JP2001356155A - Ladder type multi-point optical fiber magnetometric sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ladder type multi-point optical fiber magnetometric sensor having no electric signal line, a high expanding property and a high interchangeability of sensor systems, an excellent fail-safe property, and a stable output and capable of removing fading. SOLUTION: A plurality of sensor systems 101, 102, 103 detecting optical path difference changes based on magnetic information as light intensity signals via optical interference are arranged in the longitudinal direction of a light source optical fiber 104 connected to a light source. The light inputs of the sensor systems 101, 102, 103 are connected to the light source optical fiber 104 via optical couplers 20, 21, 22 respectively, the light outputs of the sensor systems 101, 102, 103 are connected to a light receiving optical fiber 105 separate from the light source optical fiber 104 via optical couplers 60, 61, 62 respectively, and a magnetic information extracting section 106 for extracting the magnetic information of the sensor systems from the light intensity signals is provided at one end of the light receiving optical fiber 105.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multipoint type magnetic sensor using an optical fiber, and in particular, has no electric signal line, has high expandability and compatibility of a sensor system, and has a fail-safe characteristic. The present invention relates to a ladder-type multipoint optical fiber magnetic sensor having excellent output, fading elimination, and stable output.

[0002]

2. Description of the Related Art As a sensor for detecting magnetism using light interference by an optical fiber, a multipoint sensor using electromagnetic force is known (Japanese Patent Application Laid-Open No. 1-35284). As shown in FIG. 7, this multi-point optical fiber magnetic sensor has magnetic detecting elements 10, 13, 1 made of a metal-coated optical fiber.
6 and sensor units 171, 172, 173 provided with AC power supplies (oscillators) 11, 14, 17 for supplying an AC current to the magnetic detection elements 10, 13, 16 are connected to the light source 1 via the coupler 2. A plurality of optical fibers 174 are arranged in series in the longitudinal direction of the optical fiber 174, and another optical fiber 3 connected to the light source 1 via the coupler 2 is coupled to the optical fiber 174 with the coupler 4 at the opposite end. Total 1
75 (hereinafter referred to as an optical interferometer). Light from the light source 1 passes through the sensor units 171, 172, and 173.
Magnetic detection elements 10, 13, 1 due to the interaction between an alternating current flowing through magnetic detection elements 10, 13, 16 and an external magnetic flux.
A change in the optical path difference is caused by the expansion and contraction in the longitudinal direction or the change in the lateral pressure in the radial direction of the optical interferometer 17.
5 is detected as a light intensity signal. At the output of the optical interferometer 175, there is provided a photodetector 5 for converting a signal of the intensity of light into an electric signal.
2, 173 are provided with synchronous detectors 12, 15, and 18, which perform synchronous detection of the electric signal using a carrier wave of each sensor unit and extract magnetic information of an external magnetic flux of the sensor unit. Further, in order to remove fading, the multi-point optical fiber magnetic sensor is provided with an optical phase modulation element 9 on the optical fiber 3 and another optical receiver 6 connected to the coupler 4,
The DC component of the signal difference between the two light receivers 5 and 6 is converted into an optical fiber 3
So that the DC component can be controlled so that the DC component becomes zero. Reference numeral 7 denotes a compensator for extracting a DC component of a signal difference between the two light receivers 5 and 6, and reference numeral 8 denotes an amplifier for amplifying an output of the compensator and applying the amplified output to an optical phase modulation element 9.
By controlling the DC component of the signal difference between the two light receivers 5 and 6 to be 0, fading can be removed.

[0003]

In the magnetic sensor using an optical fiber, it is highly likely that measurement over a wide area can be achieved relatively easily if the optical fibers 3 and 174 are lengthened. The multipoint technology of the optical fiber magnetic sensor has the following problems.

1) Existence of an electric signal line between each sensor unit and the base unit The base unit (not shown) includes each of the sensor units 171, 172,
173 collects the magnetic information output at the same time and performs centralized processing, and is installed, for example, near the photodetectors 5 and 6.
In the related art, for example, the following installation method can be considered for components constituting the left end sensor unit 171.

