Single-mode optical fiber magnetic field sensor based on Fabry-Perot cavity and preparation method of single-mode optical fiber magnetic field sensor

Abstract

The invention discloses a single-mode optical fiber magnetic field sensor based on a Fabry-Perot cavity and a preparation method of the single-mode optical fiber magnetic field sensor, and belongs to the technical field of optical fiber sensing and femtosecond laser two-photon polymerization micro-nano manufacturing. The front end is composed of a spring supporting seat, a tower-shaped spring, a reflection platform, an iron ball support and a magnetic sensitive iron ball. The tower-shaped spring and the magnetic sensitive iron ball are eccentrically arranged relative to the optical fiber core shaft, so that when an external magnetic field acts on the magnetic sensitive iron ball, the reflection platform is driven by an eccentric load to generate inclined deformation instead of traditional axial translation, and the effective optical path change of the Fabry-Perot cavity is remarkably amplified. The tower-shaped spring adopts a spiral structure with gradually-changed intermediate diameter, so that the inclination response capability is enhanced while the structural stability is ensured. The reflection platform is provided with a through hole, discharging of developing liquid is facilitated, and the micro-machining success rate is increased. The sensor is compact in structure, high in sensitivity and suitable for weak magnetic field detection, preparation of the sensor is based on the femtosecond laser two-photon polymerization technology, and optical fiber end face integrated machining can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing and femtosecond laser two-photon polymerization micro / nano fabrication technology, specifically relating to a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity and its fabrication method. Background Technology

[0002] High-precision magnetic field measurement has significant application value in fields such as power equipment condition monitoring, biomedical detection, space magnetic field detection, and micro / nano device characterization. With the development of micro / nano and fiber optic technologies, integrating magnetic field sensors into the fiber end face to achieve miniaturized, long-distance, and electromagnetic interference-resistant magnetic field measurement has become an important research direction in the field of fiber optic sensing.

[0003] Existing fiber optic magnetic field sensing technologies mainly include fiber optic interferometric sensors based on magnetostriction, magnetohydrodynamic modulation, and external magnetic sensing structures. Among these, Fabry-Perot cavity (FPC) fiber optic magnetic field sensors have attracted widespread attention due to their simple structure, high sensitivity, and ease of spectral demodulation. A common implementation involves introducing a reflective structure that can be displaced under the influence of a magnetic field into the fiber endface or externally. The axial translation of the reflective surface alters the length of the Fabry-Perot cavity, thereby achieving optical detection of the magnetic field. However, existing axial translation-based fiber endface magnetic field sensing structures still have certain shortcomings in practical applications: the limited axial displacement amplitude of the reflective structure under magnetic force results in a small change in cavity length and a limited spectral modulation amplitude, thus restricting further improvements in magnetic field sensing sensitivity; to obtain sufficient displacement response, it is often necessary to reduce the overall stiffness of the elastic support structure, which can easily introduce problems such as structural instability, lateral swaying, poor repeatability, and difficulty in fabrication, affecting the reliability and consistency of the sensing results. In addition, some magnetic field sensing structures introduce magnetic sensitive elements and elastic structures by external placement or multiple assembly methods, which makes the process complex and alignment difficult, and is not conducive to the miniaturization and mass production of devices. Summary of the Invention

[0004] This invention aims to overcome the problems of limited magnetostrictive displacement response amplitude, insufficient sensing sensitivity, and poor stability of end-face microstructure fabrication in existing fiber optic magnetic field sensors. It proposes a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity and its fabrication method. The reflection interference spectrum of a Fabry-Perot cavity is determined by the cavity length L and the effective refractive index n, and its resonance condition can be expressed as mλ = 2nL. When the cavity length or equivalent optical path changes, it will cause a shift in the wavelength λ corresponding to the interference peak or valley in the reflection interference spectrum. Based on the above measurement principle, this invention integrates an eccentrically arranged elastic support structure and a magnetically sensitive structural unit at the end face of the single-mode fiber. This transforms the magnetic force caused by the applied magnetic field into the tilt deformation of the reflecting end face, thereby significantly amplifying the optical path change of the Fabry-Perot cavity and achieving high-sensitivity optical detection of magnetic field signals. This invention has a compact structure and high integration, and is suitable for weak magnetic field measurement, fiber optic sensing in space-constrained environments, and high-sensitivity magnetic field detection applications, showing promising engineering application prospects.

