A single-mode-sapphire-fiber-based in-fiber michelson-interferometric high-temperature-resistant curvature sensor

Abstract

The application discloses an in-fiber Michelson interference type high-temperature-resistant curvature sensor based on a single-mode sapphire optical fiber, and belongs to the field of optical fiber sensing, which comprises a demodulation system, a single-mode optical fiber and a sapphire optical fiber sensing probe connected in sequence, a continuous single-mode light guide structure is formed in the sapphire optical fiber sensing probe, and a Bragg grating area and a Michelson interference structure area are sequentially arranged in the sapphire optical fiber sensing probe in the axial direction; the Bragg grating area is used as a temperature reference and an auxiliary compensation unit in high-temperature demodulation, and the Michelson interference structure area is used for generating an optical path difference change under the action of curvature and forming an interference signal related to the curvature. The sensor has the advantages of high-temperature resistance of sapphire material, high signal-to-noise ratio of single-mode transmission, compactness and anti-disturbance capacity of the in-fiber integrated structure, and is suitable for optical fiber sensing applications in aerospace, energy, metallurgy and other extreme high-temperature environments.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a high-temperature resistant curvature sensor based on a single-mode sapphire fiber with in-fiber Michelson interferometry. Background Technology

[0002] With the increasing demand for extreme environmental parameter measurement in aerospace, nuclear energy, metallurgy, and other energy fields, fiber optic sensing technology capable of long-term stable operation in high-temperature environments has attracted widespread attention. While traditional silica fiber possesses mature fabrication processes and low-loss characteristics, it softens above 1000℃, limiting its application in ultra-high-temperature environments. In contrast, sapphire fiber, due to its high melting point and excellent chemical stability, maintains good optical properties and mechanical strength below 1400℃, and is considered an important candidate material for high-temperature fiber optic sensing.

[0003] However, existing sapphire fibers are unclad and have a high refractive index, resulting in significant multimode effects. Multimode transmission superimposes higher-order mode interference noise into the interference signal, leading to decreased interference fringe contrast, complex spectral structure, reduced signal-to-noise ratio, and increased sensitivity to external disturbances, thus affecting the stability and demodulation accuracy of the sensing signal. In existing technologies, to mitigate the impact of multimode propagation on the interference signal, the length of the sapphire fiber is typically shortened to limit mode accumulation effects, thereby improving the recognizability of the interference signal to some extent. However, this method relies on a short effective sensing length, which not only limits the spatial deployment capability of the sensing system but also makes it difficult to balance stability and measurement range in complex high-temperature environments, failing to meet the engineering application requirements for long-distance, high-reliability sensing. Summary of the Invention

[0004] The purpose of this invention is to provide a high-temperature resistant curvature sensor based on a single-mode sapphire fiber with in-fiber Michelson interferometry, so as to achieve stable curvature sensing under high-temperature conditions.

[0005] To achieve the above objectives, the present invention provides a fiber-in-fiber Michelson interferometric high-temperature resistant curvature sensor based on single-mode sapphire fiber, comprising a demodulation system, a single-mode fiber, and a sapphire fiber sensing probe connected in sequence. The sapphire fiber sensing probe is a sapphire fiber modified to achieve single-mode by femtosecond laser direct writing, and a continuous single-mode light guiding structure is formed inside it. The sapphire fiber sensing probe has a Bragg grating region and a Michelson interferometric structure region arranged in sequence along the axial direction inside it. The Bragg grating region serves as a temperature reference and auxiliary compensation unit in high-temperature demodulation, while the Michelson interference structure region is used to generate optical path difference changes under curvature and form interference reflection signals. The demodulation system outputs incident light, which is coupled into the sapphire fiber optic sensing probe via a single-mode fiber and propagates stably within its continuous single-mode light guide structure. The beam passes through the Bragg grating region and then enters the Michelson interference structure region, forming an interference reflection signal related to curvature. The reflected wavelength signal returned from the Bragg grating region and the interference reflection signal formed by the Michelson interference structure region are returned to the demodulation system via the single-mode fiber for detection and demodulation.

[0006] Preferably, both ends of the sapphire fiber optic sensing probe are polished. The end face closer to the single-mode fiber is used for end-to-end alignment and coupling with the single-mode fiber, while the end face farther from the single-mode fiber is used to form a high-quality reflective end face.

