High-sensitivity optical fiber magnetic field sensor based on extrinsic F-P interference

Abstract

The invention discloses a high-sensitivity optical fiber magnetic field sensor based on extrinsic F-P interference, and the sensor employs Terfenol-D as a sensitive element, employs a permanent magnet to provide a bias magnetic field, and improves the conversion efficiency of a magnetostrictive rod. A high-precision F-P cavity is formed by accurately aligning the end face of the single-mode optical fiber with the surface of the permanent magnet. Experimental results show that the highest sensitivity of the sensor within the range of 0-150 [mu] T reaches 676 nm / mT, and the corresponding magnetic field resolution is 147 nT. The integrated reference FBG is used for monitoring temperature changes, and the problem of temperature cross sensitivity is effectively solved. The design based on a first-generation EFPI sensor is further optimized, an improved sensor with adjustable magnetic bias is provided, and the linearity is improved to 99.2%. The sensitivity of the improved sensor in the same magnetic field range reaches 826 nm / mT, the magnetic field resolution is 121 nT, and the improved sensor shows good linear characteristics and temperature stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensors, and particularly relates to a high-sensitivity optical fiber magnetic field sensor based on extrinsic F-P interference. BACKGROUND

[0002] Magnetic field measurement plays an important role in resource exploration, national defense security, seismogenic mechanism and other fields. With the transformation of instruments and equipment towards intelligence, magnetic field sensors are also developing towards high sensitivity, high resolution, miniaturization and easy integration. Traditional electric magnetic field sensors, such as Hall elements and magnetoresistance sensors, have been widely used, but still have certain limitations in terms of anti-electromagnetic interference, high temperature and pressure resistance, and distributed measurement.

[0003] Optical fiber sensors have been widely valued and rapidly developed in recent decades due to their unique advantages of anti-electromagnetic interference, high temperature and pressure resistance, and distributed measurement. Among them, the optical fiber magnetic field sensor based on extrinsic Fabry-Perot interference (EFPI) realizes magnetic field measurement by detecting the phase change caused by the interaction between light wave and magnetic field, and has the advantages of not being affected by electromagnetic interference and being able to work stably in harsh environments. However, the existing optical fiber magnetic field sensor based on EFPI still has the problems of low sensitivity and temperature cross-sensitivity, which limits its engineering application in the field of magnetic field measurement. SUMMARY

[0004] The application aims to provide a high-sensitivity optical fiber magnetic field sensor based on extrinsic F-P interference, which further improves the performance of the sensor by optimizing the structure of the sensor, improving the coupling efficiency of the optical fiber and the magnetic sensitive material, etc.

[0005] In order to solve the technical problem, the technical scheme of the application is:

[0006] An optical fiber magnetic field sensor based on extrinsic F-P interference, comprising: a single-mode optical fiber, a Terfenol-D rod, a sensor shell, an FBG and a permanent magnet.

[0007] The Terfenol-D rod is arranged inside a sensor shell, the single-mode optical fiber and the FBG are both located between the sensor shell and the Terfenol-D rod and are oppositely distributed, the permanent magnet is fixed at one end of the Terfenol-D rod, the other end of the Terfenol-D rod is fixed at the bottom of the sensor shell, the single-mode optical fiber passes through the sensor shell and the permanent magnet fixed at one end of the Terfenol-D rod to form a high-precision extrinsic F-P cavity; the sensor shell is made of Monel-400 alloy material which has similar thermal expansion characteristics as the Terfenol-D rod;

[0008] In the overall sensor design, two optical fibers are included, one single-mode optical fiber forms an optical interference cavity with the F-P cavity and participates in the detection of the magnetic field; the other optical fiber integrated with the FBG is used to compensate for the sensor sensitivity deviation caused by temperature changes, ensuring accurate measurement of the external magnetic field by the sensor.

[0009] Further, when the sensor is excited by an external magnetic field, the Terfenol-D rod is longitudinally elongated along the axial direction under the action of the magnetic field, and the cavity length increases with the elongation of the Terfenol-D rod length, the light beam enters from the optical fiber, part of the light enters the cavity through the end face of the optical fiber, is reflected back to the inside of the lower optical fiber from the F-P mirror, i.e. the permanent magnet, forming an interference phenomenon.

[0010] Further, based on the working principle of the EFPI optical fiber magnetic field sensor based on the multi-beam interference theory, the working principle specifically includes:

[0011] According to the multi-beam interference theory, the optical path difference ΔL of two adjacent parallel light beams can be expressed as:

[0012] ΔL = 2ndcos(θ i ) (1)

[0013] n is the refractive index in the cavity, d is the cavity length, θ i is the refraction angle of the incident light passing through the bottom optical fiber end face, and thus the phase difference δ of the interferometer is:

[0014]

[0015] λ is the wavelength corresponding to the peak for detecting the analyte, and the transformation of formula (3.2) into a function of λ is:

[0016]

[0017] Since there is no material filling in the sensor cavity, it is considered that the refractive index in the F-P cavity is basically constant, and the influence of the refractive index n on the wavelength is ignored, if the change to be measured causes the detection peak to move, then the relationship between the wavelength change Δλ corresponding to the peak for detection and the F-P cavity length change Δd is:

[0018]

[0019] When only considering the GMM axial strain, the sensor is pasted in a position parallel to the GMM rod axis, so the strain of the GMM rod under the changing magnetic field strength will cause the cavity length of the sensor to change. Assuming that the length of the GMM rod is l, the axial strain when the magnetic field strength changes is Δl, and the single-mode optical fiber and the quartz capillary are not connected in any way, and the cavity length change is approximately equal to the elongation of the GMM rod: Δd≈Δl, then:

[0020]

[0021] Therefore, the magnetic field sensitivity S of the sensor is: H Indicated as:

[0022]

[0023] Wherein, ΔH in the formula is the magnetic field change amount;

[0024] The cavity length d of the magnetic field sensor:

[0025]

[0026] As can be seen from equations (6) and (7), when the wavelength corresponding to the interference peak used for detection and the cavity length of the sensor are known, the sensitivity S of the sensor is proportional to the strain Δl of the GMM rod (where Δl is the strain of the GMM rod corresponding to ΔH); H

[0027] Since the refractive index and the grating period change with temperature, the reflected Bragg wavelength is sensitive to temperature, so temperature detection is achieved by measuring the reflected Bragg wavelength shift of the FBG. When the external temperature changes, the amount of shift of the Bragg wavelength with temperature is:

[0028]

[0029] In equation (8), n eff is the effective refractive index of the grating; Λ is the period of the FBG grating;

[0030] It is considered that the temperature change will have a great influence on the sensitivity of the magnetic field sensor, and then cause the cross-sensitivity of the temperature and the magnetic field double parameters, which seriously restricts the performance of the magnetic field sensor in practical application; Therefore, a new temperature decoupling method based on FBG is proposed, and the specific implementation steps are as follows:

[0031] Firstly, by fitting the curve of the center wavelength of the FBG changing with temperature, a temperature-wavelength mapping model is constructed. The real-time temperature data measured by the model can be used as a key reference, which can be used as a key reference for separating the temperature effect in the magnetic field signal decoupling process.​

[0032] Secondly, the temperature-induced sensor sensitivity offset phenomenon is systematically quantified and analyzed, a temperature compensation model is formed by establishing the relationship between sensitivity drift and temperature change, and the model is embedded in the signal processing flow of the temperature-magnetic field composite sensor to realize dynamic correction of the influence of environmental temperature on magnetic field measurement.

