Distributed temperature sensing optical cable adopting chirp grating

Abstract

The invention relates to a distributed temperature sensing optical cable adopting chirp gratings. The distributed temperature sensing optical cable comprises an optical fiber, a plurality of chirp gratings and a loose sheath layer, the plurality of chirp gratings are sequentially arranged along the length direction of the optical fiber; the loose sheath layer wraps the optical fiber and the plurality of chirp gratings, and enables the optical fiber to be in a loose excess length 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 temperature of the section of optical fiber is changed, the effective length and the refractive index of the optical fiber are changed, and the optical path of the reflected signal of the next chirp grating is influenced; temperature positioning and measurement are realized by measuring and comparing the phase difference of the two optical signals.

Description

Technical Field

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

[0002] Among all types of safety monitoring, fire warning is the primary goal of eliminating potential safety hazards in energy facilities. Specifically, it is necessary to achieve high-precision real-time monitoring of the temperature of each potential hazard location to achieve rapid troubleshooting and early warning of potential hazards. Limited by the working environment and cost scale, electrical sensors are difficult to apply in practice. Among various temperature sensing optical cables, fiber Bragg gratings are considered to be the most practical optical sensor device and have been widely used.

[0003] At present, the most commonly used temperature sensing grating is mainly Bragg grating, which uses the characteristic that the grating pitch changes under the influence of temperature to restore the temperature change of the external environment by real-time monitoring the change of the grating center wavelength. Patent CN105301729A proposes a high-strength temperature sensing optical cable, which combines the sensing fiber Bragg grating with a sheath structure with a metal shell. The metal sheath plays two roles at the same time: protecting the supporting optical fiber and conducting heat.

[0004] However, the above method still has certain limitations, that is, the position of temperature change must be close to the grating position, so as to minimize the time and heat loss required for heat to be transferred from the outside to the grating, and improve the sensor response speed and accuracy. This leads to: 1. In order to meet the needs of higher-precision testing, it is necessary to continuously increase the distribution density of the grating array and reduce the unresponsive effect of the blank optical fiber between the gratings on temperature changes; 2. During the installation of the sensor optical cable, the position of the grating in the optical cable must be strictly calibrated and accurately fixed to the possible hidden danger position. This has led to the continuous increase in the preparation cost and difficulty of using the temperature sensing optical cable. Summary of the invention

[0005] In view of this, it is necessary to provide a distributed temperature sensing optical cable using a chirped grating to solve the problem of the need to continuously increase the distribution density of the grating array. At the same time, the position of the grating in the optical cable needs to be strictly calibrated and accurately fixed to the possible hidden danger position, and the temperature sensing optical cable preparation cost and difficulty of use are constantly increasing.

[0006] The present invention provides a distributed temperature sensing optical cable using a chirped grating, comprising an optical fiber, a plurality of chirped gratings and a loose sheath layer; a plurality of the chirped gratings are arranged in sequence along the length direction of the optical fiber; the loose sheath layer wraps the optical fiber and the plurality of the chirped gratings, and places the optical fiber in a relaxed excess length 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 do not overlap, 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 loose sheath layer is a polymer sheath material, and the loose sheath layer is wrapped around the outside of the optical fiber by extrusion and glue bonding, and the optical fiber is in a relaxed excess length state, wherein the excess length amplitude of the optical fiber ranges from 0.1% to 5%.

[0014] Furthermore, the distributed temperature sensing optical cable also includes a protective layer sleeved on the loose sheath layer.