[0007] Preferably, the single-mode fiber and the sapphire fiber sensing probe are coaxially connected by end-to-end alignment, and the central axis of the light guiding area inside the sapphire fiber coincides with the central axis of the single-mode fiber core.

[0008] Preferably, the single-mode modification inside the sapphire fiber sensing probe is achieved by a recessed cladding structure formed by femtosecond laser direct writing. The recessed cladding structure consists of a central light-guiding area and low-refractive-index recessed cladding regions distributed around it.

[0009] Preferably, each recessed cladding region is composed of multiple modulation points, and each modulation point is arranged in accordance with a preset coordinate array rule along the radial and circumferential directions of the cross-section of the sapphire fiber, forming an equivalent low refractive index cladding structure around the central light guiding region; the region located at the center of the array and which has not undergone laser modification forms the central light guiding region, which is used for the transmission of the fundamental mode of the optical field.

[0010] Preferably, the recessed cladding region is formed inside the sapphire fiber by a femtosecond laser direct writing method, specifically: a femtosecond laser is used to scan and write point by point inside the sapphire fiber, and each modulation point is completed by a single scan or multiple repeated scans. The Bragg grating region is integrated into a continuous single-mode light guide structure and is simultaneously written into the single-mode light guide path during the process of femtosecond laser direct writing.

[0011] Preferably, the Michelson interference structure region is a lateral etched microgroove structure located on the side of the sapphire fiber away from the single-mode fiber. The lateral etched microgroove structure extends radially from the sidewall of the sapphire fiber towards the center and cuts into the continuous single-mode light guide structure, partially removing the fiber core and forming two reflective interfaces distributed in the same light guide path.

[0012] Preferably, the Michelson interference structure region is formed by wet etching. After etching, a portion of the optical fiber core is removed, while the other portion is retained to continue guiding light, forming a residual light guiding structure.

[0013] Preferably, the cross section of the lateral etched microgroove structure consists of an etched removal area and a residual fiber core area. The etched removal area removes part of the fiber core cross section, exposing the front fiber core to form a first reflection interface with the air. The residual fiber core area retains part of the light guiding cross section, and the remaining light continues to propagate forward, forming a second reflection interface at its end where it contacts the air. The two reflective interfaces form an in-fiber reflective Michelson interference structure through two reflective boundaries distributed one after the other on the same single-mode light guide path. Within the Michelson interference structure region, the light beam undergoes partial reflection at the first reflection interface, forming the first reflected light. The remaining unreflected light continues to propagate forward along the residual fiber core region and is reflected again at the second reflection interface, forming the second reflected light. The two reflected lights return along the original optical path, recouple and interfere in the continuous single-mode light guide structure, forming an interference reflection signal, which returns to the demodulation system for detection and demodulation via the single-mode fiber.

[0014] Preferably, the light intensities of the first reflected light and the second reflected light are denoted as follows: and The overall interference reflected light intensity is expressed as: ; in, The phase difference between the two reflected beams; For an in-fiber reflective Michelson interferometer structure, the phase difference between the two reflected beams satisfies: ; in, The incident light wavelength, The equivalent effective refractive index in the residual light guide path. The geometric distance between the first and second reflective interfaces is denoted as . This is the initial phase term of the system; The intensity of the overall interference reflected light is expressed as: ; The Bragg grating area is used to provide temperature reference information, wavelength calibration information, or temperature drift compensation information.

[0015] Therefore, the present invention employs the above-mentioned intrafiber Michelson interferometric high-temperature resistant curvature sensor based on single-mode sapphire fiber, which has the following beneficial effects: (1) This invention introduces a recessed cladding single-mode waveguide structure inside the sapphire fiber to suppress multimode propagation and intermode coupling noise, stabilize the optical field distribution and improve the contrast and recognizability of the interference signal, thereby significantly reducing the influence of external disturbances on the interference signal and improving the anti-disturbance capability and measurement repeatability in high-temperature complex environments.

[0016] (2) Based on the above single-mode transmission, an in-fiber reflective Michelson interference structure is constructed inside the same sapphire fiber. Two reflective interfaces are formed by etching microgrooves on the side, so that the reflected light returns along the original optical path and interferes, realizing the in-fiber integration of the sensing unit. It has the characteristics of compact structure and high integration.