A) The AC power supply 11 and the synchronous detector 12 are both installed near the magnetic detection element 10. According to this method, in addition to the power supply line (for power of each component), an electric signal line for sending an electric signal from the light receiver 5 to the sensor unit 171 and a sensor unit 171 between the base unit and the sensor unit 171. And an electric signal line for transmitting magnetic information output to the base unit.

[0006] b) The AC power supply 11 is installed near the magnetic detecting element 10 and the synchronous detector 12 is installed near the base unit. According to this method, between the base unit and the sensor unit 171, in addition to the power supply line, the AC power supply 1
An electric signal line for transmitting the signal No. 1 to the synchronous detector 12 on the base device side is required.

C) The AC power supply 11 and the synchronous detector 12 are both installed near the base station. According to this method, an electric signal line for transmitting a signal of the AC power supply 11 from the base device to the sensor unit 171 is required between the base device and the sensor unit 171 in addition to the power supply line.

As described above, in any of the installation methods, the electric signal lines exist between the sensor units 171, 172, and 173 and the base device. Since an alternating current directly involved in the detection of magnetic information flows through these electric signal lines, if the electric signal lines receive external noise, the noise causes a large error. In addition, when the arrangement interval between the sensor units and the distance between the sensor unit and the base device become longer, the length of the electric signal line to be routed becomes longer, resulting in a decrease in the S / N ratio and deterioration in workability during installation and maintenance.

2) Insufficiency of expandability and compatibility of the sensor unit The optical fiber magnetic sensor forms one sensor unit with one magnetic detecting element and one AC power supply, and the one sensor unit and the optical interferometer Thus, an optical fiber magnetic sensor that measures a single point can be configured. Such an optical fiber magnetic sensor will be referred to as a complete single sensor. In the related art, a plurality of sensor units 171, 172, and 173 are incorporated in a common optical interferometer 175 to constitute a multipoint optical fiber magnetic sensor. A part of the sensor (= sensor part) is inserted into the common optical interferometer 175. For this reason, the compatibility between the multi-point optical fiber magnetic sensor and the complete single sensor is poor. In addition, because of such poor compatibility, it is not easy to add measurement points, and expandability is poor.

3) Lack of fail-safe property Due to the increase in the number of points (expansion of the measurement space), the arms (optical fibers 3 and 174) of the optical interferometer are extended. However, if an abnormality such as disconnection or an increase in transmission loss occurs in a part of this long arm, all the sensor units 1
71, 172 and 173 become unusable.

4) Difficulty of Fading Removal When the number of points increases, the length of the arm increases, or when the number of connectors used to add the arm increases,
The amount of disturbance or polarization fluctuation added increases, or the variation of the fading and the variation of the changing frequency increase due to the decrease of the coherence of light, and it becomes difficult to remove the fading.

In the prior art, the direct current component is fed back to the optical interferometer 175 to remove fading. However, the direct current component has a difference in coupler branching ratio, which is an element other than optical fading, and an electric signal. A DC offset is also included, and the DC component is not always desirable for feedback.

Therefore, an object of the present invention is to solve the above-mentioned problems, to achieve no elimination of electric signal lines, to have high expandability and compatibility of the sensor system, to have excellent fail-safe properties, and to eliminate fading. An object of the present invention is to provide a ladder-type multipoint optical fiber magnetic sensor having a stable output.

[0014]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an optical fiber coated with a metal, which is supplied with an alternating current, and wherein the metal coated optical fiber is formed by an interaction between the alternating current and an external magnetic flux. An optical path difference change is caused by a longitudinal expansion or contraction or a radial side pressure change, and a sensor system for detecting the optical path difference change as a light intensity signal by optical interference is provided in a longitudinal direction of a light source optical fiber connected to the light source. The optical input side of each sensor system is connected to the light source optical fiber via an optical coupler, and the optical output side of each sensor system is received separately from the light source for the light source via an optical coupler. And a magnetic information extracting section for extracting magnetic information of the external magnetic flux in each sensor system from the intensity signal of the light at one end of the light receiving optical fiber.

