A method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid

Abstract

The present invention discloses a method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid, comprising the following steps: a first step of manufacturing a Fabry-Perot interferometer 1 (FPI1); a second step of manufacturing a capillary tube + single-mode optical fiber fusion splicing structure; and a third step of manufacturing a Fabry-Perot interferometer 2 (FPI2). The present invention utilizes ferromagnetic Fe3O4 nanoparticle material to design an FPI2 that is relatively sensitive to a magnetic field. Then, utilizing the principle of an optical vernier effect, a cascade structure of FPI1 and FPI2 is constructed to generate a vernier effect, thereby forming a vernier effect magnetic field sensor. The vernier effect magnetic field sensor can further amplify the magnetic field sensitivity, thereby obtaining an optical fiber magnetic field sensor that is extremely sensitive to magnetic field intensity. The optical fiber magnetic field sensor manufactured using the method of the present invention has a compact structure, is easy to manufacture and package, and has high sensitivity.

Description

Technical Field

[0001] The present invention relates to the technical field of high-sensitivity and high-precision sensors, and in particular to a method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid. Background Art

[0002] Magnetic field sensors are widely used for magnetic field measurement in aerospace, geophysical research, controlled nuclear fusion, marine resource exploration, industrial testing, and other fields. Unlike traditional magnetic field sensors, fiber optic magnetic field sensors offer advantages such as compactness, immunity to electromagnetic interference, corrosion resistance, and high sensitivity, making them widely used for field detection in the field. In recent years, fiber optic magnetic field sensors with various structures have been proposed based on the magnetic field sensing principle, including fiber Bragg grating magnetic field sensors, surface plasmon magnetic field sensors, whispering gallery mode magnetic field sensors, antiresonant optical waveguide mode magnetic field sensors, and fiber interferometer magnetic field sensors. Each of these magnetic field sensors has its own unique characteristics and varying levels of sensitivity.

[0003] Fiber-optic magnetic field sensors based on a combination of fiber interferometers and ferrofluids are currently gaining popularity. These sensors have seen rapid development due to their compact design and ease of integration. Examples include high-sensitivity magnetic field sensors constructed by filling the FP cavity of a Fabry-Perot interferometer (FPI); magnetic field sensors using ferrofluid coated on the surface of a Mach-Zehnder interferometer (MZI); vector magnetic field sensors based on U-bend optical fiber and ferrofluid; and magnetic field sensors using tapered optical fiber coated with ferrofluid. However, in most of these ferrofluid-based magnetic field sensors, the change in the ferrofluid's refractive index caused by an external magnetic field is typically small, resulting in a small shift in the sensor's spectral resonance wavelength. Consequently, the resulting magnetic field sensitivity is typically low. Furthermore, ferrofluid is a water-soluble liquid, and improper packaging can render the ferrofluid ineffective. Therefore, effectively packaging it with a fiber-optic interferometer presents a significant challenge.

[0004] Another commonly used magnetic field sensor structure combines the magnetostrictive material Terfenol-D with a grating or interferometer. Examples include magnetic field sensors combining FPI with Terfenol-D; magnetic field sensors based on the whispering gallery mode of hollow microbubble resonators and Terfenol-D; and whispering gallery mode magnetic field sensors based on double-tailed silica microspheres and Terfenol-D. While Terfenol-D magnetostrictive materials can effectively measure magnetic fields, the sensor structures are typically large, making them unsuitable for magnetic field measurements in confined spaces.

[0005] In recent years, researchers have applied the optical Vernier effect to fiber-optic magnetic field sensors, further improving their sensitivity. Examples include a high-sensitivity Vernier magnetic field sensor based on a cascade of two MZIs and a magnetic fluid combination; a high-sensitivity Vernier magnetic field sensor based on a parallel connection of two FPIs and a magnetic fluid combination; a high-sensitivity Vernier magnetic field sensor based on a cantilever FPI combined with Terfenol-D material; and an ultra-sensitive axial magnetic field sensor based on three-reflection surface interferometry and the harmonic Vernier effect. While these sensors achieve high sensitivity, they also suffer from drawbacks such as complex structure, difficulty in fabrication, lack of compactness, and large size. Summary of the Invention

[0006] The purpose of the present invention is to address the above situation and provide a method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid. The magnetic field sensor manufactured by this method has a compact structure, is easy to manufacture and package, and has high sensitivity.

[0007] The specific solution of the present invention is: a method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid, comprising the following steps:

[0008] The first step is to make a Fabry-Perot interferometer 1 (FPI1): take a small quartz capillary and burn off the protective layer on its surface with fire. The inner diameter is 100 microns and the outer diameter is 125 microns. Then, two sections of single-mode optical fiber are fused to the two ends of the capillary to form FPI1. FPI1 is formed by the interference of reflected light from the two reflection surfaces M1 and M2. The FP cavity of FPI1 is an air cavity with a length of L1.

