Accelerator magnetic field mapping measurement method and system based on optical fiber sensing

Abstract

This invention provides a method and system for measuring accelerator magnetic field mapping based on fiber optic sensing, belonging to the field of magnetic field measurement technology. The method includes: deploying an fiber optic sensing link containing at least first and second fiber optic magnetic sensing units within the aperture of the magnet of the accelerator under test, and acquiring first and second magnetic field response signals containing wavelength domain information; performing coherent detection on the first and second magnetic field response signals to extract polarization fading characteristic parameters induced by the magnetic field and magnetostrictive birefringence dispersion coefficients; and determining a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters. This invention uses dual fiber optic magnetic sensing units to perform differential demodulation measurements under steady-state and strong dynamic disturbance conditions, relies on the polarization extinction virtual reference node to complete global polarization distortion correction, and combines differential closed-loop compensation and polarization multidimensional mapping with joint vector demodulation to effectively suppress measurement errors caused by light source drift, optical path polarization fading, and equipment vibration disturbances.

Description

Technical Field

[0001] This invention relates to the field of magnetic field measurement technology, and in particular to an accelerator magnetic field mapping measurement method and system based on fiber optic sensing. Background Technology

[0002] Precise measurement of the magnetic field distribution within the aperture of an accelerator magnet is crucial for ensuring beam quality and operational stability. Taking medical proton therapy accelerators as an example, their magnet apertures are typically tens of millimeters in diameter. Within this confined space, high-resolution mapping of the magnetic field is necessary to meet the precision requirements of beam transport and energy regulation. Fiber optic magnetic field sensing technology, due to its advantages of small size, resistance to electromagnetic interference, and radiation resistance, has been explored for application in such scenarios in recent years.

[0003] In practice, it has been found that the inherent linear birefringence of optical fibers, as well as environmental disturbances such as temperature and mechanical vibration, can cause random changes in the polarization state of transmitted light, resulting in polarization fading of the measured signal. This phenomenon causes fluctuations in the amplitude of the magnetic field response signal output by the sensor, especially when the accelerator excitation current changes or the cooling system vibrates, the polarization fading is more pronounced, and it has a certain impact on the repeatability of multiple measurements at the same magnetic field point. In existing technologies, polarization-maintaining fibers or polarization controllers are typically used for passive compensation, but these methods have high requirements for hardware consistency and there is room for improvement in real-time tracking of polarization state changes in dynamic environments. Therefore, when performing synchronous measurements at multiple points within the aperture, there is still room for further improvement in the measurement stability and accuracy of each sensing unit.

[0004] For example, when mapping the magnetic field distribution within the magnet aperture of a medical proton therapy accelerator, the local magnetic field values ​​measured by fiber optic sensors based on the traditional Faraday effect fluctuate significantly due to abrupt changes in polarization state as the excitation current changes. Since the accelerator control system has stringent requirements for magnetic field stability, the measurement accuracy of existing solutions still has room for improvement compared to practical application needs. Therefore, effectively suppressing the interference of polarization fading on fiber optic magnetic field measurement signals and achieving highly stable magnetic field distribution reconstruction within a finite aperture is a technical problem worthy of further research. Summary of the Invention

[0005] This invention provides a method and system for measuring accelerator magnetic field mapping based on fiber optic sensing, which can effectively suppress the interference of polarization fading on the measurement signal and achieve highly stable magnetic field distribution reconstruction within the aperture of the accelerator magnet.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0007] A first aspect is a method for measuring accelerator magnetic field mapping based on fiber optic sensing, the method comprising:

[0008] Step 1: Layout an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and collect the first and second magnetic field response signals containing wavelength domain information.

[0009] Step 2: Perform coherent detection on the first and second magnetic field response signals, extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient; determine a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters;

[0010] Step 3: Use the polarization extinction virtual reference node to perform polarization compensation on the first and second magnetic field response signals to obtain the first and second compensated magnetic field response signals, and calculate the magnetic response difference between the first and second compensated magnetic field response signals.

[0011] Step 4: When the difference in magnetic response is less than a preset threshold, the first and second fiber magnetic sensing units are associated as a differential magnetic compensation closed loop using a polarization extinction virtual reference node, and the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node is brought to zero through feedback control to form a locked measurement baseline.

[0012] Step 5: When the difference in magnetic response is greater than or equal to a preset threshold, a multidimensional mapping relationship of polarization state is constructed based on the polarization extinction virtual reference node and the magnetostrictive birefringence dispersion coefficient. The first and second compensated magnetic field response signals are jointly vector demodulated under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution in the aperture of the accelerator magnet.

[0013] Secondly, an accelerator magnetic field mapping measurement system based on fiber optic sensing includes:

[0014] The acquisition module is used to deploy an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and to acquire the first and second magnetic field response signals containing wavelength domain information.

[0015] The evolution module is used to perform coherent detection on the first and second magnetic field response signals, extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient; and determine a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters.

[0016] The calculation module is used to perform polarization compensation on the first and second magnetic field response signals using a polarization extinction virtual reference node, to obtain the first and second compensated magnetic field response signals, and to calculate the magnetic response difference between the first and second compensated magnetic field response signals.

[0017] The compensation module is used to associate the first and second fiber magnetic sensing units into a differential magnetic compensation closed loop by using a polarization extinction virtual reference node when the magnetic response difference is less than a preset threshold, and to make the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node zero through feedback control, thereby locking the measurement baseline.

[0018] The reconstruction module is used to construct a multidimensional mapping relationship of polarization state based on the magnetostrictive birefringence dispersion coefficient, using the polarization extinction virtual reference node as a reference when the magnetic response difference is greater than or equal to a preset threshold; and to jointly demodulate the first and second compensated magnetic field response signals under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution in the aperture of the accelerator magnet.

[0019] Thirdly, a computer-readable storage medium storing a program that, when executed by a processor, implements the method.

[0020] The above-described solution of the present invention has at least the following beneficial effects:

[0021] By extracting polarization fading characteristic parameters and magnetostrictive birefringence dispersion coefficients through signal coherent detection, and constructing a virtual reference node for polarization extinction based on the polarization state evolution trajectory, a unified and stable polarization reference is established. This can specifically counteract the random polarization state drift caused by inherent linear birefringence of optical fibers, changes in ambient temperature, mechanical vibration, and dynamic excitation conditions of accelerators. It can also correct signal distortion caused by polarization fading in real time at the algorithm level, and weaken the interference of complex external conditions on the magnetic field response signal.

[0022] By employing a magnetic response difference threshold discrimination mechanism, a dual-mode adaptive measurement system is established. Under steady-state conditions, the equivalent magnetostrictive phase shift is reduced to zero through differential magnetic compensation closed-loop feedback control, establishing a stable locked measurement baseline and continuously eliminating slow polarization disturbances to ensure the repeatability and stability of steady-state magnetic field measurements. Under conditions with strong dynamic disturbances, a multi-dimensional mapping relationship of polarization states is constructed based on the magnetostrictive birefringence dispersion coefficient. Magnetic field reconstruction is achieved through multi-signal joint vector demodulation, overcoming the limitations of traditional scalar demodulation and effectively solving the problem of magnetic field measurement distortion under dynamic conditions, comprehensively covering the measurement needs of different accelerator operating states.

[0023] High-precision polarization compensation and magnetic field mapping measurement can be achieved by optimizing the link layout and algorithm of dual fiber optic magnetic sensing units, reducing the stringent requirements of the measurement system on hardware consistency. At the same time, the fiber optic sensing link is small in size and highly integrated, which can adapt to the narrow and limited space layout requirements of accelerator magnet apertures, while retaining the inherent advantages of fiber optic sensing such as resistance to electromagnetic interference and radiation.

[0024] Precise polarization compensation is achieved by using a virtual reference node for polarization extinction. After eliminating signal polarization deviation, a fine mapping of the global magnetic field distribution within the accelerator magnet aperture is realized through multi-dimensional polarization space mapping, multi-channel signal vector synthesis, and spatial interpolation reconstruction. Compared with traditional single-point, scalar measurement methods, this method can completely restore the vector distribution characteristics of the magnetic field within the confined space, capture the local magnetic field inhomogeneity, and provide precise magnetic field data support for accelerator beam transport and energy regulation, effectively ensuring the quality of the accelerator beam and the overall operational stability.

[0025] The constructed polarization extinction virtual reference node can serve as a shared benchmark, providing a unified polarization calibration standard for multiple sensing units within the fiber optic sensing link. It can rapidly expand the distributed fiber optic sensing array architecture, enabling multi-point, synchronized, and high-precision magnetic field mapping measurements within the magnet aperture, effectively improving the coverage and efficiency of magnetic field measurements. Attached Figure Description

[0026] Figure 1 This is a schematic flowchart of an accelerator magnetic field mapping measurement method based on fiber optic sensing provided in an embodiment of the present invention.

[0027] Figure 2 This is a schematic diagram of the process for determining a polarization extinction virtual reference node in the polarization state evolution trajectory based on polarization fading characteristic parameters, provided by an embodiment of the present invention.

[0028] Figure 3 This is a schematic diagram of an accelerator magnetic field mapping measurement system based on fiber optic sensing, provided in an embodiment of the present invention. Detailed Implementation

[0029] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0030] like Figure 1 As shown, embodiments of the present invention propose a method for measuring accelerator magnetic field mapping based on fiber optic sensing, the method comprising the following steps:

[0031] Step 1: Layout an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and collect the first and second magnetic field response signals containing wavelength domain information.

[0032] Step 2: Perform coherent detection on the first and second magnetic field response signals, extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient; determine a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters;

[0033] Step 3: Use the polarization extinction virtual reference node to perform polarization compensation on the first and second magnetic field response signals to obtain the first and second compensated magnetic field response signals, and calculate the magnetic response difference between the first and second compensated magnetic field response signals.

[0034] Step 4: When the difference in magnetic response is less than a preset threshold, the first and second fiber magnetic sensing units are associated as a differential magnetic compensation closed loop using a polarization extinction virtual reference node, and the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node is brought to zero through feedback control to form a locked measurement baseline.

[0035] Step 5: When the difference in magnetic response is greater than or equal to a preset threshold, a multidimensional mapping relationship of polarization state is constructed based on the polarization extinction virtual reference node and the magnetostrictive birefringence dispersion coefficient. The first and second compensated magnetic field response signals are jointly vector demodulated under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution in the aperture of the accelerator magnet.

[0036] In a preferred embodiment of the present invention, step 1, which involves arranging an optical fiber sensing link containing at least first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and acquiring first and second magnetic field response signals containing wavelength domain information, is performed as follows:

[0037] An optical fiber sensing link is deployed within the aperture of the magnet in the accelerator under test. This optical fiber sensing link refers to an optical fiber path formed by connecting one or more optical fibers in series, parallel, or a hybrid series-parallel connection, with at least two optical fiber magnetic sensing units installed on it to sense the magnetic field strength at different spatial locations. The optical fiber magnetic sensing unit refers to an optical fiber segment that has undergone special treatment (such as writing fiber gratings, coating with magnetostrictive materials, or fusion splicing special optical fiber segments) and is sensitive to changes in the magnetic field. In this embodiment, an optical fiber magnetic sensing unit based on a composite structure of a magnetostrictive material coating layer and a fiber Bragg grating (FBG) is used. Specifically, a first optical fiber magnetic sensing unit and a second optical fiber magnetic sensing unit are fabricated at two selected locations on the single-mode optical fiber. The spacing between the two units along the fiber axis is set according to the spatial resolution requirements of the magnetic field to be measured within the magnet aperture; in this embodiment, it is set to 8 mm.

