JP2006030007A - Fiber-optic current sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber-optic current sensor which reduces measurement errors caused by rotational motion. <P>SOLUTION: The fiber-optic current sensor comprises a sensing fiber 14 circling a current path, transmission optical fibers which are connected to the both exit ends of a coupler 8 for separating and a light beam of a light source into two branches and emitting it and which are connected to the both ends of the sensing fiber 14 through λ/4 elements 15, 16, respectively, a light receiving element 5 for detecting the interference of light propagating through the sensing fiber 14 in the forward/reverse direction, and a delay coil 12 provided in the midway of the transmission optical fiber for providing extra length for delay. Since the delay coil 12 comprises a part wound in one direction and a part wound in the direction opposite to the one direction, the influence of an optical gyro is canceled. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber current sensor using the Faraday effect, and more particularly to an optical fiber current sensor that eliminates measurement errors due to rotational motion.

  An optical fiber current sensor circulates a sensing fiber around a current path, connects a transmission optical fiber to both emission ends of a coupler that divides the light source light into two and emits the light. Both ends of the sensing fiber are connected via a λ / 4 element, and the light emitted from one emission end of the coupler returns to the other emission end and is received by the light receiver at the other coupler. .

  The optical fiber current sensor causes the Faraday effect to act on the light propagating through the sensing fiber in the forward and reverse directions by the magnetic field accompanying the current passing through the space surrounded by the sensing fiber, and the phase difference generated in the two propagation lights Detection is performed using interference, and current is obtained from this phase difference.

  The principle and details of this optical fiber current sensor are described in detail in Non-Patent Documents 1 to 4. That is, the detailed principle and basic configuration of the optical fiber current sensor are described in Non-Patent Document 1, the optical fiber current sensor for electric railway substations is described in Non-Patent Document 2, and the λ / 4 element is described in Non-Patent Document 3. , Shupé effect is described in Non-Patent Document 4, respectively. These replace the disclosure of the background art of the present invention.

"Plasma current measurement with Sagnac interferometric photocurrent transformer (CT)", Yuji Ichinose, Mitsushi Abe, Journal of Plasma and Fusion Research, Vol. 76, No. 6, 593-600, June 2000 "Development of an optical fiber current sensor for DC electric railway", Hitoshi Hayashiya, Tatsuya Kumagai, Masami Hino, Takeshi Endo, Masato Ando, Hideo Negishi, Proceedings of 32nd Meeting on Lightwave Technology, pages 141-146, December “Temperature and Vibration Insensitive Fiber-Optical Current Sensor”, K.K. Bohnert, P.A. Gabus, J. et al. Nehring, H.M. Brandle, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 20, no. 2, pp. 267-275, FEBRUARY 2002 “Thermally Induced Nonreciprocity in the Fiber Interferometer”, Shupe, D. et al. M.M. Appl. Opt. 19,654.1980

  FIG. 4 shows the configuration of the optical fiber current sensor of the background art, and FIG. 5 shows the configuration of the delay optical fiber.

  As shown in FIG. 4, the optical fiber current sensor is divided into a sensor head 2 and a sensor body 1, and the sensor body 1 includes a light source 4, a light receiver 5, a first-stage coupler 6, a polarizer 7, a loop. Side coupler 8, phase modulator 11, delay coil 112, and transmission optical fibers 9 and 10 are accommodated. The sensor head 2 includes a sensing fiber 14, two λ / 4 elements 15 and 16, and a transmission optical fiber. 17 and 18 are accommodated, and the optical connectors 125 and 126 for transmission are connected between the optical connectors of the sensor head 2 and the sensor main body 1.

  As shown in FIG. 5, in the delay optical fiber 112, the optical fiber 122 is wound around the body of the bobbin 124 in one direction.

  In the background art, the delay coil is wound only in one direction. It is known that when an angular velocity (angular acceleration) is generated in such a coiled optical fiber, a phase difference is generated in propagation light in both directions by the principle of an optical gyro. Therefore, also in the optical fiber current sensor, for example, if the delay coil is angularly moved by rotating the sensor main body or the like, the phase difference due to the optical gyro overlaps with the phase difference due to current measurement, and a measurement error occurs.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical fiber current sensor that solves the above-described problems and eliminates measurement errors due to rotational motion.

