Elliptical ring core optical fiber capable of realizing double-parameter sensing of bending radius and bending angle

Abstract

The invention relates to an elliptical ring core optical fiber capable of realizing bending radius and bending angle dual-parameter sensing, and belongs to a distributed sensing technology. According to the invention, the asymmetric elliptical ring core structure of the optical fiber is utilized to realize double-parameter sensing of a small bending radius and a bending angle. Through numerical simulation, the ellipticity and the ring core thickness of the optical fiber elliptical ring core are optimized, and the characteristics of the sensing bending radius and the bending angle of the optical fiber with the elliptical ring core are revealed. Brillouin scattering light excited by light field LP01 and LP11e modes is utilized to realize double-parameter sensing, the sensing range of the bending radius is 0.6 cm to 2.0 cm, and the sensing range of the bending radius is 0 degree to 90 degrees. When the bending radius is 0.6 cm and the bending angle is 0 degree, the variation of the Brillouin frequency shift is the maximum, and the value is 31.6 MHz. Meanwhile, the design ensures that the limited loss of the optical fiber is less than 1.5 * 10 <-6 > dB / m and the birefringence of the optical fiber is less than 1 * 10 <-5 >, and the optical fiber has lower limited loss and birefringence compared with the existing mainstream multi-core optical fiber. According to the invention, the bending angle and the bending radius can be sensed at the same time.

Description

Technical Field

[0001] The present invention relates to an elliptical ring core optical fiber capable of realizing dual-parameter sensing of bending radius and bending angle, which is characterized by having excellent anti-bending loss characteristics and realizing distributed dual-parameter sensing of bending radius and bending angle while utilizing only one optical fiber core, and belongs to the technical field of distributed optical fiber sensing. Background Art

[0002] In recent years, shape sensing technology has been widely studied and paid attention to in many fields, such as probes, soft manipulators, endoscopes in the medical field, deformation monitoring of key structures in the aerospace field, and online deformation measurement of bridges or shape sensing of flexible robot tentacles (Reference 1: RT Schermer, and JH Colle, "Improved bend loss formula verified for optical fiber by simulation and experiment," Ieee J Quantum Elect 43, 899-909 (2007).; Literature 2: YJ Meng, RL Sui, WFLiang et al., "Multicore Fiber Shape Sensing Based on Optical Frequency Domain Reflectometry Parallel Measurements," J Lightwave Technol 42, 3909-3917 (2024).). Among the many alternative technologies that can realize shape sensing, optical fiber sensing technology provides a new exploration direction for curvature and bending angle sensing technology due to its advantages such as strong anti-interference ability, long measurement distance, high transmission reliability and accuracy, and simple wiring (Reference 3: RT Schermer, "Mode scalability in bent optical fibers," Opt Express 15, 15674-15701 (2007).). Among them, stimulated Brillouin scattering sensing technology senses the strain of the optical fiber by observing the frequency change of the backward Brillouin scattering signal and the local reference signal during optical fiber transmission, and then uses the spatial reconstruction algorithm to process the data and convert the strain information into the curvature information and bending angle information of the optical fiber. It is reported that compared with single-mode optical fiber, the Brillouin frequency shift caused by bending in few-mode optical fiber is much larger. Nevertheless, for the reported few-mode optical fiber, the large bending loss seriously limits its minimum sensing bending radius and sensing distance. As for multi-core optical fibers, although distributed shape sensing technology has been reported, it is also limited by the bending loss that leads to a large bending radius of sensing and is easily affected by optical fiber torsion, resulting in large errors in sensing (Reference 4: ZYZhao, MASoto, M.Tang et al., "Distributed shape sensing using Brillouinscattering in multi-core fibers," Opt Express 24, 25211-25223 (2016).).For ring-core fiber, distributed curvature sensing can be realized due to its excellent bending resistance, but it is limited by the symmetrical structure of the fiber, which makes it insensitive to the bending angle (Reference 5: L.Shen, H.Wu, C.Zhao et al., "Distributed curvature sensing based on a bending loss-resistant ring-core fiber," Photonics Res 8, 165-174 (2020).). In the case of a small bending radius, the simultaneous detection of the two parameters of bending radius and bending radius is often limited by bending loss and low sensing sensitivity.

