Tapered cascaded spiral four-core optical fiber sensor and preparation method thereof

Abstract

The invention relates to a conical cascaded spiral four-core optical fiber sensor and a preparation method thereof. The conical cascaded spiral four-core optical fiber sensor comprises a single-mode optical fiber, a multimode optical fiber and a four-core optical fiber, the single-mode optical fiber, the multimode optical fiber and the four-core optical fiber are sequentially cascaded according to the following sequence: the single-mode optical fiber, the multimode optical fiber, the four-core optical fiber and the single-mode optical fiber; an optical fiber core in the middle area of the four-core optical fiber is of a spiral structure. Cascade connection among the optical fibers uses a fusion splicer to perform discharge fusion splicing. The spiral structure is manufactured by using a special optical fiber fusion splicer, and spiral structures with different parameters are manufactured by adjusting a program of the fusion splicer. The sensor is mainly used for simultaneous detection of multi-parameter sensing, the special microstructure enables the sensor to respond to parameters such as temperature, strain, curvature, torsion and the like very sensitively, and particularly, the sensor can recognize the torsion direction. The sensor is compact in structure and simple in preparation method, and can be applied to the field of multi-parameter sensing.

Description

A tapered cascaded helical four-core fiber optic sensor and its fabrication method Technical Field

[0001] This invention relates to the field of multi-core fiber optic sensing technology, and in particular to a tapered cascaded spiral four-core fiber optic sensor and its fabrication method. Background Technology

[0002] With the rapid development of modern industry and infrastructure, from the precision transmission systems of aerospace vehicles to the health monitoring of large bridge structures, there is an increasingly urgent need for high-precision, real-time monitoring of structural components.

[0003] Torsion, as a critical mechanical state, requires precise sensing to ensure structural safety, optimize system performance, and achieve predictive maintenance. Fiber optic sensors, with their advantages of electromagnetic interference resistance, high sensitivity, small size, and ease of forming distributed sensor networks, are suitable for various extreme environments. However, although fiber optic sensors are mature in measuring physical quantities such as strain, temperature, and vibration, achieving high sensitivity and high reliability in detecting torsion remains an important research direction and technical challenge in this field. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a tapered cascaded helical four-core fiber optic sensor and its fabrication method. The sensor features a novel structure, low cost, and applicability to various parameter measurements. It utilizes multimode beam expansion and fine-tapered beam contraction to achieve mode interference formation and collection within the four-core fiber. Furthermore, the birefringence effect brought about by the helical structure enhances the sensor's torsional sensitivity, combining the inherent advantages of multi-core fiber with those of helical fiber to improve the sensitivity of multi-core fiber sensing.

[0005] The technical solution adopted in this invention is as follows:

[0006] The present invention proposes a conical cascaded spiral four-core fiber optic sensor, comprising a single-mode fiber, a multimode fiber, and a four-core fiber; the single-mode fiber, multimode fiber, and four-core fiber are cascaded in the following order: single-mode fiber ~ multimode fiber ~ four-core fiber ~ single-mode fiber; the fiber core in the middle region of the four-core fiber has a spiral structure.

[0007] Furthermore, the cladding diameter of the single-mode fiber is 120~130μm, and the core diameter is 8~10μm; the cladding diameter of the multimode fiber is 120~130μm, and the core diameter is 100-110μm; the cladding diameter of the four-core fiber is 120~130μm, and the four cores are arranged in a lattice triangle structure.

[0008] Furthermore, the spacing between the middle core and the outer core of the four-core optical fiber is 25μm~35μm, the spacing between the outer cores is 35μm~55μm and they are arranged in an equilateral triangle around the middle core, and the diameter of all cores is 8μm~12μm.

[0009] Furthermore, the length of the spiral structure is 5mm to 10mm, and the spiral angle is 360° to 7200°.

[0010] Furthermore, one end of the four-core optical fiber cascaded with the single-mode optical fiber is configured as a thin tapered structure.

[0011] Furthermore, the length of the thin conical structure portion is 2mm to 5mm, and the diameter of the waist of the cone is 40μm to 80μm.

