Temperature and strain dual-parameter sensor based on variable-diameter hollow-core fiber and preparation method thereof

Abstract

The present application belongs to the technical field of test and measurement, and relates to optical fiber sensor technology, in particular to a temperature and strain dual-parameter sensor based on a variable-diameter hollow-core optical fiber and a preparation method thereof. The present application is based on a splicing structure of hollow-core optical fibers with different core diameters, that is, a first hollow-core optical fiber with a large core diameter is spliced with a second hollow-core optical fiber with a small core diameter, and then a segment of coreless optical fiber is fused on each side; the coreless optical fibers on the two sides are respectively used for exciting high-order modes and mode coupling, and the first hollow-core optical fiber and the second hollow-core optical fiber with different core diameters enable multiple modes to produce a phase difference, thereby forming a multimode interference. This structure is used for strain and temperature sensing, and has the advantages of low cross-talk, low manufacturing cost and good robustness, etc., and solves the technical defects of temperature cross-talk, high cost and fragile structure of existing optical fiber strain sensors. The present application has high precision and high reliability, and can be applied to complex scenes such as building health monitoring, aerospace, deep sea exploration, etc.

Description

Technical Field

[0001] This invention belongs to the field of testing and measurement technology, and relates to fiber optic sensor technology, particularly to a temperature strain dual-parameter sensor based on variable diameter hollow fiber and its preparation method. Background Technology

[0002] Fiber optic strain sensors have attracted increasing attention due to their advantages such as high sensitivity, strong anti-electromagnetic interference capability, compact and lightweight structure, and strong corrosion resistance.

[0003] The probes of common fiber optic strain sensors are mainly designed using principles such as interferometry, fiber gratings, anti-resonance effects, and surface plasmon resonance. When using fiber gratings for strain measurement, significant temperature crosstalk problems arise. For example, with Bragg fiber gratings, temperature changes alter the grating period and effective refractive index, causing a shift in the Bragg wavelength. Fiber optic strain sensors based on the surface plasmon resonance principle often require a metal layer deposited on the fiber surface to excite the surface plasmon resonance effect, involving complex metal layer processing, resulting in high manufacturing difficulty and cost. Fiber optic strain sensors based on the anti-resonance effect often use expensive photonic crystal fibers, further increasing the difficulty and cost of sensor manufacturing. Currently, there has been extensive research on probe structures for fiber optic strain sensors based on interferometry principles. For example, the tapered capillary structure sensing probe proposed by Mi Li et al. of Hainan University in 2025 is essentially a Fabry-Perot interferometer, achieving a strain sensitivity of 32.19 pm / με; however, the robustness and stability of the tapered structure are poor, making it difficult to handle complex scenarios.

[0004] In summary, while pursuing high sensitivity, current fiber optic strain sensors face challenges in terms of cost, manufacturing process, and structural stability. In particular, how to achieve reliable measurement in complex environments is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a temperature strain dual-parameter sensor based on variable-diameter hollow-core optical fiber and its fabrication method, so as to solve the problems existing in the prior art. The sensor uses two hollow-core optical fibers with different core diameters spliced ​​together. The coreless optical fibers on both sides are used to excite higher-order modes and mode coupling, respectively. The hollow-core optical fibers with different core diameters enable multiple modes to generate phase differences, thereby forming multimode interference. Using this structure for strain and temperature sensing has low crosstalk, low manufacturing cost, good robustness and high structural reliability.

[0006] To achieve the above objectives, the present invention provides the following solution: On one hand, the present invention provides a temperature strain dual-parameter sensor based on variable diameter hollow optical fiber, comprising: A variable-diameter fiber splicing structure includes a first hollow fiber and a second hollow fiber spliced ​​and fixed to the first hollow fiber, wherein the core diameter of the first hollow fiber is larger than the core diameter of the second hollow fiber. The first coreless optical fiber is spliced ​​and fixed to the end of the first hollow optical fiber that is away from the second hollow optical fiber; The second coreless optical fiber is spliced ​​and fixed to the end of the second hollow optical fiber that is away from the first hollow optical fiber; The input fiber is spliced ​​and fixed to one of the first coreless fiber and the second coreless fiber to serve as the sensor input end; The output optical fiber is spliced ​​and fixed to the other one of the first coreless optical fiber and the second coreless optical fiber to serve as the sensor output end.

[0007] In some implementations, the input fiber is a single-mode fiber.

[0008] In some implementations, the output fiber is a single-mode fiber.

[0009] In some implementations, the first hollow fiber and the second hollow fiber are coaxially spliced.

[0010] In some embodiments, the variable-diameter fiber splicing structure, the first coreless fiber, the second coreless fiber, the input fiber, and the output fiber are coaxially spliced.

