Fiber bragg grating low-temperature sensor and preparation method and monitoring method thereof

Abstract

The invention relates to the technical field of optical fiber sensing, in particular to a fiber bragg grating low-temperature sensor and a preparation method and a monitoring method thereof, and the fiber bragg grating low-temperature sensor makes use of the characteristics that a polyether-ether-ketone material is large in thermal expansion coefficient and has good adaptability in a frozen soil environment, so that the sensitivity enhancement effect of the packaged sensor is obvious; the device can well resist frozen soil freezing and thawing, water erosion and other influences, realizes long-term stable and high-precision temperature measurement, and is simple in structure and easy to process; according to the preparation method, the extra pressure stress acting on the fiber bragg grating in the cooling process after the thermosetting adhesive is heated and cured is counteracted through enough tensile stress, the chirp phenomenon caused by full-pasting packaging can be effectively avoided, good thermal coupling between the substrate and the fiber bragg grating is achieved, the use reliability is improved, and the service life is prolonged; the monitoring method adopts a specific mounting mode to realize good contact between the base and the mounting position, and the base can freely expand and retract under the action of temperature, so that the requirement of high longitudinal resolution of the temperature gradient of the frozen earth stratum is met.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an optical fiber grating low-temperature sensor and a preparation method and a monitoring method thereof. Background Art

[0002] Frozen soil refers to various rocks and soils below zero degrees Celsius and containing ice. Temperature is the most important physical quantity that distinguishes frozen soil from other media. Its real-time and accurate acquisition is of great significance to the safety and health of road projects in high-cold and high-altitude areas. At present, the temperature sensors used for frozen soil temperature measurement at home and abroad are generally electromagnetic sensors, which have low measurement accuracy (±0.1 ℃), short life, poor reliability, and the number of sensors connected in series on the same signal line is limited, which cannot meet the high longitudinal resolution requirements of the temperature gradient of the frozen soil layer. Fiber Bragg grating temperature sensors use the mechanism that changes in external temperature will cause changes in the grating period and refractive index, thereby causing changes in the central wavelength of the fiber Bragg grating. By detecting the changes in the central wavelength of the fiber Bragg grating, temperature information can be obtained. The application of fiber Bragg grating in the field of temperature sensing has many advantages.

[0003] Chinese invention patent CN115597742B discloses a fiber Bragg grating temperature sensor and a manufacturing and calibration method, including an optical fiber, a fiber Bragg grating wrapped in the optical fiber, and a sensitizing material of epoxy resin and / or PMMA resin arranged on the surface of the optical fiber. The sensitizing material wraps the fiber Bragg grating, and both ends of the optical fiber pass through the photosensitive material. The sensitizing material has the characteristics of a high thermal expansion coefficient, sufficient elastic modulus and good compatibility in an extremely low temperature environment. The fiber Bragg grating is encapsulated in the sensitizing material, and the thermal deformation of the material is used to achieve temperature measurement in extreme environments.

[0004] However, in the preparation process of the fiber Bragg grating temperature sensor, the temperature sensitivity of the fiber Bragg grating before the sensitizing material is cured is low, while the temperature sensitivity of the fiber Bragg grating after curing is high. After the sensitizing material wrapped in the fiber Bragg grating is cured at high temperature, it is lowered from the high temperature of the curing temperature to the room temperature of the preparation environment. The sensitizing material of the fiber Bragg grating temperature sensor shrinks, which will produce a continuous compression effect on the fiber Bragg grating, causing the central wavelength of the fiber Bragg grating temperature sensor to be significantly smaller than its initial state. If the fiber Bragg grating temperature sensor is applied to the perennial low temperature environment of frozen soil, the grating area of ​​the fiber Bragg grating will be subjected to greater compressive stress, and its central wavelength will be further shortened, which may cause the fiber Bragg grating to chirp, greatly affecting the range and linearity of the fiber Bragg grating sensor. At the same time, affected by environmental factors such as frozen soil thawing and water erosion, the service life of existing sensitizing materials is short, and it is impossible to achieve long-term stable high-precision temperature measurement in low temperature extreme environments. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiency of the fiber Bragg grating sensor in the prior art that it cannot achieve long-term stable and high-precision temperature measurement in a frozen soil low-temperature environment, and to provide a fiber Bragg grating low-temperature sensor and its preparation method and monitoring method.

[0006] In a first aspect, the present invention provides a fiber Bragg grating cryogenic sensor, comprising: The substrate is a polyetheretherketone structural part, and the substrate is provided with a through groove, and the depth and width of the groove are both greater than the diameter of the optical fiber; The optical fiber is arranged longitudinally along the groove, and the optical fiber is engraved with a fiber grating, and the fiber grating is encapsulated in the groove by a thermosetting adhesive; A continuous tensile stress is applied to the fiber Bragg grating before the thermosetting adhesive is cured, and the tensile stress is used to make the central wavelength of the fiber Bragg grating at a preset working temperature the same as or close to that before packaging.

[0007] The present invention provides a fiber Bragg grating low-temperature sensor, which adopts a polyetheretherketone material substrate, fully utilizes the large thermal expansion coefficient of the polyetheretherketone material to achieve the fiber Bragg grating sensitivity enhancement effect, greatly improves the response sensitivity of the fiber Bragg grating to the temperature drop, makes the packaged sensor sensitivity enhancement effect obvious, and can achieve high-precision temperature measurement. The polyetheretherketone material substrate is used to fully encapsulate and protect the fiber Bragg grating, and has good resistance to the influence of frozen soil thawing, water erosion, etc. in a frozen soil environment. It has good adaptability in a frozen soil environment and a long service life in a frozen soil environment. It can meet the long-term stable use in a frozen soil low-temperature extreme environment, has a simple structure, and is easy to process. At the same time, The fiber Bragg grating is fully encapsulated in the substrate groove with a near-rigidity by a thermosetting adhesive under the action of tensile stress. Sufficient tensile stress can offset the additional compressive stress on the fiber Bragg grating during the cooling process after the thermosetting adhesive is heated and cured, so that the grating area of ​​the encapsulated fiber Bragg grating is almost not subjected to additional stress. The central wavelength at the preset working temperature is closer to the central wavelength in the initial state, which can effectively avoid the chirp phenomenon that may be caused by full-paste packaging, overcome the influence of long-term large compressive stress on the fiber Bragg grating at low temperature, and make it more reliable, stable and have a higher service life when used in frozen low-temperature environments for a long time while maintaining high sensitivity.

[0008] Preferably, the bottom of the truncated surface of the groove is arc-shaped or pointed, the fiber Bragg grating is attached to the groove and located in the longitudinal middle of the groove, and the thermosetting adhesive at least covers the fiber Bragg grating. The fiber Bragg grating is fully encapsulated and protected by the thermosetting adhesive in the groove of the substrate, and has a long service life. The bottom of the fiber Bragg grating attached to the groove is located in the longitudinal middle of the substrate, and can be in close contact with the substrate, so that the response is faster, the detection is more accurate, and the sensitivity is higher.

[0009] Preferably, the thermosetting adhesive fills the grooves, so that the substrate with the grooves maintains a regular shape after the fiber grating is packaged, which is convenient for fixation and use, improves the packaging effect, increases the coupling effect between the fiber grating and the substrate, and improves the detection sensitivity.

[0010] Preferably, the thermosetting adhesive is a two-component high-temperature curing adhesive 353ND or 383ND, which can fully encapsulate the fiber grating in a nearly rigid manner after curing, ensuring close contact between the fiber grating and the substrate.

[0011] Preferably, the upper limit of the volume size of the substrate satisfies: the setting of the substrate will not affect the temperature distribution of the object to be measured; and the lower limit of the volume size of the substrate satisfies: the structural strength of the substrate can drive the fiber grating to generate additional strain.