[0012] A method for fabricating a tapered cascaded spiral four-core fiber optic sensor includes the following steps: S1: First, prepare the required single-mode fiber, multimode fiber, and four-core fiber, remove the fiber coating and clean surface impurities, then cut the fiber to the required length for later use; S2: Place the single-mode fiber and multimode fiber at both ends of a fusion splicer, and cascade them after splicing; S3: Place the cascaded single-mode fiber and multimode fiber, along with the prepared lattice four-core fiber, at both ends of a fusion splicer, and cascade them after splicing to fabricate a single-mode fiber ~ multimode fiber ~ Cascaded structure of four-core fiber optic array; S4: Place the cascaded structure of single-mode fiber ~ multimode fiber ~ four-core fiber optic array in a special fiber optic fusion splicer to create a spiral structure, with the discharge electrode located at the center of the four-core fiber optic array; Discharge parameters are set as follows: discharge current is set to 10~15mA, sweep motor moves to the right at a speed of 0.01~0.1μm / ms, and the moving time is 50000~200000ms; left motor moves to the left at a speed of 0.01~0.03μm / ms, and right motor moves to the left at a speed of... 0.005~0.03μm / ms; simultaneously, the rotary motor starts rotating at a speed of 0.02~0.06° / ms for a rotation time of 50,000~200,000ms; S5: The cascaded structure of single-mode fiber~multimode fiber~array four-core fiber is placed in the special fiber fusion splicer. A thin tapered structure is prepared at the end of the array four-core fiber, and the discharge electrode is located 2~7cm away from the multimode fiber; the discharge parameters are set as follows: discharge intensity of 10~15mA, discharge time of 1500~2500ms, discharge... After several tapering operations, the first section of the fiber is tapered, reducing the fiber diameter from 100-150 μm to 60-80 μm. Then, the discharge intensity is changed to 10-13 mA and the discharge time is 500-2000 ms. After several tapering operations, the fiber diameter is reduced from 60-80 μm to 40-60 μm. S6: The thin tapered structure prepared in step S5 is cut in the middle and then fused with a single-mode fiber using the manual fusion splicing method. Finally, a spiral four-core fiber multi-parameter sensor based on multimode beam expansion and tapered cascading is prepared.

[0013] Furthermore, in step S2, the splicing parameters of the single-mode fiber and the multimode fiber are: discharge intensity of 100~150mA, discharge time of 150~250ms, advance distance of 15~25μm, and discharge times of one.

[0014] Furthermore, in step S3, the splicing parameters of the multimode fiber and the lattice four-core fiber are: discharge intensity of 100~150mA, discharge time of 200~500ms, advance distance of 15~25μm, and discharge times of one.

[0015] Furthermore, in step S6, the splicing parameters of the fine tapered structure and the single-mode optical fiber are set as follows: discharge intensity of 100~150mA, discharge time of 200~500ms, advance distance of 15~25μm, and discharge times of one.

[0016] Compared with the prior art, the present invention has the following advantages: The present invention uses a spiral method to fabricate a spiral structure, which significantly improves its sensitivity to strain and torsion. Furthermore, since the fabrication parameters are controlled by a program, the sensor has extremely high repeatability. The sensor structure adopts a cascaded structure, so the sensor structure is relatively simple and easy to fabricate. Multimode fiber is used to expand the transmitted light beam, transmitting the transmitted light energy to all the cores and cladding of the four-core fiber. The ends of the four-core fiber are tapered to couple the transmitted light into the single-mode fiber, thereby forming a Mach-Zehnder interferometer. Moreover, the spiral four-core fiber structure is fabricated using the arc discharge method, which gives the sensor a birefringence effect. Attached Figure Description

[0017] Figure 1 is a schematic diagram of the structure of a tapered cascaded spiral four-core fiber optic sensor proposed in this invention; Figure 2 is a diagram of the end face structure of the four-core fiber optic cable in this invention; Figure 3 is a microscopic schematic diagram of the spiral structure of the tapered cascaded spiral four-core fiber optic sensor proposed in this invention; Figure 4 is a microscopic schematic diagram of the tapered four-core fiber optic cable in this invention; Figure 5 is a schematic diagram of the fusion splicing of the tapered four-core fiber optic cable and the single-mode fiber in this invention; Figure 6 is a schematic diagram of the transmission spectrum of the sensor in this invention during counterclockwise and clockwise torsion sensing experiments; Figure 7 is a schematic diagram of the result analysis at dip1 of the transmission spectrum of the sensor in this invention during counterclockwise and clockwise torsion sensing experiments; Figure 8 is a schematic diagram of the sensor in this invention. Figure 9 is a schematic diagram of the result analysis at dip2 of the transmission spectrum in the counterclockwise and clockwise torsion sensing experiment of the present invention; Figure 10 is a schematic diagram of the result analysis at dip4 of the transmission spectrum in the counterclockwise and clockwise torsion sensing experiment of the present invention; Figure 11 is a schematic diagram of the transmission spectrum in the temperature sensing experiment of the present invention; Figure 12 is a schematic diagram of the result analysis of the temperature sensing experiment of the present invention; Figure 13 is a schematic diagram of the transmission spectrum in the strain sensing experiment of the present invention; Figure 14 is a schematic diagram of the result analysis of the strain sensing experiment of the present invention.