[0015] Furthermore, the distributed temperature sensing optical cable also includes a steel wire built into the protective layer and extending along the length direction of the optical fiber; The distributed temperature sensing optical cable also includes a plurality of temperature sensitive layers embedded on the outer wall of the protective layer, the plurality of temperature sensitive layers are sequentially arranged along the length direction of the protective layer, the plurality of temperature sensitive layers are respectively arranged at positions between each two adjacent chirped gratings among the plurality of chirped gratings, and the length range of the temperature sensitive layer does not exceed the spacing distance between the two adjacent chirped gratings; The temperature sensitive layer is a temperature sensitive material, and the temperature sensitive layer is arranged in a groove opened on the outer wall of the protective layer or attached to the outer wall of the protective 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 band range. Every two adjacent chirped gratings serve as two reflection surfaces for reflecting signals in the same band, and the light signal reflected by the latter chirped grating travels a longer distance of the optical fiber between the two chirped gratings than that of the former chirped grating. When the temperature of this section of optical fiber changes, the effective length and refractive index of the optical fiber change, which affects the optical path of the reflected signal of the latter chirped grating. By measuring and comparing the phase difference of the two optical signals, the temperature change information of the position can be extracted to achieve temperature positioning and measurement, without considering how to quickly transmit the temperature change to the grating area. During the calibration and installation process, any optical fiber position can be brought into close contact with the temperature position to be measured, which greatly reduces the difficulty of installing the sensor optical cable. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic cross-sectional view of the entire distributed temperature sensing optical cable along its radial direction using a chirped grating provided in an embodiment of the present invention; Figure 2 A schematic cross-sectional view of the upper portion of a distributed temperature sensing optical cable using a chirped grating provided in an embodiment of the present invention along its axis; Figure 3 A working principle diagram of a distributed temperature optical cable using a chirped grating in a distributed temperature sensing optical cable using a chirped grating provided in an embodiment of the present invention; Figure 4 A working principle diagram of a distributed temperature optical cable using a dual-wavelength chirped grating in a distributed temperature 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 temperature sensing optical cable. Figure 2 It is a schematic diagram of the cross section of the upper part of the distributed temperature sensing optical cable along its axis, and the overall structure is centrally symmetrical. An embodiment of the present invention provides a distributed temperature sensing optical cable using a chirped grating, comprising an optical fiber 100, a plurality of chirped gratings 200 and a loose sheath layer 300; the plurality of chirped gratings 200 are arranged in sequence along the length direction of the optical fiber 100; the loose sheath layer 300 wraps the optical fiber 100 and the plurality of chirped gratings 200, and places the optical fiber 100 in a relaxed excess length 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 further distance of the optical fiber 100 between two chirped gratings 200 than that of the former chirped grating 200. When the temperature of this section of optical fiber 100 changes, 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 temperature change information of the position can be extracted to achieve temperature positioning and measurement, without considering how to quickly transmit the temperature change to the grating area. During the calibration and installation process, any optical fiber 100 position can be brought into close contact with the temperature position to be measured, which greatly reduces the difficulty of installing the sensing optical cable.

[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 optical fiber.

[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 sensing optical cable using the chirped grating 200. The distributed sensing optical cable comprises an array of equally spaced identical chirped gratings 200 (A1, A2, A3...), each with a central wavelength of , where every two adjacent chirped gratings 200 serve as a sensing unit, such as A1-A2, A2-A3, A3-A4. When a temperature change occurs at the optical fiber 100 between the gratings A1 and A2, the optical signal I1 reflected by the grating A1 does not change, and the phase information of the optical signal I2 reflected by the 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 temperature change position T is between the gratings A1 and A2, and calculating the degree of temperature change based on the degree of change in the phase difference, thereby realizing temperature sensing. Accordingly, the invention can respond to temperature changes at any location between any two gratings, thereby realizing a distributed sensing function.

[0027] In the second embodiment, the multiple chirped gratings 200 are divided into a multiple chirped gratings 200 having a first wavelength and a multiple chirped gratings 200 having a second wavelength, the first wavelength and the second wavelength are not equal and do not overlap, and the multiple chirped gratings 200 having the first wavelength and the multiple chirped gratings 200 having 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 temperature sensing optical cable using the dual-wavelength chirped grating 200. The distributed sensing optical cable includes a central wavelength of The chirped grating 200 array (A1, A2, A3...) and the central wavelength are The chirped grating 200 array (B1, B2, B3...), the sensing unit is still the adjacent grating pair with the same wavelength, such as (A1-A2, B1-B2), etc. When the temperature change position T is between B1 and A2, the optical phases of the optical signals I12 and I22 reflected by the gratings A2 and B2 change, and the phase difference information is generated between the optical signals I11 and I21 reflected by the previous gratings (A1 and B1) with the same wavelength, respectively, and thus it is determined that the temperature change position T is located in the overlapping part of the gratings A1-A2 and the gratings B1-B2, that is, between A2-B1. In this way, the positioning accuracy of the sensing optical cable can be further improved on the basis of the current resolution of the 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 having different central wavelengths and no intersection in the optical frequency signal, that is, no crosstalk occurs between the signals reflected by any two chirped gratings 200 at different wavelengths.

