Distributed strain optical cable based on ultra-weak fiber grating array

Abstract

The invention discloses a distributed strain optical cable based on an ultra-weak fiber bragg grating array. The distributed strain optical cable comprises a fiber reinforced composite optical fiber core which is located in the center and inscribed with the ultra-weak fiber bragg grating array. The steel cable is twisted and surrounded around the fiber reinforced composite optical fiber core; and the sheath is formed by thermoplastic molding after a high-density polyethylene material is coated outside the steel cable. According to the invention, long-distance distributed sensing of the ultra-weak fiber grating array is realized. Strain transmission is carried out in a layering mode, the strain on the periphery of the optical cable is effectively transmitted to the adjacent point type ultra-weak fiber bragg grating, and therefore full-optical-cable sensing is achieved, the spectrum chirp problem during optical fiber cabling is solved, the distributed monitoring effect is achieved in a quasi-distributed mode, and the optical cable is completely the same as a traditional BOTDA in appearance and purpose and can be directly replaced.

Description

Technical Field

[0001] The invention belongs to the technical field of fiber grating strain sensing, and in particular relates to a distributed strain optical cable based on an ultra-weak fiber grating array. Background Art

[0002] Ultra-weak fiber Bragg grating is a general term for fiber Bragg gratings with a reflectivity of usually less than 0.1%. It inherits the characteristics of traditional fiber Bragg grating (FBG) sensors, and also has the advantages of large multiplexing capacity, good mechanical properties, and easy cabling. A single optical fiber can accommodate up to tens of thousands of sensor units, effectively alleviating the demand for capacity in the sensor network. It is highly favored in the field of structural monitoring, especially for structural health monitoring of large buildings (such as tunnels, dams, high-rise buildings, bridges, etc.). However, like ordinary optical fibers, the body strength of bare optical fibers is low and it is very easy to be damaged, making it difficult to directly apply them to actual projects. In order to use optical fibers to monitor stress / strain normally in actual complex environments, bare optical fibers are usually packaged into cables to ensure their survival. The cabling quality of ultra-weak fiber Bragg gratings directly determines the field and scope of the promotion and application of this technology! The existing fiber packaging and cabling methods have made certain progress in improving the strength and impact resistance of sensing optical fibers. In 2016, Suo Wenbin and others twisted 0.9mm hyacinth tight-wrapped optical fibers with steel cables and then covered them with outer sheaths to make optical cables. However, this type of optical cable requires pre-tensioning and has certain strain losses, and the strain monitoring accuracy is low. In addition, due to the low compressive strength of hyacinth tight-wrapped optical fibers, additional losses and grating chirps are easily caused during the twisting process. This method is not suitable for the cabling of ultra-weak fiber grating arrays. In 2020, Shi Bin and others designed an external fixed-point structure on the outer diameter of the optical cable to facilitate fixation and pre-stretching, but its strength is relatively low, its sensitivity is low, and its dynamic response is slow. In 2022, Luo Zhihui and others developed a fixed-point strain optical cable based on ultra-weak fiber gratings, but due to the existence of fixed points, automated production is difficult, and the problems of "strain blind spots" and pre-stretching are not solved. In 2023, Luo Zhihui and others improved the above structure and invented an ultra-weak fiber grating prestressed optical cable. This optical cable has better solved the "strain blind spot" problem, but there are still problems such as limited cable length and difficulty in automated production.

[0003] Although the above research has solved the problem of "existence" or "non-existence" of ultra-weak fiber Bragg grating strain detection optical cables, it is easy to cause spectral chirp or "strain dead zone", causing degradation of sensing performance and making it difficult to meet the requirements of long-distance automated production. It is crucial to study a high-performance, efficient and automated ultra-weak fiber Bragg grating array cabling technology. Summary of the invention

[0004] The purpose of the present invention is to propose a distributed strain optical cable based on an ultra-weak fiber grating array in view of the deficiencies in the prior art. In view of the performance degradation and easy chirping of the existing ultra-weak fiber grating array during linear packaging, a distributed strain optical cable based on an ultra-weak fiber grating array is proposed. The FRP containing the ultra-weak fiber grating array is used as the central cable wire, and the distributed sensing optical cable is made in combination with the traditional steel cable twisting process. The optical cable has no "strain dead zone" in the tensile and compression test, and has good bending and impact resistance, thereby improving the strain transmission efficiency of the optical cable, and providing an excellent solution for the distributed sensing of the ultra-weak fiber grating array.