[0038] The deployment method is as follows: the optical fiber carrying the first and second fiber optic magnetic sensing units is laid out and fixed along the circumferential or axial path of the inner wall of the magnet aperture, ensuring that the sensitive direction of the fiber optic magnetic sensing unit is aligned with the principal direction of the measured magnetic field. For a dipole magnet, the principal direction of the measured magnetic field is the transverse magnetic field direction perpendicular to the beam direction (i.e., the dipole magnetic field direction). Therefore, the orientation of the fiber optic magnetic sensing unit should ensure that its sensitive axis is parallel to this direction. The fixing method can use high-temperature resistant ceramic adhesive or mechanical clamps. For ease of subsequent magnetic field vector description, the coordinate system is defined as follows: the beam direction is the Z-axis, the horizontal radial direction is the X-axis, and the vertical direction is the Y-axis. The origin of the coordinate system is set at the center of the magnet aperture, ensuring that the spatial position of the sensing unit does not shift during accelerator operation.

[0039] After the fiber optic sensing link is deployed, one end of the fiber is connected to a broadband light source (in this embodiment, a superluminescent diode (SLD) light source with a center wavelength of 1550 nm and a full width at half maximum (FWHM) of approximately 50 nm is used), and the other end is connected to a spectral acquisition and analysis device (in this embodiment, a spectrometer or high-speed spectrometer is used, with a wavelength scanning range of 1510 nm to 1590 nm and a wavelength resolution better than 0.02 nm). The broadband light emitted by the light source is transmitted through the fiber to each fiber optic magnetic sensing unit. Under the influence of the magnetic field, the wavelength of the light signal reflected or transmitted by each sensing unit is shifted, carrying the magnetic field information at the corresponding location.

[0040] The wavelength domain information refers to the distribution of optical signal intensity recorded with the wavelength as the independent variable, i.e., spectral data. For fiber optic magnetic sensing units based on FBG, a reflection peak appears in its reflection spectrum at a specific wavelength. The center wavelength of this reflection peak shifts with the applied magnetic field. Therefore, the wavelength position and intensity of the reflection peak in the spectrum constitute the basic information carrier of the magnetic field response signal.

[0041] During the acquisition process, the spectral acquisition and analysis device performs a full-spectrum scan of the optical signal output from the fiber optic sensing link, recording the light intensity values ​​at different wavelength points to form a first magnetic field response signal and a second magnetic field response signal containing wavelength domain information. The first magnetic field response signal corresponds to the spectral data output by the first fiber optic magnetic sensing unit, specifically a one-dimensional data sequence composed of the light intensity values ​​of the reflected light at each scanning wavelength point at the first fiber optic magnetic sensing unit. The second magnetic field response signal corresponds to the spectral data output by the second fiber optic magnetic sensing unit, specifically a one-dimensional data sequence composed of the light intensity values ​​of the reflected light at each scanning wavelength point at the second fiber optic magnetic sensing unit. The first and second magnetic field response signals are transmitted in the same optical fiber and identified separately through wavelength division multiplexing (WDM) or time division multiplexing (TDM). This embodiment uses WDM, with the center wavelength of the FBG of the first fiber optic magnetic sensing unit designed to be around 1535 nm and the center wavelength of the FBG of the second fiber optic magnetic sensing unit designed to be around 1555 nm. The two signals are naturally separated spectrally and can be acquired simultaneously by the spectral acquisition and analysis device in a single full-spectrum scan.

[0042] This step achieves simultaneous acquisition of magnetic field signals from multiple measurement points within a confined space by deploying fiber optic sensing links containing at least first and second fiber optic magnetic sensing units within the accelerator magnet aperture, and acquiring first and second magnetic field response signals that include wavelength domain information. The dual-sensor unit link architecture provides the data foundation for subsequent polarization compensation and differential demodulation, enabling multi-point measurement capabilities within the aperture without additional hardware, effectively improving the coverage efficiency and spatial resolution of magnetic field mapping; simultaneously, the acquisition of wavelength domain information preserves the full-band response characteristics under a broadband light source.

[0043] like Figure 2 As shown, in a preferred embodiment of the present invention, step 2, coherent detection of the first and second magnetic field response signals, extraction of polarization fading characteristic parameters induced by the magnetic field and magnetostrictive birefringence dispersion coefficient; the process of determining a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters is as follows:

[0044] The optical signal corresponding to the first magnetic field response signal is input to a polarization beamsplitter. The polarization beamsplitter is an optical device capable of splitting incident light into two beams of outgoing light with mutually perpendicular polarization directions. In this embodiment, a Gran-Thompson prism-type polarization beamsplitter is used, with an extinction ratio better than 30 dB. The polarization beamsplitter decomposes the first magnetic field response signal into a first horizontal polarization component and a first vertical polarization component. Specifically, the X-axis of the coordinate system corresponding to the radial direction of the accelerator magnet aperture is defined as the fixed reference horizontal direction; the Y-axis of the coordinate system corresponding to the direction perpendicular to the radial direction of the magnet aperture within the beam cross-section is defined as the fixed reference vertical direction. The two reference directions are orthogonal, and the included angle is fixed at 90°. After fixed beam splitting by the polarization beamsplitter, the first magnetic field response signal is decomposed into two orthogonal polarization components, including the first horizontal polarization component, specifically the linearly polarized light component with a polarization direction parallel to the fixed reference horizontal direction; and the first vertical polarization component, specifically the linearly polarized light component with a polarization direction parallel to the fixed reference vertical direction.

[0045] Two orthogonally polarized light components are respectively connected to independently paired photodetectors to complete linear photoelectric conversion. In this embodiment, a photodiode is used as the photoelectric conversion device. The fixed parameters of the device are: spectral response band of 1500nm to 1600nm, and photoresponsivity R0. g =0.9A / W. The first horizontal polarization component is entirely incident on the first photodiode, and the first vertical polarization component is entirely incident on the second photodiode. The two detection channels are physically independent, with no crosstalk in the optical path and no coupling interference in the circuit. The photodetector achieves linear conversion of optical power to current signal based on the photoelectric effect. That is, the optical power of the incident polarized light at time t is defined as P(t) (unit: W), and the detector output analog photocurrent is I(t) (unit: A). The fixed conversion formula is: I(t) = R g ×P(t), where the responsiveness R g The constant value is 0.9A / W. The downstream load resistor R is a fixed value. load The signal conditioning circuit realizes the linear conversion of photocurrent to analog voltage signal, and the conversion formula is: U(t)=I(t)×R load The voltage signal U(t) (unit: V) and the incident light power P(t) have a one-to-one linear relationship.

[0046] The system employs a synchronous sampling clock to synchronously acquire two voltage signals. The sampling clock frequency is fixed, and both detection channels share the same trigger sampling sequence. Voltage signals U1(t) and U2(t) output from the first and second photodetectors are synchronously acquired at exactly the same sampling time t, ensuring timing alignment and a completely consistent number of sampling points. To eliminate system errors caused by light intensity drift and optical path loss fluctuations, standard point-to-point normalization is performed on the original voltage signal. The normalized light intensity value is calculated for each sampling point. The specific normalization calculation formula is: Inorm1 (t)=U1(t) / (U1(t+U2(t)); I norm2 (t)=U2(t) / (U1(t)+U2(t)); in the formula, I norm1 (t) represents the normalized light intensity value of the first horizontal polarization component at time t, I norm2 (t) represents the normalized light intensity value of the first vertical polarization component at time t. After normalization, the sum of the two light intensity values ​​at a single sampling time is always 1, thus eliminating global light intensity fluctuation interference.

[0047] Normalized light intensity values ​​corresponding to all synchronous sampling times are arranged sequentially according to sampling time to construct two sets of one-dimensional time-series numerical sequences with unified dimensions and precise temporal matching, defined as the first horizontal light intensity signal and the first vertical light intensity signal, respectively. The first horizontal light intensity signal is composed of I values ​​from all sampling times. norm1 The time-series data sequence composed of (t) represents the relative optical power variation characteristics of the first horizontal polarization component at each sampling time; the first vertical optical intensity signal is composed of I at all sampling times. norm2 The time-series data sequence (t) represents the relative optical power variation characteristics of the first vertical polarization component at each sampling time. The two signals have the same sampling interval, the same data dimension, and are completely synchronized in time.

[0048] Based on the first horizontal light intensity signal and the first vertical light intensity signal, the first polarization fading signal is calculated. The polarization fading signal refers to a characteristic signal reflecting the fluctuations in the polarization state of transmitted light in the optical fiber due to external factors (such as magnetic fields, stress, temperature changes, etc.) over time or wavelength. Essentially, it is a quantitative expression of the intensity ratio or phase relationship during the evolution of the polarization state. In this embodiment, the first polarization fading signal is calculated as follows:

[0049] The first horizontal light intensity signal and the first vertical light intensity signal are acquired, which are light intensity values ​​obtained from the first photodetector and the second photodetector respectively at the same sampling time. For each synchronous sampling time t, the first horizontal light intensity value and the first vertical light intensity value that have been normalized at that time are retrieved; the sum of the normalized first horizontal light intensity value and the normalized first vertical light intensity value is calculated to obtain the total light intensity reference value of the two light intensities; then the difference between the normalized first horizontal light intensity value and the normalized first vertical light intensity value is calculated to obtain the differential light intensity value of the two light intensities; the ratio of the differential light intensity value of the two light intensities to the total light intensity reference value of the two light intensities is calculated to obtain the first polarization fading signal value F1(t) at that sampling time. The range of this ratio is limited to the interval [-1, 1], describing the proportion of the distribution deviation of light energy in the two fixed orthogonal polarization directions at the same time.

