Integrated optical fiber sensor for measuring temperature strain at extreme high temperature and preparation method thereof

Abstract

The invention discloses an integrated optical fiber sensor for measuring temperature strain at extreme high temperature, and the sensor comprises a crystal optical fiber which comprises a first optical fiber segment, a second optical fiber segment and a third optical fiber segment which are connected in sequence; the first optical fiber grating is formed in the first optical fiber section of the crystal optical fiber; the second optical fiber grating is formed in the second optical fiber section of the crystal optical fiber; wherein the diameters of the first optical fiber section and the third optical fiber section are the same, and the diameters of the second optical fiber section are different from those of the first optical fiber section and the second optical fiber section. The integrated optical fiber sensor can measure the temperature and the strain at the same time in an extremely high temperature environment. The invention further discloses a preparation method of the integrated optical fiber sensor.

Description

Technical Field

[0001] The invention relates to grating sensing technology, and in particular to an integrated optical fiber sensor for measuring temperature strain under extremely high temperature and a preparation method thereof. Background Art

[0002] With the continuous development of modern industrial technology, the demand for temperature strain measurement in high temperature environments is increasing. In high-end manufacturing fields such as aerospace, nuclear energy, and petrochemicals, especially large and important equipment such as skins, rudders, and aircraft engines of hypersonic vehicles, they often work under extreme high temperatures, and their structural safety and performance stability are directly related to the reliability and safety of the entire system. Therefore, sensing technology that can accurately monitor strain changes in high temperature environments is crucial for applications in these fields.

[0003] Optical fiber sensors have great potential in the field of high-temperature strain measurement due to their advantages such as high temperature resistance, anti-electromagnetic interference and small size. The Chinese patent application number CN201210351483.0 discloses a method for simultaneously measuring temperature and stress by etching fiber gratings of different diameters. The method uses hydrofluoric acid to partially etch a single-mode optical fiber so that the etched and unetched parts of the single-mode optical fiber have different diameters, and writes fiber gratings in the etched and unetched parts of the single-mode optical fiber respectively, so as to decouple temperature and strain by utilizing the characteristics that fiber gratings of different diameters have different responses to strain but the same response to temperature, so as to achieve simultaneous measurement of temperature and strain.

[0004] However, single-mode optical fiber is a quartz optical fiber. Due to the low softening temperature of quartz, it can only simultaneously measure temperature and strain below a maximum of 800°C. It cannot simultaneously measure temperature and strain in extreme environments with higher temperatures. Summary of the invention

[0005] In order to solve the above-mentioned deficiencies of the prior art, the present invention provides an integrated optical fiber sensor capable of simultaneously measuring temperature and strain in an extremely high temperature environment.

[0006] The present invention also provides a method for preparing the integrated optical fiber sensor.

[0007] The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0008] An integrated optical fiber sensor for measuring temperature strain under extremely high temperatures, comprising:

[0009] A crystal optical fiber, comprising a first optical fiber segment, a second optical fiber segment and a third optical fiber segment connected in sequence;

[0010] A first fiber grating formed in a first fiber segment of the crystal fiber;

[0011] A second fiber grating is formed in a second fiber segment of the crystal fiber;

[0012] The first optical fiber segment has the same diameter as the third optical fiber segment, and the second optical fiber segment has a different diameter from the first optical fiber segment and the second optical fiber segment.

[0013] Furthermore, the temperature sensitivity and strain sensitivity of the first fiber Bragg grating are K T1 and K ε1 The temperature sensitivity and strain sensitivity of the second fiber Bragg grating are K T2 and K ε2 , then the measured temperature change ΔT and strain Δε satisfy the following formula:

[0014]

[0015] Wherein, Δλ1 and Δλ2 are wavelength drifts of the first fiber grating and the second fiber grating respectively.

[0016] Furthermore, the diameters of the first optical fiber segment and the third optical fiber segment are the first diameter, the diameter of the second optical fiber segment is the second diameter, the total length of the first optical fiber segment and the third optical fiber segment is the first length, and the length of the second optical fiber segment is the second length; the first diameter is smaller than the second diameter, and the first length is greater than the second length.

[0017] Further, the diameters of the first optical fiber segment and the third optical fiber segment are the first diameter, the diameter of the second optical fiber segment is the second diameter, the total length of the first optical fiber segment and the third optical fiber segment is the first length, and the length of the second optical fiber segment is the second length; the first diameter is smaller than the second diameter, and the first length is greater than the second length

[0018] Furthermore, the diameters of the first optical fiber segment and the third optical fiber segment are the first diameter, the diameter of the second optical fiber segment is the second diameter, the total length of the first optical fiber segment and the third optical fiber segment is the first length, and the length of the second optical fiber segment is the second length; the first diameter is greater than the second diameter, and the first length is greater than the second length.

