Liquid injection microstructure photonic crystal fiber capable of realizing temperature and refractive index sensing

Abstract

The invention provides a liquid injection microstructure photonic crystal fiber capable of realizing temperature and solution refractive index dual-parameter sensing. A cladding of the optical fiber adopts a multi-layer air hole structure to enhance light field constraint, and a liquid injection hole is formed in the center of a fiber core to enhance interaction between an evanescent field and liquid to be detected; a fiber core region adopts a design of doping two layers of GeO2 with different concentrations, so that a fundamental mode LP01 is coupled with a plurality of acoustic modes to generate a multi-peak Brillouin spectrum, and the multi-peak Brillouin spectrum can be used for realizing dual-parameter sensing. According to the invention, synchronous measurement in a temperature range (20-70 DEG C) and a refractive index range (1.317-1.342) is realized by utilizing bimodal Brillouin frequency shift coupled by an LP01 mode and an L01 and L03 acoustic mode. Meanwhile, parameters such as the doping concentration of the fiber core and the radius of the liquid injection hole are optimized through numerical simulation, and the influence rule of the parameters on the refractive index sensitivity and the double-peak characteristic is revealed. Compared with a tapered optical fiber or a D-shaped optical fiber and other structures for refractive index sensing, the optical fiber provided by the invention can realize higher refractive index sensitivity and temperature-refractive index dual-parameter sensing, and has good application potential.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing based on Brillouin scattering, and designs a liquid-filled microstructure photonic crystal fiber. Its main feature is that it has excellent solution refractive index sensing capability, and can realize simultaneous sensing of temperature and solution refractive index. Background Technology

[0002] In recent years, with the development of fiber optic technology, distributed fiber optic sensors based on Brillouin scattering have gradually become a research hotspot due to their advantages such as long-distance sensing capability, high spatial resolution, wide dynamic range and resistance to electromagnetic interference (Reference 1: Xu, Pengbai, et al. Bending-loss-resistant distributed temperature and strain-discriminative Brillouin sensor based on 98mol%Germania-doped few-modefiber. Journal of Lightwave Technology 41.14(2023):4854-4861.). In addition, the measurement of refractive index of a medium solution has a wide range of applications in chemical analysis, biomedical diagnosis, environmental monitoring and industrial production. However, the refractive index of a general solution changes with temperature, so a distributed Brillouin fiber optic sensor that can simultaneously measure temperature and refractive index is very necessary (Reference 2: Li, Kaifeng, et al. Dual parameter sensor for RI and temperature detection by cascading Ag / WO3 film PCF and Ag / MoS2 / PDMS film PCF. Optics Express 32.16(2024):27710-27722.).Among the reported articles, Brillouin fiber sensors based on tapered optical fibers can achieve good sensing performance. However, due to the inherent structural limitations of tapered optical fibers, they suffer from significant transmission loss (Reference 3: Minardo, Aldo, et al. Quasi-distributed refractive index sensing by stimulated Brillouin scattering in tapered optical fibers. Journal of Lightwave Technology 40.8(2022):2619-2624.). Additionally, Brillouin fiber sensors based on few-mode optical fibers have also been proposed, but these require special fiber treatment, resulting in poor sensing performance and increased transmission loss (Reference 4: Bernini, Romeo, et al. Refractive index sensing by Brillouin scattering in side-polished optical fibers. Optics Letters 43.10(2018):2280-2283.). In contrast, photonic crystal fibers, due to the flexibility of their porous structure design, offer significant advantages in both sensing performance and fiber transmission loss.

[0003] This invention utilizes the advantages of photonic crystal fibers combined with a microstructured core to design a liquid-filled microstructured core photonic crystal fiber, enabling simultaneous dual-parameter sensing of temperature and solution refractive index based on Brillouin scattering. Numerical simulations were used to investigate the sensing performance of this fiber. Due to limitations in the fiber material and solution (Reference 5: Kleis, SJ, and LA Sanchez. "Dependence of speed of sound on salinity and temperature in concentrated NaCl solutions." Solar Energy 45.4(1990):201-206.), the temperature range that this invention can sense is 20-70℃, and the refractive index range of the medium solution is 1.317-1.342. This invention provides a new direction for high-sensitivity refractive index sensing. Summary of the Invention

[0004] This invention relates to a photonic crystal fiber with a liquid-filled microstructure core. The multi-layer air-hole cladding structure of the photonic crystal fiber is used to confine the light field, and the solution to be tested is injected into the core holes to enhance photoacoustic interaction. At the same time, the liquid-filled and two-layer GeO2-doped microstructure core causes two peaks to appear in the Brillouin gain spectrum, thereby enabling the fiber structure to sense both temperature and refractive index simultaneously.

