Three-layer step type optical fiber structure with wide bending sensing range and high sensitivity

Abstract

The invention belongs to the field of optical fiber sensing, and particularly relates to a three-layer step type optical fiber structure which has a wide bending radius sensing range and high bending sensitivity on the premise that bending loss can be resisted. By regulating and controlling the refractive index of the second layer of the fiber core, balance between low bending loss and high bending sensitivity is achieved. And meanwhile, two groups of spatially separated acoustic modes are excited under a specific refractive index condition, so that double Brillouin gain peaks can be formed. Specifically, when the refractive index of the second layer of the fiber core changes, the characteristics of optical and acoustic modes and opto-acoustic interaction are studied, and finally a W-type optical fiber structure with excellent performance is determined. The structure has a wider bending radius sensing range and higher bending sensitivity while keeping a good bending loss resistance characteristic. In a word, the optical fiber structure provided by the invention has important application value in the field of optical fiber double-physical parameter distributed sensing, and a new thought is provided for realizing high-performance Brillouin bending sensing.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of sensing fiber design, and particularly relates to a W-shaped fiber structure with excellent sensing performance. Its characteristics are to realize wide bending radius sensing range and high bending sensitivity on the premise of resisting bending loss. BACKGROUND

[0002] With the rapid development of structural health monitoring, oil pipeline safety early warning, intelligent biological medicine, deep space exploration and marine and terrestrial earthquake monitoring, the demand for optical fiber sensing technology capable of high-precision and multi-parameter detection in complex environments is increasing. Optical fiber sensors based on Brillouin scattering have become a research hotspot due to their advantages of distributed measurement, resistance to electromagnetic interference and remote transmission.

[0003] Traditional single-parameter Brillouin fiber sensors are usually used for independent measurement of strain or temperature, but in practical applications, temperature and strain often change simultaneously, causing serious cross-sensitivity problem of single-parameter sensing, which limits its application in complex environments. In recent years, multi-parameter Brillouin sensing technology has been proposed, which realizes the simultaneous discrimination and decoupling measurement of multiple parameters by designing special fiber structure with double Brillouin gain peaks (Ref. 1: A. Rjeb, A. M. Ragheb, M. A. Esmail, H. Fathallah and S. A. Alshebeili. Discriminative strain and temperature sensing using a ring-hyperbolic tangent fiber sensor. Opt Express 30, 34612-34628 (2022).; Ref. 2: Y. P. Xu, M. Q. Ren, Y. Lu, P. Lu, P. Lu, X. Y. Bao, L. X. Wang, Y. Messaddeq and S. LaRochelle. Multi-parameter sensor based on stimulated Brillouin scattering in inverse-parabolic graded-index fiber. Opt Lett 41, 1138-1141 (2016).). Few-mode fiber is applied to Brillouin curvature radius sensing, but few-mode fiber is prone to large optical loss under high bending conditions (Ref. 3: H. Wu, M. Tang, M. Wang, C. Zhao, Z. Y. Zhao, R. X. Wang, R. L. Liao, S. N. Fu, C. Yang, W. J. Tong, P. P. Shum and D. M. Liu. Few-mode optical fiber based simultaneously distributed curvature and temperature sensing. Opt Express 25, 12722-12732 (2017).), which limits its stable application in space-limited or vibration environment.Currently, some reported multimodal Brillouin fibers have achieved a certain degree of sensing function of bending radius and can resist bending loss, but the upper limit of the bending radius sensing range of these fibers is small, and the bending sensitivity is low (Literature 3: P. B. Xu, H. P. Guo, X. L. Wang, L. Shen, K. H. Wen, Y. H. Sun, D. X. Ba, Y. K. Dong, X. Y. Dong, J. Yang and Y. W. Qin. Ring-core few-mode fiber and DPP-BOTDA-based distributed large-curvature sensing eligible for shape reconstruction. Opt Express 30, 42553-42563 (2022).; Literature 4: L. Shen, H. Wu, C. Zhao, L. Shen, R. Zhang, W. J. Tong, S. N. Fu and M. Tang. Distributed curvature sensing based on a bending loss-resistant ring-core fiber. Photonics Res 8, 165-174 (2020).).