[0019] Furthermore, the stress generated by the first optical fiber segment, the second optical fiber segment and the third optical fiber segment is inversely proportional to their cross-sectional areas, while the cross-sectional areas of the first optical fiber segment, the second optical fiber segment and the third optical fiber segment are directly proportional to the square of their diameters.

[0020] Furthermore, the crystal fiber also includes a first fiber gradient segment and a second fiber gradient segment, the first fiber gradient segment is connected between the first fiber segment and the second fiber segment, and the second fiber gradient segment is connected between the second fiber segment and the third fiber segment; the diameters of the first fiber gradient segment and the second fiber gradient segment gradually change.

[0021] Furthermore, the diameter change rate of the first optical fiber gradient section and the second optical fiber gradient section satisfies the following formula:

[0022]

[0023] Wherein, β1 is the local propagation constant of the fundamental mode of the crystal fiber, β2 is the local propagation constant of the high-order mode of the crystal fiber, and dr is the diameter change of the first optical fiber gradient section and the second optical fiber gradient section within a unit length dz.

[0024] Furthermore, the crystal fiber may be, but is not limited to, sapphire fiber, zirconium oxide fiber, lutetium oxide fiber or YAG fiber.

[0025] The method for preparing the above-mentioned integrated optical fiber sensor comprises the following steps:

[0026] Step 100: Designing the length and diameter of the first optical fiber segment, the second optical fiber segment, and the third optical fiber segment;

[0027] Step 200: using an acidic solution to corrode the crystal fiber, so that the crystal fiber forms the first fiber segment, the second fiber segment and the third fiber segment respectively;

[0028] Step 300: Using a femtosecond laser to write the first optical fiber segment and the second optical fiber segment to form the first optical fiber grating and the second optical fiber grating in the first optical fiber segment and the second optical fiber segment, respectively.

[0029] Furthermore, in step 100, the strain sensitivity difference between the first fiber grating and the second fiber grating is adjusted by designing the diameter ratio between the first fiber segment and the second fiber segment.

[0030] Furthermore, the strain sensitivity of the first fiber grating and the second fiber grating satisfies the following formula:

[0031]

[0032] Among them, K ε1 and K ε2 are the strain sensitivities of the first fiber Bragg grating and the second fiber Bragg grating, respectively; d1 and d2 are the diameters of the first fiber segment and the second fiber segment, respectively.

[0033] Furthermore, in step 100, the strain sensitivity, strain resolution and strain range of the first fiber grating and the second fiber grating are adjusted by designing the length ratio between the second fiber segment and the total length of the first fiber segment and the third fiber segment.

[0034] Furthermore, in step 200, the step of corroding the crystal optical fiber with an acidic solution is as follows:

[0035] Step 210: placing a glass protection tube on the non-corroded portion of the crystal optical fiber;

[0036] Step 220: Immerse the corroded portion of the crystal optical fiber in a first acidic solution and corrode it to a desired diameter;

[0037] Step 230: using a second acidic solution to corrode and remove the glass protection tube outside the crystal optical fiber.

[0038] Furthermore, in step 220, the end of the glass protection tube close to the corrosion portion is also immersed in the first acidic solution, so that the first acidic solution corrodes the end of the non-corrosion portion close to the corrosion portion into a fiber gradient section.

[0039] The present invention has the following beneficial effects:

[0040] The integrated optical fiber sensor of the present invention adopts the crystal fiber for optical signal transmission, and the crystal fiber is made into a first optical fiber segment, a second optical fiber segment and a third optical fiber segment with different diameters, and then the first optical fiber grating and the second optical fiber grating are respectively formed in the first optical fiber segment and the second optical fiber segment, and the temperature and strain are decoupled by utilizing the characteristics that the first optical fiber grating and the second optical fiber grating with different diameters have different responses to strain but the same response to temperature, so as to realize the simultaneous measurement of temperature and strain; due to the unique crystal structure of the crystal fiber, it has a high melting point (about 2045°C) and excellent mechanical properties, and the first optical fiber grating and the second optical fiber grating adopt the crystal fiber as a substrate, and can measure temperature and strain simultaneously in an extreme high temperature environment (such as 1600°C), and has many advantages such as high temperature resistance, compact structure, low crosstalk, etc., and has important application value in the fields of aerospace, nuclear reactors, metallurgy, etc.

