Distributed optical fiber sensing structure, method and sensing system

Abstract

The invention discloses a distributed optical fiber sensing structure, a method and a sensing system. When the distributed optical fiber sensing structure capable of simultaneously measuring temperature and refractive index is used for detection, a tunable laser light source emits detection light of linear sweep tuning; due to the design of the light reflection structure and the reflection film, the detection light is reflected to the third fiber core from the second fiber core of the double-core single-mode optical fiber; a refractive index sensitive area is formed on the outer side of the cladding of the second fiber core due to the design of a groove, so that the second fiber core is sensitive to both the temperature and the refractive index, and the third fiber core is only sensitive to the temperature; the change of the refractive index and the temperature can cause the deviation of the frequency spectrum, so that the refractive index value and the temperature value at the same position can be obtained through algorithm processing; and the third fiber core not only measures the temperature, but also performs temperature compensation on the second fiber core at the same position, so that the measured refractive index value is more accurate.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed optical fiber sensors, and particularly to a distributed optical fiber sensing structure, a manufacturing method and a parameter measurement method capable of simultaneously detecting temperature and refractive index, and a sensing system having the distributed optical fiber sensing structure. Background Art

[0002] With the advancement of the informatization process, the industrial field is evolving towards intelligence and networking. As a key link, information perception needs to accurately obtain external information in a timely manner. For example, in scenarios such as oil and gas pipeline leakage detection and chemical process monitoring, it is necessary to sense abnormal changes in the temperature and refractive index of the surrounding medium of the pipeline in real time to accurately locate tiny leakage points or dynamically track the diffusion path of pollutants. Currently, fiber optic sensing technology faces the problems of distributed and simultaneous multi-parameter sensing.

[0003] Patent Document 1 [CN205262638U] discloses a dual-core photonic crystal fiber sensor for simultaneous measurement of temperature and strain. This patent uses refractive index matching liquid to fill the air holes around one of the cores, achieving simultaneous measurement of temperature and strain. It utilizes the characteristics of the dual-core photonic crystal fiber. The air holes around one core are filled with refractive index matching liquid, making it respond differently to changes in temperature and strain. Then, the change amount of a single parameter is calculated through a demodulation method.

[0004] Patent Document 2 [CN105954231A] discloses a hole-assisted dual-core fiber sensor based on the mechanism of mode coupling. By performing operations such as chemical etching or laser micromachining on the eccentric core of the hole-assisted dual-core fiber to expose it to the outside, the sensing and measurement of physical quantities such as refractive index and gas concentration are realized. This sensor has the advantages of low processing cost, simple manufacturing process, and small device insertion loss, and can conveniently form a microfluidic channel device to achieve multi-parameter sensing.

[0005] Patent Document 3 [CN112665658A] discloses an optical fiber sensor and a preparation method for simultaneous measurement of refractive index and temperature. It uses a double-clad fiber and forms a tapered fiber through the fused biconical taper technique, deposits a gold film on the surface to form an SPR sensing region, and realizes simultaneous measurement of refractive index and temperature using a dual-channel sensing technique. It uses a double-clad fiber with a high-low-high refractive index distribution at the end face as the sensing fiber. The middle low-refractive-index cladding enables each channel to work independently without interference, effectively avoiding the cross-sensitivity problem during temperature and refractive index measurement, and the manufacturing process is relatively simple and the cost is low.

[0006] The above three patent documents all disclose point-type sensing technologies. Although this technology has the advantages of multi-parameters and high sensitivity, it cannot achieve spatial positioning, has a low spatial resolution, and a limited measurement distance.

[0007] Although distributed sensing technology has a large number of sensing points, it has a single parameter and low sensitivity. How to achieve distributed sensing and multi-parameter sensing simultaneously with a single sensing system has become a technical problem that urgently needs to be solved in the field of optical fiber sensing. Traditional refractive index sensors are mostly limited to single-point or discrete measurements, which are difficult to meet the distributed monitoring requirements in complex environments, and cannot achieve multi-parameter sensing with the same sensor.

[0008] Patent Document 4 [CN105092535B] discloses a distributed surface plasmon resonance optical fiber sensor. The distributed surface plasmon resonance optical fiber sensor in this solution processes paired V-grooves on a dual-core optical fiber, and a sensing layer is deposited on the inclined surface of the V-groove. By using the reflection of light in the V-groove and the principle of exciting SPR, distributed sensing is achieved. It can fabricate multiple groups of sensing areas on the side of the optical fiber, and use the special structure of the optical fiber to connect multiple sensing areas in series, thereby realizing multi-channel real-time measurement.

