An all-fiber current sensor for linear measurements

Abstract

The application provides a linear measurement all-fiber current sensor, linearly polarized light is obtained through a polarizer, and is divided into two mutually orthogonal linearly polarized lights, and is further converted into left-handed and right-handed circularly polarized light to input a sensing optical fiber, a Faraday magneto-optic effect of current is used to make the two circularly polarized lights have a phase difference, the circularly polarized light is reflected by a mirror at the end of the sensing optical fiber, and then the phase difference is doubled through the sensing optical fiber, and the circularly polarized light is combined into a linearly polarized light through a λ / 4 wave plate, a modulator secondary phase modulation and a 45° fusion splice point, meanwhile, the phase difference of the circularly polarized light is converted into the rotation of the linearly polarized light polarization plane, and the rotation of the linearly polarized light polarization plane is converted into the synchronous translation of a light spot through a coupler, a beam expander and a radial polarization grating, and the phase difference is obtained by positioning the light spot through an image sensor; the application realizes linear demodulation of a Faraday magneto-optic rotation angle, has the advantages of being irrelevant to the measurement result and optical power, being beneficial to the separation compensation of linear birefringence, realizing non-distortion harmonic measurement and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical measurement, in particular to an all-optical fiber current sensor for linear measurement. Background Art

[0002] Current transformers are essential tools for sensing the operating status of power grids and form the foundation for grid protection, control, monitoring, and metering. Current transformers currently in operation primarily utilize electromagnetic oil insulation and SF6 gas insulation. These insulators are costly, present risks of oil and gas leaks, open circuits, and explosions, require extensive maintenance, and have poor harmonic measurement capabilities. Fiber Optic Current Transducers (FOCTs), which use optical fiber as both a sensing and transmission medium, offer reliable insulation, eliminate the risks of oil and gas leaks, open circuits, and explosions, are easy to install, and require no maintenance, representing the next generation of current transformers.

[0003] However, most existing FOCTs implement nonlinear demodulation of light intensity based on Malus's law, which leads to several difficult-to-solve problems, including direct correlation with optical power, difficulty in eliminating linear birefringence, inability to measure harmonics, and complex signal modulation and demodulation mechanisms. Therefore, achieving linear measurement is the key to overcoming the failure of FOCT in practical application. Linear measurement avoids the nonlinear demodulation based on Malus's law and can directly obtain the Faraday magneto-optical rotation angle. Therefore, the measurement result is independent of optical power, and all problems related to optical power in FOCT are solved. The magneto-optical rotation angle obtained by linear measurement is independent of linear birefringence, which is conducive to the separation and compensation of linear birefringence. Since it is independent of optical power, a temperature-insensitive and long-life LED (Light-Emitting Diode) light source can be used. Based on the principle of linear superposition, the harmonic components can be truly separated to achieve distortion-free harmonic measurement. Summary of the Invention

[0004] The present invention proposes an all-fiber current sensor for linear measurement, namely a linear all-fiber current sensor (Liner Fiber Optical Current Transducer, LFOCT), which realizes linear measurement, is independent of optical power, and is fully compensated for temperature drift and linear birefringence. It also allows the use of an LED light source with a long working life and has the ability to measure harmonics without distortion. The present invention is an all-fiber current sensor for linear measurement, which realizes the linear measurement of the all-fiber current sensor based on a radial polarization grating polarization analysis method. The light emitted by the LED light source is converted into linearly polarized light by a polarizer, enters the optical fiber loop through a coupler, achieves beat length matching through a delay line, and is split into two mutually orthogonal beams of linearly polarized light at a 45° fusion point. The light is initially phase modulated by a modulator and converted into left-handed and right-handed circularly polarized light by a λ / 4 wave plate. After entering the sensing optical fiber, the Faraday magneto-optical effect of the primary current causes the two circularly polarized light beams to The light generates a phase difference. The circularly polarized light is reflected by the reflector at the end of the sensing fiber, where it doubles the phase difference. It then passes through a λ / 4 wave plate, undergoes secondary phase modulation by a modulator, and undergoes a 45° fusion splice to combine into a single beam of linearly polarized light. Simultaneously, the phase difference of the circularly polarized light is converted into a rotation of the polarization plane of the linearly polarized light. This rotation is then converted into a synchronous translation of the light spot through a coupler, beam expander, and radial polarization grating. An image sensor locates the light spot to determine the phase difference. The image sensor also uses an algorithm to filter out linear birefringence, yielding the current to be measured. The secondary phase modulation by the modulator prevents polarization mismatch in the reflected linearly polarized light before reaching the radial polarization grating. This detection mode is suitable for measuring both AC and DC currents.