[0012] Preferably, the length of the substrate is 5-25 mm, the length of the grating region of the fiber grating is 2-10 mm, and the length of the substrate is greater than the length of the grating region by 3-23 mm, so that the overall volume of the fiber grating low temperature sensor is small.

[0013] Preferably, the base is a cylindrical structural member, the groove is extended along the axial direction of the base and penetrates the radial side wall of the base, and the diameter of the base is 8-20 times the diameter of the optical fiber.

[0014] Preferably, the base is a rectangular parallelepiped structural member, the groove extends along the length direction of the base and penetrates the side wall of the base in the thickness direction, the width of the base is 10-20 times the diameter of the optical fiber, and the thickness of the base is 8-18 times the diameter of the optical fiber.

[0015] Preferably, the base is a tubular structure, the channel includes the inner space of the base, the channel partially penetrates the radial side wall of the base, the wall thickness of the base is more than 2 times the diameter of the optical fiber, and the outer diameter of the base is more than 6 times the diameter of the optical fiber. The structure in which the channel partially penetrates the base makes the base of the tubular structure have sufficient structural strength, and at the same time, facilitates the uniform coating of the thermosetting adhesive in the base of the tubular structure.

[0016] Preferably, when the wall thickness of the base exceeds 2 times the diameter of the optical fiber, the channel includes a limit groove arranged in the inner space of the tube, and the optical fiber is arranged in the limit groove. The structural strength of the base of the tubular structural member is ensured by thickening the wall thickness, and the contact stability between the optical fiber grating and the inner wall of the base is improved by setting the limit groove.

[0017] Preferably, the depth and width of the limiting groove are greater than the diameter of the optical fiber, and the optical fiber fits the bottom of the limiting groove and is packaged in the limiting groove, so that the optical fiber Bragg grating is packaged within the wall thickness range of the base of the tubular structure, further improving the contact stability between the optical fiber Bragg grating and the base.

[0018] Preferably, within the operating temperature range of -40°C to 60°C, the sensitivity of the fiber Bragg grating low-temperature sensor is not less than 57 pm / °C, which is about 6 times that of a bare fiber Bragg grating.

[0019] In a second aspect, the present invention provides a method for preparing a fiber Bragg grating low-temperature sensor, comprising: Step 1: a longitudinally penetrating groove is formed on a polyetheretherketone structural member to obtain a substrate; Step 2, calculating the central wavelength offset of the fiber Bragg grating, applying tensile stress to the optical fiber according to the calculation result, the offset is related to the curing temperature of the thermosetting adhesive, the ambient temperature of the fiber Bragg grating during preparation, and the coupling coefficient between the substrate strain and the fiber Bragg grating strain; Step 3, matching the relative positions of the fiber Bragg grating and the substrate, filling the groove with a thermosetting adhesive, and encapsulating the fiber Bragg grating; Step 4: Heat the whole body until the thermosetting adhesive is cured; Step 5: Cool naturally.

[0020] The present invention provides a method for preparing a fiber Bragg grating low-temperature sensor. According to the curing temperature of the thermosetting adhesive, the ambient temperature of the fiber Bragg grating during preparation, and the coupling coefficient between the substrate strain and the fiber Bragg grating strain, the specific parameters of the tensile stress applied to the fiber Bragg grating are accurately determined, so that the fiber Bragg grating is fully encapsulated under sufficient tensile stress. The tensile stress is applied accurately and sufficiently, and good thermal coupling between the substrate and the fiber Bragg grating is achieved. The fiber Bragg grating low-temperature sensor can achieve long-term stable and high-precision temperature measurement in an extremely low-temperature environment.

[0021] Preferably, in step 2, the central wavelength offset of the fiber Bragg grating satisfies: under the action of tensile stress, it increases by at least ; Where: is the thermo-optical coefficient of the fiber material; is the coefficient of thermal expansion of the optical fiber material; c is the coupling coefficient between substrate strain and fiber Bragg grating strain, and the value range of c is 0.5-0.7; is the effective elastic-optical coefficient of the optical fiber; is the coefficient of thermal expansion of the base material; is the ambient temperature of the fiber Bragg grating during preparation; is the curing temperature of the thermosetting adhesive; is the central wavelength of the fiber Bragg grating in the free state at ambient temperature.

[0022] Preferably, step 3 comprises: Step 3.1, preheat the whole to 40℃; Step 3.2, coating thermosetting adhesive between the fiber Bragg grating and the groove, and filling the groove until at least the fiber Bragg grating is covered; Step 3.3, heating the whole to 80-120°C; Step 3.4, heat preservation and curing.

[0023] Preferably, the optical fiber has an acrylate coating, and the fiber grating is obtained by partially removing the acrylate coating and then writing with an ultraviolet laser, or the optical fiber has a polyimide coating, and the fiber grating is obtained by writing with a femtosecond laser without removing the coating.

[0024] Preferably, during preparation, the central wavelength of the fiber Bragg grating is 1555.61 nm, the ambient temperature of the fiber Bragg grating is 20° C., and the curing temperature of the thermosetting adhesive is 80° C., and the central wavelength of the fiber Bragg grating increases to 1559.115 nm under tensile stress.

[0025] In a third aspect, the present invention provides a method for monitoring temperature in a frozen soil area using a fiber Bragg grating cryogenic sensor, comprising: Determine the installation position of the fiber Bragg grating cryogenic sensor; The fiber Bragg grating low temperature sensor is installed at the installation position by adopting the method of fixing the substrate at one end, and a plurality of fiber Bragg grating low temperature sensors are connected in series through optical fibers; One side of the base of the fiber grating low-temperature sensor is in contact with the installation position, and the other side is the through side of the groove.

[0026] The present invention provides a method for monitoring temperature in a frozen soil area using a fiber grating low-temperature sensor. The method adopts a single-end fixing method for the base for installation and fixing. The base can achieve good contact with the installation position and can expand and contract freely under the action of temperature, thereby improving the measurement sensitivity and achieving accurate temperature measurement.

[0027] Preferably, the single-end fixing method includes: bonding, welding, magnetic attraction, bundling, crimping or clamping the area on the substrate that is misaligned with the optical fiber Bragg grating.

[0028] Preferably, the installation positions are distributed and arranged according to the stratified characteristics of the temperature changes in the soil in the frozen soil area or in the engineering structure.