[0005] This invention is achieved through the following technical solution:

[0006] A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity includes a single-mode fiber 1 and a magnetic field sensing front end 2 disposed on one end face of the single-mode fiber 1. The magnetic field sensing front end 2 includes a spring support 21, a tower-shaped spring 22, a reflective platform 23, an iron ball support 24 disposed on the reflective platform 23, and a magnetically sensitive iron ball 25 disposed on the iron ball support 24. The spring support 21 is fixed to the end face of the single-mode fiber 1. The reflective platform 23 is connected to the spring support 21 through the tower-shaped spring 22 and is disposed opposite to the end face of the single-mode fiber 1. The reflective platform 23 and the end face of the single-mode fiber 1 together constitute a Fabry-Perot cavity structure. When an external magnetic field is applied to the magnetically sensitive iron ball 25, the magnetically sensitive iron ball 25, under the action of magnetic force, drives the reflective platform 23 to tilt and deform under the elastic constraint of the tower-shaped spring 22 through the iron ball support 24, thereby causing changes in the cavity length and effective optical path within the Fabry-Perot cavity. The magnetic field measurement is achieved by detecting the changes in the reflection interference spectrum. The tilting deformation is a non-axial deformation primarily characterized by an overall angular deflection of the reflective platform 23. Its deformation direction deviates from the axial direction of the single-mode fiber 1, unlike the pure axial translational deformation along the core axis of the single-mode fiber 1. Specifically, under external force, the reflective platform 23 rotates under the elastic constraint of the tower-shaped spring 22, causing it to tilt at a certain angle relative to the end face of the single-mode fiber 1. This results in uneven changes in the gap between the reflective platform 23 and the end face of the single-mode fiber 1 at various radial positions, introducing an asymmetric cavity length distribution and effective optical path variation into the Fabry-Perot cavity.

[0007] Furthermore, the central axis of the tower-shaped spring 22 is eccentrically arranged relative to the core axis of the single-mode fiber 1, and the iron ball support 24 is disposed on the reflection platform 23 at a position corresponding to the eccentric arrangement, so that the magnetically sensitive iron ball 25 is aligned with the core axis of the single-mode fiber 1 in the initial state. Through the above-mentioned eccentric arrangement, the magnetically sensitive iron ball 25 applies a non-axial load to the reflection platform 23 under the action of the magnetic field, so that the reflection platform 23 preferentially produces tilting deformation under the elastic support of the tower-shaped spring 22. Compared with the axial translation response, a larger equivalent displacement can be obtained, thereby increasing the spectral modulation amplitude of the Fabry-Perot cavity and thus achieving sensitization.

[0008] Furthermore, the offset of the eccentric arrangement is referenced to the geometric center of the magnetic field sensing front end 2, and the radial offset of the magnetic sensitive iron ball 25 relative to the geometric center of the magnetic field sensing front end 2 is 5 μm to 15 μm; the radial offset of the central axis of the tower-shaped spring 22 relative to the geometric center of the magnetic field sensing front end 2 is matched with the radial offset of the magnetic sensitive iron ball 25, so as to achieve tilt response amplification while ensuring structural stability.

[0009] Furthermore, the tower-shaped spring 22 is a frustum-shaped helical spring structure with a gradually changing mean diameter along the axial direction. The mean diameter at the end near the spring support 21 is larger than the mean diameter at the end near the reflective platform 23; the mean diameter is the diameter of the circle formed by the center line of the spring coil. This gives the tower-shaped spring 22 high overall stability in the axial direction and makes it easy to drive the reflective platform 23 to produce controllable tilting deformation under eccentric load, thus balancing structural stability and sensing sensitivity.

