Deformation sensor and deformation detection system based on double-core tilted fiber bragg grating

Abstract

The invention relates to a deformation sensor based on a double-core tilted fiber grating and a deformation detection system. Belongs to the technical field of safety monitoring. The sensor is composed of a double-core optical fiber, a single-frequency laser, a detector and a deformation carrier, wherein the double-core optical fiber comprises a first fiber core and a second fiber core which are inscribed with 45-degree inclined fiber gratings. The sensor detects light intensity attenuation caused by bending deformation of the deformation carrier by detecting the light intensity change of the second fiber core. The fiber bragg grating design of the sensor meets phase matching conditions, and accurate output of a deformation value is realized through a light intensity-deformation mapping algorithm. The deformation sensor can be widely applied to the fields of structural health monitoring, geological disaster early warning and the like. The invention further provides a deformation monitoring system based on the sensor. Real-time monitoring and early warning of high-risk areas such as landslide are achieved through a multi-point distributed monitoring system.

Description

Technical Field

[0001] The present invention belongs to the technical field of safety monitoring, and in particular relates to a deformation sensor based on a 45-degree dual-core tilted optical fiber Bragg grating. Background Art

[0002] With the development of modern industry, construction, infrastructure, and other fields, deformation monitoring has become an important means to ensure structural safety and improve project quality. Especially in projects that require long-term monitoring, such as bridges, tunnels, dams, pipelines, and areas at high risk of geological disasters (such as landslides and earthquake faults), obtaining real-time and accurate deformation information of the structure or geological environment is crucial for promptly identifying potential safety hazards and preventing accidents.

[0003] Existing deformation monitoring technologies include traditional methods such as strain gauges, lidar, and displacement sensors. While these methods have achieved some success in certain application scenarios, they still have limitations. For example, strain gauges are mostly used for local monitoring and are easily affected by factors such as ambient temperature and humidity over long periods of use, resulting in reduced measurement accuracy. While lidar can perform remote monitoring, it is easily interfered with by factors such as climate and obstacles in complex environments, and its monitoring accuracy is limited. Displacement sensors are generally suitable for small-scale measurements, but are difficult to deploy and costly for large-scale, distributed monitoring.

[0004] Therefore, with the advancement of fiber-optic sensing technology, fiber-optic deformation sensors have gradually become a research hotspot. Fiber-optic sensors offer advantages such as immunity to electromagnetic interference, adaptability to harsh environments, high precision, and ease of distributed deployment. Fiber Bragg grating (FBG) technology, in particular, has been widely used in fields such as structural health monitoring and geological disaster early warning, thanks to its excellent response characteristics to physical quantities such as temperature, pressure, and deformation.

[0005] Traditional fiber Bragg grating (FBG) sensors typically use a single-core structure. The relationship between the grating period and the strain of the fiber is relatively simple, and they are easily affected by temperature fluctuations, resulting in inaccurate monitoring data. To improve the accuracy and robustness of fiber optic sensors, dual-core FBG technology has emerged. By integrating two optical cores within the same fiber, dual-core FBG sensors can effectively reduce the impact of temperature fluctuations on measurement results. Furthermore, by coupling optical signals from different cores, they can more accurately measure deformation.

[0006] Furthermore, by using a tilted fiber Bragg grating (FBG) structure, the phase matching conditions of the FBG are optimized by adjusting the grating's tilt angle, further improving the sensor's sensitivity and measurement accuracy. This new fiber Bragg grating sensor combines the advantages of dual-core fiber with the characteristics of tilted FBG, enabling high-precision and high-sensitivity deformation monitoring. It is widely used in building structures, geological disaster warnings, smart infrastructure, and other fields.

[0007] However, existing dual-core fiber Bragg grating (FBG) deformation sensors still have certain deficiencies in data processing and application in deformation monitoring. In particular, efficient processing of data from multiple sensors and providing real-time warnings in large-scale monitoring systems remains a challenge. Therefore, developing a high-precision, high-reliability deformation sensor based on a tilted dual-core fiber Bragg grating (FBG), combined with advanced data processing algorithms, can significantly improve the performance and reliability of monitoring systems. Summary of the Invention

[0008] This invention aims to address the shortcomings of existing technologies by proposing a deformation sensor and deformation detection system based on a dual-core tilted fiber Bragg grating (FBG). These sensors enable high-precision, long-term, distributed deformation monitoring in harsh environments, making them particularly suitable for structural health monitoring and geological disaster early warning. Using a light intensity-deformation mapping algorithm, they enable precise deformation data acquisition, and a multi-point synchronous monitoring system enables large-scale monitoring and early warning, further improving monitoring efficiency and early warning capabilities.