[0029] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention provides a fiber Bragg grating low-temperature sensor, which adopts a polyetheretherketone material as a substrate, fully utilizes the large thermal expansion coefficient of the polyetheretherketone material to achieve the fiber Bragg grating sensitivity enhancement effect, greatly improves the fiber Bragg grating response sensitivity to temperature reduction, makes the packaged sensor sensitivity enhancement effect obvious, and can achieve high-precision temperature measurement; 2. The present invention provides a fiber Bragg grating low-temperature sensor, the substrate of which is made of polyetheretherketone material and can better resist the influence of frozen soil thawing, water erosion, etc. in a frozen soil environment, has good adaptability in a frozen soil environment, has a long service life in a frozen soil environment, and can meet the requirements of long-term stable use in a low-temperature extreme environment; 3. The present invention provides a fiber Bragg grating low-temperature sensor with a simple structure and easy processing; 4. The present invention provides a fiber Bragg grating low-temperature sensor. The fiber Bragg grating is fully encapsulated in a substrate groove by a thermosetting adhesive in a nearly rigid manner under the action of tensile stress. The additional compressive stress acting on the fiber Bragg grating during the cooling process after the thermosetting adhesive is heated and cured is offset by sufficient tensile stress. This can effectively avoid the chirp phenomenon that may be caused by full-paste encapsulation, and can make the grating area of ​​the encapsulated fiber Bragg grating almost free from additional stress. The central wavelength at a preset working temperature is closer to the central wavelength in the initial state, and the reliability, stability and service life of the fiber Bragg grating in an extreme low-temperature environment for a long time will be higher. 5. The present invention provides a method for preparing a fiber Bragg grating low-temperature sensor. According to the curing temperature of the thermosetting adhesive, the ambient temperature of the fiber Bragg grating during preparation, and the coupling coefficient between the substrate strain and the fiber Bragg grating strain, the specific parameters of the tensile stress applied to the fiber Bragg grating are accurately determined, so that the fiber Bragg grating is fully encapsulated under sufficient tensile stress, the tensile stress is applied accurately and sufficiently, and good thermal coupling between the substrate and the fiber Bragg grating is achieved, so that the fiber Bragg grating low-temperature sensor can achieve long-term stable and high-precision temperature measurement in an extreme low-temperature environment; 6. The present invention provides a method for monitoring temperature in frozen soil areas using a fiber grating low-temperature sensor. By installing and fixing the base at a single end, the base can be in good contact with the installation position, and can expand and contract freely under the action of temperature, thereby improving measurement sensitivity and achieving accurate temperature measurement. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 A schematic diagram of the structure of a fiber Bragg grating low-temperature sensor based on the cylindrical structural member in Example 1; Figure 2 A schematic cross-sectional view of a fiber Bragg grating low-temperature sensor of the cylindrical structural member substrate in Example 1; Figure 3 A schematic diagram of the structure of a fiber Bragg grating low-temperature sensor based on a rectangular parallelepiped structural member in Example 1; Figure 4 A schematic cross-sectional view of a fiber Bragg grating low-temperature sensor of the rectangular parallelepiped structural member substrate in Example 1; Figure 5 A schematic diagram of the position of a fiber Bragg grating of a fiber Bragg grating low-temperature sensor on the rectangular parallelepiped structural member substrate in Example 1; Figure 6A schematic diagram of the structure of a fiber Bragg grating low-temperature sensor based on the tubular structural member in Example 1; Figure 7 A schematic diagram of the position of a fiber Bragg grating of a fiber Bragg grating low-temperature sensor on the tubular structural member substrate in Example 1; Figure 8 A schematic diagram of a cross section of a fiber Bragg grating low-temperature sensor of the tubular structure substrate in Example 1 Figure 1 ; Fig. 9 A schematic diagram of a cross section of a fiber Bragg grating low-temperature sensor of the tubular structure substrate in Example 1 Figure 2 ; Fig.10 A schematic diagram of a cross section of a fiber Bragg grating low-temperature sensor of the tubular structure substrate in Example 1 Figure 3 ; Fig.11 The temperature rise and fall curve of a fiber Bragg grating low-temperature sensor in Example 1 at a temperature of -40°C to 20°C; Fig.12 The temperature rise and fall curves of the copper-based fiber Bragg grating low-temperature sensor for comparison; Fig.13 The temperature rise and fall curves of the copper-based fiber Bragg grating low-temperature sensor for comparison; Fig.14 is the temperature rise and fall curve of another fiber Bragg grating low-temperature sensor with different central wavelength at a temperature of -40℃~20℃; Fig.15 A schematic diagram of a process for preparing a fiber Bragg grating low-temperature sensor according to Example 2; Fig.16 is a schematic structural diagram of the clamping member described in Example 3; Fig.17 This is a schematic diagram of the end portion of a fiber Bragg grating low-temperature sensor that is clamped and fixed in Example 3; Fig.18 A top view of a fiber Bragg grating cryogenic sensor fixed and clamped in Example 3; Markings in the figure: 1-base, 11-groove, 12-limiting groove, 13-hollow section, 2-optical fiber, 21-optical fiber grating, 3-thermosetting adhesive, 4-clamping piece, 41-travel channel, 42-arc surface. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with specific embodiments. However, this should not be understood as the scope of the above subject matter of the present invention being limited to the following embodiments, and all technologies realized based on the content of the present invention belong to the scope of the present invention.

[0032] Unless otherwise specified, in the description of the specific embodiments of the present invention, the terms indicating the orientation or position relationship such as "up", "down", "left", "right", "center", "inside", "outside", etc. are all expressions based on the orientation or position relationship shown in the drawings, or are the orientation or position relationship when the invented product / equipment / device is usually used. These terms of orientation or position relationship are only for the convenience of describing the scheme of the present invention or simplifying the description in the specific embodiments, so as to facilitate the technicians to quickly understand the scheme, and do not indicate or imply that a specific device / component / element must have a specific orientation, or be constructed and operated in a specific position relationship, and therefore cannot be understood as a limitation on the present invention.

[0033] In addition, if the terms "horizontal", "vertical", "overhanging", "parallel" and the like appear, it does not mean that the corresponding devices / components / elements are required to be absolutely horizontal or vertical or overhanging or parallel, but may be slightly tilted or have deviations. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but may be slightly tilted. Alternatively, it can be simplified to mean that the corresponding devices / components / elements are set in directions such as "horizontal", "vertical", "overhanging", "parallel", etc., and can have an error / deviation of ±10% relative to the corresponding direction setting, more preferably an error / deviation within ±8%, more preferably an error / deviation within ±6%, more preferably an error / deviation within ±5%, and more preferably an error / deviation within ±4%. As long as the corresponding device / component / element is within the error / deviation range, it can still achieve its role in the scheme of the present invention.

[0034] In addition, the expressions “first”, “second”, “third”, etc., which appear in the terms, are merely used to distinguish the description of the same or similar components and should not be understood as emphasizing or implying the relative importance of specific components.

[0035] In addition, in the description of the embodiments of the present invention, "several", "plurality" and "a number" represent at least 2. It can be any number such as 2, 3, 4, 5, 6, 7, 8, 9, and even more than 9.

[0036] In addition, in the description of the technical solution of the present invention, unless otherwise clearly specified / defined / restricted, the terms "set", "install", "connect", "connected", "provided with", "laid", and "arranged" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection, and can be welding, riveting, bolting, threading, and other commonly used connection means in the field. This connection can be a mechanical connection, an electrical connection, or a communication connection; it can be a direct connection, or an indirect connection through an intermediate medium, and it can be the internal connection of two elements.

[0037] Example 1 like Figure 1-Figure 10 As shown, a fiber Bragg grating low temperature sensor comprises a substrate 1 and an optical fiber 2, a fiber Bragg grating 21 is engraved on the optical fiber 2, the substrate 1 is a polyetheretherketone structural member, a through groove 11 is provided on the substrate 1, the depth and width of the groove 11 are both greater than the diameter of the optical fiber 2, the optical fiber 2 is arranged longitudinally along the groove 11, the fiber Bragg grating 21 is encapsulated in the groove 11 by a thermosetting adhesive 3 in a state of being in contact with the inner bottom surface of the groove 11, and a continuous tensile stress is applied to the fiber Bragg grating 21 before the thermosetting adhesive 3 is cured, and the tensile stress is used to make the central wavelength of the fiber Bragg grating 21 at a preset working temperature the same as or close to that before encapsulation.

[0038] In the present embodiment, a fiber Bragg grating low-temperature sensor has a substrate 1 which is an integrated structure of polyetheretherketone. The substrate 1 made of polyetheretherketone can fully utilize the large thermal expansion coefficient of the polyetheretherketone material to achieve the sensitivity enhancement effect of the fiber Bragg grating 21, greatly improve the response sensitivity of the fiber Bragg grating 21 to the temperature drop, make the packaged sensor sensitivity enhancement effect obvious, and achieve high-precision temperature measurement. At the same time, the substrate 1 which is an integrated structure of polyetheretherketone material is adopted, and the fiber Bragg grating 21 is packaged through the groove 11, which can better resist the influence of frozen soil thawing, water erosion, etc. in a frozen soil environment, has good adaptability in a frozen soil environment, has a long service life in a frozen soil environment, and can meet the long-term stable use in a low-temperature extreme environment.