[0009] Patent Document 4 focuses on the distributed measurement of parameters such as refractive index or gas concentration through the SPR effect, and there is no direct description and corresponding structural design for simultaneously measuring other parameters such as temperature, and temperature compensation is not achieved, and the measured value is easily affected by temperature. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide a distributed optical fiber sensing structure, manufacturing method and sensing system for simultaneously measuring temperature and refractive index, so as to solve the problem that traditional sensors are difficult to meet the distributed detection requirements in complex environments.

[0011] The technical solution adopted by the present invention to solve the above technical problems is as follows: A distributed optical fiber sensing structure includes a single-core single-mode optical fiber and a dual-core single-mode optical fiber; the single-core single-mode optical fiber includes a first core and a first cladding; the dual-core single-mode optical fiber includes a second core, a third core and a second cladding;

[0012] The first core at the end of the single-core single-mode optical fiber is coaxially aligned and connected with the second core at the beginning of the dual-core single-mode optical fiber; at least one groove is processed on the side of the second cladding close to the second core, and the bottom of the groove is processed to the surface or inside of the first core of the dual-core single-mode optical fiber;

[0013] The end of the dual-core single-mode optical fiber is processed into a light reflection structure; the light reflection structure enables the transmitted light in the dual-core single-mode optical fiber to be reflected from one core to the other core for transmission.

[0014] The preferred technical solution adopted by the present invention to solve the above technical problems is: The second core and the third core of the dual-core single-mode optical fiber are symmetrically arranged about the central axis.

[0015] The preferred technical solution adopted by the present invention to solve the above technical problems is as follows: the light reflection structure is a conical structure; the light reflection structure has a first reflection surface and a second reflection surface; there is an included angle between each reflection surface and the normal plane of the central axis of the dual-core single-mode optical fiber; the transmitted light in the dual-core single-mode optical fiber is transmitted from one core to the first reflection surface, reflected by the first reflection surface to the second reflection surface, and then reflected by the second reflection surface to be transmitted in the other core.

[0016] The preferred technical solution adopted by the present invention to solve the above technical problems is as follows: a reflective film is coated on the light reflection structure.

[0017] The preferred technical solution adopted by the present invention to solve the above technical problems is as follows: the light reflection structure is a conical structure.

[0018] The preferred technical solution adopted by the present invention to solve the above technical problems is as follows: the single-core single-mode optical fiber and the dual-core single-mode optical fiber have the same diameter.

[0019] Another technical solution of the present invention to solve the above technical problems is: a manufacturing method of a distributed optical fiber sensing structure, including the following steps:

[0020] Align and connect the first core of the single-core single-mode optical fiber with the second core at the starting end of the dual-core single-mode optical fiber;

[0021] Process the end of the dual-core single-mode optical fiber to form a light reflection structure;

[0022] Process at least one groove on the side of the dual-core single-mode optical fiber close to the second core.

[0023] The preferred solution adopted by another technical solution of the present invention to solve the above technical problems is: prepare a reflective film on the surface of the light reflection structure.

[0024] Another technical solution of the present invention to solve the above technical problems is: a distributed optical fiber sensing system, wherein the detection light source is connected to the input end of the first optical fiber coupler, and the output end of the first optical fiber coupler is respectively connected to the second optical fiber coupler and the third optical fiber coupler;

[0025] One output end of the second optical fiber coupler is connected to the first input end of the fourth optical fiber coupler; the other output end of the second optical fiber coupler is connected to the second input end of the fourth optical fiber coupler through a delay optical fiber; the output end of the fourth optical fiber coupler is connected to the first photodetector;

[0026] The first output end of the third optical fiber coupler is connected to an optical fiber circulator, and the optical fiber circulator is connected to the distributed optical fiber sensing structure described above; the output end of the optical fiber circulator is connected to a first polarization beam splitter; the output ends of the first polarization beam splitter are respectively connected to the input ends of a fifth optical fiber coupler and a sixth optical fiber coupler;

[0027] The second output end of the third optical fiber coupler is connected to a second polarization beam splitter, and the second polarization beam splitter is respectively connected to the input ends of the fifth optical fiber coupler and the sixth optical fiber coupler;

[0028] The output end of the fifth optical fiber coupler is connected to a second photodetector; the output end of the sixth optical fiber coupler is connected to a third photodetector;

[0029] The output ends of the first photodetector, the second photodetector and the third photodetector are connected to a data acquisition card, and the data acquisition card is connected to a processor.

[0030] Another technical solution of the present invention for solving the above technical problems is: a parameter measurement method using the distributed optical fiber sensing structure described above, including the following steps:

[0031] Demodulate the sensing signals of several reference positions on the third fiber core to obtain the temperatures of several reference positions on the third fiber core; the reference positions on the third fiber core are symmetrically arranged one by one with the target positions of the corresponding fiber core parts of the second fiber core in the groove;

[0032] Determine the temperature of the reference position on the third fiber core as the temperature of the corresponding target position;

[0033] Decouple the sensing signal of the target position according to the temperature of the target position to obtain the refractive index information of the target position.