[0005] 1. The technical problem to be solved by the present invention

[0006] (1) The fiber structures (tapered fiber, specially treated fiber) used in the refractive index fiber sensors that have been proposed so far have high transmission loss, which limits their application in long-distance sensing. More importantly, the above sensors have poor refractive index sensing performance for medium solutions.

[0007] (2) Most of the reported articles on Brillouin fiber optic sensors only focus on sensing the refractive index. However, in practical applications, the refractive index of a solution changes with temperature, so a Brillouin fiber optic sensor that can simultaneously sense both temperature and refractive index is essential.

[0008] 2. Technical solution of the present invention

[0009] (1) In order to solve the problem of high transmission loss in the Brillouin fiber sensor based on tapered fiber and specially treated few-mode fiber for refractive index sensing, the present invention adopts a multi-layer air hole structure in the cladding of photonic crystal fiber to confine the light field to the core region, reduce transmission loss, and make it suitable for long-distance distributed fiber sensing.

[0010] (2) To achieve high-performance refractive index sensing of a dielectric solution, this invention incorporates a porous structure within the fiber core, consisting of six pores, each with a diameter of 1.62 μm and a center-to-center distance of 2 μm from the fiber core center. In sensing applications, the dielectric solution is injected into these pores. This design enhances the penetration of the evanescent field into the solution, strengthens photoacoustic interaction, and thus achieves highly sensitive refractive index sensing of the dielectric solution.

[0011] (3) Furthermore, the fiber core region of the optical fiber structure used in this invention is designed with two layers of GeO2 doping at different concentrations, based on a porous structure. The inner side of the porous core is designated as a region with a higher concentration of GeO2 doping, and the refractive index n... core1 The refractive index n is 1.454; while the outer core region where the porous structure is located is doped with a lower concentration of GeO2, the refractive index n is 1.454. core2The value is 1.445. The two doped regions are separated by a low-refractive-index injection hole buffer layer. This hybrid core structure can generate two sets of acoustic modes from the optical field fundamental mode, with their distribution concentrated in the core center and the outer core region, respectively. The L01 acoustic modes of the two sets of modes are selected to achieve dual-parameter sensing of temperature and refractive index.

[0012] 3. Advantages of the present invention

[0013] (1) The structure designed in this invention can achieve a refractive index sensitivity of 810 MHz / RIU when detecting a medium solution with a refractive index of 1.317-1.342. Compared with some previously reported articles, the sensing performance of this structure is improved by 140%.

[0014] (2) Compared with the previously proposed tapered optical fiber and specially treated few-mode optical fiber, the present invention can simultaneously realize dual-parameter sensing of temperature and refractive index. Attached Figure Description

[0015] Figure 1 The image shows a two-dimensional cross-sectional view of the designed liquid-filled photonic crystal fiber. We enhanced the interaction between the measured liquid and the evanescent field by designing injection holes in the fiber core, thereby improving sensing performance. Simultaneously, by designing a double-layer core with different concentrations of GeO2 doping, we obtained a bimodal Brillouin spectrum to achieve two-parameter sensing. The structural parameters of the fiber cross-section are: r core1 =0.90μm, injection hole radius r liquid =0.81μm, radius r of the lower doping concentration region on the periphery of the fiber core core2 =4μm, cladding air pore radius r air =1.40μm, cladding radius r clad =62.5μm.

[0016] Figure 2 The image shows the Brillouin gain spectrum of the designed structure. It can be seen that the two Brillouin peaks, peak 1 and peak 3, corresponding to sound fields L01 and L03, have high peak gain. Peaks 1 and 3 are selected for dual-parameter sensing.