[0011] In some embodiments, the outer diameters of the first coreless optical fiber, the second coreless optical fiber, the input optical fiber, and the output optical fiber are the same; the outer diameter of the second coreless optical fiber is equal to the outer diameter of the second hollow optical fiber.

[0012] In some embodiments, the outer diameter of the first hollow fiber is larger than the outer diameter of the second hollow fiber.

[0013] In some embodiments, the core diameter of the first hollow optical fiber is 80 micrometers to 110 micrometers, and the outer diameter is 120 micrometers to 150 micrometers; The core diameter of the second hollow optical fiber is 35 micrometers to 55 micrometers, and the outer diameter is 100 micrometers to 130 micrometers.

[0014] In some embodiments, the length of the first hollow fiber is 90 micrometers to 110 micrometers; the length of the second hollow fiber is 190 micrometers to 210 micrometers; and the lengths of the first coreless fiber and the second coreless fiber are 2.5 millimeters to 3.2 millimeters.

[0015] On the other hand, this invention proposes a method for fabricating the aforementioned temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber, comprising the following steps: S1. Splice the input fiber with the first coreless fiber, and cut the first coreless fiber to a predetermined length after splicing. S2. Splice the first coreless optical fiber with the first hollow optical fiber, and cut the first hollow optical fiber to a predetermined length after splicing. S3. Splice the first hollow fiber and the second hollow fiber together, and cut the second hollow fiber to a predetermined length after splicing. S4. Splice the second hollow fiber with the second coreless fiber, and cut the second coreless fiber to a predetermined length after splicing. S5. Splice the second coreless optical fiber with the output optical fiber.

[0016] In some implementations, fusion splicing is used to fix adjacent optical fibers in steps S1 to S5.

[0017] The present invention achieves the following technical effects compared to the prior art: This invention proposes a dual-parameter temperature and strain sensor based on variable-diameter hollow-core optical fibers. It utilizes a spliced ​​structure of hollow-core optical fibers with different core diameters, specifically splicing a large-diameter first hollow-core fiber with a small-diameter second hollow-core fiber, and then fusing a section of coreless fiber on each side. The coreless fibers on both sides are used to excite higher-order modes and mode coupling, respectively. The different core diameters of the first and second hollow-core fibers allow multiple modes to generate a phase difference, thus forming multimode interference. Applying this structure to strain and temperature sensing offers advantages such as low crosstalk, low manufacturing cost, and good robustness, overcoming the technical shortcomings of existing fiber optic strain sensors, including temperature crosstalk, high cost, and structural fragility. Specific beneficial effects are as follows: (i) While retaining the high sensitivity advantage of the interferometer, the fragile conical structure is abandoned. The entire sensor adopts a non-conical linear structure with good coaxiality, which is more robust and structurally reliable.

[0018] (ii) It can simultaneously measure two parameters, strain and temperature, and can fundamentally solve the temperature crosstalk problem by integrating two interference signals with different response characteristics into one sensor.

[0019] (iii) Sensors are fabricated by fusion splicing hollow optical fibers, coreless optical fibers, and single-mode optical fibers, which have simple structures and low manufacturing costs.

[0020] The temperature strain dual-parameter sensor based on variable diameter hollow optical fiber proposed in this invention has both high precision and high reliability, as well as high mechanical strength and low manufacturing cost, and can be applied to complex scenarios such as building health monitoring, aerospace, and deep-sea exploration. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the structure of a temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber disclosed in an embodiment of the present invention; Figure 2 This is a schematic diagram of the optical transmission principle of a temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber disclosed in an embodiment of the present invention; Figure 3 This is a diagram of a test system based on a temperature strain dual-parameter sensor using variable-diameter hollow optical fiber, as disclosed in an embodiment of the present invention. Figure 4 This is a spectrum-strain relationship diagram disclosed in an embodiment of the present invention; Figure 5 This is a graph showing the fitting results of the strain-light intensity relationship disclosed in an embodiment of the present invention; Figure 6 The figure shows the strain reversibility test results disclosed in the embodiments of the present invention; Figure 7 This is a spectrum-temperature relationship diagram disclosed in an embodiment of the present invention; Figure 8 This is a graph showing the fitting results of the relationship between temperature and trough wavelength disclosed in an embodiment of the present invention; Figure 9 This is a fitting graph of the relationship between temperature and light intensity disclosed in an embodiment of the present invention; Figure 10 This is a fitting diagram of the relationship between strain and trough wavelength disclosed in an embodiment of the present invention.