[0017] (3) Combining the excellent high temperature resistance of sapphire material with the stable light transmission capability brought by single-mode transmission, it is beneficial to improve the reliability of system engineering integration, making the present invention applicable to curvature monitoring applications in high temperature and extreme environments such as aerospace, energy, metallurgy and nuclear industry.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a system structure diagram of an in-fiber Michelson interferometric high-temperature resistant curvature sensor based on single-mode sapphire fiber, according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the overall structure of the sapphire fiber optic sensing probe in an embodiment of the present invention; Figure 3 This is a partially enlarged schematic diagram of the sapphire fiber Michelson interference structure region in an embodiment of the present invention; Figure 4 This is a schematic cross-sectional view of the sapphire fiber single-mode waveguide region in an embodiment of the present invention; Figure 5 This is a schematic cross-sectional view of the section where the lateral etched microgroove structure is located in an embodiment of the present invention; Figure Labels 1. Demodulation system; 2. Single-mode fiber; 3. Sapphire fiber sensing probe; 3-1. Bragg grating region; 3-2. Michelson interference structure region; 3-2-1. First reflection interface; 3-2-1-1. Etched region; 3-2-1-2. Substrate region; 3-2-2. Second reflection interface; 3-2-2-1. Etched removal region; 3-2-2-2. Residual fiber core region. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0021] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0022] Example like Figure 1 As shown, this invention provides a fiber-interferometric high-temperature resistant curvature sensor based on single-mode sapphire fiber, comprising a demodulation system 1, a single-mode fiber 2, and a sapphire fiber sensing probe 3 connected in sequence. The demodulation system 1 outputs incident light and detects and demodulates the returned interference reflection signal; the single-mode fiber 2 provides the optical connection between the demodulation system 1 and the sapphire fiber sensing probe 3; the sapphire fiber sensing probe 3 serves as the sensing core, used for optical signal transmission, reflection interference, and curvature response in a high-temperature environment.

[0023] like Figure 2 As shown, the sapphire fiber sensing probe 3 is a sapphire fiber modified for single-mode operation via femtosecond laser direct writing, forming a continuous single-mode light guiding structure inside. Inside the sapphire fiber sensing probe 3, a Bragg grating region 3-1 and a Michelson interference structure region 3-2 are sequentially arranged along the axial direction. The Bragg grating region 3-1 provides temperature reference, wavelength calibration, or temperature drift compensation information; the Michelson interference structure region 3-2 generates optical path difference changes under curvature and forms interference reflection signals.

[0024] In this embodiment, a single-crystal sapphire fiber with a diameter of 125μm is selected as the substrate material. Both ends of the sapphire fiber sensing probe 3 are polished. The end face closer to the single-mode fiber 2 is used for end-face alignment and coupling with the single-mode fiber 2; the end face farther from the single-mode fiber 2 is used to form a high-quality reflective end face to improve the optical quality of the end face and ensure the overall structural integrity of the probe.

[0025] The single-mode fiber 2 and the sapphire fiber sensing probe 3 are coaxially connected using end-to-end alignment. During the connection process, the center of the single-mode fiber 2 core is precisely aligned with the center of the light-guiding area inside the sapphire fiber, ensuring that the central axis of the light-guiding area coincides with the central axis of the single-mode fiber 2 core. This guarantees mode field matching between the two fibers and achieves a low-loss connection. In this way, incident light can be stably coupled from the single-mode fiber 2 into the continuous single-mode light-guiding path inside the sapphire fiber sensing probe 3 for propagation.