[0039] In order to further verify the adaptability of the polyetheretherketone material substrate 1 of this embodiment in the extreme environment of frozen soil and low temperature, this embodiment uses a copper material substrate 1, an aluminum material substrate 1 and a polyetheretherketone material substrate 1 to prepare a fiber Bragg grating low-temperature sensor, and conducts comparative tests at an experimental working temperature of -40°C to 15°C or -40°C to 20°C. The experimental working temperature is selected according to the experimental environment temperature. Figure 11-Figure 14 It can be seen that the fiber grating low-temperature sensor prepared by the present embodiment, which is an integrated structural component made of polyetheretherketone material and is coupled with the fiber grating 21 under the action of thermosetting adhesive 3 and tensile stress, has higher measurement sensitivity, better sensitivity and linearity than the temperature sensor with conventional copper substrate 1 and aluminum substrate 1, indicating that the polyetheretherketone material substrate 1 has good adaptability to the frozen soil low-temperature environment with the corresponding working temperature.

[0040] In a fiber Bragg grating low-temperature sensor of the present embodiment, the fiber Bragg grating 21 is fully encapsulated in the groove 11 of the substrate 1 in a nearly rigid manner through the thermosetting adhesive 3 under sufficient tensile stress. Not only can the substrate 1 encapsulation improve its ability to resist freeze-thaw, water erosion, etc., but also the sufficiently large tensile stress can be used to offset the additional compressive stress acting on the fiber Bragg grating 21 during the cooling process after the thermosetting adhesive 3 is heated and cured. The chirp phenomenon that may be caused by the full-paste encapsulation can be effectively avoided, and the grating area of ​​the encapsulated fiber Bragg grating 21 is almost not subjected to additional stress, and the central wavelength at the working temperature is closer to the central wavelength in the initial state.

[0041] Specifically, compared with conventional fiber Bragg grating temperature sensors, the fiber Bragg grating 21 of conventional fiber Bragg grating temperature sensors will be subjected to additional compressive stress after the thermosetting adhesive 3 is heated and cured and then cooled. If used in a high-temperature environment, the higher temperature will cause the substrate 1 and the fiber Bragg grating 21 to stretch, increasing the central wavelength in the working state, thereby offsetting the impact of the compressive stress. Therefore, conventional fiber Bragg grating temperature sensors do not consider applying a tensile stress sufficient to make the central wavelength of the fiber Bragg grating 21 at the working temperature close to the central wavelength in the initial state during preparation. If the conventional fiber Bragg grating temperature sensor is used in a frozen soil low-temperature environment, the lower temperature will further cause the substrate 1 and the fiber Bragg grating 21 to shrink, resulting in the optical The central wavelength of the fiber Bragg grating 21 is further shortened, and its grating area will be subjected to greater compressive stress, which will affect its reliability and service life and cause chirp. This is also the main reason why the fiber Bragg grating temperature sensor of the same type in the prior art cannot be used for long-term, high-sensitivity temperature detection in extremely low temperatures in frozen soil environments. Therefore, the fiber Bragg grating low-temperature sensor of this embodiment applies sufficient tensile stress during packaging to make the central wavelength of the fiber Bragg grating 21 at the operating temperature closer to the central wavelength in the initial state. The reliability, stability and service life of the fiber Bragg grating 21 in an extreme low temperature environment for a long time will be higher, thereby overcoming the influence of the large compressive stress on the fiber Bragg grating 21 at low temperatures on its range, linearity and life.

[0042] A fiber Bragg grating low temperature sensor of the present embodiment adopts a single substrate 1 structure to open a groove 11 and fill a thermosetting adhesive 3 to form a single substrate full packaging structure of a fiber Bragg grating 21. The structure is simple and easy to process, which is conducive to improving the preparation efficiency of the fiber Bragg grating low temperature sensor and reducing the preparation difficulty.

[0043] A fiber Bragg grating low-temperature sensor of the present embodiment adopts the substrate 1 of the above-mentioned material and structure, is packaged with thermosetting adhesive 3, and sufficient tensile stress is applied during packaging, so that the sensitivity of the fiber Bragg grating low-temperature sensor is not less than 57 pm / ℃ within the working temperature range of -40℃ to 60℃. Compared with the sensitivity of 9-12 pm / ℃ of conventional bare fiber Bragg grating temperature sensors, the overall sensitivity of the fiber Bragg grating low-temperature sensor of the present embodiment is improved by about 6 times.

[0044] In one or more embodiments, the substrate 1 is a three-dimensional structure with the characteristics of being long in the axial direction and narrow in the width direction. The groove 11 is arranged along the axial direction of the substrate 1 to match the long and thin characteristics of the optical fiber 2. The groove 11 is a lateral through-side that penetrates the substrate 1 in the lateral direction perpendicular to the axial direction, that is, the opening side of the groove 11. The fiber grating 21 can be placed in the substrate 1 from the lateral through-position of the groove 11, and the thermosetting adhesive 3 can be directly applied from the lateral through-position of the groove 11. Such a design can facilitate the uniform application of the thermosetting adhesive 3.

[0045] In one or several embodiments, the bottom of the cross-section of the groove 11 is arc-shaped or pointed-angle-shaped, and the fiber Bragg grating 21 is located in the longitudinal middle of the groove 11 and contacts the bottom surface of the groove 11, which can reduce the distance between the optical fiber 2 and the substrate 1 as much as possible, increase the spatial contact area between the optical fiber 2 and the substrate 1, and make the contact between the fiber Bragg grating 21 and the substrate 1 stable. At the same time, the space formed between the optical fiber 2 and the bottom surface of the groove 11 can still be filled with thermosetting adhesive 3, and the fiber Bragg grating 21 can be wrapped and packaged in the space of the groove 11 with less thermosetting adhesive 3, so that the fiber Bragg grating 21 is fully packaged and protected by the thermosetting adhesive 3 in the groove 11 of the substrate 1 and the structure of the substrate 1, and has a long service life. It also realizes good coupling between the fiber Bragg grating 21 and the substrate 1, and is more sensitive to the central wavelength change caused by the strain of the substrate 1 under temperature changes, which can realize high-sensitivity and accurate detection of temperature changes by low-temperature sensors.

[0046] In an optional embodiment, the thermosetting adhesive 3 fills the space inside the groove 11 and is filled to be flush with the opening of the groove 11. The thermosetting adhesive 3 fills the structural defects formed by the grooves of the substrate 1, making the overall structure of the substrate 1 regular and convenient for installation and use.

[0047] In an optional implementation manner, the bottom of the cross-section of the groove 11 is arc-shaped, and the fiber grating 21 forms a contact line with the bottom surface of the groove 11 .

[0048] In an optional embodiment, the bottom of the cross-section of the groove 11 is a right-angled or wedge-shaped sharp-angled shape, and the fiber grating 21 forms at least two contact lines with the bottom surface of the groove 11 .

[0049] In one or several embodiments, the upper limit of the volume size of the substrate 1 satisfies: the setting of the substrate 1 will not affect the temperature distribution of the object to be measured; the lower limit of the volume size of the substrate 1 satisfies: the structural strength of the substrate 1 can drive the fiber grating 21 to produce additional strain, so that the volume size of the substrate 1 can be designed to be smaller, adapted to the object to be measured, and convenient for installation and use, and after being affected by temperature, it can have sufficient strength to drive the fiber grating 21 packaged thereon to produce a central wavelength offset change, thereby realizing accurate temperature monitoring.

[0050] In an optional embodiment, the length of the substrate 1 is 5-25 mm, the length of the grating area of ​​the fiber grating 21 is 2-10 mm, and the length of the substrate 1 is greater than the length of the grating area by 3-23 mm, so that the volume of the substrate 1 is small, and it is convenient to install and use on different objects to be tested.