[0005] The technical purpose of the present invention is achieved through the following technical solutions: A distributed strain optical cable based on ultra-weak fiber grating array, comprising a fiber-reinforced composite optical fiber core with an ultra-weak fiber grating array inscribed on the center; A steel cable twisted and wrapped around the fiber-reinforced composite optical fiber core; The steel cable is externally coated with a high-density polyethylene material and then thermoplasticized to form a sheath.

[0006] Preferably, the fiber-reinforced composite optical fiber core is formed by pre-coating a bend-resistant optical fiber with a temperature-resistant layer and then hot-pultruding the fiber-reinforced composite material and resin.

[0007] Preferably, the ultra-weak fiber grating array includes a plurality of ultra-weak fiber gratings with a reflectivity lower than 0.1%, and the spacing between two adjacent ultra-weak fiber gratings is 0.1m-5m.

[0008] Preferably, the ultra-weak fiber grating array is prepared online using a drawing tower process, and the multi-wavelength sensing unit is prepared by automatically replacing the mask.

[0009] Preferably, the core diameter of the fiber-reinforced composite optical fiber is 1 mm, or 2 mm.

[0010] Preferably, the diameter of the steel cable is the same as that of the fiber-reinforced composite optical fiber core, and the steel cable is twisted and surrounded in a spiral form around the fiber-reinforced composite optical fiber core.

[0011] Preferably, the sheath is formed by thermoplasticization of high-density polyethylene and is in close contact with the steel cable.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a distributed strain optical cable based on ultra-weak fiber grating array, which has a simple structure, can be mass-produced to realize the automated preparation of optical cables, and has high cost performance. It is compatible with the process of twisting traditional steel cables, has a simple structure, and has high production efficiency; the whole process is machine-produced quickly, no manual control of process parameters is required, the optical cable loss is low, and long-distance sensing can be achieved.

[0013] The present invention realizes long-distance distributed sensing of ultra-weak fiber grating arrays. Through layered strain transmission, the strain on the periphery of the optical cable is effectively transmitted to the adjacent point-type ultra-weak fiber grating, thereby realizing full optical cable sensing, solving the problem of spectral chirp during optical fiber cabling, and realizing distributed monitoring effect in a quasi-distributed manner. The appearance and use of the optical cable are exactly the same as those of the traditional BOTDA, and can be directly replaced.

[0014] The present invention overcomes the long-standing strain "blind zone problem" of distributed optical cables, and the consistency of the sensor is good. The strain on the sheath is efficiently transferred to the fiber core by using the FRP rigid strain transmission mechanism, and there is no low modulus material as an intermediate layer, which completely solves the strain "blind zone problem" of traditional distributed optical cables. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the structure of the present invention.

[0016] Figure 2 This is a strain-wavelength curve of the strain optical cable proposed in the present invention.

[0017] Figure 3 This is a temperature-wavelength curve of the strained optical cable proposed by the present invention.

[0018] In the above figures: 1. ultra-weak fiber grating array; 2. fiber-reinforced composite optical fiber core; 3. steel cable; 4. sheath. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] As a preferred embodiment of the present invention, Figure 1 As shown, this embodiment provides a distributed strain optical cable based on ultra-weak fiber grating array, such as Figure 1 As shown, including: A fiber-reinforced composite optical fiber core 2 with an ultra-weak fiber grating array 1 inscribed thereon, located in the center; A steel cable 3 twisted and surrounded around the fiber-reinforced composite optical fiber core 2; The steel cable 3 is externally coated with a sheath 4 formed by thermoplasticization of a high-density polyethylene material.

[0021] In some embodiments, the fiber reinforced composite material 2 is pre-coated with a heat-resistant layer by the bend-resistant optical fiber, and then hot-pultruded with the fiber reinforced composite material and resin. Fiber reinforced composite material, called Fiber Reinforced Polymer, abbreviated as FRP, has the following characteristics: high specific strength, large specific modulus; designable material properties; good corrosion resistance and durability; thermal expansion coefficient is similar to that of concrete. These characteristics enable FRP materials to meet the needs of modern structures to develop towards large spans, towering, heavy loads, light weight and high strength, and work under harsh conditions, while also meeting the requirements of the industrial development of modern construction. Therefore, it is widely used in various civil buildings, bridges, highways, oceans, hydraulic structures, and underground structures. This embodiment uses high-strength glass fibers and resin glue to mix first, and then pultrusion molding and heat curing into a rigid structure, which solves the problem of "strain dead zone" that exists after the optical fiber is prepared into a cable.