[0033] Finally, based on the relationship between the zero-point drift of the FBG center wavelength and the sensor sensitivity drift, the calculated value of the magnetic field is obtained.

[0034] Further, the process of making fiber connection, single-mode fiber, fiber Bragg grating (FBG) and tail fiber equipped with FC / APC type connector are reliably connected, the specific steps are:

[0035] Use the fiber cutting knife: cut the end face of FBG, single-mode fiber and tail fiber connector, ensure that each end face is flat, smooth and defect-free to ensure good optical signal transmission;

[0036] Fix the optical fiber sample: place the treated optical fiber in the optical fiber clamp of the fusion machine, ready for fusion;

[0037] Start the fusion program: complete the butt joint of the optical fiber through the fusion machine;

[0038] Heat treatment and packaging: after the fusion point is completed, put it into a high-speed heater for heat treatment, and at the same time, package a protective sleeve to enhance the mechanical strength and long-term reliability of the fusion point.

[0039] Further, the preparation process of F-P cavity includes:

[0040] Remove the optical fiber coating: use wire strippers to remove the coating of the single-mode fiber to ensure that the fiber end face is clear and available;

[0041] Fix the assembly: firmly fix the Terfenol-D rod, magnetic field sensor shell and permanent magnet on one side of the optical three-dimensional moving platform, and fix the cut single-mode fiber on the other side, and use the precise groove in the sensor shell to ensure optical collimation;

[0042] Adjust the position of the optical fiber: adjust the X, Y and Z axis positions on the three-dimensional moving platform to make the distance between the optical fiber and the two reflecting surfaces of the F-P cavity meet the design requirements, and at the same time, monitor the formation of a stable spectrum in real time through the upper computer;

[0043] Integrate FBG: select FBG with a center wavelength of 1548nm, cut and integrate it into the magnetic field sensor, and then put the entire structure into a non-magnetic sleeve to complete the sensor manufacturing.

[0044] A high-sensitivity fiber-optic magnetic field sensor based on extrinsic F-P interference, comprising: an EFPI interference cavity, an alloy shell, an FBG and a permanent magnet;

[0045] The EFPI interference cavity comprises: a single-mode optical fiber, an F-P cavity mirror and a Terfenol-D rod, one end of the single-mode optical fiber directly faces the F-P cavity mirror and forms a cavity therebetween, the F-P cavity mirror is fixed on the upper end of the alloy shell, and the Terfenol-D rod is fixed inside the alloy shell coaxially with the single-mode optical fiber and coupled with the other end of the single-mode optical fiber;

[0046] The permanent magnet is arranged at the lower end of the Terfenol-D rod and is spaced apart to provide an optimal bias magnetic field of 20-50 mT, so that the Terfenol-D material works in the best magnetic permeability interval; the FBG is integrated inside the magnetic field sensor and close to the Terfenol-D rod; the alloy shell adopts a Monel-400 alloy material having similar thermal expansion characteristics as the Terfenol-D rod;

[0047] The whole EFPI fiber sensor is fixed inside the non-magnetic sleeve; the whole sensor design includes two optical fibers, one single-mode optical fiber forms an optical interference cavity with the F-P cavity and participates in the detection of the magnetic field, and the other optical fiber integrated with the FBG is used to compensate for the sensor sensitivity deviation caused by temperature changes and ensure accurate measurement of the external magnetic field.

[0048] Further, when the sensor is excited by an external magnetic field, the Terfenol-D rod is elongated along the longitudinal direction under the action of the magnetic field, resulting in a decrease in the distance between the end face of the single-mode optical fiber and the end face of the extrinsic F-P cavity, and the precise measurement of the cavity length change realizes the precise measurement of the external magnetic field strength.

[0049] Further, the F-P cavity mirror is a precision ring of non-magnetic stainless steel, and the surface roughness is controlled to be 0.4 after super-precision polishing treatment, and the single-mode optical fiber end face is accurately aligned to form a high-quality F-P interference cavity.

[0050] Further, the manufacturing process of the sensor comprises:

[0051] First, the coating of the single-mode optical fiber needs to be removed with a wire stripper, and then the length of the coating removal part is cut to 10 mm with a fiber cutting knife to ensure the end face to be flat and smooth;

[0052] The assembled Terfenol-D rod, the magnetic field sensor shell and the ring serving as the F-P cavity mirror are fixed on one side of the optical three-dimensional moving platform by a clamp, and the cut single-mode optical fiber is fixed on the other side of the three-dimensional moving platform, and the optical collimation is realized through the precise groove in the mechanical structure of the magnetic field sensor shell;

[0053] By adjusting the X, Y and Z directions of the three-dimensional frame, the adjustment effect is observed in real time in the upper computer spectrum monitoring system to form a good spectrum;

[0054] The FBG with a central wavelength of 1546 nm is selected as the temperature sensitive element, in order to ensure the accuracy of the temperature measurement result, the FBG is cut at the end face of the grating area, the cut FBG is assembled into the magnetic field sensor, and then the whole structure is vertically inserted into the sensing head to monitor the spectrum change of the sensor in real time.

[0055] Compared with the prior art, the advantages of the present application are that:

[0056] High sensitivity: in the magnetic field range of 0-150 muT, the sensitivity of the sensor reaches 826 nm / mT, which is greatly improved compared with the traditional sensor, and the weak magnetic field change can be accurately detected.

[0057] Excellent magnetic field resolution: the magnetic field resolution of 121 nT is realized, so that it is suitable for high-precision magnetic field measurement applications.

[0058] Linear response characteristic: the adjustable permanent magnet bias device is introduced, so that the measurement linearity reaches 99.2%, effectively solving the nonlinear response problem of the traditional sensor in the weak magnetic field region.