[0010] Furthermore, the ratio of the lower end mid-diameter to the upper end mid-diameter of the tower-shaped spring 22 is in the range of 7 / 9 to 1 / 2.

[0011] Furthermore, the tower-shaped spring 22 is made of four to eight spring wires wound together, with a diameter of 2 μm for each spring wire and a winding angle of 270°; the lower end of the tower-shaped spring 22 has a median diameter of 90 μm, the upper end has a median diameter of 70 μm, and the height is 60 μm.

[0012] Furthermore, the reflective platform 23 has a thickness of 3 μm and has multiple through holes 231 extending through its upper and lower surfaces. The through holes 231 are used to drain the developing solution during the developing process, so as to reduce the influence of the surface tension of the developing solution on the reflective platform and the elastic support structure.

[0013] Furthermore, the height of the spring support 21 is 15 μm; the thickness of the reflective platform 23 is 3 μm; and the height of the iron ball bracket 24 is 30 μm.

[0014] On the other hand, the present invention also provides a method for fabricating a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity, comprising the following steps:

[0015] S1. After flattening and cleaning the end face of the single-mode fiber, fix it on the substrate and coat its end face with photoresist.

[0016] S2. Using a femtosecond laser two-photon polymerization processing system, the photoresist is scanned and exposed according to a preset three-dimensional structural model, so that it is polymerized to form a microstructure containing a spring support, a tower-shaped spring, a reflective platform and an iron ball support. Position correction is performed before or during processing to ensure that the central axis of the tower-shaped spring is offset by a preset amount relative to the core axis of the single-mode fiber.

[0017] S3. Perform development processing on the magnetic field sensing front end to remove unpolymerized photoresist, and obtain the magnetic field sensing front end integrated on the end face of the single-mode fiber; fix the magnetic sensitive iron ball on the iron ball support.

[0018] Furthermore, in step S1, the photoresist is SU-8 2005 photoresist; pre-baking and post-baking processes are performed before and after step S2 respectively to improve the structural stability and curing degree of the magnetic field sensing front end 2.

[0019] Furthermore, the heating temperature for the pre-baking and post-baking processes is 80–100°C, and the heating time is 3–4 hours, to adapt to the needs of different structural dimensions and processing conditions.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] 1. Unlike traditional fiber optic Fabry-Perot magnetic field sensing structures that primarily respond to axial translation of the reflective end face, this invention introduces eccentrically arranged magnetic sensing units and elastic support structures at the fiber end face, causing the reflective end face to preferentially undergo tilting deformation under the action of an external magnetic field. This results in a more significant spectral response under the same magnetic field disturbance conditions, which is beneficial for improving magnetic field detection sensitivity.

[0022] 2. Compared with end-face sensors that use equal-diameter springs or simple cantilever structures, this invention uses a tower-shaped helical spring with a gradually changing median diameter along the axial direction as an elastic support unit. While ensuring the overall structural stability, it enhances the angular response capability of the reflecting end face to eccentric loads, which is beneficial to improving the consistency and repeatability of the sensing response.

[0023] 3. Compared with existing fiber end-face microstructure fabrication schemes, this invention introduces a through-hole structure on the reflective end face to facilitate the discharge of the developer, reduce the adverse effects of liquid surface tension on the microscale elastic structure during the development process, and improve the stability and success rate of fiber end-face three-dimensional microstructure fabrication. At the same time, the sensing structure can be integrated into the fiber end face through femtosecond laser two-photon polymerization technology, resulting in a compact structure suitable for miniaturization and integrated applications. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0025] Figure 1 This is a schematic diagram of the first structure of a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity according to the present invention.

[0026] Figure 2 This is a schematic diagram of the second structure of a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity according to the present invention.

