Preparation method of optical fiber stress sensor and optical fiber stress sensor

Abstract

The invention relates to the technical field of optical fiber mechanical sensing, in particular to a preparation method of an optical fiber stress sensor and the optical fiber stress sensor. The invention aims to solve the problems of complex structure and high manufacturing difficulty of an optical fiber mechanical sensor. The preparation method comprises the following steps: welding the tail end of the first single-mode optical fiber with the front end of the hollow-core optical fiber; cutting the hollow-core optical fiber to enable the hollow-core optical fiber welded with the first single-mode optical fiber to remain a preset length; welding the tail end of the hollow-core optical fiber with the reserved preset length with the front end of a second single-mode optical fiber; at least one of the tail end of the fiber core of the first single-mode fiber and the front end of the fiber core of the second single-mode fiber is in core shift welding with the fiber core of the hollow-core fiber, and radial dislocation is formed at the corresponding welding position. In addition, the invention further provides the optical fiber stress sensor which is prepared by the method, the first single-mode optical fiber serves as a signal light leading-in optical fiber of the optical fiber stress sensor, and the second single-mode optical fiber serves as a signal light leading-out optical fiber of the optical fiber stress sensor.

Description

Technical Field

[0001] This invention relates to the field of fiber optic mechanical sensing technology, and more specifically, to a method for fabricating a fiber optic stress sensor and the fiber optic stress sensor itself. Background Technology

[0002] A stress sensor is a device that converts stress (or strain) inside or on the surface of an object into a measurable electrical signal (such as changes in voltage, resistance, or capacitance). Among these, fiber optic force sensing technology exhibits significant advantages in the field of weak force measurement due to its ultra-low transmission loss, flexibility, strong resistance to electromagnetic interference, light weight, and ease of miniaturization.

[0003] Patent document CN118190210A discloses a residual stress measurement system based on fiber optic sensing. The device includes: an array of fiber Bragg gratings, a heating element, a circulator, a radiation source, a resonant cavity, a photodetector, and a power supply. The fiber Bragg gratings are positioned on the surface of the object under test in both a stress-free region and a stress-laden region. The heating element heats both the stress-laden and stress-free regions. A second radiation source is connected to the circulator; the circulator is connected to the first resonant cavity; and the second resonant cavity is connected to the first photodetector, thereby achieving stress measurement. This method requires heating and temperature control before measurement at different temperatures, making it difficult to apply under normal operating temperatures. Furthermore, the need for a heating element results in a relatively complex structure.

[0004] Patent document CN119197366A discloses a fiber optic strain sensor based on the vernier effect and its fabrication method. The fiber optic strain sensor based on the vernier effect consists of a 1-to-2 coupler, a high-sensitivity Fabry-Perot interferometer used for sensing, and a low-sensitivity Fabry-Perot interferometer used for reference, all fused in parallel with the 1-to-2 coupler. The fiber optic strain sensor produced by this method is cumbersome to fabricate, involves complex processes, and requires a high level of skill from the fabrication personnel.

[0005] In summary, existing fiber optic sensors still suffer from problems such as complex structure and high manufacturing difficulty when applied to stress measurement. Summary of the Invention

[0006] The present invention aims to overcome at least one of the defects of the prior art and provide a method for preparing an optical fiber stress sensor and an optical fiber stress sensor, thereby solving the problems of complex structure and high manufacturing difficulty.

[0007] The first objective of this invention is to provide a method for fabricating an optical fiber stress sensor, comprising: The end of the first single-mode fiber is fused to the front end of the hollow fiber. Cut the hollow fiber so that the hollow fiber fused with the first single-mode fiber retains a preset length; The end of the hollow fiber, retaining a preset length, is fused to the front end of the second single-mode fiber. In this embodiment, at least one of the core end of the first single-mode fiber and the core front end of the second single-mode fiber is eccentrically fused with the core of the hollow fiber, and radial misalignment is formed at the corresponding fusion point.