[0050] When the optical fiber link is free from magnetic field disturbances and polarization drift, and the polarization state is in a stable and constant state, the intensity distribution ratio of the two orthogonally polarized light paths is fixed, and F1(t) is a constant without fluctuation. When factors such as changes in the accelerator magnetic field, micro-disturbances in the optical path, and stress deformation cause rotation and distortion of the light polarization state, the light energy in the orthogonal polarization direction is dynamically redistributed. Specifically, let the instantaneous deflection angle of the polarization principal axis relative to the fixed horizontal reference direction be θ, and the normalized total light intensity be constant at 1. Then, the proportion of horizontally polarized light intensity is equal to cos²θ, and the proportion of vertically polarized light intensity is equal to sin²θ, satisfying the energy conservation relationship of cos²θ + sin²θ = 1. When the polarization principal axis deflects towards the horizontal reference direction and the angle θ decreases, the value of cos²θ monotonically increases and the value of sin²θ monotonically decreases, corresponding to an increase in the proportion of horizontally polarized light intensity and a decrease in the proportion of vertically polarized light intensity. When the polarization principal axis deflects towards the vertical reference direction and the angle θ increases, the value of cos²θ monotonically decreases and the value of sin²θ monotonically increases, corresponding to a precise increase in the proportion of vertically polarized light intensity and a precise decrease in the proportion of horizontally polarized light intensity. This quantitative energy redistribution process occurs in real time with the continuous change of magnetic field parameters. The angle deflection and the rise and fall of light intensity have a continuous trigonometric function relationship, ultimately causing F1(t) to exhibit regular continuous periodic or quasi-periodic temporal fluctuations based on the quantitative dynamic change of light intensity, following the dynamic parameters of the magnetic field. Among them, the magnitude of the real-time fluctuation amplitude of F1(t) directly corresponds to the strength of polarization fading. The larger the fluctuation amplitude, the higher the degree of distortion and rotation of the polarization state due to the magnetic field and its associated disturbances, the greater the imbalance in the energy distribution of orthogonally polarized light, and the more significant the polarization fading phenomenon. The smaller the fluctuation amplitude, the better the stability of the polarization state and the weaker the polarization fading interference.

[0051] The above ratio calculation is performed point by point along the time axis, and the ratios of all sampling times are arranged in chronological order to obtain the first polarization fading signal sequence. Each sampling time corresponds to a wavelength scan point, and the sampling interval is determined by the wavelength scan step size of the spectral acquisition and analysis device. Finally, the first polarization fading signal is output, which is a one-dimensional numerical sequence that varies with time (equivalent to the wavelength scan order).

[0052] The second magnetic field response signal is processed synchronously using a process identical to that of the first magnetic field response signal, ultimately yielding a sequence of second horizontal light intensity signals, second vertical light intensity signals, and second polarization fading signals with perfectly matched timing. Specifically, the optical signal of the second magnetic field response signal is input into a Gran-Thompson prism-type polarization beamsplitter of the same specifications as described above. Using a unified and fixed orthogonal reference coordinate system, orthogonal second horizontal polarization components and second vertical polarization components are obtained. The two polarization components are respectively connected to the third and fourth photodetectors with identical parameters, and linear photoelectric conversion is performed based on the standard photoelectric effect, converting the polarized light signal into a distortion-free analog voltage signal that dynamically changes with the sampling time. The system uses a globally unified synchronous sampling clock to ensure that the sampling timing of the two signals is perfectly aligned. Point-to-point normalization processing is performed on the original voltage signal to eliminate system interference caused by light source intensity drift and optical path loss fluctuations, resulting in stable second horizontal and second vertical normalized light intensity values. Changes in the magnetic field, micro-vibrations in the optical path, and fiber stress deformation continuously alter the spatial angle of the polarization principal axis of the transmitted light, disrupting the steady-state energy distribution balance of polarized light. This leads to a continuous and conserved dynamic redistribution of light energy between fixed orthogonal horizontal and vertical polarization directions. The greater the deflection angle of the polarization principal axis, the higher the degree of imbalance in the intensity ratio of the two orthogonal polarized light paths, ultimately causing the intensity distribution ratio to exhibit a regular dynamic change with polarization distortion. Based on the normalized intensity value, the second polarization fading signal value with a value range of [-1, 1] is calculated point by point, characterizing the degree of polarization distortion and energy distribution imbalance in the second sensing path. By arranging the signal values ​​at all sampling times continuously in time sequence, a one-dimensional time sequence with uniform dimension and time sequence aligned with the first polarization fading signal is constructed, which is the second polarization fading signal sequence.

[0053] Polarization fading characteristic parameters are extracted from the first polarization fading signal. The polarization fading characteristic parameters refer to the core parameter set in the polarization fading signal that can quantitatively characterize the polarization state change law. In this embodiment, they include two parameters: fading depth and fading period.

[0054] The fading depth is extracted as follows: An extremum search is performed on the first polarization fading signal sequence to find all local maxima and local minima in the sequence; for each pair of adjacent local maxima and local minima (i.e., a peak-valley pair), the difference between the maxima and the minima is calculated, and this difference is the peak-valley amplitude within that fading period; the average peak-valley amplitude of all peak-valley pairs is taken as the fading depth. The fading depth reflects the maximum range of change in the polarization fading signal amplitude when the polarization state is disturbed; the greater the fading depth, the greater the fluctuation amplitude of the polarization state.

[0055] The fading period is extracted as follows: The number of sampling points between two adjacent local maxima in the first polarization fading signal sequence is recorded. This number of sampling points is multiplied by the sampling time interval to obtain the time span between two adjacent local maxima. The average of the time spans between all adjacent local maxima is taken as the fading period. The fading period reflects the average time required for a complete polarization state fluctuation. The shorter the fading period, the more frequent the polarization state changes.

[0056] The magneto-induced birefringence dispersion coefficient is calculated as follows: The magneto-induced birefringence dispersion coefficient refers to the rate of change of the difference between the ordinary and extraordinary refractive indices of light transmitted in an optical fiber with respect to the wavelength under the action of an external magnetic field, and its unit is nm. －1 This coefficient reflects the difference in birefringence response of light of different wavelengths under the influence of a magnetic field. The specific calculation process is as follows:

[0057] Obtain the first and second polarization fading signal sequences, along with their corresponding wavelength scanning ranges (e.g., 1510nm to 1590nm). To eliminate the boundary effects caused by the Hilbert transform on finite-length signals, discard 2% of the data points at each end of the wavelength scanning range, thus limiting the effective analysis wavelength range to 1516nm to 1584nm. All subsequent calculations involving the wavelength domain (including instantaneous phase extraction, magnetostrictive birefringence dispersion coefficient fitting, polarization compensation, and differential operations) are based on this effective range. Perform the Hilbert transform on both polarization fading signal sequences to filter out low-frequency noise and DC components, extracting the instantaneous phase of the signal corresponding to each wavelength sampling point. Let the wavelength sampling point be λ, and define the original real discrete signals of the first and second polarization fading signals at wavelength λ as F1(λ) and F2(λ), respectively. Perform the Hilbert transform on the original real signals to construct complex analytic signals, eliminating the ambiguity problem of traditional real signal phase calculation. The calculation formulas for the first and second analytic signals are as follows: , ;

[0058] In the formula: , These are the analytical signals corresponding to the two polarization fading signals; Here, is the Hilbert transform operator; j is the imaginary unit. Based on the constructed complex analytic signal, the instantaneous phase is solved point-by-point along the wavelength. The instantaneous phase is the arctangent of the ratio of the imaginary part to the real part of the complex analytic signal, and the solution formula is: , ;

[0059] The instantaneous phase is measured in rad. Here, arctan² is the four-quadrant arctangent function, which can completely reconstruct the true phase value in the interval [-π, π], avoiding the phase jump defect of the ordinary arctangent function. The instantaneous phase is a unique characteristic of the polarization oscillation state at each wavelength point, reflecting the degree of polarization state shift at different wavelengths.

[0060] The instantaneous phase difference between the two signals is calculated point by point along each wavelength. That is, for each wavelength point λ, the instantaneous phase difference of the first signal corresponding to that wavelength is calculated. Subtract the instantaneous phase of the second path The instantaneous phase difference corresponding to wavelength λ is obtained. The unit is rad. By performing phase difference operations on a wavelength-by-wavelength basis, the fixed phase bias introduced by hardware differences between the two sensing channels and fixed optical path deviations can be eliminated, resulting in a continuous, non-jumping, and pure phase difference-wavelength sequence corresponding to the scanning wavelength. This sequence retains only the effective phase fluctuation information caused by the magnetic field and dispersion effect.

[0061] For the phase difference-wavelength sequence within the complete effective measurement wavelength range of 1516nm to 1584nm, a global linear fitting was performed using the least squares method to construct a linear fitting model of phase difference versus wavelength. The fitting formula is as follows: =k×λ+b; where λ is the independent variable of the spectral scanning wavelength, ranging from 1516nm to 1584nm (effective analysis wavelength range), in nm; k is the global fitting slope of the phase difference with wavelength, in rad / nm, which characterizes the overall average rate of change of the polarization phase difference of the two sensing signals with wavelength, directly reflecting the wavelength sensitivity characteristics of magnetostrictive birefringence; b is the fitting constant bias, in rad, used to compensate for the static fixed phase offset of the system.

[0062] Based on the unique slope parameter k obtained from the fitting, the magnetostrictive birefringence dispersion coefficient D is solved, i.e., D = |k| / 2πL; where 2π is the phase period normalization coefficient, used to convert the absolute phase change into a single-period normalized change; L is the fixed axial spacing between the two fiber optic magnetic sensing units. In this embodiment, L = 8mm is fixed to eliminate the influence of sensing distance differences on dispersion calculation and to achieve unit length standardization. The finally solved magnetostrictive birefringence dispersion coefficient is a standardized quantitative parameter, with units of 1000 kJ / m². This parameter characterizes the change in magneto-birefringence per unit length of sensing fiber and per unit wavelength interval. The final output is a scalar form of the magneto-birefringence dispersion coefficient. This parameter is a dedicated, fixed, quantitative parameter characterizing the magneto-optical dispersion characteristics of this measurement system. It can quantify the differences in magneto-polarization response of different wavelengths of light under a broadband light source, compensating for the deficiency that single-wavelength detection cannot characterize dispersion deviation.

[0063] The Bonga sphere is a normalized three-dimensional unit spherical model characterizing the complete evolution of optical polarization states. Any coordinate point on the sphere uniquely corresponds to a complete polarization state, quantifying the dynamic changes of various polarization states such as linear polarization, elliptical polarization, and circular polarization. The Bonga sphere uses three pairs of orthogonal Stokes coordinate axes (S1, S2, and S3) to form a normalized polarization space coordinate system. The S1 axis characterizes the energy difference between horizontal and vertical orthogonal linear polarization components, the S2 axis characterizes the energy difference between ±45° orthogonal linear polarization components, and the S3 axis characterizes the energy difference between right-handed and left-handed circular polarization components. Specifically, the entire equatorial region of the Bonga sphere corresponds to linear polarization states at various angles, the north pole corresponds to a right-handed circular polarization state, the south pole corresponds to a left-handed circular polarization state, and the remaining regions of the sphere correspond to elliptical polarization states with different ellipticities, thus fully covering all polarization evolution forms of optical fiber transmitted light.

[0064] Based on the first and second polarization fading signal sequences that have undergone normalization and have a one-to-one wavelength correspondence as described above, global synchronous timing matching and feature parameter extraction are performed. The system uses a unified global synchronous sampling clock to trigger the parallel acquisition of the two signals throughout the process, eliminating channel timing offset and sampling misalignment issues, ensuring that the timing and wavelength dimensions of the two polarization fading signals are aligned, so that the two sets of signal data corresponding to each matched sampling point correspond to the magnetic field disturbance and optical path polarization distortion state at the same time and wavelength. After timing alignment is completed, the core feature parameters of the two signals at each matched sampling point are extracted point by point, including the polarization signal amplitude ratio and the instantaneous phase difference. The polarization signal amplitude ratio is the ratio of the absolute value of the first polarization fading signal to the absolute value of the second polarization fading signal at the same sampling point, which is used to quantify the difference in the amplitude of the polarization energy distribution imbalance between the two sensing paths. The instantaneous phase difference is the difference between the instantaneous phase of the first polarization fading signal and the instantaneous phase of the second polarization fading signal, which are solved by Hilbert transform at the same sampling point. The unit of measurement is rad, which is used to characterize the difference in polarization phase shift caused by magneto-induced birefringence dispersion, optical path perturbation, and stress deformation in the two sensing paths. The two types of parameters together completely characterize the multidimensional polarization distortion features of a single sampling point.

