Distributed strain sensing optical cable adopting chirp grating

Abstract

The invention relates to a distributed strain sensing optical cable adopting chirp gratings. The distributed strain sensing optical cable comprises an optical fiber, a plurality of chirp gratings and a tight wrapping sheath layer, the plurality of chirp gratings are sequentially arranged along the length direction of the optical fiber; the tight wrapping sheath layer wraps the optical fiber and the plurality of chirp gratings, and enables the optical fiber to be in a stretched and tightened state; chirp gratings are continuously written into an optical fiber, each chirp grating can be regarded as a signal reflection platform in a specific wave band range, and every two adjacent chirp gratings serve as two reflection surfaces for reflecting signals in the same wave band. The optical signal reflected by the next chirp grating is longer than that of the previous chirp grating by the distance of the section of optical fiber between the two chirp gratings, when the section of optical fiber is strained, the effective length and the refractive index of the optical fiber are changed, the optical path of the reflected signal of the next chirp grating is influenced, and the phase difference of the two optical signals is measured and compared, so that the optical distance of the next chirp grating is determined. Strain positioning and strain measurement are realized, metal armoring is not needed, and the preparation cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber grating sensing, and in particular to a distributed strain sensing optical cable using a chirped grating. Background Art

[0002] Safety monitoring of large buildings mainly involves detecting the growth and spread of cracks caused by various stress fatigue during long-term use, and preventing the final strain from leading to irreversible damage. Since strain will be generated during crack growth and structural deformation, the function of health monitoring can be achieved by strain detection of important parts. At present, strain sensing optical cables have many advantages such as strong networking capabilities, wide coverage, and low cost per meter, and have become the main solution.

[0003] Among various strain sensing optical cables, distributed array gratings have become the main research object due to their good linear response to strain and high reliability. Patent CN116400466A proposes to use an equally spaced Bragg grating array, use a spiral metal armor tube as a strain transmission medium, and apply the strain at any point on the spiral armor tube to the grating between the glue points by means of equally spaced glue dots, thereby achieving distributed strain sensing.

[0004] However, this method has certain limitations: 1. The presence of metal armor increases the structure and manufacturing cost of the optical cable, limiting the application of longer distances; 2. In the long-distance continuous dispensing process, it is easy to cause the grating position to be dotted on the spiral armor, increasing the transmission loss and changing the linear response amplitude of the strain. Therefore, it is necessary to propose a distributed strain sensing optical cable with a simpler structure and lower cost. Summary of the invention

[0005] In view of this, it is necessary to provide a distributed strain sensing optical cable using a chirped grating to solve the problem that the presence of metal armor increases the structure and manufacturing cost of the optical cable and limits its application over longer distances; at the same time, in the process of long-distance continuous dispensing, the grating position is easily caused to be dotted onto the spiral armor, increasing the transmission loss and changing the linear response amplitude during strain.

[0006] The present invention provides a distributed strain sensing optical cable using a chirped grating, comprising an optical fiber, a plurality of chirped gratings and a tight sheath layer; the plurality of chirped gratings are arranged in sequence along the length direction of the optical fiber; the tight sheath layer wraps the optical fiber and the plurality of chirped gratings, and puts the optical fiber in a stretched and taut state.

[0007] Furthermore, the diameter of the optical fiber is 80-200 µm, and the optical fiber is configured to support writing of various types of gratings under single pulse online conditions.

[0008] Furthermore, a plurality of the chirped gratings are continuously written on the optical fiber through a drawing tower single pulse online writing system to form an array, the length of the chirped grating is 5 mm to 15 mm, the reflectivity of the chirped grating is 0.005% to 0.1%, and the 3dB bandwidth range of the chirped grating is 1 nm to 5 nm.

[0009] Furthermore, the multiple chirped gratings are subjected to reflectivity compensation so that the reflectivities of the multiple chirped gratings are the same during the writing process.

[0010] Furthermore, the wavelengths of the multiple chirped gratings are the same.

[0011] Furthermore, the plurality of chirped gratings are divided into a plurality of chirped gratings having a first wavelength and a plurality of chirped gratings having a second wavelength, the first wavelength and the second wavelength are not equal, and the plurality of chirped gratings having the first wavelength and the plurality of chirped gratings having the second wavelength are alternately arranged along the length direction of the optical fiber.

[0012] Furthermore, the interval between two adjacent chirped gratings is not less than 10 cm, and two adjacent chirped gratings with the same wavelength form a sensing unit, and the length of the sensing unit is 1 m to 30 m.

[0013] Furthermore, the tight sheath layer is a polymer sheath material, and the tight sheath layer is wrapped around the outside of the optical fiber by extrusion and glue bonding, and the optical fiber is in a stretched and taut state, wherein the pre-stretching amplitude range of the optical fiber is 0.01%~0.1%.