[0026] like Figure 4 As shown, the single-mode modification inside the sapphire fiber optic sensing probe 3 is achieved through a recessed cladding structure formed by femtosecond laser direct writing. This structure consists of a central light-guiding region and a low-refractive-index recessed cladding region distributed around it. The central light-guiding region includes an etched region 3-2-1-1 and a substrate region 3-2-1-2. The etched region 3-2-1-1 is the area etched away by acid, which is air; the substrate region 3-2-1-2 is not etched by acid and is sapphire. That is, half of the fiber core is etched away (leaving air, corresponding to etched region 3-2-1-1), and half of the fiber core is retained (corresponding to substrate region 3-2-1-2). The diameter of the central light-guiding region is set to 14 μm, and the equivalent cladding diameter formed by the recessed cladding region is set to 50 μm. The size of a single laser modulation point is approximately 0.67 μm × 4.25 μm. These modulation points are arranged regularly around the central light-guiding region according to a preset cross-sectional coordinate array, thus forming an equivalent low-refractive-index recessed cladding within the sapphire matrix. The equivalent refractive index modulation of the recessed cladding is approximately -0.001. Through this structural design, a continuous single-mode light-guiding structure extending axially is formed inside the sapphire fiber, used to constrain light field propagation and suppress multimode propagation and intermode coupling.

[0027] In the specific processing, the recessed cladding region consists of multiple modulation points. These modulation points are arranged radially and circumferentially along the cross-section of the sapphire fiber according to a preset coordinate array rule, thus forming an equivalent low-refractive-index cladding structure around the central light-guiding region. The region located at the center of the array and not subjected to laser modification forms the central light-guiding region, used for the fundamental mode transmission of the optical field. The aforementioned recessed cladding region is formed inside the sapphire fiber using a femtosecond laser direct-writing method. Specifically, a femtosecond laser is used to scan and write point by point inside the sapphire fiber, and each modulation point is completed in a single scan. That is, laser energy is applied only in the forward direction of the laser scan to achieve material modification, and no writing is performed during the scan return process to avoid repeated exposure from adversely affecting the consistency and uniformity of the modulation points. Each modulation point is written sequentially according to a preset spatial coordinate. The writing order on the z-axis adopts a bottom-up point-by-point writing method, that is, the modulation points located deeper inside the sapphire fiber are processed first, and then the remaining modulation points are gradually written towards the fiber surface.

[0028] In this embodiment, the Bragg grating region 3-1 is integrated into the continuous single-mode light guide structure and is simultaneously written during the femtosecond laser direct writing process to form the single-mode light guide path. This grating region, written onto the single-mode light guide path, has a length of 2 mm and a central reflection wavelength located near 1550 nm. During subsequent demodulation, the Bragg grating region 3-1 is used to provide temperature reference information, wavelength calibration information, or temperature drift auxiliary compensation information, thereby improving the accuracy and stability of sensor signal demodulation under high-temperature environments.

[0029] like Figure 3 As shown, the Michelson interference structure region 3-2 is located on the side of the sapphire fiber away from the single-mode fiber 2. Its core structure is a laterally etched microgroove structure formed on the sidewall of the sapphire fiber. This microgroove structure extends radially from the sidewall of the sapphire fiber towards the center and cuts into the continuous single-mode light guide structure, partially removing the fiber core, thereby forming two reflective interfaces distributed front and back on the same light guide path. In this embodiment, the length of the laterally etched microgroove structure is 50 μm, and the groove width is 14 μm. After etching, a portion of the fiber core is removed, while the other portion is retained to continue guiding light, thus forming a residual light guide structure.

[0030] The Michelson interference structure region 3-2 is formed by wet etching. By controlling the etching position, etching length and etching depth, a stable lateral etched microgroove structure can be locally formed at the far end of the sapphire fiber, thereby obtaining a structure in which the exposed fiber core on the front side and the residual fiber core on the rear side coexist, and further forming the first reflection interface 3-2-1 and the second reflection interface 3-2-2.

[0031] like Figure 5 As shown, the cross-section of the lateral etched microgroove structure consists of an etched removal region 3-2-2-1 and a residual fiber core region 3-2-2-2. The etched removal region 3-2-2-1 removes part of the fiber core cross-section, exposing the front fiber core to form a first reflective interface 3-2-1 with the air. The residual fiber core region 3-2-2-2 retains part of the light-guiding cross-section, allowing the remaining light to continue propagating forward, and forms a second reflective interface 3-2-2 at its end where it contacts the air. In other words, in this embodiment, the two reflective interfaces are formed by two reflective boundaries distributed along the same single-mode light-guiding path, constituting an in-fiber reflective Michelson interferometer structure.

