Manufacturing method of ultrahigh-sensitivity optical fiber current sensor

Abstract

The invention discloses a method for manufacturing an ultra-high-sensitivity optical fiber current sensor. The method specifically comprises the following steps of: 1, manufacturing a cascaded Fabry-Perot interferometer structure, namely FPI (Fabry-Perot Interferometer); 2, dimethyl silicone oil with the refractive index n equal to 1.40 is injected into the large capillary tube and the small capillary tube of the cascade structure; 3, uniformly coating the conductive silver adhesive mixture on the surfaces of the large capillary tube C1 and the small capillary tube C2 to form a sensor sensitive to current; fourthly, in order to prevent Joule heat generated by the conductive silver adhesive from being diffused or convected by the environment, a transparent hard plastic shell is arranged on the outer surface of the structure in a sleeving mode, the heat generated by the conductive silver adhesive can be rapidly absorbed by the temperature sensor, and high-sensitivity measurement of the current is indirectly achieved; the current sensor manufactured by the method is particularly compact in structure, simple to manufacture, easy to package and high in sensitivity.

Description

Technical Field

[0001] This invention relates to the field of precision sensor manufacturing technology, and in particular to a method for manufacturing an ultra-high sensitivity fiber optic current sensor. Background Technology

[0002] Current sensors are indispensable key components in the monitoring, protection, online control, and system diagnostics of modern power electronic systems. The sensitivity, measurement accuracy, measurement range, and reliability of current sensors are crucial for the safe operation of the power grid. Simultaneously, current sensors can accurately measure weak currents in some low-voltage systems. Compared to other types of current sensors, fiber optic current sensors are lightweight, compact, electrically insulated, highly sensitive, reliable in operation, easy to remotely control, and readily integrated into other electrical devices. Typically, fiber optic current sensors do not require direct measurement of large currents (or high voltages), avoiding damage to the device from direct measurement. Furthermore, when using fiber optic current sensors to measure current, it is often possible to indirectly measure the current by measuring the magnetic field or heat (temperature).

[0003] Based on detection principles, numerous fiber optic current sensors have been invented. Firstly, magneto-optical materials, magnetofluids, and magnetostrictive materials can easily measure magnetic effects, thus indirectly achieving current measurement. Early fiber optic current sensors primarily utilized the Faraday effect of magneto-optical materials, but these sensors typically have complex structures and large volumes, and are also affected by the material's Wilder constant. Currently, magnetofluid-based fiber optic current sensors have been extensively studied. Their working principle utilizes the change in magnetic field caused by a change in current, which in turn leads to a change in the refractive index of the magnetofluid, thus achieving current measurement. Due to the liquid nature of magnetofluids, there are some technical challenges in sensor fabrication and sealing. Once the magnetofluid dries out, the sensor loses its function. Magnetostrictive materials are particularly sensitive to magnetic effects, and current sensors based on magnetostrictive materials have also been extensively studied. Researchers combine the magnetostrictive material Terfenol-D with fiber optic gratings or fiber optic interferometers to design current sensors. However, these sensors also suffer from drawbacks such as large structural size, susceptibility to magnetic saturation, and the brittleness and fragility of Terfenol-D.

[0004] Finally, the method of indirectly measuring current using Joule heat generated by electric current is also commonly used in fiber optic current sensors. The sensing head is typically made of materials that readily conduct and generate heat, and have high thermal conductivity, such as copper wire, aluminum wire, chromium nickelate wire, copper rod, solder balls, and power resistors. They are usually small in size and tightly integrated with the fiber optic sensor. They convert the current into Joule heat and efficiently transfer it to the fiber optic sensor. The fiber optic sensor sensitively senses thermal changes, similar to the operating characteristics of a fiber optic temperature sensor. This type of fiber optic current sensor has a good linear response to the square of the current. Such current sensors are generally simple in structure, easy to manufacture, and inexpensive, but their sensitivity is typically lower and they have a slow response. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned issues by providing a method for manufacturing an ultra-sensitive fiber optic current sensor. The current sensor manufactured using this method has a particularly compact structure, is simple to manufacture, easy to package, and has high sensitivity.