[0005] The present invention adopts the following technical solutions.

[0006] A linear measurement all-fiber current sensor, wherein the detection end of the sensor includes a sensing fiber; the sensing fiber forms a fiber ring structure, and when the sensor performs linear measurement on a circuit to be measured passing through the fiber ring structure, the sensor comprises the following steps:

[0007] Step S1, inputting left-handed circularly polarized light and right-handed circularly polarized light into the starting end of the sensing optical fiber, so that the optical fiber ring outputs a polarized light signal for expressing the measurement result based on the Sagnac effect and the Faraday magneto-optical effect;

[0008] Step S2: Processing the polarized light signal with the modulator of the sensor, and then passing the polarization plane of the light signal through the coupler, beam expander, and radial polarization grating of the sensor in sequence, converting the rotation of the polarization plane of the light into a synchronous translation of the light spot of the polarized light signal;

[0009] Step S3: Use an image sensor to locate the light spot to obtain a phase difference, and then use an algorithm to filter out the linear birefringence to obtain the current to be measured.

[0010] The sensor also includes an LED light source, a coupler, a polarizer, a delay line, a 45° fusion point, a modulator, and a λ / 4 wave plate;

[0011] In step S1, when generating left-handed circularly polarized light and right-handed circularly polarized light, the light emitted by the LED light source is first converted into linearly polarized light by a polarizer and then output through a coupler. The linearly polarized light is then beat-length matched by a delay line and then split into two mutually orthogonal linearly polarized beams at a 45° fusion point. Subsequently, the two beams of polarized light are initially phase-modulated by a modulator, converted into left-handed circularly polarized light and right-handed circularly polarized light by a λ / 4 wave plate and input into the sensing optical fiber.

[0012] The line to be tested is a power transmission line; a reflector is provided at the end of the sensing optical fiber;

[0013] In step S1, when the optical fiber ring is placed at the line to be measured, the magnetic field generated by the current in the transmission line acts on the sensing optical fiber. The resulting Faraday magneto-optical effect causes the polarization planes of the two circularly polarized light beams in the optical fiber ring to rotate, resulting in a phase difference. With linear birefringence δ; after the two circularly polarized lights with phase difference reach the end of the sensing optical fiber, they are reflected by the reflector and input into the optical fiber ring again to double the phase difference, forming a polarized light signal emitted from the starting end of the sensing optical fiber for expressing the current measurement result.

[0014] In step S2, the polarized light signal output from the starting end of the sensing fiber returns to the modulator through the λ / 4 wave plate and delay line. After secondary phase adjustment by the modulator, it passes through the 45° fusion point and the coupler to reach the beam expander and radial polarization grating. The sensor uses the beam expander and radial polarization grating to convert the rotation of the polarization plane of the polarized light signal into a synchronous translation of the light spot.

[0015] In step S3, the image sensor locates the light spot to obtain the phase difference, that is, the image sensor locates the light spot to obtain the phase delay angle At the same time, the image sensor filters out the linear birefringence δ based on the algorithm, so that the sensor can measure the translation of the light spot and obtain the current value to be measured.

[0016] In step S2, the modulator performs secondary phase adjustment on the polarized light signal output from the beginning of the sensing fiber. This is to avoid polarization mismatch in the reflected linearly polarized light before it reaches the S-wave plate.

[0017] The sensor includes detection modes suitable for AC current measurement and DC current measurement.