The sensor system comprises: an optical coupler for splitting input light; a plurality of metal-coated optical fibers connected to the optical coupler; a plurality of AC power supplies for supplying power to the metal-coated optical fibers; An optical phase modulation element that performs reference modulation for fading detection on the metal-coated optical fiber, an AC power supply that drives the optical phase modulation element, and multiplexes and outputs optical signals from the plurality of metal-coated optical fibers. Optical coupler, a photodetector that converts a part of the multiplexed optical signal into an electric signal, a synchronous detector that detects a fading component from the electric signal, and the detected signal to the other metal-coated optical fiber. It may be composed of a feedback optical phase modulation element.

The magnetic information extracting section includes a light receiver connected to the light receiving optical fiber, and a carrier for detecting a plurality of carrier waves for each sensor system from the electric signal obtained by converting the light intensity signal of the light. It may be constituted by a detector and a synchronous detector for extracting magnetic information of an external magnetic flux for each sensor system by performing synchronous detection from the electric signal using each of the plurality of carriers.

The optical path difference between the sensor systems is set to be sufficiently large with respect to the optical path difference in the sensor system, and the coherence length of the light source is sufficiently larger than the optical path difference in the sensor system. The spectral line width of the light source may be controlled so as to be sufficiently smaller than the path difference.

The branch ratio of each optical coupler interposed between the optical fiber for the light source and the optical input side of each sensor system may be changed according to the number of branches passing from the light source.

[0019]

An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

As shown in FIG. 1, a ladder-type multipoint optical fiber magnetic sensor according to the present invention has a sensor system 101, 102, which detects a change in an optical path caused by a metal-coated optical fiber as a light intensity signal by optical interference. 103,.
9 are arranged in the longitudinal direction of the optical fiber 104 for the light source, and the light input side of each sensor system 101, 102, 103 is connected to the optical fiber 104 for the light source via the optical couplers 20, 21, 22,. Connect each sensor system 101, 1
02 and 103 are connected to optical couplers 60 and 6 respectively.
, 62,... Are connected to another light receiving optical fiber 105 for the light source, and one end of the light receiving optical fiber 105 is connected to one of the sensor systems 101, 102,.
A magnetic information extracting unit 106 for extracting magnetic information of the external magnetic flux in 103 is provided.

Since each sensor system has the same configuration, the sensor system 101 will be described.

The sensor system 101 includes an optical coupler 23 for splitting input light, and two metal-coated optical fibers (hereinafter, referred to as magnetic detecting elements) 2 connected to the optical coupler 23.
6, 27, two AC power supplies (oscillators) 32, 33 for energizing the magnetic detection elements 26, 27, and an optical phase modulation element 41 for performing reference modulation on one of the magnetic detection elements 26 for fading detection. , This optical phase modulation element 4
1 and an optical coupler 5 for multiplexing and outputting optical signals from the magnetic detecting elements 26 and 27.
4, a photodetector 53 for monitoring a part of the multiplexed optical signal and converting it to an electric signal, a synchronous detector 47 for detecting a fading component from the electric signal, and an amplifier 48 for amplifying the detected signal. And an optical phase modulation element 42 that feeds back the amplified detection signal to the other magnetic detection element 27. The optical coupler 23 forming the light input side of the sensor system 101 is connected to the optical coupler 20 in the light source optical fiber 104, and the optical coupler 54 forming the light output side is connected to the optical coupler 60 in the light receiving optical fiber 105. Will be done.

The magnetic information extracting section 106 detects a light receiver 59 connected to the light receiving optical fiber 105 and detects two carrier waves for each sensor system from the electric signal obtained by converting the light intensity signal of the light. Carrier detectors 63 and 2 each
Synchronous detectors 64, 65, and 66 for extracting magnetic information of external magnetic flux for each sensor system by performing synchronous detection from the electric signal using one carrier wave.