[0065] For each sampling point with perfect time and wavelength matching, the three-dimensional coordinates of the corresponding polarization state in the Bonga sphere space are solved point by point. The specific transformation steps are as follows:

[0066] For a single matched sampling point, the absolute values ​​of the first and second polarization fading signals are extracted, and the amplitude ratio of the two signals is calculated. The amplitude ratio is then subjected to arctangent calculation to obtain the azimuth parameter of the Bonga sphere equatorial plane, which ranges from 0 to π. This azimuth angle is twice the angle between the major axis of the polarization ellipse and the fixed horizontal reference direction of the system. It characterizes the overall spatial deflection direction of the linear polarization component and uniquely corresponds to the horizontal deflection characteristics of the principal axis of polarization in the sensing optical path after being disturbed by magnetic fields and stress. It is a core parameter for determining the orientation of the polarization state plane.

[0067] Based on the aforementioned azimuth parameters, a normalized conversion is performed to solve for the standardized auxiliary angle, which is equal to half the arctangent of the amplitude ratio. The unit of the standardized auxiliary angle is rad. This standardized auxiliary angle serves as an intermediate transitional quantity in the calculation of polarization ellipsoidity, effectively linking the energy ratio between the linear and circular polarization components in the polarization state. It establishes a quantitative correlation bridge between the plane polarization deflection angle and the three-dimensional spatial polarization distortion. The parameter solution rules are fixed and there is no adaptive adjustment, ensuring the reproducibility of the entire process.

[0068] Retrieve the instantaneous phase difference between two signals obtained by Hilbert transform at the same sampling point. Combining the standardized auxiliary angles obtained from the previous solution, the calculation is completed by substituting them into the fixed elliptic declination angle conversion formula. The calculation formula is as follows: In the formula: This is the elliptic declination parameter, in rad, with a value range limited to -π / 4 to π / 4; This is the standardized auxiliary angle corresponding to the current sampling point. The ellipticity parameter is used to characterize the degree of spatial elevation angle distortion of the polarization state from pure linear polarization. When the parameter is positive, it corresponds to a right-handed elliptic polarization state; when the parameter is negative, it corresponds to a left-handed elliptic polarization state; when the parameter is zero, it corresponds to a standard undistorted linear polarization state. It can completely distinguish the rotation direction and distortion amplitude of various types of elliptic polarization and fully cover the spatial polarization distortion characteristics caused by magnetic field disturbances.

[0069] Based on the solved azimuth and ellipticity parameters, and using the Bonga sphere-normalized three-dimensional spherical coordinate system, the three-dimensional Cartesian coordinates (S1, S2, S3) of the polarization state are solved point-by-point using a fixed transformation formula. This uniquely determines the polarization spatial position of a single sampling point. The complete calculation formula for the three components is as follows: S1 = S2= S3= In the formula: S1, S2, and S3 are the three-dimensional orthogonal Stokes coordinate components of the Bonga sphere, which are dimensionless and satisfy the normalization constraint condition S1. 2 +S2 2 +S3 2 =1; The azimuth parameter is denoted by . The three sets of Stokes components are orthogonal in dimension, which can reproduce the multidimensional polarization state shift information caused by dynamic disturbances in the accelerator magnetic field, optical path perturbations, and stress deformation.

[0070] By traversing all time- and wavelength-matched sampling points, the three-dimensional Stokes polarization coordinates of the Bonga sphere corresponding to each sampling point are obtained. Following a unified sampling time sequence (equivalent to the dynamic change time sequence of the accelerator magnetic field), all discrete polarization coordinates are sequentially connected in a point-by-point continuous manner, generating a continuous spatial curve on the normalized unit sphere of the Bonga sphere, which is the polarization state evolution trajectory. Each point on this trajectory represents the actual polarization state of the optical fiber transmitted light at that sampling time and wavelength. The spatial orientation of the trajectory reflects the continuous deflection path of the polarization state on the Bonga sphere. The magnitude of the local curvature at each point on the trajectory corresponds to the drastic change in the polarization state at that moment (the greater the curvature, the more significant the abrupt change in polarization state caused by magnetic field or environmental disturbance). The amplitude of the trajectory fluctuation corresponds to the intensity range of the polarization fading signal, and the periodicity of the trajectory corresponds to the frequency of the polarization state cyclical change. The above-mentioned curvature magnitude, fluctuation amplitude, and periodicity are the quantization results of mapping the polarization fading characteristic parameters (fading depth and fading period) extracted in step 2 onto the Bonga sphere trajectory.

[0071] A polarization extinction virtual reference node is a specific point selected on the polarization state evolution trajectory where the polarization extinction ratio reaches a local extremum (i.e., a local maximum value). This indicates that the polarization state separation is the highest and the polarization direction is the most definite at this point, making it suitable as a reference for subsequent polarization compensation and magnetic field demodulation.

[0072] Specifically, the fading depth and fading period in the polarization fading characteristic parameters are mapped to local curvature parameters of the polarization state evolution trajectory on the Bonga sphere, quantifying the intensity of polarization distortion at each position of the trajectory. These local curvature parameters measure the degree of local bending at each position of the Bonga sphere's polarization state evolution trajectory; a larger parameter value indicates a more severe spatial distortion of the polarization state caused by magnetic field and optical path disturbances at that position. The mapping rule is as follows: calculate the ratio of fading depth to fading period, and use this ratio as a curvature reference quantity to characterize the average disturbance intensity of polarization state temporal fluctuations.

[0073] For any sampling point to be solved on the polarization state evolution trajectory, symmetrical neighboring sampling points are selected on the trajectory. In this embodiment, five points are selected before and after the current point. The center neighboring point is taken to form a stable ternary calculation point group. The forward neighboring point, the current point to be solved, and the backward neighboring point without collinear interference are determined. The geometric curvature of the current point is solved by a three-point circular arc fitting method. The specific calculation process is as follows: calculate the spatial vector between the current point to be solved and the forward neighboring point, and simultaneously calculate the spatial vector between the backward neighboring point and the current point to be solved; perform a cross product operation on the two sets of spatial vectors. The calculation involves determining the magnitude of the cross product (magnitude of the cross product of spatial vectors); calculating the magnitudes of two sets of spatial vectors to obtain the corresponding side lengths; simultaneously calculating the spatial distances between the forward and backward neighboring points to obtain a third set of side lengths; multiplying these three sets of side lengths to obtain the product; and then ratioing this product to twice the magnitude of the cross product of the spatial vectors to obtain the local radius of curvature corresponding to the current sampling point; finally, taking the reciprocal of the local radius of curvature to obtain the geometric curvature of the current trajectory sampling point. The geometric curvature obtained above is a dimensionless parameter used to visually characterize the degree of local bending in the Bonga sphere polarization state evolution trajectory. All coordinate data involved in the calculation are Bonga sphere normalized Stokes coordinates. Multiplying the aforementioned curvature reference quantity with the geometric curvature of the current sampling point yields the comprehensive local curvature parameter of the current sampling point. This comprehensive local curvature parameter uniquely corresponds to the strength of polarization distortion at each position of the trajectory; a larger parameter value indicates a more significant spatial distortion of the polarization state caused by magnetic field and optical path disturbances at the corresponding position.

[0074] Using local curvature parameters as the core, a search direction vector is constructed along the tangent direction of the polarization state evolution trajectory. This is achieved by performing a difference operation on the three-dimensional Stokes coordinates of adjacent sampling points on the current sampling point on the polarization state evolution trajectory to obtain the local original tangent vector at the current position of the trajectory, representing the local original extension trend of the trajectory. The local original tangent vector is then normalized by calculating its spatial magnitude and then proportionaling each dimensional component of the original tangent vector to this magnitude to obtain the normalized standard trajectory tangent direction, representing the natural extension direction of the trajectory at the current position. The local curvature parameters obtained from the current sampling point are extracted and used as the weight step size of the tangent direction. The local curvature parameters are multiplied by the normalized standard trajectory tangent direction to obtain the search direction vector corresponding to that sampling point. This search direction vector can adaptively point towards regions where the trajectory curvature gradually increases and the polarization distortion characteristics are more significant.

[0075] Along the polarization state evolution trajectory, each trajectory point is examined one by one in the direction indicated by the search direction vector, and the extinction ratio at that point is calculated. The extinction ratio is the logarithm (in decibels) of the ratio of the maximum transmitted light power to the minimum transmitted light power in that polarization state, used to measure the purity of the polarization state and the degree of polarization separation. The extinction ratio is calculated as follows:

[0076] Based on Stokes coordinates of a single sampling point Solving for the normalized amplitude of the linearly polarized component, i.e., calculating... The square value, calculation The square value of the two square values ​​is obtained; the sum of the squares is calculated, and the square root of the sum of squares is obtained to get the normalized amplitude S of the linear polarization component. LPi Extinction ratio When |S 3i |<10 －6 To avoid division by zero errors, EXT is directly retrieved. i =60dB (cutoff upper limit).

[0077] In the formula: EXT i S represents the polarization extinction ratio at the i-th sampling point, in dB. LPi S represents the normalized amplitude of the linearly polarized component. 3i These are the Stokes coordinates of the circular polarization component. The entire domain is traversed through all trajectory sampling points. Sampling points satisfying the maximum condition (i.e., the extinction ratio of the current point is greater than the extinction ratios of its immediate neighbors) are searched by comparing neighboring numerical values. Finally, all local extrema of the extinction ratio are selected.

[0078] To select the optimal and most stable polarization reference, the extreme point with the largest local curvature parameter among all local maxima of extinction ratio is selected and formally calibrated as the virtual reference node for polarization extinction. This selection rule ensures that the final reference point possesses both high polarization purity and significant polarization distortion characterization capabilities, resulting in stronger reference stability and representativeness. The characteristic parameters of this virtual reference node are uniformly recorded, including the three-dimensional Stokes coordinates of the Bonga sphere, the corresponding sampling time and wavelength, and the extreme extinction ratio value.

[0079] This step involves coherently detecting the first and second magnetic field response signals to extract polarization fading characteristic parameters and magnetostrictive birefringence dispersion coefficients. Based on the polarization state evolution trajectory, a virtual reference node for polarization extinction is determined, achieving algorithm-level construction of the polarization reference. Compared to traditional schemes relying on hardware-based polarization-maintaining fibers or polarization controllers, this virtual reference node requires no additional physical reference source. It adaptively extracts the optimal polarization reference point directly from the measurement data, effectively overcoming the impact of hardware consistency differences and environmental drift on the reference stability.