[0006] The specific solution of this invention is: a method for manufacturing an ultra-high sensitivity fiber optic current sensor, which specifically includes the following steps:

[0007] The first step is to fabricate the cascaded Fabry-Perot interferometer structure, i.e., FPI: Take a section of large quartz capillary C1, burn off its protective layer with fire, then take a section of small capillary C2, and similarly burn off its protective layer with fire. Then take a section of single-mode fiber SMF, strip off its coating and cut the end face flat. Then, fuse them together in the order of SMF-C1-C2. Finally, use oxyhydrogen flame discharge technology to process the large quartz capillary C1, so that it slowly forms a large bubble, approximately spherical, with a diameter of L1.

[0008] The second step involves injecting dimethyl silicone oil with a refractive index of n=1.40 into the large and small capillaries of the cascaded structure. Specifically, a syringe is used to inject the dimethyl silicone oil into C1 and C2. The syringe needle is made by drawing another capillary C3 to a diameter large enough to pass through C2 using a fiber optic fusion tapering machine. To avoid generating microbubbles in C1, the needle is deeply inserted into the end of C1, and the dimethyl silicone oil is slowly injected while simultaneously being slowly removed from C2 until it is partially filled. The silicone oil liquid filling the large and small capillaries can be considered as its liquid core, where light is transmitted and reflected. Finally, a layer of UV adhesive is coated on the end face of the small capillary C2, and then the UV adhesive is cured by irradiating it with a UV lamp for 1 minute, completely sealing the small capillary C2. A total of four reflective surfaces are formed: the weld surface between the SMF and the large capillary, the weld surface between the large and small capillaries, the silicone oil liquid surface in the small capillary, and the reflective surface of the UV adhesive.

[0009] The third step involves uniformly coating the conductive silver paste mixture onto the surfaces of the large capillary C1 and the small capillary C2 to form a current-sensitive sensor. The conductive silver paste mixture is prepared by mixing conductive silver paste and epoxy resin in a 95:5 mass ratio, i.e., 95% conductive silver paste and 5% epoxy resin (AB glue) are mixed and stirred to form a homogeneous liquid. Finally, the prepared conductive silver paste mixture is uniformly coated onto the outer surfaces of the large and small capillaries, maintaining a coating thickness of 1 mm. The fabricated structure is then left to air dry at room temperature for two hours, and then placed in a drying oven at 40°C. 。 Dry at C for 12 hours, and then leave at room temperature for half a month to allow the coated material to dry completely;

[0010] Fourth, to prevent the Joule heat generated by the conductive silver paste from being diffused or convectioned by the environment, a transparent hard plastic shell is placed over the above structure so that the heat generated by the conductive silver paste can be quickly absorbed by the temperature sensor, thereby indirectly achieving high-sensitivity measurement of the current.

[0011] Furthermore, in this invention, the large capillary C1 has an inner diameter of 75 micrometers and an outer diameter of 150 micrometers; the small capillary C2 has an inner diameter of 20 micrometers and an outer diameter of 150 micrometers.

[0012] Furthermore, the needle of the syringe described in this invention uses a capillary C3 with an inner diameter of 30 micrometers and an outer diameter of 150 micrometers.

[0013] Furthermore, in the second step of this invention, the four reflective surfaces formed by the welding surface of SMF and large capillary, the welding surface of large and small capillary, the silicone oil surface in small capillary, and the UV adhesive reflective surface are M1, M2, M3, and M4, respectively.

[0014] Furthermore, in the third step of this invention, positive and negative electrodes are respectively provided at both ends of the outer wall of the conductive silver paste coating layer.