In the ladder-type multi-point optical fiber magnetic sensor shown in FIG.
22, the respective sensor systems 101, 1
02, 103. Optical couplers 23, 24, 25
Is a coupler corresponding to the optical coupler 2 of the Mach-Zehnder interferometer in FIG. 7 and serving as an entrance of each of the sensor systems 101, 102, and 103. The light split into the sensor systems 101, 102, and 103, respectively, after taking in magnetic information, is multiplexed by the optical couplers 60, 61, and 62 and sent to the light receiver 59. Thus, the ladder-type multipoint optical fiber magnetic sensor is characterized in that the connection structure of the optical fibers is a ladder type.

In this ladder type structure, the adjacent sensor systems also form a Mach-Zehnder interferometer, which may cause unnecessary interference. Therefore, in order to avoid unnecessary interference, the optical path difference between the sensor systems is set to be sufficiently large with respect to the optical path difference in the sensor system. Then, sufficient interference occurs, and no interference occurs between the sensor systems.

The operation of the ladder type multipoint optical fiber magnetic sensor shown in FIG. 1 will be described in detail below.

The light whose spectral line width is controlled is transmitted from the light source 19 to the optical coupler 20, and the light transmitted to the first sensor system 101 by the optical coupler 20 and the second and subsequent sensor systems 10.
It is split into light to be sent to 2,103.

The light transmitted to the first sensor system 101 is split by the optical coupler 23 into two optical fibers (unsigned) which are two optical paths of the Mach-Zehnder interferometer.
In each optical path, the magnetic detecting elements 26, which are driven at different frequencies by AC power supplies 32, 33, respectively.
27 captures magnetic information of each frequency component as an optical path difference change.

The light emitted from the magnetic detection element 26 is modulated at a constant frequency and a constant amplitude by an optical phase modulation element 41 driven by an AC power supply 38. Interference occurs when the lights of the two optical paths are combined by the optical coupler 54.
The output at one end of the optical coupler 54 is converted into an electric signal by the light receiver 53. This electric signal is synchronously detected by the synchronous detector 47 with a frequency component twice as high as the frequency of the AC power supply 38. This detection signal is amplified by the amplifier 48, and the amplified detection signal is applied to the optical phase modulation element 42. As a result, a feedback loop is established, and 2 in the sensor system is formed.
The optical path difference between the two optical paths is controlled to a point shifted by 波長 wavelength. This stabilizes the operating point so that the sensitivity of the fundamental component of the modulation frequency is maximized.

On the other hand, the output of the other end of the optical coupler 54 is
The second and subsequent sensor systems 10 guided to the optical coupler 60
2, 103 light is multiplexed with the multiplexed light, and
Is input to The electric signal converted by the light receiver 59 is sent to the carrier detector 63. This carrier detector 63
In, frequency detection of frequency components including magnetic information is performed, and respective frequency signals are generated. The reproduced carrier signal and the electric signal from the light receiver 59 are sent to the synchronous detectors 64, 65, 66. Each synchronous detector 64, 6
At 5, 66, magnetic information included in each frequency component is extracted.

In the ladder-type multi-point optical fiber magnetic sensor described above, the sensor system 101 constituting the Mach-Zehnder interferometer from the optical coupler 23 to the optical coupler 54 is stored in one housing, and the sensor system 101 Since the optical path is not extended, it is less susceptible to disturbance. Further, only a power supply line (for power of each component) is connected to each of the sensor systems 101, 102, and 103, and an electric signal line from the base device is not connected. Therefore, there is no exchange of electric signals between each sensor system and the base device, and it is not necessary to route electric signal lines. Further, even if one sensor system fails, a signal of another sensor system is output, so that fail-safe performance is improved. Further, the sensor systems are interchangeable with each other, and it is not necessary to cut off the optical interferometer and insert a component as in the related art when adding a sensor system, which is excellent in expandability.