[0032] The demodulation system 1 outputs incident light, which is coupled into the sapphire fiber optic sensing probe 3 via single-mode fiber 2 and propagates stably within its continuous single-mode light guide structure. The beam first passes through the Bragg grating region 3-1, then enters the Michelson interference structure region 3-2, forming a curvature-related interference reflection signal. The reflected wavelength signal returned from the Bragg grating region 3-1 and the interference reflection signal formed in the Michelson interference structure region 3-2 are returned to the demodulation system 1 via the single-mode fiber 1 for detection and demodulation. Specifically, within the Michelson interference structure region 3-2, the beam undergoes partial reflection at the first reflection interface 3-2-1, forming the first reflected light. The remaining unreflected light continues to propagate forward along the residual fiber core region 3-2-2-2 and is reflected again at the second reflection interface 3-2-2, forming the second reflected light. The two reflected lights return along their original paths, recouple and interfere within the continuous single-mode light guide structure, ultimately forming the interference reflection signal.

[0033] To facilitate quantitative analysis of the interference reflection signal, the light intensities of the first and second reflected lights can be denoted as follows: and Then the overall interference reflected light intensity can be expressed as: ; in, This represents the phase difference between the two reflected beams.

[0034] For the fiber-reflective Michelson interferometer structure in this embodiment, the phase difference between the two reflected beams satisfies: ; in, The incident light wavelength, The equivalent effective refractive index in the residual light guide path. The geometric distance between the first reflective interface 3-2-1 and the second reflective interface 3-2-2 is... This is the initial phase term of the system.

[0035] Therefore, the overall interference reflected light intensity can be further expressed as: ; When the curvature of the optical fiber changes, the equivalent optical path difference between the first reflecting interface 3-2-1 and the second reflecting interface 3-2-2 will change, causing a drift in the wavelength, phase, or spectral characteristics of the interference reflection signal. Under high-temperature conditions, the thermal expansion and thermo-optic effects of the sapphire material will jointly affect this equivalent optical path difference. When the Michelson interference structure region 3-2 bends with the sapphire fiber, its local geometric path length and stress distribution will change, further affecting the equivalent optical path difference between the two reflecting interfaces. The demodulation system 1 performs wavelength demodulation and phase demodulation on the returned interference reflection signal, and combines the reference wavelength information provided by the Bragg grating region 3-1 for temperature reference, wavelength calibration, or temperature drift compensation, thereby achieving curvature measurement under high-temperature conditions and improving demodulation accuracy and system stability.

[0036] Compared with traditional non-single-mode sapphire fiber sensing structures, this invention simultaneously constructs a continuous single-mode light guide structure, a Bragg grating region 3-1, and a Michelson interference structure region 3-2 within the same sapphire fiber. This not only effectively suppresses multimode propagation and intermode coupling noise, but also achieves integrated sensing unit within the fiber. It features a compact structure, good high-temperature resistance, strong anti-disturbance capability, and suitability for measurement in extreme environments.

[0037] Therefore, this invention employs a high-temperature resistant curvature sensor based on a single-mode sapphire fiber with in-fiber Michelson interferometry. By introducing a single-mode transmission mechanism into the sapphire fiber, the optical field distribution is effectively constrained and higher-order mode interference noise is suppressed, thereby significantly improving the transmission stability of light in the sapphire fiber. Furthermore, a Michelson interferometer structure is constructed within the same sapphire fiber, enabling the sensing unit to be integrated within the fiber. This results in interference signals with high contrast and good stability, which is beneficial for extending the effective working length range of the sapphire fiber and improving the overall stability and engineering applicability of the sensing system when performing curvature measurements in complex high-temperature environments.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry, characterized in that: It includes a demodulation system, a single-mode fiber and a sapphire fiber sensing probe connected in sequence. The sapphire fiber sensing probe is a sapphire fiber that has been modified to be single-mode by direct writing with a femtosecond laser. It forms a continuous single-mode light guiding structure inside. The sapphire fiber sensing probe has a Bragg grating region and a Michelson interference structure region arranged in sequence along the axial direction inside. The Bragg grating region serves as a temperature reference and auxiliary compensation unit in high-temperature demodulation, while the Michelson interference structure region is used to generate optical path difference changes under curvature and form interference reflection signals. The demodulation system outputs incident light, which is coupled into the sapphire fiber optic sensing probe via a single-mode fiber and propagates stably within its continuous single-mode light guide structure. The beam passes through the Bragg grating region and then enters the Michelson interference structure region, forming an interference reflection signal related to curvature. The reflected wavelength signal returned from the Bragg grating region and the interference reflection signal formed by the Michelson interference structure region are returned to the demodulation system via the single-mode fiber for detection and demodulation.