[0059] Temperature compensation capability: the integrated reference FBG can monitor the temperature change in real time, effectively solving the temperature cross-sensitivity problem and improving the stability and measurement accuracy of the sensor.

[0060] Structural optimization: by improving the coupling structure of magnetostrictive material and optical fiber, the long-term stability of the sensor is enhanced, and the reliability in actual application is ensured.

[0061] Simplified design and operation: standardized optical fiber fusion and precise adjustment are adopted, which is conducive to realizing high-precision production and assembly and reducing manufacturing complexity.

[0062] Wide application potential: suitable for wide application in scientific research, industrial monitoring, geological exploration and environmental monitoring and other fields. BRIEF DESCRIPTION OF DRAWINGS

[0063] Figure 1 , the internal structure diagram of the EFPI optical fiber magnetic field sensor in embodiment 1;

[0064] Figure 2, overall structure diagram of the EFPI optical fiber magnetic field sensor in Example 1;

[0065] Figure 3 , spectrum of the EFPI optical fiber magnetic field sensor in Example 1;

[0066] Figure 4 , Figure 1: EFPI optical fiber magnetic field sensor test platform;

[0067] Figure 5 , internal structure diagram of the improved EFPI optical fiber magnetic field sensor in Example 3;

[0068] Figure 6 , the overall structure diagram of the improved EFPI optical fiber magnetic field sensor in Example 3;

[0069] Figure 7 , spectrum of the improved EFPI optical fiber magnetic field sensor in Example 3;

[0070] Figure 8 , sensitivity curves of the improved EFPI optical fiber magnetic field sensor at different temperatures in Example 4;

[0071] Figure 9 , Curve of the improved magnetic field sensitivity changing with temperature in Example 4. DETAILED DESCRIPTION

[0072] The specific implementation of the present invention is described below in conjunction with examples:

[0073] It should be noted that the structures, proportions, sizes, etc. shown in this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the conditions under which the present invention can be implemented. Any structural modification, change in proportional relationship or adjustment of size should still fall within the scope of the technical content disclosed in the present invention without affecting the efficacy and purpose that can be achieved by the present invention.

[0074] At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" quoted in this specification are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0075] The working principle of the fiber F-P interferometer is analyzed first. Based on the multi-beam interference theory, the correspondence between the cavity length change and the optical signal change is analyzed, and a sensitivity model containing the magnetostrictive characteristic structure parameters is constructed. In view of the temperature cross-sensitivity problem, a temperature decoupling method based on a fiber Bragg grating (FBG) is proposed to compensate the temperature of the sensor and correct the magnetic field measurement result.

[0076] Then a high-sensitivity fiber magnetic field sensor based on EFPI is designed. The sensor uses Terfenol-D to make a sensitive element, uses a permanent magnet to provide a bias magnetic field to improve the conversion efficiency of the magnetostrictive rod, and forms a high-precision extrinsic F-P interferometer by accurately aligning the end face of the single-mode optical fiber with the smooth surface of the permanent magnet. When the external magnetic field changes, the length of the magnetostrictive rod will change, which in turn causes the change of the F-P cavity length. Experimental results show that within 0-150 μT, the highest sensitivity of the sensor reaches 676 nm / mT, and the corresponding magnetic field resolution is 147 nT. In the packaging structure of the magnetic field sensor, a reference FBG is integrated, which only senses temperature changes and is not affected by the magnetic field. By monitoring the change caused by temperature in real time and correcting the magnetic field measurement value, the temperature cross-sensitivity problem of the sensor can be effectively solved.

[0077] Finally, based on the first generation of EFPI-based fiber magnetic field sensor, the bias magnetic field and the magneto-optical coupling structure are further optimized, and an improved EFPI fiber magnetic field sensor with adjustable magnetic bias is designed. First, by introducing a bias magnetic field device with adjustable size of permanent magnet, the non-linear response problem of the traditional sensor in the weak magnetic field region is effectively overcome, and the measurement linearity is improved to 99.2%. Second, the coupling structure of magnetostrictive material and optical fiber is optimized to improve the long-term stability of the sensor. Experimental results show that the improved EFPI-based fiber magnetic field sensor has a sensitivity of 826 nm / mT within 0-150 μT, and the corresponding magnetic field resolution is 121 nT. The sensor shows good linearity and temperature stability.

[0078] Embodiment 1:

[0079] This embodiment proposes an EFPI-based fiber magnetic field sensor based on super magnetostrictive material. The sensor uses Terfenol-D material to make a sensitive element, and uses a NdFeB permanent magnet ring to provide a bias magnetic field to improve the conversion efficiency of the Terfenol-D rod. First, a theoretical model of the EFPI-based fiber magnetic field sensor is established to analyze the working principle of the sensor. Second, the manufacturing steps of the F-P cavity and the assembly of the sensor are introduced. Finally, a magnetic field sensor test system is built and its performance is tested.

[0080] Figure 1 and 2 The structure diagram of EFPI-based fiber-optic magnetic field sensor. The sensor is mainly composed of a single-mode optical fiber, a magnetostrictive rod, an EFPI sensor shell, a permanent magnet, and an external non-magnetic sleeve.

[0081] Among them, the sensing element selects Terfenol-D rod, which has the advantages of significant magnetostrictive effect, fast response speed, and high coupling coefficient. In addition, a NdFeB permanent magnet is introduced in the sensor as a bias magnetic field to improve the conversion efficiency of Terfenol-D material.

[0082] In order to minimize the influence of temperature change on the performance of the sensor, the EFPI sensor shell adopts Monel-400 alloy material with similar thermal expansion coefficient to the Terfenol-D rod. At the same time, FBG is integrated on the other side of the sensor for measuring temperature change, thereby effectively solving the temperature crosstalk problem.

[0083] The entire EFPI fiber-optic sensor is fixed in the non-magnetic sleeve. When the external magnetic field changes, the length of the Terfenol-D rod will change, causing the length of the extrinsic F-P cavity to change. By measuring the cavity length change of the EFPI fiber-optic magnetic field sensor, the strength of the external magnetic field can be accurately calculated. The specific parameters of each part of the EFPI fiber-optic sensor are listed in Table 1.