[0008] In this invention, by staggering the cores of the first single-mode fiber and the hollow-core fiber, when signal light enters the hollow-core fiber from the first single-mode fiber, some optical energy is coupled into the cladding of the hollow-core fiber, exciting multiple higher-order modes. These higher-order modes, along with the fundamental mode propagating in the core, are transmitted together in the hollow-core fiber. By staggering the cores of the second single-mode fiber and the hollow-core fiber, the higher-order modes and the fundamental mode can be recoupled back to the core of the second single-mode fiber at the second staggered fusion point, resulting in multimode interference. Besides the staggered ends, this invention can also employ an asymmetric connection method of 'front-end staggered-back-end aligned' or 'front-end aligned-back-end staggered'. Specifically, the former involves staggering the cores of the first single-mode fiber and the hollow-core fiber, and aligning the cores of the second single-mode fiber and the hollow-core fiber. In this method, the fiber stress sensor utilizes the central aperture effect of the output fiber to extract the interference signal. The latter specifically involves core alignment fusion of the first single-mode fiber and the hollow-core fiber, and core misalignment fusion of the second single-mode fiber and the hollow-core fiber. This type of fiber optic stress sensor utilizes mode field mismatch at the input end to excite multimode signals and acquires interference signals through off-axis sampling at the output end. When subjected to external axial stress, all three configurations of the fiber optic stress sensor cause slight changes in the physical length and effective refractive index of the hollow-core fiber, leading to a change in the phase difference between different modes. This results in a wavelength shift in the characteristic peaks or valleys of the output interference spectrum. By monitoring this wavelength shift, accurate measurement of the corresponding force can be achieved. This invention has a simple structure and requires only two fusion operations in the radial misalignment during fabrication, significantly reducing the fabrication difficulty.

[0009] In some embodiments, the radial misalignment is 4-8 μm.

[0010] Based on the misalignment range of the present invention, it is possible to ensure the excitation of higher-order modes, improve the sensitivity of the sensor, and avoid excessive misalignment from increasing insertion loss and reducing signal-to-noise ratio.

[0011] In some implementations, the preset length is 3-5 mm.

[0012] In this invention, based on the preset length, it can ensure effective interference between modes, avoid excessively low output power, and prevent excessive noise interference from affecting detection accuracy.

[0013] In some embodiments, a first radial misalignment is formed between the core tip of the hollow fiber and the core end of the first single-mode fiber through eccentric fusion splicing, and a second radial misalignment is formed between the core end of the hollow fiber and the core tip of the second single-mode fiber through eccentric fusion splicing. The directions of the first radial misalignment and the second radial misalignment are symmetrical about the hollow fiber.

[0014] In this invention, by symmetrically setting the two ends of the hollow fiber to be misaligned, the output power of the signal light can be avoided to ensure that the output signal can be detected.

[0015] In some embodiments, the hollow optical fiber is a capillary hollow optical fiber with an axial through cavity, and the core of the hollow optical fiber is formed by air filling the axial through cavity.

[0016] In some embodiments, the core diameter of the hollow optical fiber is 25-35 μm, and the cladding diameter of the hollow optical fiber is 100-150 μm.

[0017] In some embodiments, the core diameter of the first single-mode fiber is 6-12 μm, and the cladding diameter of the first single-mode fiber is 100-150 μm.

[0018] In some embodiments, the core diameter of the second single-mode fiber is 6-12 μm, and the cladding diameter of the second single-mode fiber is 100-150 μm.

[0019] A second objective of this invention is to provide an optical fiber stress sensor, which is prepared using the method described above, wherein the first single-mode optical fiber serves as the signal light guide fiber of the optical fiber stress sensor, and the second single-mode optical fiber serves as the signal light exit fiber of the optical fiber strain sensor.

[0020] In some embodiments, the hollow optical fiber, along with a portion of the first single-mode optical fiber and a portion of the second single-mode optical fiber, is suspended during stress detection to form the sensing part of the optical fiber stress sensor.

[0021] In some embodiments, the length of the sensing element is 6-40 cm.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Simple to manufacture and low cost: The standard optical fiber fusion splicing process is adopted, without the need for complex steps such as optical fiber tapering, chemical corrosion or additional coating. The preparation process is simple, highly repeatable, and the device is robust and durable.

[0023] (2) High sensitivity and adjustable performance: By optimizing structural parameters such as misalignment, hollow fiber length and suspension length, the sensitivity and range can be flexibly adjusted. Experiments show that the minimum measurable force can reach 0.02N, the measurement accuracy reaches the millinewton level, and the highest sensitivity can reach -0.82nm / N.

[0024] (3) Good stability and resistance to electromagnetic interference: The sensor is an all-fiber structure made of quartz, which has excellent chemical and temperature stability and is completely immune to electromagnetic interference, making it suitable for complex industrial environments.

[0025] (4) The system is easy to integrate: the demodulation of the sensor can be completed through standard spectral analysis equipment, which is easy to integrate with existing fiber optic communication and sensor networks, and facilitates distributed sensing and automated remote monitoring. Attached Figure Description

[0026] Figure 1 This is a structural diagram of an optical fiber stress sensor according to some embodiments of the present invention.