[0051] In one or more embodiments, Figure 1-Figure 2 As shown, the base 1 is a cylindrical structural member, the groove 11 is extended along the axial direction of the base 1 and penetrates the radial side wall of the base 1 , and the diameter of the base 1 is 8-20 times the diameter of the optical fiber 2 .

[0052] In an optional embodiment, the base 1 is a cylindrical structural member, the base 1 is 15 mm long and 3 mm in diameter, a groove 11 with a circular arc bottom and a width and depth of 1 mm is opened on the outer wall of the base 1 along the axial direction, the optical fiber 2 has a diameter of 0.25 mm, and a fiber grating 21 with a grating area length of 5 mm is provided on the optical fiber 2. The fiber grating 21 fits the bottom of the groove 11 and is arranged in the longitudinal middle of the groove 11. The two ends of the fiber grating 21 are respectively 5 mm away from the axial end wall of the base 1. The thermosetting adhesive 3 is filled until it is flush with the opening of the groove 11, so as to achieve full bonding and packaging of the fiber grating 21 and the local optical fiber 2 in the groove 11, thereby forming a fiber grating low-temperature sensor with a cylindrical structural member with a regular shape.

[0053] In one or more embodiments, Figure 3-Figure 5 As shown, the base 1 is a rectangular structure, the groove 11 extends along the length direction of the base 1 and penetrates the side wall of the base 1 in the thickness direction, the width of the base 1 is 10-20 times the diameter of the optical fiber 2, and the thickness of the base 1 is 8-18 times the diameter of the optical fiber 2.

[0054] In an optional embodiment, the substrate 1 is a rectangular structural member with a length of 15 mm, a width of 4 mm, and a thickness of 3 mm. A groove 11 with a circular arc bottom and a width and depth of 1 mm is opened along the length direction on the long and wide surfaces of the substrate 1. The diameter of the optical fiber 2 is 0.25 mm. A fiber grating 21 with a grating area length of 5 mm is provided on the optical fiber 2. The thermosetting adhesive 3 is filled until it is flush with the opening of the groove 11, forming a fiber grating low-temperature sensor with a rectangular structural member with a regular shape.

[0055] In one or more embodiments, Figure 6-Figure 10 As shown, the base 1 is a tubular structure, the groove 11 includes the inner space of the base 1, the groove 11 partially penetrates the radial side wall of the base 1, the wall thickness of the base 1 is more than twice the diameter of the optical fiber 2, and the outer diameter of the base 1 is more than 6 times the diameter of the optical fiber 2.

[0056] In an optional embodiment, if Figure 6As shown, the middle part of the groove 11 passes through the radial side wall of the base 1, and the two axial ends of the base 1 maintain an annular tubular structure, so as to ensure the overall structural strength of the base 1 of the tubular structure; a hollow section 13 is formed in the middle area of ​​the base 1 of the tubular structure, and the hollow section 13 is located in the longitudinal middle part of the base 1, corresponding to the grating area of ​​the fiber grating 21. The hollow section 13 is used to facilitate the uniform coating of the thermosetting adhesive 3 and fully expose the grating area of ​​the fiber grating 21, so that the fiber grating 21 arranged in the tubular structure is in stable contact with the base 1, and can produce a central wavelength change with the strain and temperature change of the base 1, thereby realizing sensitive detection of temperature.

[0057] In an optional embodiment, the depth of the hollow section 13 does not exceed half of the diameter of the tubular structural member base 1 to ensure that after the hollow section 13 is set, the structural strength of the corresponding hollow portion of the tubular structural member base 1 is still sufficient to drive the fiber grating 21 to generate additional strain.

[0058] In an optional embodiment, the length of the hollow section 13 is at least 5 mm longer than the grating area of ​​the fiber grating 21 to fully expose the fiber grating 21 and ensure good coating of the thermosetting adhesive 3 in the grating area.

[0059] In an optional embodiment, if Fig. 9 As shown, when the wall thickness of the substrate 1 exceeds 2 times the diameter of the optical fiber 2, the channel 11 further includes a limiting groove 12 arranged in the inner space of the tube, and the optical fiber 2 is arranged in the limiting groove 12. At this time, the wall thickness of the substrate 1 is relatively thick, and the opening of the limiting groove 12 will not affect the structural strength of the substrate 1. The optical fiber 2 is arranged in the limiting groove 12, which is conducive to the stable contact of the optical fiber grating 21 in the substrate 1, avoiding displacement during the packaging process, ensuring the quality of the obtained optical fiber grating low-temperature sensor, and by increasing the contact area between the optical fiber grating 21 and the substrate 1, it is conducive to further improving the detection sensitivity, such as Figure 8 As shown, when the limiting groove 12 is not provided in the base 1 , the optical fiber 2 is provided on the bottom surface of the inner space of the base 1 .

[0060] In an optional embodiment, if Fig.10 As shown, the depth and width of the limiting groove 12 are greater than the diameter of the optical fiber 2, and the optical fiber 2 is fitted to the bottom of the limiting groove 12 and packaged in the limiting groove 12. At this time, the wall thickness of the substrate 1 meets the structural strength after the limiting groove 12 with sufficient depth and width is opened. The fiber Bragg grating 21 is encapsulated in the limiting groove 12 as a whole and coupled with the thick wall area of ​​the substrate 1 of the tubular structure, which can further improve the coupling effect between the fiber Bragg grating 21 and the substrate 1 of the tubular structure under the action of the thermosetting adhesive 3, and improve the structural stability and detection sensitivity of the obtained fiber Bragg grating low-temperature sensor.

[0061] In an optional embodiment, after the groove 11 partially penetrates the radial side wall of the substrate 1, at least a section of the complete tubular substrate 1 is retained in the area on the substrate 1 that is misaligned with the grating area of ​​the optical fiber grating 21, so as to ensure the structural strength of the low-temperature sensor of the tubular structural member substrate 1, that is, the position of the hollow section 13 formed by the groove 11 partially penetrating the radial side wall of the substrate 1 can be adjusted according to actual conditions.

[0062] In an optional embodiment, if Figure 6 As shown, the base 1 is a tubular structure with an outer diameter of 5 mm, an inner diameter of 1 mm, and a wall thickness of 4 mm. A 1 mm diameter arc groove 11 parallel to the axis is processed on the base 1. The diameter of the optical fiber 2 is 0.25 mm, and a fiber grating 21 with a grating area length of 5 mm is provided on the optical fiber 2.

[0063] In one or several embodiments, the substrate 1 may also be a three-dimensional structure having obvious axial length and width-to-narrow characteristics, such as a prism or a strip. The cross-sectional shape and size of the groove 11 may also be adjusted according to the grating area length of the fiber grating 21 to achieve high-stability full encapsulation of the fiber grating 21 in the groove 11 of the substrate 1.

[0064] In one or several embodiments, the length of the substrate 1 can be extended according to actual conditions, the number of grooves 11 on a single substrate 1 can be increased according to actual conditions, and one or more fiber gratings 21 can be encapsulated in the groove 11 according to actual conditions, so that multiple fiber gratings 21 are connected in series through optical fibers 2 and encapsulated in one substrate 1, thereby expanding the optional shape of the substrate 2 to form fiber grating low-temperature sensors with different structural forms to meet the requirements of different usage environments.

[0065] In one or more embodiments, the thermosetting adhesive 3 is a two-component high-temperature curing adhesive 353ND or 383ND. Conventional two-component high-temperature curing adhesive 353ND or 383ND can be cured by heating at 80°C to achieve stable contact between the fiber grating 21 and the substrate 1. This embodiment uses a method different from the conventional heating and curing operation when encapsulating the thermosetting adhesive 3, so that the thermosetting adhesive 3 can fully encapsulate the fiber grating 21 in a nearly rigid manner after curing.