  In order to achieve the above-mentioned object, the present invention circulates a sensing fiber around a current path, connects a transmission optical fiber to both emission ends of a coupler that divides and emits light source light into two, and transmits both of them. Each end of the sensing fiber is connected to each optical fiber via a λ / 4 element, and a receiver is provided to detect interference of light propagating in the forward and reverse directions on the sensing fiber. In the optical fiber current sensor in which a coil is formed to have a delay extra length, the delay coil is composed of a portion wound in one direction and a portion wound in the opposite direction.

  In the delay coil, the transmission optical fiber is folded back at an intermediate point, and one side portion and the opposite side portion are wound along each other from the intermediate point, so that the winding directions of both side portions are mutually different. It may be the opposite.

  The delay coil may be formed by winding two optical fibers along one another and then fusing one end of both optical fibers on the same side.

  The present invention exhibits the following excellent effects.

  (1) The measurement error due to the rotational motion is eliminated.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, the optical fiber current sensor according to the present invention is divided into a sensor body 1, a sensor head 2, and an optical cord 3 that connects the sensor body 1 and the sensor head 2 to the sensor body 1. .

  The sensor body 1 includes a light source 4 that generates light for causing the Faraday effect, a light receiver 5 that receives interference light, and guides the light source light to a coupler 8 on the loop side, and interference light from the coupler 8 on the loop side. The first-stage coupler 6 that guides the light to the light receiver 5, the polarizer 7 that polarizes the light source light from the first-stage coupler 6, and the light source light that has passed through the polarizer 7 is split into two to be emitted and two return lights are emitted. A loop-side coupler 8 that is combined (interfered) and returned to the first-stage coupler 6, transmission optical fibers 9 and 10 respectively connected to both emission ends a and b of the coupler 8, and one transmission optical fiber 10. And a delay coil 12 formed in the middle of one transmission optical fiber 10 are accommodated in one chassis 13.

  The sensor head 2 includes a sensing fiber 14 made of a twisted fiber, two λ / 4 elements 15 and 16, and transmission optical fibers 17 and 18 in one chassis 19. The sensing fiber 14 is installed so as to circulate around a conductor cable (a cable cross section is shown in the figure) 20 which is an example of a current path to be measured.

  As shown in FIGS. 2 and 3, the delay coil 12 includes a portion wound in one direction (CW) and a portion wound in the opposite direction (CCW). Here, the winding direction means a direction in which light emitted from one of the specific emission ends a (or b) of the loop-side coupler 8 rotates in the delay coil 12. The light propagating in the optical fiber of FIG. 3 is indicated by a mark indicating that it is directed toward the front of the paper, and the mark indicated by x is indicated that it is directed behind the paper.

  The delay coil 12 is folded at a position where the transmission optical fiber 10 becomes the intermediate point 21 of the delay coil 12, and the one side portion 22 and the opposite side portion 23 are wound along the intermediate point 21 along each other. Thus, the winding directions of the both side portions 22 and 23 are opposite to each other.

  When manufacturing such a delay coil 12, instead of winding one optical fiber in one direction and then rewinding it in the opposite direction, an intermediate point is appropriately placed in the transmission optical fiber as shown in FIG. (Note: For the sake of clarity, it is shown to protrude to the extreme, but it is not particularly necessary.) 21 is determined, and the one side portion 22 and the opposite side portion 23 turned back at this midpoint 21 are aligned in parallel. Then, it may be wound around the bobbin 24. Alternatively, after winding two separate optical fibers along each other, one end of both optical fibers on the same side (location corresponding to the intermediate point 21) is fused to form one. A portion wound in one direction and a portion wound in the opposite direction may be formed on one optical fiber.

