Anti-torsion temperature, strain and curvature three-parameter sensor based on D-type multi-core fiber Brillouin scattering

Abstract

The invention belongs to the field of optical fiber sensor design, and particularly relates to an anti-torsion temperature, strain and curvature three-parameter sensor based on D-type multi-core optical fiber Brillouin scattering. According to the invention, the multi-core optical fiber is designed to be of a D-shaped structure, so that the sensor has torsion resistance, and the space division multiplexing technology is utilized to respectively sense temperature and strain through double Brillouin peaks of the inner fiber core and sense curvature through the outer fiber core. Specifically, when the multi-core optical fiber is of a D-shaped structure, the multi-core optical fiber has bending deflection due to the asymmetry of the structure, sensing errors caused by optical fiber torsion are reduced, the inner fiber core is insensitive to bending, temperature and strain can be sensed by using double Brillouin peaks, and the outer fiber core is sensitive to bending due to the fact that the core position is far away from the neutral axis of the optical fiber. And curvature sensing can be realized through temperature and strain compensation of the inner core. The space division multiplexing D-type multi-core fiber Brillouin scattering-based anti-torsion temperature, strain and curvature three-parameter sensor designed by the invention has important significance on the design of a multi-parameter sensor.

Description

Technical Field

[0001] The present invention belongs to the field of optical fiber sensor design, and specifically relates to a torsion-resistant temperature, strain and curvature three-parameter sensor based on D-type multi-core optical fiber Brillouin scattering. The characteristic of the sensor is that it can resist optical fiber torsion and can simultaneously distribute and sense three physical parameters. Background Art

[0002] Distributed fiber optic sensors based on Brillouin scattering are widely used in structural health monitoring, oil pipeline early warning, intelligent biomedicine, earthquake early warning and other fields. (Reference 1: XYBao, ZCZhou, and Y.Wang, "Review: distributed time-domain sensors based on Brillouin scattering and FWMenhanced SBS for temperature, strain and acoustic wave detection," Photonix2(2021).). In recent years, the rise of multi-core optical fibers has brought new vitality to Brillouin multi-parameter sensors. The use of space-division multiplexed multi-core optical fiber to sense temperature and strain dual parameters has been verified (Reference 2: ZYZhao, M.Tang, SNFu, WJTong, and DMLiu, "Distributed and discriminative Brillouin optical fiber sensingbased on heterogeneous multicore fiber," 2017Optical Fiber Communications Conference and Exhibition (Ofc) (2017).; Reference 3: ZYZhao, YLDang, M.Tang, BRLi, L.Gan, SNFu, HFWei, WJTong, P.Shum, and DMLiu, "Spatial-divisionmultiplexed Brillouin distributed sensing based on aheterogeneous multicorefiber," Opt Lett 42, 171-174 (2017).).In addition, the Brillouin curvature sensor based on multi-core optical fiber uses the outer core far away from the neutral axis as the sensing core to expand the curvature sensing range (Reference 4: YL Dang, ZY Zhao, M. Tang, C. Zhao, L. Gan, SN Fu, TQ Liu, WJ Tong, PPShum, and DM Liu, "Towards large dynamic range and ultrahigh measurement resolution in distributed fibersensing based on multicore fiber," Opt Express 25, 20183-20193 (2017).; Reference 5: ZY Zhao, M A Soto, M. Tang, and L. Thévenaz, "Distributed shape sensing using Brillouin scattering in multi-core fibers," Opt Express 24, 25211-25223 (2016).). Generally, multi-core optical fibers are susceptible to fiber torsion, which reduces the sensing accuracy (Reference 6: A. Zafeiropoulou, A. Masoudi, A. Zdagkas, L. Cooper, and G. Brambilla, "Curvature sensing with a D-shaped multicore fibre and Brillouin optical time-domain reflectometry," Opt Express 28, 1291-1299 (2020).). However, the reported Brillouin sensors based on multi-core optical fibers can usually only sense one curvature parameter or two parameters, temperature and strain. The potential of multi-core optical fiber space division multiplexing in Brillouin multi-parameter sensors needs to be further explored.