[0023] In the figure, the labels are as follows: 100 - temperature strain dual-parameter sensor based on variable diameter hollow fiber; 200 - broadband light source; 300 - spectrometer; 400 - constant temperature heating stage; 1-Variable diameter fiber splicing structure; 11-First hollow fiber; 12-Second hollow fiber; 2- First coreless optical fiber; 3-Second coreless optical fiber; 4-Input fiber optic cable; 5- Output fiber optic cable. Detailed Implementation

[0024] The technical solutions of the embodiments 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 scope of protection of the present invention.

[0025] One objective of this invention is to provide a temperature and strain dual-parameter sensor based on variable-diameter hollow-core optical fiber to address the problems existing in the prior art. This sensor uses two hollow-core optical fibers with different core diameters spliced ​​together. The coreless optical fibers on both sides are used to excite higher-order modes and mode coupling, respectively. The hollow-core optical fibers with different core diameters enable multiple modes to generate phase differences, thereby forming multimode interference. Using this structure for strain and temperature sensing has low crosstalk, low manufacturing cost, good robustness, and high structural reliability.

[0026] Another objective of this invention is to provide a method for fabricating a temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber.

[0027] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0028] Example 1 like Figure 1 and Figure 2As shown, this embodiment provides a temperature strain dual-parameter sensor 100 based on variable-diameter hollow optical fiber, which includes a variable-diameter optical fiber splicing structure 1, a first coreless optical fiber 2, a second coreless optical fiber 3, an input optical fiber 4, and an output optical fiber 5. The variable-diameter optical fiber splicing structure 1 is the core component of the sensor, which includes a first hollow optical fiber 11 and a second hollow optical fiber 12 spliced ​​and fixed to the first hollow optical fiber 11. The core diameter (i.e., inner diameter) of the first hollow optical fiber 11 is larger than the core diameter (i.e., inner diameter) of the second hollow optical fiber 12. The variable-diameter optical fiber splicing structure 1 achieves the variable-diameter structure by splicing two hollow optical fibers with different core diameters along the axial direction. The first coreless fiber 2 is spliced ​​and fixed to the end of the first hollow fiber 11 furthest from the second hollow fiber 12, and the second coreless fiber 3 is spliced ​​and fixed to the end of the second hollow fiber 12 furthest from the first hollow fiber 11. By splicing the first coreless fiber 2 and the second coreless fiber 3 at both ends of the variable-diameter fiber splicing structure 1, a sandwich structure consisting of the first coreless fiber 2, the variable-diameter fiber splicing structure 1, and the second coreless fiber 3 is formed. An input fiber 4 and an output fiber 5 are spliced ​​at both ends of this sandwich structure, thus forming a temperature strain dual-parameter sensing probe. This part can be used as an independent temperature strain dual-parameter sensor, or as a sensing probe in some testing systems, where it, together with a light source and a spectrometer, constitutes a complete sensor. This temperature strain dual-parameter sensor is an optical fiber sensor based on a splicing structure of hollow fibers with different core diameters. In specific applications, the input fiber 4 is spliced ​​and fixed to one of the first coreless fiber 2 and the second coreless fiber 3 to serve as the sensor input end; the output fiber 5 is spliced ​​and fixed to the other of the first coreless fiber 2 and the second coreless fiber 3 to serve as the sensor output end. Figure 1 and Figure 2 The diagram shown is a structural schematic of the input fiber 4 spliced ​​with the first coreless fiber 2 and the output fiber 5 spliced ​​with the second coreless fiber 3.

[0029] The first hollow-core fiber 11 and the second hollow-core fiber 12 are both technologically mature hollow-core fibers, and the first coreless fiber 2 and the second coreless fiber 3 are both technologically mature coreless fibers; the input fiber 4 and the output fiber 5 are both preferably technologically mature single-mode fibers. The specific structure, functional principle and operating characteristics of each type of fiber will not be described in detail.

[0030] The aforementioned temperature strain dual-parameter sensor 100 based on variable-diameter hollow-core optical fiber splices two hollow-core optical fibers with different core diameters, and splices a section of coreless optical fiber on each side of this structure to form a sandwich structure. Single-mode optical fibers are spliced ​​to the coreless optical fibers at both ends as the input and output ends, respectively, forming a complete sensor probe. This structure utilizes the principle of multimode interference, causing the intensity of the interference troughs to change when strain is applied at both ends of the structure. During temperature sensing measurements, the trough wavelengths drift with temperature changes. This structure offers advantages such as low crosstalk, low manufacturing cost, good robustness, and high reliability.

