embodiment, since a He—Ne laser with a wavelength of 633 nm is used, the Verde constant is 6 times the 45 degrees / 200 kA described above, and the current coil and the second sensor fiber are configured to have 1000 turns and 5 turns, respectively. Therefore, the current that must be passed through the current coil is
0.5 A (= 200 kA / (45 / 3.4) / (6 × 1000 × 5))
Is calculated. This value is realistic, and it has been shown that the optical fiber current sensor to which the present invention is applied can control the optical bias with high accuracy in the temperature range of 100 ° C. at the wavelength of 633 nm.

Thus, according to the present embodiment, the Faraday rotator 30 and the polarization control means 20 are provided in the optical fiber current sensor, and the polarization control means 20 includes the second sensor fiber 21 and the second sensor fiber. And a current coil 22 installed so as to surround 21. Linearly polarized light that has been subjected to Faraday rotation by the Faraday rotator 30 and the polarization control means 20 is incident on the first sensor fiber 11 that measures the current I to be measured. Then, the polarization plane of linearly polarized light incident on the first sensor fiber 11 is controlled by adjusting the control current _Ictrl flowing through the current coil 22. Thereby, the polarization plane can be controlled with high accuracy by adjusting the control current _Ictrl .

As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to
For example, the material of the first sensor fiber and the second sensor fiber in the present invention is not limited, and appropriate materials other than lead glass fibers can be selected and used.
Further, the wavelength band of light to be used does not preclude the use of the 1550 nm band that has been widely used in the past.
Furthermore, the measured current I to be measured can be either direct current or alternating current.

1 is a configuration diagram of an optical fiber current sensor according to a first embodiment of the present invention. It is a block diagram which shows the modification of the polarization control means in 1st Embodiment. It is a block diagram of the reflection type optical fiber current sensor which is the 2nd Embodiment of this invention. It is a block diagram of the reflection type optical fiber current sensor which is the 3rd Embodiment of this invention. It is a block diagram of the reflection type optical fiber current sensor in a prior art example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... 1st sensor fiber 12 ... Light source 13 ... Light receiving element 14 ... Signal processing part 16 ... Analyzer 18 ... Polarization separation element 19 ... Optical circulator 20 ... Polarization control means 21 ... 2nd sensor fiber 22 ... Current coil 30 ... Faraday Rotor 31 ... Faraday element 32 ... Permanent magnet 60 ... Current control unit 61 ... DC power supply 62 ... Temperature sensor 71 ... Sending fiber 72 ... Receiving fiber 100 ... Conductor 111 ... Reflecting part

Claims (11)

Faraday rotation of the linearly polarized light induced by a magnetic field generated by a measured current flowing through a conductor installed around the first sensor fiber, the linearly polarized light being input to the first sensor fiber In the optical fiber current sensor that measures the current to be measured by detecting a corner,
A Faraday rotator that imparts a predetermined Faraday rotation to linearly polarized light that passes through the first sensor fiber;
The second sensor fiber is connected to the first sensor fiber and receives linearly polarized light, and a current coil surrounding the second sensor fiber. The current flowing through the current coil is adjusted, and the first magnetic field is generated by the magnetic field generated by the current. A polarization control means for imparting Faraday rotation to the linearly polarized light in the two-sensor fiber,
A desired Faraday rotation is imparted to the linearly polarized light passing through the first sensor fiber by the Faraday rotator and the polarization control means. An optical fiber current sensor.   2. The optical fiber current sensor according to claim 1, wherein an influence of a characteristic variation of the Faraday rotator is compensated by adjusting a current flowing through the current coil.   The optical fiber current sensor according to claim 2, wherein the characteristic variation is a change in a Faraday rotation angle depending on temperature.   4. The optical fiber current sensor according to claim 1, wherein the Faraday rotator is a paramagnetic material or a ferromagnetic material magnetized by applying a magnetic field. 5.   5. The optical fiber current sensor according to claim 4, wherein the paramagnetic material is one of TGG or TAG which is a paramagnetic crystal, or FR5 which is paramagnetic glass.   The polarization control means includes the linear second sensor fiber and one or a plurality of windings of the current coil installed so that the second sensor fiber penetrates the second sensor fiber. The optical fiber current sensor according to any one of claims 1 to 5.   The polarization control means includes the coil-shaped second sensor fiber and one or a plurality of windings of the current coil installed so as to interlink with the second sensor fiber. The optical fiber current sensor according to any one of claims 1 to 5. The first sensor fiber has a reflecting portion at the other end opposite to the incident end to which linearly polarized light is input,
An optical bias setting means comprising the Faraday rotator and the polarization control means is installed on the incident end side of the first sensor fiber,
The light input from the light source is converted into linearly polarized light and output to the optical bias setting means, and the light reflected from the reflecting portion of the first sensor fiber is separated into two polarization components orthogonal to each other. The optical fiber current sensor according to any one of claims 1 to 7, further comprising a polarization separation element disposed on the light source side with respect to the optical bias setting means.   9. The optical bias setting unit sets the optical bias so that a rate of change in intensity of light output from the analyzer is maximized with respect to a change in Faraday rotation angle. Fiber optic current sensor.   The optical fiber current sensor according to any one of claims 1 to 9, wherein at least one of the first sensor fiber and the second sensor fiber is a lead glass fiber. By inputting linearly polarized light into the first sensor fiber and detecting a Faraday rotation angle of the linearly polarized light induced by a magnetic field generated by a current to be measured flowing through a conductor installed around the first sensor fiber, In the current measurement method for measuring the current to be measured,
A Faraday rotator that imparts a predetermined Faraday rotation to linearly polarized light that passes through the first sensor fiber;
The second sensor fiber is connected to the first sensor fiber and receives linearly polarized light, and a current coil surrounding the second sensor fiber. The current flowing through the current coil is adjusted, and the first magnetic field is generated by the magnetic field generated by the current. Polarization control means for imparting Faraday rotation to linearly polarized light in two sensor fibers;
A desired Faraday rotation is imparted to the linearly polarized light passing through the first sensor fiber using a current measuring method.

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