[0084] Table 1

[0085]

[0086] Fiber-optic magnetic field sensing principle based on EFPI:

[0087] The permanent magnet is fixed at one end of the Terfenol-D rod by epoxy resin glue, and the other end of the Terfenol-D rod is fixed at the bottom of the cylindrical Monel-400 alloy shell. The single-mode optical fiber passes through the Monel-400 alloy shell and the permanent magnet fixed at the other end of the magnetic rod to form a high-precision extrinsic F-P cavity. Its working principle is that when the sensor is excited by an external magnetic field, the Terfenol-D rod is elongated along the axial direction under the action of the magnetic field, and the cavity length increases with the elongation of the Terfenol-D rod length. The light beam enters from the optical fiber, part of the light enters the cavity through the optical fiber end face, and is reflected back to the inside of the lower optical fiber from the F-P mirror surface, forming an interference phenomenon. According to the multi-beam interference theory, the optical path difference ΔL of two adjacent parallel light beams can be expressed as:

[0088] ΔL = 2ndcos(θ i ) (3.1)

[0089] n is the refractive index of the cavity, d is the cavity length, and θi The refraction angle of the incident light passing through the bottom fiber end face is θi. Therefore, the phase difference δ of the interferometer is:

[0090]

[0091] λ is the wavelength corresponding to the detection peak of the analyte. Transforming equation (3.2) into a function of λ is:

[0092]

[0093] Since there is no material filling in the sensor cavity, it can be considered that the refractive index in the F-P cavity is basically constant, and the effect of refractive index n on wavelength can be ignored. If the change to be measured causes the detection peak to move, the relationship between the change Δλ of the wavelength corresponding to the detection peak and the change Δd of the F-P sensor cavity length is:

[0094]

[0095] When only considering the axial strain of the GMM, the sensor is pasted in a position parallel to the axial direction of the GMM rod. Therefore, the strain of the GMM rod under the changing magnetic field strength will cause the cavity length of the sensor to change. Assuming that the length of the GMM rod is l, the axial strain amount when the magnetic field strength changes is Δl. And the single-mode optical fiber and the quartz capillary tube are not connected in any way, and the cavity length change is approximately equal to the elongation of the GMM rod: Δd≈Δl, then:

[0096]

[0097] Therefore, the magnetic field sensitivity S of the sensor is: H is expressed as:

[0098]

[0099] wherein ΔH is the change in magnetic field.

[0100] The cavity length d of the magnetic field sensor is according to equation (2.27) in section 2.3:

[0101]

[0102] From equations (3.6) and (3.7), it can be seen that when the wavelength corresponding to the detection peak of the sensor and the cavity length are known, the sensitivity S of the sensor is proportional to the strain Δl of the GMM rod (where Δl is the strain of the GMM rod corresponding to ΔH). H Therefore, improving the conversion efficiency between the strain change amount of the GMM rod and the cavity length change can effectively improve the magnetic field sensitivity of the sensor.

[0103] Because the refractive index and the grating period change with temperature, the reflected Bragg wavelength is sensitive to temperature. Therefore, temperature detection can be achieved by measuring the reflected Bragg wavelength shift of the FBG. When the external temperature changes, the amount of Bragg wavelength shift with temperature is:

[0104]

[0105] In formula (3.8), n eff is the effective refractive index of the grating; and Λ is the period of the FBG grating.

[0106] Temperature changes have a great impact on the sensitivity of the magnetic field sensor, and further cause the cross-sensitivity of temperature and magnetic field dual parameters, which seriously restricts the performance of the magnetic field sensor in practical applications. Therefore, a new temperature decoupling method based on FBG is proposed, and the specific implementation steps are as follows:

[0107] Firstly, the temperature-wavelength mapping model is constructed by fitting the curve of the FBG center wavelength change with temperature. The real-time temperature data measured by the model can be used as a key reference, which can be used as a key reference for temperature effect separation in the magnetic field signal decoupling process.

[0108] Secondly, the sensor sensitivity shift phenomenon caused by temperature is quantitatively analyzed. By establishing the relationship between sensitivity drift and temperature change, a temperature compensation model is formed, and the model is embedded in the signal processing process of the temperature-magnetic field composite sensor to realize the dynamic correction of the influence of environmental temperature on magnetic field measurement.

[0109] Finally, the magnetic field calculation value is obtained based on the relationship between the zero-point drift of the FBG center wavelength and the sensor sensitivity drift.

[0110] Fabrication of EFPI-based fiber-optic magnetic field sensor

[0111] In the experimental operation link, since all the fibers used are bare fibers, the single-mode fiber, FBG and tail fiber equipped with FC / APC connector need to be reliably connected by fusion splicing technology to ensure the stability and efficiency of optical signal transmission. In this experiment, Sumitomo TYPE-81C intelligent fiber fusion splicer is selected for fusion splicing operation. The device has the functions of bidirectional fusion and synchronous packaging of high-speed heat shrink sleeve, and its integrated design can realize the integration of fiber fusion and protective sleeve installation. The specific operation steps are as follows:

[0112] (1) Use the fiber cutting knife to cut the FBG, single-mode fiber and FC / APC tail fiber connector end face to ensure that the end face is flat and defect-free;

[0113] (2) Fix the treated fiber samples in the fiber clamps on the left and right sides of the fusion splicer;

[0114] (3) Start the fusion procedure to complete the optical fiber butt joint;

[0115] (4) Place the fusion point in the high-speed heater of the device for heat treatment, and complete the packaging of the protective sleeve.

[0116] This series of standardized operations not only ensures the low-loss characteristics of the optical fiber connection, but also ensures the mechanical strength and long-term reliability of the joint.

[0117] The preparation of the F-P cavity requires the establishment of a precise optical adjustment system, with an optical three-dimensional moving platform as the core adjustment device. This platform is equipped with X, Y, Z three-axis precision translation stages, with a resolution of 1 μm in each dimension. Through micro-displacement adjustment, the relative position of the optical fiber end face can be accurately controlled, ensuring that the length and parallelism of the F-P cavity meet the design requirements. In the experiment, the single-mode optical fiber used to prepare the sensor is 9 / 125 μm in size; the Terfenol-D is produced by Shijiazhuang Saining Electronics Technology Co., Ltd., with a size of Φ5mm×34mm. The entire preparation process is carried out on a vibration isolation platform, with strict control of environmental vibration and temperature fluctuations. The final F-P cavity has stable cavity length characteristics and excellent fineness, meeting the application requirements of laser frequency stabilization and high-sensitivity sensing. The steps for making the sensor are as follows:

[0118] (1) First, remove the coating of the single-mode optical fiber with wire strippers, then cut the length of the stripped coating part to 10 mm with a fiber cutting knife to ensure a smooth and flat end face.

[0119] (2) Securely fix the assembled Terfenol-D rod, magnetic field sensor housing, and permanent magnet on one side of the optical three-dimensional moving platform with a clamp, and fix the cut single-mode optical fiber on the other side of the three-dimensional moving platform. Achieve optical collimation through the precise grooves in the magnetic field sensor housing mechanical structure, as shown in Figure 2 .