[0004] Therefore, in order to make the optical fiber improve the upper limit of the sensing range and the bending sensitivity on the premise of resisting bending loss, the present application evolves from W-type optical fiber to M-type optical fiber, studies the change from two groups of spatially separated acoustic modes to the fusion of two groups of acoustic modes into one group, and selects the optical fiber structure with the best sensing performance. The designed optical fiber structure is W-shaped, which exhibits a wider bending radius sensing range and higher bending sensitivity on the premise of resisting bending loss. The optical fiber has great prospects in the field of distributed double-parameter sensing (such as temperature and curvature), and is an ideal solution to solve such measurement needs. SUMMARY

[0005] In the process of evolving from W-type optical fiber to M-type optical fiber through three-layer step-type optical fiber, based on the principle of stimulated Brillouin scattering, the special optical and acoustic mode characteristics are studied, and a simple three-layer W-type optical fiber structure is proposed, which can exhibit a wider bending radius sensing range and higher bending sensitivity on the premise of resisting bending loss.

[0006] 1. Specific content of the present application

[0007] There is an inherent contradiction between bending sensitivity and anti-bending loss performance. Generally, to improve bending sensitivity, the light field needs to be enhanced in the cladding or interface to amplify the change in Brillouin frequency shift caused by bending. However, such design will cause the light energy to concentrate in the outer layer area, thereby significantly increasing the bending loss, intensifying the attenuation of the optical signal, and limiting the sensing distance and stability. How to balance between anti-bending loss and good sensing performance is the core difficulty of structural design.

[0008] (1) The three-layer core structure is designed, and the evolution of the refractive index of the optical fiber from W type to M type is as follows: Figure 1 The refractive index of the pure silica cladding (n clad ) is 1.444, and the core structure from inside to outside is first layer, second layer and third layer. The three-layer core is germanium-doped, wherein the refractive index (n1) of the first layer core is 1.4524, the refractive index (n3) of the third layer core is 1.4552, and the refractive index (n2) of the second layer core ranges from 1.444 to 1.4552 with a step of 0.0002. The cladding diameter is 150 μm, and the diameters of the three-layer core from inside to outside are 8.6 μm, 13.8 μm and 20 μm.

[0009] (2) In the evolution process from W type optical fiber to M type optical fiber, that is, as the refractive index of the second layer core increases, the effective refractive index of the basic optical mode monotonically increases, and the effective area monotonically increases first and then monotonically decreases.

[0010] (3) The structure has a unique characteristic in the aspect of photoacoustic interaction. When the optical fiber is in W type structure, the light field distribution presents an inner-outer double-peak characteristic, and the energy is concentrated in the first layer and the third layer core, corresponding to two groups of spatially separated acoustic modes. One group of acoustic energy is mainly concentrated in the first layer core, and the other group is concentrated in the third layer core, forming two groups of independent photoacoustic interaction zones. As the refractive index of the second layer core increases, the distribution of the two groups of acoustic modes begins to approach and energy coupling occurs, and gradually fuses into one group of acoustic modes, and the fusion threshold is about n2=1.45. When the optical fiber structure evolves into M type, that is, n2=1.4524, the two groups of acoustic modes are completely fused into one group. At the same time, the basic modes of the two groups of acoustic modes can maintain a large peak spacing under the condition of large peak gain, and the W structure of the double-peak Brillouin gain spectrum can be determined to realize high gain in the evolution of the Brillouin gain spectrum.

