A method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI

Abstract

The present invention discloses a method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI. The method comprises the following steps: a first step of manufacturing a Fabry-Perot interferometer 1 (FPI1); a second step of uniformly coating the Terfenol-D powder on the surface of a large capillary of the FPI1 to form the FPI1 that is sensitive to a magnetic field; a third step of manufacturing a Fabry-Perot interferometer 2 (FPI2); and a fourth step of fusing the single-mode optical fiber ends of the FPI1 and the FPI2 together using a fusion splicer, with the two FPIs spaced 5 mm to 1 cm apart. The two FPIs are cascaded to form a first harmonic vernier effect sensor. The method of the present invention absorbs the advantages of existing sensors and overcomes their disadvantages. By utilizing a quartz capillary, a single-mode optical fiber, Terfenol-D powder, epoxy resin glue, ultraviolet glue, the FPI, and the first harmonic vernier effect, a compact optical fiber magnetic field sensor can be manufactured, which is easy to manufacture, easy to package, and highly sensitive. The sensor is suitable for measuring magnetic fields in confined spaces and has excellent practical application value.

Description

Technical Field

[0001] The present invention relates to the technical field of high-precision sensor equipment manufacturing process methods, in particular to a method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI. Background Art

[0002] Accurately measuring magnetic fields is crucial in fields such as nuclear testing, ocean exploration, geophysics, and space exploration. Compared to traditional magnetic field sensors, fiber optic magnetic field sensors utilize a combination of optical fiber and magnetically sensitive materials. They offer advantages such as compact structure, ease of operation, immunity to electromagnetic interference, suitability for use in harsh environments, and high sensitivity. They have been widely used in field detection of magnetic fields.

[0003] Fiber-optic magnetic field sensors have been developed in a variety of configurations, depending on the detection principle and magnetically sensitive materials. Examples include fiber Bragg grating (FBG) magnetic field sensors, fiber surface plasmon (SP) magnetic field sensors, fiber whispering gallery mode magnetic field sensors, fiber antiresonant 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.

[0004] The first and most common fiber-optic magnetic field sensors are based on a combination of fiber-optic interferometers and ferrofluids. These sensors have experienced rapid development due to their compact structure 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 in which ferrofluid is wrapped around the surface of a Mach-Zehnder interferometer (MZI); and vector magnetic field sensors based on U-bend optical fibers and ferrofluid wrapping. In these ferrofluid-based magnetic field sensors, the change in the ferrofluid's refractive index caused by the external magnetic field is small, resulting in a small shift in the sensor's spectral resonance wavelength and, consequently, a generally low magnetic field sensitivity. Furthermore, ferrofluid is a water-soluble liquid, and improper packaging can render the ferrofluid ineffective. Therefore, effectively packaging it with the fiber-optic interferometer presents a challenging challenge.

[0005] The second common magnetic field sensor structure utilizes the magnetostrictive material Terfenol-D in combination with fiber Bragg gratings (FBGs) or fiber interferometers (FIBs). Examples include magnetic field sensors combining FPI with Terfenol-D, magnetic field sensors based on the whispering gallery mode of a hollow microbubble resonator and Terfenol-D, and magnetic field sensors combining Terfenol-D with fiber Bragg gratings (FBGs). While the magnetostrictive material Terfenol-D can effectively measure magnetic fields, the resulting sensor is typically large due to the large size of the Terfenol-D rods, making it unsuitable for magnetic field measurements in confined spaces.

[0006] In recent years, to improve the sensitivity of magnetic field sensors, researchers have also applied optical Vernier effects and harmonic Vernier effects to fiber-optic magnetic field sensors. For example, a high-sensitivity Vernier magnetic field sensor based on a cascade of two MZIs and a magnetic fluid combination has been developed; 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 magnetic field sensor based on a combination of three-reflection surface interferometry, the harmonic Vernier effect, and Terfenol-D material. While these sensors achieve extremely high sensitivity, they also suffer from structural complexity, difficulty in fabrication, and lack of compactness. Summary of the Invention

[0007] 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 Terfenol-D powder and FPI. This method can produce a magnetic field sensor with high sensitivity, large measurement range, small size, and suitable for magnetic field measurement in confined spaces.

[0008] The specific solution of the present invention is: a method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI, the method comprising the following steps:

[0009] The first step is to create a Fabry-Perot interferometer 1, or FPI1. A large quartz capillary is prepared and its protective coating is burned off. The inner diameter is 150 microns and the outer diameter is 360 microns. Two stripped, clean-end-cut single-mode optical fibers are inserted into the capillary from either end. The distance between the two single-mode fibers is adjusted. When the spectrum meets the requirements, the single-mode fibers are glued together at both ends of the capillary using UV glue. The capillary is then irradiated with UV light for one minute. The capillary and single-mode fibers are then glued together to form an FPI1. The FPI1 is formed by the interference of reflected light from two reflective surfaces, M1 and M2. The FP cavity of the FPI1 is an air cavity with a length of L1. In the FPI1 structure, the length between the bonding points of the two capillaries is L. The single-mode fibers inserted from both sides of the capillary form a cantilever beam structure within the capillary.