[0041] The integrated optical fiber sensor of the present invention designs the second optical fiber segment with a larger middle diameter to be relatively short, and the first optical fiber segment and the third optical fiber segment with smaller diameters at both ends to be relatively long, so that the first optical fiber segment and the third optical fiber segment with smaller diameters at both ends occupy most of the length of the crystal optical fiber, and the stress acting on the crystal optical fiber can be dispersed in the first optical fiber segment and the third optical fiber segment, which can increase the strain range and make the strain measurement range closer to the tensile strength value of the crystal optical fiber itself;

[0042] The integrated optical fiber sensor of the present invention designs the second optical fiber segment with a smaller diameter in the middle to be relatively short, and the first optical fiber segment and the third optical fiber segment with larger diameters at both ends to be relatively long, so that the first optical fiber segment and the third optical fiber segment with larger diameters at both ends occupy most of the length of the crystal optical fiber, and the stress acting on the crystal optical fiber can be concentrated on the second optical fiber segment, which can improve the strain sensitivity and is suitable for high-sensitivity static and dynamic strain sensing;

[0043] The integrated optical fiber sensor of the present invention provides a first optical fiber gradient segment and a second optical fiber gradient segment with gradually changing diameters between the first optical fiber segment and the second optical fiber segment and between the second optical fiber segment and the third optical fiber segment, respectively, so as to reduce the high coupling loss caused by the change in optical fiber diameter when the optical signal enters the second optical fiber segment from the first optical fiber segment and enters the third optical fiber segment from the second optical fiber segment, thereby improving the spectral signal-to-noise ratio of the optical fiber Bragg grating with a smaller diameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic structural diagram of the integrated optical fiber sensor provided by the present invention.

[0045] Figure 2 for Figure 1 The length of each part of the integrated optical fiber sensor is shown.

[0046] Figure 3 This is a schematic structural diagram of another integrated optical fiber sensor provided by the present invention.

[0047] Figure 4 for Figure 3 The length of each part of the integrated optical fiber sensor is shown.

[0048] Figure 5 A flowchart of the steps of the method for preparing the integrated optical fiber sensor provided by the present invention.

[0049] Figure 6 This is a flowchart of step 200 in the method for preparing the integrated optical fiber sensor provided by the present invention.

[0050] Figure 7 This is a schematic diagram of etching in step 200 in the method for preparing the integrated optical fiber sensor provided by the present invention. DETAILED DESCRIPTION

[0051] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments, examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0052] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0053] In addition, the terms "first", "second", and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", and "third" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0054] In the present invention, unless otherwise clearly specified and limited, the terms "install", "connect", "connect", "fix", "set" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0055] Embodiment 1

[0056] like Figure 1 and 3 As shown, an integrated optical fiber sensor for measuring temperature strain under extremely high temperature comprises:

[0057] The crystal fiber 10 comprises a first fiber segment 11, a second fiber segment 12 and a third fiber segment 13 connected in sequence;

[0058] A first fiber grating 20 formed in the first fiber segment 11 of the crystal fiber 10;

[0059] A second fiber grating 30 formed in the second fiber segment 12 of the crystal fiber 10;

[0060] The first optical fiber segment 11 and the third optical fiber segment 13 have the same diameter, and the second optical fiber segment 12 has a different diameter from the first optical fiber segment 11 and the second optical fiber segment 12 .

[0061] The integrated optical fiber sensor of the present invention adopts the crystal fiber 10 for optical signal transmission, and the crystal fiber 10 is made into a first optical fiber segment 11, a second optical fiber segment 12 and a third optical fiber segment 13 with different diameters, and then the first optical fiber grating 20 and the second optical fiber grating 30 are respectively formed in the first optical fiber segment 11 and the second optical fiber segment 12, and the temperature and strain are decoupled by utilizing the characteristics that the first optical fiber grating 20 and the second optical fiber grating 30 with different diameters have different responses to strain but the same response to temperature, so as to realize the simultaneous measurement of temperature and strain; due to the unique crystal structure of the crystal fiber 10, it has a high melting point (about 2045°C) and excellent mechanical properties. The first optical fiber grating 20 and the second optical fiber grating 30 use the crystal fiber 10 as a substrate, and can measure temperature and strain simultaneously in an extreme high temperature environment (such as 1600°C), and has many advantages such as high temperature resistance, compact structure, low crosstalk, etc., and has important application value in the fields of aerospace, nuclear reactors, metallurgy, etc.

[0062] The central wavelengths of the first fiber grating 20 and the second fiber grating 30 are different. During measurement, an optical signal is coupled into the crystal fiber 10, and an optical signal emitted after passing through the first fiber grating 20 and the second fiber grating 30 in sequence is received, thereby obtaining a resonance spectrum of the integrated optical fiber sensor, wherein the resonance spectrum has a first resonance peak corresponding to the central wavelength of the first fiber grating 20, and a second resonance peak corresponding to the central wavelength of the second fiber grating 30.