[0027] Figure 2 This is a structural diagram of an optical fiber stress sensor according to other embodiments of the present invention.

[0028] Figure 3 This is a structural diagram of the stress measurement system of the present invention.

[0029] Figure 4 The graph shows the spectral response characteristics and wavelength-stress variation of the sensor device with single-end misaligned welding in Test Example 1 under different axial stresses.

[0030] Figure 5 The graph shows the characteristic peak wavelength of the fiber optic stress sensor with double-ended misaligned fusion splicing in Test Example 2 as a function of strain magnitude.

[0031] Figure 6 The graph shows the characteristic peak wavelength of the fiber optic stress sensor in Test Example 3 as a function of stress magnitude under different misalignment amplitudes at the single end of a hollow fiber.

[0032] Figure 7 To test Example 4, the optical fiber stress sensor with double-ended misaligned fusion splicing exhibits spectral response characteristics and wavelength-stress variation patterns under different axial stresses. Figure 7 (a) Transmission spectra under different stresses. Figure 7 (b) is the selected option. Figure 7 (a) A magnified view of a local spectral characteristic peak. Figure 7 (c) shows the measurement results of the characteristic peak wavelength selected under the corresponding conditions as a function of stress.

[0033] Figure 8The graph shows the characteristic peak wavelength of the fiber stress sensor with double-ended misaligned fusion splicing in Test Example 5 as a function of stress magnitude under different hollow fiber lengths.

[0034] Figure 9 The graph shows the characteristic peak wavelength of the fiber optic stress sensor with double-ended misaligned fusion splicing in Test Example 6 as a function of stress magnitude when different sensing element lengths are set. Figure 9 (a) is a graph showing the variation of the characteristic peak wavelength with stress when the sensor length is 300 mm. Figure 9 (b) is a graph showing the variation of characteristic peak wavelength with stress when the sensor length is 400 mm.

[0035] Reference numerals in the attached figures: 1. Signal light guide fiber; 2. Hollow-core fiber; 3. Signal light effluent fiber; 4. Signal light transmitter; 5. First displacement platform; 51. First fiber clamp; 6. Second displacement platform; 61. Second fiber clamp; 7. Spectrum analyzer; 8. Sensor unit. Detailed Implementation

[0036] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the invention. To better illustrate the following embodiments, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions; it is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0037] Furthermore, in this invention, unless otherwise explicitly specified and limited, the terms "welding," "fixing," etc., should be interpreted broadly. For example, "fixing" can mean fixed welding, detachable welding, or integral bonding; it can mean direct connection or indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0038] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. When the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0039] Example 1 This invention first provides a method for fabricating an optical fiber stress sensor, comprising the following steps: S1. Fusion splice the end of the first single-mode fiber to the front end of the hollow fiber; S2. Cut the hollow fiber so that the hollow fiber fused with the first single-mode fiber retains a preset length; S3. The end of the hollow fiber, which retains a preset length, is fused to the front end of the second single-mode fiber.

[0040] In some embodiments, the hollow-core fiber is connected to the first single-mode fiber and the second single-mode fiber via a single-end eccentric misalignment fusion splice. In one implementation, the first single-mode fiber and the hollow-core fiber are eccentrically misaligned fused together. In this case, step S1 includes: eccentrically fused between the fiber core end of the first single-mode fiber and the fiber core of the hollow-core fiber, and forming a radial misalignment at the corresponding fusion splice. It can be understood that, under this method, the resulting fiber optic stress sensor includes a first single-mode fiber, a hollow-core fiber, and a second single-mode fiber connected end-to-end, and the end of the first single-mode fiber and the front end of the hollow-core fiber are eccentrically misaligned fused together.

[0041] In another implementation, the hollow-core fiber and the second single-mode fiber are eccentrically misaligned fused together. In this case, step S3 includes: eccentrically fused between the front end of the second single-mode fiber and the end of the hollow-core fiber, forming a radial misalignment at the corresponding fusion joint. It can be understood that, in this method, the resulting fiber optic stress sensor includes a first single-mode fiber, a hollow-core fiber, and a second single-mode fiber connected end-to-end, and the front end of the second single-mode fiber and the end of the hollow-core fiber are eccentrically misaligned fused together.