[0014] Furthermore, the distributed strain sensing optical cable also includes a strain protection layer sleeved on the tight sheath layer.

[0015] Furthermore, the distributed strain sensing optical cable also includes a plurality of strain-sensitizing layers embedded on the outer wall of the strain protection layer, the plurality of strain-sensitizing layers are sequentially arranged along the length direction of the strain protection layer, the plurality of strain-sensitizing layers are respectively arranged at positions between each two adjacent chirped gratings in the plurality of chirped gratings, and the length range of the strain-sensitizing layers does not exceed the spacing distance between two adjacent chirped gratings; The strain-sensitizing layer is a strain-sensitive material, and the strain-sensitizing layer is arranged in a groove opened on the outer wall of the strain protection layer or attached to the outer wall of the strain protection layer.

[0016] Compared with the prior art, chirped gratings are continuously written into the optical fiber, and each chirped grating can be regarded as a signal reflection platform in a specific wavelength range. Every two adjacent chirped gratings serve as two reflection surfaces for reflecting signals in the same wavelength band, and the light signal reflected by the latter chirped grating travels a further distance of the optical fiber between the two chirped gratings than that of the former chirped grating. When the optical fiber section is strained, the effective length and refractive index of the optical fiber change, which affects the optical path of the reflection signal of the latter chirped grating. By measuring and comparing the phase difference of the two optical signals, the strain change information at the position can be extracted to achieve strain positioning and strain measurement. The above-mentioned distributed strain sensing optical cable does not require metal armor, simplifies the structure of the existing strain distributed sensing optical cable, and reduces the preparation cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a radial cross section of a distributed strain sensing optical cable using a chirped grating provided in an embodiment of the present invention; Figure 2 A schematic cross-sectional view of the upper half of a distributed strain sensing optical cable using a chirped grating provided in an embodiment of the present invention along its axial direction; Figure 3 A working principle diagram of a distributed strain optical cable using a chirped grating in a distributed strain sensing optical cable using a chirped grating provided in an embodiment of the present invention; Figure 4 A working principle diagram of a distributed strain optical cable using a dual-wavelength chirped grating in a distributed strain sensing optical cable using a chirped grating provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0019] like Figure 1-2 As shown, Figure 1 is a schematic diagram of the radial cross section of the distributed strain sensing optical cable. Figure 2 It is a schematic cross-sectional view of the upper part of the distributed strain sensing optical cable along its axial direction, and the overall structure is centrally symmetrical. An embodiment of the present invention provides a distributed strain sensing optical cable using a chirped grating, comprising an optical fiber 100, a plurality of chirped gratings 200 and a tight sheath layer 300; the plurality of chirped gratings 200 are arranged in sequence along the length direction of the optical fiber 100; the tight sheath layer 300 wraps the optical fiber 100 and the plurality of chirped gratings 200, and puts the optical fiber 100 in a stretched and taut state.

[0020] During implementation, chirped gratings 200 are continuously written into the optical fiber 100, and each chirped grating 200 can be regarded as a signal reflection platform within a specific wavelength range. Every two adjacent chirped gratings 200 serve as two reflection surfaces for reflecting signals in the same wavelength range, and the optical signal reflected by the latter chirped grating 200 travels a distance of the optical fiber 100 between two chirped gratings 200 more than that of the former chirped grating 200. When the section of optical fiber 100 is strained, the effective length and refractive index of the optical fiber 100 change, which affects the optical path of the reflected signal of the latter chirped grating 200. By measuring and comparing the phase difference of the two optical signals, the strain change information at the position can be extracted to achieve strain positioning and strain measurement. The above-mentioned distributed strain sensing optical cable does not require metal armor, simplifies the structure of the existing strain distributed sensing optical cable, and reduces the preparation cost.

[0021] The diameter of the optical fiber 100 in this embodiment is 80-200 µm, and the optical fiber 100 is configured to support writing of various types of gratings under single pulse online conditions. The optical fiber 100 can be single-mode, multi-mode, few-mode, thin-diameter, etc.

[0022] In this embodiment, a plurality of chirped gratings 200 are arranged in sequence along the length direction of the optical fiber 100 .

[0023] In one embodiment, a plurality of chirped gratings 200 are continuously written on the optical fiber 100 by a drawing tower single pulse online writing system to form an array, the length of the chirped grating 200 is 5 mm to 15 mm, the reflectivity of the chirped grating 200 is 0.005% to 0.1%, and the 3dB bandwidth range of the chirped grating 200 is 1 nm to 5 nm.

[0024] In one embodiment, multiple chirped gratings 200 are subjected to reflectivity compensation so that the reflectivity of the multiple chirped gratings 200 is the same during the writing process. Specifically, the chirped grating 200's grating parameters such as reflectivity and bandwidth need to remain consistent during the writing process, and reflectivity compensation can be performed according to the actual application range, that is, the reflectivity of the chirped grating 200 that is farther away from the incident light direction is appropriately enhanced during the preparation process to achieve a uniform intensity of the light signal reflected by the entire array.