  In the delay coil 12 of FIGS. 2 and 3, the portion wound in one direction (CW) and the portion wound in the opposite direction (CCW) have the same optical fiber length, the same number of turns, and a coil cross-sectional area. Preferably equal.

  In the optical fiber current sensor of FIG. 1, all the transmission optical fibers 9, 10, 17, 18, 25, and 26 including the delay coil 12 are polarization-maintaining optical fibers. In other words, all the optical fiber portions are polarization-maintaining optical fibers except that the sensing fiber 14 is a twisted fiber. This is because an optical fiber having a low birefringence is used for the sensing fiber 14 and a polarization-preserving optical fiber is used for the other optical fibers to reduce drift (interference noise) due to fluctuations in the polarization plane.

  In the optical fiber current sensor of FIG. 1, the control of the light source and the phase modulator and the control of calculating the measurement current from the light reception output of the light receiver are well-known matters and will not be described in this specification.

  The λ / 4 elements 15 and 16 in FIG. 1 are constructed by twisting about 45 ° around the optical fiber axis at a circumferential angle in a state where a part of the transmission optical fibers 17 and 18 is melted. The details of this processing are as disclosed in FIG. 6 (FIG. 6) of Non-Patent Document 3.

  As shown in FIG. 1, the transmission optical fibers 25 and 26 are communication optical fibers 25 and 26 that communicate between the sensor body 1 and the sensor head 2. The two communication optical fibers 25 and 26 are integrated. That is, the two communication optical fibers 25 and 26 are attached to each other so that they are in close contact with each other, and are bundled or bonded together to form a single cord 3. The one cord 3 is branched only at both ends, and an optical connector coupled to the optical connector of the sensor body 1 and an optical connector of the sensor head 2 are respectively connected to both ends of the connecting optical fibers 25 and 26 branched into two. And an optical connector to be coupled to each other.

  The basic operation and effect of the optical fiber current sensor according to the present invention will be described below.

  The light emitted from the light source 4 passes through the polarizer 7 from the first-stage coupler 6 and becomes linearly polarized light. The linearly polarized light is branched by the coupler 8 on the loop side and emitted from both emission ends a and b, one through the transmission optical fibers 9, 25 and 17 and the other through the transmission optical fibers 10, 26 and 18. / 4 elements 15 and 16 and enter the sensing fiber 14 as clockwise light (CW light) and counterclockwise light (CCW light). When the two linearly polarized light passes through the λ / 4 elements 15 and 16, they become circularly polarized light having the same rotation direction of the polarization angle.

  Since the sensing fiber 14 is installed so as to go around the conductor cable 20 which is an example of the measurement object, the light in the sensing fiber 14 is affected by the external magnetic field generated from the current flowing through the conductor cable 20. This phenomenon is called the Faraday effect. When the light affected by this is circularly polarized light, its phase changes depending on the strength of the magnetic field. When the CW light undergoes a + θ phase change, the CCW light undergoes a −θ phase change. As a result, a phase difference of 2θ is generated between the two light beams. The bi-directional light that exits the sensing fiber 14 and returns to the coupler 8 is coupled by the coupler 8 and interferes with each other. The intensity of the interference light is detected by the light receiver 5.

  Since the following equation is established between the phase difference θ represented by the intensity of the interference light and the current flowing through the conductor cable 20, the current value can be obtained.

θ = 2nVeI
Where θ: phase difference caused by Faraday effect (rad)
Ve: Verde number (rad / A)
n: Number of turns of sensing fiber 14 (times)
I: Current value of conductor cable 20 (A)
It is.

  However, the conventional optical fiber current sensor has a problem that when the delay coil is angularly moved, the phase difference caused by the optical gyro overlaps with the phase difference caused by the current measurement, resulting in a measurement error. Therefore, in the present invention, the delay coil 12 has a portion wound in one direction and a portion wound in the opposite direction, and preferably both portions have the same optical fiber length, the same number of turns, and a coil cross-sectional area. To be equal. The phase difference of propagating light according to the principle of the optical gyro cancels out the phase difference generated in the portion wound in one direction and the phase difference generated in the portion wound in the opposite direction. For this reason, even if the delay coil 12 is angularly moved, the influence does not appear in the light intensity of the interference light detected by the light receiver 5. Therefore, no measurement error occurs in the current measurement.