[0063] When the temperature changes, the first fiber Bragg grating 20 and the second fiber Bragg grating 30 expand and contract along the axial direction of the optical fiber due to the thermal expansion and contraction effect, thereby causing the center wavelengths of the two to drift. Since the degree of thermal expansion and contraction mainly depends on the thermal expansion coefficient of the material and is less affected by the diameter of the material, the difference in the center wavelength drift caused by the first fiber Bragg grating 20 and the second fiber Bragg grating 30 under the same temperature change is small (only on the order of millimeters) and can be ignored. Therefore, even if the first fiber Bragg grating 20 and the second fiber Bragg grating 30 have different diameters, their responses to temperature changes are the same.

[0064] When the strain changes, the first fiber grating 20 and the second fiber grating 30 are stretched along the axial direction of the optical fiber due to the strain, thereby causing the central wavelengths of the two to drift. Since the degree of strain stretching depends not only on the elastic coefficient of the material, but is also greatly affected by the diameter of the material, the first fiber grating 20 and the second fiber grating 30 cause a large difference in the amount of central wavelength drift under the same strain change. Therefore, the first fiber grating 20 and the second fiber grating 30 have different responses to strain changes due to their different diameters.

[0065] Assume that the temperature sensitivity and strain sensitivity of the first fiber Bragg grating 20 are K T1 and K ε1 The temperature sensitivity and strain sensitivity of the second fiber Bragg grating 30 are K T2 and K ε2 , then the measured temperature change ΔT and strain Δε satisfy the following formula:

[0066]

[0067] Wherein, Δλ1 and Δλ2 are wavelength drifts of the first fiber grating 20 and the second fiber grating 30 respectively.

[0068] The crystal fiber 10 may be, but is not limited to, sapphire fiber, zirconium oxide fiber, lutetium oxide fiber or YAG fiber.

[0069] The first fiber grating 20 and the second fiber grating 30 may be, but are not limited to, fiber Bragg gratings, chirped fiber gratings, phase-shifted fiber gratings, and the like.

[0070] Embodiment 2

[0071] As an optimization solution of the first embodiment, in this embodiment, if Figure 1 and 2 As shown, the diameters of the first optical fiber segment 11 and the third optical fiber segment 13 are the first diameter, the diameter of the second optical fiber segment 12 is the second diameter, the total length of the first optical fiber segment 11 and the third optical fiber segment 13 is the first length L1+L3, and the length of the second optical fiber segment 12 is the second length L2; the first diameter is smaller than the second diameter, and the first length L1+L3 is larger than the second length L2.

[0072] The stress generated by the first optical fiber segment 11, the second optical fiber segment 12 and the third optical fiber segment 13 is inversely proportional to their cross-sectional areas, while the cross-sectional areas of the first optical fiber segment 11, the second optical fiber segment 12 and the third optical fiber segment 13 are proportional to the square of their diameters.

[0073] The integrated optical fiber sensor of the present invention designs the second optical fiber segment 12 with a larger middle diameter to be relatively short, while the first optical fiber segment 11 and the third optical fiber segment 13 with smaller diameters at both ends are designed to be relatively long, so that the first optical fiber segment 11 and the third optical fiber segment 13 with smaller diameters at both ends occupy most of the length of the crystal fiber 10, and the stress acting on the crystal fiber 10 can be dispersed in the first optical fiber segment 11 and the third optical fiber segment 13, which can increase the strain range and make the strain measurement range closer to the tensile strength value of the crystal fiber 10 itself.

[0074] Embodiment 3

[0075] As another optimization solution of the first embodiment, in this embodiment, if Figure 3 and 4 As shown, the diameters of the first optical fiber segment 11 and the third optical fiber segment 13 are the first diameter, the diameter of the second optical fiber segment 12 is the second diameter, the total length of the first optical fiber segment 11 and the third optical fiber segment 13 is the first length L1+L3, and the length of the second optical fiber segment 12 is the second length L2; the first diameter is greater than the second diameter, and the first length L1+L3 is greater than the second length L2.

[0076] The stress generated by the first optical fiber segment 11, the second optical fiber segment 12 and the third optical fiber segment 13 is inversely proportional to their cross-sectional areas, while the cross-sectional areas of the first optical fiber segment 11, the second optical fiber segment 12 and the third optical fiber segment 13 are proportional to the square of their diameters.

[0077] The integrated optical fiber sensor of the present invention designs the second optical fiber segment 12 with a smaller middle diameter to be relatively short, while the first optical fiber segment 11 and the third optical fiber segment 13 with larger diameters at both ends are designed to be relatively long, so that the first optical fiber segment 11 and the third optical fiber segment 13 with larger diameters at both ends occupy most of the length of the crystal fiber 10, which can concentrate the stress acting on the crystal fiber 10 on the second optical fiber segment 12, improve the strain sensitivity, and is suitable for high-sensitivity static and dynamic strain sensing.