[0015] This invention draws on the advantages of existing fiber optic current sensors while overcoming their disadvantages. It utilizes two cascaded Fabry-Perot interferometers (FPIs) made of two quartz capillary tubes of different inner diameters, single-mode optical fiber, UV adhesive, and dimethyl silicone oil. Similar to the working principle of a mercury thermometer, the expansion and amplification effect of the thermally sensitive liquid column of silicone oil within the capillary tubes of the two cascaded FPIs allows for the design of a temperature sensor with fast temperature response, high sensitivity, and high measurement accuracy. A mixture of conductive silver paste and AB adhesive is then uniformly coated onto the surface of the temperature sensor. When the sensor head is energized, the conductive paste generates a large amount of Joule heat, which is rapidly converted into temperature. This temperature acts on the temperature sensor, causing it to produce a highly sensitive response to the applied current, indirectly achieving rapid measurement of the current with high sensitivity and high accuracy. Simultaneously, the optical path difference measurement method of the FPIs simplifies the analysis of the sensor results.

[0016] The sensor produced by the method of this invention has a particularly compact structure, is simple to manufacture, easy to package, has high sensitivity, a large measurement range, small size, and can be used in confined spaces. Attached Figure Description

[0017] Figure 1 This is a cross-sectional schematic diagram of the overall structure of the present invention.

[0018] In the diagram: 1—single-mode optical fiber, 2—positive electrode, 3—dimethyl silicone oil, 4—large capillary, 5—transparent rigid plastic shell, 6—negative electrode, 7—ultraviolet adhesive, 8—small capillary, 9—conductive silver paste mixture. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention. In the description of the present invention, it should be noted that the terms "upper," "lower," "inner," "outer," etc., indicating orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is 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 orientation, be constructed or operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0020] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," and "connect" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] See Figure 1 This invention relates to a method for fabricating an ultra-sensitive fiber optic current sensor, which specifically includes the following steps:

[0022] The first step is to fabricate the cascaded Fabry-Perot interferometer structure, i.e., FPI: Take a section of large quartz capillary C1, burn off its protective layer with fire, then take a section of small capillary C2, and similarly burn off its protective layer with fire. Then take a section of single-mode fiber SMF, strip off its coating and cut the end face flat. Then, fuse them together in the order of SMF-C1-C2. Finally, use oxyhydrogen flame discharge technology to process the large quartz capillary C1, so that it slowly forms a large bubble, approximately spherical, with a diameter of L1.

[0023] The second step involves injecting dimethyl silicone oil with a refractive index of n=1.40 into the large and small capillaries of the cascaded structure. Specifically, a syringe is used to inject the dimethyl silicone oil into C1 and C2. The syringe needle is made by drawing another capillary C3 to a diameter large enough to pass through C2 using a fiber optic fusion tapering machine. To avoid generating microbubbles in C1, the needle is deeply inserted into the end of C1, and the dimethyl silicone oil is slowly injected while simultaneously being slowly removed from C2 until it is partially filled. The silicone oil liquid filling the large and small capillaries can be considered as its liquid core, where light is transmitted and reflected. Finally, a layer of UV adhesive is coated on the end face of the small capillary C2, and then the UV adhesive is cured by irradiating it with a UV lamp for 1 minute, completely sealing the small capillary C2. A total of four reflective surfaces are formed: the weld surface between the SMF and the large capillary, the weld surface between the large and small capillaries, the silicone oil liquid surface in the small capillary, and the reflective surface of the UV adhesive.