[0120] (3) Adjust the X, Y, Z directions of the three-dimensional frame, and observe the adjustment effect in real time on the host computer spectrum monitoring system to form a good spectrum. The spectrum of the completed sensor is shown in Figure 3 .

[0121] (4) Select an FBG with a center wavelength of 1548 nm as the temperature sensitive element. To ensure the accuracy of the temperature measurement results, cut the FBG at the end face of the grating region. Integrate the cut FBG into the magnetic field sensor, then vertically insert the entire structure into the non-magnetic sleeve to complete the sensor production, as shown in Figure 3 .

[0122] After the preparation of F-P cavity and the assembly of FBG, the sensor needs to be packaged to form a complete sensor structure. To ensure the quality of the package, the thermal expansion coefficient of the selected adhesive should match the packaging material, and the thickness of the adhesive layer should be strictly controlled at the nanometer level (usually tens of nanometers) to reduce the influence of thermal stress on the performance of the sensor. Based on the above requirements, the experiment selects UV glue for packaging, and uses a handheld JDS UV curing lamp to cure the adhesive layer. The UV light source can provide stable light intensity output, and the maximum irradiance can reach 3166mW / cm 2 , which can realize efficient and rapid curing of UV glue, thereby ensuring the mechanical stability and long-term reliability of the packaging structure.

[0123] Example 2:

[0124] This embodiment 2 is applied to embodiment 1, and this embodiment 2 specifically builds an EFPI-based optical fiber magnetic field sensor test system.

[0125] Magnetic field test system: the EFPI optical fiber magnetic field sensor test platform includes a magnetic field generating system and an optical fiber sensing system, and the system structure is as shown in Figure 4 .

[0126] The magnetic field system adopts a zero magnetic space control structure, and a static "zero magnetic field" environment (residual magnetism <20nT) is constructed through multi-layer composite shielding (two layers of aluminum electric shielding + eight layers of permalloy magnetic shielding) without power supply. The geomagnetic shielding is realized by high magnetic permeability refraction effect. The standard coil is installed at the center of the zero magnetic space, and the magnetic field intensity can be accurately controlled through a constant current source. The system is composed of a constant current power supply, an AC / DC power supply and a digital multimeter. The current-magnetic field conversion is based on the coil constant, which ensures the accuracy of field strength control.

[0127] The optical fiber sensing system is composed of an EFPI sensor, an optical fiber demodulator and an upper computer. The sensor is placed at the center of the standard coil, the spectral signal is collected by the demodulator, and the cavity length is recorded by the upper computer with the change of the magnetic field. In the experiment, the system is kept closed and constant temperature to exclude interference and evaluate the performance of the sensor.

[0128] Magnetic field-temperature composite environment test system: in order to study the influence of temperature on the performance of the sensor, a magnetic field-temperature composite test environment is established in the zero magnetic space. The test structure is: the outer layer is magnetic field shielding, and the inner layer is a 30mm thick foam heat insulation box, and the temperature change is realized by ice bags.

[0129] Temperature measurement adopts double-channel mode (±0.1℃ / ±0.5℃) of FBG and thermocouple, and the data is compared in real time. The sensor is placed at the center of the coil, and the cavity length and magnetic field response are observed by gradually increasing the ice bag to reduce the temperature, and the magnetic-temperature coupling effect is studied.

[0130] Temperature calibration test system: To calibrate FBG temperature data, a high-precision temperature control test system was established, which was composed of a fiber demodulator, a host computer and a high-precision temperature control box (MQ-TH1000F-2N). The temperature control accuracy was ±0.01℃, and the control range was -70~170℃.

[0131] In the experiment, the FBG was placed in the temperature control box, and the wavelength change was recorded in real time and the temperature sensitivity of the FBG was fitted, which provided accurate support for the decoupling of magnetic field and temperature.

[0132] Performance test and analysis of EFPI fiber magnetic field sensor:

[0133] Sensor output response test: In the range of 0~150μT, the magnetic field was increased and decreased by 20μT, and the F-P cavity length change was recorded. The experiment showed that the cavity length increased approximately linearly with the magnetic field, and the preliminary sensitivity was 148nm / mT, and the resolution was 675nT.

[0134] Considering that the bias magnetic field was not optimized, the sensor was reconstructed and the magnet parameters were adjusted, and the sensitivity was improved to 676nm / mT, and the resolution was improved to 147nT. The forward and reverse stroke curves did not coincide, showing hysteresis phenomenon, which was mainly caused by the magnetic hysteresis and residual strain of Terfenol-D material.

[0135] Sensor temperature characteristic test: The magnetic field response at different temperatures was tested by ice bag cooling method. The results showed that the cavity length increased linearly with the magnetic field, and the sensitivity increased with the increase of temperature, which reflected the temperature cross-sensitivity.

[0136] By integrating FBG temperature measurement, the spectrum drift caused by temperature was obtained, which was distinguished from the EFPI output, and the temperature decoupling was realized, and the magnetic field measurement accuracy was improved.

[0137] Sensor directivity test: The magnetic field direction was fixed (150μT), and the sensor was rotated by 30° as a step, and the F-P cavity length change at different angles was recorded. The experimental results showed an "8" type polar coordinate response graph, which indicated that the sensor had good uniaxial directivity, and the maximum response was located at 0° / 180°, and the minimum response was at 90° / 270°. The response was related to the anisotropic coupling characteristics of magnetostrictive axis, and a small amount of cross-sensitivity was caused by material anisotropy and structural non-ideality.

[0138] Based on the implementation of Example 1 and Example 2, this embodiment establishes the structural model of the EFPI optical fiber magnetic field sensor, analyzes the relationship between the sensor F-P cavity length and the sensitivity change, builds the EFPI optical fiber magnetic field sensor test system and tests its performance. The experimental results show that the sensor has good magnetic field sensing characteristics. In 0-150 μT, the highest sensitivity of the sensor reaches 676 nm / mT, and the corresponding magnetic field resolution is 147 nT, which is significantly higher than most reported magnetic field sensors. The FBG temperature decoupling method is proposed to compensate the temperature of the magnetic field sensor and correct the magnetic field measurement results, which solves the cross-sensitivity problem caused by temperature. However, the designed sensor still needs to be improved, such as the optimal working point of the optical fiber F-P magnetic field sensor is not determined, and the sensor packaging process needs to be further improved.