[0078] Embodiment 4

[0079] A single-mode optical fiber includes a core and a cladding. The optical signal is transmitted only in the core. Its numerical aperture is small and the transmission mode only has a fundamental mode. Therefore, the prior art partially corrodes the cladding of the single-mode optical fiber to form optical fibers with different diameters without affecting the transmission of the optical signal in the core.

[0080] However, the crystal fiber 10 is a cladding-free structure, and the optical signal is transmitted throughout the interior of the crystal fiber 10. It has a large numerical aperture and a transmission mode that includes both a fundamental mode and a high-order mode. Therefore, the present invention partially corrodes the crystal fiber 10 to form optical fibers with different diameters, which affects the transmission of the optical signal in the crystal fiber 10. When the optical signal is transmitted in the crystal fiber 10, the mode field mismatch causes high coupling loss, thereby causing the spectral signal-to-noise ratio of the fiber grating with a smaller diameter to deteriorate.

[0081] In order to solve the above technical problems, as a further optimization solution of Embodiment 1, Embodiment 2 or Embodiment 3, in this embodiment, if Figure 1 and 3 As shown, the crystal fiber 10 also includes a first optical fiber gradient segment 14 and a second optical fiber gradient segment 15, wherein the first optical fiber gradient segment 14 is connected between the first optical fiber segment 11 and the second optical fiber segment 12, and the second optical fiber gradient segment 15 is connected between the second optical fiber segment 12 and the third optical fiber segment 13; the diameters of the first optical fiber gradient segment 14 and the second optical fiber gradient segment 15 change gradually.

[0082] The integrated optical fiber sensor of the present invention provides a first optical fiber gradient segment 14 and a second optical fiber gradient segment 15 with gradually changing diameters between the first optical fiber segment 11 and the second optical fiber segment 12 and between the second optical fiber segment 12 and the third optical fiber segment 13, respectively, so as to reduce the high coupling loss caused by the change in optical fiber diameter when the optical signal enters the second optical fiber segment 12 from the first optical fiber segment 11 and enters the third optical fiber segment 13 from the second optical fiber segment 12, thereby improving the spectral signal-to-noise ratio of the optical fiber Bragg grating with a smaller diameter.

[0083] When the diameter of the second optical fiber segment 12 is larger than the diameters of the first optical fiber segment 11 and the third optical fiber segment 13, the diameter of the first optical fiber gradient segment 14 gradually increases, and the diameter of the end connected to the first optical fiber segment 11 is the smallest (the same as the first diameter of the first optical fiber segment 11), and the diameter of the end connected to the second optical fiber segment 12 is the largest; the diameter of the second optical fiber gradient segment 15 gradually decreases, and the diameter of the end connected to the second optical fiber segment 12 is the largest, and the diameter of the end connected to the third optical fiber segment 13 is the smallest.

[0084] When the diameter of the second optical fiber segment 12 is smaller than the diameters of the first optical fiber segment 11 and the third optical fiber segment 13, the diameter of the first optical fiber gradient segment 14 gradually decreases, and the diameter of the end connected to the first optical fiber segment 11 is the largest, and the diameter of the end connected to the second optical fiber segment 12 is the smallest; the diameter of the second optical fiber gradient segment 15 gradually increases, and the diameter of the end connected to the second optical fiber segment 12 is the smallest, and the diameter of the end connected to the third optical fiber segment 13 is the largest.

[0085] Preferably, in order not to excite a high-order leakage mode and cause a large optical loss, it is necessary to reduce the cone angle of the first optical fiber gradient section 14 and the second optical fiber gradient section 15 as much as possible, improve the signal-to-noise ratio of the resonance peak of the resonance spectrum, and the diameter change rate of the first optical fiber gradient section 14 and the second optical fiber gradient section 15 satisfies the following formula:

[0086]

[0087] Wherein, β1 is the local propagation constant of the fundamental mode of the crystal fiber 10, β2 is the local propagation constant of the high-order mode of the crystal fiber 10, and dr is the diameter change of the first optical fiber gradient section 14 and the second optical fiber gradient section 15 within a unit length dl.

[0088] Embodiment 5

[0089] like Figure 5 As shown, a method for preparing an integrated optical fiber sensor is used to prepare the integrated optical fiber sensor described in Example 1, Example 2, Example 3 or Example 4; the preparation method comprises the following steps:

[0090] Step 100 : Designing the lengths and diameters of the first optical fiber segment 11 , the second optical fiber segment 12 , and the third optical fiber segment 13 .