[0003] Therefore, in order to resist the torsion of multi-core optical fiber and increase the number of sensing parameters, the present invention makes full use of the space division multiplexing characteristics of multi-core optical fiber, introduces optical fiber bending bias by designing the multi-core optical fiber as a D-type structure, resists the problem of large sensing error caused by optical fiber torsion, and senses temperature and strain through the double peaks of the bending-insensitive inner core Brillouin scattering, and senses curvature by compensating temperature and strain for the outer core far away from the neutral axis, and finally realizes the three-parameter sensing of temperature, strain and curvature by combining the inner and outer core Brillouin scattering. In short, the Brillouin sensor extends the common two-parameter sensor to three-parameter, can cope with more complex sensing environments, and provides an important reference for the design and application of distributed three-parameter sensors. Summary of the invention

[0004] The present invention ingeniously utilizes the bending bias caused by the asymmetry of the D-type structure and the natural space-division multiplexing characteristics of the multi-core optical fiber. By studying the cross-sensitivity between the temperature, strain and curvature of the inner and outer cores, and combining the Brillouin scattering spectra in the inner and outer cores, a torsion-resistant temperature, strain and curvature three-parameter sensor based on D-type multi-core optical fiber Brillouin scattering is proposed.

[0005] 1. Specific content of the present invention

[0006] The torsion of multi-core optical fibers seriously affects the sensing accuracy of Brillouin sensors. It is crucial to solve the cross-sensitivity problem among the three parameters of temperature, strain and curvature. It is challenging to use Brillouin scattering in multi-core optical fibers to achieve simultaneous distributed sensing of temperature, strain and curvature that resists optical fiber torsion.

[0007] (1) The present invention realizes a double-peak Brillouin scattering spectrum by designing a D-type multi-core optical fiber, such as Figure 1 , an optimized design of a D-type multi-core optical fiber can achieve resistance to torsion and excitation of double-peak Brillouin scattering spectrum. The refractive index of the pure silica cladding is 1.444, and the refractive index of the germanium-doped core 1 (core1) is 1.452. The refractive index of core 2 (core2) can be increased from 1.444 to 1.452 to optimize the Brillouin scattering spectrum and obtain a better Brillouin double peak. The cladding diameter is 170μm, the core diameters are 3.2 and 9μm respectively, the distance from the center of the fiber to the D-type boundary is 65μm, and the core spacing is 44μm. The refractive index of core 2 was finally determined to be 1.4484.

[0008] (2) By designing the Brillouin scattering double peak, the cross sensitivity between temperature and strain in the inner and outer cores was calculated and simulated. It was found that the influence of temperature or strain change on the strain or temperature sensitivity of the main peak (i.e., the interaction between the fundamental optical mode and the fundamental acoustic mode) was only 8.344×10 -6 MHz / με / ℃ or 8.344×10 -6 MHz / ℃ / με, indicating that changes in temperature or strain have little effect on the corresponding sensitivity, and the same is true for the secondary peak.

[0009] (3) Further calculation and simulation of the cross-sensitivity between temperature and bending, with a bending radius ranging from 6 cm to 12 cm, found that temperature has almost no effect on bending sensitivity, and bending has almost no effect on temperature sensitivity. The mutual independence of temperature, strain, and curvature sensitivity makes it possible to establish a classic three-dimensional sensing matrix to demodulate the three physical parameters.

[0010] 2. The advantages of the present invention are as follows:

[0011] (1) The present invention adopts a D-type multi-core optical fiber structure with bending bias to overcome the sensing error introduced by optical fiber twisting and improve the accuracy of Brillouin multi-parameter sensing.

[0012] (2) The present invention utilizes the space division multiplexing characteristics of multi-core optical fiber to realize the simultaneous sensing of temperature, strain and curvature based on Brillouin scattering.

[0013] 3. The principles of the present invention are as follows:

[0014] (1) Double Brillouin scattering peak design and optimization based on photoacoustic interaction theory.