[0080] In a preferred embodiment of the present invention, step 3, which involves using a polarization extinction virtual reference node to perform polarization compensation on the first and second magnetic field response signals to obtain the first and second compensated magnetic field response signals, and calculating the difference in magnetic response between the first and second compensated magnetic field response signals, is performed as follows:

[0081] After determining the polarization extinction virtual reference node, the first and second magnetic field response signals are polarized and compensated using this node to eliminate the interference of polarization state drift on the signal amplitude. The difference in magnetic response between the two compensated signals is calculated to provide a basis for the selection of subsequent measurement modes.

[0082] Extract the polarization state vector of the polarization extinction virtual reference node on the Bonga sphere. The polarization state vector is a three-dimensional column vector originating from the center of the Bonga sphere and ending at the polarization extinction virtual reference node; its three components are the three coordinate values ​​of the node on the Bonga sphere (S...). 1ref ,S 2ref ,S 3ref The polarization state vector is converted into a polarization compensation matrix. The polarization compensation matrix is ​​a 3x3 square matrix used to correct signals affected by polarization fading to a polarization coordinate system based on a polarization extinction virtual reference node.

[0083] The rules for generating the polarization compensation matrix are as follows:

[0084] Obtain the polarization state vector (S) of the polarization extinction virtual reference node. 1ref ,S 2ref ,S 3ref The spatial magnitude of the polarization state vector is calculated. Since the Bonga sphere is a unit normalized sphere, the theoretical value of the vector magnitude is always 1, and in actual engineering calculations it is 1 or infinitely close to 1. The three components of the polarization state vector are divided by their corresponding magnitudes to complete the normalization process, resulting in a standard unit polarization state vector. This unit polarization state vector is used as the target alignment direction for polarization compensation. A 3×3 pure rotation matrix R is constructed. This matrix only realizes the spatial rotation transformation of the polarization state without signal scaling, and can rotate any original polarization state vector to a direction that is completely aligned with the above unit polarization state vector. First, the calculation parameters are defined: the S1 axis of the Bonga sphere (reference direction (1,0,0)) is taken as a fixed reference axis, and the spatial rotation angle between the unit polarization state vector and the S1 reference axis is defined as α, satisfying α=arccos(S 1ref The corresponding rotation angle cosine is cosα and rotation angle sine is sinα. Based on the normalized unit polarization state vector three components (S... 1ref ,S 2ref ,S 3ref The calculation rules for the nine elements of the rotation matrix R, including the sine and cosine values ​​of the rotation angle, are as follows:

[0085] In the first row, the element R in the first column... 11 =S 1ref ²+(1-S 1ref ²)·cosα; element R in the first row and second column 12 =S 1ref S 2ref·(1-cosα)-S 3ref sinα; element R in the first row and third column 13 =S 1ref ·S 3ref ·(1-cosα)+S 2ref ·sinα; In the second row, the element in the first column of the second row is R. 21 =S1ref·S2ref·(1-cosα)+S3ref·sinα; Element R in the second row and second column 22 =S 2ref ²+(1-S 2ref ²)·cosα; element R in the second row and third column 23 =S 2ref ·S 3ref ·(1-cosα)-S 1ref ·sinα; In the third row, the element R in the first column of the third row. 31 =S 1ref ·S 3ref ·(1-cosα)-S 2ref ·sinα; element R in the third row and second column 32 =S 2ref ·S 3ref ·(1-cosα)+S 1ref sinα; element R in the third row and third column 33 =S 3ref ²+(1-S 3ref ²)·cosα. The 3×3 rotation matrix R obtained by solving according to the above fixed operation rules has a determinant that is always 1, and is a distortion-free pure rotation matrix. This matrix can be directly used as the polarization compensation matrix for polarization error correction.

[0086] Based on the previously generated 3×3 polarization compensation pure rotation matrix R, the polarization state distortion of the two original magnetic field response signals is corrected globally to eliminate signal amplitude distortion caused by optical path polarization drift and magnetostrictive birefringence disturbances. For each wavelength sampling point of the spectral scan (corresponding one-to-one with the time-series sampling points), the three-dimensional original Stokes polarization state vectors corresponding to the first and second magnetic field response signals are extracted and denoted as the first original polarization state column vectors, respectively. Second original polarization state column vector The vector components satisfy the Bonga sphere normalization constraint. Polarization compensation is achieved using the standard rotation transformation method of matrix left multiplication, that is, the polarization compensation matrix R is left-multiplied by the two original polarization state vectors respectively, realizing the alignment correction of any original polarization state to the reference polarization direction of the polarization extinction virtual reference node, thus eliminating polarization fading interference. The corresponding polarization compensation calculation formula is: , ;

[0087] The compensated polarization state vector corresponding to the first sensing unit at the same wavelength after polarization matrix correction has been stripped of polarization distortion error, and only the effective magnetic field response polarization information is retained. This is the compensated polarization state vector corresponding to the second sensing unit at the same wavelength after polarization matrix correction. After this pure rotation matrix correction, the two compensated polarization state vectors retain only the effective response information corresponding to the magnetic field action, eliminating distortion errors introduced by polarization drift and optical path disturbances. The vector magnitude still maintains normalized characteristics, and there are no signal scaling or amplitude distortion problems.

[0088] Three-dimensional vector difference operations are performed based on two compensated polarization state vectors aligned point-by-point along the wavelength dimension to quantify the difference in magnetic field response between two fiber optic magnetic sensing units. This effectively avoids the problems of information loss, error omission, and incomplete representation that exist in traditional single Stokes component difference operations. For each wavelength sampling point within the effective wavelength range (1516nm to 1584nm), three sets of orthogonal Stokes components corresponding to the first and second compensated polarization state vectors are extracted simultaneously. Component-by-component difference operations are then performed in three independent dimensions, and the corresponding differences between the compensated polarization components of the first and second sensing units are calculated sequentially in the first, second, and third Stokes dimensions. After obtaining the component differences in three orthogonal dimensions, the three-dimensional deviation is fused and calculated based on the Euclidean distance calculation principle. First, the differences corresponding to the three dimensions are squared respectively. Then, all the squared results are summed. Finally, the square root of the summed value is taken to obtain the three-dimensional comprehensive vector difference value of the two sensing signals at a single wavelength sampling point. This value can comprehensively and without blind spots characterize the overall deviation of the real-time magnetic field response of the two fiber optic magnetic sensing units in the three-dimensional polarization space. All effective wavelength sampling points in the measurement band are traversed sequentially, and the three-dimensional comprehensive vector difference value at each wavelength position is calculated one by one. All single-wavelength comprehensive difference values ​​are arranged and integrated in an orderly manner according to the scanning time sequence and arrangement order of the spectral wavelengths. Finally, a global differential signal sequence with complete data dimensions, perfect wavelength and time sequence matching, and continuous and without missing data is constructed.

[0089] The root mean square value of the differential signal sequence is calculated as the magnetic response difference. The magnetic response difference is a statistical index that measures the overall deviation of the magnetic field response signals between the first and second fiber optic magnetic sensing units; a larger value indicates a greater difference between the two signals.

[0090] The calculated magnetic response difference is compared with a preset threshold. This preset threshold distinguishes between steady-state and dynamic disturbance conditions. Under typical steady-state conditions with a constant excitation current, multiple differential signal sequences are continuously acquired and their root mean square (RMS) values ​​are calculated. The upper statistical limit of these RMS values ​​(e.g., the mean plus three standard deviations) is used as the initial threshold value. Simultaneously, this threshold must meet the accelerator control system's requirements for magnetic field stability (better than 0.1% of the full-scale magnetic field amplitude (i.e., the maximum magnetic field amplitude under the 10A to 200A excitation conditions in this embodiment)). The larger of the two values ​​is used as the final preset threshold. When the magnetic response difference is less than this preset threshold, it indicates good consistency between the two sensor signals, and the system is in a steady-state or slowly changing condition, entering the differential magnetic compensation closed-loop mode in step 4. When the magnetic response difference is greater than or equal to the preset threshold, it indicates a strong dynamic polarization disturbance causing a significant increase in the difference between the two signals, and the system is in a strong dynamic disturbance condition, entering the polarization state multidimensional mapping and joint vector demodulation mode in step 5.

[0091] This step utilizes a polarization extinction virtual reference node to perform polarization compensation on the two magnetic field response signals and calculates the difference in magnetic response, achieving precise correction of polarization distortion and quantitative evaluation of the consistency of the two signals. By unifying the two signals to the same polarization reference, the inherent birefringence of the optical fiber and polarization fading interference introduced by external disturbances are effectively eliminated, ensuring that the compensated signal truly reflects the information of the magnetic field. Simultaneously, the difference in magnetic response serves as the core criterion for subsequent steady-state / dynamic dual-mode switching, enabling adaptive selection of the measurement strategy and ensuring that the system maintains optimal measurement performance under different operating conditions.

[0092] In a preferred embodiment of the present invention, step 4, when the difference in magnetic response is less than a preset threshold, uses a polarization extinction virtual reference node to associate the first and second fiber magnetic sensing units into a differential magnetic compensation closed loop, and uses feedback control to bring the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node to zero, forming a locked measurement baseline. The execution process is as follows:

[0093] When the difference in magnetic response is less than a preset threshold, the system determines that it is currently in a steady state or a slowly changing operating condition and initiates the differential magnetic compensation closed-loop control mode. The goal of this mode is to use closed-loop feedback to bring the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node to zero, thereby establishing a stable locked measurement baseline and continuously eliminating slow polarization disturbances.

[0094] The polarization extinction virtual reference node finally calibrated in step 2 is used as the closed-loop locking reference point. The closed-loop locking reference point refers to the reference point in the closed-loop feedback control system that serves as the control target. The goal of the control system is to maintain a certain controlled physical quantity (equivalent magnetostrictive phase shift in this embodiment) at the reference point at the target value (zero state in this embodiment).

[0095] A differential magnetic compensation closed-loop circuit is constructed based on a closed-loop locked reference point. This differential magnetic compensation closed-loop circuit refers to a closed-loop control circuit that includes a first fiber optic magnetic sensing unit and a second fiber optic magnetic sensing unit, using the difference between the two sensing signals as feedback signals. The specific configuration of the circuit includes: using the first and second fiber optic magnetic sensing units as two sensing nodes in the circuit; coaxially arranging an adjustable compensation magnetic field source (miniature electromagnetic coil) at the magnetic field sensitive center of the second fiber optic magnetic sensing unit, so that the generated compensation magnetic field effectively covers the magnetic sensitive area of ​​the sensing unit (in this embodiment, a miniature electromagnetic coil with a wire diameter of approximately 0.5 mm, approximately 200 turns, and an inner diameter of approximately 5 mm is used, which can generate an adjustable magnetic field of up to approximately 50 mT); inputting the real-time current value of the differential signal sequence as an error signal into a proportional-integral-derivative (PID) controller; and connecting the output of the PID controller to the drive circuit of the adjustable compensation magnetic field source, thus forming a complete negative feedback closed-loop circuit.