[0031] In some feasible implementations, the first hollow fiber 11 and the second hollow fiber 12 are axially spliced, meaning that both ends of the first hollow fiber 11 and the second hollow fiber 12 are respectively set as flat-cut end faces, which are perpendicular to the axis of the hollow fiber. The close-proximity end faces of the first hollow fiber 11 and the second hollow fiber 12 are in seamless contact and connected and fixed. The connection and fixing methods include, but are not limited to, bonding. Specifically, the bonding method can be direct bonding with adhesive, or fusion splicing using a fiber optic fusion splicer. Fusion splicing can improve the structural connection reliability of the first hollow fiber 11 and the second hollow fiber 12 without affecting the signal transmission between them. Therefore, fusion splicing of the first hollow fiber 11 and the second hollow fiber 12 is generally preferred.

[0032] Similarly, the axial ends of the first coreless fiber 2, the second coreless fiber 3, the input fiber 4, and the output fiber 5 are preferably cut with flat, perpendicular end faces to ensure the splicing stability and signal transmission reliability between adjacent fiber segments. The input fiber 4 is preferably bonded to the first coreless fiber 2, the first coreless fiber 2 to the first hollow fiber 11, the second hollow fiber 12 to the second coreless fiber 3, and the second coreless fiber 3 to the output fiber 5. The bonding method can be direct bonding with adhesive or fusion splicing using a fiber optic fusion splicer. Fusion splicing improves the structural connection reliability without affecting signal transmission between the fibers. Therefore, it is generally preferred that the input fiber 4 is fixed and spliced ​​to the first coreless fiber 2, the first coreless fiber 2 to the first hollow fiber 11, the second hollow fiber 12 to the second coreless fiber 3, and the second coreless fiber 3 to the output fiber 5 using fusion splicing.

[0033] In some feasible implementations, the first hollow-core fiber 11 and the second hollow-core fiber 12 are preferably coaxially spliced ​​to ensure the stability of the electrical connection between them. Simultaneously, the variable-diameter fiber splicing structure 1, the first coreless fiber 2, the second coreless fiber 3, the input fiber 4, and the output fiber 5 are all coaxially spliced ​​together to ensure the stability of the electrical connection between the fiber segments in the entire sensor probe. Besides this coaxial splicing method, radial offset between adjacent fiber segments is permissible, as long as the electrical connection between them is maintained.

[0034] Furthermore, considering the variable diameter relationship between the first hollow fiber 11 and the second hollow fiber 12, the end faces of both are perpendicular to the fiber axis. Regarding the splicing end faces between the input fiber 4 and the first coreless fiber 2, and between the second coreless fiber 3 and the output fiber 5, if the outer diameters of adjacent fibers are the same, the splicing end faces can be either perpendicular to the fiber axis or beveled. For example, if the outer diameters of the input fiber 4 and the first coreless fiber 2 are the same, they can be coaxially spliced ​​using beveled surfaces. Similarly, if the outer diameters of the second coreless fiber 3 and the output fiber 5 are the same, they can be coaxially spliced ​​using beveled surfaces.

[0035] Considering that vertical end face splicing is easier to operate and has a better connection and fixation effect, in practical applications, vertical end face splicing is preferred between the input fiber 4 and the first coreless fiber 2, between the first coreless fiber 2 and the first hollow fiber 11, between the second hollow fiber 12 and the second coreless fiber 3, and between the second coreless fiber 3 and the output fiber 5, especially when the outer diameters of adjacent fiber segments are different.

[0036] In addition to the aforementioned beveled end face, the two end faces of adjacent fiber segments that are close to each other can also be provided with serrated end faces that fit together, as long as seamless splicing of adjacent fiber segments can be achieved.

[0037] In some feasible implementations, the outer diameter relationship of the first coreless fiber 2, the second coreless fiber 3, the input fiber 4, and the output fiber 5 is not limited, as long as reliable electrical connection between adjacent fiber segments can be guaranteed. In practical applications, it is preferable that the outer diameters of the first coreless fiber 2, the second coreless fiber 3, the input fiber 4, and the output fiber 5 are the same. Simultaneously, the outer diameter of the second coreless fiber 3 is equal to the outer diameter of the second hollow fiber 12. This design minimizes the splicing of different diameter fibers in the sensor, thereby minimizing problems such as complex manufacturing processes and unreliable connections caused by splicing different diameter fibers, and helping to improve the measurement accuracy and performance of the sensor.

[0038] In some feasible implementations, the relationship between the outer diameters of the first hollow fiber 11 and the second hollow fiber 12 is not limited, as long as their core diameters are different. In most cases, the outer diameter of the first hollow fiber 11, which has a larger core diameter, is greater than the outer diameter of the second hollow fiber 12, which has a smaller core diameter.