[0034] Compared with the prior art, the advantages of the present invention are: when detecting with the distributed optical fiber sensing structure for simultaneous measurement of temperature and refractive index, the tunable laser light source emits linearly swept and tuned detection light. Due to the design of the optical reflection structure and the reflection film, the detection light is reflected from the second fiber core of the dual-core single-mode optical fiber to the third fiber core. The outer side of the cladding of the second fiber core forms a refractive index sensitive area due to the design of the groove, while the third fiber core is only sensitive to temperature. Both the refractive index and temperature changes can cause the spectrum to shift. Therefore, the refractive index value and temperature value at the same position can be obtained through algorithm processing. The third fiber core not only measures temperature, but also compensates the temperature of the second fiber core at the same position, making the measured refractive index value more accurate. The present invention realizes the distributed measurement of temperature and refractive index dual parameters by using a dual-core optical fiber as the sensing optical fiber and carrying out microstructural design on the sensing optical fiber, overcoming the problem that multiple parameters cannot be demodulated in traditional distributed measurement. Description of the Drawings

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and preferred embodiments. However, those skilled in the art will appreciate that these drawings are only drawn for the purpose of explaining the preferred embodiments and should not be construed as limiting the scope of the present invention. In addition, unless otherwise specified, the drawings only schematically show the composition or structure of the described object and may include exaggerated displays, and the drawings are not necessarily drawn to scale.

[0036] Figure 1 Schematic diagram of an embodiment of a distributed optical fiber sensing structure;

[0037] Figure 2 Cross-sectional view of an embodiment of a distributed optical fiber sensing structure;

[0038] Figure 3 Schematic diagram of another embodiment of a distributed optical fiber sensing structure;

[0039] Figure 4 Cross-sectional view of another embodiment of a distributed optical fiber sensing structure;

[0040] Figure 5 Schematic diagram of a distributed optical fiber sensing system;

[0041] Figure 6 Flowchart of a manufacturing method of a distributed optical fiber sensing structure.

[0042] Reference numerals:

[0043] Distributed optical fiber sensing structure 20; single-core single-mode optical fiber 100; dual-core single-mode optical fiber 200; first core 101;

[0044] First cladding 102; second core 201; third core 202; second cladding 203; groove 300; optical reflection structure 400; reflective film 500; detection light source 1; first optical fiber coupler 2; second optical fiber coupler 3; third optical fiber coupler 4; fourth optical fiber coupler 5; fifth optical fiber coupler 6; sixth optical fiber coupler 7; delay optical fiber 8; optical fiber circulator 10; first polarization beam splitter 11; second polarization beam splitter 12; first photodetector 9; second photodetector 13; third photodetector 14; data acquisition card 15; processor 16. Detailed implementation manners

[0045] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Those skilled in the art will appreciate that these descriptions are only descriptive and exemplary and should not be construed as limiting the protection scope of the present invention.

[0046] It should be noted that similar reference numerals represent similar items in the following drawings. Therefore, once an item is defined in one drawing, it will not be further defined or explained in subsequent drawings. For a clearer presentation of the structure, the proportions of the components in the drawings are not actual proportions.

[0047] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed during use. It is 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 construed as a limitation on the present invention. The terms "first" and "second" are only for the convenience of description and have no other directional meaning, and should not be construed as a limitation on the present invention.

[0048] As Figures 1-4 shown, this embodiment provides a distributed optical fiber sensing structure 20. The sensing optical fiber includes a single-core single-mode optical fiber 100 and a dual-core single-mode optical fiber 200. The single-core single-mode optical fiber 100 includes a first core 101 and a first cladding 102. The dual-core single-mode optical fiber 200 includes a second core 201, a third core 202, and a second cladding 203.

[0049] As Figures 1-4 shown, the first core 101 at the end of the single-core single-mode optical fiber 100 is connected to the second core 201 at the beginning of the dual-core single-mode optical fiber 200.

[0050] On one side of the second cladding 203 of the dual-core single-mode optical fiber 200 close to the second core 201, a plurality of grooves 300 are processed. The bottom of the groove 300 is processed to the surface or inside of the first core 101 of the dual-core single-mode optical fiber 200. The end of the dual-core single-mode optical fiber 200 is processed into an optical reflection structure 400. The optical reflection structure 400 enables the transmitted light in the dual-core single-mode optical fiber 200 to be reflected from one core to the other core for transmission.