[0010] The second step is to evenly apply Terfenol-D powder to the surface of the large capillary of FPI1 to form an FPI1 that is sensitive to magnetic fields. Specifically, Terfenol-D powder and epoxy resin glue are first prepared in a powder-to-glue mass ratio of 95:5, that is, 95% Terfenol-D powder and 5% epoxy resin glue are mixed and stirred to form a uniform mixed colloid for later use; the prepared Terfenol-D mixed glue is then evenly applied to the outer surface of the large capillary in FPI1, covering the entire outer surface of the large capillary; the prepared structure is first left to air at room temperature for two hours, then placed in a drying oven at 40°C for eight hours, and finally left at room temperature for half a month to allow the coated material to completely dry;

[0011] The third step is to make a Fabry-Perot interferometer 2, or FPI2: take a small quartz capillary and burn off the protective layer on its surface with fire. Its inner diameter is 75 microns and the outer diameter is 125 microns. Then, use a fiber fusion splicer to fuse a section of single-mode optical fiber with its coating stripped and end face cut flat to the small quartz capillary. Use a fiber cleaver to cut the capillary to the designed length L2. Then, fuse another section of single-mode optical fiber with its coating stripped and end face flat to the other end of the small capillary. Finally, use a fiber cleaver to cut the end of the single-mode optical fiber into an oblique surface. This completes the production of FPI2. FPI2 is formed by the interference of reflected light from the two reflection surfaces M3 and M4. The FP cavity of FPI2 is also an air cavity with a length of L2.

[0012] In the fourth step, the single-mode optical fiber ends of FPI1 and FPI2 are fused together using a fusion splicer. The two FPIs are 5 mm to 1 cm apart. The two FPIs are cascaded to produce a first harmonic vernier effect sensor.

[0013] Furthermore, the FPI1 and FPI2 described in the present invention are both extrinsic low-fineness FPIs.

[0014] Furthermore, the diameter of the Terfenol-D powder in the second step of the present invention is 100 microns to 150 microns.

[0015] Furthermore, in the present invention, the thickness of the Terfenol-D mixed solution coating on the outer wall of the large capillary in the second step is maintained at 0.8 to 1.2 mm.

[0016] Furthermore, the FP cavity length in FPI1 and FPI2 of the present invention is L I and L2, L2 is approximately twice that of L1, and the reflectance spectra of the two FPIs have similar extinction ratios, and their free spectral ranges are approximately twice that of each other.

[0017] The method of the present invention absorbs the advantages of existing sensors and overcomes their disadvantages. It utilizes quartz capillaries, single-mode optical fibers, Terfenol-D powder, epoxy resin glue, ultraviolet glue, FPI and the first harmonic vernier effect to produce an optical fiber magnetic field sensor that is particularly compact, easy to manufacture and package, and highly sensitive. The sensor is suitable for measuring magnetic fields in confined spaces and has excellent practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the structure of the sensor produced by the method of the present invention.

[0019] In the figure: 1, 5 and 7 are single-mode optical fibers, 2 is a large capillary, 3 is a Terfenol-D mixture coating, 4 is an ultraviolet adhesive, and 6 is a small capillary. DETAILED DESCRIPTION

[0020] 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.

[0021] 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.

[0022] See also Figure 1 The present invention is a method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI, the method comprising the following steps:

[0023] The first step is to create a Fabry-Perot interferometer 1, or FPI1: Take a large quartz capillary, or large capillary 2, and burn off the protective coating on its surface. The inner diameter is 150 microns and the outer diameter is 360 microns. Then, two stripped and smooth-end-cut single-mode optical fibers, 1 and 5, are inserted into the large capillary from either end. The distance between the two single-mode optical fibers is adjusted. When the spectrum meets the requirements, the single-mode optical fibers are bonded to the large capillary using UV glue 4. After irradiation with a UV lamp for one minute, the large capillary and the single-mode optical fibers are bonded together to form 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. In the FPI1 structure, the length between the bonding points of the two capillaries is L. The single-mode optical fibers inserted from both sides of the capillary form a cantilever beam structure within the capillary.