[0024] The third step involves uniformly coating the conductive silver paste mixture onto the surfaces of the large capillary C1 and the small capillary C2 to form a current-sensitive sensor. The conductive silver paste mixture is prepared by mixing conductive silver paste and epoxy resin in a 95:5 mass ratio, i.e., 95% conductive silver paste and 5% epoxy resin (AB glue) are mixed and stirred to form a homogeneous liquid. Finally, the prepared conductive silver paste mixture is uniformly coated onto the outer surfaces of the large and small capillaries, maintaining a coating thickness of 1 mm. The fabricated structure is then left to air dry at room temperature for two hours, and then placed in a drying oven at 40°C. 。Dry at C for 12 hours, and then leave at room temperature for half a month to allow the coated material to dry completely;

[0025] Fourth, to prevent the Joule heat generated by the conductive silver paste from being diffused or convectioned by the environment, a transparent hard plastic shell is placed over the above structure so that the heat generated by the conductive silver paste can be quickly absorbed by the temperature sensor, thereby indirectly achieving high-sensitivity measurement of the current.

[0026] Furthermore, in this invention, the large capillary C1 has an inner diameter of 75 micrometers and an outer diameter of 150 micrometers; the small capillary C2 has an inner diameter of 20 micrometers and an outer diameter of 150 micrometers. Furthermore, the needle of the syringe in this invention uses a capillary C3 with an inner diameter of 30 micrometers and an outer diameter of 150 micrometers. Furthermore, in the second step of this invention, the four reflective surfaces formed by the fusion surface of the SMF and the large capillary, the fusion surface of the large and small capillary, the silicone oil surface in the small capillary, and the UV adhesive reflective surface are M1, M2, M3, and M4, respectively. Furthermore, in the third step of this invention, positive and negative electrodes are respectively provided at both ends of the outer wall of the conductive silver paste coating layer.

[0027] The sensor structure designed in this invention is similar to the principle of a mercury thermometer. The temperature-sensing material, dimethyl silicone oil, is filled in two cascaded capillary tubes of different inner diameters: 75 micrometers and 20 micrometers, respectively. The design aims to amplify minute changes in the volume of the silicone oil, making the change in the silicone oil column within the smaller capillary tube significant, thereby improving reading accuracy and achieving extremely high-sensitivity temperature sensing. Then, when the conductive silver paste is energized, it generates significant Joule heat, causing the temperature sensor to withstand a significant temperature change, indirectly achieving highly sensitive and accurate measurement of the energized current. The actual fabrication steps are as follows:

[0028] The first step is to fabricate a cascaded Fabry-Perot interferometer (FPI) structure. Take a section of large quartz capillary tube 4 (C1) and burn off its protective layer with fire. Its inner diameter is 75 micrometers and its outer diameter is 150 micrometers. Then take a section of small capillary tube 8 (C2) and burn off its protective layer with fire as well. Its inner diameter is 20 micrometers and its outer diameter is 150 micrometers. Next, take a section of single-mode fiber 1 (SMF), remove its coating, and cut its end face flat. Then, fuse them together in the order SMF - C1 - C2. Finally, use oxyhydrogen flame discharge technology to process C1, allowing it to slowly form a large bubble, approximately spherical, with a diameter of about L1.

[0029] The second step involves injecting dimethyl silicone oil (referred to as silicone oil) with a refractive index of n=1.40 into the capillaries of the aforementioned cascade structure. Silicone oil is a transparent liquid with a light transmittance greater than 95% and an extremely high coefficient of thermal expansion, making it an excellent heat-sensitive material. A syringe is used to inject the silicone oil into capillaries C1 and C2. The syringe needle is made by drawing another capillary C3 (30 micrometers inner diameter and 150 micrometers outer diameter) to a diameter sufficient to pass through C2 using a fiber optic fusion tapering machine. To avoid creating microbubbles in C1, the needle is deeply inserted into the end of C1, and the silicone oil is slowly injected while simultaneously being slowly withdrawn from C2 until C2 is partially filled. The silicone oil liquid filling the capillary can be considered its liquid core, within which light is transmitted and reflected. Finally, a thin layer of UV adhesive 7 is coated onto the end face of the small capillary C2, and then the UV adhesive is cured by irradiating it with a UV lamp for about 1 minute, completely sealing the small capillary C2. UV adhesive is an industrial-grade high-refractive-index optical UV adhesive that bonds structures exceptionally strongly. For example... Figure 1 As shown, there are four reflective surfaces: the weld surface between the SMF and the large capillary, the weld surface between the large and small capillary, the silicone oil surface in the small capillary, and the UV adhesive reflective surface. Theoretically, this can form three cascaded Fabry-Perot interferometers (FPIs). Since the UV adhesive reflective surface emits very weak light, it is actually a cascade of two FPIs. Thus, the large and small capillary and silicone oil constitute a cascaded FPI ultrasensitive temperature sensor.