[0039] In some feasible implementations, the core diameter of the first hollow fiber 11 is 80 micrometers to 110 micrometers, and the outer diameter is 120 micrometers to 150 micrometers; correspondingly, the core diameter of the second hollow fiber 12 is 35 micrometers to 55 micrometers, and the outer diameter is 100 micrometers to 130 micrometers.

[0040] The lengths of the first hollow fiber 11, the second hollow fiber 12, the first coreless fiber 2, the second coreless fiber 3, the input fiber 4, and the output fiber 5 are unlimited and are often flexibly set according to the actual application scenario and application requirements. The entire sensor can be spliced ​​on-site according to the actual application scenario and application requirements, which enhances the application flexibility of the sensor.

[0041] In some feasible implementations, the length of the first hollow fiber 11 is 90 micrometers to 110 micrometers; the length of the second hollow fiber 12 is 190 micrometers to 210 micrometers. The lengths of the first coreless fiber 2 and the second coreless fiber 3 are 2.5 millimeters to 3.2 millimeters. The lengths of the input fiber 4 and the output fiber 5 are cut to preset lengths as required.

[0042] refer to Figure 1 and Figure 2 The sensor employs a structure in which the input fiber 4, the first coreless fiber 2, the first hollow fiber 11, the second hollow fiber 12, the second coreless fiber 3, and the output fiber 5 are sequentially spliced ​​from left to right. In actual fabrication, the splicing order of the fiber segments is not restricted, as long as it ensures that a complete structure can be formed after splicing. Figure 1 The structure shown is sufficient. For example, it can be arranged as follows: Figure 1 The structure can be constructed by splicing the input fiber 4, the first coreless fiber 2, the first hollow fiber 11, the second hollow fiber 12, the second coreless fiber 3, and the output fiber 5 from left to right. Alternatively, the input fiber 4, the first coreless fiber 2, the first hollow fiber 11, the second hollow fiber 12, the second coreless fiber 3, and the output fiber 5 can be spliced ​​in reverse order from right to left. Alternatively, the first hollow fiber 11 and the second hollow fiber 12 can be spliced ​​first, and then coreless fibers and single-mode fibers can be spliced ​​at their respective ends. Alternatively, the input fiber 4, the first coreless fiber 2, and the first hollow fiber 11 can be spliced ​​first (the splicing order is not limited), the second hollow fiber 12, the second coreless fiber 3, and the output fiber 5 can be spliced ​​first (the splicing order is not limited), and then the two parts can be spliced ​​together.

[0043] In some specific examples, the available dimensional parameters of each fiber segment in the sensor can be as follows: the inner diameter (i.e., core diameter) of the first hollow fiber 11 and the second hollow fiber 12 are 100 micrometers and 50 micrometers, respectively, and the outer diameters are 140 micrometers and 125 micrometers, respectively. The length of the first hollow fiber 11 with an inner diameter of 100 micrometers is approximately 100 micrometers, and the length of the second hollow fiber 12 with an inner diameter of 50 micrometers is approximately 200 micrometers. After splicing the first hollow fiber 11 and the second hollow fiber 12, a first coreless fiber 2 and a second coreless fiber 3 are fused to both ends of this spliced ​​structure, respectively. The lengths of the first coreless fiber 2 and the second coreless fiber 3 can be 3 millimeters. Finally, single-mode fibers are connected to the ends of the coreless fibers on both sides, serving as the input fiber 4 and the output fiber 5 of the sensor, respectively.

[0044] refer to Figure 1 and Figure 2 The technical principle of the temperature strain dual-parameter sensor 100 based on variable-diameter hollow optical fiber, as shown in the diagram, is as follows: In the temperature strain dual-parameter sensor 100 based on variable-diameter hollow optical fiber, the input fiber 4 is used to connect to an external light source, and the output fiber 5 is used to connect to the corresponding detection equipment. Taking a broadband light source 200 and a spectrometer 300 as examples: When the broadband light source 200 is input, the incident light reaches the first coreless fiber 2 via the input fiber 4. Since the first coreless fiber 2 has no core, the light entering the first coreless fiber 2 will be excited into higher-order modes. Subsequently, the light spot expands and propagates to the splicing interface (also called the interface) between the first coreless fiber 2 and the first hollow fiber 11. Part of the light enters the cladding of the first hollow fiber 11 and couples to the cladding of the subsequent spliced ​​second hollow fiber 12, denoted as I1; another part enters the air cavity. Since the core diameters of the spliced ​​first hollow fiber 11 and the second hollow fiber 12 are different, the light entering the air cavity will couple into the cladding of the second hollow fiber 12, denoted as I2. The remaining part continues to propagate along the axial direction of the air cavity of the second hollow fiber 12, denoted as I3. These three parts of light reach the second coreless fiber 3 and couple together to produce interference, and finally are output to the spectrometer 300 via the output fiber 5. This structure is essentially a Mach-Zehnder multimode interferometer, considered as a three-beam interference. The light transmission schematic diagram is shown below. Figure 2 As shown, the intensity of the interference light can be expressed as: in, The phase difference between I1 and I2, The phase difference between I1 and I3 The phase difference between I2 and I3 can be expressed as follows: = , = , = ,in λ is the effective refractive index of the cladding mode of the first and second hollow fiber, n0 is the effective refractive index of the core (i.e., the air core of the hollow fiber) of the first and second hollow fiber, L1 and L2 are the lengths of the first hollow fiber 11 and the second hollow fiber 12, respectively, and λ is the wavelength of light.