[0027] Figure 3 This is a schematic diagram illustrating the reflection interference spectrum variation of a single-mode fiber magnetic field sensor based on a Fabry-Perot cavity under different magnetic induction intensities according to the present invention.

[0028] Figure 4 This is a schematic diagram showing the relationship between the magnetic induction intensity and the wavelength response of the interference peak of a single-mode fiber magnetic field sensor based on a Fabry-Perot cavity according to the present invention.

[0029] Figure 5 This is a schematic diagram of the magnetic field calibration results of a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity according to the present invention.

[0030] In the figure: 1. Single-mode fiber; 2. Magnetic field sensing front end; 21. Spring support base; 22. Tower-shaped spring; 23. Reflection platform; 24. Iron ball bracket; 25. Magnetic sensitive iron ball; 231. Through hole. Detailed Implementation

[0031] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0032] Example 1

[0033] like Figure 1 and Figure 2As shown, this embodiment provides a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity. The magnetic field sensor includes a single-mode fiber 1 and a magnetic field sensing front-end 2 disposed on the end face of the single-mode fiber 1. The magnetic field sensing front-end 2 is fixed to the end face of the single-mode fiber 1 and is used to generate a Fabry-Perot cavity optical path change that can be demodulated optically under the action of an external magnetic field, thereby realizing the detection of the magnetic field signal. The magnetic field sensing front-end 2 includes a spring support 21, a tower-shaped helical spring 22, a reflective platform 23, an iron ball support 24, and a magnetically sensitive iron ball 25. The spring support 21 is fixedly disposed on the end face of the single-mode fiber 1, serving as a mechanical connection and structural support between the magnetic field sensing front-end 2 and the single-mode fiber 1. In this embodiment, the height of the spring support 21 is 15 μm. The tower-shaped helical spring 22 is disposed between the spring support 21 and the reflective platform 23, providing elastic support for the reflective platform 23. The tower-shaped helical spring 22 is a helical elastic structure with a gradually changing median diameter, exhibiting a tower-like geometric shape that gradually contracts from bottom to top. In this embodiment, the tower-shaped helical spring 22 is formed by winding four spring wires, each with a diameter of 2 μm and a winding angle of 270°. The lower end of the tower-shaped helical spring 22 has a diameter around its axis of 90 μm, the upper end has a diameter around its axis of 70 μm, and the spring section has a height of 60 μm. By adopting a tower-shaped helical spring structure with a gradually changing mid-diameter, the elastic support stiffness can be effectively reduced while maintaining the overall structural stability. Furthermore, it facilitates the reflection platform 23 to generate a response mode dominated by tilting deformation under external forces, thereby enhancing the optical path modulation capability of the Fabry-Perot cavity.

[0034] The reflective platform 23 is disposed at the upper end of the tower-shaped helical spring 22, opposite to the end face of the single-mode fiber 1, and together they constitute a Fabry-Perot cavity structure. The reflective platform 23 serves as the reflective end face of the Fabry-Perot cavity. When it shifts or tilts relative to the end face of the single-mode fiber 1, it causes changes in the cavity length and the effective optical path within the cavity, resulting in a drift in the reflection interference spectrum. In this embodiment, the thickness of the reflective platform 23 is 3 μm. To reduce the adverse effects of the surface tension of the developing solution on the microstructure during the developing process, this embodiment provides through holes 231 on the reflective platform 23. The through holes 231 are used to drain or release the developing solution above and below the reflective platform 23 during the developing process, reducing the pulling force and additional stress caused by liquid retention. In this embodiment, there are four through holes 231, each with a diameter of 8 μm.

[0035] The iron ball support 24 is disposed on the upper surface of the reflective platform 23, used to fix the magnetically sensitive iron ball 25 above the reflective platform 23 and realize the mechanical transmission of magnetic force to the reflective platform 23. In this embodiment, the height of the iron ball support 24 is 30 μm. The magnetically sensitive iron ball 25 is fixedly disposed on the top of the iron ball support 24 with UV-curable adhesive, used to generate a magnetic force response under the action of an external magnetic field. In this embodiment, the material of the magnetically sensitive iron ball 25 is a ferromagnetic material, and its diameter can be selected according to application requirements.