[0011] (4) Compared with other optical fibers, the designed W type optical fiber has higher bending sensitivity and wider bending radius sensing range under the premise of anti-bending loss characteristics, and has made a breakthrough in solving the inherent contradiction between bending sensitivity and anti-bending loss performance.

[0012] 2. The advantages of the present application are as follows:

[0013] (1) The simple W-shaped optical fiber structure adopted by the present application has excellent bending loss resistance and high bending sensitivity.

[0014] (2) The simple W-shaped optical fiber structure adopted by the present application has a wider bending radius sensing range than other optical fibers, and is more adaptable to measurement.

[0015] 3. The principle of the present application is as follows:

[0016] (1) In optical fiber bending sensing based on the Brillouin scattering effect, bending causes changes in the core refractive index distribution and the light field distribution, resulting in changes in the Brillouin frequency shift (BFS). Bending sensitivity mainly depends on the degree of energy distribution of the light field in the outer layer of the core; when the light field leakage is enhanced, the influence of bending disturbance on the effective refractive index is amplified, and the sensitivity is improved. However, light field leakage will also cause a significant increase in bending loss, limiting the sensing distance and system stability. Therefore, how to balance between high bending sensitivity and low loss is the core problem of optical fiber structure design. The three-layer germanium-doped core structure proposed by the present application, the second layer of core refractive index is adjustable, is the key layer to control the light field leakage and the acoustic mode distribution; the third layer of core is used to guide part of the light energy to distribute outward, improving the response ability to bending disturbance. By adjusting the second layer of core refractive index n2, the optical fiber refractive index distribution can be continuously evolved between W-shaped structure and M-shaped structure. During this evolution process, the spatial distribution of the light field and its coupling relationship with the acoustic mode change systematically.

[0017] (2) The gain of Brillouin scattering is mainly determined by the coupling strength between light wave and acoustic wave. In a multi-layer core structure, the spatial distribution of acoustic mode and the matching degree of light field mode directly affect the shape and sensitivity of Brillouin gain spectrum. Under the W-shaped structure, due to the existence of two high refractive index regions, the acoustic mode presents the characteristics of two groups of acoustic modes spatially separated: the first group of acoustic modes concentrates energy in the first layer of core (circular region), and the second group of acoustic modes concentrates energy in the third layer of core (annular region). These two groups of acoustic modes interact with the corresponding light field in the corresponding region, forming a double-peak Brillouin gain spectrum. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 (a) is a schematic diagram of a three-layer step-shaped optical fiber structure. Figure 1 (b) is a schematic diagram of the evolution of the refractive index of the optical fiber from W-shaped to M-shaped. The pure silica cladding (n cladThe refractive index of the W-type fiber is 1.444, the refractive index of the first layer of the core (n1) is 1.4524, the refractive index of the third layer of the core (n3) is 1.4552, and the refractive index of the second layer of the core (n2) ranges from 1.444 to 1.4552 with a step of 0.0002. The cladding diameter is 150 μm, and the diameters of the three layers of the core from the inside out are 8.6 μm, 13.8 μm, and 20 μm, respectively. Figure 1 (c) is a plot of the effective refractive index and the effective mode area of the simulated fundamental optical mode (LP01) as a function of the second layer core refractive index (n2).

[0019] Figure 2 is a plot of the BFS variation and comparison of the first five acoustic modes. Among them Figure 2 (a-b) are the BFS variation plots of the fundamental acoustic mode (Ring-L01) and the high-order acoustic mode (Ring-L02) of the acoustic mode concentrated in the third layer of the core (i.e., the annular region), respectively. Figure 2 (c-d) are the BFS variations of the fundamental acoustic mode (Circular-L01) and the high-order acoustic mode (Circular-L02) of the acoustic mode concentrated in the first layer of the core (i.e., the circular region) evolving into high-order acoustic modes (Combined-L03 and Combined-L04) of the fused acoustic mode group as the second layer core refractive index (n2) increases, respectively. Figure 2 (e) is the BFS variation of the annular region high-order acoustic mode (Ring-L03) evolving into the high-order acoustic mode (Combined-L05) of the fused acoustic mode group. Figure 2 (f) is a comparison plot of the BFS variations of the above five acoustic modes as the second layer core refractive index (n2) increases.