2. The high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 1, characterized in that: Both ends of the sapphire fiber optic sensing probe are polished. The end face closer to the single-mode fiber is used for end-to-end alignment and coupling with the single-mode fiber, while the end face farther from the single-mode fiber is used to form a high-quality reflective end face.

3. The high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 2, characterized in that: The single-mode fiber and the sapphire fiber sensing probe are coaxially connected using end-to-end alignment, and the central axis of the light-guiding area inside the sapphire fiber coincides with the central axis of the single-mode fiber core.

4. The high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 1, characterized in that: The single-mode modification inside the sapphire fiber optic sensing probe is achieved through a recessed cladding structure formed by femtosecond laser direct writing. The recessed cladding structure consists of a central light-guiding area and low-refractive-index recessed cladding regions distributed around it.

5. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 4, characterized in that: Each recessed cladding region consists of multiple modulation points, which are arranged in a preset coordinate array along the radial and circumferential directions of the sapphire fiber cross-section, forming an equivalent low-refractive-index cladding structure around the central light-guiding region. The region located at the center of the array and which has not undergone laser modification forms the central light-guiding region, which is used for the transmission of the fundamental mode of the optical field.

6. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 5, characterized in that: The recessed cladding region is formed inside the sapphire fiber by a femtosecond laser direct writing method. Specifically, a femtosecond laser is used to scan and write point by point inside the sapphire fiber, and each modulation point is completed by a single scan or multiple repeated scans. The Bragg grating region is integrated into a continuous single-mode light guide structure and is simultaneously written into the single-mode light guide path during the process of femtosecond laser direct writing.

7. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 1, characterized in that: The Michelson interference structure region is a lateral etched microgroove structure located on the side of the sapphire fiber away from the single-mode fiber. The lateral etched microgroove structure extends radially from the sidewall of the sapphire fiber towards the center and cuts into the continuous single-mode light guide structure, partially removing the fiber core and forming two reflective interfaces distributed front and back on the same light guide path.

8. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 7, characterized in that: The Michelson interference structure region is formed by wet etching. After etching, part of the optical fiber core is removed, while the other part is retained to continue guiding light, forming a residual light guiding structure.

9. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 8, characterized in that: The cross section where the lateral etched microgroove structure is located consists of an etched removal area and a residual fiber core area. The etched removal area removes part of the fiber core cross section, allowing the exposed fiber core on the front side to form a first reflection interface with the air. The residual core area retains part of the light-guiding cross section, and the remaining light continues to propagate forward, forming a second reflection interface at its end where it contacts the air; The two reflective interfaces form an in-fiber reflective Michelson interference structure through two reflective boundaries distributed one after the other on the same single-mode light guide path. Within the Michelson interference structure region, the light beam undergoes partial reflection at the first reflection interface, forming the first reflected light. The remaining unreflected light continues to propagate forward along the residual fiber core region and is reflected again at the second reflection interface, forming the second reflected light. The two reflected lights return along the original optical path, recouple and interfere in the continuous single-mode light guide structure, forming an interference reflection signal, which returns to the demodulation system for detection and demodulation via the single-mode fiber.

10. A high-temperature resistant curvature sensor based on single-mode sapphire fiber with in-fiber Michelson interferometry according to claim 9, characterized in that: Let the light intensities of the first reflected light and the second reflected light be denoted as follows: and The overall interference reflected light intensity is expressed as: ； in, The phase difference between the two reflected beams; For an in-fiber reflective Michelson interferometer structure, the phase difference between the two reflected beams satisfies: ； in, The incident light wavelength, The equivalent effective refractive index in the residual light guide path. The geometric distance between the first and second reflective interfaces is denoted as . This is the initial phase term of the system; The intensity of the overall interference reflected light is expressed as: ； The Bragg grating area is used to provide temperature reference information, wavelength calibration information, or temperature drift compensation information.

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