[0042] In some embodiments, the hollow-core optical fiber is fused with the first single-mode optical fiber and the second single-mode optical fiber in a double-ended eccentric misalignment fusion splice. In this case, step S1 further includes: eccentric fusion splicing between the core end of the first single-mode optical fiber and the core of the hollow-core optical fiber, forming a radial misalignment at the corresponding fusion splice; simultaneously, step S3 further includes: eccentric fusion splicing between the core front end of the second single-mode optical fiber and the core end of the hollow-core optical fiber, forming a radial misalignment at the corresponding fusion splice. It can be understood that, in this manner, the resulting optical fiber stress sensor includes a first single-mode optical fiber, a hollow-core optical fiber, and a second single-mode optical fiber connected end-to-end, and both the end of the first single-mode optical fiber and the front end of the hollow-core optical fiber, as well as the front end of the second single-mode optical fiber and the end of the hollow-core optical fiber, are eccentric misalignment fusion splices.

[0043] In practice, the first single-mode fiber and the second single-mode fiber are both standard single-mode fibers with the coating removed and then cleaned with alcohol; similarly, the hollow-core fiber is a standard hollow-core fiber with the coating removed and then cleaned with anhydrous alcohol. That is, in this embodiment, the signal light guide fiber, the hollow-core fiber, and the signal light guide fiber all consist only of the fiber core and the cladding covering the outer periphery of the fiber core.

[0044] In step S2, the cutting includes: using a precision fiber optic cleaver to ensure that the end face is flat, vertical and free of chipped edges or gaps.

[0045] The following describes an implementation method involving a double-ended offset fusion splice between a hollow-core optical fiber and two single-mode and second single-mode optical fibers. In step S1, the splicing includes: fixing the cut first single-mode and hollow-core optical fibers onto the clamps on both sides of the fusion splicer; applying a preset radial offset amount through a displacement platform; and controlling the fiber fusion splicer to perform discharge fusion processing on the two flat end faces, thereby forming a first radial offset fusion point between the end of the first single-mode optical fiber and the front end of the hollow-core optical fiber.

[0046] Similarly, in step S3, the fusion splicing includes: fixing the cut second single-mode fiber and hollow fiber onto the clamps on both sides of the fusion splicer, applying a preset radial misalignment amount through the displacement platform, and controlling the fiber fusion splicer to perform discharge fusion treatment on the two flat end faces, thereby forming a second radial misalignment fusion point between the front end of the second single-mode fiber and the end of the hollow fiber.

[0047] In the double-ended misalignment implementation, the direction of the first radial misalignment and the direction of the second radial misalignment are symmetrical about the hollow fiber. This prevents the output power of the signal light from being too low, ensuring that the output signal can be detected.

[0048] In this invention, the core diameter of the first single-mode fiber is 6-12 μm, and the cladding diameter is 100-150 μm. The core diameter of the second single-mode fiber is also 6-12 μm, and the cladding diameter is 100-150 μm. In this embodiment, the first and second single-mode fibers use the same fiber structure, which simplifies the overall structure of the fiber optic stress sensor and reduces the fabrication difficulty. For example, the cladding diameter of both the first and second single-mode fibers is 125 μm, and the core diameter is 9 μm.

[0049] In this invention, the hollow optical fiber is implemented using a capillary hollow optical fiber. It can be understood that the capillary hollow optical fiber has an axial through cavity. By filling the axial through cavity with air, the core of the hollow optical fiber is formed. In specific implementation, the cladding of the hollow optical fiber is made of quartz material.

[0050] In this invention, the core diameter of the hollow-core optical fiber is 25-35 μm, and the cladding diameter is 100-150 μm. For example, the core diameter of the hollow-core optical fiber is 30 μm, and the cladding diameter is 125 μm.

[0051] In this invention, the length of the hollow fiber is 3-5mm. Within this range, the situation where the hollow fiber is too short and therefore cannot form a significant interference phenomenon can be avoided; it can also prevent the output power from being significantly attenuated due to the hollow fiber being too long, resulting in a low output spectral power. Thus, the accuracy and reliability of stress detection can be ensured.