[0003] The present invention combines the performance advantages of ring-core optical fiber and introduces an asymmetric structure to design an elliptical ring-core optical fiber, which can realize the simultaneous sensitivity of Brillouin frequency shift to bending angle and bending radius. Based on numerical simulation, we explored the sensing characteristics of elliptical ring-core optical fiber to bending radius and bending angle. The present invention can sense a bending radius range of 0.6cm to 2.0cm, and a bending direction range of 0° to 90°. When the bending radius is 0.6cm and the bending angle is 0°, the maximum value of the change in Brillouin frequency shift is 31.6MHz. At the same time, our design ensures that the limiting loss is less than 1.5×10 -6 dB / m and birefringence less than 1×10 -5 The invention provides a new direction for realizing distributed shape sensing under small bending radius. Summary of the invention

[0004] The present invention is an elliptical ring core optical fiber with an asymmetric structure, which uses the asymmetry of its structure to achieve sensitive response to bending angles, and uses the anti-bending characteristics of the ring core structure to achieve sensing of small bending radii. Combined with the above characteristics, the invention has the potential to achieve dual-parameter sensing of bending angles and bending radius.

[0005] 1. Specific content of the present invention

[0006] Having excellent anti-bending performance while realizing dual-parameter sensing of bending radius and bending angle is crucial for distributed shape sensing.

[0007] (1) The present invention designs an elliptical ring core structure that can achieve anti-bending loss, such as Figure 1 As shown, after optimization design, the optimal ellipticity is found Where i = 1, 2. Define the parameters After optimization, γ=0.55, which is the result of taking into account both bending loss and sensor sensitivity.

[0008] (2) When the designed optical fiber is incident with a 1550nm plane wave light source, when the minimum sensing bending radius (R) is 0.6cm, the limiting loss (CL) and birefringence (B) obtained by numerical simulation vary with the bending angle (θ b )’s changing trend is as follows Figure 2 As shown in the figure, the smaller the bending radius of the optical fiber, the greater the limiting loss and birefringence of the optical fiber. Therefore, we only need to ensure that the loss and birefringence of the optical fiber meet the limit at the minimum curvature radius to ensure distributed sensing.

[0009] (3) LP using light field 01 and LP 11e The frequency shift variation of the generated Brillouin scattering signal is used to jointly demodulate the information of the bending angle and the bending radius.

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

[0011] (1) Compared with the ring-core optical fiber sensor, the present invention can simultaneously realize the bending radius and the bending radius dual parameter sensing.

[0012] (2) Compared with multi-core optical fiber sensors, the present invention has improved anti-bending loss performance, so the present invention can sense a smaller bending radius.

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

[0014] (1) Taking the center of the fiber cross section as the origin of polar coordinates, the coordinates of any position in the interface are (r, θ). Assuming the bending radius R, the strain in the fiber cross section caused by bending can be expressed as: ε = -r·cos(θ-θ b ) / R. The change in power center caused by bending is represented by the vector (d x ,d y ) indicates that

[0015]

[0016] From this, we can get the change in Brillouin frequency shift (Δν B )for

[0017]

[0018] (2) The sensitivity of optical fiber strain to bending angle can be achieved through the asymmetric design of the ring core. This is because the power center change of the fundamental mode of the optical field (d x ,d y As the ellipticity increases or the thickness of the ring core decreases, the sensitivity of the Brillouin frequency shift to the bending radius and bending angle increases.

[0019] (3) By appropriately increasing the refractive index of the elliptical ring core, the light field can be better confined in the bending state to reduce the loss, and the sensitivity of the Brillouin frequency shift to strain can be improved.

[0020] (4) Using deep neural networks to construct the Δν of two Brillouin scattered light B With bending radius R and bending angle θ b The one-to-one mapping relationship between the bending radius R and the bending angle θ is realized. b Dual parameter sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 (a) is a two-dimensional cross-sectional view of the designed elliptical ring core fiber. We increase the refractive index by doping the ring core with GeO2. The structural parameters of the fiber cross section are: ellipticity η = 1.3, γ = 0.55, a1 = 7.125μm, r = 62.5μm. Figure 1 (b) and Figure 1 (c) are the distribution diagrams of the refractive index in the optical fiber cross section along the major axis and minor axis of the elliptical ring core.