[0017] Figure 3 (a) and (b) show the peak gain trends of peaks 1 and 3 as temperature and refractive index change within the sensing range when the liquid refractive index and temperature reach their maximum values. It can be seen that both peaks maintain good peak gain during this process, which verifies the reliability of the structure for sensing.

[0018] Figure 4 (a) shows that when the liquid refractive index is 1.317, the temperature sensitivities of peaks 1 and 3 in the Brillouin spectrum are 1.06 MHz / ℃ and 1.12 MHz / ℃, respectively. Figure 4(b) shows that at room temperature (20°C), the refractive index sensitivities of peaks 1 and 3 in the Brillouin spectrum are 774 MHz / RIU and 816 MHz / RIU, respectively, indicating that the structure has excellent sensing performance.

[0019] Figure 5 Figures (a) and (b) illustrate the cross-sensitivity characteristics between temperature and refractive index when the fiber optic structure of this invention performs simultaneous temperature and refractive index sensing. From the figures, the temperature coefficients of the refractive index sensitivity of the two peaks in the Brillouin spectrum are -0.0148 MHz / (RIU·℃) and -0.0165 MHz / (RIU·℃), respectively; while the refractive index coefficients of the temperature sensitivity are consistent with the above values, namely -0.0148 MHz / (℃·MHz) and -0.0165 MHz / (℃·MHz).

[0020] Figure 6 (a) and (b) show the prediction results of temperature and strain using a deep neural network (DNN), respectively. The RMSE of the temperature prediction is 0.03979℃. 2 =0.9993, RMSE of predicted refractive index =0.00002, R 2 =0.9992, which has a high accuracy rate. Detailed Implementation

[0021] To make the technical problems, technical solutions, and advantages of this invention clearer, the specific structure, principle, and sensing characteristics of the invention will be described below in conjunction with the accompanying drawings.

[0022] This invention optimizes the design by adjusting the radius of the air holes in the fiber cladding, the radius of the liquid injection holes in the fiber core, and the concentration of GeO2 doping in both layers of the fiber core. It utilizes the liquid-injected hybrid core structure of the photonic crystal fiber to achieve dual-parameter sensing of temperature and refractive index. The final designed fiber structure is as follows: Figure 1 As shown. The radius r of the region with higher doping concentration at the center of the fiber core is... core1 =0.90μm, the refractive index of this region is n core1 =1.454, injection hole radius r liquid =0.81μm, radius r of the lower doping concentration region on the periphery of the fiber core core2 =4μm, the refractive index of this region is n core2 =1.445, cladding air hole radius r air =1.40μm, cladding radius r clad =62.5μm. In the final optimized fiber structure, the Brillouin gain spectrum obtained by coupling the optical field fundamental mode with multiple acoustic modes is as follows: Figure 2 As shown, the peak gain of sound field L01 corresponding to peak 1 is 0.340m. -1 W -1The peak gain of sound field L03 corresponding to peak 3 is 0.106m. -1 W -1 Both peaks showed significant gains, and peaks 1 and 3 were selected for dual-parameter sensing of temperature and refractive index.

[0023] To optimize the fiber structure, we considered the sensitivity of the Brillouin frequency shift to the refractive index in the fiber Brillouin gain spectrum as the primary optimization objective, and the peak gain of peaks 1 and 3 used for dual-parameter sensing as secondary objectives. While maintaining good peak gain of the Brillouin peak within the specified sensing range (temperature: 20-70℃, refractive index: 1.317-1.342), we optimized the refractive index sensing performance of the fiber as much as possible. Figure 2 The image shows the Brillouin gain spectrum of the optical fiber at room temperature (20°C) and with the sensing liquid having a refractive index of 1.317. Figure 3 (a) shows the variation trends of peak gain of peaks 1 and 3 in the Brillouin spectrum with temperature when the liquid refractive index is 1.342. It is found that temperature has no significant effect on the peak gain of the two peaks, and the gains of peaks 1 and 3 remain at 0.471m. -1 W -1 and 0.068m -1 W -1 Left and right; by Figure 3 (b) It can be seen that at a temperature of 70℃, the peak gains of the two peaks in the Brillouin spectrum show opposite trends with the change of the liquid refractive index, and the lowest peak gains of the two peaks during this process are 0.343m. -1 W -1 and 0.068m -1 W -1 It can be observed that peaks 1 and 3 used for sensing in the Brillouin spectrum of the optimized fiber structure maintain good peak gain throughout the entire sensing range.