[0020] Figure 3 (a) is an evolution plot of the light-acoustic coupling area of the first five acoustic modes. Figure 3 (b) is an evolution plot of the light-acoustic coupling efficiency of the first five acoustic modes. Figure 3 (c) is an evolution plot of the Brillouin gain of the first five acoustic modes. Figure 3 (d) is a double-peak Brillouin gain spectrum excited in the designed W-type fiber structure.

[0021] Figure 4 (a) is a plot of the variation of the light field energy distribution of the designed W-type fiber when bent. Figure 4 (b-d) are comparison plots of the fiber structure and other reported fiber structures in terms of the range of sensing bending radius, bending sensitivity, and bending loss, respectively. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present application clearer, the specific structure, principle and performance of the present application will be further described below with reference to the drawings:

[0023] In order to balance the high bending sensitivity and low loss, the present application proposes a three-layer germanium-doped core structure, as shown in Figure 1 The second layer core refractive index can be adjusted, which is the key layer to control the light field leakage and acoustic mode distribution; the third layer core is used to guide part of the light energy to distribute outward, improving the response ability to bending disturbance. By adjusting the second layer core refractive index n2, the optical fiber refractive index distribution can be continuously evolved between the W-type structure and the M-type structure. With the increase of n2, the light field gradually expands from the inner layer core to the outer layer region; when n2 further increases to close to the third layer refractive index, the light field is re-constrained by the outer layer core, realizing the re-concentration of the light field distribution and energy balance.

[0024] Figure 2 The BFS evolution process of the first five acoustic modes is shown. When the refractive index distribution is a typical "W type, the acoustic modes exhibit the characteristics of two groups of mode space separation. Moreover, with the increase of the second layer core refractive index n2, due to the change of the light field distribution, the characteristics of the light-acoustic interaction are also changing. The acoustic modes originally separated in the third layer annular core region and the first layer circular core region gradually approach and merge into a group. Specifically, the basic mode Ring-L01 of the annular acoustic mode group becomes the basic mode Combined-L01 of the combined acoustic mode group, and its high-order acoustic modes Ring-L02 and Ring-L03 evolve into the high-order acoustic modes Combined-L02 and Combined-L05 of the combined acoustic mode group, respectively; the basic mode Circular-L01 and the high-order mode Circular-L02 of the inner circular acoustic mode group evolve into the high-order acoustic modes Combined-L03 and Combined-L04 of the combined acoustic mode group, respectively. The entire evolution process starts from n2=1.45, and ends when the second layer core refractive index increases to the same as the first layer core refractive index, at which time the optical fiber structure becomes M-type, showing the characteristics of a typical M-type optical fiber.

[0025] At the same time, during the evolution process, the BFS of the first five groups of acoustic modes changes relatively consistently, so it can better illustrate the authenticity of the evolution process. In addition, the BFS of the first four acoustic modes shows a trend of first increasing and then decreasing, while the BFS of the fifth acoustic mode shows a trend of continuously decreasing. Finally, the characteristic frequency interval of the first five modes becomes smaller compared to the initial state. The BFS change is determined by the effective refractive index of the basic optical mode and the effective acoustic velocity of the acoustic mode. The frequency change exhibited by the present optical fiber may be due to the continuous decrease of the effective acoustic velocity of the acoustic mode, and the high-order mode is more unstable, and its effective acoustic velocity decreases faster.