[0139] Example 3

[0140] Based on Example 1 and Example 2, this embodiment 3 further optimizes the bias magnetic field and the magnetic-optical coupling structure, and designs an improved EFPI optical fiber magnetic field sensor with adjustable magnetic bias. By introducing the adjustable permanent magnet bias magnetic field device, the nonlinear response problem of traditional sensors in the weak magnetic field region is effectively overcome, and the performance evaluation system of the EFPI optical fiber magnetic field sensor is further improved. First, the optimal bias magnetic field experiment is added to determine the optimal working point of the EFPI optical fiber magnetic field sensor and maximize its monitoring performance. Second, the cavity length resolution experiment of the magnetic field sensor is improved, which provides important support for the accuracy determination of the magnetic field sensor.

[0141] Figure 5 and 6 The structure diagram of the improved EFPI optical fiber magnetic field sensor. The EFPI optical fiber magnetic field sensor is composed of a single-mode optical fiber, a FBG, a magnetostrictive rod, an EFPI optical fiber sensor shell, a non-magnetic stainless steel ring, a permanent magnet and an external non-magnetic cylinder.

[0142] The core sensing element of the EFPI optical fiber magnetic field sensor is high-performance Terfenol-D magnetostrictive material, which has the advantages of large magnetostrictive coefficient, fast response speed and large coupling coefficient. The sensor uses a non-magnetic stainless steel precision machined ring as the F-P cavity mirror surface, which is precisely aligned with the end face of the single-mode optical fiber after super-precision polishing to form a high-quality F-P interference cavity. To improve the magnetic conversion efficiency, the system integrates an NdFeB permanent magnet array to provide an optimal bias magnetic field of 20-50 mT, which makes the Terfenol-D material work in the best magnetic permeability interval.

[0143] In order to minimize the impact of temperature, EFPI fiber-optic magnetic field sensor shell using with Terfenol-D rod thermal expansion coefficient similar to Monel-400 alloy material. In the sensor integration FBG, temperature changes measured by FBG, so as to solve the temperature crosstalk problem. Will be the whole EFPI fiber-optic magnetic field sensor fixed in a non-magnetic sleeve, when the external magnetic field changes, Terfenol-D rod length changes, making the EFPI cavity length changes, by measuring the EFP fiber-optic magnetic field sensor cavity length, can be calculated the external magnetic field. The designed EFPI fiber-optic magnetic field sensor parts specific parameters as shown in table 2.

[0144] Table 2

[0145]

[0146] Improved EFPI fiber-optic magnetic field sensor as F-P cavity mirror ring fixed in the sensor shell, Terfenol-D rod fixed in the sensor shell inside, single mode fiber and Terfenol-D rod directly coupled, its working principle is that when the sensor is excited by external magnetic field, Terfenol-D rod in the magnetic field along the longitudinal elongation, resulting in single mode fiber end face and the distance between the end face of the non-intrinsic F-P cavity decreases, through the precise measurement of cavity length changes, can realize the precise measurement of external magnetic field intensity.

[0147] Improved EFPI fiber-optic magnetic field sensor production: in order to make the sensor has good performance, need to adjust the F-P cavity length through optical three-dimensional moving platform. Three-dimensional moving platform each dimension resolution is 1 μm, through adjusting the optical three-dimensional moving platform and real-time observation of host computer spectrum, so as to accurately adjust the F-P cavity length, for the preparation of sensor single mode fiber size is 9 / 125 μm; Terfenol-D by shijiazhuang sail ning electronic technology co., LTD, size is Φ5mm × 55mm. The production steps of the sensor are as follows:

[0148] (1) first, the coating of single mode fiber needs to be removed by wire stripper, and then the length of the coating removed part is cut to 10 mm by fiber cutting knife, to ensure the end face smooth and flat.

[0149] (2) the assembled Terfenol-D rod, magnetic field sensor shell and F-P cavity mirror ring are fixed on one side of the optical three-dimensional moving platform by clamp, and the cut single mode fiber is fixed on the other side of the three-dimensional moving platform. Through the precise groove in the mechanical structure of the magnetic field sensor shell to realize optical collimation.

[0150] (3) by adjusting the three-dimensional frame X, Y, Z direction, real-time observation of the adjustment effect in the host computer spectrum monitoring system to form a good spectrum.

[0151] (4) FBG with a central wavelength of 1546 nm is selected as the temperature sensitive element. In order to ensure the accuracy of the temperature measurement results, the FBG gate end face is cut and the cut FBG is assembled into the magnetic field sensor. Then the whole structure is vertically inserted into the sensor head to monitor the spectral changes of the sensor in real time. Figure 7 shown.

[0152] After the precise fabrication of the FP cavity and accurate assembly of the FBG, the components must be integrated into a complete sensor structure through packaging. This experiment uses a light-curing process for packaging: First, a prepared UV adhesive is evenly applied to the key joints of the sensor. Then, a UV curing lamp is used to irradiate the adhesive in a targeted manner for at least 30 seconds to ensure complete curing.

[0153] Example 4:

[0154] This embodiment is applied to Example 3. This embodiment performs a performance test on an improved EFPI optical fiber magnetic field sensor.

[0155] Optimal bias magnetic field testing: To optimize the sensor's magnetic field response, a systematic bias magnetic field tuning scheme was employed. The sensor was precisely positioned at the center of the standard coil, ensuring that the Terfenol-D rod's axis was collinear with the coil's axis. The bias magnetic field was adjusted using an NdFeB permanent magnet. The magnetic field strength was varied by adding small magnets, with a gradient adjustment method employed. After each adjustment, the bias magnetic field was stabilized for one minute and the FP cavity length data was recorded. Experiments showed that a bias magnetic field of 35 mT was the most sensitive to external magnetic fields, thus representing the optimal bias magnetic field.

[0156] Sensor output response test: The sensor output response was tested in a magnetic field range of 0 to 150 μT. During the experiment, the magnetic field was gradually increased from 0 μT to 150 μT in steps of 20 μT. Data was recorded after stabilization for 1 minute, and then the magnetic field was gradually reduced, maintaining the same interval. The experimental results show that the FP cavity length changes approximately linearly with the magnetic field, but the forward and reverse strokes do not completely overlap, which is due to the hysteresis characteristics of the Terfenol-D rod. After analysis, the sensor sensitivity is 826 nm / mT, and the linearity is good (R 2 =0.992).

[0157] Sensor resolution testing: The resolution of a fiber optic magnetic field sensor refers to the minimum magnetic field change that can be reliably detected. The test consisted of a warm-up phase and a noise floor phase. Under zero magnetic field and zero stress conditions, the FP cavity length variation was continuously recorded for 30 minutes. The results showed that the cavity length fluctuation range did not exceed 1.5 nm, demonstrating the sensor's excellent stability. By calculating the difference between the data, the sensor resolution was determined to be 121 nT.