[0009] The second step is to make a capillary tube + single-mode optical fiber fusion splice structure: First, the Fe3O4 nanoparticles and ultraviolet glue are fully mixed to form a uniform mixture for use; a small capillary tube with a length of 1 mm is taken, the surface protective layer is burned off, the inner diameter is 100 microns, and the outer diameter is 125 microns. It is fused together with a section of single-mode optical fiber to form a capillary tube + single-mode optical fiber structure; then, the prepared Fe3O4 nanoparticle + ultraviolet glue mixture is filled into the capillary tube of the capillary tube + single-mode optical fiber structure until the capillary tube is completely filled; finally, the capillary tube is irradiated with an ultraviolet lamp for about one minute until the Fe3O4 nanoparticle + ultraviolet glue mixture in the capillary tube is completely solidified and dried, forming a capillary tube + single-mode optical fiber structure;

[0010] The third step is to fabricate Fabry-Perot interferometer 2 (FPI2): insert the capillary segment of the capillary tube + single-mode fiber structure into a large quartz capillary with an inner diameter of 150 microns and glue them together at the tail end with ultraviolet glue. Then, insert the single-mode fiber end of the fabricated FPI1 into the large capillary from the other end. Use an optical spectrum analyzer to observe the reflection spectrum of the structure. If the requirements are met, glue the single-mode fiber and the large capillary together at the tail end with ultraviolet glue, finally forming FPI2. The FP cavity of FPI2 is also an air cavity with a length of L2.

[0011] Furthermore, the FPI1 and FPI2 described in the present invention are both extrinsic low-fineness FPIs.

[0012] Furthermore, in the second step of the present invention, when the Fe3O4 nanoparticles+ultraviolet glue mixture is filled into the capillary, the capillary should be placed vertically upward and the outer wall of the capillary should be wiped clean.

[0013] Furthermore, in the present invention, the size of the Fe3O4 nanoparticles in the second step is 8 to 12 nanometers; the UV glue used is an optical UV glue with a refractive index of 1.57.

[0014] Furthermore, FPI2 in the third step of the present invention is formed by interference of reflected light between the contact surface M3 between the single-mode optical fiber inserted into the large capillary side of FPI1 and the air and the contact surface M4 between Fe3O4 nanoparticles + ultraviolet glue and the air.

[0015] Furthermore, the FP cavity length L in FPI1 and FPI2 of the present invention is I Close to but not equal to L2.

[0016] The present invention uses ferromagnetic Fe3O4 nanoparticles to design an FPI2 that is relatively sensitive to magnetic fields. Then, using the principle of optical vernier effect, a cascade structure of FPI1 and FPI2 is constructed to produce a vernier effect, forming a vernier effect magnetic field sensor. This can further amplify the magnetic field sensitivity, resulting in an optical fiber magnetic field sensor that is extremely sensitive to magnetic field intensity. The optical fiber magnetic field sensor manufactured using the method of the present invention has a compact structure, is easy to manufacture and package, and has high sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The figure is a schematic diagram of the structure of the optical fiber magnetic field sensor produced by the method of the present invention.

[0018] In the figure: 1 and 7—single-mode optical fibers, 2 and 6—small capillaries, 3—UV glue, 4—large capillary, 5—Fe3O4 nanoparticles + UV glue mixture. DETAILED DESCRIPTION

[0019] The technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. It is obvious that the embodiments described are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention. In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the drawings, or the directions or positional relationships in which the invented product is usually placed when in use. They are only for the convenience of describing the present invention or simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed or operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0020] In the description of the present invention, it should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," and "connected" should be understood broadly. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0021] See also Figure 1 The present invention is a method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid, comprising the following steps:

[0022] The first step is to make a Fabry-Perot interferometer 1, or FPI1: Take a small quartz capillary, or capillary 2, and burn off the protective layer on its surface. The inner diameter is 100 microns and the outer diameter is 125 microns. Then, two sections of single-mode optical fiber 1 are fused to the ends of the capillary to form an FPI1. FPI1 is formed by the interference of reflected light from two reflective surfaces, M1 and M2. The FP cavity of FPI1 is an air cavity with a length of L1.