[0024] The second step is to evenly apply Terfenol-D powder to the surface of the large capillary of FPI1 to form an FPI1 that is sensitive to magnetic fields. Specifically, Terfenol-D powder and epoxy resin glue are first prepared in a powder-to-glue mass ratio of 95:5, that is, 95% Terfenol-D powder and 5% epoxy resin glue are mixed and stirred to form a uniform mixed colloid for later use; the prepared Terfenol-D mixed glue is then evenly applied to the outer surface of the large capillary in FPI1, and the entire outer surface of the large capillary is coated to form a Terfenol-D mixed glue coating 3; the prepared structure is first placed at room temperature for two hours, then placed in a drying oven at 40°C for 8 hours, and finally left at room temperature for half a month to allow the coated material to completely dry; further, the diameter of the above-mentioned Terfenol-D powder is 100 microns to 150 microns; further, the thickness of the Terfenol-D mixed glue coating on the outer wall of the above-mentioned large capillary is maintained at 0.8 to 1.2 mm;

[0025] The third step is to make a Fabry-Perot interferometer 2, or FPI2: take a small quartz capillary, or small capillary 6, and burn off the protective layer on its surface with fire. Its inner diameter is 75 microns and the outer diameter is 125 microns. Then, use a fiber fusion splicer to fuse a section of single-mode optical fiber 7 with its coating stripped and end face cut flat to the small quartz capillary. Use a fiber cleaver to cut the capillary to the designed length L2. Then, fuse another section of single-mode optical fiber with its coating stripped and end face flat to the other end of the small capillary. Finally, use a fiber cleaver to cut the end of the single-mode optical fiber into an oblique surface. This completes the production of FPI2. FPI2 is formed by the interference of reflected light from the two reflection surfaces M3 and M4. The FP cavity of FPI2 is also an air cavity with a length of L2.

[0026] In the fourth step, the single-mode optical fiber ends of FPI1 and FPI2 are fused together using a fusion splicer. The two FPIs are 5 mm to 1 cm apart. The two FPIs are cascaded to produce a first harmonic vernier effect sensor.

[0027] Furthermore, the FPI1 and FPI2 in this embodiment are both extrinsic low-fineness FPIs. Furthermore, the FP cavity length in the FPI1 and FPI2 in the present invention is L I and L2, L2 is approximately twice that of L1, and the reflectance spectra of the two FPIs have similar extinction ratios, and their free spectral ranges are approximately twice that of each other.

[0028] 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:

[0029]

[0030] Here, λ FPI1 and λ FPI2 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.

[0031] Since the reflectance spectra of FPI1 and FPI2 are both periodic waveforms that approximate sine waves, the difference between the wavelengths of two adjacent troughs in their spectra is defined as their free spectral range (FSR). Therefore, the approximate calculation formulas for the FSR of FPI1 and FPI2 are:

[0032]

[0033] In FPI1, due to the existence of the cantilever beam structure, it increases the effective action length of the strain acting on FPI1 from L1 to L, so the strain sensitivity of FPI1 is increased by L / L1 times. If L is designed to be much larger than L1, FPI1 will have a high sensitivity to axial strain. In addition, since Terfenol-D powder is a material that can produce linear expansion and contraction under the action of a magnetic field, it is now coated on the surface of FPI1. When an increased magnetic field is applied to both ends of FPI1, the Terfenol-D material will stretch along the direction of the magnetic field, which is equivalent to applying an axial strain to FPI1. Since FPI1 is very sensitive to axial strain, FPI1 is also sensitive to the applied magnetic field. By demodulating the trough wavelength drift of FPI1, the external magnetic field can be measured. The magnetic field sensitivity of FPI1 can be simply expressed as follows:

[0034]

[0035] FPI2 is an interferometer formed by two reflecting surfaces between a single-mode optical fiber and air. Its surface is not adhered with the magnetostrictive material Terfenol-D. Therefore, under the action of an external magnetic field, its refractive index n2 and cavity length L2 do not change, making it insensitive to external magnetic fields.

[0036] Finally, when FPI1 and FPI2 are cascaded together, since the FSR of FPI1 is designed to be nearly twice the FSR of FPI2, according to the principle of optical harmonic vernier effect, their spectra will be superimposed to form large periodic inner and outer envelope spectra. If the wavelength of the intersection of the inner envelope of the spectrum is used for measurement, it will have a greater amplification effect on the sensitivity of the sensing unit FPI1. This is the so-called optical harmonic vernier effect. In the harmonic vernier effect sensor formed by the cascade of FPI1 and FPI2, FPI1, which is sensitive to magnetic fields, serves as the sensing element, and FPI2, which is insensitive to magnetic fields, serves as the reference element. According to the principle of optical harmonic vernier effect, it can be deduced that the inner envelope of the first harmonic vernier effect spectrum as follows:

[0037]

[0038] Therefore, using the optical harmonic vernier effect amplification principle, when we examine the change in wavelength of the envelope crossover point within the spectrum with the magnetic field, we find that the magnetic field causes a small shift in the wavelength of the FPI1 spectrum trough, which will cause the wavelength drift of the envelope crossover point within the spectrum to be amplified by M times. M is the amplification factor of the wavelength drift, which is also the amplification factor of the magnetic field sensitivity. It can be defined as:

[0039]

[0040] in, is the magnetic field sensitivity of the first harmonic vernier effect sensor.