[0015] (2) Multi-core optical fiber sensing The sensing of temperature, strain and curvature is based on space division multiplexing technology and double scattering peak characteristics. The sensing principle is as follows:

[0016] First, since the curvature change is intrinsically derived from the strain change, assuming that the Brillouin frequency shift caused by temperature, strain, and curvature can be linearly superimposed, a three-dimensional sensing matrix is ​​established:

[0017]

[0018] in and represent the total Brillouin frequency shift (BFS) caused by temperature (T), strain (S), and bending (R), respectively. and They represent the temperature, strain and curvature sensitivities of the inner core peak 1 (peak1) and peak 2 (peak2), respectively. and They represent the three parameter sensitivities corresponding to the outer core peak 1. The Brillouin frequency shift caused by the inner core bending alone can be measured by C S Δd / R calculation, where C S and Δd represent the strain sensitivity and light field offset of the straight fiber, respectively. The light field offset of the inner core under a large bending radius is extremely small (maximum 0.035 μm). Therefore, relative to temperature and strain, the BFS change caused by bending is very small (maximum 0.028 MHz), and its corresponding sensitivity is about 0.249 MHz / cm -1 , it can be considered that: Since the refractive index distribution of the inner and outer cores is the same, and And the BFS caused by the bending in the outer core can be obtained by C S (d+Δd) / R≈C S d / R calculation, where d represents the core spacing (44μm), which is much larger than the optical field offset Δd (maximum 0.035μm). Therefore, Formula 1 can be further simplified to:

[0019]

[0020] The inverse matrix of formula 2 can be used to obtain temperature, strain and curvature:

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] (1) Figure 1 This is a schematic diagram of the optimization of the double peaks of Brillouin scattering in D-type multi-core optical fiber. By changing the core refractive index ncore2, the core supports double Brillouin scattering peaks. The spectrum evolution process is shown in Figure 1 (a). The optimized structure is: the refractive index of the pure silica cladding is 1.444, the diameter is 170μm, the two-step germanium doped core ncore1=1.452, ncore2=1.4884, the diameters are 3.2 and 9μm respectively. The D-type boundary is 65μm from the center of the fiber, and the core spacing is 44μm. Figure 1 (c) is the light field distribution of the inner and outer cores.

[0023] Figure 2 It is the result of the cross-sensitivity study of temperature and strain. Figure 2 (a) and (b) are the simulated BFS of peak 1 and peak 2 when T and S vary in the range of 20-70℃ and 0-1000με. Figure 2 (c) and (d) are the relationship diagrams of the temperature sensitivity of peak 1 and peak 2 as a function of strain, respectively. Figure 2 (e) and (f) are the temperature changes of the strain sensitivity of peak 1 and peak 2, respectively. It can be seen that the temperature / strain sensitivity changes very little.

[0024] Figure 3 This is the result of the cross-sensitivity study of temperature and curvature. Figure 3 (a) and (b) are the simulated BFS of peak 1 and peak 2 when T and R vary in the range of 20-70°C and 6-12 cm. Figure 3 (c) and (d) are the relationship diagrams of the temperature sensitivity of peak 1 and peak 2 as a function of the bending radius, respectively. Figure 3 (e) and (f) are the temperature changes of the curvature sensitivity of peak 1 and peak 2. It can be seen that the temperature / curvature sensitivity changes are very small.

[0025] Figure 4 This is a comparison of the curvature sensitivity of the inner core and the outer core. It can be seen that the curvature sensitivity of the outer core is 840 times greater than that of the inner core. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention clearer, the specific structure, principle and performance of the present invention are further described below in conjunction with the accompanying drawings:

[0027] In order to achieve torsion-resistant temperature, strain and curvature three-parameter sensing, the invention designs the multi-core optical fiber into a D-type structure with bending bias and utilizes its space division multiplexing characteristics. Figure 1 As shown, the present invention optimizes ncore1 to obtain a better double Brillouin peak for sensing.