[0026] Figure 3 The photoacoustic coupling effective area, efficiency and Brillouin gain of the first five acoustic modes are shown respectively. When n2<1.45, the acoustic modes mainly coupled with the fundamental optical mode are Ring-L01 and Circular-L01, and the coupling relationship is relatively stable. When n2 increases, other acoustic modes gradually participate in the coupling. In particular, when n2=1.4524, the photoacoustic coupling effective area, efficiency and Brillouin gain of the three evolved modes Circular-L01, Circular-L02 and Ring-L03 all reach a peak, among which the gain of Circular-L01 reaches the maximum value and the gains of the other two modes reach the minimum value. Such performance can be attributed to the fact that the light field distribution also expands to a peak when n2=1.4524 while the acoustic field expands outward, and the light field distribution starts to shrink after n2>1.4524, and is re-constrained by the outer core.

[0027] Figure 4 (a) shows the light field distribution of the optical fiber after bending, and the center of light field energy gradually deviates as the bending radius decreases. Figure 4 (b-d) show the comparison of the present optical fiber structure with the reported optical fibers in terms of bending radius sensing range, bending sensitivity and bending loss. The present optical fiber has a wider bending radius sensing range and higher bending sensitivity under the premise of resisting bending loss.

[0028] The present application designs a three-layer step-type core structure. Through the analysis of the Brillouin frequency shift evolution of acoustic modes and the photoacoustic coupling characteristics, the structure with the strongest comprehensive sensing performance is determined. The design takes into account the high bending sensitivity and low loss characteristics, significantly widens the bending radius sensing range, and improves the stability and measurement accuracy of the bending response. The present application provides a new structure design idea and effective technical approach for realizing high-performance and wide dynamic range Brillouin bending sensing optical fiber.

Claims

1. A three-layer step index fiber structure with wide bending radius sensing range and high bending sensitivity simultaneously under the premise of resistance to bending loss, characterized in that: The fiber core has a three-layer structure, and from inside to outside, it is respectively a first layer, a second layer and a third layer. The second layer fiber core refractive index (n2) rises from the cladding refractive index to a higher third layer fiber core refractive index, the whole structure evolves from a W-type optical fiber to an M-type optical fiber, and in the refractive index regulation process, the controllable excitation and fusion of the two groups of spatially separated acoustic modes are realized, and the Brillouin gain spectrum also evolves specifically. The fiber structure size parameters are: the cladding diameter is 150 μm, and the diameters of the three layers of the fiber core from inside to outside are respectively 8.6 μm, 13.8 μm and 20 μm. ​ 2. The second layer refractive index as claimed in claim 1, wherein: The second layer fiber core refractive index ranges from 1.444 to 1.4552, with a step of 0.0002. In this evolution process, the optical fiber evolves from a W-type structure to an M-type structure.

3. The optical fiber structure and specificity evolution of Brillouin gain spectrum according to claim 1, 2, characterized in that: As the W-type optical fiber evolves into an M-type optical fiber, the excited Brillouin gain spectrum shows a transformation from a double-peak to a triple-peak and then back to a single-peak. The optical mode effective refractive index, effective area and acoustic mode characteristic frequency, Brillouin gain involved in the interaction of acousto-optics all show unique change trends.

4. The two sets of spatially separated acoustic modes as defined in claim 1 wherein: When the fiber structure is W-type, the interaction of acousto-optics will excite two groups of spatially separated acoustic modes in the first and third layers of the fiber core with higher refractive index, respectively. As the W-type optical fiber evolves into an M-type optical fiber, the two groups of acoustic modes gradually fuse into one group.

5. The wide bending sensing range, high bending sensitivity as claimed in claim 1, wherein: Compared with the single-core optical fiber reported for bend radius sensing, the W-type optical fiber has a wider sensing range. In the process of evolving from a W-type optical fiber to an M-type optical fiber, the structure with the optimal sensing range and sensitivity performance is selected to determine a simple W-type optical fiber.

6. The bend loss resistant optical fiber of claim 1, wherein: Compared with the reported optical fibers for sensing, the designed W-type optical fiber has bend loss resistance ability.

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