Further, the coherence length of the light source is determined by the optical path difference in the sensor system (in the first sensor system 101, the optical path difference between the two optical paths from the optical coupler 23 to the optical coupler 54). The light source is set to be sufficiently large and sufficiently smaller than the optical path difference between the sensor systems (substantially equal to the sum of the optical path from the optical coupler 20 to the optical coupler 21 and the optical path from the optical coupler 61 to the optical coupler 60). Since the 19 spectral line widths are controlled, coherent noise between the sensor systems can be reduced.

When a ladder-type branch is configured, if an optical coupler having an input-to-output ratio of 1: 2 is used for the optical couplers 20, 21, and 22, which are branch couplers between the sensor systems, the n-th sensor system The intensity of the input light to the first sensor system 101
Input light intensity to 2 ^-n . Therefore, the branching ratio of each of the optical couplers 20, 21, 22 is changed in accordance with the number of branch passages from the light source 19. By adopting the branching ratio in which the distribution of the output connected to the terminal is increased, the degree of weakening of the intensity of the input light to the sensor system on the terminal side can be reduced, and the sensitivity difference between the sensor systems can be reduced.

A ladder-type two-point optical fiber magnetic sensor as shown in FIG. 2 was actually produced. As shown in the figure, a first sensor system # is provided between a light source optical fiber connected to a constant polarization light source and a light receiving optical fiber connected to a light receiver.
1. A second sensor system # 2 was provided. Sensor system # 1,
Although the internal configuration of # 2 is shown simply, it has the same configuration as the sensor system of FIG. The light source optical fiber and the light receiving optical fiber between the sensor systems # 1 and # 2 were 5 m extension cables.

FIG. 3 shows the output of the ladder type two-point optical fiber magnetic sensor of FIG. The external magnetic flux to be measured is geomagnetism. The output of sensor system # 2 is 21.9 kHz, sensor system #
The output of No. 1 was 23.1 kHz, and the output intensity as shown in each figure was obtained. As a result, the geomagnetism could be detected with high sensitivity.

Next, various characteristics of the ladder type multipoint optical fiber magnetic sensor of the present invention will be considered.

1. Reduction of coherent noise The causes of noise in a Mach-Zehnder interferometer are: 1) phase noise of a light source, 2) coherence of light, 3) polarization fluctuation, 4)
Return light, 5) a change in the intensity of the light source, and the like. Here, 1) to 4) that are considered to have a large effect will be considered.

1) Phase noise of the light source The phase noise of the light source is caused by the phase (frequency) fluctuation of the light source, and is determined by the spectral line width. In a Mach-Zehnder interferometer, in an area where the optical path difference is sufficiently smaller than the coherence length of light, phase noise increases in proportion to the optical path difference. If the optical path difference is ideally zero, no noise is generated in the signal output. The phase noise dφ _n (x) of the light source at the multiplexing point of the Mach-Zehnder interferometer is given by Expression 1. Here, 2Δλ _w : light source spectrum half width (wavelength value) x: optical path difference λ ₀ : light source center wavelength λ ₀ >> Δλ _w

[0039]

(Equation 1)

2) Coherence of light The coherence of light is related as a coefficient to all of the phenomena caused by interference. This coefficient V _C is given as in Equation 2. Here, n: fiber refractive index 2Δλ _w : light source spectral half width (wavelength value) x: optical path difference λ ₀ : light source center wavelength λ ₀ >> Δλ _w

[0041]

(Equation 2)

3) Polarization State (Polarization Fluctuation) The polarization state of two lights at the time of optical coupling does not directly contribute to noise, but when the angle difference between the two polarization planes becomes zero, When the coherence of the two lights is maximum and they are orthogonal,
The coherence of the two lights is minimized. That is, the difference between the polarization angles acts as a coefficient indicating the coherence. Although the two lights are actually elliptically polarized light, they are regarded as linearly polarized light for the sake of simplicity, and if the angle difference between their polarization planes is η, the coefficient of coherence V _p is given by Can be

[0043]

(Equation 3)

4) Return light In the Mach-Zehnder interferometer, unlike the ring interferometer,
Since there is no signal light propagating in the opposite direction in one optical fiber, it is considered that the influence of the return light directly on the signal is small.