[0018] In the attached figures, the following labels are used: 1-single-mode fiber; 2-multimode fiber; 3-four-core fiber; 4-spiral structure; 5-fine tapered structure. Detailed Implementation

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] It should be noted that in the description of this invention, the terms "upper", "lower", "top", "bottom", "one side", "the other side", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not mean that the device or element must have a specific orientation, or be constructed and operated in a specific orientation.

[0021] Referring to Figures 1-5, the conical cascaded spiral four-core fiber optic sensor proposed in this invention includes a single-mode fiber 1, a multimode fiber 2, and a four-core fiber 3. The single-mode fiber 1, multimode fiber 2, and four-core fiber 3 are cascaded in the following order: single-mode fiber 1 ~ multimode fiber 2 ~ four-core fiber 3 ~ single-mode fiber 1; in this invention, the fiber core in the middle region of the four-core fiber 3 has a spiral structure 4.

[0022] The single-mode fiber 1 has a cladding diameter of 120-130 μm and a core diameter of 8-10 μm; the multimode fiber 2 has a cladding diameter of 120-130 μm and a core diameter of 100-110 μm.

[0023] The cladding diameter of the four-core optical fiber 3 is 120~130μm, and the four cores are arranged in a lattice triangle structure. The spacing between the middle core and the three outer cores of the four-core optical fiber 3 is 25μm~35μm, the spacing between the outer cores is 35μm~55μm, the three outer cores are arranged in an equilateral triangle around the middle core, and the diameter of the four cores is 8μm~12μm.

[0024] The length of the four parts of the spiral structure is 5mm to 10mm, and the spiral angle is 360° to 7200°.

[0025] In this invention, one end of the four-core optical fiber 3 cascaded with the single-mode optical fiber 1 is configured as a thin tapered structure 5; the length of the thin tapered structure 5 is 2mm~5mm, and the diameter of the waist is 40μm~80μm.

[0026] The fabrication process of the above sensor is described below through specific embodiments: a special optical fiber fusion splicer is used to fabricate the four-core optical fiber helical structure 4 and the fine conical structure 5 in the sensor. This type of sensor has a simple, compact structure and is easy to fabricate. At the same time, compared with other multi-core optical fiber sensors, this invention is more sensitive to stress, torsion and other types of axial forces.

[0027] The physical basis of this sensor configuration lies in the complex interference mechanism formed in the helical four-core fiber after multimode expansion. This mechanism simultaneously includes interference between core modes caused by multimode fiber expansion, interference between core and cladding modes, and the birefringence effect introduced by the helical structure. It is the combined effect of these mechanisms that causes different wavelengths in the sensor's transmission spectrum to exhibit different responses to torsion, thus enabling better decoupling and separation of torsion parameters. The relatively short, tapered structure allows the transmitted light energy in the four-core fiber to be better focused into the single-mode fiber, while also allowing for better contact between the transmitted light and the external environment, thereby improving the sensor's sensing sensitivity.

[0028] The operation of fabricating a four-core fiber spiral structure sensor using the arc discharge method is as follows: A method for fabricating a tapered cascaded spiral four-core fiber sensor includes the following steps: S1: First, prepare the required single-mode fiber 1, multimode fiber 2, and lattice four-core fiber 3. The parameters of each fiber are as follows: the cladding diameter of the single-mode fiber is 125 μm, and the core diameter is 9 μm; the cladding diameter of the multimode fiber is 125 μm, and the core diameter is 105 μm; the cladding diameter of the four-core fiber is 125 μm, the four cores are arranged in a lattice triangular structure, the spacing between the outer cores is 56.4 μm, and they form an equilateral triangle surrounding the middle core. The diameter of each of the four cores is 8.5 μm. Second, use fiber strippers to remove the fiber coating and wipe with alcohol to remove impurities. Finally, use a cleaver to cut the fiber to the required length for later use. The length of the multimode fiber is 0.05~0.5 cm, which is 0.3 cm in this embodiment, and the length of the four-core fiber is 2~7 cm, which is 4 cm in this embodiment.