[0096] The equivalent magnetostrictive phase shift refers to the equivalent phase shift of the polarization state of transmitted light in the fiber optic magnetic sensing unit caused by an applied magnetic field under the combined effects of magnetostriction and magneto-optical Faraday effects. The global differential signal sequence obtained in step 3 is used as the closed-loop real-time error signal and input into the differential magnetic compensation closed-loop circuit to complete quantization adjustment. The processing rules for the error signal are as follows:

[0097] Error quantization and adjustment are achieved using a PID control algorithm based on a fixed-period iteration. The fixed control period of this system is 1ms, and the formula for calculating the total PID control quantity within a single period is as follows: ;

[0098] In the formula: u(t) is the compensation magnetic field control output of the current control cycle; e(t) is the real-time magnetic response differential error value of the current cycle; e(t-1) is the differential error value of the previous control cycle; K p K i K d These are preset fixed proportional, integral, and derivative gain coefficients, assuming a proportional gain coefficient of 2.5, an integral gain coefficient of 0.08, and a derivative gain coefficient of 0.15; T is a fixed control period, taken as 1 ms; the above PID gain coefficients are obtained by tuning using the Ziegler-Nichols critical proportional gain method, the specific steps of which are: first set K... i =K d =0, gradually increase K p Simultaneously, the output response of the closed-loop system is monitored. When the peak-to-peak fluctuation of the system output within 10 consecutive oscillation cycles does not exceed ±5% relative to the average peak-to-peak value, it is determined that a stable constant-amplitude oscillation has occurred, and the critical gain K is recorded at this time. c and critical period T c(Unit: ms). Then tune K according to the ZN tuning formula. p =0.6K c K i =2K p / T c K d =K p ·T c The final PID gain coefficient is calculated by / 8. The proportional term follows the current differential error in real time to complete the immediate correction, the integral term accumulates to eliminate steady-state residual deviation, and the derivative term suppresses small fluctuations. The three terms are superimposed to output a unique corresponding compensation magnetic field control quantity.

[0099] Within a fixed control cycle of 1 ms, the system updates the control output according to the aforementioned PID formula, driving the miniature electromagnetic compensation coil to dynamically adjust the strength and direction of the output compensation magnetic field. The direction of the compensation magnetic field is opposite to the direction of the main magnetic field being measured by the accelerator, thus canceling the equivalent magnetostrictive phase shift of the second fiber optic magnetic sensing unit, continuously reducing the polarization response difference between the two sensing units, and pushing the system error towards zero. The final output is the compensation magnetic field strength value corresponding to the PID control quantity, which serves as the closed-loop execution result and is continuously fed back to the compensation coil drive circuit to achieve closed-loop steady-state regulation.

[0100] The three-dimensional vector dot product algorithm is used to monitor the zero-state of the equivalent magnetostrictive phase shift in real time. The formula for real-time calculation of the equivalent magnetostrictive phase shift is as follows: ;

[0101] In the formula: Real-time equivalent magnetostrictive phase shift, unit: rad; S ref The initial three-dimensional Stokes polarization state vector of the polarization extinction virtual reference node recorded at the moment of closed-loop startup; S now The real-time polarization state vector of the virtual reference node acquired during the current control cycle; It is the dot product of two sets of vectors; These are the spatial magnitudes of the two sets of vectors, respectively. Due to the normalization property of the Bonga sphere, the magnitude is always equal to 1. This formula can quantify the polarization state shift angle, enabling unambiguous real-time monitoring of the equivalent magnetostrictive phase shift.

[0102] When the equivalent magnetostrictive phase shift value is monitored in real time and remains close to zero for several consecutive control cycles (e.g., 100 cycles) (e.g., absolute value less than 0.01 radians), it is determined that the equivalent magnetostrictive phase shift has returned to zero. At this point, the PID controller output tends to stabilize, the compensation amount of the adjustable compensation magnetic field source no longer changes significantly, and the differential signal sequence also tends to a steady state. The current operating state of the differential magnetic compensation closed-loop circuit is locked, that is, the current compensation magnetic field output is maintained.

[0103] Record the locking measurement baseline. The locking measurement baseline refers to a complete set of system state parameters recorded in the stable state after the equivalent magnetostrictive phase shift returns to zero. It includes the following three data items: First, the polarization state information of the closed-loop locking reference point (i.e., the polarization extinction virtual reference node) at the locking moment: the three-dimensional coordinates (S1lock, S2lock, S3lock) on the Bonga sphere, the extinction ratio, and the corresponding wavelength value; Second, the compensation amount of the adjustable compensation magnetic field source at the locking moment, including the direction parameter and real-time intensity value of the compensation magnetic field. The unit of measurement is uniformly Tesla. Among them, the direction of the compensation magnetic field is obtained by reverse matching with the real-time magnetic field direction of the main magnetic field of the accelerator under test, and is always opposite to the direction of the magnetopolarization disturbance of the main magnetic field acting on the sensing unit, in order to cancel the polarization phase shift; the strength value of the compensation magnetic field is the constant output control quantity of the PID controller at the steady-state lock time, which is the steady-state constant compensation value after the closed loop converges to zero. This compensation quantity can quantify the magnitude of the polarization interference amplitude caused by the static birefringence of the optical fiber and the slow environmental disturbance that needs to be canceled in the steady state of the system; thirdly, the zero steady-state value of the differential signal sequence at the lock time is the average value of the comprehensive differential values ​​before and after the lock time (e.g., 50 each), which should be close to zero.

[0104] The above three data points together constitute the locked measurement baseline. Once the locked measurement baseline is established, continuous and stable magnetic field measurements can be performed at each measuring point within the magnet aperture under this baseline state. The measurement results have high repeatability and consistency and are not affected by slow polarization drift. If the equivalent magnetostrictive phase shift is detected to exceed the zero range during subsequent measurements (i.e., the system deviates from the locked state), the closed-loop adjustment can be restarted based on the existing locked measurement baseline to restore the system to locked state.

[0105] In this step, when the difference in magnetic response is less than a preset threshold, a differential magnetic compensation closed loop is constructed using a polarization extinction virtual reference node. Feedback control is then used to bring the equivalent magnetostrictive phase shift to zero and establish a locked measurement baseline, achieving continuous suppression of slow polarization disturbances under steady-state conditions. The closed-loop feedback mechanism can track and offset polarization shifts introduced by slow environmental changes such as temperature drift and mechanical creep in real time, keeping the system in an optimal locked state for a long period. The establishment of the locked measurement baseline provides a unified reference standard for subsequent continuous measurements, improving the repeatability and long-term stability of magnetic field measurements.

[0106] In a preferred embodiment of the present invention, step 5, when the difference in magnetic response is greater than or equal to a preset threshold, uses the polarization extinction virtual reference node as a reference and constructs a multidimensional mapping relationship of polarization state based on the magnetostrictive birefringence dispersion coefficient; the first and second compensated magnetic field response signals are jointly vector demodulated under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution within the aperture of the accelerator magnet. The execution process is as follows:

[0107] When the difference in magnetic response obtained in step 3 is greater than or equal to a preset threshold, the system determines that it is currently under a strong dynamic disturbance condition (such as a rapid step in excitation current, abnormal vibration of the cooling system, etc., causing a rapid and significant change in polarization state). The slow adjustment of the differential magnetic compensation closed loop cannot keep up with such a rapid change in polarization state. At this time, the system switches to the polarization state multidimensional mapping and joint vector demodulation mode. This mode does not rely on closed-loop feedback adjustment, but instead constructs a multidimensional mapping relationship of polarization state based on the magnetostrictive birefringence dispersion coefficient. The first and second compensation magnetic field response signals are jointly vector demodulated in a unified multidimensional polarization space to directly reconstruct the magnetic field distribution.

[0108] During the operation of step 5, the system continuously monitors the difference in magnetic response. If the difference in magnetic response is less than the preset threshold for 100 consecutive sampling cycles, it automatically switches back to step 4 to re-establish the differential magnetic compensation closed-loop lock, in order to adapt to the situation where the magnetic field condition recovers from dynamic disturbance to steady state.

[0109] In this step, all sampling points in the wavelength domain correspond one-to-one with sampling points in the time domain. The spectrometer used in this system has a scanning speed of 100Hz (a single full-spectrum scan takes 10ms), while the frequency of accelerator excitation current changes is usually below 10Hz. Therefore, the magnetic field state can be considered approximately constant within one scan cycle, and the wavelength domain and time domain information can be regarded as synchronous. For faster dynamic disturbance scenarios, it can be expanded to a parallel multi-wavelength detection method to achieve real-time analysis.

[0110] The coordinates of the polarization extinction virtual reference node in the multidimensional polarization space are extracted and set as the mapping origin. The multidimensional polarization space refers to the four-dimensional space with coordinates (S1, S2, S3) as coordinates (S1, S2, S3, λ), consisting of the three coordinate axes of the Bonga sphere (S1, S2, S3) and an extended polarization space with wavelength as an additional dimension (λ being the light wavelength). 1ref ,S 2ref ,S 3ref ,λ ref ), where the first three terms are the coordinates of the node on the Banga sphere, λ ref The center wavelength value corresponding to this node represents the effective wavelength range of 1516nm to 1584nm. This four-dimensional coordinate system is set as the mapping origin U. Q .

[0111] The magnetostrictive birefringence dispersion coefficient is decomposed by projection along the orthogonal basis vectors of the multidimensional polarization space to obtain the dispersive projection components corresponding to each orthogonal basis vector direction. To avoid dimensional conflicts, the magnetostrictive birefringence dispersion coefficient (unit: ...) is first decomposed by projection along the orthogonal basis vectors of the multidimensional polarization space to obtain the dispersive projection components corresponding to each orthogonal basis vector direction. ) Perform dimensionless normalization: Define reference wavelength λ0 = 1550 nm, reference length L0 = 1 mm, and construct dimensionless parameters. =D×λ0×L0, this parameter is dimensionless and characterizes the normalized dispersion intensity per unit reference wavelength and reference length.

[0112] Calculate the unit vector of the projection direction of the polarization state vector of the polarization extinction virtual reference node onto the S1-S2 plane, i.e., calculate S 1ref The square value and S 2ref The square of the two squares is added together, and the square root of the sum is taken to obtain the projection modulus S of the polarization state vector on the S1-S2 plane. 12 ; Calculate S 1ref With S 12 The quotient is used to obtain the unit projection coefficient e in the S1 direction. S1 ; Calculate S 2ref With S 12 The quotient is used to obtain the unit projection coefficient e in the S2 direction. S2 (If S) 12 <10 －6 Then take e S1 =1,e S2 =0). The dimensionless dispersive projection components of each orthogonal basis vector direction are calculated as follows:

[0113] Normalized magnetobirefringence dispersion coefficient Multiply by e S1 The dispersive projection component d in the S1 direction is obtained. S1 ; normalized magneto-birefringence dispersion coefficient Multiply by e S2 The dispersive projection component d in the S2 direction is obtained. S2 ; normalized magneto-birefringence dispersion coefficient Multiplied by the S3 coordinate component S of the polarization extinction virtual reference node 3ref (Because the coupling between the circular polarization component and dispersion is directly proportional to S) 3ref ), to obtain the dispersive projection component d in the S3 direction. S3 Take the normalized magneto-birefringence dispersion coefficient It itself yields the dispersive projection component d in the wavelength direction. λ (Since the dispersion coefficient itself is the rate of change with respect to wavelength, it becomes itself after normalization). Finally, the dispersive projection components corresponding to each orthogonal basis vector direction are obtained, i.e., four numerical values ​​(d...). S1 ,d S2 ,d S3 ,d λ ), respectively corresponding to e S1 e S2 e S3 and e λ The coordinate increment of the direction.