[0030] The third step involves uniformly coating the conductive silver paste mixture onto the surfaces of the large capillary C1 and the small capillary C2 to form a current-sensitive sensor. The conductive silver paste possesses high conductivity, strong adhesion, and excellent thermal conductivity. The conductive silver paste and epoxy resin are mixed at a mass ratio of 95:5, i.e., 95% conductive silver paste and 5% epoxy resin (i.e., AB glue) are mixed and stirred to form a homogeneous liquid for later use. The AB glue ratio can be further reduced during preparation. Mixing the conductive silver paste with the AB glue further increases the adhesion of the conductive silver paste to the capillary surface. Finally, the prepared conductive silver paste mixture is uniformly coated onto the outer surfaces of the two capillary tubes, maintaining a thickness of approximately 1 mm. The fabricated structure is first left to air dry at room temperature for two hours, then placed in a drying oven at 40°C for 12 hours, and finally left to dry at room temperature for half a month to allow the coated conductive silver paste mixture to completely dry.

[0031] Fourth, to prevent the Joule heat generated by the conductive silver paste from being diffused or convectioned by the environment, a transparent hard plastic shell 5 is placed over the outer surface of the structure, so that the heat generated by the conductive silver paste can be quickly absorbed by the temperature sensor, thereby indirectly achieving high-sensitivity measurement of the current.

[0032] In summary, when fabricating the sensor structure, in order to enable the sensor to achieve a magnification effect similar to that of a mercury thermometer, the key technology is to control the length L1 of the large bubble (microsphere), the length L2 of the liquid column in the small capillary, and the length L3 of the air cavity in the small capillary, so that they form an optimal matching relationship, achieve maximum drift of the temperature-sensitive silicone oil in the small capillary, and achieve maximum current sensitivity.

[0033] The working principle of the sensor of this invention:

[0034] Figure 1 A schematic diagram of the proposed current sensor is shown. It consists of quartz capillaries with different inner diameters, which are then sequentially fused together with a single-mode fiber, a large capillary, and a small capillary. The single-mode fiber also serves as the input and output channel for the sensor's optical signal. The inside of the capillaries is filled with silicone oil, and their outer surfaces are coated with conductive silver paste, a current-sensitive material. The capillary with the larger inner diameter is labeled C1, and it is completely filled with silicone oil, while the capillary with the smaller inner diameter is labeled C2, and it is partially filled with silicone oil. A small amount of UV adhesive is used to seal C2.

[0035] exist Figure 1 In this process, light from a broadband source is transmitted to the sensor via a single-mode fiber (SMF). Due to refractive index or inner diameter mismatch at the interfaces, it is partially reflected at interfaces M1, M2, M3, and M4. This light then re-enters the SMF and converges, producing typical multi-beam interference. However, the reflected light at reflective surfaces M2 and M4 is actually very weak; therefore, the Fabry-Perot interference (FPI) caused by M1 and M3 dominates the sensor's total reflection spectrum.