[0041] In summary, a magnetic field-sensitive FPI1 was designed using magnetic Terfenol-D powder material and FPI. Then, using the principle of optical harmonic vernier effect, a cascade structure of FPI1 and FPI2 was constructed to produce a harmonic vernier effect, forming a harmonic 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.

[0042] The method of the present invention absorbs the advantages of existing sensors and overcomes their disadvantages. It utilizes quartz capillaries, single-mode optical fibers, Terfenol-D powder, epoxy resin glue, ultraviolet glue, FPI and the first harmonic vernier effect to produce an optical fiber magnetic field sensor that is particularly compact, easy to manufacture and package, and highly sensitive. The sensor is suitable for measuring magnetic fields in confined spaces and has excellent practical application value.

Claims

1. A method for manufacturing an optical fiber magnetic field sensor composed of Terfenol-D powder and FPI, characterized in that: The method comprises the following steps: The first step is to create a Fabry-Perot interferometer 1, or FPI1. A large quartz capillary is prepared and its protective coating is burned off. The inner diameter is 150 microns and the outer diameter is 360 microns. Two stripped, clean-end-cut single-mode optical fibers are inserted into the capillary from either end. The distance between the two single-mode fibers is adjusted. When the spectrum meets the requirements, the single-mode fibers are glued together at both ends of the capillary using UV glue. The capillary is then irradiated with UV light for one minute. The capillary and single-mode fibers are then glued together to form an FPI1. The FPI1 is formed by the interference of reflected light from two reflective surfaces, M1 and M2. The FP cavity of the FPI1 is an air cavity with a length of L1. In the FPI1 structure, the length between the bonding points of the two capillaries is L. The single-mode fibers inserted from both sides of the capillary form a cantilever beam structure within the capillary. The second step is to evenly apply Terfenol-D powder to the surface of the large capillary of FPI1 to form an FPI1 that is sensitive to magnetic fields. Specifically, Terfenol-D powder and epoxy resin glue are first prepared in a powder-to-glue mass ratio of 95:5, that is, 95% Terfenol-D powder and 5% epoxy resin glue are mixed and stirred to form a uniform mixed colloid for later use; the prepared Terfenol-D mixed glue is then evenly applied to the outer surface of the large capillary in FPI1, covering the entire outer surface of the large capillary; the prepared structure is first left to air at room temperature for two hours, then placed in a drying oven at 40°C for eight hours, and finally left at room temperature for half a month to allow the coated material to dry completely; The third step is to make a Fabry-Perot interferometer 2, or FPI2: take a small quartz capillary and burn off the protective layer on its surface with fire. Its inner diameter is 75 microns and the outer diameter is 125 microns. Then, use a fiber fusion splicer to fuse a section of single-mode optical fiber with its coating stripped and end face cut flat to the small quartz capillary. Use a fiber cleaver to cut the capillary to the designed length L2. Then, fuse another section of single-mode optical fiber with its coating stripped and end face flat to the other end of the small capillary. Finally, use a fiber cleaver to cut the end of the single-mode optical fiber into an oblique surface. This completes the production of FPI2. FPI2 is formed by the interference of reflected light from the two reflection surfaces M3 and M4. The FP cavity of FPI2 is also an air cavity with a length of L2. In the fourth step, the single-mode optical fiber ends of FPI1 and FPI2 are fused together using a fusion splicer. The two FPIs are 5 mm to 1 cm apart. The two FPIs are cascaded to produce a first harmonic vernier effect sensor.

2. The method for manufacturing a fiber optic magnetic field sensor composed of Terfenol-D powder and FPI according to claim 1, characterized in that: Both FPI1 and FPI2 are extrinsic low-fineness FPIs.

3. The method for manufacturing a fiber optic magnetic field sensor composed of Terfenol-D powder and FPI according to claim 1, characterized in that: The diameter of the Terfenol-D powder in the second step is 100 microns to 150 microns.

4. The method for manufacturing a fiber optic magnetic field sensor composed of Terfenol-D powder and FPI according to claim 1, characterized in that: The thickness of the Terfenol-D mixed adhesive coating on the outer wall of the large capillary in the second step is maintained at 0.8 to 1.2 mm.

5. The method for manufacturing a fiber optic magnetic field sensor composed of Terfenol-D powder and FPI according to claim 1, characterized in that: The FP cavity length in FPI1 and FPI2 is L I and L2, L2 is approximately twice that of L1, and the reflectance spectra of the two FPIs have similar extinction ratios, and their free spectral ranges are approximately twice that of each other.

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