[0066] In one or several embodiments, the central wavelength of the fiber Bragg grating 21 is 1510-1590 nm. The fiber Bragg grating 21 can be formed by writing on an optical fiber with a portion of the acrylate coating removed, or can be directly written on an optical fiber with a polyimide coating. The fiber Bragg grating 21 prepared in different ways can meet the requirements of use, which is convenient for designing and preparing fiber Bragg grating low-temperature sensors according to actual conditions.

[0067] Example 2 like Fig.15 As shown, a method for preparing a fiber Bragg grating low-temperature sensor comprises: Step 1, a longitudinally penetrating groove 11 is formed on a polyetheretherketone structural member to obtain a substrate 1; Step 2, calculating the center wavelength offset of the fiber Bragg grating 21, applying tensile stress to the optical fiber 2 according to the calculation result, the center wavelength offset is related to the curing temperature of the thermosetting adhesive 3, the ambient temperature of the fiber Bragg grating 21 during preparation, and the coupling coefficient between the strain of the substrate 1 and the strain of the fiber Bragg grating 21; Step 3, matching the relative positions of the fiber Bragg grating 21 and the substrate 1, filling the groove 11 with a thermosetting adhesive 3, and encapsulating the fiber Bragg grating 21; Step 4, heating the whole body until the thermosetting adhesive 3 is solidified; Step 5: Cool naturally.

[0068] A method for preparing a fiber Bragg grating low-temperature sensor according to the embodiment of the present invention accurately determines the specific parameters of the tensile stress applied to the fiber Bragg grating 21 according to the curing temperature of the thermosetting adhesive 3, the ambient temperature of the fiber Bragg grating 21 during preparation, and the coupling coefficient between the strain of the substrate 1 and the strain of the fiber Bragg grating 21, so that the fiber Bragg grating 21 is fully encapsulated under sufficient tensile stress, and the tensile stress is applied accurately and sufficiently to achieve good thermal coupling between the substrate 1 and the fiber Bragg grating 21. The highly sensitive and highly stable fiber Bragg grating low-temperature sensor thus prepared can be used after being calibrated in a high-precision constant temperature bath within the working temperature range, and can achieve long-term stable and high-precision temperature measurement of the fiber Bragg grating low-temperature sensor in an extreme low-temperature environment.

[0069] Specifically, in step 2, the central wavelength offset of the fiber Bragg grating 21 satisfies: under the action of tensile stress, it increases by at least ; Where: is the thermo-optical coefficient of the fiber 2 material; is the thermal expansion coefficient of the fiber 2 material; is the coupling coefficient between the strain of substrate 1 and the strain of fiber Bragg grating 21, and the value range of c is 0.5-0.7; is the effective elastic-optical coefficient of optical fiber 2; is the thermal expansion coefficient of the substrate 1 material; is the ambient temperature of the fiber Bragg grating 21 during preparation; is the curing temperature of the thermosetting adhesive 3; is the central wavelength of the fiber Bragg grating 21 in a free state at ambient temperature.

[0070] The determination mechanism of the above tensile stress is: On the one hand, when the ambient temperature of the sensor decreases, the thermal expansion effect and the thermo-optical effect of the optical fiber 2 material cause the relative wavelength shift of the central wavelength of the optical fiber grating 21 to be expressed as: ; In the formula, is the central wavelength of the fiber Bragg grating 21 in a free state at ambient temperature; is the change in temperature relative to the initial state; is the drift of the central wavelength of the fiber grating 21 due to the temperature drop; is the thermo-optical coefficient of the fiber 2 material; is the thermal expansion coefficient of the fiber 2 material; Without sensitization of the fiber Bragg grating 21, its independent temperature sensitivity is: ; When the ambient temperature of the fiber Bragg grating 21 does not change, but the strain of the grating region changes due to external force, the center wavelength drift of the fiber Bragg grating 21 is: ; In the formula, is the drift of the central wavelength due to the strain in the grating region of the fiber Bragg grating 21; is the effective elastic-optical coefficient of optical fiber 2; is the strain of the fiber Bragg grating 21. If the grating region is subjected to tensile stress, it is positive; if the grating region is subjected to compressive stress, it is negative.

[0071] On the other hand, considering that the thermal expansion coefficient of the substrate 1 is greater than that of the optical fiber 2, when the temperature decreases, the unit length of the substrate 1 will become shorter than that of the optical fiber 2, and the optical fiber Bragg grating 21 is fixed to the substrate 1 by the thermosetting adhesive 3 in an approximately rigid manner. The decrease in temperature will force the optical fiber Bragg grating 21 to obtain additional negative strain. The relative wavelength drift of the optical fiber Bragg grating 21 caused by this axial strain satisfies Formula 2: ; In the formula, is the drift of the central wavelength of the fiber grating 21 caused by the thermal expansion of the substrate 1; is the central wavelength of the fiber Bragg grating 21 in a free state at ambient temperature; is the effective elastic-optical coefficient of optical fiber 2; is the strain of the fiber Bragg grating 21; is the thermal expansion coefficient of the substrate 1 material; is the thermal expansion coefficient of the fiber 2 material; is the change in temperature relative to the initial state; is the coupling coefficient between the strain of the substrate 1 and the strain of the fiber grating 21.

[0072] Specifically, the coupling coefficient is related to factors such as the thermal expansion coefficient, Young's modulus, and structural dimensions of the substrate 1, the optical fiber 2, and the thermosetting adhesive 3. This embodiment uses a substrate 1 of a polyetheretherketone integrated structural member of a specific structural dimension, combined with a specific thermosetting adhesive 3 and a preparation method, and obtains the coupling coefficient through theoretical research. The value range of can be 0.5-0.7, preferably 0.55-0.65.

[0073] Therefore, the relative wavelength drift of the fiber Bragg grating 21 caused by the decrease in ambient temperature is the sum of equation 1 and equation 2, and the final relative wavelength drift is satisfy: ; Assuming that the initial temperature of the fiber Bragg grating 21 is room temperature, and the temperature of the thermosetting adhesive 3 during high temperature curing is, when the fiber Bragg grating 21 is heated to reach a temperature without external stress, its wavelength drift is: ; When the fiber Bragg grating 21 is rigidly bonded to the substrate 1 material at a temperature by the thermosetting adhesive 3, its temperature sensitivity increases. When it returns to room temperature again, its wavelength drift relative to temperature is: ; Then the center wavelength drift of the fiber Bragg grating 21 relative to the initial state after returning to room temperature is: ; because , the central wavelength at this time will become smaller than the initial state, that is, the fiber Bragg grating 21 is subjected to compressive stress. This embodiment considers eliminating / reducing compressive stress. Therefore, it is considered to apply the same prestress as the fiber Bragg grating 21 before it is rigidly bonded to the substrate 1 material, and the wavelength drift caused by the prestress needs to be, therefore, at the curing temperature, this embodiment uses the central wavelength at the initial temperature and room temperature as a reference, and by applying sufficient prestress, the central wavelength of the fiber Bragg grating 21 is increased by at least Thus, after the thermosetting adhesive 3 is cured and the fiber grating 21 returns to room temperature, its central wavelength will be very close to the initial state.

[0074] Therefore, by applying accurate tensile stress adapted thereto, the fiber Bragg grating 21 is packaged under sufficient tensile stress, and the additional compressive stress acting on the fiber Bragg grating 21 during the cooling process after the thermosetting adhesive 3 is heated and cured is offset, that is, the grating area of ​​the packaged fiber Bragg grating 21 is not subjected to additional stress, and the central wavelength at the working temperature is closer to the central wavelength in the initial state, and the chirp phenomenon that may be caused by the full-paste package can be effectively avoided, thereby making the obtained fiber Bragg grating low-temperature sensor more reliable, stable and having a higher service life when used in an extreme low-temperature environment for a long time, greatly improving the response sensitivity of the fiber Bragg grating 21 to temperature reduction, and solving the problem of high-precision temperature measurement in an extreme low-temperature environment well, and being able to overcome the influence of the fiber Bragg grating 21 being subjected to large compressive stress for a long time at low temperatures on its range, linearity and service life.