[0051] As Figures 1-2 shown, one groove is processed on one side of the second cladding 203 close to the second core 201. As Figures 3-4 shown, a plurality of spaced-apart grooves 300 are processed on one side of the second cladding 203 close to the second core 201. The setting of multiple grooves can achieve multi-channel measurement based on measuring one object through one channel. For example, it can measure the parameters of different liquids.

[0052] The opening size and processing depth of the grooves 300 are the same, and the spacing between adjacent grooves 300 is the same. Of course, in other solutions, the opening size and processing depth of each groove 300 can be different, and the spacing between adjacent grooves 300 can be the same or different.

[0053] The structure provided by this embodiment facilitates multi-parameter demodulation during subsequent modulation. Through the optical reflection structure, the backscattered Rayleigh light in the third core 202 is sequentially transmitted back to the second core 201 and the first core 101. Since the third core 202 is always enclosed in the second cladding 203, it is only sensitive to the ambient temperature. The temperatures of the second and third cores 201 and 202 at any location on the dual-core single-mode optical fiber 200 can be considered equal. Therefore, during subsequent demodulation, the temperature of the corresponding sensing point on the third core 202 can be demodulated to obtain the temperature of the corresponding sensing point on the second core 201. Since the light transmitted by the core portion of the second core 201 corresponding to the groove is sensitive to the refractive index of the external medium and the ambient temperature, when demodulating the measurement results of the corresponding sensing point on the second core 201, the temperature of the corresponding sensing point can be used to compensate for the measurement results, thereby obtaining refractive index information, thereby achieving demodulation of both refractive index and temperature information.

[0054] The first fiber core 101 at the end of a single-core single-mode optical fiber 100 is coaxially aligned and connected to the second fiber core 201 at the beginning of a dual-core single-mode optical fiber 200. This coaxial connection ensures efficient transmission of optical signals between the fibers, reduces light loss and reflections at the connection, and improves the stability and signal quality of the entire sensing structure.

[0055] Specifically, the light-reflecting structure 400 comprises a first reflective surface and a second reflective surface; each reflective surface forms an angle with the normal plane to the central axis of the dual-core single-mode optical fiber 200; the transmitted light in the dual-core single-mode optical fiber 200 is transmitted from one core to the first reflective surface, reflected from the first reflective surface to the second reflective surface, and then reflected from the second reflective surface to the other core for transmission. When light enters from the single-core single-mode optical fiber 100, the configuration of the groove 300 allows the evanescent field of the light wave transmitted within the second core 201 of the dual-core single-mode optical fiber 200 to penetrate the external medium, thereby making the second core 201 sensitive to the refractive index, improving the optical fiber's ability to sense refractive index, and thus enabling accurate detection of subtle changes in the refractive index of the surrounding environment, expanding the application potential of optical fibers in fields such as chemical and biological sensing.

[0056] The third core 202 is coated in the second cladding 203. It is only sensitive to temperature and serves as a dedicated temperature measurement core. It can accurately obtain ambient temperature information, is independent of the refractive index measurement core, and obtains a set of accurate temperature data independent of the refractive index. Since the sensing optical fiber has both a refractive index measurement core and a temperature measurement core at the same position, the temperature data measured by the temperature measurement core can be directly used to compensate for the drift value generated by the refractive index measurement core due to temperature changes. Through real-time temperature compensation, the influence of temperature fluctuations on the refractive index measurement result can be effectively eliminated, the accuracy and reliability of the refractive index measurement can be significantly improved, and the measurement performance of the sensor in a complex environment can be made more stable.

[0057] Preferably, a reflective film 500 is plated on the optical reflection structure 400. The reflective film 500 further improves the reflection effect and can play an isolation role to prevent the liquid environment from affecting the Fresnel reflection.

[0058] As Figure 1 shown, at the groove 300, each unit distance is an independent sensing unit as the sensing area, and distributed sensing can be realized along the length direction of the optical fiber. This distributed characteristic can continuously monitor the refractive index and temperature changes at different positions along the optical fiber, providing an effective solution for long-distance and multi-point environmental parameter monitoring. For example, full-distributed and real-time physical quantity monitoring can be realized in fields such as large-scale infrastructure (bridges, tunnels, pipelines, etc.) and environmental monitoring.

[0059] It should be noted that the design concept and working principle of this sensing structure are universal. According to different application scenarios and measurement requirements, by adjusting parameters such as the geometric shape, size, distribution density of the groove 300, and the length of the optical fiber, a distributed optical fiber sensor that meets specific requirements can be flexibly customized to meet diverse monitoring needs.