[0036] In this embodiment, the central axis of the tower-shaped helical spring 22 is eccentrically arranged relative to the core axis of the single-mode fiber 1, and the iron ball support 24 is positioned eccentrically on the reflective platform 23, aligning the magnetically sensitive iron ball 25 with the core axis of the single-mode fiber 1 in the initial state. In this embodiment, the eccentricity is 10 μm. Through the above-mentioned eccentric structure design, when an external magnetic field is applied to the magnetically sensitive iron ball 25, the magnetically sensitive iron ball 25 applies an eccentric load to the reflective platform 23 through the iron ball support 24 under the action of magnetic force, causing the reflective platform 23 to preferentially tilt under the elastic constraint of the tower-shaped helical spring 22. Compared with the structural response mode dominated by axial translation, this embodiment, through the mechanical amplification mechanism of "eccentric load - platform tilt", makes the optical path change of the Fabry-Perot cavity more significant, thereby obtaining a more obvious spectral response under a smaller magnetic force and improving the magnetic field sensing sensitivity.

[0037] Example 2

[0038] This embodiment provides a method for fabricating the above-mentioned Fabry-Perot cavity-based single-mode fiber optic magnetic field sensor, specifically including the following steps:

[0039] S1. The end face of the single-mode fiber 1 is trimmed to ensure it is flat, and then cleaned with alcohol. The single-mode fiber 1 is then fixed onto a cover glass slide, and photoresist is applied to the end face of the single-mode fiber 1. In this embodiment, the photoresist is SU-8 2005.

[0040] S2. Place the photoresist-coated sample in a 90 ℃ environment and heat for 3 hours for pre-baking treatment to remove the solvent in the photoresist and improve its adhesion stability.

[0041] S3. The pre-baked sample is placed on a femtosecond laser two-photon polymerization processing platform. Processing parameters are set, and scanning processing is performed according to a preset three-dimensional structural path, causing the photoresist to polymerize according to the designed structure, forming a magnetic field sensing front end 2, including microstructures such as a spring support 21, a tower-shaped helical spring 22, a reflective platform 23, and an iron ball support 24. Before processing, the laser focus and spatial position are corrected by adjusting the platform position, so that the central axis of the tower-shaped helical spring 22 is arranged in a preset eccentric relationship with respect to the core axis of the single-mode fiber 1.

[0042] S4. After processing, the sample is heated in a 90 ℃ environment for 3 hours for post-baking treatment to further improve the crosslinking degree and mechanical stability of the polymer structure.

[0043] S5. The post-baked sample is developed to remove unpolymerized photoresist, resulting in the formed magnetic field sensing front end 2. During development, the through-hole 231 on the reflective platform 23 is used for developer drainage, thereby reducing the adverse effects of surface tension on the microstructure and improving the fabrication success rate. Finally, the magnetically sensitive iron ball is fixed to the iron ball support.

[0044] Example 3 Sensor Performance Testing

[0045] In the specific operation of magnetic field detection, the light source and spectral demodulation device are connected to the single-mode fiber 1. Detection light is emitted to the magnetic field sensing front end 2 through the single-mode fiber 1, and the Fabry-Perot cavity reflection interference spectrum formed by the end face of the single-mode fiber 1 and the reflection platform 23 is collected. When the magnetic field sensor is placed in the environment of the magnetic field to be measured, an external magnetic field acts on the magnetically sensitive iron ball 25. Under the action of the magnetic force, the magnetically sensitive iron ball 25 applies an eccentric load to the reflection platform 23 through the iron ball support 24, causing the reflection platform 23 to preferentially tilt and deform under the elastic constraint of the tower-shaped helical spring 22.