[0091] In step 100, the relationship between the stress and diameter of the crystal fiber 10 is as follows:

[0092]

[0093] Wherein, E is the Young's modulus of the crystal fiber 10, F is the axial tension applied to the crystal fiber 10, A is the cross-sectional area of ​​the crystal fiber 10, ΔL is the length change of the crystal fiber 10, L0 is the original length of the crystal fiber 10, σ is the stress generated by the crystal fiber 10, and ε is the axial strain of the crystal fiber 10.

[0094] It can be seen from the above formula that the stress σ generated by the crystal fiber 10 is inversely proportional to the cross-sectional area A of the crystal fiber 10, and the cross-sectional area A of the crystal fiber 10 is directly proportional to the square of the diameter. Therefore, the strain sensitivity difference between the first fiber grating 20 and the second fiber grating 30 can be adjusted by designing the diameter ratio of the first fiber segment 11 and the second fiber segment 12.

[0095] The strain sensitivity of the first fiber grating 20 and the second fiber grating 30 satisfies the following formula:

[0096]

[0097] Among them, Kε1 and K ε2 are the strain sensitivities of the first fiber grating 20 and the second fiber grating 30 , respectively; d1 and d2 are the diameters of the first fiber segment 11 and the second fiber segment 12 , respectively.

[0098] The diameters of the first optical fiber segment 11 and the third optical fiber segment 13 are the first diameter, the diameter of the second optical fiber segment 12 is the second diameter, the total length of the first optical fiber segment 11 and the third optical fiber segment 13 is the first length L1+L3, and the length of the second optical fiber segment 12 is the second length L2.

[0099] When the first diameter is smaller than the second diameter, and the first length L1+L3 is larger than the second length L2, the first fiber segment 11 and the third fiber segment 13 with smaller diameters at both ends occupy most of the length of the crystal fiber 10, so that the stress acting on the crystal fiber 10 can be dispersed in the first fiber segment 11 and the third fiber segment 13, thereby increasing the strain range and making the strain measurement range closer to the tensile strength value of the crystal fiber 10 itself; when the first diameter is larger than the second diameter, and the first length L1+L3 is larger than the second length L2, the first fiber segment 11 and the third fiber segment 13 with larger diameters at both ends occupy most of the length of the crystal fiber 10, thereby concentrating the stress acting on the crystal fiber 10 on the second fiber segment 12, thereby increasing the strain sensitivity.

[0100] Therefore, the strain sensitivity, strain resolution and strain range of the first fiber grating 20 and the second fiber grating 30 can be adjusted by designing the length ratio between the second fiber segment 12 and the total length of the first fiber segment 11 and the third fiber segment 13 .

[0101] Step 200: using an acidic solution to corrode the crystal fiber 10, so that the crystal fiber 10 forms the first fiber segment 11, the second fiber segment 12 and the third fiber segment 13 respectively.

[0102] like Figure 6 As shown, in step 200, the steps of corroding the crystal optical fiber 10 with an acidic solution are as follows:

[0103] Step 210 : placing a glass protection tube over the non-corroded portion of the crystal optical fiber 10 .

[0104] In this step 210, the crystal fiber 10 includes a non-corroded portion 11' and a corroded portion 12', wherein the non-corroded portion 11' corresponds to the fiber segment with a larger diameter, and the corroded portion 12' corresponds to the fiber segment with a smaller diameter. That is, if the diameter of the second fiber segment 12 is larger than the diameters of the first fiber segment 11 and the third fiber segment 13, the non-corroded portion 11' corresponds to the second fiber segment 12, and the corroded portion 12' corresponds to the first fiber segment 11 and the third fiber segment 13; if the diameter of the second fiber segment 12 is smaller than the diameters of the first fiber segment 11 and the third fiber segment 13, the non-corroded portion 11' corresponds to the first fiber segment 11 and the third fiber segment 13, and the corroded portion 12' corresponds to the second fiber segment 12.

[0105] If the crystal fiber 10 needs to be corroded into a structure with a large diameter in the middle and small diameters at both ends, there is only one glass protection tube 40, which is sleeved outside the non-corroded part 11' in the middle of the crystal fiber 10; if the crystal fiber 10 needs to be corroded into a structure with a small diameter in the middle and large diameters at both ends, there are two glass protection tubes 40, which are respectively sleeved outside the non-corroded parts 11' at both ends of the crystal fiber 10.

[0106] Step 220: Immerse the corroded portion 12' of the crystal optical fiber 10 in a first acidic solution and corrode it to a desired diameter.

[0107] In step 220 , the crystal fiber 10 and the glass protection tube 40 can be corroded by different acid solutions respectively due to different materials. The first acid solution can only corrode the crystal fiber 10 but cannot corrode the glass protection tube 40 .