[0023] The second step is to make a capillary + single-mode optical fiber fusion splicing structure: First, the Fe3O4 nanoparticles are fully mixed with the UV glue, as shown in the figure, which is the Fe3O4 nanoparticle + UV glue mixture 5, to form a uniform mixture for standby use; take a small capillary 6 with a length of 1 mm, burn off the surface protective layer, and the inner diameter is 100 microns and the outer diameter is 125 microns. It is fused together with a section of single-mode optical fiber 7 to form a capillary + single-mode optical fiber structure; then, the configured Fe3O4 nanoparticle + UV glue mixture is filled into the capillary of the capillary + single-mode optical fiber structure, and the capillary is fused. The capillary is completely filled; finally, an ultraviolet lamp is used to irradiate the capillary for about one minute, and the Fe3O4 nanoparticles + ultraviolet glue mixed colloid in the capillary is completely solidified and dried to form a capillary + single-mode optical fiber structure; further, in the second step of the present invention, when the Fe3O4 nanoparticles + ultraviolet glue mixture is filled into the capillary, the capillary should be placed vertically upward, and the outer wall of the capillary should be wiped clean; further, the size of the Fe3O4 nanoparticles in the second step of the present invention is 8 to 12 nanometers; the ultraviolet glue used is an optical ultraviolet glue with a refractive index of 1.57;

[0024] The third step is to fabricate a Fabry-Perot interferometer 2, or FPI2. The capillary segment of the fabricated capillary tube + single-mode fiber structure is inserted into a large quartz capillary tube with an inner diameter of 150 microns (4). The two are glued together at their tail ends with ultraviolet glue (3). Next, the single-mode fiber end of the fabricated FPI1 is inserted into the large capillary tube from the other end. The reflection spectrum of the structure is observed using a spectrum analyzer. If the reflection spectrum meets the requirements, the single-mode fiber and the large capillary tube are glued together at their tail ends with ultraviolet glue (3). Finally, an FPI2 is formed. The FP cavity of FPI2 is also an air cavity with a length of L2. Furthermore, the FPI2 in the third step of the present invention is formed by the interference of reflected light between the contact surface M3 between the single-mode fiber inserted into the large capillary side of FPI1 and the air, and the contact surface M4 between the Fe3O4 nanoparticles + ultraviolet glue and the air.

[0025] Furthermore, the FPI1 and FPI2 described in this embodiment are both extrinsic low-fineness FPIs; further, the FP cavity length L in FPI1 and FPI2 of the present invention is I Close to but not equal to L2.

[0026] The working principle of the sensor made by the present invention is as follows:

[0027] exist Figure 1 In the structure of , based on the FPI theory, it can be deduced that the trough wavelengths in the reflection spectra of FPI1 and FPI2 are:

[0028] , , (m =1, 2, 3, …) (1)

[0029] here, and They represent the trough wavelengths of the reflection spectra of FPI1 and FPI2, respectively; m is a positive integer; n1 and n2 are the refractive indices of the air in the FP cavities of FPI1 and FPI2, respectively; L1 and L2 are the cavity lengths of the FP cavities of FPI1 and FPI2, respectively.

[0030] Since the reflectance spectra of FPI1 and FPI2 are both periodic waveforms that approximate sine waves, the difference between the two trough wavelengths in their spectra is defined as their free spectral range (FSR). Therefore, the approximate calculation formulas for the FSR of FPI1 and FPI2 are:

[0031] , (2)

[0032] FPI1 is an interferometer formed by the reflection surface between a single-mode optical fiber and air. Under the action of an external magnetic field, its refractive index n1 and cavity length L1 do not change, so it is insensitive to the external magnetic field.

[0033] One of the FPI2's reflective surfaces is composed of a colloidal Fe3O4 nanoparticle. When a magnetic field is applied to both ends of the FPI2, the refractive index of the ferromagnetic Fe3O4 nanoparticles changes, and the length of the Fe3O4 nanoparticle colloidal particles expands and contracts along the magnetic field. This causes the optical path difference of light propagating through the FPI2 to change, resulting in a linear shift in the trough wavelength in the FPI2's reflection spectrum. By demodulating the FPI2's trough wavelength shift, the external magnetic field can be measured. The FPI2's magnetic field sensitivity can be expressed as follows:

[0034] (3)

[0035] In addition, when FPI1 and FPI2 are cascaded together, since the FSRs of FPI1 and FPI2 are similar but not equal, according to the principle of optical vernier effect, their spectra will be superimposed to form a large periodic spectral envelope. If the trough of the spectral envelope is used for measurement, the sensitivity will be amplified. This is the so-called optical vernier effect. In the vernier effect sensor formed by the cascade of FPI1 and FPI2, FPI2, which is sensitive to magnetic fields, acts as the sensing element, and FPI1, which is insensitive to magnetic fields, acts as the reference element. According to the principle of optical vernier effect, the spectral envelope can be deduced. as follows:

[0036] (4)

[0037] Therefore, using the optical vernier effect amplification principle, when we examine the change in the trough of the spectral envelope with the magnetic field, we find that the magnetic field causes a small shift in the wavelength of the FPI2 spectrum trough, which results in the trough drift of the spectral envelope being amplified by M times. M is the amplification factor of the wavelength shift, which is also the amplification factor of the magnetic field sensitivity. It can be defined as:

[0038] (5)

[0039] in, is the magnetic field sensitivity of the Vernier effect sensor.