[0114] Mapping origin U QStarting from the origin, and using the normalized dispersion projection components corresponding to the directions of each orthogonal basis vector as coordinate increments, a mapping tensor is constructed pointing from the origin to the signal domains of the first and second compensated magnetic field response signals. To maintain dimensional consistency and avoid ambiguity in the physical meaning of off-diagonal intersection terms, this embodiment uses a diagonally corrected form of the mapping tensor M (4×4 matrix), applying independent first-order dispersion corrections only to each dimension and ignoring higher-order coupling terms (coupling effects can be ignored when dispersion is small; if higher-order terms need to be considered, refer to the extended form of the Jacobian matrix): ;

[0115] All elements of this matrix are dimensionless. The elements on the main diagonal are the normalized dispersion corrections corresponding to the superposition of unity gains in each dimension, enabling mapping and conversion from polarization space to the magnetic field signal domain. The mapping origin is U. Q The position remains unchanged after the action of M (because the coordinates of the origin are (0,0,0,λ)). ref When ), the fourth dimension is mapped to λ. ref ×(1+d λ To achieve this, the fourth dimension of the origin needs to be set to 0, i.e., λ = λ. ref The relative offset is zero at this point; in actual mapping, the relative wavelength Δλ = λ - λ is used. ref As the fourth dimension of the coordinate system, the origin is (0,0,0,0) in the relative coordinate system, and remains zero after mapping, thus ensuring the stability of the reference.

[0116] Based on the polarization state multidimensional mapping relationship and the global constant mapping tensor M obtained from the aforementioned solution, the first and second compensated magnetic field response signals completed in step 3 are projected sequentially and wavelength-wise onto a unified four-dimensional multidimensional vector space to complete the accurate mapping conversion from polarization signal to magnetic field vector. The specific projection operation rule is as follows: For each sampling point with matching time and wavelength, the four-dimensional polarization space coordinate vector V1(Δλ) corresponding to the first compensated magnetic field response signal is extracted. The four components of this vector are, in order, the first compensated S1 coordinate component, the first compensated S2 coordinate component, the first compensated S3 coordinate component, and the wavelength Δλ relative to the current sampling point, used to characterize the wavelength shift for dispersion correction. Multidimensional mapping demodulation is completed using the standard matrix operation method of left-multiplying the mapping tensor by the vector. The calculation formula is: In the formula: The first sensing unit maps out a four-dimensional vector, with the first three dimensions representing the magnetic field spatial vector components and the fourth dimension representing the wavelength dispersion correction. The effective components of the first three dimensions of this output vector are extracted to construct the first multi-dimensional magnetic field vector component, as shown in the formula: This uniquely corresponds to the three-dimensional magnetic field vector information of the spatial location of the first fiber optic magnetic sensing unit. Similarly, the four-dimensional polarization space coordinate vector V2(Δλ) corresponding to the second compensated magnetic field response signal is extracted. Its four components are, in order, the second compensated S1 coordinate component, the second compensated S2 coordinate component, the second compensated S3 coordinate component, and the current wavelength value. (Same as the first path, since the timing of the two paths is synchronized), perform the same-source mapping demodulation operation, and the calculation formula is: .

[0117] Extract the first three effective components of the output vector to construct the second multidimensional magnetic field vector component, as shown in the formula. The three-dimensional magnetic field vector information characterizing the spatial location of the second fiber optic magnetic sensing unit; For any relative wavelength The second multidimensional magnetic field vector component, obtained by demodulation through multidimensional tensor mapping, is a three-dimensional spatial vector characterizing the true magnetic field at the measurement point of the second fiber optic magnetic sensing unit. The two vector mapping operations share a unified benchmark and consistent algorithm, completely eliminating dynamic polarization distortion and dispersion coupling interference.

[0118] Based on two multidimensional magnetic field vector components at the same sampling time and wavelength dimension, standard three-dimensional vector synthesis is performed to fuse the magnetic field spatial information of the dual-channel sensor signals, thereby improving the signal-to-noise ratio and spatial resolution of magnetic field measurements under dynamic conditions. The vector synthesis follows the three-dimensional Euclidean space vector addition criterion, and the specific component synthesis calculation is as follows:

[0119] The X-axis magnetic field component of the first sensing unit X-axis magnetic field component of the second sensing unit Adding them together, we obtain the X-axis composite magnetic field component B. x ( The Y-axis magnetic field component of the first sensing unit Y-axis magnetic field component of the second sensing unit Adding them together, we obtain the Y-direction composite magnetic field component B. y ( The Z-axis magnetic field component of the first sensing unit Z-axis magnetic field component of the second sensing unit Adding them together, we obtain the Z-direction composite magnetic field component B. z ( ); Integration of B x ( B y ( B z ( ) is a three-dimensional column vector, from which the high-precision synthesized magnetic field vector B( is obtained at a single sampling point and a single wavelength) is obtained. ).

[0120] Traversing the effective wavelength range (1516nm to 1584nm, corresponding to the Δλ range of -34nm to 34nm, because λ ref The above mapping and vector synthesis operations are performed point by point on all sampling points within 1550nm and all time-series sampling points to generate a global synthetic magnetic field vector sequence, which serves as the original magnetic field vector dataset to be interpolated and reconstructed.

[0121] Spatial interpolation and three-dimensional magnetic field reconstruction were performed on the global synthetic magnetic field vector sequence to obtain a continuous and complete spatial distribution of the magnetic field within the accelerator magnet aperture. A three-dimensional linear spatial interpolation algorithm was used to complete the magnetic field reconstruction. This algorithm relies on the known spatial measurement points of the two fiber optic magnetic sensing units and the high-precision synthetic magnetic field vector. Based on the continuous and gradual change characteristics of the magnetic field in space, it calculates the magnetic field vector of all unknown spatial grid points within the aperture through a linear weighted fitting method. The specific interpolation and reconstruction process is as follows:

[0122] Input fixed known parameters, including the fixed spatial coordinates of the first and second fiber optic magnetic sensing units, P1(x1,y1,z1) and P2(x2,y2,z2), respectively; and simultaneously input the composite magnetic field vector data of the two measurement points corresponding to the full time sequence and the full wavelength. , The dataset contains three components of the magnetic field at each measuring point, serving as the interpolation reference sample dataset. The spatial distance between the two measuring points is fixed at 8 mm, and the magnetic field parameters are synchronized in time and aligned in wavelength dimension. The three-dimensional space of the magnet aperture (40 mm in diameter and 100 mm in axial length) is discretized into a regular spatial grid of 40×40×100 with a uniform step size of 1 mm. All grid nodes represent unknown spatial points to be interpolated, uniformly covering the entire measured area of ​​the magnet aperture to ensure the consistency of the spatial resolution of the reconstructed magnetic field. Based on the physical characteristic of continuous and gradual spatial variation of the magnetic field, the magnetic field vector of any unknown node within the aperture changes linearly and smoothly with its spatial position, without abrupt changes, satisfying the application prerequisite of linear interpolation.

[0123] For any unknown node P(x,y,z) in the mesh to be interpolated within the aperture, calculate the Euclidean spatial distances h1 and h2 (in mm) from this unknown node to two known sensing points. Based on the principle of inverse weighting of spatial distance, construct linear interpolation weight coefficients. The closer the distance to the measuring point, the higher the contribution weight of the magnetic field data of the corresponding measuring point. The weights are calculated as follows: calculate the sum of h1 and h2 to obtain the total distance; calculate the quotient of h2 divided by the total distance to obtain the interpolation weight w1 of the first sensing point; calculate the quotient of h1 divided by the total distance to obtain the interpolation weight w2 of the second sensing point.

[0124] The weighting coefficients satisfy the normalization constraint w1+w2=1, with no weight bias and no data distortion. Based on the normalized weighting coefficients, linear weighted interpolation is performed on the three components of the magnetic field, and the magnetic field vector components of the unknown grid nodes are solved dimension by dimension. The complete interpolation calculation process is as follows: The following interpolation calculation is performed on the synthetic magnetic field vector under a certain fixed relative wavelength Δλ, and the resulting spatial magnetic field distribution is the instantaneous magnetic field state corresponding to that wavelength.

[0125] Specifically, the X-axis magnetic field component of the first sensing point Multiplying by weight w1 yields the first X-axis weighted value; the X-axis magnetic field component of the second sensing point is then... Multiplying by weight w2 yields the second X-axis weighted value; adding the first and second X-axis weighted values ​​yields the X-axis magnetic field component B of the unknown node. x ( The Y-axis magnetic field component of the first sensing point Multiplying by weight w1 yields the first Y-axis weighted value; the Y-axis magnetic field component of the second sensing point is then... Multiplying by weight w2 yields the second Y-axis weighting value; adding the first and second Y-axis weighting values ​​yields the Y-axis magnetic field component B of the unknown node. y ( The Z-axis magnetic field component of the first sensing point Multiplying by weight w1 yields the first Z-axis weighted value; the Z-axis magnetic field component of the second sensing point is then... Multiplying by weight w2 yields the second Z-axis weighting value; adding the first and second Z-axis weighting values ​​gives the Z-axis magnetic field component B of the unknown node. z ( ); Integration of B x ( B y ( B z ( ) is a three-dimensional column vector, which yields the complete three-dimensional composite magnetic field vector B(x,y,z) at any unknown grid node P(x,y,z).

[0126] By traversing all grid nodes within the aperture and performing linear interpolation calculations point-by-point, magnetic field vector parameters for all spatial locations are generated globally. After completing the linear interpolation calculations for all grid nodes, the magnetic field modulus of each grid node is solved, i.e., the X-direction magnetic field component B of that node is obtained. x The square value of the value, and the Y-axis magnetic field component B of the node. y The square value of the value, and the Z-direction magnetic field component B of the node. zThe square value is calculated, the three sets of square values ​​are added together to obtain the sum of squares, and the square root of the sum of squares is calculated to obtain the magnetic field modulus |B| of that node. Finally, the complete magnetic field parameters of the entire three-dimensional mesh of the accelerator magnet aperture are output, including the three-dimensional magnetic field components B of each node. x B y B z By using the magnetic field modulus |B|, the complete three-dimensional magnetic field distribution within the accelerator magnet aperture under dynamic disturbance conditions is obtained, fully reproducing the spatial strength distribution and vector direction distribution characteristics of the magnetic field under dynamic disturbance.