[0009] The present invention proposes a deformation sensor based on dual-core tilted fiber Bragg gratings. The core of the sensor is to achieve high-sensitivity measurement through the asymmetric deformation coupling effect of the dual-core optical fiber and eliminate temperature interference by utilizing the temperature consistency of the dual-core structure.

[0010] The present invention is implemented by the following technical solutions:

[0011] A dual-core optical fiber comprises a first fiber core and a second fiber core, wherein tilted fiber gratings are respectively engraved in the first fiber core and the second fiber core;

[0012] A single-frequency laser is connected to the input end of the first fiber core and is used to emit a single-frequency laser that meets the tilted fiber Bragg grating phase matching condition;

[0013] The detector is connected to the output end of the second fiber core and is used to detect the intensity of the optical signal coupled to the second fiber core;

[0014] The dual-core optical fiber is fixed on the surface or inside of the deformation carrier.

[0015] When the deformable carrier is bent and deformed, the grating periods of the first fiber core and the second fiber core change in opposite directions due to stretching or compression, causing the light intensity of the detector to attenuate.

[0016] The initial period of the tilted fiber Bragg grating satisfies the phase matching condition:

[0017] λ=2n eff Λ / cosθ

[0018] λ is the wavelength of the single-frequency laser, Λ is the grating period, θ = 45°, and neff is the effective refractive index of the fiber core.

[0019] The grating period Λ is 500 to 1500 nm, and the length is 10 to 30 mm.

[0020] The first fiber core and the second fiber core are in the same cladding, with a spacing of ≤50 μm, and the core materials of the two fiber cores are the same, so as to achieve the synchronous influence of temperature change on the gratings of the two fiber cores.

[0021] The tilt angle of the tilted fiber Bragg grating is 45°±5°.

[0022] The deformation carrier is a cantilever beam, a geological fracture insert or a pipe surface.

[0023] The dual-core optical fiber is fixed to the bending-sensitive area of the deformation carrier by epoxy resin glue or laser welding.

[0024] It also includes a signal processing unit, which is connected to the detector and has a built-in light intensity-deformation mapping algorithm. It outputs the deformation value by calibrating the correspondence between the light intensity attenuation rate and the deformation curvature.

[0025] A deformation monitoring system based on the deformation sensor based on the dual-core tilted fiber Bragg grating as described in claim 1, wherein the deformation sensor of the dual-core tilted fiber Bragg grating is connected in series or in parallel through optical fibers to form a distributed network and connected to a central processing terminal, thereby realizing synchronous monitoring of multi-point deformation.

[0026] The present invention achieves high-sensitivity measurement through the asymmetric deformation coupling effect of a dual-core optical fiber, and utilizes the temperature consistency of the dual-core structure to eliminate temperature interference. The dual-core structure naturally offsets the influence of temperature on the coupling conditions, eliminating the need for additional temperature compensation devices. By directly demodulating deformation through light intensity, complex wavelength tracking equipment is avoided, reducing costs. The dual-core spacing d and grating length can optimize sensitivity and range. The present invention achieves high-precision, low-cost deformation sensing through the asymmetric deformation response and temperature self-compensation characteristics of the dual-core tilted fiber Bragg grating. It is particularly suitable for complex environments such as geological disaster monitoring and has significant technical advantages and market application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a structural diagram of the dual-core optical fiber of the present invention.

[0028] Figure 2 This is the structural diagram of the sensor of the present invention

[0029] Figure 3 Schematic diagram of the deformation of the bent lower grating of the present invention DETAILED DESCRIPTION

[0030] As shown in the accompanying figure, dual-core fiber (DCF) is a fiber structure with two independent cores. Its key feature is that each fiber contains two independent optical transmission channels. Compared to traditional single-core fiber, DCF can transmit multiple optical signals within the same fiber, offering greater flexibility and versatility. The design of DCF enables it to carry multiple signal streams or sensor outputs simultaneously, making it widely used in communications, sensing, fiber optic amplifiers, and other fields.

[0031] In a dual-core optical fiber, two cores are arranged in the same cladding at a certain distance, and the structure and material of these cores are usually the same. The core of each optical fiber can independently transmit optical signals, and the propagation path and propagation characteristics of the optical signal can be optimized according to actual needs. Dual-core optical fiber has the following advantages in sensing: (1) Synchronous temperature response: Since the two cores are usually made of the same material and arranged in the same cladding, the impact of temperature changes on the two optical fibers is almost the same. This feature is particularly important in deformation sensors, which can effectively reduce the errors caused by temperature fluctuations and improve measurement accuracy. (2) Improved signal transmission efficiency: Dual-core optical fiber can transmit multiple signal streams simultaneously, thereby improving the data transmission capacity of the system. In some application scenarios, such as distributed sensing systems, dual-core optical fiber can realize data collection and transmission of different sensors in the same optical fiber line, simplifying the layout and improving system efficiency.