[0045] The above phase difference can be simplified as follows: = Where Δn is the effective refractive index difference, and L is the cavity length of the first and second hollow fibers. Therefore, the wavelength of the interference trough can be expressed as... Where m is a positive integer. Differentiating both sides of the above equation with respect to temperature, we obtain the temperature sensitivity as... = ,in Thermo-optic coefficient, This is the coefficient of thermal expansion. Temperature changes can affect the effective refractive index difference and the length of the hollow fiber, causing a shift in the wavelength of the interference valley. Furthermore, changes in strain can also alter the length and effective refractive index of the hollow fiber, thereby changing the intensity and wavelength of the interference valley.

[0046] As described above, this invention is based on a spliced ​​structure of hollow optical fibers with different core diameters. Specifically, a large-diameter first hollow optical fiber 11 is spliced ​​with a small-diameter second hollow optical fiber 12, and then a section of coreless fiber is fused to each side. The coreless fibers on both sides are used to excite higher-order modes and mode coupling, respectively. The different core diameters of the first and second hollow optical fibers 11 and 12 allow multiple modes to generate a phase difference, thereby forming multimode interference. Using this probe structure for strain and temperature sensing offers advantages such as low crosstalk, low manufacturing cost, and good robustness.

[0047] The following test system, consisting of a temperature-strain dual-parameter sensor 100 based on variable-diameter hollow optical fiber, a broadband light source 200, a spectrometer 300, and a constant-temperature heating stage 400, is used to test the above-mentioned sensing structure to verify and illustrate the sensor performance and effect of this embodiment: like Figure 3As shown, broadband light emitted from a broadband light source (wavelength range: 1526~1606nm) travels through a single-mode fiber to a temperature-strain dual-parameter sensor 100 based on a variable-diameter hollow-core fiber, and then outputs to a spectrometer. The temperature-strain dual-parameter sensor 100 based on the variable-diameter hollow-core fiber is placed on a constant-temperature heating stage 400 for temperature testing, or placed between two spiral micro-displacement stages for strain testing. The broadband light source 200, spectrometer 300, constant-temperature heating stage 400 (a Xinhaomai model can be used), and spiral micro-displacement stages are all common testing equipment; their specific structures and functional principles will not be elaborated here. The temperature-strain dual-parameter sensor 100 based on the variable-diameter hollow-core fiber is hereinafter referred to as the "sensing structure".

[0048] I. Strain Sensitivity Test Results The sensing structure was placed between two helical micro-displacement stages, and strain was applied to the sensing structure by turning the knob. The two stages sandwiched the sensing structure, with a spacing of approximately 2.2 cm between them. The micro-displacement stages moved in 5 μm increments, increasing by 227 με each time. The measured relationship between the spectrum and strain is as follows: Figure 4 Using the third trough as the tracking point, it can be observed that the light intensity at the trough gradually decreases with increasing strain. Fitting the relationship between light intensity and strain yields the following results: Figure 5 As shown, the strain sensitivity was -8.6673 dB / mε, and the linearity was 0.99691.

[0049] Then, strain release was performed to conduct a reversibility test. The test results are as follows: Figure 6 As shown, the strain sensitivity of the strain release process is -8.732 dB / mε, and the linearity is 0.99719. The strain application and strain release processes have good consistency and good reversibility.

[0050] II. Temperature Sensitivity Test Results The sensing structure was fixed on a constant-temperature heating stage, and the temperature was gradually increased from 30°C to 60°C at 5°C intervals. The spectral changes during this process were recorded. The resulting spectrum versus temperature graph is shown below. Figure 7 As can be seen, a redshift occurs in the interference trough as the temperature increases. Fitting the relationship between the trough wavelength and temperature yields the following results: Figure 8 As shown, the temperature sensitivity was 43.029 pm / ℃ and the linearity was 0.99936.