[0052] In this invention, for the implementation of the two-end misaligned fusion splicing, by misaligning the cores of the first single-mode fiber and the hollow fiber, when the signal light enters the hollow fiber from the first single-mode fiber, some of the optical energy is coupled into the cladding of the hollow fiber, exciting multiple higher-order modes. These higher-order modes, along with the fundamental mode propagating in the core, are transmitted together in the hollow fiber. By misaligning the cores of the second single-mode fiber and the hollow fiber, the higher-order modes and the fundamental mode can be recoupled back to the core of the second single-mode fiber at the second misaligned fusion splice point, resulting in multimode interference. For the 'front-end misalignment-back-end alignment' implementation, i.e., the cores of the first single-mode fiber and the hollow fiber are misaligned fused, and the cores of the second single-mode fiber and the hollow fiber are aligned fused, the fiber stress sensor in this method utilizes the central aperture effect of the output fiber to extract the interference signal. In the 'front-end alignment-back-end misalignment' implementation, i.e., the cores of the first single-mode fiber and the hollow fiber are aligned and fused together, while the cores of the second single-mode fiber and the hollow fiber are misaligned and fused together, the fiber stress sensor in this method utilizes the mode field mismatch at the input end to excite multimode signals and obtains interference signals through off-axis sampling at the output end. Studies have shown that when subjected to external axial stress, the three types of fiber stress sensors described in this invention can all cause slight changes in the physical length and effective refractive index of the hollow fiber, leading to a change in the phase difference between different modes. This results in a wavelength shift in the characteristic peaks or valleys of the output interference spectrum. By monitoring this wavelength shift, accurate measurement of the corresponding force can be achieved. This invention has a simple structure and requires only two fusion operations in the radial misalignment stage during fabrication, significantly reducing the fabrication difficulty.

[0053] Based on the above preparation method, refer to Figure 1 , 2 This embodiment also provides an optical fiber stress sensor, including a signal light guiding fiber 1, a hollow fiber 2, and a signal light discharging fiber 3. The front end of the hollow fiber 2 is connected to the end of the signal light guiding fiber 1, and the end of the hollow fiber 2 is connected to the front end of the signal light discharging fiber 3. Among them, at least one of the fiber core end of the signal light guide fiber 1 and the fiber core front end of the signal light detach fiber 3 is connected to the fiber core of the hollow fiber 2 in an off-core connection, and a radial misalignment is formed at the corresponding connection.

[0054] For example, refer to Figure 1In this embodiment, the hollow-core fiber 2 is spliced ​​with the signal light guide fiber 1 and the signal light exit fiber 3 using a double-ended off-center splice. Specifically, the cladding diameters of the hollow-core fiber 2, the signal light guide fiber 1, and the signal light exit fiber 3 are the same, and the cores of all three are located at the center of the cladding. Therefore, it can be understood that the off-center distance between the hollow-core fiber 2 and the signal light guide fiber 1 or the signal light exit fiber 3 is equivalent to the radial misalignment distance d between the corresponding cores. In this embodiment, the radial misalignment distance d is 4-8 μm. Within this misalignment range, it ensures the excitation of higher-order modes, improves the sensor's sensitivity, and avoids excessive misalignment that could increase insertion loss and reduce the signal-to-noise ratio. Furthermore, in order to avoid the output power of the signal light being too low, in this embodiment, the hollow fiber 2 and the signal light guide fiber 1 are fused together with an eccentric misalignment, forming a first radial misalignment at the fusion joint. The hollow fiber 2 and the signal light output fiber 3 are fused together with an eccentric misalignment, forming a second radial misalignment at the fusion joint. The directions of the first radial misalignment and the second radial misalignment are symmetrical about the hollow fiber 2, thereby ensuring that the output signal can be detected during stress testing.

[0055] refer to Figure 2 In this embodiment, the hollow-core fiber 2 and the signal light guide fiber 1 are core-to-core fusion spliced, while the hollow-core fiber 2 and the signal light guide fiber 3 are single-end off-core misaligned fusion spliced. Specifically, a radial misaligned fusion splice is formed between the fiber core end of the hollow-core fiber 2 and the fiber core front end of the signal light guide fiber 3.

[0056] It is understood that in some other embodiments, the fiber optic stress sensor may also employ a core-to-core fusion splice between the hollow-core fiber 2 and the signal light guiding fiber 3, while the hollow-core fiber 2 and the signal light guiding fiber 1 are connected by a single-end off-core misaligned fusion splice. Specifically, a radial misaligned fusion splice is formed between the front end of the core of the hollow-core fiber 2 and the rear end of the core of the signal light guiding fiber 1.

[0057] It is understood that in this invention, the first single-mode fiber serves as the signal light input fiber 1, and the second single-mode fiber serves as the signal light output fiber 3. In use, the signal light input fiber 1 is connected to the signal light transmitter 4, and the signal light output fiber 3 is connected to the spectrum analyzer 7.