[0075] In one or more embodiments, step 3 specifically includes: Step 3.1, preheat the whole to 40℃; Step 3.2, coating the thermosetting adhesive 3 between the fiber grating 21 and the groove 11, and filling the groove 11 until at least the fiber grating 21 is covered; Step 3.3, heating the whole to 80-120°C; Step 3.4, heat preservation and curing.

[0076] Conventional two-component high-temperature curing adhesive 353ND or 383ND can be cured by heating at 80°C for at least 30 minutes for the fiber Bragg grating package obtained by partially removing the acrylate coating and then writing with a UV laser. For the fiber Bragg grating 21 obtained by writing with a polyimide coating on an optical fiber without removing the coating by a femtosecond laser, the thermosetting adhesive 3 can be cured by heating at 80°C for at least 30 minutes, or at 100°C for 10 minutes, or at 120°C for 5 minutes.

[0077] In an optional implementation, step 3 of this embodiment uses a thermosetting adhesive 3 to encapsulate the fiber Bragg grating 21, and the fiber Bragg grating 21 is bonded and encapsulated with the polyetheretherketone integrated substrate 1 by preheating to 40°C before the initial coating of the thermosetting adhesive 3, and heating the entire substrate to 120°C for 30 minutes after the thermosetting adhesive 3 is coated. This technical solution is different from the use of conventional two-component high-temperature curing adhesive. It adopts an operation mode of preheating first, then coating, and then heating and curing, so that the thermosetting adhesive 3 can be more evenly coated in the groove 11, so that the fiber Bragg grating 21 and the substrate 1 are more stably encapsulated, and the coupling effect is better.

[0078] In an optional embodiment, the preparation of a fiber grating low-temperature sensor on a cylindrical structural component substrate 1 is taken as an example to illustrate, the optical fiber 2 is an optical fiber with acrylate, the diameter of the optical fiber 2 is 0.25 mm, a coating layer of 10 mm in length is peeled off the optical fiber 2, and a fiber grating 21 with a grating area length of 5 mm is written by ultraviolet laser. The length of the substrate 1 is 15 mm and the diameter is 3 mm. An arc-shaped groove 11 with a width and depth of 1 mm is opened on the substrate 1. The initial central wavelength of the fiber grating 21 at room temperature of the preparation environment is 1549.825 nm. After prestressing to 1552.852 nm, it is packaged with thermosetting glue 3 and then naturally cooled to form.

[0079] In an optional embodiment, during preparation, the optical fiber 2 is a quartz optical fiber, the central wavelength of the fiber grating 21 is 1555.61 nm, the ambient temperature of the fiber grating 21 is 20°C, and the curing temperature of the thermosetting adhesive 3 is 80°C. The central wavelength of the fiber grating 21 increases to 1559.115 nm under the action of tensile stress, that is, the central wavelength of the fiber grating 21 increases by about 3.5 nm.

[0080] Example 3 A method for monitoring temperature in a frozen soil area using a fiber grating low-temperature sensor, comprising: Determine the installation position of the fiber Bragg grating cryogenic sensor; The fiber Bragg grating low temperature sensor is installed at the installation position by adopting the single-end fixing method of the base 1, and a plurality of fiber Bragg grating low temperature sensors are connected in series through the optical fiber 2; One side of the base 1 of the fiber Bragg grating low-temperature sensor is in contact with the installation position, and the other side is the through side of the groove 11 .

[0081] The present embodiment provides a method for monitoring temperature in a frozen soil area using a fiber Bragg grating low-temperature sensor. The substrate 1 is fixed at a single end for installation and fixing. This allows the substrate 1 to be in good contact with the installation position and to be able to expand and contract freely under the action of temperature, thereby improving the measurement sensitivity and achieving highly sensitive temperature measurement. At the same time, the bottom of the substrate 1 is attached to the object to be measured, and the groove 11 is installed with the side through the groove 11 facing outward. This ensures that the fixing of the substrate 1 will not bring additional compressive stress to the fiber Bragg grating 21 and cause chirp, thereby ensuring the normal operation of the fiber Bragg grating low-temperature sensor.

[0082] In one or more embodiments, the single-end fixing method includes: bonding, welding, magnetic attraction, bundling, crimping or clamping the area on the substrate 1 that is misaligned with the grating area of ​​the fiber Bragg grating 21. The basic principle is to achieve good contact between the packaged fiber Bragg grating low-temperature sensor and the object to be measured, and to make its expansion and deformation after being affected by temperature unrestricted, and to facilitate the replacement of the fiber Bragg grating low-temperature sensor.

[0083] In an optional embodiment, the adhesive fixation includes applying glue to one end of a side of the substrate 1 away from the groove 11, and sticking it to the object to be measured by the glue to achieve temperature measurement of the object to be measured, which is suitable for fixing the substrate 1 of a rectangular structural member.

[0084] In an optional embodiment, the magnetic fixation includes embedding a magnet on a side surface of the substrate 1 away from the groove 11, and the embedding depth of the magnet does not exceed 1 / 3 of the thickness of the substrate 1. When in use, the fiber grating low-temperature sensor is installed on the ferromagnetic material object to be measured by placing the substrate 1 on the ferromagnetic material object to be measured.

[0085] In an optional embodiment, if Figure 16-Figure 18 As shown, the clamping and fixing includes setting a clamping member 4 at the installation position. The clamping member 4 is made of a material with good elasticity and has a mounting hole at the bottom. It can be fixed to the object to be measured by screws or by gluing. The clamping member 4 is formed with two relatively arranged arc-shaped spring pieces to form a passage 41 for moving in or out of the substrate 1. The height of the arc-shaped spring piece is greater than twice the diameter or height of the substrate 1. The initial width of the passage 41 is greater than the width dimension of the groove 11 and smaller than the width dimension of the substrate 1. When the fiber Bragg grating low-temperature sensor is clamped into the clamping member 4, the side surface of the arc-shaped spring piece bent toward the inside of the passage 41 can press the substrate 1 into the clamping member 4, and the width dimension between the arc-shaped spring pieces can fully expose the grating area of ​​the fiber Bragg grating 21, thereby avoiding the influence of thermal expansion deformation on the grating area of ​​the fiber Bragg grating 21.

[0086] In an optional embodiment, the crimping fixation includes setting a crimping part at the installation position. The crimping part can be a sheet-like structural part such as a thin steel sheet or an iron sheet, which can be fixed by spot welding and has a thickness of 0.15-0.25 mm. It can constrain and crimp the area on the substrate 1 that is misaligned with the fiber grating 21, and can enable the fiber grating low-temperature sensor to effectively and reliably transfer the deformation caused by its thermal expansion to the fiber grating 21 when the temperature changes, thereby achieving high sensitivity and high stability of the sensor.

[0087] In one or more embodiments, the installation position is set according to the distribution of the temperature change layering characteristics of the soil in the frozen soil area or the engineering structure. That is, the fiber Bragg grating low temperature sensor string connected by the optical fiber 2 is vertically extended into the soil or engineering structure for burial, and the layout spacing between each fiber Bragg grating low temperature sensor is adjusted according to the temperature change layering situation of the soil in the frozen soil area or the engineering structure, so that each fiber Bragg grating low temperature sensor can sensitively monitor the temperature of the corresponding layered position, which can meet the high longitudinal resolution requirements of the temperature gradient of the frozen soil layer, and realize the high sensitivity and high stability of the fiber Bragg grating low temperature sensor. It is preferred that the fiber Bragg grating low temperature sensor can be laid out according to this principle in natural foundations, roadbeds, and bridge pile foundations.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A fiber Bragg grating low temperature sensor, characterized in that: include: The base (1) is a polyetheretherketone structural member, and the base (1) is provided with a through channel (11), and the depth and width of the channel (11) are both greater than the diameter of the optical fiber (2); An optical fiber (2) is arranged longitudinally along the groove (11), a fiber grating (21) is engraved on the optical fiber (2), and the fiber grating (21) is encapsulated in the groove (11) by a thermosetting adhesive (3); A continuous tensile stress is applied to the optical fiber grating (21) before the thermosetting adhesive (3) is cured, and the tensile stress is used to make the central wavelength of the optical fiber grating (21) at a preset working temperature the same as or close to that before packaging.

