High-sensitivity vector magnetic field sensing device based on three-core optical fiber

Abstract

The invention provides a high-sensitivity vector magnetic field sensing device based on a three-core optical fiber. The high-sensitivity vector magnetic field sensing device is composed of a broadband light source, a sensing element, an adjustable permanent magnet, a spectrograph and a gauss meter. The sensing element is formed by directly welding the two ends of a three-core optical fiber with single-mode optical fibers, and the device utilizes the three-core optical fiber sensing element packaged by magnetic fluid to realize vector measurement of the size and direction of a magnetic field. When the sensor is wrapped in a capillary filled with magnetic fluid, an external magnetic field can change the refractive index of the magnetic fluid, so that an output optical signal of the magnetic fluid is modulated by the magnetic field intensity, and different from other methods in which the optical fiber needs to be processed to form non-circular symmetric optical field distribution, three fiber cores of the three-core optical fiber are arranged in a triangular shape on the cross section of the optical fiber; the sensor has natural circle asymmetry, so that the direction of an external magnetic field can be detected. The device has the advantages of high sensitivity, strong directional response, electromagnetic interference resistance, low power consumption and the like, and is suitable for vector magnetic field measurement in the technical field of optical fiber sensing.

Description

(I) Technical field

[0001] The invention relates to a high-sensitivity vector magnetic field sensing device based on three-core optical fibers, belonging to the technical field of optical fiber sensing. (II) Background technology

[0002] Magnetic field sensors are of great significance in modern technological applications and are widely used in military, power, medical and space systems. Traditional magnetic field sensors, such as giant magnetoresistance-based sensors, inductive magnetometers and Hall magnetic sensors, have some limitations, including complex structures, high costs and insufficient anti-interference capabilities. In contrast, optical fiber magnetic field sensors have attracted increasing attention due to their superior properties, including compact size, strong anti-electromagnetic interference capabilities, and high sensitivity and resolution.

[0003] Magnetic fluid (MF) is a colloid consisting of magnetic nanoparticles (Fe 3 O 4 ) and has liquid fluidity and solid magnetism. In an environment without a magnetic field, the magnetic particles in the magnetic fluid are evenly distributed and do not exhibit magnetic polarization. However, when an external magnetic field is applied, the magnetic particles will present a magnetic link structure aligned with the direction of the magnetic field, resulting in an uneven distribution of the refractive index of the magnetic fluid. Since magnetic fluids have some unique magneto-optical properties, including adjustable refractive index, Faraday effect, and birefringence effect, magnetic fluids are ideal optical materials for magnetic field sensors.

[0004] In recent years, the fiber optic magnetic field sensing technology based on the combination of optical fiber and magnetic fluid (MF) has attracted more and more attention from researchers. The magnetic field is a vector physical quantity, which has not only the magnetic field amplitude but also the magnetic field direction. To realize magnetic vector sensing, a non-circular symmetric light field distribution must be used. The first method is to excite surface plasma resonance by coating a metal film on the surface of the optical fiber; the second method is to write a grating in the optical fiber structure, thereby destroying its inherent light field symmetry; the third method is based on the core offset fusion fiber structure, that is, the misaligned fusion of the optical fiber; the fourth method is to bend the optical fiber to generate a non-centrally symmetric light field at the optical fiber cross section; the fifth is a side-polished optical fiber structure vector magnetic field sensor; the sixth is to use a fiber structure with circular asymmetry to realize magnetic vector sensing.

[0005] The main purpose of the present invention is to provide a three-stage optical fiber vector magnetic field sensor device with simple and compact structure and high sensitivity. The sensor with this structure can make up for the shortcomings of existing optical fiber vector magnetic field sensors in practical applications, such as complex preparation process, high production cost and low sensitivity. (III) Summary of the invention

[0006] The invention provides a high-sensitivity vector magnetic field sensing device based on three-core optical fibers.

[0007] The present invention is achieved in that:

[0008] A high-sensitivity vector magnetic field sensing device based on a three-core optical fiber is characterized in that it is composed of a broadband light source (1), a sensing element (2), an adjustable permanent magnet (3), a spectrometer (4) and a Gauss meter (5). The sensing element (3) is composed of single-mode optical fibers directly fused at both ends of the three-core optical fiber.

[0009] The present invention designs an all-optical fiber vector magnetic field sensor element based on the Mach-Zehnder interference effect.

[0010] Manufacturing process of the sensing element: In order to enhance the ability of the sensing area to detect external factors, the surface coating of the single-mode fiber and the three-core fiber is removed and cleaned with alcohol. Subsequently, the single-mode fiber-three-core fiber-single-mode fiber is continuously fused to form a single-mode fiber-three-core fiber-single-mode fiber sandwich-shaped fiber composite structure. The sensing structure is then placed in a capillary, filled with magnetic fluid, and sealed with UV adhesive to prevent leakage.