[0046] Unlike the Fabry-Perot cavity structure, which primarily responds to axial translation of the reflecting end face, when the reflecting platform 23 tilts, the equivalent optical path changes at different radial positions within the cavity become inconsistent. This introduces both cavity length variation and radially uneven phase modulation effects into the Fabry-Perot cavity, resulting in a more significant wavelength drift response of the reflected interference spectrum to minute mechanical disturbances. Compared to structures that rely solely on axial compression or stretching to change the cavity length, this "eccentric load-tilt response" working mode can produce a more pronounced spectral change amplitude under the same magnetic field, which is beneficial for detecting weak magnetic fields.

[0047] In this embodiment, the magnetic field sensitivity S of the Fabry-Perot cavity-based single-mode fiber optic magnetic field sensor can be defined as the ratio of the wavelength shift in the reflected interference spectrum to the change in the magnetic field, i.e.

[0048]

[0049] in, This represents the wavelength shift corresponding to the interference peak (or valley) in the reflection interference spectrum. This represents the change in the strength of the applied magnetic field.

[0050] As the applied magnetic field strength gradually changes, reflection interference spectra under different magnetic induction intensities are collected using a spectral demodulation device, such as... Figure 3As shown, with the increase of magnetic induction intensity, the overall reflection interference spectrum undergoes a regular shift, and the position of the interference peak moves towards longer wavelengths as the magnetic field strengthens. Figure 3 It can be intuitively observed that the position of the interference peak exhibits a continuous and monotonic variation trend under different magnetic induction intensities, indicating that the sensing structure has stable spectral response characteristics to magnetic field disturbances. Furthermore, the center wavelength of the interference peak corresponding to each magnetic induction intensity is extracted, and the relationship curve between magnetic induction intensity and interference peak wavelength is plotted, as shown below. Figure 4 As shown. By Figure 4 It is evident that within the measured magnetic flux density range, the sensor response curve exhibits a monotonically increasing trend and can be divided into low-sensitivity and high-sensitivity regions. In the high-sensitivity region, the wavelength changes more significantly with magnetic flux density. Based on this, linear calibration analysis was performed on the experimental data within the high-sensitivity region, such as... Figure 5 As shown, by plotting magnetic flux density on the x-axis and interference peak wavelength on the y-axis, a linear fit is performed on the experimental data to obtain a quantitative relationship between the interference peak wavelength and magnetic flux density. In this calibration process, the slope of the linear fit curve characterizes the response coefficient of the reflected interference spectrum wavelength to the change in magnetic field, i.e., the magnetic field sensitivity. Based on the linear fit results, the magnetic field sensitivity of this Fabry-Perot cavity-based single-mode fiber optic magnetic field sensor is calculated to be approximately 0.257 nm / mT.

[0051] Experimental results show that the mechanical amplification and tilt response mechanism formed by combining the eccentric structure with the tower-shaped helical spring in this embodiment can obtain significant and approximately linear spectral response changes under small magnetic field disturbances, making it suitable for high-sensitivity detection of weak magnetic fields.

[0052] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0053] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0054] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity, characterized in that, The device includes a single-mode optical fiber (1) and a magnetic field sensing front end (2) disposed on one end face of the single-mode optical fiber (1); the magnetic field sensing front end (2) includes a spring support (21), a tower-shaped spring (22), a reflective platform (23), an iron ball bracket (24) disposed on the reflective platform (23), and a magnetically sensitive iron ball (25) disposed on the iron ball bracket (24); the spring support (21) is fixed to the end face of the single-mode optical fiber (1), and the reflective platform (23) is connected to the spring support by the tower-shaped spring (22). The base (21) is connected and is set opposite to the end face of the single-mode fiber (1). The reflection platform (23) and the end face of the single-mode fiber (1) together constitute the Fabry-Perot cavity structure. When an external magnetic field is applied to the magnetic sensitive iron ball (25), the magnetic sensitive iron ball (25) drives the reflection platform (23) to tilt and deform under the elastic constraint of the tower spring (22) through the iron ball support (24) under the action of magnetic force, thereby causing changes in the cavity length and effective optical path of the Fabry-Perot cavity. The magnetic field measurement is realized by detecting the change in the reflection interference spectrum.