[0031] The loose sheath 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 relaxed excess length state.

[0032] In one embodiment, the loose sheath layer 300 is a polymer sheath material, and is wrapped around the outside of the optical fiber 100 by extrusion bonding, and the optical fiber 100 is in a relaxed excess length state, wherein the excess length amplitude of the optical fiber 100 ranges from 0.1% to 5%.

[0033] In this embodiment, the distributed temperature sensing optical cable also includes a protective layer 400 sleeved on the loose sheath layer 300, wherein the protective layer 400 is made of materials such as high-density polyethylene, and 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 temperature sensing optical cable is installed in a relatively friendly indoor environment and has high requirements for temperature measurement sensitivity, the protective layer 400 can be removed and the optical fiber 100 with only the loose sheath layer 300 can be directly attached to the temperature measurement area, and anti-strain interference treatment can be done.

[0035] In one embodiment, the distributed temperature sensing optical cable further includes a steel wire 410 built into the protective layer 400 and extending along the length direction of the optical fiber 100. The diameter of the steel wire 410 is in the range of 0.1 mm to 0.5 mm, which is used to further improve the ability of the optical cable to resist tensile deformation. The number of the steel wires 410 can be set to two, and the two steel wires 410 are relatively arranged on both sides of the steel wire 410.

[0036] In one embodiment, the distributed temperature sensing optical cable further includes a plurality of temperature sensitive layers 420 embedded on the outer wall of the strain protection layer 400, the plurality of temperature sensitive layers 420 are arranged in sequence along the length direction of the protection layer 400, the plurality of temperature sensitive layers 420 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 temperature sensitive layer 420 does not exceed the spacing distance between the two adjacent chirped gratings 200.

[0037] The temperature sensitive layer 420 is a temperature 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 high strength, significant heat conduction effect or is easy to absorb heat.

[0038] 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 temperature of this section of optical fiber 100 changes, 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 temperature change information at that position can be extracted to achieve temperature positioning and measurement.

[0039] Compared with existing technologies: 1) The present invention has the ability of strain distribution measurement 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 loose sheath layer 300 and the protective layer 400). There is no need to consider how to quickly transmit the temperature change to the grating area. During the calibration installation process, any position of the optical fiber 100 can be closely contacted with the temperature position to be measured, which greatly reduces the difficulty of installing the sensor optical cable; 2) In the present invention, the chirped grating 200 is only used as a signal reflection plane. On the basis of ensuring that the structure of the optical cable region where the chirped grating 200 is located is solid and reliable, the optical cable structure between adjacent chirped gratings 200 can be flexibly modified according to application requirements, thereby expanding the application scope of the sensor optical cable; 3) The present invention can truly realize distributed temperature measurement, rather than the quasi-distributed measurement of ordinary grating arrays, and can respond quickly to temperature changes of any size range.

[0040] The above are only preferred specific implementations 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 temperature 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 loose sheath layer wraps the optical fiber and the plurality of chirped gratings and puts the optical fiber in a relaxed excess length state.

2. The distributed temperature 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 temperature 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 temperature 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 temperature 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 temperature 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 temperature 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 temperature sensing optical cable using chirped grating according to claim 1, characterized in that: The loose sheath layer is a polymer sheath material, and is wrapped around the outside of the optical fiber by extrusion and glue bonding, and the optical fiber is in a relaxed excess length state, wherein the excess length amplitude of the optical fiber ranges from 0.1% to 5%.

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

10. The distributed temperature sensing optical cable using chirped grating according to claim 9, characterized in that: The distributed temperature sensing optical cable also includes a steel wire built into the protective layer and extending along the length direction of the optical fiber; The distributed temperature sensing optical cable also includes a plurality of temperature sensitive layers embedded on the outer wall of the protective layer, the plurality of temperature sensitive layers are sequentially arranged along the length direction of the protective layer, the plurality of temperature sensitive layers are respectively arranged at positions between each two adjacent chirped gratings among the plurality of chirped gratings, and the length range of the temperature sensitive layer does not exceed the spacing distance between two adjacent chirped gratings; The temperature sensitive layer is a temperature sensitive material, and the temperature sensitive layer is arranged in a groove opened on the outer wall of the protective layer or attached to the outer wall of the protective layer.

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