[0036] exist Figure 1 In the device, the silicone oil has a refractive index of n = 1.40. It is a transparent liquid with a light transmittance greater than 95%. The liquid in the capillary can be considered its liquid core, where light propagates and is reflected. For example... Figure 1 As shown, L1 and L2 represent the liquid column lengths of C1 and C2, respectively, with L1 being much larger than L2. L3 is the distance between the liquid surface in C2 and the reflective surface of the cured UV adhesive, i.e., the length of the air cavity providing space for the expansion of the liquid core. Based on optical principles, the optical path difference (OPD) for the Fabry-Perot interference caused by reflective surfaces M1 and M3 can be expressed as:

[0037] (1)

[0038] Where n represents the refractive index of the silicone oil in the liquid core.

[0039] When the ambient temperature changes, the sensor heats up, causing its temperature to rise. This leads to thermal expansion of the silicone oil and quartz capillary (C1 and C2), resulting in a change in the sensor's optical path difference (OPD). Furthermore, the thermo-optical effect of the silicone oil is much smaller than its thermal expansion effect, and can be ignored here. Therefore, the sensor's temperature sensitivity (S) based on the change in OPD... T )for:

[0040] (2)

[0041] Furthermore, compared to quartz capillary tubes, silicone oil has a higher coefficient of thermal expansion (0.934 × 10⁻³ / ℃), therefore the change in L2 mainly depends on the thermal expansion of the silicone oil. The sensor structure is similar to a glass tube liquid thermometer, and L2 changes significantly with temperature.

[0042] Assuming the coefficient of volume expansion of silicone oil is Where V is the sum of the volumes of silicone oil inside the two capillaries: Where V1 is the volume of the microsphere. A2 is the cross-sectional area of ​​C2 (minimal). Since V1 and A2 are fixed values, the change in the volume of silicone oil inside the capillary is entirely converted into a change in the length L2 of the liquid core in C2. This can be derived from the formula above. Equation (2) can be rewritten as:

[0043] (3)

[0044] According to equation (3), the temperature sensitivity can be significantly improved by increasing L1 and L2. The structure of the device is similar to that of a capillary mercury thermometer, with C1 and C2 acting as the bulb and valve stem, respectively.

[0045] Furthermore, the capillary tubes C1 and C2 of the sensor are coated with conductive silver paste. According to thermodynamic theory, the Joule heat Q emitted by the conductive silver paste when energized is proportional to the square of the current. This Joule heat Q is absorbed by the sensor surface and converted into a temperature change in the sensor. The relationship between the sensor's temperature change (ΔT) and the heat and current can be derived as follows:

[0046] (4)

[0047] Where k1 and k2 are proportionality constants, which are related to the properties of the conductive silver paste and the surface area of ​​the sensor. As can be seen from equation (4), the temperature change (ΔT) of the sensor is proportional to the square of the current.

[0048] Therefore, the current square sensitivity of the sensor is:

[0049] (5)

[0050] in, This indicates the dependence of OPD changes on sensor temperature. This indicates the dependence of temperature change on the square of the current.

[0051] This invention draws on the advantages of existing fiber optic current sensors while overcoming their disadvantages. It utilizes two cascaded Fabry-Perot interferometers (FPIs) made of two quartz capillaries of different inner diameters, single-mode optical fiber, UV adhesive, and dimethyl silicone oil. Similar to the working principle of a mercury thermometer, the expansion and amplification effect of the thermally sensitive liquid column of silicone oil within the capillaries of the two cascaded FPIs allows for the design of a temperature sensor with fast temperature response, high sensitivity, and high measurement accuracy. A mixture of conductive silver paste and AB adhesive is then uniformly coated onto the surface of the temperature sensor. When the sensor head is energized, the conductive paste generates a large amount of Joule heat Q, which is proportional to the square of the current. Joule heat Q is rapidly converted into temperature, which acts on the temperature sensor, causing it to produce a highly sensitive response to the applied current, indirectly achieving rapid measurement of the current with high sensitivity and high accuracy. Simultaneously, the optical path difference measurement method of the FPIs simplifies the analysis of the sensor results. The sensor produced by the method of this invention has a particularly compact structure, is simple to manufacture, easy to package, has high sensitivity, a large measurement range, small size, and can be used in confined spaces.