[0022] Figure 2 (a) and Figure 2 (b) shows the limiting loss (CL) and birefringence (B) with the bending angle (θ) when the minimum bending radius (R) is 0.6 cm. b ). The wavelength of the incident light is 1550nm. According to the simulation results, the loss and birefringence are both within the acceptable range.

[0023] Figure 3 (a) and (b) are the light field LP in the designed elliptical ring core fiber. 01 and LP 11o The variation curve of the excited Brillouin frequency shift with strain. According to the numerical simulation results, the strain sensitivity (C ε ) are 0.04698MHz / με and 0.04695MHz / με.

[0024] Figure 4 (a) and Figure 4 (b) represent the light field LP 01 The change in the Brillouin frequency shift (Δv B ) with the bending radius (R) and bending angle (θ b ) trend. Figure 4 The result of function fitting in (a), Δv B is proportional to the curvature (1 / R). Figure 4 The result of function fitting in (b), Δv B With θ bIt shows a sinusoidal function relationship.

[0025] Figure 5 (a) and Figure 5 (b) represent the light field LP 11e In the excited Brillouin scattering signal, Δv B With R and θ b According to the changing trend of Figure 5 The result of function fitting in (a), Δv B Proportional to the curvature (1 / R). Figure 5 (b) shows Δv B With θ b The process of increasing from negative value to positive value, that is, when v is bent along the direction close to the long axis of the elliptical ring core of the optical fiber, B decreases; when bending along the direction close to the short axis, v B Increase.

[0026] Figure 6 (a) and Figure 6 (b) are respectively composed of light field LP 01 And light field LP 11e In the excited Brillouin scattering signal, Δv B With R and θ b The function of changing the fitting surface. Compare Figure 6 (a) and Figure 6 (b), we find that there is an obvious nonlinear relationship between the two functions. This condition is to achieve R and θ b This lays the foundation for dual-parameter sensing, indicating that the optical fiber can use two Brillouin scattering spectra to achieve dual-parameter sensing of bending radius and bending angle.

[0027] Figure 7 (a) and Figure 7 (b) shows the predictions of the deep neural network we trained for the test data, where the root mean square error (RMSE) of the predicted bending radius R is 0.0016; the predicted bending angle θ b The root mean square error (RMSE) is 0.5828. DETAILED DESCRIPTION

[0028] In order to make the objectives, technical solutions and advantages of the present invention more clear, the specific structure, principle and sensing characteristics of the present invention are further described below in conjunction with the accompanying drawings.

[0029] The present invention optimizes the design by adjusting the ellipticity and thickness of the fiber core cross section, and utilizes the asymmetric structure of the elliptical core fiber to achieve distributed dual-parameter sensing of the bending radius and the bending angle. Figure 1(a). The major axis a1 = 7.125 μm, the ellipticity η = 1.3, the elliptical ring core ratio γ = 0.55, and the cladding radius r = 62.5 μm. The refractive index distribution of the designed optical fiber cross section is shown in Figure 1 (b) where the refractive index of the ring core is n ring =1.457, and the refractive index of the cladding n ring =1.444. Figure 1 (b) is the refractive index distribution of the optical fiber cross section in the long axis direction. Figure 1 (a) is the refractive index distribution of the optical fiber cross section in the short axis direction.

[0030] In order to optimize the design structure of the optical fiber, we mainly considered the limiting loss and birefringence as optimization constraints, and took the sensitivity of the Brillouin frequency shift to the bending radius and bending angle as the optimization target. Specifically, we used the conformal transformation technology to convert the bent optical fiber into a straight optical fiber, and in order to prevent the loss of the cladding during the modal calculation process, the cladding refractive index was truncated to a larger value. According to the structure we designed, we numerically calculated the change curve of the limiting loss with the bending angle when the bending radius is 0.6cm. Figure 2 As shown in (a), we find that the maximum loss occurs at 90°, that is, the bending radius is along the short axis. The trend of birefringence with bending angle is shown in Figure 2 As shown in (b), unlike the trend of limiting loss, birefringence reaches its maximum value at 0°. According to the extreme value characteristics of loss and birefringence curves, the limiting conditions in the optimization only need to analyze the two angles of 0° and 90°, which greatly reduces the computational complexity. After determining the fiber structure, we used numerical simulation to calculate the strain sensitivity of the two Brillouin scattering spectra. Figure 3 At room temperature, the light field LP of the present invention 01 Strain sensitivity C of Brillouin scattering spectrum coupled with acoustic field ε About 0.04698MHz / με, light field LP 11e Strain sensitivity C of Brillouin scattering spectrum coupled with acoustic field ε About 0.04695MHz / με.