[0158] Temperature testing of the sensor: Due to the thermal expansion coefficient of the Terfenol-D rod, the sensor has temperature cross-sensitivity issues. Temperature changes were measured by integrating an FBG and calculating the effect of temperature on the spectrum. Experimental results show that the FBG's temperature sensitivity is 0.011nm / °C. At different temperatures, the sensor's FP cavity length increases linearly with increasing magnetic field, and increasing temperature leads to increased sensitivity, such as Figure 8 The temperature decoupling method can effectively reduce the influence of temperature on magnetic field measurement and accurately calculate the magnetic field value.

[0159] Sensor Directivity Testing: To verify the sensor's directivity, the sensor was placed at the center of a standard coil and its magnetic field response was measured at an adjustable angle. The test results showed that the FP cavity length changed the most when the magnetic field aligned with the axis of the Terfenol-D rod; the change was minimal when the magnetic field was perpendicular. The response curve exhibited a typical figure-8 distribution, but due to experimental error, hysteresis, and the magnetocrystalline anisotropy of the Terfenol-D rod, the curve exhibited some deviation. Future improvements in measurement accuracy could be achieved through the use of a high-precision rotating platform and magnetic domain preprocessing.

[0160] As can be understood, this embodiment established a sensor structural model, analyzed the relationship between the FBG central wavelength, sensitivity, temperature, and the sensor's magnetic field response curve, and conducted performance tests using a magnetic field sensor test system. The experiments showed that within the 0-150μT range, the sensor had a sensitivity of 826nm / mT and a magnetic field accuracy of 121nT. The sensor's directivity generally conformed to the cosine law, demonstrating good temperature compensation.

[0161] Example 5:

[0162] Example 2-3 proposes a high-sensitivity EFPI fiber optic magnetic field sensor based on extrinsic FP interferometry. The sensor uses Terfenol-D as the sensitive element and a permanent magnet to provide a bias magnetic field to improve the conversion efficiency of the magnetostrictive rod. By precisely aligning the end face of the single-mode optical fiber with the smooth surface of the permanent magnet, a high-fineness extrinsic FP cavity is formed. When the external magnetic field changes, the length of the magnetostrictive rod changes accordingly, causing the length of the FP cavity to change. Experimental results show that within the range of 0 to 150 μT, the sensor's maximum sensitivity reaches 676 nm / mT, corresponding to a magnetic field resolution of 147 nT. A reference FBG is integrated into the packaging structure of the magnetic field sensor. This FBG only senses temperature changes and is not affected by the magnetic field. By monitoring the temperature changes in real time and correcting the magnetic field measurements, the temperature cross-sensitivity problem of the sensor is effectively solved.

[0163] Embodiment 4-5 is based on the first generation EFPI fiber-optic magnetic field sensor, and further optimizes the bias magnetic field and the magneto-optic coupling structure. An improved EFPI fiber-optic magnetic field sensor with adjustable magnetic bias is designed. First, the problem of nonlinear response of the traditional sensor in the weak magnetic field region is effectively solved by introducing an adjustable permanent magnet bias device, and the measurement linearity is improved to 99.2%. Second, the coupling structure of magnetostrictive material and optical fiber is optimized to improve the long-term stability of the sensor. Experimental results show that the improved EFPI fiber-optic magnetic field sensor has a sensitivity of 826 nm / mT in the range of 0-150 μT, and the corresponding magnetic field resolution is 121 nT. The sensor shows good linearity and temperature stability.

[0164] The preferred embodiments of the present application are described in detail above, but the present application is not limited to the above-described embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the spirit of the present application.

[0165] Many other changes and modifications can be made without departing from the spirit and scope of the present application. It should be understood that the present application is not limited to a particular embodiment, and the scope of the present application is defined by the appended claims.

Claims

1. A fiber optic magnetic field sensor based on extrinsic FP interferometry, characterized in that: include: Single-mode optical fiber, Terfenol-D rod, sensor housing, FBG, and permanent magnet; The Terfenol-D rod is disposed inside the sensor housing, the single-mode optical fiber and the FBG are located between the sensor housing and the Terfenol-D rod and are relatively distributed, the permanent magnet is fixed to one end of the Terfenol-D rod, and the other end of the Terfenol-D rod is fixed to the bottom of the sensor housing, and the single-mode optical fiber passes through the sensor housing and forms a high-precision extrinsic FP cavity with the permanent magnet fixed to one end of the Terfenol-D rod; The sensor housing is made of Monel-400 alloy material which has similar thermal expansion characteristics to the Terfenol-D rod; The overall sensor design includes two optical fibers: one single-mode fiber, which forms an optical interference cavity with the FP cavity and participates in the detection of the magnetic field; the other optical fiber integrated with FBG is used to compensate for the sensor sensitivity offset caused by temperature changes, ensuring that the sensor accurately measures the external magnetic field.

2. The optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 1, characterized in that: When the sensor is excited by an external magnetic field, the Terfenol-D rod extends longitudinally along the axis under the action of the magnetic field. The length of the cavity increases with the extension of the Terfenol-D rod. The light beam enters from the optical fiber, and part of the light enters the cavity through the end face of the optical fiber. It is reflected from the FP mirror, i.e., the permanent magnet, back into the interior of the optical fiber below, forming an interference phenomenon.