Here, the magnitudes of the signal and the noise were obtained for the multipoint Mach-Zehnder interferometer model as shown in FIG. Parameters relating to the light source are λ ₀ : light source center wavelength 2Δλ _w : light source spectrum half width (wavelength value), and parameters relating to the optical fiber are: ΔLm: optical path difference in the sensor system ΔLs: optical path difference between the sensor systems η: It is defined as the angle difference between the polarization planes of the two lights at the combining point. At this time, the signal S of the sensor system and the noise N generated by the interference between the sensor systems are expressed by the following equations.

[0046]

(Equation 4)

By substituting equation (1) into equation (4),
Calculation simulation was performed by substituting equation (3) into equation (5). FIG. 5 shows the relationship between the signal S and the noise N when the optical path difference between the sensor systems is changed by 1 m between 1 m and 100 m.
Shown in Table 1 shows the calculation conditions at this time. It is known that the spectral line width of the light source is about 10 ⁻⁶ to 10 ⁻¹¹ of the center wavelength in the case of a semiconductor laser. Therefore, two types of light source spectral line widths (A = 0.03, B = 0.3) estimated to be close to the current experiment were calculated.

[0048]

[Table 1]

In FIG. 5, the signal S is indicated by a 0 dB line, the noise N at A is indicated by a solid line, and the noise N at B is indicated by a broken line.
Comparing the noise N of A and B, it is presumed that, under these calculation conditions, if the spectral line width of the light source is increased by 10 times, the noise N can be reduced by about 20 dB when the optical path difference is 10 m.

2. As shown in FIG. 6, a plurality of sensors 1, sensor 2,..., Sensor n are provided between the optical fiber extended from the light source and the optical fiber connected to the light receiver. When a ladder-type multi-point sensor was configured by arranging them in parallel, the branching ratio of input light to each sensor by each coupler was set to (sensor side) :( optical fiber extension side) = 1: r. At this time, the intensity ai of the input light to the i-th sensor is expressed by Expression 5.

[0051]

(Equation 5)

When r = 1 and r = 9, the intensity ai of the input light to the i-th sensor is as shown in Expression 6.

[0053]

(Equation 6)

When r is increased, the initial value is reduced, but the reduction rate at which the light intensity decreases in the subsequent stage can be reduced to some extent. For example, if the dynamic range of the detection unit can be secured at about 20 dB, it is considered that about 20 points can be obtained by setting r = 9.

[0055]

The present invention exhibits the following excellent effects.

(1) An AC power supply is provided for each sensor system, the optical output signals of each sensor system are multiplexed and transmitted, and the carrier is detected from the electric signal on the light receiving side. There is no electric signal line.

(2) The sensor system is configured as a complete single sensor including an optical interferometer, and the sensor system is arranged in a ladder form, so that the expandability and compatibility of the sensor system are high.

(3) Since the failure of the optical fiber in each sensor system does not affect the entire optical transmission, excellent fail-safe property is obtained.

(4) Since fading is eliminated in the sensor system, fading can be eliminated irrespective of an increase in the entire optical transmission distance or the number of optical elements.

(5) By setting the optical path difference and controlling the spectral line width, sufficient interference occurs in the sensor system and no interference occurs between the sensor systems, so that a stable output with little noise can be obtained. it can.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a ladder-type multipoint optical fiber magnetic sensor showing one embodiment of the present invention.

FIG. 2 is a configuration diagram when a ladder-type multipoint optical fiber magnetic sensor of the present invention is actually created.

FIG. 3 is a diagram showing a detection result of terrestrial magnetism by the ladder-type multipoint optical fiber magnetic sensor of FIG. 2;

FIG. 4 is a diagram showing a multipoint model for noise analysis according to the present invention.

FIG. 5 is a characteristic diagram of optical path difference versus output obtained as a result of noise analysis according to the present invention.