[0060] As Figure 2 shown, preferably, the single-core single-mode optical fiber 100 is a standard single-mode optical fiber, and the first core 101 is located at the exact middle position of the cross-section. The second core 201 and the third core 202 of the dual-core single-mode optical fiber 200 are symmetrically arranged about the central axis. There is no mutual coupling of energy between the two cores. The optical reflection structure 400 is a conical structure.

[0061] In this embodiment, as Figures 1-4 shown, the single-core single-mode optical fiber 100 and the dual-core single-mode optical fiber 200 have the same diameter. The second core 201 and the third core 202 are respectively located at one-third of the diameter of the dual-core single-mode optical fiber 200. It should be noted that the distance between the second core 201 and the third core 202 of the dual-core single-mode optical fiber 200 exceeds the penetration depth of the evanescent field, and the distance between the cores can be set according to actual design requirements such as the target loss.

[0062] Preferably, the cone angle of the light reflection structure 400 with a conical structure is 90 degrees. The light reflection structure 400 adopts a conical structure, and its processing technology is relatively simple and mature. Compared with complex curved surfaces or special-shaped structures, the processing accuracy of a cone is easy to control, which can effectively reduce production costs and improve production efficiency.

[0063] Furthermore, the reflective film can be a metal film or a multi-layer dielectric film. A metal film is preferred, such as a high-reflectivity film of gold, silver, etc. The high-reflectivity characteristic can significantly improve the light reflection efficiency. When light is incident on the light reflection structure 400 of the dual-core single-mode fiber 200, the high-reflectivity film can reflect most of the incident light. Compared with the ordinary reflective film 500, it can effectively reduce the loss of light energy during the reflection process, enabling more light energy to be reflected from one core and enter the other core for continued transmission. This ensures the intensity and quality of the optical signal, providing a more sufficient light energy basis for subsequent sensing measurements. Since the high-reflectivity film reduces energy loss and enhances the reflected light energy, the optical signal reflected from the second core 201 into the third core 202 is more stable and its intensity is easier to accurately detect. In distributed fiber optic sensing, the strength and stability of the optical signal directly affect the measurement accuracy of physical quantities such as refractive index and temperature. The application of the high-reflectivity film can reduce signal attenuation and distortion problems caused by light energy loss, thereby improving the detection sensitivity of the entire sensing system to refractive index and temperature changes, making the measurement results more accurate and reliable, and being able to more subtly reflect the actual changes in the measured environmental parameters.

[0064] Specifically, the method for measuring the parameters of the distributed fiber optic sensing structure includes the following steps:

[0065] First, demodulate the sensing signals at several reference positions on the third core 202 to obtain the temperatures at several reference positions on the third core 202. The reference positions on the third core 202 are symmetrically arranged one-to-one with the target positions of the corresponding core part of the second core 201 in the groove 300. Then, determine the temperature at the reference position on the third core 202 as the temperature at the corresponding target position. Finally, decouple the sensing signal at the target position based on the temperature at the target position to obtain the refractive index information at the target position.

[0066] As Figure 6 shown, this embodiment provides a manufacturing method for this distributed fiber optic sensing structure 20, including the following steps:

[0067] Step S1: Align and connect the first core 101 of the single-core single-mode fiber 100 with the second core 201 at the starting end of the dual-core single-mode fiber 200.

[0068] Step S2: Process the end of the dual-core single-mode fiber 200 to form the light reflection structure 400.

[0069] Step S3: Prepare a reflective film 500 on the surface of the optical reflection structure 400; this step is not necessary and is an optional technical means.

[0070] Step S4: Process a plurality of grooves 300 on one side of the dual-core single-mode fiber 200 close to the second core 201, so that the evanescent field of the light wave transmitted in the second core 201 of the dual-core single-mode fiber 200 can penetrate into the external medium.

[0071] In a specific implementation, in step S1, an optical fiber fusion splicer is used to achieve precise alignment and fusion of the first core 101 of the single-core single-mode fiber 100 and the second core 201 at the starting end of the dual-core single-mode fiber 200. The optical fiber fusion splicer has high-precision core alignment and fusion functions, which can ensure perfect butt joint between the cores, thereby effectively reducing the optical loss and reflection loss at the connection, and ensuring the efficient transmission of the optical signal at the fiber connection.

[0072] In step S2, an optical fiber end face polishing machine is used to process the end of the dual-core single-mode fiber 200 to form an optical reflection structure 400. This device can precisely control the shape and surface flatness of the end face. Through the fine processing of the optical fiber end face polishing machine, the geometric shape and surface quality of the optical reflection structure 400 are optimized, ensuring the stability and directivity of the reflected light, providing a reliable structural basis for subsequent optical reflection and transmission, and avoiding light scattering and energy loss caused by poor processing of the reflection structure.