[0058] refer to Figure 3To conduct stress testing on the fiber optic stress sensor, the present invention also provides a stress measurement system. The stress measurement system includes a signal light transmitter 4, a first displacement platform 5, a second displacement platform 6, a spectrum analyzer 7, and a fiber optic stress sensor. The first displacement platform 5 and the second displacement platform 6 are arranged back and forth along the length direction of the fiber optic stress sensor. The first displacement platform 5 holds the signal light in the fiber optic guide 1 through a first fiber optic clamp 51, and the second displacement platform 6 holds the signal light out of the fiber optic guide 3 through a second fiber optic clamp 61. The portion of the fiber optic stress sensor located between the first fiber optic clamp 51 and the second fiber optic clamp 61 is suspended. It can be understood that this suspended portion of the fiber optic stress sensor includes a complete hollow fiber 2, a portion of the signal light inlet fiber 1, and a portion of the signal light outlet fiber 3. This portion constitutes the sensing part 8 of the fiber optic stress sensor. The output end of the signal light transmitter 4 is connected to the front end of the signal light guide fiber 1, and the input end of the spectrum analyzer 7 is connected to the end of the signal light output fiber 3.

[0059] In this embodiment, the signal light transmitter 4 generates a broadband light source.

[0060] In practical implementation, both the first displacement platform 5 and the second displacement platform 6 are precision displacement platforms. The first fiber clamp 51 is fixedly connected to the first displacement platform 5, and the second fiber clamp 61 is fixedly connected to the second displacement platform 6. Since the first fiber clamp 51 clamps the input single-mode fiber, and the second fiber clamp 61 clamps the output single-mode fiber, axial stress can be applied to the sensing part 8 of the fiber optic stress sensor by moving the micrometers of the first and second displacement platforms 5 and 6. This axial stress causes a slight change in the physical length and effective refractive index of the hollow fiber 2, resulting in a change in the phase difference between different modes. This causes a wavelength shift in the characteristic peaks or valleys of the output interference spectrum. By monitoring the amount of this wavelength shift using the spectrum analyzer 7, accurate measurement of the corresponding force can be achieved. It should be noted that since the Young's modulus, length, cross-sectional area, and other parameters of the hollow fiber 2 are all known, the magnitude of the stress applied to the sensing part 8 of the fiber optic strain sensor under different strain magnitudes can be easily calculated using Hooke's law, thereby achieving force measurement.

[0061] Specifically, axial strain is generated according to Hooke's law. The strain-stress relationship of the sensing unit 8 can be expressed as: Where E is the Young's modulus of hollow fiber 2 (quartz), A is the cross-sectional area of ​​hollow fiber 2, and strain is... The ratio of the changed length of the hollow fiber 2 to its original length is denoted as . Given the fiber suspension length L0 and the Young's modulus of the hollow fiber. E And the cross-sectional area A. Therefore, the axial stress F applied to the optical fiber can be calculated from the elongation ΔL.

[0062] It is understood that by adjusting the position of the first displacement platform 5 and the distance between the second displacement platform 6, the positions of the first fiber optic clamp 51 and the second fiber optic clamp 61 can be adjusted, thereby adjusting the length of the sensing part 8. Thus, the sensitivity of the fiber optic stress sensor is adjustable. In a preferred embodiment, the length of the sensing part 8 is 6-40 cm. During stress detection, increasing the length of the sensing part 8 amplifies the strain under a unit force, thereby improving the sensitivity of the fiber optic stress sensor, but simultaneously reduces its maximum measurable mechanical range. Based on this length range, the minimum measurable force of the fiber optic sensor of the present invention can reach 0.02 N, the measurement accuracy reaches the millinewton level, and the highest sensitivity can reach -0.82 nm / N.

[0063] Test Example 1 refer to Figure 2 In this test example, the fiber optic stress sensor has a single-end misalignment structure. Specifically, by eccentrically fusing the front end of the core of the second single-mode fiber with the end of the core of the hollow fiber 2, a radial misalignment is created between the end of the core of the hollow fiber 2 and the front end of the core of the optical signal output fiber 3. In this test example, the radial misalignment distance between the hollow fiber 2 and the optical signal output fiber 3 is d = 4 μm, the length of the sensing part 8 is L0 = 6 cm, and the length of the hollow fiber 2 is L... HCF =3mm. Figure 4 This study investigates the spectral response characteristics and wavelength-stress variation of the fiber optic stress sensor under different axial stresses. Figure 4 (a) Transmission spectra under different stresses. Figure 4 (b) is a magnified view of the selected spectral characteristic peaks. Figure 4 (c) shows the measurement results of the characteristic peak wavelength selected under the corresponding conditions as a function of stress. For example... Figure 4 (a) and Figure 4 As shown in (b), with the gradual increase of axial stress, the interference spectrum shifts to the short-wavelength direction (blue shift), and the position of the interference peaks drifts continuously without abrupt changes, indicating that the fiber optic stress sensor maintains good spectral stability during stress application. Figure 4 (c) shows that the characteristic peaks exhibit a highly linear response relationship, with a linear fit correlation coefficient R0. 2 A value >0.99 indicates that within a given stress range, the wavelength drift and stress follow a good linear relationship.