[0031] On this basis, the sensing performance was analyzed according to formula (1.3). Figure 4 (a) and (b) show the wavelength of 1550nm, respectively, by LP 01 Brillouin scattering spectrum coupled with the acoustic field, where Δv B With R and θ b 's changing trend. Figure 5 (a) and (b) show the wavelength of 1550nm, respectively, by LP 11e The Brillouin scattering spectrum coupled with the acoustic field, Δv BWith R and θ b Under different R, Δv B With θ b The curve relationship is different, indicating that θ b There is also the problem of cross-sensitivity between R, which brings challenges to dual-parameter sensing.

[0032] Finally, based on the optimized structure, we fitted the Δv of the two Brillouin scattering spectra respectively. B The bending radius R and the bending angle θ b The functional relationship is as follows Figure 6 (a) and (b). According to the fitted function relationship, we found that there is a linear relationship between the Brillouin frequency shift and the curvature (1 / R) and the b There is an approximate sinusoidal function relationship between them. Within the sensing range, the light field LP 01 In the excited Brillouin scattering spectrum, its Δv B The maximum value is 31.6MHz and the minimum value is 1MHz. b Conditional light field LP 01 and LP 11e The excited Brillouin scattering spectrum is used as training data, and the deep neural network is used to learn the features in the data. The final test results of the neural network are as follows: Figure 7 As shown, a good accuracy rate was obtained.

Claims

1. An elliptical ring core optical fiber that can realize dual-parameter sensing of bending radius and bending angle is proposed, which is characterized by: using the asymmetric structure of the optical fiber elliptical ring core to realize sensitive sensing of the bending angle, and then using the optical fiber ring core structure to realize sensing of small bending radius. In the ring core area, we increase the refractive index to 1.457 by doping GeO2 into SiO2, while the cladding is pure SiO2 material with a refractive index of 1.

444. The increase in the refractive index of the ring core has a good limiting effect on the energy of the light field, which can improve the bending loss resistance of the optical fiber and reduce birefringence, which further helps us realize the sensing of the bending radius at a small bending radius.

2. The asymmetric structure of the elliptical ring core according to claim 1, characterized in that: The ellipticity of the ring core region is 1.3, and the ratio (γ) of the major axis (or minor axis) radius of the inner ellipse to the outer ellipse is 0.

55. Under the condition that the major axis radius of the outer ellipse is determined to be 7.125 μm, the minor axis radius of the outer ellipse is determined to be 5.481 μm, the major axis radius of the inner ellipse is 3.919 μm, and the minor axis radius is 3.014 μm.

3. According to the design of asymmetric elliptical ring core optical fiber sensing bending radius and bending angle according to claim 1, it is characterized in that: In the asymmetric structure, different bending angles will cause different strains in the optical fiber, and then the bending angle is demodulated according to the sensitivity of the Brillouin frequency shift to strain to achieve the sensing process. The simulation results show that the minimum Brillouin frequency shift change caused by bending is greater than 1MHz, which will be helpful in detecting the change of Brillouin frequency shift in the optical fiber caused by bending in actual sensing.

4. According to the contents of claims 1, 2 and 3, it is characterized in that: Through the geometric structure of the elliptical ring core fiber cross section and the doping characteristics, the birefringence of the fiber can be less than 10 -5 And limit the loss to less than 5×10 -6 The dual-parameter sensing curves of bending radius and bending angle of the present invention are obtained by simulation, which lays a foundation for realizing dual-parameter sensing of bending radius and bending angle.

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