[0018] The detection method of the sensor is based on Jones matrix theory and includes the following steps:

[0019] Step A1: Ignore the linear birefringence δ of the sensing fiber. The incident linear polarized light is split into two orthogonal linear polarized light beams at the 45° fusion point. Suppose the electric vectors of the two polarized light beams are:

[0020]

[0021] After being converted into circularly polarized light by the λ / 4 wave plate, the Jones vector of the λ / 4 wave plate is:

[0022]

[0023] Step A2: The circularly polarized light passes through the sensing optical fiber and returns after passing through the reflector at the end of the optical fiber. The Jones vector of the sensing optical fiber in the incident light path and the reflected light path is:

[0024]

[0025] in The Jones vector of the reflector is:

[0026]

[0027] Step A3: The reflected light emitted from the sensing fiber is converted into orthogonal linear polarized light through a λ / 4 wave plate. The Jones vector of the λ / 4 wave plate in the return optical path is:

[0028]

[0029] After passing through the 45° splice, the orthogonal linearly polarized light beams are combined into a beam of linearly polarized light. The Jones vector of the 45° splice in the return optical path is:

[0030]

[0031] E x For example, after passing through the λ / 4 wave plate, sensing fiber, reflector, sensing fiber, λ / 4 wave plate, and 45° fusion point, the Jones vector of the outgoing polarized light is:

[0032]

[0033] Similarly, E y The outgoing light vector is:

[0034]

[0035] E x-out With E y-out It is synthesized into a beam of linearly polarized light, whose superposition amplitude E out Expressed as:

[0036]

[0037] Eout The component along the x-axis is The component along the y-axis is E out The angle θ between the polarization plane and the x-axis satisfies the following relationship:

[0038]

[0039] E out After entering the radial polarization grating, θ is converted into a linear translation of the stripe spot image; the Jones vector of the radial polarization grating is:

[0040]

[0041] where t TM is the diffraction efficiency (i.e., transmittance) of the TM wave; ξ is the phase difference between the TM wave and the TE wave; α is the direction of the radial polarization grating strips, ranging from -50° to 50°;

[0042] After demodulation by the radial polarization grating, the Jones vector of the outgoing light is:

[0043]

[0044] The expression of the outgoing light intensity is:

[0045]

[0046] Order I out Taking the minimum value, we get:

[0047]

[0048] When the light intensity transmitted from the corresponding grid unit is the minimum, its location is the center of the dark stripe; when When the dark stripes change, they move linearly along the bars. Since α = -50° to 50°, The range of variation is 0°~200°; the range of dark fringe translation is the length of the grating, i.e. 0~l;

[0049] Assumptions When the center of the dark fringe is on the 0° grid unit, the spot displacement Δx is related to The relationship satisfies:

[0050] but:

[0051]

[0052] By detecting the displacement distance Δx of the LFOCT output light spot, the linear measurement of the Faraday magneto-optical rotation angle can be achieved.

[0053] Based on MATLAB simulation, the LFOCT output dark pattern and The relationship is as follows Figure 2 shown.

[0054] Taking into account the linear birefringence δ of the sensing fiber, since the reflected light does not pass through the polarizer, are independent of δ, so:

[0055] After filtering out δ through signal processing, we can use formula (19) to achieve Linear measurement.

[0056] The sensing optical fiber uses a rotating optical fiber, and the operating wavelength of the rotating optical fiber matches the operating wavelength of the image sensor. When the current sensor is used to measure small currents, the length of the rotating optical fiber is more than several meters. When the current sensor is used to measure large currents, the length of the rotating optical fiber is more than tens of meters.

[0057] When LFOCT is used to measure transmission lines in substations, the LFOCT's fiber loop is placed around the conductor carrying the current to be measured. The magnetic field generated by the measured current acts on the sensing fiber. The Faraday magneto-optical effect generated by the magnetic field causes the polarization plane of the polarized light passing through the fiber to rotate. Based on the linear demodulation mode, the LFOCT first converts the Faraday magnetorotation angle into a linear translation of the light spot image. The Faraday magnetorotation angle is then directly measured by detecting the amount of linear translation. The magnitude of the measured current is then determined based on the corresponding relationship between the Faraday magnetorotation angle and the current value.