[0011] The incident light is excited by a broadband light source and passes through the three-core fiber through the introduction single-mode fiber. Due to the mode field mismatch between the single-mode fiber and the three-core fiber, different high-order modes are excited. The three-core fiber acts as a coupler, coupling the light into the three cores of the three-core fiber at the first interface between the single-mode fiber and the three-core fiber, and coupling part of the energy into the cladding of the three-core fiber, thereby exciting multiple high-order modes (core mode and cladding mode) of the three-core fiber. Multiple modes propagate along the three-core fiber and meet in the second single-mode fiber. Multiple phase differences along different optical paths will produce multipath interference, resulting in multipath MZI. The optical signal is coupled to the core of the lead-out single-mode fiber through the second single-mode fiber. The output spectrum will be a superposition of multiple interference modes, and the output transmitted light intensity can be expressed by formula (1):

[0012]

[0013] Where I represents the total light intensity. 1 and I 2 denote the intensity of the core mode and cladding mode respectively. φ denotes the phase difference between the modes.

[0014] When the sensor is encapsulated in a capillary filled with magnetic fluid, the external magnetic field changes the refractive index of the magnetic fluid. This causes the effective refractive index difference to change, resulting in a shift in the wavelength corresponding to the interference valley. The degree of change in the interference valley wavelength relative to the MF refractive index can be expressed by formula (2):

[0015]

[0016] Under different magnetic field directions, the magnetic nanoparticles are unevenly distributed in the magnetic fluid, resulting in a positive correlation between the refractive index of the magnetic fluid and the aggregation degree of the magnetic nanoparticles. When the different cores of the three-core optical fiber pass through the parallel lines of the direction of the external magnetic field, the uneven distribution of the magnetic nanoparticle aggregation leads to an uneven distribution of the refractive index of the magnetic fluid around the three-core optical fiber, which changes with the intensity and direction of the magnetic field. Therefore, the direction of the external magnetic field can be dynamically determined by observing the wavelength drift and the intensity of the interference valley in the transmission spectrum.

[0017] Compared with the prior art, the advantages of the present invention are as follows:

[0018] 1. The manufacturing process of the sensor device of the present invention is simple, and the sensor element used only involves two simple weldings, which is low in cost, compact in structure and high in sensitivity.

[0019] 2. The three-core optical fiber with circular asymmetry is used to measure the direction of the magnetic field. It is not only cheaper than the surface plasmon resonance (SPR) and the internal grating structure, but also simpler to make than the side polishing and offset fusion structure. It has excellent reproducibility. In the magnetic field intensity range of 35-44mT, the sensor can achieve a magnetic field sensitivity of -1230pm / mT and a maximum directional sensitivity of 730pm / °. It is easy to make, suitable for large-scale production, has high detection efficiency, and is of great significance to the future development of optical fiber vector magnetic field sensors. (IV) Description of the drawings

[0020] Figure 1 The invention is a schematic diagram of a high-sensitivity vector magnetic field sensing device based on three-core optical fiber, which is composed of a broadband light source (1), a sensing element (2), an adjustable permanent magnet (3), a spectrometer (4) and a gauss meter (5).

[0021] Figure 2 It is a cross-sectional view of a three-core optical fiber, which is characterized by: the core radius R col =4.1μm, radius of the fiber cladding R cll =62.5μm, the distance between the cores is 54.4μm, and the refractive indices of the core and cladding are n core =1.4625, n clad =1.4575.

[0022] Figure 3 The sensing unit of the present invention, wherein 1 is a single-mode optical fiber, 2 is UV glue, 3 is a capillary, 4 is a three-core optical fiber, and 5 is a magnetic fluid.

[0023] Figure 4 This is the spectrum diagram of the sensor device under different magnetic field strengths. It can be seen from the figure that with the increase of magnetic field strength, the interference peak wavelength drifts toward the shorter wavelength direction.

[0024] Figure 5 This is the magnetic field intensity-wavelength fitting diagram of the sensor device. It can be seen from the figure that the magnetic field sensitivity is -1.23nm / mT in the magnetic field range of 35mT to 44mT, the linearity is good, and R 2 It is 0.97.

[0025] Figure 6 The spectrum diagram of the sensor device under different magnetic field directions and the magnetic field direction-wavelength fitting diagram are shown in Figure 2. Figure 6 As shown in (a), as the direction of the magnetic field changes, the wavelength of the interference peak undergoes periodic redshift and blueshift. Figure 6 (b) is a polar coordinate diagram of the wavelength changing with the direction of the magnetic field. Figure 6 (c) is the linear fitting curve of the magnetic field direction sensitivity, with a maximum sensitivity of 0.73nm / °, indicating that the sensing structure performs well in measuring the direction of the external magnetic field. (V) Specific implementation methods

[0026] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation methods.

[0027] This paper proposes a high-sensitivity vector magnetic field sensing device based on three-core optical fiber. Figure 1 As shown: it is composed of a broadband light source (1), a sensor element (2), an adjustable permanent magnet (3), a spectrometer (4) and a Gauss meter (5). The sensor element (2) is composed of a single-mode optical fiber directly fused at both ends of a three-core optical fiber and combined with a magnetic fluid package.

[0028] The broadband light source (1) used is a broadband light source with a wavelength range of 600nm to 1700nm.