[0029] S2: Place the single-mode fiber 1 (with the coating removed and wiped with alcohol) and the 0.3cm long multimode fiber 2 at both ends of the fusion splicer, and then cascade them after splicing; wherein, the splicing parameters are set as follows: discharge intensity is 100~150mA, which is 100mA in this embodiment; discharge time is 150~250ms, which is 210ms in this embodiment; advance distance is 15~25μm, which is 19μm in this embodiment; and the number of discharges is one.

[0030] S3: Place the cascaded structure of single-mode fiber 1 and multimode fiber 2 prepared in step S2 and the prepared 4cm lattice four-core fiber 3 at both ends of the fusion splicer and cascade them after fusion splicing; wherein, the fusion splicing parameters are set as follows: discharge intensity is 100~150mA, which is 110mA in this embodiment; discharge time is 200~500ms, which is 300ms in this embodiment; advance distance is 15~25μm, which is 17μm in this embodiment; discharge once, and a cascaded structure of single-mode fiber~multimode fiber~lattice four-core fiber is prepared.

[0031] S4: Place the cascaded structure of single-mode fiber, multi-mode fiber, and four-core fiber array prepared in step S3 into a special fiber fusion splicer to create a spiral structure 4. The discharge electrode is located at the center of the four-core fiber 3. The discharge parameters are set as follows: the discharge current is set to 10~15mA (12.6mA in this embodiment), the sweep motor moves to the right at a speed of 0.01~0.1μm / ms (0.05μm / ms in this embodiment), and the moving time is 50000~200000ms. For example, the time is 150,000 ms. The left motor moves to the left at a speed of 0.01~0.03 μm / ms, which is 0.015 μm / ms in this embodiment. The right motor moves to the left at a speed of 0.005~0.03 μm / ms, which is 0.01 μm / ms in this embodiment. Simultaneously, the rotary motor starts rotating at a speed of 0.02~0.06° / ms, which is 0.05° / ms in this embodiment. The rotation time is 50,000~200,000 ms, which is 150,000 ms in this embodiment. The spiral structure 4 of the four-core optical fiber is fabricated through the above steps.

[0032] S5: The cascaded structure of single-mode fiber, multimode fiber, and four-core fiber array prepared in step S4 is placed in a special fiber fusion splicer. A thin tapered structure 5 is prepared at the outer end of the four-core fiber 3. The discharge electrode is located 2-7 cm away from the multimode fiber 2 (4 cm in this embodiment). The discharge parameters are set as follows: discharge intensity of 10-15 mA (13.3 mA in this embodiment), and discharge time of 1500-2500 ms (2000 ms in this embodiment). The fiber is tapered several times. The first segment taper operation involves pulling the fiber diameter from 125 μm to 80 μm, then changing the discharge intensity to 10-13 mA (12.3 mA in this embodiment) and the discharge time to 500-2000 ms (1500 ms in this embodiment). The second segment taper operation involves pulling the fiber diameter from 80 μm to 60 μm. Through the above steps, a thin tapered structure 5 can be prepared at the end of the four-core fiber 3.

[0033] S6: Cut the thin tapered structure 5 prepared in step S5 in the middle, and then use the manual fusion splicing method to fusion splice it with the single-mode fiber 1. The fusion splicing parameters are set as follows: discharge intensity of 100~150mA (110mA in this embodiment), discharge time of 200~500ms (300ms in this embodiment), advance distance of 15~25μm (17μm in this embodiment), and one discharge cycle. Finally, a helical four-core fiber multi-parameter sensor based on multimode beam expansion and tapered cascading is fabricated.

[0034] Temperature, stress, and torsion experiments were conducted on the fabricated helical four-core fiber optic multi-parameter sensor based on multimode beam expansion and conical cascade. A temperature, strain, and torsion experimental system was built, with a light source and a spectrometer connected to both ends of the sensor. The light source used was a broadband light source, and the spectrometer used was an AQ6375 optical spectrometer. The temperature, strain, and torsion sensing experiments of the sensor were conducted in a laboratory temperature environment of 23℃.