[0108] If the diameter of the second optical fiber segment 12 is larger than the diameters of the first optical fiber segment 11 and the third optical fiber segment 13, the corroded portion 12' forms the first optical fiber segment 11 and the third optical fiber segment 13 after corrosion; if the diameter of the second optical fiber segment 12 is smaller than the diameters of the first optical fiber segment 11 and the third optical fiber segment 13, the corroded portion 12' forms the second optical fiber segment 12 after corrosion.

[0109] In this embodiment, the first acidic solution is a mixture of sulfuric acid and phosphoric acid.

[0110] Preferably, in step 220, the end of the glass protection tube 40 close to the corrosion portion 12' is also immersed in the first acidic solution, so that the first acidic solution corrodes the end of the non-corrosion portion 11' close to the corrosion portion 12' into a fiber gradient section.

[0111] like Figure 7As shown, during corrosion, as the first acidic solution corrodes the corroded portion 12' of the crystal optical fiber 10, the diameter of the corroded portion 12' gradually decreases, and the end side surface 13' close to the corroded portion 12' in the non-corroded portion 11' wrapped by the glass protection tube 40 will gradually be exposed, and the exposed end side surface 13' will be corroded after contacting with the first acidic solution, so that the first acidic solution not only has a corrosive effect on the corroded portion 12' along the diameter direction, but also has a corrosive effect on the non-corroded portion 11' along the axial direction of the optical fiber, and the closer the position of the non-corroded portion 11' is to the corroded portion 12', the greater the degree of corrosion and the smaller the diameter, finally forming a fiber gradient section with a gradual diameter.

[0112] The optical fiber gradient section comprises two sections, namely a first optical fiber gradient section 14 connected between the first optical fiber section 11 and the second optical fiber section 12 , and a second optical fiber gradient section 15 connected between the second optical fiber section 12 and the third optical fiber section 13 .

[0113] Step 230: using a second acidic solution to corrode and remove the glass protection tube 40 outside the crystal optical fiber 10 .

[0114] In step 230 , the second acidic solution can only corrode the glass protection tube 40 but cannot corrode the crystal optical fiber 10 .

[0115] In this embodiment, the second acidic solution is a hydrofluoric acid solution.

[0116] Step 300 : using a femtosecond laser to write the first optical fiber segment 11 and the second optical fiber segment 12 , so as to form the first optical fiber grating 20 and the second optical fiber grating 30 in the first optical fiber segment 11 and the second optical fiber segment 12 , respectively.

[0117] In step 300, a femtosecond laser processing system is constructed using a high repetition rate femtosecond laser, a high-precision electrically controlled three-dimensional displacement platform, a high-power objective lens, etc. to respectively write the first fiber grating 20 and the second fiber grating 30 in the first fiber segment 11 and the second fiber segment 12 of the crystal fiber 10.

[0118] According to the different diameters of the first optical fiber segment 11 and the second optical fiber segment 12, the femtosecond laser beam is focused on different depths of the first optical fiber segment 11 and the second optical fiber segment 12, respectively, and parameters such as writing cycle, writing speed, and writing length are set.

[0119] Since the first fiber grating 20 and the second fiber grating 30 adopt wavelength demodulation, it is necessary to reasonably set the respective grating periods to ensure that the resonance peaks of the two do not overlap in the resonance spectrum.

[0120] When the wavelength bandwidth is limited, the fiber grating with a larger period can be written in the fiber segment with a smaller diameter, and the fiber grating with a smaller period can be written in the fiber segment with a larger diameter. In the fiber segment with a smaller diameter, the fiber grating can be written point by point or line by line, and in the fiber segment with a larger diameter, the fiber grating can be written in a multi-layer line by line or surface by surface manner, so as to increase the refractive index modulation area of ​​the fiber grating, improve the reflectivity of the fiber grating, offset the light leakage caused by the light signal passing through the first fiber gradient segment 14 and the second fiber gradient segment 15 to a certain extent, and improve the signal-to-noise ratio.

[0121] Finally, one end of the crystal fiber 10 of the integrated optical fiber sensor is fusion-connected to the multimode quartz optical fiber, and the dual grating signal of the integrated optical fiber sensor is tested and demodulated through the multimode quartz optical fiber using a broadband light source, an optical fiber spectrometer, a 3dB multimode optical fiber coupler, a mode scrambler, a flange, etc.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention rather than to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An integrated optical fiber sensor for measuring temperature strain under extremely high temperatures, characterized in that: include: A crystal optical fiber, comprising a first optical fiber segment, a second optical fiber segment and a third optical fiber segment connected in sequence; A first fiber grating formed in a first fiber segment of the crystal fiber; A second fiber grating is formed in a second fiber segment of the crystal fiber; The first optical fiber segment has the same diameter as the third optical fiber segment, and the second optical fiber segment has a different diameter from the first optical fiber segment and the second optical fiber segment.