[0064] Test Example 2 refer to Figure 1This test example focuses on a fiber optic stress sensor with a double-ended misalignment structure. Specifically, during the fabrication process, the core end of the first single-mode fiber is eccentrically fused with the core end of the hollow-core fiber 2, and the core end of the second single-mode fiber is eccentrically fused with the core end of the hollow-core fiber 2. Thus, in the fabricated fiber optic stress sensor, there is a first radial misalignment between the core end of the hollow-core fiber 2 and the core end of the signal light guide fiber 1, and a second radial misalignment between the core end of the hollow-core fiber 2 and the core end of the optical signal output fiber 3. In this test example, the distance between the first and second radial misalignments is d = 4 μm, the length of the sensing part 8 is L0 = 6 cm, and the length of the hollow-core fiber 2 is L... HCF =3mm. Reference Figure 5 The characteristic peak wavelength generated by the fiber optic stress sensor varies with the strain magnitude, and the characteristic peak wavelength and strain follow a good linear relationship, indicating that the fiber optic strain sensor of the present invention can accurately measure strain.

[0065] Test Example 3 This test example uses two fiber optic stress sensors. These two fiber optic strain sensors employ the same single-end misaligned structure as in Test Example 1. Specifically, the length L0 of the sensing part 8 of both fiber optic stress sensors is 6 cm, and the length L of the hollow fiber 2 is... HCF Both are 3mm, the difference being that the radial misalignment distances d between the two fiber optic stress sensors are 4μm and 8μm, respectively. (Reference) Figure 6 Tests show that the characteristic peak undergoes a stable blue shift with increasing stress; furthermore, the sensitivity of the fiber optic strain sensor increases from -0.39 nm / N to -0.47 nm / N with increasing radial misalignment distance. This indicates that increasing the radial misalignment distance between the hollow fiber 2 and the single-mode fiber can improve the sensitivity of the fiber optic stress sensor, as the stronger difference in mode energy distribution exacerbates the response of the interference optical path change to external disturbances. However, it should be noted that an excessively large radial misalignment distance will lead to a significant attenuation of the initial spectral power and a decrease in the signal-to-noise ratio, while also reducing the system range.

[0066] Test Example 4 The fiber optic stress sensor tested here uses the same double-ended misalignment structure as in Test Example 2. Specifically, in this test example, the fiber optic stress sensor has a first radial misalignment and a second radial misalignment d of 4 μm, and the length L of the sensing part 8 is... 0= 6cm, hollow fiber 2 length L HCF= 3mm. Reference Figure 7 As shown in (a) and 7(b), similar to single-end dislocation, the spectrum exhibits a continuous blue shift trend with increasing axial stress. (Refer to...) Figure 7 (c) It can be seen that the characteristic peak drift of the fiber optic stress sensor in this test example has a good linear relationship with the stress change, and the correlation coefficient R of the linear fit is...2 A value greater than 0.99 indicates that within a given stress range, the wavelength drift and stress follow a good linear relationship.

[0067] Test Example 5 This test example uses three fiber optic stress sensors. These three fiber optic strain sensors employ the same double-ended misalignment structure as in Test Example 2. Specifically, in these three double-ended misalignment fiber optic stress sensors, the first radial misalignment and the second radial misalignment d are both 4 μm, the length L0 of the sensing part 8 is 6 cm, and the length L of the hollow fiber 2 in these three double-ended misalignment fiber optic stress sensors is... HCF The thicknesses are 3mm, 4mm, and 5mm respectively. (Reference) Figure 8 Tests show that the characteristic peaks of the three fiber optic stress sensors all exhibit a highly linear response relationship, with a linear fitting correlation coefficient R0. 2 All values ​​were above 0.99, indicating that within the given stress range, wavelength drift and stress followed a good linear relationship. When the lengths of hollow fiber 2 were 3mm, 4mm, and 5mm, the sensitivities were -0.37nm / N, -0.40nm / N, and -0.46nm / N, respectively, corresponding to measurement ranges of 0-5.32N, 0-5.985N, and 0-5.32N. It should be noted that the upper limit of the measurement range mainly depends on the mechanical fracture strength of the misaligned fusion splice structure of the sensor, rather than the limitation of optical principles. Due to slight individual differences in the fusion splicing process, the mechanical tolerance limit (i.e., fracture threshold) of each sensor exhibits a certain degree of random dispersion and is not linearly correlated with the length of the hollow fiber. Therefore, in this test example, the lengths of hollow fiber 2 (3mm and 5mm) differed, but the measurement ranges were consistent. Overall, these data show that, under the experimental conditions, the sensitivity of the fiber optic stress sensor of the present invention gradually increases with the increase of the length of the hollow fiber 2. This phenomenon is mainly attributed to the fact that as the length of the hollow fiber 2 increases, the interference phase difference increases, making the phase shift and wavelength drift caused by the same stress more significant.