[0073] In step S3, femtosecond laser etching, focused ion beam etching or optical fiber side polishing is used to process the grooves 300. These existing processing technologies can achieve processing accuracy at the micron or even nanometer level, and can precisely control geometric parameters such as the depth, width and spacing of the grooves 300. This enables the bottom of the grooves 300 to be accurately processed to the surface or inside of the first core 101 of the dual-core single-mode fiber 200, thereby optimizing the interaction between the evanescent field of the light wave and the external medium, further improving the sensitivity of the optical fiber to refractive index changes and the measurement accuracy, and providing more accurate physical quantity detection capabilities for distributed sensing.

[0074] In step S4, a magnetron sputtering method is used to prepare a reflective film 500 on the surface of the optical reflection structure 400. This method can achieve high-precision thin film deposition and precisely control the thickness and uniformity of the reflective film 500. The uniform and dense reflective film 500 can effectively improve the surface reflectivity, reduce energy loss, and enhance the light energy reflected from one core into the other core, thereby ensuring the test accuracy. At the same time, the magnetron sputtering technology can ensure good adhesion between the reflective film 500 and the optical reflection structure 400, making it not easy to fall off or be damaged during long-term use, further improving the stability and reliability of the reflective film 500, and extending the service life of the sensor.

[0075] AsFigure 5 As shown in the figure, this embodiment provides a distributed optical fiber sensing system applying such a distributed optical fiber sensing structure 20. It includes a detection light source 1, a first optical fiber coupler 2, a second optical fiber coupler 3, a third optical fiber coupler 4, a fourth optical fiber coupler 5, a fifth optical fiber coupler 6, a sixth optical fiber coupler 7, a delay optical fiber 8, an optical fiber circulator 10, a first polarization beam splitter 11, a second polarization beam splitter 12, a first photodetector 9, a second photodetector 13, a third photodetector 14, a data acquisition card 15, a processor 16, and a distributed optical fiber sensing structure 20.

[0076] In terms of structure, as Figure 4 shown, the detection light source 1 is connected to the input end of the first optical fiber coupler 2, and the output end of the first optical fiber coupler 2 is respectively connected to the second optical fiber coupler 3 and the third optical fiber coupler 4.

[0077] One output end of the second optical fiber coupler 3 is connected to the first input end of the fourth optical fiber coupler 5; the other output end is connected to the second input end of the fourth optical fiber coupler 5 through the delay optical fiber 8. The output end of the fourth optical fiber coupler 5 is connected to the first photodetector 9.

[0078] The first output end of the third optical fiber coupler 4 is connected to the optical fiber circulator 10, and the optical fiber circulator 10 is connected to the distributed optical fiber sensing structure 20. The output end of the optical fiber circulator 10 is connected to the first polarization beam splitter 11. The output ends of the first polarization beam splitter 11 are respectively connected to the input ends of the fifth optical fiber coupler 6 and the sixth optical fiber coupler 7. The second output end of the third optical fiber coupler 4 is connected to the second polarization beam splitter 12, and the second polarization beam splitter 12 is respectively connected to the input ends of the fifth optical fiber coupler 6 and the sixth optical fiber coupler 7. The output end of the fifth optical fiber coupler 6 is connected to the second photodetector 13; the output end of the sixth optical fiber coupler 7 is connected to the third photodetector 14.

[0079] The output ends of the first photodetector 9, the second photodetector 13, and the third photodetector 14 are connected to the data acquisition card 15, and the data acquisition card 15 is connected to the processor 16.

[0080] The following further elaborates on the specific working method of this distributed optical fiber sensing system.

[0081] The detection light source 1 emits linearly tuned laser light, which is divided into sensing light and sampling clock light by the first optical fiber coupler 2. The sensing light is divided into signal light and reference light by the third optical fiber coupler 4.

[0082] The signal light enters the single-core single-mode optical fiber 100 of the sensing optical fiber through the optical fiber circulator 10, and is transmitted successively through the first core 101 of the single-core single-mode optical fiber 100 and the second core 201 of the dual-core single-mode optical fiber 200; then it is reflected by the optical reflection structure 400 and the reflection film 500 and enters the third core 202 of the dual-core single-mode optical fiber 200 for transmission.

[0083] The backward scattered light of the sensing optical fiber formed by the transmission of the signal light is output from the end of the single-core single-mode optical fiber 100 to the optical fiber circulator 10; and returns from the optical fiber circulator 10 to reach the first polarization beam splitter 11; the first polarization beam splitter 11 divides the returned signal light into signal polarization p-light and signal polarization s-light.