[0022] In some embodiments, the ultra-weak fiber grating array 1 includes multiple ultra-weak fiber gratings with a reflectivity lower than 0.1%, and the spacing between two adjacent ultra-weak fiber gratings is 0.1m-5m, and the optional spacing is 0.2m, 0.4m, 0.8m, 0.9m, 1.0m, 1.1, 1.2m, 1.3m, 1.4m, 1.5m, 1.6m, 2m or 3m, etc., and can also be other spacings.

[0023] In some preferred embodiments, the ultra-weak fiber grating array 1 is prepared online by a drawing tower process, and the multi-wavelength sensing unit is prepared by automatic mask replacement. Generally, three wavelengths are alternated. When the reflectivity of the grating is less than 0.1%, an ultra-weak fiber grating array can multiplex thousands of gratings at the same wavelength.

[0024] In this embodiment, the diameter of the fiber-reinforced composite optical fiber core 2 is 1 mm or 2 mm.

[0025] In some embodiments, the diameter of the steel cable 3 is the same as that of the fiber-reinforced composite optical fiber core 2 , and the steel cable 3 is twisted and surrounded in a spiral form around the fiber-reinforced composite optical fiber core 2 .

[0026] In some preferred embodiments, the sheath 4 is formed of high-density polyethylene thermoplastic, and is in close contact with the steel cable 3. The thickness of the sheath 4 is about 1 mm to enhance the mechanical properties of the optical cable.

[0027] The distributed strain optical cable based on ultra-weak fiber grating array provided by the present invention is put into a tensile machine. After the tensile test, the strain-wavelength curve of the steel cable distributed strain optical cable is as follows: Figure 2As shown, the horizontal axis is the strain applied to the optical cable, and the vertical axis is the central wavelength of the grating. The test results show that the strain is linearly related to the wavelength, and the strain sensitivity is better than 0.99, the linearity is better than 0.99, the strain transmission efficiency is high, and there is no "strain dead zone".

[0028] The distributed strain optical cable based on ultra-weak fiber grating array provided by the present invention is placed in a constant temperature box and connected to an ultra-weak fiber grating demodulator to conduct a temperature experiment. The temperature-wavelength curve of the steel cable distributed strain optical cable is shown in FIG. Figure 3 As shown in the figure, the horizontal axis is temperature and the vertical axis is the central wavelength of the grating. The test results show that the temperature and wavelength are linearly related, and the temperature sensitivity is , and the linearity reaches 99.93%.

[0029] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A distributed strain optical cable based on ultra-weak fiber grating array, characterized in that: include: A fiber-reinforced composite optical fiber core (2) with an ultra-weak fiber grating array (1) inscribed thereon, located at the center; A steel cable (3) twisted and wrapped around the fiber-reinforced composite optical fiber core (2); The steel cable (3) is externally coated with a high-density polyethylene material and then thermoplasticized to form a sheath (4).

2. A distributed strain optical cable based on ultra-weak fiber grating array according to claim 1, characterized in that: The fiber-reinforced composite optical fiber core (2) is formed by pre-coating a bend-resistant optical fiber with a temperature-resistant layer, and then hot-pultruding the fiber-reinforced composite material and resin.

3. A distributed strain optical cable based on ultra-weak fiber grating array according to claim 1, characterized in that: The ultra-weak fiber grating array (1) comprises a plurality of ultra-weak fiber gratings with a reflectivity lower than 0.1%, and the spacing between two adjacent ultra-weak fiber gratings is 0.1 m-5 m.

4. A distributed strain optical cable based on ultra-weak fiber grating array according to claim 1, characterized in that: The ultra-weak fiber grating array (1) is prepared online using a drawing tower process, and the preparation of a multi-wavelength sensing unit is achieved through automatic replacement of a mask.

5. A distributed strain optical cable based on ultra-weak fiber grating array according to claim 4, characterized in that: The fiber-reinforced composite optical fiber core (2) has a diameter of 1 mm or 2 mm.

6. The distributed strain optical cable based on ultra-weak fiber grating array according to claim 1, characterized in that: The diameter of the steel cable (3) is the same as that of the fiber-reinforced composite optical fiber core (2), and is twisted and surrounded in a spiral form around the fiber-reinforced composite optical fiber core (2).

7. The distributed strain optical cable based on ultra-weak fiber grating array according to claim 1, characterized in that: The sheath (4) is formed by thermoplasticization of high-density polyethylene and is in close contact with the steel cable (3).

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