[0051] III. Temperature Crosstalk Test In strain sensing applications, it is necessary to eliminate crosstalk caused by temperature changes. Therefore, the temperature crosstalk of this sensing structure was tested, and the fitting results of the relationship between temperature and light intensity were obtained as follows: Figure 9The temperature sensitivity is -0.045037 dB / ℃, and the linearity is 0.99418. The calculated temperature cross-sensitivity is 0.00516 mε / ℃, therefore this sensing structure has low temperature crosstalk.

[0052] IV. Strain Crosstalk Test In temperature sensing applications, it is necessary to eliminate crosstalk caused by strain changes. Therefore, the strain crosstalk of this sensing structure was tested, and the fitting results of the relationship between strain and trough wavelength are as follows. Figure 10 The strain sensitivity was found to be -1.3486 pm / με, and the linearity was 0.95353. The calculated strain crosstalk sensitivity was -0.0313 ℃ / με, indicating that the sensing structure exhibits low strain crosstalk at the test temperature.

[0053] Therefore, this sensing probe structure has low crosstalk, allowing the valley light intensity to be used to demodulate strain information, and the valley wavelength to demodulate temperature information, thus achieving dual-parameter sensing of strain and temperature. Furthermore, its low manufacturing cost and good robustness are also significant advantages.

[0054] In summary, this invention addresses the technical shortcomings of existing fiber optic strain sensors, such as temperature crosstalk, high cost, and fragile structure, by creatively designing the aforementioned high-sensitivity interferometric fiber optic strain sensor based on a robust structure. The specific advantages of this sensor are as follows: (i) While retaining the high sensitivity advantage of the interferometer, the fragile conical structure is abandoned. The entire sensor adopts a non-conical linear structure with good coaxiality, which is more robust.

[0055] (ii) It can simultaneously measure two parameters, strain and temperature, and can fundamentally solve the temperature crosstalk problem by integrating two interference signals with different response characteristics into one sensor.

[0056] (iii) Sensors are fabricated by fusion splicing hollow optical fibers, coreless optical fibers, and single-mode optical fibers, which have simple structures and low manufacturing costs.

[0057] Example 2 This embodiment presents a method for fabricating the temperature strain dual-parameter sensor 100 based on variable-diameter hollow fiber as described in Embodiment 1, including the following steps: S1. Splice the input fiber 4 with the first coreless fiber 2, and cut the first coreless fiber 2 to a predetermined length after splicing. S2. Splice the first coreless optical fiber 2 with the first hollow optical fiber 11, and cut the first hollow optical fiber 11 to a predetermined length after splicing. S3. Splice the first hollow fiber 11 and the second hollow fiber 12 together, and cut the second hollow fiber 12 to a predetermined length after splicing. S4. Splice the second hollow fiber 12 with the second coreless fiber 3, and cut the second coreless fiber 3 to a predetermined length after splicing. S5. Connect the second coreless optical fiber 3 to the output optical fiber 5.

[0058] In some feasible implementations, fusion splicing is used to fix adjacent optical fibers in steps S1 to S5.

[0059] The preparation method described above will be explained with examples below, using specific operational procedures. The operational procedures are as follows: (1) A section of single-mode fiber (i.e., input fiber 4) with a flattened end face was spliced ​​with the first coreless fiber 2 using a fiber optic fusion splicer (FITEL, S178A). The manual mode was used, and the fusion program was set with a discharge intensity of 100 bits and a discharge time of 1500 ms. After the fusion was completed, it was placed under a microscope and cut to a predetermined length using a fiber optic cleaver. The length of the cut first coreless fiber 2 was approximately 3 mm.

[0060] In practice, at least one fusion splicing operation can be performed on the single-mode fiber (i.e., input fiber 4) and the first coreless fiber 2 based on the splicing result, with each splicing involving one discharge; multiple discharges can be performed to perform multiple splices, thereby enhancing the stability of the spliced ​​structure. Generally, a single discharge splicing is sufficient for the single-mode fiber (i.e., input fiber 4) and the first coreless fiber 2.

[0061] (2) One end of the cut first coreless fiber 2 is fused to a section of the first hollow fiber 11 with an inner diameter of 100 μm and an outer diameter of 140 μm. The fusion splicing program is set with a discharge intensity of 40 bits and a discharge time of 300 ms. The fusion splicing process is performed twice to enhance the structural strength. Then, with the help of a microscope, the first hollow fiber 11 is cut to a length of 100 μm using a fiber optic cleaver.

[0062] The discharge interval can be flexibly set as needed.