[0032] The dual-core optical fiber of the present invention is composed of a first fiber core and a second fiber core. Each fiber core is respectively inscribed with a 45° tilted fiber grating, such as Figure 1 The tilt angle of these fiber Bragg gratings is 45°±5°.

[0033] The grating period Λ satisfies the phase matching condition

[0034] λ=2n eff Λ / cosθ (1)

[0035] Where λ is the wavelength of the single-frequency laser, Λ is the grating period, θ is the grating inclination angle, θ = 45°, n eff is the effective refractive index of the fiber core.

[0036] Sensor structure such as Figure 2 As shown,

[0037] A single-frequency laser injects a laser with a wavelength of λ into the first fiber core, where λ is phase-matched with the grating;

[0038] The detector receives the optical signal output from the second fiber core and detects the attenuation of the light intensity;

[0039] The dual-core optical fiber is fixed on the surface of structures such as cantilever beams and geological crack embedments. The deformation of the structure due to geological disasters such as landslides can cause the surface of the structure to bend, thereby causing the optical fiber to bend.

[0040] When the single-frequency laser wavelength λ meets the phase matching condition of the two-core gratings, the light is coupled from the first core and enters the second core for transmission, and the detector receives the maximum light intensity I0.

[0041] When the deformable carrier bends, the outer core of the dual-core fiber is stretched (the grating period increases by ΔΛ1), and the inner core is compressed (the grating period decreases by ΔΛ2), which destroys the phase matching condition, reduces the coupling efficiency, and attenuates the detector light intensity to I.

[0042] By calibrating the relationship between the light intensity attenuation rate (I0-I) / I0 and the deformation variable ΔL, the deformation information is directly output.

[0043] The initial phase matching of the dual-core tilted grating satisfies equation (1):

[0044] When the fiber bending curvature radius is R, the grating period of the two fiber cores will change due to the bending, such as Figure 3 As shown, the change is:

[0045]

[0046] Wherein ΔΛ1 is the change of the grating period in the fiber core (11), ΔΛ2 is the change of the grating period in the fiber core (12), d is the distance between the two cores, and the deformation direction is perpendicular to the direction of the line connecting the two cores.

[0047] Position mismatch caused by bending deformation:

[0048]

[0049] According to coupled mode theory, the relationship between coupling efficiency η and phase mismatch is:

[0050]

[0051] The relationship between the detector light intensity I and the initial light intensity I0 is:

[0052]

[0053] When the deformation is small (ΔL∝1 / R<<1), the Taylor expansion approximation can be performed on formula (5):

[0054]

[0055] in L0 is the initial length of the deformation carrier (such as a cantilever beam). After the k value is determined through calibration experiments, the deformation ΔL can be directly calculated from the light intensity attenuation:

[0056]

[0057] Since the distance between the two cores is extremely small (≤50μm) and the cladding materials are the same, the temperature change affects the gratings on the two cores synchronously, thus achieving temperature self-compensation.

[0058] The dual-core optical fiber used was SM-28C-2C-50 / 125 (core spacing = 50 μm, cladding diameter 125 μm); grating parameters: 45° tilted fiber Bragg gratings (TFBGs) were inscribed in both cores, with a period of Λ = 1060 nm and a length of 20 mm, using the UV laser phase mask method, and a tilt angle of 45 ± 1°.

[0059] The dual-core optical fiber (grating region) was laser welded to the surface of a stainless steel cantilever beam (length L0 = 1.0 m, thickness 5 mm), with the weld points spaced 10 cm apart to ensure synchronous bending of the fiber and the beam. The end of the cantilever beam was connected to a high-precision piezoelectric ceramic translation stage (PIP-621.1CD, 0.1 μm displacement resolution) to apply a controllable deformation ΔL.

[0060] Single-frequency laser: Santec TSL-570, wavelength λ = 1550 nm, wavelength stability ≤ 0.005 nm, output power 10 mW; detector: Thorlabs PDA20HC2, bandwidth 150 MHz, sensitivity 0.9 A / W, connected to a NIPXIe-6366 data acquisition card (sampling rate 2 MS / s).

[0061] Calibration experiment process

[0062] (1) Initial calibration

[0063] The translation stage is reset to ΔL=0, the laser wavelength is adjusted to 1550 nm, and the initial light intensity of the detector is recorded.