[0068] Test Example 6 This test example uses two fiber optic stress sensors, employing the same double-ended misalignment structure as in Test Example 2. Specifically, in these two fiber optic stress sensors, the first radial misalignment and the second radial misalignment d are both 4 μm, and the length L of the hollow fiber 2 is... HCF= 3mm. Furthermore, by adjusting the distance between the first displacement platform 5 and the second displacement platform 6 in the stress measurement system, the positions of the first fiber optic clamp 51 and the second fiber optic clamp 61 are adjusted, thereby controlling the length of the sensing part 8 of the fiber optic stress sensor. In this test example, the lengths L0 of the sensing parts 8 of the two fiber optic stress sensors are 30cm and 40cm, respectively. (Reference) Figure 9It can be seen that, with the radial misalignment distance and the length of the hollow fiber 2 being consistent, changing the length of the sensing element 8 results in a good linear response between the characteristic peak wavelength and the applied stress. Specifically, when the length of the sensing element 8 L0 = 30 cm, the fitting slope is -0.73 nm / N, and the correlation coefficient R² reaches 0.998. When the length of the sensing element 8 L0 = 40 cm, the fitting slope increases to -0.82 nm / N, and the correlation coefficient R² is 0.993. Therefore, increasing the length of the suspended sensing element 8 can significantly improve the sensitivity of the fiber optic stress sensor while maintaining extremely high linearity, but the corresponding measurement range shrinks from 0-3.06 N to 0-1.19 N.

[0069] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solution of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A method of making a fiber optic strain sensor, comprising: The method comprises: fusing an end of a first single-mode optical fiber with a front end of a hollow-core optical fiber; cutting the hollow-core optical fiber so that the hollow-core optical fiber fused with the first single-mode optical fiber retains a preset length; fusing an end of the hollow-core optical fiber retaining the preset length with a front end of a second single-mode optical fiber; wherein at least one of the end of the core of the first single-mode optical fiber and the front end of the core of the second single-mode optical fiber is offset-fused with the core of the hollow-core optical fiber, and a radial offset is formed at the corresponding fusion.

2. The production method according to claim 1, characterized by, The offset amount of the radial offset is 4-8 μm.

3. The preparation method according to claim 1, characterized in that, The preset length is 3-5 mm.

4. The method of claim 1, wherein, The front end of the core of the hollow-core optical fiber and the end of the core of the first single-mode optical fiber are offset-fused to form a first radial offset, and the end of the core of the hollow-core optical fiber and the front end of the core of the second single-mode optical fiber are offset-fused to form a second radial offset, and the direction of the first radial offset is symmetrical to the direction of the second radial offset about the hollow-core optical fiber.

5. The method of any one of claims 1-4, wherein, The hollow-core optical fiber is a capillary-type hollow-core optical fiber having an axially-through cavity, and the core of the hollow-core optical fiber is formed by air filled in the axially-through cavity.

6. The method of any one of claims 1-4, wherein, The core diameter of the hollow-core optical fiber is 25-35 μm, and the cladding diameter of the hollow-core optical fiber is 100-150 μm.

7. The method of any one of claims 1-4, wherein, The core diameter of the first single-mode optical fiber is 6-12 μm, and the cladding diameter of the first single-mode optical fiber is 100-150 μm; and / or, The core diameter of the second single-mode optical fiber is 6-12 μm, and the cladding diameter of the second single-mode optical fiber is 100-150 μm.

8. An optical fiber stress sensor, characterized by, The optical fiber stress sensor is prepared by the method of any one of claims 1-7, wherein the first single-mode optical fiber is used as a signal light input optical fiber of the optical fiber stress sensor, and the second single-mode optical fiber is used as a signal light output optical fiber of the optical fiber stress sensor.

9. The optical fiber strain sensor of claim 8, wherein, The whole of the hollow-core optical fiber and part of the first single-mode optical fiber and part of the second single-mode optical fiber are suspended during stress detection to form a sensing part of the optical fiber stress sensor.

10. The optical fiber strain sensor of claim 9, wherein, The length of the sensing part is 6-40 cm.

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