[0029] The cross-section of the three-core optical fiber used is shown in Figure 2 As shown, its core radius R col =4.1μm, radius of the fiber cladding R cll =62.5μm, the distance between the cores is 54.4μm, and the refractive indices of the core and cladding are n core =1.4625, n clad =1.4575. The sensor element (2) used is as follows Figure 3 As shown, it is composed of three-core optical fibers directly fused to single-mode optical fibers at both ends and combined with magnetic fluid packaging. The single-mode optical fiber cladding diameter in the sensor element is 125μm, and the core diameter is 9μm.

[0030] Preparation of the sensor element used: In order to enhance the ability of the sensing area to detect external factors, the coating layer of the single-mode optical fiber and the three-core optical fiber is removed and cleaned with alcohol. Use a fusion splicer to fuse a section of single-mode optical fiber with one end of the three-core optical fiber, and fix the fused sample on a fixed-length cutting device. The length of the three-core optical fiber is preferably 10,000 μm. After cutting, another section of the three-core optical fiber is fused to complete the preparation of the fiber interferometer. Then, straighten the two ends of the prepared fiber interferometer and put them into the capillary without touching the capillary wall. Then, inject the magnetic nanofluid into the capillary, and finally use ultraviolet UV glue to seal the two ends of the capillary to prevent the magnetic fluid from flowing out. At this point, the sensor element we invented is completed.

[0031] The adjustable permanent magnet (3) used is generated by a pair of cylindrical magnets placed in parallel. By adjusting the upper pole plate, the magnetic field induction intensity can be adjusted. And the built-in 3D printed rotating head can rotate the sensor element.

[0032] The used Gauss meter (5) is composed of a probe and a measuring instrument, wherein the probe and the sensor element are placed in an adjustable permanent magnet.

[0033] Specific operation examples:

[0034] After the sensor element is manufactured according to the above method, the sensor element is placed flat in the adjustable permanent magnet and fixed. One end of the sensor element is connected to the broadband light source through a single-mode optical fiber, and the other end is connected to the spectrometer through a single-mode optical fiber. The broadband light source is turned on, and the emitted light passes through the sensor element to generate an interference spectrum and is displayed on the spectrometer. At this time, by adjusting the adjustable permanent magnet and combining with the Gauss meter to accurately control the magnetic field strength of the sensor element in real time, the interference spectrum under different magnetic field strengths is obtained as shown in the figure. Figure 4 As shown, the software then calculates the magnetic field sensitivity to be -1.23nm / mT. Figure 5 At this time, fix a magnetic field strength, adjust the rotating stage, make the sensor element rotate in the magnetic field, and obtain the interference spectrum under different magnetic field directions as shown in Figure 6 (a) is shown in Figure 2. The polarization diagram is drawn by the software as shown in Figure 2. Figure 6 (b), and then calculate its magnetic field direction sensitivity to be 0.73nm / °. Figure 6 (c) is shown. Figure 4 , Figure 5 , Figure 6 It can be seen that the interference spectrum has a sensitive response to the magnetic field intensity and direction, and has good linearity. All these show that our high-sensitivity vector magnetic field sensor device based on three-core optical fiber can complete the measurement of vector magnetic field.

Claims

1. A high-sensitivity vector magnetic field sensing device based on three-core optical fiber. Its characteristics are: It is composed of a broadband light source (1), a sensor element (2), an adjustable permanent magnet (3), a spectrometer (4) and a Gauss meter (5). The sensor element (3) is composed of single-mode optical fibers directly fused at both ends of a three-core optical fiber.

2. A high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1, characterized in that: The light emitted by the broadband light source (1) is transmitted to the sensor element (2) via a single-mode optical fiber. The sensor element (2) is placed in an adjustable permanent magnet (3). A gauss meter (5) is used to read the magnetic field strength of the sensor element. The other end of the sensor element (2) is connected to a spectrometer (4) via a single-mode optical fiber.

3. The high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1 is characterized in that: The broadband light source (1) is a broadband light source of 600nm to 1700nm.

4. The high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1 is characterized in that: The sensor element (2) is made of a three-core optical fiber with two ends fused to a single-mode optical fiber. The two sections of single-mode optical fiber are prepared by a fixed-length cutter, and then the three-core optical fiber and the two sections of single-mode optical fiber are successively fused using an optical fiber fusion splicer.

5. The high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1 is characterized in that: The lengths of the two sections of single-mode optical fiber in the sensor element (2) are both L SMF =2000μm, the length of the three-core optical fiber is L TCF =10000μm, core radius R col =4.1μm, radius R of the fiber cladding cll =62.5μm, the distance between the cores is 54.4μm, and the refractive indices of the core and cladding are n core =1.4625, n clad =1.4575.

6. The high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1 is characterized in that: The adjustable permanent magnet (3) is composed of a pair of cylindrical magnets placed in parallel. The magnetic field induction intensity can be adjusted by adjusting the upper pole plate.

7. The high-sensitivity vector magnetic field sensing device based on three-core optical fiber according to claim 1 is characterized in that: The Gauss meter (5) is composed of a probe and a measuring instrument, and the probe and the sensor element are placed in an adjustable permanent magnet.

Images