[0040] In summary, we used ferromagnetic Fe3O4 nanoparticles to design an FPI2 that is sensitive to magnetic fields. Then, using the principle of optical vernier effect, we constructed a cascade structure of FPI1 and FPI2 to produce a vernier effect, forming a vernier effect magnetic field sensor. This can further amplify the magnetic field sensitivity and obtain a fiber optic magnetic field sensor that is extremely sensitive to magnetic field strength.

[0041] The present invention uses ferromagnetic Fe3O4 nanoparticles to design an FPI2 that is relatively sensitive to magnetic fields. Then, using the principle of optical vernier effect, a cascade structure of FPI1 and FPI2 is constructed to produce a vernier effect, forming a vernier effect magnetic field sensor. This can further amplify the magnetic field sensitivity, resulting in an optical fiber magnetic field sensor that is extremely sensitive to magnetic field intensity. The optical fiber magnetic field sensor manufactured using the method of the present invention has a compact structure, is easy to manufacture and package, and has high sensitivity.

Claims

1. A method for manufacturing an optical fiber magnetic field sensor composed of Fe3O4 ferromagnetic fluid, characterized in that: The following steps are involved: The first step is to make a Fabry-Perot interferometer 1 (FPI1): take a small quartz capillary and burn off the protective layer on its surface with fire. The inner diameter is 100 microns and the outer diameter is 125 microns. Then, two sections of single-mode optical fiber are fused to the two ends of the capillary to form FPI1. FPI1 is formed by the interference of reflected light from the two reflection surfaces M1 and M2. The FP cavity of FPI1 is an air cavity with a length of L1. The second step is to make a capillary tube + single-mode optical fiber fusion splice structure: First, the Fe3O4 nanoparticles and ultraviolet glue are fully mixed to form a uniform mixture for use; a small capillary tube with a length of 1 mm is taken, the surface protective layer is burned off, the inner diameter is 100 microns, and the outer diameter is 125 microns. It is fused together with a section of single-mode optical fiber to form a capillary tube + single-mode optical fiber structure; then, the prepared Fe3O4 nanoparticle + ultraviolet glue mixture is filled into the capillary tube of the capillary tube + single-mode optical fiber structure until the capillary tube is completely filled; finally, the capillary tube is irradiated with an ultraviolet lamp for about one minute until the Fe3O4 nanoparticle + ultraviolet glue mixture in the capillary tube is completely solidified and dried, forming a capillary tube + single-mode optical fiber structure; The third step is to fabricate Fabry-Perot interferometer 2 (FPI2): insert the capillary segment of the capillary tube + single-mode fiber structure into a large quartz capillary with an inner diameter of 150 microns and glue them together at the tail end with ultraviolet glue. Then, insert the single-mode fiber end of the fabricated FPI1 into the large capillary from the other end. Use an optical spectrum analyzer to observe the reflection spectrum of the structure. If the requirements are met, glue the single-mode fiber and the large capillary together at the tail end with ultraviolet glue, finally forming FPI2. The FP cavity of FPI2 is also an air cavity with a length of L2.

2. The method for manufacturing a fiber optic magnetic field sensor composed of Fe3O4 ferromagnetic fluid according to claim 1, characterized in that: The FPI1 and FPI2 are both extrinsic low-fineness FPIs.

3. The method for manufacturing a fiber optic magnetic field sensor composed of Fe3O4 ferromagnetic fluid according to claim 1, characterized in that: In the second step, when the Fe3O4 nanoparticles + UV glue mixture is filled into the capillary, the capillary should be placed vertically upward and the outer wall of the capillary should be wiped clean.

4. The method for manufacturing a fiber optic magnetic field sensor composed of Fe3O4 ferromagnetic fluid according to claim 1, characterized in that: The size of the Fe3O4 nanoparticles in the second step is 8 to 12 nanometers; the UV glue used is an optical UV glue with a refractive index of 1.

57.

5. The method for manufacturing a fiber optic magnetic field sensor composed of Fe3O4 ferromagnetic fluid according to claim 1, characterized in that: The FPI2 in the third step is formed by the interference of the reflected light between the contact surface M3 between the single-mode optical fiber inserted into the large capillary side of FPI1 and the air and the contact surface M4 between Fe3O4 nanoparticles + ultraviolet glue and the air.

6. The method for manufacturing a fiber optic magnetic field sensor composed of Fe3O4 ferromagnetic fluid according to claim 1, characterized in that: FP cavity length L in FPI1 and FPI2 I Close to but not equal to L2.

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