FIG. 6 is a diagram showing a branch ratio model in the ladder-type multipoint sensor of the present invention.

FIG. 7 is a configuration diagram of a conventional multipoint optical fiber magnetic sensor.

[Explanation of symbols]

Reference Signs List 19 light source 20, 21, 22 optical coupler for branching optical fiber for light source 23, 24, 25 optical coupler on optical input side of sensor system 26, 27, 28, 29, 30, 31 metal-coated optical fiber (magnetic detecting element) 32, 33, 34, 35, 36, 37 AC power supply (oscillator) for driving the magnetic detection element 38, 39, 40 AC power supply (oscillator) for reference modulation 41, 43, 45 Optical phase modulation element for reference modulation 42, 44, 46 Optical phase modulation elements for feedback 47, 49, 51 Synchronous detectors for fading component detection 48, 50, 52 Amplifiers 53, 55, 57 Monitor light receivers 54, 56, 58 Light of sensor system Output side optical coupler 59 Optical receiver for magnetic information extraction unit 60, 61, 62 Optical coupler for multiplexing optical fiber for light reception 63 Carrier detector 64, 65, 66 For extraction of magnetic information Synchronous detectors 101, 102, 103 Sensor system 104 Optical fiber for light source 105 Optical fiber for light reception 106 Magnetic information extraction unit

Claims (5)

[Claims] 1. An AC current is applied to an optical fiber coated with metal, and an optical path difference changes due to a longitudinal expansion or contraction or a radial side pressure change of the metal-coated optical fiber due to an interaction between the AC current and an external magnetic flux. A plurality of sensor systems for detecting the change in the optical path difference as a signal of the intensity of light by optical interference are arranged in the longitudinal direction of the optical fiber for the light source connected to the light source, and the light input side of each of the sensor systems is optically controlled. Connected to the light source optical fiber via a coupler, the light output side of each sensor system is connected to a different light receiving optical fiber from the light source for the light source via an optical coupler, respectively, at one end of this light receiving optical fiber A ladder-type multipoint optical fiber magnetic sensor, comprising: a magnetic information extracting unit for extracting magnetic information of the external magnetic flux in each sensor system from the light intensity signal. 2. The optical system according to claim 1, wherein the sensor system includes an optical coupler that branches the input light, a plurality of the metal-coated optical fibers connected to the optical coupler, and a plurality of AC power supplies that supply current to the metal-coated optical fibers. An optical phase modulation element that performs reference modulation on one of the metal-coated optical fibers for fading detection, and an AC power supply that drives the optical phase modulation element,
An optical coupler that multiplexes and outputs optical signals from the plurality of metal-coated optical fibers, a photodetector that converts a part of the multiplexed optical signal into an electric signal, and detects a fading component from the electric signal 2. The ladder-type multi-point optical fiber magnetic sensor according to claim 1, comprising a synchronous detector that performs the detection and an optical phase modulation element that returns the detection signal to the other metal-coated optical fiber. 3. The magnetic information extracting unit detects a plurality of carrier waves for each sensor system from a light receiver connected to the light receiving optical fiber, and the light receiver converts an electric signal obtained by converting an intensity signal of the light. And a synchronous detector for extracting magnetic information of an external magnetic flux for each sensor system by performing synchronous detection from the electric signal using each of the plurality of carrier waves. Or the ladder-type multipoint optical fiber magnetic sensor according to 2. 4. An optical path difference between the sensor systems is set to be sufficiently large with respect to the optical path difference in the sensor system, and the coherence length of the light source is sufficiently larger than the optical path difference in the sensor system. The spectral line width of the light source is controlled so as to be sufficiently smaller than the optical path difference.
The ladder-type multipoint optical fiber magnetic sensor according to any one of the above. 5. The branch ratio of each optical coupler interposed between the optical fiber for the light source and the optical input side of each sensor system is changed according to the number of branch passes from the light source. 5. The ladder-type multi-point optical fiber magnetic sensor according to any one of items 4 to 4.