[0063] (3) The cut first hollow fiber 11 is fused together with a second hollow fiber 12 with an inner diameter of 50 μm and an outer diameter of 125 μm. The fusion procedure is the same as in step (2), and two discharges are performed. After the fusion is completed, the second hollow fiber 12 with a length of 200 μm is cut under a microscope.

[0064] The discharge interval can be flexibly set as needed.

[0065] (4) Then, fusion splice one end of the cut second hollow fiber 12 with a section of second coreless fiber 3. The fusion procedure is the same as step (2). After the fusion is completed, cut out a second coreless fiber 3 about 3mm long in the same way as step (1).

[0066] The welding process involves two discharges, and the discharge interval can be flexibly set as needed.

[0067] (5) Finally, one end of the cut second coreless fiber 3 is fused with another single-mode fiber (i.e., the output fiber 5) to form the output end. The fusion procedure is the same as step (1). After the fusion is completed, the output fiber 5 is connected to the fiber optic patch cord and then easily connected to the spectrometer 300.

[0068] By adopting the above-mentioned specific sequence of preparation steps, the sensor can be operated on-site, and each fiber segment can be cut and spliced ​​immediately, avoiding the fiber end face from being left for too long after cutting, which would cause impurities to adhere to the fiber end face and thus affect the sensing performance.

[0069] It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art and to facilitate understanding. They are not intended to limit the scope of the invention and therefore have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to size, without affecting the effectiveness and purpose of the invention, should still fall within the scope of the technical content disclosed herein. Furthermore, the terms "upper," "lower," "left," "right," "middle," and "one" used in this specification are merely for clarity and not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention's implementation.

[0070] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A dual-parameter temperature strain sensor based on variable-diameter hollow-core optical fiber, characterized in that, include: A variable-diameter fiber splicing structure includes a first hollow fiber and a second hollow fiber spliced ​​and fixed to the first hollow fiber, wherein the core diameter of the first hollow fiber is larger than the core diameter of the second hollow fiber. The first coreless optical fiber is spliced ​​and fixed to the end of the first hollow optical fiber that is away from the second hollow optical fiber; The second coreless optical fiber is spliced ​​and fixed to the end of the second hollow optical fiber that is away from the first hollow optical fiber; The input fiber is spliced ​​and fixed to one of the first coreless fiber and the second coreless fiber to serve as the sensor input end; The output optical fiber is spliced ​​and fixed to the other one of the first coreless optical fiber and the second coreless optical fiber to serve as the sensor output end.

2. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 1, characterized in that, The input fiber is a single-mode fiber.

3. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 1, characterized in that, The output fiber is a single-mode fiber.

4. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to any one of claims 1 to 3, characterized in that, The first hollow fiber and the second hollow fiber are coaxially spliced.

5. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 4, characterized in that, The variable-diameter fiber splicing structure, the first coreless fiber, the second coreless fiber, the input fiber and the output fiber are coaxially spliced.

6. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 5, characterized in that, The first coreless optical fiber, the second coreless optical fiber, the input optical fiber, and the output optical fiber have the same outer diameter; the outer diameter of the second coreless optical fiber is equal to the outer diameter of the second hollow optical fiber.

7. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to any one of claims 1 to 3, characterized in that, The core diameter of the first hollow optical fiber is 80 micrometers to 110 micrometers, and the outer diameter is 120 micrometers to 150 micrometers; The core diameter of the second hollow optical fiber is 35 micrometers to 55 micrometers, and the outer diameter is 100 micrometers to 130 micrometers.

8. The temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 7, characterized in that, The length of the first hollow fiber is 90 micrometers to 110 micrometers; the length of the second hollow fiber is 190 micrometers to 210 micrometers; and the lengths of the first coreless fiber and the second coreless fiber are 2.5 millimeters to 3.2 millimeters.

9. The method for fabricating a temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to any one of claims 1 to 8, characterized in that, Including the following steps: S1. Splice the input fiber with the first coreless fiber, and cut the first coreless fiber to a predetermined length after splicing. S2. Splice the first coreless optical fiber with the first hollow optical fiber, and cut the first hollow optical fiber to a predetermined length after splicing. S3. Splice the first hollow fiber and the second hollow fiber together, and cut the second hollow fiber to a predetermined length after splicing. S4. Splice the second hollow fiber with the second coreless fiber, and cut the second coreless fiber to a predetermined length after splicing. S5. Splice the second coreless optical fiber with the output optical fiber.

10. The method for fabricating a temperature strain dual-parameter sensor based on variable-diameter hollow optical fiber according to claim 9, characterized in that, In steps S1 to S5, fusion splicing is used to fix adjacent optical fibers.

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