[0025] In the first embodiment, the wavelengths of the plurality of chirped gratings 200 are the same.

[0026] like Figure 3 The figure shows the working principle of the distributed strain optical cable using chirped gratings. The distributed strain sensing optical cable contains an array of equally spaced identical chirped gratings (A1, A2, A3…), with a central wavelength of , where every two adjacent chirped gratings serve as a sensing unit, such as A1-A2, A2-A3, A3-A4. When strain is generated at the optical fiber 100 between the chirped gratings A1 and A2, as shown in the strain position P in the figure, the optical signal I1 reflected by the chirped grating A1 does not change, and the phase information of the optical signal I2 reflected by the chirped grating A2 changes due to the change in the optical path, resulting in a change in the phase difference between the optical signal I1 and the optical signal I2, thereby determining that the position where the strain occurs is between the chirped gratings A1 and A2, and calculating the deformation amount of the optical fiber 100 according to the degree of change in the phase difference, thereby realizing strain sensing. Accordingly, the invention can respond to the strain at any position between any two chirped gratings to realize the distributed strain sensing function.

[0027] In the second embodiment, the multiple chirped gratings 200 are divided into multiple chirped gratings 200 with a first wavelength and multiple chirped gratings 200 with a second wavelength, the first wavelength and the second wavelength are not equal, and the multiple chirped gratings 200 with the first wavelength and the multiple chirped gratings 200 with the second wavelength are alternately arranged along the length direction of the optical fiber 100.

[0028] like Figure 4 The figure shows the working principle of the distributed strain optical cable using dual-wavelength chirped grating. The distributed strain sensing optical cable contains a central wavelength of The chirped grating array (A1, A2, A3…) and the central wavelength are The chirped grating array (B1, B2, B3...) is used, and the sensing unit is still an adjacent grating pair with the same wavelength, such as (A1-A2, B1-B2), etc. When the strain position P is between B1 and A2, the optical phases of the optical signals I12 and I22 reflected by the chirped gratings A2 and B2 change, and the phase difference information is generated between the optical signals I11 and I21 reflected by the previous chirped gratings (A1 and B1) of the same wavelength, and thus the strain position is determined to be located in the overlapping part of the chirped gratings A1-A2 and B1-B2, that is, between A2-B1. In this way, the spatial resolution of the positioning strain of the sensing optical cable can be improved on the basis of the resolution of the current demodulation equipment.

[0029] The spacing between two adjacent chirped gratings 200 is not less than 10 cm, and two adjacent chirped gratings 200 with the same wavelength form a sensing unit, and the length of the sensing unit is 1m~30m, so that the positioning accuracy of the sensing optical cable can be improved by alternately writing chirped gratings 200 with different wavelengths.

[0030] It should be noted that the chirped gratings 200 at different wavelengths refer to two chirped gratings 200 with different central wavelengths and no overlap, and no intersection exists in the optical frequency signal, that is, no crosstalk occurs between the signals reflected by any two chirped gratings 200 with different wavelengths.

[0031] The tight jacket layer 300 in this embodiment wraps the optical fiber 100 and the plurality of chirped gratings 200, and puts the optical fiber 100 in a stretched and taut state.

[0032] In one embodiment, the tight jacket layer 300 is a polymer jacket material, and the tight jacket layer 300 is wrapped around the outside of the optical fiber 100 by extrusion and glue bonding, and the optical fiber 100 is in a stretched and taut state, wherein the pre-stretching amplitude range of the optical fiber 100 is 0.01%~0.1%.

[0033] In one embodiment, the distributed strain sensing optical cable further includes a strain protection layer 400 sleeved on the tight jacket layer 300. The strain protection layer 400 is made of materials such as high-density polyethylene, which has the advantages of high strength, good toughness and strong rigidity, and protects the optical cable from erosion and damage in harsh outdoor environments.

[0034] It is understandable that when the distributed strain sensing optical cable is placed in an environment such as indoors where the environment is relatively friendly and has high requirements for strain measurement sensitivity, the strain protection layer 400 can be removed and the optical fiber 100 with only the tight sheath layer 300 can be directly attached to the strain measurement area.

[0035] In one embodiment, the distributed strain sensing optical cable also includes a plurality of strain-sensitizing layers 410 embedded on the outer wall of the strain protection layer 400, and the plurality of strain-sensitizing layers 410 are arranged in sequence along the length direction of the strain protection layer 400. The plurality of strain-sensitizing layers 410 are respectively disposed at positions between each two adjacent chirped gratings 200 among the plurality of chirped gratings 200, and the length range of the strain-sensitizing layer 410 does not exceed the spacing distance between two adjacent chirped gratings 200.