[0058] The LFOCT of the present invention realizes linear measurement, light power independence, and full compensation of temperature drift and linear birefringence. It allows the use of LED light sources with long service life and has the ability to measure harmonics without distortion. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0060] Attachment Figure 1 It is a schematic diagram of the principle of the present invention;

[0061] Attachment Figure 2 This is a schematic diagram of the simulation results of the LFOCT output light spot. DETAILED DESCRIPTION

[0062] As shown in the figure, a linear measurement all-fiber current sensor is provided. The detection end of the sensor includes a sensing fiber; the sensing fiber forms a fiber ring structure. When the sensor performs a linear measurement on a circuit to be measured passing through the fiber ring structure, the sensor includes the following steps:

[0063] Step S1, inputting left-handed circularly polarized light and right-handed circularly polarized light into the starting end of the sensing optical fiber, so that the optical fiber ring outputs a polarized light signal for expressing the measurement result based on the Sagnac effect and the Faraday magneto-optical effect;

[0064] Step S2: Processing the polarized light signal with the modulator of the sensor, and then passing the polarization plane of the light signal through the coupler, beam expander, and radial polarization grating of the sensor in sequence, converting the rotation of the polarization plane of the light into a synchronous translation of the light spot of the polarized light signal;

[0065] Step S3: Use an image sensor to locate the light spot to obtain a phase difference, and then use an algorithm to filter out the linear birefringence to obtain the current to be measured.

[0066] The sensor also includes an LED light source, a coupler, a polarizer, a delay line, a 45° fusion point, a modulator, and a λ / 4 wave plate;

[0067] In step S1, when generating left-handed circularly polarized light and right-handed circularly polarized light, the light emitted by the LED light source is first converted into linearly polarized light by a polarizer and then output through a coupler. The linearly polarized light is then beat-length matched by a delay line and then split into two mutually orthogonal linearly polarized beams at a 45° fusion point. Subsequently, the two beams of polarized light are initially phase-modulated by a modulator, converted into left-handed circularly polarized light and right-handed circularly polarized light by a λ / 4 wave plate and input into the sensing optical fiber.

[0068] The line to be tested is a power transmission line; a reflector is provided at the end of the sensing optical fiber;

[0069] In step S1, the optical fiber ring is placed on the conductor that transmits the current to be measured. The magnetic field generated by the current to be measured acts on the sensing optical fiber. The resulting Faraday magneto-optical effect causes the polarization planes of the two circularly polarized light beams in the optical fiber ring to rotate, resulting in a phase difference. With linear birefringence δ; after the two circularly polarized lights with phase difference reach the end of the sensing optical fiber, they are reflected by the reflector and input into the optical fiber ring again to double the phase difference, forming a polarized light signal emitted from the starting end of the sensing optical fiber for expressing the current measurement result.

[0070] In step S2, the polarized light signal output from the starting end of the sensing fiber returns to the modulator through the λ / 4 wave plate and delay line. After secondary phase adjustment by the modulator, it passes through the 45° fusion point and the coupler to reach the beam expander and radial polarization grating. The sensor uses the beam expander and radial polarization grating to convert the rotation of the polarization plane of the polarized light signal into a synchronous translation of the light spot.

[0071] In step S3, the image sensor locates the light spot to obtain the phase difference, that is, the image sensor locates the light spot to obtain the phase delay angle At the same time, the image sensor filters out the linear birefringence δ based on the algorithm, so that the sensor can measure the translation of the light spot and obtain the current value to be measured.

[0072] In step S2, the modulator performs secondary phase adjustment on the polarized light signal output from the beginning of the sensing fiber. This is to avoid polarization mismatch in the reflected linearly polarized light before it reaches the S-wave plate.

[0073] The sensor includes detection modes suitable for AC current measurement and DC current measurement.

[0074] The detection method of the sensor is based on Jones matrix theory and includes the following steps:

[0075] Step A1: Ignore the linear birefringence δ of the sensing fiber. The incident linear polarized light is split into two orthogonal linear polarized light beams at the 45° fusion point. Suppose the electric vectors of the two polarized light beams are:

[0076]

[0077] After being converted into circularly polarized light by the λ / 4 wave plate, the Jones vector of the λ / 4 wave plate is:

[0078]

[0079] Step A2: The circularly polarized light passes through the sensing optical fiber and returns after passing through the reflector at the end of the optical fiber. The Jones vector of the sensing optical fiber in the incident light path and the reflected light path is:

[0080]

[0081] in The Jones vector of the reflector is:

[0082]

[0083] Step A3: The reflected light emitted from the sensing fiber is converted into orthogonal linear polarized light through a λ / 4 wave plate. The Jones vector of the λ / 4 wave plate in the return optical path is:

[0084]

[0085] After passing through the 45° splice, the orthogonal linearly polarized light beams are combined into a beam of linearly polarized light. The Jones vector of the 45° splice in the return optical path is:

[0086]

[0087] E x For example, after passing through the λ / 4 wave plate, sensing fiber, reflector, sensing fiber, λ / 4 wave plate, and 45° fusion point, the Jones vector of the outgoing polarized light is:

[0088]

[0089] Similarly, E y The outgoing light vector is:

[0090]

[0091] E x-out With E y-out It is synthesized into a beam of linearly polarized light, whose superposition amplitude E out Expressed as:

[0092]

[0093] E out The component along the x-axis is The component along the y-axis is E out The angle θ between the polarization plane and the x-axis satisfies the following relationship:

[0094]

[0095] E out After entering the radial polarization grating, θ is converted into a linear translation of the stripe spot image; the Jones vector of the radial polarization grating is:

[0096]

[0097] where t TM is the diffraction efficiency (i.e., transmittance) of the TM wave; ξ is the phase difference between the TM wave and the TE wave; α is the direction of the radial polarization grating strips, ranging from -50° to 50°;

[0098] After demodulation by the radial polarization grating, the Jones vector of the outgoing light is:

[0099]

[0100] The expression of the outgoing light intensity is:

[0101]

[0102] Order I out Taking the minimum value, we get:

[0103]

[0104] When the light intensity transmitted from the corresponding grid unit is the minimum, its location is the center of the dark stripe; when When the dark stripes change, they move linearly along the bars. Since α = -50° to 50°, The range of variation is 0°~200°; the range of dark fringe translation is the length of the grating, i.e. 0~l;

[0105] Assumptions When the center of the dark fringe is on the 0° grid unit, the spot displacement Δx is related to The relationship satisfies:

[0106] but:

[0107]

[0108] By detecting the displacement distance Δx of the LFOCT output light spot, the linear measurement of the Faraday magneto-optical rotation angle can be achieved.

[0109] Based on MATLAB simulation, the LFOCT output dark pattern and The relationship is as follows Figure 2 shown.

[0110] Taking into account the linear birefringence δ of the sensing fiber, since the reflected light does not pass through the polarizer, are independent of δ, so:

[0111] After filtering out δ through signal processing, we can use formula (19) to achieve Linear measurement.

[0112] The sensing optical fiber uses a rotating optical fiber, and the operating wavelength of the rotating optical fiber matches the operating wavelength of the image sensor. When the current sensor is used to measure small currents, the length of the rotating optical fiber is more than several meters. When the current sensor is used to measure large currents, the length of the rotating optical fiber is more than tens of meters.

[0113] When LFOCT is used to measure transmission lines in substations, the LFOCT's fiber loop is placed around the conductor carrying the current to be measured. The magnetic field generated by the measured current acts on the sensing fiber. The Faraday magneto-optical effect generated by the magnetic field causes the polarization plane of the polarized light passing through the fiber to rotate. Based on the linear demodulation mode, the LFOCT first converts the Faraday magnetorotation angle into a linear translation of the light spot image. The Faraday magnetorotation angle is then directly measured by detecting the amount of linear translation. The magnitude of the measured current is then determined based on the corresponding relationship between the Faraday magnetorotation angle and the current value.

[0114] Example 1:

[0115] This example proposes an all-fiber current sensor for linear measurement, based on the Sagnac effect, the Faraday magneto-optical effect, and a radial polarization grating analysis method. The technical principle is as follows: Light emitted from a light source passes through a polarizer to generate linearly polarized light. This light is then split into two orthogonal linearly polarized beams through a coupler, a delay line, and a 45° fusion splice. These beams undergo initial phase modulation by a modulator and are converted into left-handed and right-handed circularly polarized light by a λ / 4 wave plate. Upon entering the sensing fiber, the Faraday magneto-optical effect of the primary current causes a phase difference between the two circularly polarized beams. After reflection from a reflector at the end of the sensing fiber, the phase difference is doubled by the sensing fiber. The beam is then combined into a single linearly polarized beam after passing through a λ / 4 wave plate, secondary phase modulation by a modulator, and a 45° fusion splice. This phase difference is then converted into a rotation of the linearly polarized beam's plane of polarization. After secondary phase modulation by the modulator, the beam passes through a coupler, a beam expander, and a radial polarization grating, converting the rotation into a synchronous translation of the light spot. An image sensor locates the light spot and determines the phase difference.