[0024] After determining the fiber structure, numerical simulations were used to calculate the temperature and refractive index sensitivity of the two Brillouin peaks in the Brillouin gain spectrum. The results showed that the refractive index sensitivities of the two peaks in the fiber Brillouin spectrum at room temperature (20℃) were 774 MHz / RIU and 816 MHz / RIU, respectively, and the temperature sensitivities of the two peaks at a liquid refractive index of 1.317 were 1.06 MHz / ℃ and 1.12 MHz / ℃, respectively. Figure 4 As shown in (a) and (b), data were collected simultaneously, and the cross-sensitivity characteristics of temperature and refractive index were analyzed. The temperature coefficients of the refractive index sensitivity for peaks 1 and 3 in the Brillouin spectrum were obtained as -0.0148 MHz / (RIU·℃) and -0.0165 MHz / (RIU·℃), respectively. The refractive index coefficient of the temperature sensitivity is consistent with the above values, as shown in (a) and (b). Figure 5 As shown in (a) and (b).

[0025] Finally, due to the cross-sensitivity between temperature and refractive index, a neural network was used to improve the sensing accuracy. Brillouin gain spectrum data of the optical field fundamental mode excited under numerical simulation conditions of 20-70℃ and refractive indices of 1.317-1.342 were used as the training dataset to build a deep neural network, train the neural network model, and use the trained model to predict the temperature and refractive index parameters. The prediction results are as follows. Figure 6 As shown in (a) and (b), the RMSE of the neural network for temperature prediction is 0.03979℃, R 2 =0.9993, and the predicted RMSE of the refractive index =0.00002, R 2 =0.9992, which has a high accuracy rate.

Claims

1. A liquid-filled microstructure photonic crystal fiber capable of dual-parameter sensing of temperature and solution refractive index is proposed. The main features of this fiber are: precise temperature sensing is achieved by utilizing the sensitivity of the Young's modulus, Poisson's ratio, and solution refractive index of SiO2 material to temperature; simultaneously, the sensitivity of solution refractive index sensing is enhanced by utilizing the liquid-filled hole structure in the fiber core. Furthermore, the bimodal Brillouin spectrum generated by two regions with different refractive indices in the fiber core allows for simultaneous measurement of temperature and refractive index. In the fiber core design, the refractive index is increased by doping SiO2 with GeO2, and the fiber core is divided into inner and outer regions according to different doping concentrations. The refractive index of the inner high-doped region is 1.454, and the refractive index of the outer low-doped region is 1.

445. The cladding design employs a multi-layered air-hole structure to enhance the confinement of the optical field.

2. The specific spatial distribution of the injection holes and the two doped regions as described in claim 1 is as follows: The fiber core is divided into two regions, an inner region (refractive index 1.454) and an outer region (refractive index 1.445), based on different doping concentrations. Within the outer region, six injection holes are uniformly distributed at equidistant points from the center of the fiber core, with a radius of 0.81 μm.

3. The high-sensitivity solution refractive index sensing based on the fiber core injection hole as described in claim 1, characterized in that: On the one hand, changes in external temperature cause changes in the refractive index of SiO2 material and solution, which in turn affect the Brillouin frequency shift through changes in the effective refractive index of the mode, and temperature measurement is achieved through the Brillouin frequency shift. On the other hand, filling the sample solution into the six air holes in the microstructure core, the multi-layer air hole structure in the highly doped core and cladding enhances the optical fiber's ability to confine the optical field, improves the acousto-optic interaction between the optical field and the sample solution, and thus improves the refractive index sensitivity of the optical fiber. In addition, the dual-parameter sensing achieved by the two-layer doped regions described in claim 1 specifically refers to the following: different concentrations of GeO2 doping in the fiber core divide the optical fiber into two regions, so that the acoustic field energy generated during Brillouin scattering is concentrated in these two doped regions respectively. The coupling of the optical field fundamental mode LP01 with the two acoustic fields of different frequencies generates a bimodal Brillouin spectrum, which ultimately makes dual-parameter sensing of temperature and solution refractive index possible.

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