[0036] The strain-sensitizing layer 410 is a strain-sensitive material, and is disposed in a groove formed on the outer wall of the strain protection layer 400 or attached to the outer wall of the strain protection layer 400. The strain-sensitive material has the characteristics of low strength, high elasticity and easy stretching.

[0037] Working process: chirped gratings 200 are continuously written into the optical fiber 100. Each chirped grating 200 can be regarded as a signal reflection platform in a specific wavelength range. Every two adjacent chirped gratings 200 serve as two reflection surfaces for reflecting signals in the same wavelength range. The optical signal reflected by the latter chirped grating 200 travels a further distance of the optical fiber 100 between two chirped gratings 200 than that of the former chirped grating 200. When the optical fiber 100 is strained, the effective length and refractive index of the optical fiber 100 change, which affects the optical path of the reflected signal of the latter chirped grating 200. By measuring and comparing the phase difference of the two optical signals, the strain change information at that position can be extracted to achieve strain positioning and strain measurement.

[0038] Compared with existing technologies: 1) The present invention has the ability to measure strain distribution when preparing the array grating optical fiber 100. The subsequent optical cable processing is only used as a means of protecting the optical fiber 100 (i.e., installing the tight jacket layer 300 and the strain protection layer 400). There is no need to consider how to accurately transfer the structural strain to the grating area. During the calibration and installation process, it is only necessary to determine that any position of the optical fiber 100 is in close contact with the strain structure, which greatly improves the application range of the strain optical cable and reduces the difficulty of installation. 2) In the present invention, the chirped grating 200 is only used as a signal reflection plane. On the basis of ensuring the structural firmness of the optical cable region where the chirped grating 200 is located, the optical cable structure between adjacent chirped gratings 200 can be flexibly modified according to application requirements, including but not limited to strain sensitization, temperature, humidity and other parameters converted into expansion strain, etc., which expands the application scope of strain gratings; 3) The present invention greatly simplifies the structure of the existing strain distributed sensing optical cable and reduces the preparation cost.

[0039] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A distributed strain sensing optical cable using a chirped grating, characterized in that: include: optical fiber; A plurality of chirped gratings are sequentially arranged along the length direction of the optical fiber; A tight sheath layer wraps the optical fiber and the plurality of chirped gratings and keeps the optical fiber in a stretched and taut state.

2. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The diameter of the optical fiber is 80-200 µm, and the optical fiber is configured to support writing of various types of gratings under single pulse online conditions.

3. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: A plurality of the chirped gratings are continuously written on the optical fiber through a drawing tower single pulse online writing system to form an array, the length of the chirped grating is 5 mm to 15 mm, the reflectivity of the chirped grating is 0.005% to 0.1%, and the 3dB bandwidth range of the chirped grating is 1 nm to 5 nm.

4. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The multiple chirped gratings are subjected to reflectivity compensation so that the reflectivities of the multiple chirped gratings are the same during the writing process.

5. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The wavelengths of the multiple chirped gratings are the same.

6. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The multiple chirped gratings are divided into multiple chirped gratings with a first wavelength and multiple chirped gratings with a second wavelength, the first wavelength and the second wavelength are not equal and do not overlap, and the multiple chirped gratings with the first wavelength and the multiple chirped gratings with the second wavelength are alternately arranged along the length direction of the optical fiber.

7. The distributed strain sensing optical cable using chirped grating according to claim 5 or 6, characterized in that: The distance between two adjacent chirped gratings is not less than 10 cm, and two adjacent chirped gratings with the same wavelength form a sensing unit, and the length of the sensing unit is 1 m to 30 m.

8. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The tight sheath layer is made of a polymer sheath material, and is wrapped around the outside of the optical fiber by extrusion and glue bonding, and puts the optical fiber in a stretched and taut state, wherein the pre-stretching amplitude range of the optical fiber is 0.01% to 0.1%.

9. The distributed strain sensing optical cable using chirped grating according to claim 1, characterized in that: The distributed strain sensing optical cable also includes a strain protection layer sleeved on the tight sheath layer.

10. The distributed strain sensing optical cable using chirped grating according to claim 9, characterized in that: The distributed strain sensing optical cable further comprises a plurality of strain-sensitizing layers embedded on the outer wall of the strain protection layer, the plurality of strain-sensitizing layers being arranged in sequence along the length direction of the strain protection layer, the plurality of strain-sensitizing layers being respectively arranged at positions between each two adjacent chirped gratings among the plurality of chirped gratings, and the length range of the strain-sensitizing layers does not exceed the spacing distance between two adjacent chirped gratings; The strain-sensitizing layer is a strain-sensitive material, and the strain-sensitizing layer is arranged in a groove opened on the outer wall of the strain protection layer or attached to the outer wall of the strain protection layer.

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