[0116] This example realizes the linear demodulation of Faraday magneto-optical rotation angle, which has the advantages of independent measurement results and optical power, being conducive to the separation compensation of linear birefringence, and realizing distortion-free harmonic measurement.

[0117] Example 2:

[0118] In this example, the linear measurement all-fiber current sensor proposed in Example 1 is used, and the linear measurement of the all-fiber current sensor is achieved based on the radial polarization grating polarization analysis method.

[0119] Specifically, the light emitted by the [LED light source] is converted into linearly polarized light through the [polarizer], enters the optical fiber loop through the [coupler], achieves beat length matching through the [delay line], is split into two mutually orthogonal linearly polarized light beams through the [45°] fusion point, undergoes initial phase modulation by the [modulator], and is converted into left-handed and right-handed circularly polarized light through the [λ / 4 wave plate]. After entering the [sensing optical fiber], the Faraday magneto-optical effect of the primary current causes a phase difference between the two circularly polarized light beams. The circularly polarized light is reflected by the [reflector] at the end of the sensing optical fiber and then passes through the [sensing optical fiber] The phase difference is doubled and then merged into a beam of linearly polarized light after secondary phase modulation by the [λ / 4 wave plate] and the [modulator] and the [45°] welding point. At the same time, the phase difference of the circularly polarized light is converted into the rotation of the polarization plane of the linearly polarized light. After secondary phase modulation by the [modulator], the rotation of the polarization plane of the linearly polarized light is converted into the synchronous translation of the [light spot] through the [coupler], [beam expander], and [radial polarization grating]. The [image sensor] positions the light spot to obtain the phase difference. At the same time, the [image sensor] filters out the linear birefringence based on the algorithm to obtain the current to be measured.

[0120] In this example, the function of the secondary phase modulation of the modulator is to avoid polarization mismatch of the returned linearly polarized light during its transmission before reaching the radial polarization grating.

[0121] The detection mode in this example is suitable for measuring AC current and DC current.

Claims

1. An all-fiber current sensor for linear measurement, characterized by: The detection end of the sensor includes a sensing optical fiber; the sensing optical fiber forms an optical fiber ring structure, and the sensor also includes an LED light source, a coupler, a polarizer, a delay line, a 45° fusion point, a modulator, and a λ / 4 wave plate; The sensor realizes linear measurement of all-fiber current sensor based on radial polarization grating polarization analysis method; The light emitted by the LED light source is converted into linearly polarized light by the polarizer, enters the optical fiber loop through the coupler, achieves beat length matching through the delay line, and is split into two mutually orthogonal linearly polarized light beams through the 45° fusion point. After initial phase modulation by the modulator, it is converted into left-handed and right-handed circularly polarized light by the λ / 4 wave plate. After entering the sensing fiber, the Faraday magneto-optical effect of the primary current causes the two circularly polarized light beams to produce a phase difference. ， The circularly polarized light is reflected by the reflector at the end of the sensing fiber, and then passes through the sensing fiber to double the phase difference. λ The / 4 wave plate, modulator secondary phase modulation and 45° welding point are combined into a beam of linearly polarized light. At the same time, the phase difference of the circularly polarized light is converted into the rotation of the polarization plane of the linearly polarized light. The rotation of the polarization plane of the linearly polarized light is converted into the synchronous translation of the light spot through the coupler, beam expander and radial polarization grating. The image sensor locates the light spot to obtain the phase difference. At the same time, the image sensor filters out the linear birefringence based on the algorithm to obtain the current to be measured; The function of the secondary phase modulation of the modulator is to avoid polarization mismatch in the transmission process of the returned linearly polarized light before it reaches the radial polarization grating. The sensing optical fiber adopts a rotating optical fiber, and the working wavelength of the rotating optical fiber matches the working wavelength of the image sensor.

2. The all-fiber current sensor for linear measurement according to claim 1, characterized in that: The detection mode is suitable for measuring AC current and DC current.

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