2. The single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The central axis of the tower spring (22) is eccentrically arranged relative to the core axis of the single-mode fiber (1). The iron ball support (24) is set on the reflection platform (23) at a position corresponding to the eccentric arrangement, so that the magnetically sensitive iron ball (25) is aligned with the core axis of the single-mode fiber (1) in the initial state. Through the above-mentioned eccentric arrangement, the magnetically sensitive iron ball (25) applies a non-axial load to the reflection platform (23) under the action of the magnetic field, so that the reflection platform (23) preferentially produces tilt deformation under the elastic support of the tower spring (22). Compared with the axial translation response, a larger equivalent displacement can be obtained, which increases the spectral modulation amplitude of the Fabry-Perot cavity, thereby achieving sensitization.

3. The single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The offset of the eccentric arrangement is referenced to the geometric center of the magnetic field sensing front end (2). The radial offset of the magnetic sensitive iron ball (25) relative to the geometric center of the magnetic field sensing front end (2) is 5 μm to 15 μm. The radial offset of the central axis of the tower spring (22) relative to the geometric center of the magnetic field sensing front end (2) is matched with the radial offset of the magnetic sensitive iron ball (25) to achieve tilt response amplification while ensuring structural stability.

4. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The tower-shaped spring (22) is a frustum-shaped helical spring structure with a gradually changing mid-diameter along the axial direction. The mid-diameter of the end near the spring support (21) is larger than the mid-diameter of the end near the reflective platform (23). The mid-diameter is the diameter of the circle formed by the center line of the spring coil. This gives the tower-shaped spring (22) high overall stability in the axial direction and makes it easy to drive the reflective platform (23) to produce controllable tilting deformation under eccentric load, thus balancing structural stability and sensing sensitivity.

5. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The ratio of the lower end mid-diameter of the tower-shaped spring (22) to the upper end mid-diameter is in the range of 7 / 9 to 1 / 2.

6. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The tower-shaped spring (22) is made of four to eight spring wires, with a diameter of 2 μm for each spring wire and a winding angle of 270°. The lower end of the tower-shaped spring (22) has a median diameter of 90 μm, the upper end has a median diameter of 70 μm, and the height is 60 μm.

7. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The reflective platform (23) has a thickness of 3 μm and has multiple through holes (231) that penetrate its upper and lower surfaces. The through holes (231) are used to drain the developer during the development process to reduce the influence of the surface tension of the developer on the reflective platform and the elastic support structure.

8. A single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, The height of the spring support (21) is 15 μm; the thickness of the reflective platform (23) is 3 μm; and the height of the iron ball bracket (24) is 30 μm.

9. The method for fabricating a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 1, characterized in that, Includes the following steps: S1. After flattening and cleaning the end face of the single-mode fiber, fix it on the substrate and coat its end face with photoresist. S2. Using a femtosecond laser two-photon polymerization processing system, the photoresist is scanned and exposed according to a preset three-dimensional structural model, so that it is polymerized to form a microstructure containing a spring support, a tower-shaped spring, a reflective platform and an iron ball support. Position correction is performed before or during processing to ensure that the central axis of the tower-shaped spring is offset by a preset amount relative to the core axis of the single-mode fiber. S3. Perform development processing on the magnetic field sensing front end to remove unpolymerized photoresist, and obtain the magnetic field sensing front end integrated on the end face of the single-mode fiber; fix the magnetic sensitive iron ball on the iron ball support.

10. The method for fabricating a single-mode fiber optic magnetic field sensor based on a Fabry-Perot cavity as described in claim 9, characterized in that, In step S1, the photoresist is SU-8 2005 photoresist; Pre-baking and post-baking processes are performed before and after step S2 to improve the structural stability and curing degree of the magnetic field sensing front end; the heating temperature for pre-baking and post-baking is 80–100℃, and the heating time is 3–4 hours.

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