2. The integrated optical fiber sensor according to claim 1, characterized in that: The temperature sensitivity and strain sensitivity of the first fiber Bragg grating are K T1 and K ε1 The temperature sensitivity and strain sensitivity of the second fiber Bragg grating are K T2 and K ε2 , then the measured temperature change ΔT and strain Δε satisfy the following formula: Wherein, Δλ1 and Δλ2 are wavelength drifts of the first fiber grating and the second fiber grating respectively.

3. The integrated optical fiber sensor according to claim 1, characterized in that: The diameters of the first optical fiber segment and the third optical fiber segment are the first diameter, the diameter of the second optical fiber segment is the second diameter, the total length of the first optical fiber segment and the third optical fiber segment is the first length, and the length of the second optical fiber segment is the second length; the first diameter is smaller than the second diameter, and the first length is greater than the second length.

4. The integrated optical fiber sensor according to claim 1, characterized in that: The diameters of the first optical fiber segment and the third optical fiber segment are the first diameter, the diameter of the second optical fiber segment is the second diameter, the total length of the first optical fiber segment and the third optical fiber segment is the first length, and the length of the second optical fiber segment is the second length; the first diameter is greater than the second diameter, and the first length is greater than the second length.

5. The integrated optical fiber sensor according to claim 3 or 4, characterized in that: The stress generated by the first optical fiber segment, the second optical fiber segment and the third optical fiber segment is inversely proportional to their cross-sectional areas, while the cross-sectional areas of the first optical fiber segment, the second optical fiber segment and the third optical fiber segment are proportional to the square of their diameters.

6. The integrated optical fiber sensor according to any one of claims 1 to 4, characterized in that: The crystal fiber also includes a first fiber gradient segment and a second fiber gradient segment, wherein the first fiber gradient segment is connected between the first fiber segment and the second fiber segment, and the second fiber gradient segment is connected between the second fiber segment and the third fiber segment; the diameters of the first fiber gradient segment and the second fiber gradient segment change gradually.

7. The integrated optical fiber sensor according to claim 6, characterized in that: The diameter change rates of the first optical fiber gradient section and the second optical fiber gradient section satisfy the following formula: Wherein, β1 is the local propagation constant of the fundamental mode of the crystal fiber, β2 is the local propagation constant of the high-order mode of the crystal fiber, and dr is the diameter change of the first optical fiber gradient section and the second optical fiber gradient section within a unit length dz.

8. The integrated optical fiber sensor according to claim 1, characterized in that: The crystal fiber may be, but is not limited to, a sapphire fiber, a zirconium oxide fiber, a lutetium oxide fiber or a YAG fiber.

9. The method for preparing the integrated optical fiber sensor according to claim 1, characterized in that: The steps include: Step 100: Designing the length and diameter of the first optical fiber segment, the second optical fiber segment, and the third optical fiber segment; Step 200: using an acidic solution to corrode the crystal fiber, so that the crystal fiber forms the first fiber segment, the second fiber segment and the third fiber segment respectively; Step 300: Using a femtosecond laser to write the first optical fiber segment and the second optical fiber segment to form the first optical fiber grating and the second optical fiber grating in the first optical fiber segment and the second optical fiber segment, respectively.

10. The preparation method according to claim 9, characterized in that: In step 100, the strain sensitivity difference between the first fiber grating and the second fiber grating is adjusted by designing the diameter ratio of the first fiber segment and the second fiber segment.

11. The preparation method according to claim 10, characterized in that: The strain sensitivity of the first fiber Bragg grating and the second fiber Bragg grating satisfies the following formula: Among them, K ε1 and K ε2 are the strain sensitivities of the first fiber Bragg grating and the second fiber Bragg grating, respectively; d1 and d2 are the diameters of the first fiber segment and the second fiber segment, respectively.

12. The preparation method according to claim 9, characterized in that: In step 100, the strain sensitivity, strain resolution and strain range of the first fiber grating and the second fiber grating are adjusted by designing the length ratio of the second fiber segment to the total length of the first fiber segment and the third fiber segment.

13. The preparation method according to claim 9, characterized in that: In step 200, the steps of corroding the crystal optical fiber with an acidic solution are as follows: Step 210: placing a glass protection tube on the non-corroded portion of the crystal optical fiber; Step 220: Immerse the corroded portion of the crystal optical fiber in a first acidic solution and corrode it to a desired diameter; Step 230: using a second acidic solution to corrode and remove the glass protection tube outside the crystal optical fiber.

14. The preparation method according to claim 13, characterized in that: In step 220, the end of the glass protection tube close to the corrosion portion is also immersed in the first acidic solution, so that the first acidic solution corrodes the end of the non-corroded portion close to the corrosion portion into an optical fiber gradient section.

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