[0084] The reference light is divided into reference polarization p-light and reference polarization s-light by the second polarization beam splitter 12. The signal polarization p-light and the reference polarization p-light enter the fifth optical fiber coupler 6 to form a mixed-frequency p-light; the signal polarization s-light and the reference polarization s-light enter the sixth optical fiber coupler 7 to form a mixed-frequency s-light. The mixed-frequency p-light and the mixed-frequency s-light are respectively detected by the third photodetector 14 and the second photodetector 13; the data acquisition card 15 is used to collect the signals of the third photodetector 14 and the second photodetector 13.

[0085] The sampling clock light is divided into the first sampling clock light and the second sampling clock light by the second optical fiber coupler 3; the first sampling clock light is directly connected to one input end of the fourth optical fiber coupler 5; the second sampling clock light reaches the other input end of the fourth optical fiber coupler 5 through the delay optical fiber 8. The first sampling clock light and the second sampling clock light generate a mixed-frequency optical signal at the output end of the fourth optical fiber coupler 5; the mixed-frequency optical signal is collected by the first photodetector 9 as a sampling clock signal and input to the data acquisition card 15 as the sampling clock of the data acquisition card 15.

[0086] The processor 16 receives and processes the signals collected by the data acquisition card 15, and demodulates the temperature and refractive index distributions on the sensing optical fiber through the spectral demodulation method.

[0087] Based on the detection characteristics of the linearly tunable light source, the backward scattered beat signal generated by the mixing of the signal light and the reference light has the characteristic of position correlation. The beat frequency is proportional to the transmission time delay difference and the transmission distance. That is to say, the frequency value has a linear relationship with the transmission time delay difference and the distance parameter. The spatial positioning of the event can be completed by analyzing the frequency domain information. Therefore, the distributed sensing of the entire optical fiber can be realized.

[0088] The system acquires dual-channel data and performs fast Fourier transforms on each channel, converting the optical frequency domain signal into a spatial domain signal. Windowing and inverse Fourier transforms are then performed to obtain two sets of local Rayleigh scattering spectrum data. When external physical quantities act on specific locations on the optical fiber, the corresponding local Rayleigh scattering spectrum will produce an optical frequency shift. Using a cross-correlation algorithm to analyze this spectral shift, the degree of parameter change can be precisely quantified.

[0089] Notably, the system features a dual-core design for parameter decoupling: the third fiber core 202, the temperature measurement core, is dedicated to temperature monitoring and simultaneously provides a temperature compensation reference for the second fiber core 201, the refractive index measurement core. This effectively eliminates the cross-sensitivity effects of temperature on refractive index measurement, significantly improving refractive index detection accuracy. Furthermore, the system employs a dual-polarization receive architecture, effectively suppressing polarization noise through polarization diversity processing, improving the system's signal-to-noise ratio and thus ensuring the reliability of measurement results.

[0090] Specifically, the signal demodulation method uses spectral demodulation. Spectral demodulation involves capturing the acquired time-domain signal. A fixed moving window is used to intercept the signal segments at the same location of the two sets of signals, representing a sensing unit. The time-domain signal of a sensing unit is Fourier transformed to obtain the frequency-domain signal of the optical fiber at that location. This is done by mapping the signal to the frequency domain. Because the frequency domain is linearly related to distance, each component in the frequency domain corresponds to a specific location on the optical fiber. The relationship between beat frequency and distance is shown in the following equation:

[0091]

[0092] where f beat is the beat frequency corresponding to distance l, f s is the sweep rate of the detection light source 1, n and c are the effective refractive index of the optical fiber and the speed of light, respectively.

[0093] The frequency domain signal is inverse Fourier transformed to convert it into a time domain signal to obtain the distance-frequency shift relationship curve. Each sensing unit is calculated one by one to obtain the Rayleigh scattering spectrum of the corresponding position of the entire optical fiber. The cross-correlation operation is used to obtain the spectrum drift, and the corresponding temperature and refractive index changes are demodulated.

[0094] Furthermore, changes in temperature or refractive index will cause changes in the local Rayleigh scattering spectrum. Under the combined effect of temperature and refractive index changes, the spectrum drift is:

[0095] Δv=K n Δn+K T ΔT (2)

[0096] Where Δv is the spectrum drift, K n and K TThey are the sensitivity responses corresponding to the refractive index and temperature changes respectively. Δn and ΔT correspond to the refractive index change and temperature change respectively.

[0097] By keeping the refractive index unchanged and performing temperature calibration through temperature tests, the drift amount of the spectrum at different temperatures can be obtained, and thus the K value can be obtained. T Similarly, with the temperature unchanged, by changing the refractive index value and measuring the spectrum drift amount, the K value can be obtained. n value.