  In addition, in the optical gyro, if there is a temperature change in the coil, the output is equivalent to the rotation due to the thermal expansion and contraction of the optical fiber and the temperature change in the refractive index (a phase difference that appears as if it is rotated but not rotating). Will occur. This is called the Suppe effect. Accordingly, a winding method in which the opposite-side portion 23 is overlapped and wound around the entire circumference of the layer (layer thickness = optical fiber diameter × plurality) in which the one-side portion 22 is overlapped and wound around the bobbin 24 of FIGS. Then, a difference in temperature change occurs between the layer of the one side portion 22 and the layer of the opposite side portion 23. As a result, the phase difference due to the Suppe effect differs between the two optical fibers, and if a delay coil is created by this winding method, a current measurement error due to a temperature change occurs. In that respect, in the present invention, since the one side portion 22 and the opposite side portion 23 are aligned in parallel and wound around the bobbin 24, the temperature change generated in the one side portion 22 and the opposite side portion 23 is the same. Therefore, the optical fiber current sensor of the present invention does not cause a current measurement error due to a temperature change.

  Moreover, since all the optical fiber parts other than the sensing fiber 14 are polarization plane preserving optical fibers, this optical fiber current sensor can reduce drift (interference light noise) due to polarization fluctuations.

  Further, this optical fiber current sensor has two communication optical fibers (transmission optical fibers) 25 and 26 combined into a single cord 3. Therefore, the two transmission optical fibers included in the optical cord 3 are placed in an environment where the same disturbance is applied.

  In the above description, the sensing fiber 14 is a twisted fiber, but a communication single mode fiber annealed at a high temperature may be used.

  Further, although the delay coil 12 is formed only on one transmission optical fiber 10, it may be formed on the other transmission optical fiber 9. In this case, the sensing fiber 14 can be arranged in the middle of the loop formed between the two emission ends a and b of the coupler 8 (the distance is the same from the emission end a and the emission end b). In addition to this, when a loop-shaped optical fiber surrounding the same area as the sensing fiber 14 is provided in the sensor head 2, disturbance and rotation applied to the sensor head 2 can be canceled.

  In addition, although the optical fiber current sensor uses the current flowing through the conductor cable 20 as a measurement target, it can also measure a spatial current.

  The optical fiber current sensor can also be used by being mounted on a moving body. In this case, it is advantageous that the influence of the optical gyro is removed.

It is a block diagram of the optical fiber electric current sensor which shows one Embodiment of this invention. It is a perspective view of the delay coil used for this invention. It is sectional drawing of the coil for delay used for this invention. It is a block diagram of the conventional optical fiber current sensor. It is sectional drawing of the conventional delay coil.

Explanation of symbols

4 light source 5 light receiver 8 coupler 9, 10, 17, 18, 25, 26 transmission optical fiber 12 delay coil 14 sensing fiber 15, 16 λ / 4 element

Claims (3)

  A sensing fiber is circulated around the current path, a transmission optical fiber is connected to both emission ends of the coupler that divides the light source light into two and emits, and both transmission optical fibers are respectively connected via λ / 4 elements. Connect both ends of the sensing fiber, provide a receiver that detects the interference of light propagating through the sensing fiber in the forward and reverse directions, and form a delay coil in the middle of the transmission optical fiber to reduce the extra delay length. An optical fiber current sensor, comprising: a portion in which the delay coil is wound in one direction and a portion wound in the opposite direction.   In the delay coil, the transmission optical fiber is folded back at an intermediate point, and one side portion and the opposite side portion are wound along each other from the intermediate point, so that the winding directions of both side portions are mutually different. 2. The fiber optic current sensor of claim 1, wherein the optical fiber current sensor is reversed. 3. The optical fiber current according to claim 1, wherein the delay coil is formed by winding two optical fibers along each other and then fusing one end of both optical fibers on the same side. Sensor.

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