2. A fiber Bragg grating low temperature sensor according to claim 1, characterized in that: The bottom of the cross-section of the channel (11) is arc-shaped or pointed-angle-shaped, the optical fiber grating (21) fits the channel (11) and is located in the longitudinal middle of the channel (11), and the thermosetting adhesive (3) at least covers the optical fiber grating (21).

3. A fiber Bragg grating low temperature sensor according to claim 2, characterized in that: The thermosetting adhesive (3) fills the groove.

4. The fiber Bragg grating low temperature sensor according to claim 3, characterized in that: The thermosetting adhesive (3) is a two-component high temperature curing adhesive 353ND or 383ND.

5. The fiber Bragg grating low temperature sensor according to claim 1, characterized in that: The upper limit of the volume size of the substrate (1) satisfies that: the setting of the substrate (1) will not affect the temperature distribution of the object to be measured; and the lower limit of the volume size of the substrate (1) satisfies that: the structural strength of the substrate (1) can drive the optical fiber Bragg grating (21) to generate additional strain.

6. The fiber Bragg grating low temperature sensor according to claim 5, characterized in that: The length of the substrate (1) is 5-25 mm, the length of the grating region of the optical fiber grating (21) is 2-10 mm, and the length of the substrate (1) is greater than the length of the grating region by 3-23 mm.

7. The fiber Bragg grating low temperature sensor according to claim 6, characterized in that: The base (1) is a cylindrical structural member, the groove (11) is extended along the axial direction of the base (1) and penetrates the radial side wall of the base (1), and the diameter of the base (1) is 8-20 times the diameter of the optical fiber (2).

8. The fiber Bragg grating low temperature sensor according to claim 6, characterized in that: The base (1) is a rectangular parallelepiped structural member, the groove (11) is extended along the length direction of the base (1) and penetrates the side wall of the base (1) in the thickness direction, the width of the base (1) is 10-20 times the diameter of the optical fiber (2), and the thickness of the base (1) is 8-18 times the diameter of the optical fiber (2).

9. The fiber Bragg grating low temperature sensor according to claim 6, characterized in that: The base (1) is a tubular structural member, the channel (11) includes the inner space of the base (1), the channel (11) partially penetrates the radial side wall of the base (1), the wall thickness of the base (1) is more than twice the diameter of the optical fiber (2), and the outer diameter of the base (1) is more than six times the diameter of the optical fiber (2).

10. The fiber Bragg grating low temperature sensor according to claim 9, characterized in that: When the wall thickness of the base (1) exceeds twice the diameter of the optical fiber (2), the channel (11) comprises a limiting groove (12) arranged in the space inside the tube, and the optical fiber (2) is arranged in the limiting groove (12).

11. The fiber Bragg grating low temperature sensor according to claim 10, characterized in that: The depth and width of the limiting groove (12) are both greater than the diameter of the optical fiber (2), and the optical fiber (2) fits the bottom of the limiting groove (12) and is encapsulated in the limiting groove (12).

12. The fiber Bragg grating low temperature sensor according to claim 6, characterized in that: In the operating temperature range of -40℃~60℃, the sensitivity of the fiber Bragg grating low-temperature sensor is not less than 57pm / ℃.

13. A method for preparing a fiber Bragg grating low-temperature sensor according to any one of claims 1 to 12, characterized in that: include: Step 1: Opening a groove (11) on a polyetheretherketone structural member to obtain a substrate (1); Step 2, calculating the central wavelength offset of the fiber Bragg grating (21), applying tensile stress to the optical fiber (2) according to the calculation result, the offset is related to the curing temperature of the thermosetting adhesive (3), the ambient temperature of the fiber Bragg grating (21) during preparation, and the coupling coefficient between the strain of the substrate (1) and the strain of the fiber Bragg grating (21); Step 3, matching the relative positions of the optical fiber Bragg grating (21) and the substrate (1), filling the groove (11) with a thermosetting adhesive (3), and encapsulating the optical fiber Bragg grating (21); Step 4, heating the whole until the thermosetting adhesive (3) is solidified; Step 5: Cool naturally.

14. The method for preparing a fiber Bragg grating low-temperature sensor according to claim 13, characterized in that: In step 2, the central wavelength offset of the fiber Bragg grating (21) satisfies: under the action of tensile stress, it increases by at least ; Where: is the thermo-optical coefficient of the fiber material; is the coefficient of thermal expansion of the optical fiber material; c is the coupling coefficient between the strain of the substrate (1) and the strain of the fiber Bragg grating (21), and the value range of c is 0.5-0.7; is the effective elastic-optical coefficient of the optical fiber (2); is the coefficient of thermal expansion of the substrate (1) material; is the ambient temperature of the fiber Bragg grating (21) during preparation; is the curing temperature of the thermosetting adhesive (3); is the central wavelength of the fiber Bragg grating (21) in a free state at ambient temperature.

15. The method for preparing a fiber Bragg grating low-temperature sensor according to claim 13, characterized in that: Step 3 specifically includes: Step 3.1, preheat the whole to 40℃; Step 3.2, coating the thermosetting adhesive (3) between the optical fiber grating (21) and the groove (11), and filling the groove (11) until at least the optical fiber grating (21) is covered; Step 3.3, heating the whole to 80-120°C; Step 3.4, heat preservation and curing.

16. The method for preparing a fiber Bragg grating low-temperature sensor according to claim 13, characterized in that: The optical fiber (2) has an acrylate coating layer, and the optical fiber grating (21) is obtained by partially removing the acrylate coating layer and then writing with an ultraviolet laser, or the optical fiber (2) has a polyimide coating layer, and the optical fiber grating (21) is obtained by writing with a femtosecond laser without removing the coating layer.

17. The method for preparing a fiber Bragg grating low-temperature sensor according to claim 13, characterized in that: During preparation, the central wavelength of the fiber Bragg grating (21) is 1555.61 nm, the ambient temperature of the fiber Bragg grating (21) is 20° C., and the curing temperature of the thermosetting adhesive (3) is selected to be 80° C., and the central wavelength of the fiber Bragg grating (21) increases to 1559.115 nm under the action of tensile stress.

18. A method for monitoring temperature in a frozen soil area using the fiber Bragg grating cryogenic sensor according to any one of claims 1 to 12, characterized in that: include: Determine the installation position of the fiber Bragg grating cryogenic sensor; A fiber Bragg grating low-temperature sensor is installed at the installation position by adopting a single-end fixing method of the base (1), and a plurality of fiber Bragg grating low-temperature sensors are connected in series via optical fibers (2); One side of the bottom of the base (1) of the fiber grating low-temperature sensor is in contact with the installation position, and the other side is the lateral through-going side of the groove (11).

19. A method for monitoring temperature in frozen soil areas using a fiber Bragg grating cryogenic sensor according to claim 18, characterized in that: The single-end fixing method includes: bonding, welding, magnetic attraction, bundling, crimping or clamping the area on the substrate (1) that is misaligned with the grating area of ​​the optical fiber grating (21).

20. The method for monitoring temperature in frozen soil areas using a fiber Bragg grating cryogenic sensor according to claim 18, characterized in that: The installation location is set according to the stratified distribution characteristics of temperature changes in the soil in the frozen soil area or the engineering structure.

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