[0127] This step, when the difference in magnetic response is greater than or equal to a preset threshold, uses a polarization extinction virtual reference node as a benchmark to construct a multidimensional mapping relationship of polarization states. It then reconstructs the magnetic field distribution through joint vector demodulation, achieving rapid and high-fidelity extraction of magnetic field information under strong dynamic disturbance conditions. Compared to closed-loop feedback, multidimensional mapping demodulation does not rely on a feedback convergence process and can directly map polarization state changes into magnetic field vectors, effectively overcoming the problem of closed-loop response delay under transient disturbances such as rapid excitation steps and vibration shocks. Simultaneously, joint vector demodulation fully utilizes the full polarization information of the dual-channel sensor signals, breaking through the information limitations of traditional scalar demodulation. Combined with spatial interpolation three-dimensional reconstruction, it achieves high-resolution and refined restoration of the magnetic field vector distribution within the accelerator magnet aperture.

[0128] like Figure 3 As shown, embodiments of the present invention also provide an accelerator magnetic field mapping measurement system based on fiber optic sensing, comprising:

[0129] The acquisition module is used to deploy an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and to acquire the first and second magnetic field response signals containing wavelength domain information.

[0130] The evolution module is used to perform coherent detection on the first and second magnetic field response signals, extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient; and determine a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters.

[0131] The calculation module is used to perform polarization compensation on the first and second magnetic field response signals using a polarization extinction virtual reference node, to obtain the first and second compensated magnetic field response signals, and to calculate the magnetic response difference between the first and second compensated magnetic field response signals.

[0132] The compensation module is used to associate the first and second fiber magnetic sensing units into a differential magnetic compensation closed loop by using a polarization extinction virtual reference node when the magnetic response difference is less than a preset threshold, and to make the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node zero through feedback control, thereby locking the measurement baseline.

[0133] The reconstruction module is used to construct a multidimensional mapping relationship of polarization state based on the magnetostrictive birefringence dispersion coefficient, using the polarization extinction virtual reference node as a reference when the magnetic response difference is greater than or equal to a preset threshold; and to jointly demodulate the first and second compensated magnetic field response signals under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution in the aperture of the accelerator magnet.

[0134] It should be noted that this system is a system corresponding to the above method. All implementation methods in the above method embodiments are applicable to this embodiment and can achieve the same technical effect.

[0135] Embodiments of the present invention also provide a computer-readable storage medium storing instructions that, when executed on a computer, cause the computer to perform the method described above. All implementations in the above method embodiments are applicable to this embodiment and can achieve the same technical effects.

[0136] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for measuring accelerator magnetic field mapping based on fiber optic sensing, characterized in that, The method includes: Step 1: Layout an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and collect the first and second magnetic field response signals containing wavelength domain information. Step 2: Perform coherent detection on the first and second magnetic field response signals, extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient; determine a polarization extinction virtual reference node in the polarization state evolution trajectory based on the polarization fading characteristic parameters; Step 3: Use the polarization extinction virtual reference node to perform polarization compensation on the first and second magnetic field response signals to obtain the first and second compensated magnetic field response signals, and calculate the magnetic response difference between the first and second compensated magnetic field response signals. Step 4: When the difference in magnetic response is less than a preset threshold, the first and second fiber magnetic sensing units are associated as a differential magnetic compensation closed loop using a polarization extinction virtual reference node, and the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node is brought to zero through feedback control to form a locked measurement baseline. Step 5: When the difference in magnetic response is greater than or equal to a preset threshold, a multidimensional mapping relationship of polarization state is constructed based on the polarization extinction virtual reference node and the magnetostrictive birefringence dispersion coefficient. The first and second compensated magnetic field response signals are jointly vector demodulated under the multidimensional mapping relationship of polarization state to reconstruct the magnetic field distribution in the aperture of the accelerator magnet.

2. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 1, characterized in that, Coherent detection is performed on the first and second magnetic field response signals to extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient, including: The first magnetic field response signal is polarized and split to obtain a first horizontal polarization component and a first vertical polarization component. After photoelectric conversion, a first horizontal light intensity signal and a first vertical light intensity signal are obtained. Based on the first horizontal light intensity signal and the first vertical light intensity signal, a first polarization fading signal is calculated. The first polarization fading signal is used to characterize the polarization state change of the first magnetic field response signal. The second magnetic field response signal is polarized and beam split to obtain the second horizontal polarization component and the second vertical polarization component. After photoelectric conversion, the second horizontal light intensity signal and the second vertical light intensity signal are obtained. Based on the second horizontal light intensity signal and the second vertical light intensity signal, the second polarization fading signal is calculated. The fading depth and fading period of the first polarization fading signal are extracted as polarization fading characteristic parameters, and the slope of the phase difference between the first polarization fading signal and the second polarization fading signal as a function of wavelength is calculated. The slope is then converted into the magneto-birefringence dispersion coefficient.

3. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 2, characterized in that, A virtual reference node for polarization extinction is determined in the polarization state evolution trajectory based on polarization fading characteristic parameters, including: The fading depth and fading period in the polarization fading characteristic parameters are mapped to the local curvature parameters of the polarization state evolution trajectory on the Bonga sphere; Based on the amplitude ratio and phase difference of the first polarization fading signal and the second polarization fading signal at each sampling point, calculate the corresponding polarization state coordinates and draw the polarization state evolution trajectory on the Bonga sphere. A search direction vector is constructed using local curvature parameters. Local extreme points of extinction ratio are searched along the polarization state evolution trajectory. These local extreme points of extinction ratio are then determined as virtual reference nodes for polarization extinction.

4. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 3, characterized in that, Based on the amplitude ratio and phase difference of the first and second polarization fading signals at each sampling point, the corresponding polarization state coordinates are calculated, and the polarization state evolution trajectory is plotted on the Bonga sphere, including: The first polarization fading signal and the second polarization fading signal are sampled synchronously, and the amplitude ratio and phase difference of the first polarization fading signal and the second polarization fading signal at each sampling point are extracted. For each sampling point, the amplitude ratio is converted into an azimuth parameter on the Bonga sphere, and an auxiliary angle is determined using the amplitude ratio. The phase difference and the auxiliary angle are substituted into the elliptic deflection conversion function to obtain the elliptic deflection parameter on the Bonga sphere. The polarization state coordinates of the corresponding sampling point on the Bonga sphere are determined by the azimuth parameter and the elliptic deflection parameter. The polarization state coordinates of each sampling point are connected sequentially according to the sampling time to form a continuous polarization state evolution trajectory.

5. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 4, characterized in that, Step 3 includes: Extract the polarization state vector of the polarization extinction virtual reference node on the Bonga sphere, and generate a polarization compensation matrix from the polarization state vector; The first magnetic field response signal is multiplied by the polarization compensation matrix to obtain the first compensated magnetic field response signal; the second magnetic field response signal is multiplied by the polarization compensation matrix to obtain the second compensated magnetic field response signal. The first and second compensated magnetic field response signals are differentially processed to obtain a differential signal sequence. The root mean square value of the differential signal sequence is calculated as the magnetic response difference.

6. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 5, characterized in that, Step 4 includes: The polarization extinction virtual reference node is used as the closed-loop locking reference point, and a differential magnetic compensation closed-loop circuit containing the first fiber magnetic sensing unit and the second fiber magnetic sensing unit is constructed based on the closed-loop locking reference point. The differential signal sequence is used as an error signal input to the differential magnetic compensation closed loop, driving the adjustable compensation magnetic field source to apply a reverse compensation magnetic field to the first fiber magnetic sensing unit or the second fiber magnetic sensing unit, so as to adjust the equivalent magnetostrictive phase shift at the closed-loop locked reference point. The equivalent magnetostrictive phase shift is monitored in real time to reach zero. When the equivalent magnetostrictive phase shift reaches zero, the current working state of the differential magnetic compensation closed loop is locked. The polarization state information of the closed loop locking reference point at the locking moment, the compensation amount of the adjustable compensation magnetic field source, and the zero steady-state value of the differential signal sequence are recorded together as the locking measurement baseline.

7. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 6, characterized in that, When the difference in magnetic response is greater than or equal to a preset threshold, a multidimensional mapping relationship of polarization states is constructed based on the magneto-induced birefringence dispersion coefficient, using the polarization extinction virtual reference node as a benchmark, including: When the difference in magnetic response is greater than or equal to a preset threshold, the coordinates of the polarization extinction virtual reference node in the multidimensional polarization space are extracted, and the coordinates are set as the mapping origin. The magnetostrictive birefringence dispersion coefficient is decomposed by projection along each orthogonal basis vector direction in the multidimensional polarization space to obtain the dispersion projection component corresponding to each orthogonal basis vector direction. Starting from the origin of the mapping, and using the dispersive projection components corresponding to each orthogonal basis vector direction as coordinate increments, a mapping tensor is constructed pointing from the origin of the mapping to the signal domain where the first and second compensated magnetic field response signals are located. The mapping tensor is used as a multidimensional mapping relationship of polarization state.

8. The accelerator magnetic field mapping measurement method based on fiber optic sensing according to claim 7, characterized in that, The first and second compensated magnetic field response signals are jointly vector demodulated under the polarization state multidimensional mapping relationship to reconstruct the magnetic field distribution within the accelerator magnet aperture, including: Based on the polarization state multidimensional mapping relationship, the first compensation magnetic field response signal and the second compensation magnetic field response signal are projected onto the same multidimensional vector space to obtain the first multidimensional magnetic field vector component and the second multidimensional magnetic field vector component. The first multidimensional magnetic field vector component and the second multidimensional magnetic field vector component are vector synthesized to obtain the composite magnetic field vector field. Spatial interpolation and three-dimensional reconstruction of the synthesized magnetic field vector field are performed to obtain the magnetic field distribution within the aperture of the accelerator magnet.

9. An accelerator magnetic field mapping measurement system based on fiber optic sensing, the system implementing the method as described in any one of claims 1 to 8, characterized in that, include: The acquisition module is used to deploy an optical fiber sensing link containing at least the first and second optical fiber magnetic sensing units within the aperture of the magnet of the accelerator under test, and to acquire the first and second magnetic field response signals containing wavelength domain information. The evolution module is used to perform coherent detection on the first and second magnetic field response signals and extract the polarization fading characteristic parameters induced by the magnetic field and the magneto-birefringence dispersion coefficient. A virtual reference node for polarization extinction is determined in the polarization state evolution trajectory based on polarization fading characteristic parameters; The calculation module is used to perform polarization compensation on the first and second magnetic field response signals using a polarization extinction virtual reference node, to obtain the first and second compensated magnetic field response signals, and to calculate the magnetic response difference between the first and second compensated magnetic field response signals. The compensation module is used to associate the first and second fiber magnetic sensing units into a differential magnetic compensation closed loop by using a polarization extinction virtual reference node when the magnetic response difference is less than a preset threshold, and to make the equivalent magnetostrictive phase shift at the polarization extinction virtual reference node zero through feedback control, thereby locking the measurement baseline. The reconstruction module is used to construct a multidimensional mapping relationship of polarization state based on the magneto-birefringence dispersion coefficient, using the polarization extinction virtual reference node as a reference, when the magnetic response difference is greater than or equal to a preset threshold. The first and second compensated magnetic field response signals are jointly vector demodulated under the polarization state multidimensional mapping relationship to reconstruct the magnetic field distribution within the accelerator magnet aperture.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a program that, when executed by a processor, implements the method as described in any one of claims 1 to 8.

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