Claims

1. A method for fabricating an ultra-high sensitivity fiber optic current sensor, characterized in that, Specifically, it includes the following steps: The first step is to fabricate the cascaded Fabry-Perot interferometer structure, i.e., FPI: Take a section of large quartz capillary C1, burn off its protective layer with fire, then take a section of small capillary C2, and similarly burn off its protective layer with fire. Then take a section of single-mode fiber SMF, strip off its coating and cut the end face flat. Then, fuse them together in the order of SMF-C1-C2. Finally, use oxyhydrogen flame discharge technology to process the large quartz capillary C1, so that it slowly forms a large bubble, approximately spherical, with a diameter of L1. The second step involves injecting dimethyl silicone oil with a refractive index of n=1.40 into the large and small capillaries of the cascaded structure. Specifically, a syringe is used to inject the dimethyl silicone oil into C1 and C2. The syringe needle is made by drawing another capillary C3 to a diameter large enough to pass through C2 using a fiber optic fusion tapering machine. To avoid generating microbubbles in C1, the needle is deeply inserted into the end of C1, and the dimethyl silicone oil is slowly injected while simultaneously being slowly removed from C2 until it is partially filled. The silicone oil liquid filling the large and small capillaries can be considered as its liquid core, where light is transmitted and reflected. Finally, a layer of UV adhesive is coated on the end face of the small capillary C2, and then the UV adhesive is cured by irradiating it with a UV lamp for 1 minute, completely sealing the small capillary C2. A total of four reflective surfaces are formed: the weld surface between the SMF and the large capillary, the weld surface between the large and small capillaries, the silicone oil liquid surface in the small capillary, and the reflective surface of the UV adhesive. The third step involves uniformly coating the conductive silver paste mixture onto the surfaces of the large capillary C1 and the small capillary C2 to form a current-sensitive sensor. The conductive silver paste mixture is prepared by mixing conductive silver paste and epoxy resin in a 95:5 mass ratio, i.e., 95% conductive silver paste and 5% epoxy resin (AB glue) are mixed and stirred to form a homogeneous liquid. Finally, the prepared conductive silver paste mixture is uniformly coated onto the outer surfaces of the large and small capillaries, maintaining a coating thickness of 1 mm. The fabricated structure is then left to air dry at room temperature for two hours, and then placed in a drying oven at 40°C. 。 Dry at C for 12 hours, and then leave at room temperature for half a month to allow the coated material to dry completely; Fourth, to prevent the Joule heat generated by the conductive silver paste from being diffused or convectioned by the environment, a transparent hard plastic shell is placed over the above structure so that the heat generated by the conductive silver paste can be quickly absorbed by the temperature sensor, thereby indirectly achieving high-sensitivity measurement of the current.

2. The method for fabricating an ultra-high sensitivity fiber optic current sensor according to claim 1, characterized in that: The large capillary C1 has an inner diameter of 75 micrometers and an outer diameter of 150 micrometers; the small capillary C2 has an inner diameter of 20 micrometers and an outer diameter of 150 micrometers.

3. The method for fabricating an ultra-high sensitivity fiber optic current sensor according to claim 1, characterized in that: The syringe needle uses a capillary C3 with an inner diameter of 30 micrometers and an outer diameter of 150 micrometers.

4. The method for fabricating an ultra-high sensitivity fiber optic current sensor according to claim 1, characterized in that: In the second step above, the four reflective surfaces formed by the welded surfaces of SMF and large capillary, the welded surfaces of large and small capillary, the silicone oil surface in the small capillary, and the UV adhesive reflective surface are M1, M2, M3, and M4, respectively.

5. The method for fabricating an ultra-high sensitivity fiber optic current sensor according to claim 1, characterized in that: In the third step above, positive and negative electrodes are respectively provided at both ends of the outer wall of the conductive silver paste coating layer.

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