[0035] Figure 6 shows the drift of the sensor under different torsion rates. During the torsion sensing experiment, as the torsion angle increases, the drift phenomena at each trough are different in the counterclockwise and clockwise experiments, as shown in Figures 7-10. Analysis of the results at dip1 shows that in the counterclockwise torsion experiment, the torsion sensitivity is -295 pm / (rad / m) in the torsion rate range of 0 to -9.3 rad / m; and -381 pm / (rad / m) in the torsion rate range of -9.3 rad / m to -20.93 rad / m, with a linear fitting degree of 0.992. In the clockwise torsion experiment, within the torsion rate range of 0–10.66 rad / m, the torsion sensitivity was -252 pm / (rad / m), with a linear fit of 0.999; within the torsion rate range of 10.66 rad / m–20.93 rad / m, the torsion sensitivity was 341 pm / (rad / m), with a linear fit of 0.995. Analysis of the results at dip2 shows that in the counterclockwise torsion experiment, within the torsion rate range of 0–-20.93 rad / m, the torsion sensitivity was 277 pm / (rad / m), with a linear fit of 0.994. In the clockwise torsion experiment, within the torsion rate range of 0–20.93 rad / m, the torsion sensitivity was -143 pm / (rad / m), with a linear fit of 0.995. Analysis of the results at dip3 shows that in the counterclockwise torsion experiment, within the torsion rate range of 0 to -20.93 rad / m, the torsional sensitivity is 32 pm / (rad / m), and the linear fit is 0.991. In the clockwise torsion experiment, within the torsion rate range of 0 to 20.93 rad / m, the torsional sensitivity is 203 pm / (rad / m), and the linear fit is 0.987. Analysis of the results at dip4 shows that in the counterclockwise torsion experiment, within the torsion rate range of 0 to -20.93 rad / m, the torsional sensitivity is -229 pm / (rad / m), and the linear fit is 0.996. In the clockwise torsion experiment, within the torsion rate range of 0 to 20.93 rad / m, the torsional sensitivity is -144 pm / (rad / m), and the linear fit is 0.993.

[0036] Figure 11 shows the drift of the sensor at different temperatures. During the temperature sensing experiment, the resonance peak showed a redshift phenomenon. As shown in Figure 12, within the temperature range of 25℃ to 100℃, the temperature sensitivity at dip1 was 9.8 pm / ℃ with a linear fit of 0.997, the temperature sensitivity at dip2 was 32.7 pm / ℃ with a linear fit of 0.998, the temperature sensitivity at dip3 was 36.6 pm / ℃ with a linear fit of 0.995, and the temperature sensitivity at dip1 was 32 pm / ℃ with a linear fit of 0.994.

[0037] Figure 13 shows the drift of the sensor under different strains. During the strain sensing experiment, the resonance peak exhibits a blue shift as the strain increases. As shown in Figure 14, within the strain range of 0~1000με, the strain sensitivity at dip1 is -10.17 nm / mε with a linear fit of 0.992, the strain sensitivity at dip2 is -2.74 nm / mε with a linear fit of 0.998, the strain sensitivity at dip3 is -0.795 nm / mε with a linear fit of 0.992, and the strain sensitivity at dip4 is -2.65 nm / mε with a linear fit of 0.997.

[0038] Referring to Figures 6-14, the sensor prepared using this invention can achieve multi-parameter sensing and detection. Furthermore, due to its different sensing principles, combined with the torsion-enhancing effect of the spiral structure 4 and the strain-enhancing effect of the fine conical structure 5, the sensor proposed in this invention can better detect torsion and strain parameters. Therefore, this sensor has advantages such as novel and compact structure, high sensitivity, and simultaneous detection of multiple parameters, and can be widely used in the fields of aviation safety, transportation pipelines, and structural safety.

[0039] Matters not covered in this invention are common knowledge.

[0040] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A conical cascaded spiral four-core fiber optic sensor, characterized in that: The sensor includes a single-mode fiber, a multimode fiber, and a four-core fiber; the single-mode fiber, multimode fiber, and four-core fiber are cascaded in the following order: single-mode fiber ~ multimode fiber ~ four-core fiber ~ single-mode fiber; the fiber core in the middle region of the four-core fiber has a spiral structure.