[0064] I0=8.32V;

[0065] Verify temperature consistency: Place the sensor in a constant temperature box (25℃±0.5℃), change the temperature to 40℃, and the light intensity fluctuation is <0.5%.

[0066] (2) Deformation loading and data acquisition

[0067] The deformation was graded with a step size of ΔL = 0.2 mm to 5.0 mm, with a dwell time of 30 s at each level. ΔL (Keyence LK-G5000 laser rangefinder) and light intensity I were recorded simultaneously (Table 1).

[0068] Reverse unloading deformation to ΔL = 0, verify repeatability, the maximum hysteresis error is <2%.

[0069] Table 1: Calibration experimental data (partial)

[0070] ΔL(mm) I(V) <![CDATA[I0-I]]> <![CDATA[ΔL 2 (mm 2 )]]> 0.0 8.32 0.000 0.00 0.2 8.30 0.0024 0.04 0.4 8.25 0.0084 0.16 ... ... ... ... 5.0 5.12 0.385 25.00

[0071] ΔL 2 The horizontal axis is the light intensity attenuation rate, and the vertical axis is the linear fitting slope k = 0.0153mm -2

[0072] Deformation calculation formula:

[0073]

[0074] The calibrated sensor was buried in a crack (depth of 1.5m) in a landslide in a mountainous area. The cantilever beam was perpendicular to the crack direction, and the optical fiber was led out to the monitoring station through an armored protective sleeve.

[0075] The initial light intensity I0 = 8.30 V. After continuous monitoring for 72 hours, the light intensity was stable at 8.28-8.31 V (fluctuation < 0.4%), indicating no temperature interference.

[0076] At the 48th hour, the light intensity dropped sharply to 6.85V, and ΔL was calculated to be 4.2mm, triggering an alarm.

[0077] On-site investigation confirmed that the landslide displacement was 4.5 mm (error 6.7%), verifying the effectiveness of the sensor.

[0078] The present invention is applicable to scenes with geological disasters, oil and gas pipelines, etc., where there are drastic temperature changes and a lot of vibration interference.

[0079] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form or substance. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the present invention, and these improvements and supplements should also be regarded as the scope of protection of the present invention. Any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the spirit and scope of the present invention by using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A deformation sensor based on a dual-core tilted fiber Bragg grating, characterized in that: include: A dual-core optical fiber (1) comprises a first fiber core (11) and a second fiber core (12), wherein tilted fiber gratings are respectively inscribed in the first fiber core (11) and the second fiber core (12); The single-frequency laser (2) is connected to the input end of the first fiber core (11) and is used to emit a single-frequency laser that meets the tilted fiber grating phase matching condition; The detector (3) is connected to the output end of the second fiber core (12) and is used to detect the intensity of the optical signal coupled to the second fiber core (12); The dual-core optical fiber (1) is fixed on the surface or inside of the deformation carrier (4).

2. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: When the deformable carrier (4) is bent and deformed, the grating periods of the first fiber core (11) and the second fiber core (12) change inversely due to stretching or compression, causing the light intensity of the detector (3) to attenuate.

3. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The initial period of the tilted fiber Bragg grating satisfies the phase matching condition: λ=2n eff Λ / cosθ λ is the wavelength of the single-frequency laser, Λ is the grating period, θ = 45°, and neff is the effective refractive index of the fiber core.

4. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1 or 3, characterized in that: The grating period Λ is 500 to 1500 nm, and the length is 10 to 30 mm.

5. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The first fiber core (11) and the second fiber core (12) are in the same cladding, with a spacing of ≤50 μm, and the core materials of the two fiber cores are the same, so as to achieve synchronous influence of temperature change on the gratings of the two fiber cores.

6. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The tilt angle of the tilted fiber Bragg grating is 45°±5°.

7. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The deformation carrier (4) is a cantilever beam, a geological fracture embedment or a pipeline surface.

8. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The dual-core optical fiber (1) is fixed to the bending-sensitive area of the deformation carrier (4) by epoxy resin glue or laser welding.

9. The deformation sensor based on dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The device also includes a signal processing unit (5), which is connected to the detector (3) and has a built-in light intensity-deformation mapping algorithm. The light intensity attenuation rate and the deformation curvature are calibrated to output the deformation value.

10. A deformation monitoring system based on the deformation sensor based on the dual-core tilted fiber Bragg grating according to claim 1, characterized in that: The deformation sensors of dual-core tilted fiber Bragg gratings are connected in series or in parallel through optical fibers to form a distributed network and connected to a central processing terminal, which can realize the synchronous monitoring of multi-point deformation.

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