3. The optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 2, characterized in that: The working principle of the EFPI fiber optic magnetic field sensor based on multi-beam interference theory includes: According to the multi-beam interference theory, the optical path difference ΔL between two adjacent parallel beams can be expressed as: ΔL=2ndcos(θ i ) (2) n is the refractive index in the cavity, d is the cavity length, θ i is the refraction angle of the incident light passing through the bottom fiber end face, from which we can know that the phase difference δ of the interferometer is: λ is the wavelength corresponding to the peak used to detect the analyte. The function that transforms Equation (3.2) into λ is: Since there is no material filling the sensor cavity, the refractive index in the FP cavity is assumed to be basically constant. The effect of the refractive index n on the wavelength is ignored. If the change to be measured causes the detection peak to move, the relationship between the wavelength change Δλ corresponding to the detection peak and the FP cavity length change Δd is: When only the axial strain of the GMM is considered, the sensor is attached parallel to the GMM rod's axis. Therefore, the strain of the GMM rod under varying magnetic field strength causes the sensor's cavity length to change. Assuming the GMM rod length is l, the axial strain with varying magnetic field strength is Δl. Since the single-mode fiber is not connected to the quartz capillary, the change in cavity length is approximately equal to the elongation of the GMM rod: Δd ≈ Δl. Therefore: Therefore, the magnetic field sensitivity S of the sensor H Expressed as: Where ΔH is the change in magnetic field; Cavity length d of the magnetic field sensor: From equations (6) and (7), it can be seen that when the wavelength corresponding to the interference peak used for detection and the cavity length of the sensor are known, the sensitivity of the sensor S is H Proportional to the strain Δl of the GMM rod (where Δl is the strain of the GMM rod corresponding to ΔH); Since the refractive index and grating period change with temperature, the reflected Bragg wavelength is sensitive to temperature. Therefore, temperature detection is achieved by measuring the reflected Bragg wavelength shift of the FBG. When the external temperature changes, the shift of the Bragg wavelength with temperature is: In formula (8), n eff is the effective refractive index of the grating; Λ is the period of the FBG grating; Considering that temperature changes have a significant impact on the sensitivity of magnetic field sensors, which in turn causes cross-sensitivity between temperature and magnetic field, this phenomenon seriously restricts the performance of magnetic field sensors in practical applications. To this end, a new temperature decoupling method based on FBG is proposed. The specific implementation steps are as follows: First, a temperature-wavelength mapping model is constructed by fitting the curve of the FBG central wavelength changing with temperature. The real-time temperature data measured by this model can be used as a key reference for separating the temperature effect during the magnetic field signal decoupling process. Secondly, the phenomenon of sensor sensitivity drift caused by temperature is systematically quantified and analyzed. By establishing the relationship between sensitivity drift and temperature change, a temperature compensation model is formed. This model is embedded in the signal processing flow of the temperature-magnetic field composite sensor to achieve dynamic correction of the influence of ambient temperature on magnetic field measurement. Finally, the magnetic field calculation value is obtained based on the relationship between the FBG central wavelength zero-point drift and the sensor sensitivity drift.

4. The optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 1, characterized in that: The fiber optic connection production process reliably connects single-mode fiber, fiber Bragg grating (FBG) and pigtail equipped with FC / APC type connector. The specific steps are as follows: Use a fiber cleaver to cut the end faces of FBG, single-mode optical fiber, and pigtail connectors, ensuring that each end face is flat, smooth, and free of defects to ensure good optical signal transmission; Fix the optical fiber sample: Place the processed optical fibers in the optical fiber clamps of the fusion splicer, ready for fusion splicing; Start the fusion splicing process: complete the fiber optic connection through the fusion splicer; Heat treatment and packaging: After the welding point is completed, it is placed in a high-speed heater for heat treatment and the protective sleeve is encapsulated at the same time to enhance the mechanical strength and long-term reliability of the welding point.

5. The optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 1, characterized in that: The preparation process of the FP cavity includes: Remove the fiber coating: Use wire strippers to remove the coating of the single-mode fiber, ensuring that the fiber end face is clear and usable; Fixed assembly: The Terfenol-D rod, magnetic field sensor housing, and permanent magnet are firmly fixed to one side of the optical 3D motion platform. The cleaved single-mode optical fiber is fixed to the other side. The precision groove in the sensor housing is used to ensure optical alignment. Adjust the fiber position: Adjust the X, Y, and Z axis positions on the three-dimensional mobile platform to ensure that the distance between the fiber and the two reflective surfaces of the FP cavity meets the design requirements. At the same time, the host computer monitors the real-time formation of a stable spectrum. Integrated FBG: An FBG with a central wavelength of 1548 nm is selected, cut, and integrated into the magnetic field sensor. The entire structure is then placed in a non-magnetic sleeve to complete the sensor fabrication.

6. A high-sensitivity optical fiber magnetic field sensor based on extrinsic FP interferometry, characterized in that: include: EFPI interferometer cavity, alloy housing, FBG and permanent magnet; The EFPI interferometer cavity includes: a single-mode optical fiber, an FP cavity mirror, and a Terfenol-D rod. One end of the single-mode optical fiber directly faces the FP cavity mirror, forming a cavity therebetween. The FP cavity mirror is fixed to the upper end of the alloy housing. The Terfenol-D rod is fixed inside the alloy housing, coaxial with the single-mode optical fiber, and coupled to the other end of the single-mode optical fiber. The permanent magnets are arranged at intervals at the lower end of the Terfenol-D rod to provide an optimized bias magnetic field of 20 to 50 mT, allowing the Terfenol-D material to operate in an optimal permeability range. The FBG is integrated into the magnetic field sensor, close to the Terfenol-D rod. The alloy housing is made of Monel-400 alloy material with similar thermal expansion characteristics to the Terfenol-D rod. It also includes a non-magnetic sleeve, which fixes the entire EFPI fiber optic sensor inside. The overall sensor design includes two optical fibers: one single-mode fiber, which forms an optical interference cavity with the FP cavity and participates in the detection of the magnetic field; the other optical fiber integrated with FBG is used to compensate for the sensor sensitivity offset caused by temperature changes, ensuring that the sensor accurately measures the external magnetic field.

7. The high-sensitivity optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 6, characterized in that: When the sensor is excited by an external magnetic field, the Terfenol-D rod stretches longitudinally under the action of the magnetic field, causing the distance between the end face of the single-mode fiber and the end face of the extrinsic FP cavity to decrease. By accurately measuring the change in cavity length, accurate measurement of the external magnetic field strength is achieved.

8. The high-sensitivity optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 6, characterized in that: The FP cavity mirror is a non-magnetic stainless steel precision-machined ring, and after ultra-precision polishing, the surface roughness is controlled at 0.4, and it is precisely aligned with the end face of the single-mode optical fiber to form a high-quality FP interference cavity.

9. The high-sensitivity optical fiber magnetic field sensor based on extrinsic FP interferometry according to claim 6, characterized in that: The manufacturing process of the sensor includes: First, the coating of the single-mode optical fiber needs to be removed with a wire stripper, and then the length of the stripped coating part needs to be cut to 10mm with a fiber cutter to ensure that the end face is flat and smooth; The assembled Terfenol-D rod, magnetic field sensor housing, and the ring that serves as the FP cavity mirror are firmly fixed to one side of the optical three-dimensional mobile platform using a clamp. At the same time, the cut single-mode optical fiber is fixed to the other side of the three-dimensional mobile platform. Optical alignment is achieved through the precise grooves in the mechanical structure of the magnetic field sensor housing. By adjusting the X, Y, and Z directions of the three-dimensional frame, the upper computer spectrum monitoring system can observe the adjustment effect in real time to form a good spectrum; An FBG with a central wavelength of 1546 nm was selected as the temperature sensitive element. To ensure the accuracy of the temperature measurement results, the FBG gate end face was cut and assembled into the magnetic field sensor. The entire structure was then vertically inserted into the sensor head to monitor the sensor's spectral changes in real time.

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