2. The conical cascaded spiral four-core fiber optic sensor according to claim 1, characterized in that: The single-mode fiber has a cladding diameter of 120~130μm and a core diameter of 8~10μm; the multimode fiber has a cladding diameter of 120~130μm and a core diameter of 100~110μm; the four-core fiber has a cladding diameter of 120~130μm and the four cores are arranged in a lattice triangle structure.

3. The conical cascaded spiral four-core fiber optic sensor according to claim 2, characterized in that: The spacing between the middle core and the outer core of the four-core optical fiber is 25μm~35μm, and the spacing between the outer cores is 35μm~55μm, arranged in an equilateral triangle around the middle core. The diameter of all cores is 8μm~12μm.

4. A conical cascaded spiral four-core fiber optic sensor according to claim 2, characterized in that: The length of the spiral structure is 5mm to 10mm, and the spiral angle is 360° to 7200°.

5. A conical cascaded spiral four-core fiber optic sensor according to claim 1, characterized in that: One end of the four-core optical fiber cascaded with the single-mode optical fiber is configured with a thin tapered structure.

6. A conical cascaded spiral four-core fiber optic sensor according to claim 5, characterized in that: The length of the thin conical structure is 2mm to 5mm, and the diameter of the waist is 40μm to 80μm.

7. The method for fabricating a conical cascaded spiral four-core fiber optic sensor according to claim 5, characterized in that: The method includes the following steps: S1: First, prepare the required single-mode fiber, multimode fiber, and four-core fiber, remove the fiber coating and clean surface impurities, then cut the fiber to the required length for later use; S2: Place the single-mode fiber and multimode fiber at both ends of the fusion splicer, and cascade them after splicing; S3: Place the cascaded single-mode fiber and multimode fiber, along with the prepared lattice four-core fiber, at both ends of the fusion splicer, and cascade them after splicing to prepare a cascaded structure of single-mode fiber ~ multimode fiber ~ lattice four-core fiber; S4: ... A cascaded structure of single-mode fiber, multi-mode fiber, and a four-core fiber optic array is placed in a special fiber optic fusion splicer to create a spiral structure. The discharge electrode is located at the center of the four-core fiber. The discharge parameters are set as follows: discharge current of 10~15mA, sweep motor moving to the right at a speed of 0.01~0.1μm / ms for a time of 50,000~200,000ms, left motor moving to the left at a speed of 0.01~0.03μm / ms, and right motor moving to the left at a speed of 0.005~0.03μm / ms. Simultaneously, the rotary motor begins to rotate at a speed of 0.02~0.06° / ms, with a rotation time of 50000~200000ms; S5: The cascaded structure of single-mode fiber~multimode fiber~array four-core fiber is placed in the special fiber fusion splicer. A thin tapered structure is prepared at the end of the array four-core fiber, and the discharge electrode is located 2~7cm away from the multimode fiber; the discharge parameters are set as follows: discharge intensity of 10~15mA, discharge time of 1500~2500ms, and several tapered discharge cycles. The first fiber tapering operation is performed to pull the fiber diameter from 100~150μm to 60~80μm. Then, the discharge intensity is changed to 10~13mA and the discharge time is 500~2000ms. The tapering is performed several times to pull the fiber diameter from 60~80μm to 40~60μm. S6: The thin tapered structure prepared in step S5 is cut in the middle and then fused with a single-mode fiber using the manual fusion splicing method. Finally, a spiral four-core fiber multi-parameter sensor based on multimode beam expansion and tapered cascading is prepared.

8. The method for fabricating a conical cascaded spiral four-core fiber optic sensor according to claim 7, characterized in that: In step S2, the splicing parameters of single-mode fiber and multimode fiber are: discharge intensity of 100~150mA, discharge time of 150~250ms, advance distance of 15~25μm, and discharge times of one.

9. The method for fabricating a conical cascaded spiral four-core fiber optic sensor according to claim 7, characterized in that: In step S3, the splicing parameters of the multimode fiber and the lattice four-core fiber are: discharge intensity of 100~150mA, discharge time of 200~500ms, advance distance of 15~25μm, and discharge times of one.

10. The method for fabricating a conical cascaded spiral four-core fiber optic sensor according to claim 7, characterized in that: In step S6, the splicing parameters of the fine tapered structure and the single-mode fiber are set as follows: discharge intensity of 100~150mA, discharge time of 200~500ms, advance distance of 15~25μm, and one discharge cycle.

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