[0098] Based on this, by demodulating the signals of the third core 202 - the temperature measurement core, the temperature data at each position can be obtained. Based on the temperature data obtained from the third core 202 - the temperature measurement core and the spectrum drift amount of the second core 201 - the refractive index measurement core, the refractive index change value at the same position can be obtained, thereby completing the simultaneous detection of temperature and refractive index along the sensing structure.

[0099] A distributed optical fiber sensing structure, manufacturing method, parameter measurement method, and sensing system provided by the present invention are introduced. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the present invention and its core idea. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A distributed optical fiber sensing structure, characterized in that: It includes a single-core single-mode optical fiber and a dual-core single-mode optical fiber; the single-core single-mode optical fiber includes a first core and a first cladding; the dual-core single-mode optical fiber includes a second core, a third core and a second cladding; The first core at the end of the single-core single-mode optical fiber is coaxially aligned and connected with the second core at the beginning of the dual-core single-mode optical fiber; at least one groove is machined on one side of the second cladding close to the second core, and the bottom of the groove is machined to the surface or inside of the first core of the dual-core single-mode optical fiber; The end of the dual-core single-mode optical fiber is processed into an optical reflection structure; the optical reflection structure enables the transmitted light in the dual-core single-mode optical fiber to be reflected from one core to the other core for transmission.

2. The distributed optical fiber sensing structure according to claim 1, characterized in that: The second core and the third core of the dual-core single-mode optical fiber are symmetrically arranged about the central axis.

3. The distributed optical fiber sensing structure according to claim 1, characterized in that: The optical reflection structure is a conical structure; the optical reflection structure has a first reflection surface and a second reflection surface; each reflection surface has an angle with the normal plane of the central axis of the dual-core single-mode optical fiber; the transmitted light in the dual-core single-mode optical fiber is transmitted from one core to the first reflection surface, reflected by the first reflection surface to the second reflection surface, and reflected by the second reflection surface to the other core for transmission.

4. The distributed optical fiber sensing structure according to claim 1, characterized in that: A reflective film is plated on the optical reflection structure.

5. The distributed optical fiber sensing structure according to claim 1, characterized in that: The optical reflection structure is a conical structure.

6. The distributed optical fiber sensing structure according to claim 1, characterized in that: The single-core single-mode optical fiber and the dual-core single-mode optical fiber have the same diameter.

7. A manufacturing method of a distributed optical fiber sensing structure as described in any one of claims 1-6, characterized in that: It includes the following steps: Align and connect the first core of the single-core single-mode optical fiber with the second core at the beginning of the dual-core single-mode optical fiber; Process the end of the dual-core single-mode optical fiber to form an optical reflection structure; Machine at least one groove on one side of the dual-core single-mode optical fiber close to the second core.

8. The manufacturing method of a distributed optical fiber sensing structure according to claim 7, characterized in that: Prepare a reflective film on the surface of the optical reflection structure.

9. A distributed optical fiber sensing system, characterized in that: The detection light source is connected to the input end of the first optical fiber coupler, and the output end of the first optical fiber coupler is respectively connected to the second optical fiber coupler and the third optical fiber coupler; One output end of the second optical fiber coupler is connected to the first input end of the fourth optical fiber coupler; the other output end of the second optical fiber coupler is connected to the second input end of the fourth optical fiber coupler through a delay optical fiber; the output end of the fourth optical fiber coupler is connected to the first photodetector; The first output end of the third optical fiber coupler is connected to an optical fiber circulator, and the optical fiber circulator is connected to a distributed optical fiber sensing structure as described in any one of claims 1-6; the output end of the optical fiber circulator is connected to a first polarization beam splitter; the output ends of the first polarization beam splitter are respectively connected to the input ends of a fifth optical fiber coupler and a sixth optical fiber coupler; The second output end of the third optical fiber coupler is connected to a second polarization beam splitter, and the second polarization beam splitter is respectively connected to the input ends of a fifth optical fiber coupler and a sixth optical fiber coupler; The output end of the fifth optical fiber coupler is connected to a second photodetector; the output end of the sixth optical fiber coupler is connected to a third photodetector; The output ends of the first photodetector, the second photodetector and the third photodetector are connected to a data acquisition card, and the data acquisition card is connected to a processor.

10. A parameter measurement method using the distributed optical fiber sensing structure as described in any one of claims 1-6, characterized in that: Demodulate the sensing signals at several reference positions on the third fiber core to obtain the temperatures at the several reference positions on the third fiber core; the reference positions on the third fiber core are symmetrically arranged one by one with the target positions of the corresponding fiber core parts of the second fiber core in the grooves; Determine the temperature at the reference position on the third fiber core as the temperature at the corresponding target position; Decouple the sensing signal at the target position according to the temperature at the target position to obtain the refractive index information at the target position.

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