[0028] Figure 2 This indicates that temperature and strain changes have little effect on the corresponding strain and temperature sensitivities. The effect of temperature or strain changes on the strain or temperature sensitivity of peaks 1 and 2 is only 8.344×10 -6 MHz / με / ℃ or 8.344×10 -6 MHz / ℃ / με and 8.469×10 -6 MHz / με / ℃ or 8.469×10 -6 MHz / ℃ / με. This is consistent with the classic temperature and strain dual parameter sensing system, which can be demodulated using a two-dimensional sensing matrix.

[0029] Figure 3 It is shown that the temperature and bending radius changes have little effect on the corresponding curvature and temperature sensitivity. Figure 3 (c) and (d) show that for the bending radius in the range of 6-12 cm, the temperature sensitivity change of peak 1 and peak 2 is only or 3.606×10 - 6 MHz / ℃ and 3.660×10 -6 MHz / ℃. Figure 3 (e) and (f) show that for temperatures in the range of 20–70 °C, the curvature sensitivity of the two peaks changes by only 2.164 × 10 -6 MHz / cm -1 and 2.196×10 -6 MHz / cm -1 . Figure 2 and Figure 3 It shows that the three parameters of temperature, strain and curvature can be demodulated using the classical three-dimensional sensing matrix.

[0030] Figure 4 It is shown that the curvature sensitivity of the outer core is much greater than that of the inner core, and the curvature sensitivity of the inner core is close to 0, which has little effect on temperature and strain sensing. Therefore, the Brillouin scattering spectra of the inner and outer cores can be used together to establish a three-dimensional sensing matrix, with the inner core used for temperature and strain demodulation and the outer core used for curvature demodulation.

[0031] In summary, the present invention proposes a temperature, strain and curvature three-parameter sensor based on D-type multi-core optical fiber Brillouin scattering that resists torsion. The D-type structure design makes the optical fiber have bending bias, resists torsion loss and error, and establishes a three-dimensional sensing matrix by multiplexing the Brillouin scattering spectrum of the inner and outer cores to achieve temperature, strain and curvature demodulation. The present invention provides an important reference for Brillouin three-parameter sensing.

Claims

1. A torsion-resistant three-parameter sensor of temperature, strain and curvature based on space-division multiplexing D-type multi-core optical fiber Brillouin scattering, characterized in that: The D-type structure can resist optical fiber torsion and reduce the impact of torsion on three-parameter sensing. The space division multiplexing technology can realize temperature, strain and curvature sensing through the combination of inner and outer cores.

2. The D-type multi-core optical fiber according to claim 1, characterized in that: The designed multi-core optical fiber has 1 inner core, 3 outer cores, a core spacing of 44μm, a distance from the fiber center to the D-type boundary of 65μm, a pure silica cladding diameter of 170μm, and the core diameters of the GeO2-doped two-step refractive index distribution are 3.2 and 9μm, respectively, and the refractive indices are 1.4484 and 1.452, respectively.

3. The method for resisting optical fiber torsion according to claim 1, characterized in that: The multi-core optical fiber is designed as a D-type structure, which makes the optical fiber structure asymmetric so that the multi-core optical fiber has a bending preference during the bending process, reduces the sensing error caused by torsion, and improves the accuracy of three-parameter sensing.

4. The three-parameter sensor according to claim 1, characterized in that: First, the cross-sensitivity of temperature, strain, and curvature is verified. The inner fiber core is insensitive to bending and the supported double Brillouin gain peak is used to simultaneously sense temperature and strain. The outer fiber core far away from the neutral axis of the multi-core optical fiber is very sensitive to bending, and curvature sensing can be achieved through inner core temperature and strain compensation, thereby realizing three-parameter sensing through space-division multiplexing of the inner and outer core combined spectrum.

5. The D-type multi-core optical fiber according to claims 1, 2 and 3, characterized in that: The D-type multi-core fiber structure provides strong support for temperature, strain and curvature sensing that is resistant to fiber torsion, and expands the number of Brillouin sensing parameters.

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