Simple M-type optical fiber structure capable of realizing equipower double Brillouin gain peaks and resisting bending loss

Abstract

The invention belongs to the field of sensing optical fiber design, and particularly relates to a simple M-type optical fiber structure capable of achieving equal-power double Brillouin gain peaks and bending loss resistance. According to the equipower double-Brillouin gain peak design based on the M-type optical fiber, the optical fiber ring core is evolved into the round core, a plurality of strong light and sound interactions are excited, and the equipower double-Brillouin gain peak design based on the M-type optical fiber is Specifically, when a ring core is evolved into a round core, firstly, changes of the effective refractive index and the effective mode area of an optical basic mode are researched, then, acoustic frequency evolution of an acoustic mode is researched, and finally, opto-acoustic coupling is researched. According to the invention, equipower double Brillouin gain peaks are realized in the evolution process, and a simple M-type optical fiber structure is determined. In addition, the structure also has the advantage of high bending loss resistance. In a word, the simple M-type optical fiber with the equal-power double-Brillouin gain peaks, designed by the invention, has important significance in two-parameter distributed sensing for resisting bending loss.

Description

Technical Field

[0001] The invention belongs to the field of sensing optical fiber design, and specifically relates to a simple M-type optical fiber structure with equal-power double Brillouin gain peaks. The structure is simple, and the structure can resist bending loss while having two equal-power Brillouin gain peaks. Background Art

[0002] Design and optimize Brillouin scattering-based sensing optical fiber for wide applications in structural health monitoring, oil pipeline early warning, intelligent biomedicine, deep space exploration, and marine and land earthquake early warning. (Reference 1: A. Geilen, A. Popp, D. Das, S. Junaid, CG Poulton, M. Chemnitz, C. Marquardt, M. A. Schmidt, and B. Stiller, "Extreme thermodynamics in nanolitre volumes through stimulated Brillouin-Mandelstam scattering," Nat Phys 19, 1805-+(2023).; Reference 2: 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).). In recent years, single-parameter sensing has increasingly failed to meet the requirements of realistic complex environments. Optical fibers with dual / multi-Brillouin gain peaks have been designed and used for dual-parameter sensing (Reference 3: YPXu, MQRen, Y.Lu, P.Lu, P.Lu, XYBao, LXWang, Y.Messaddeq, and S.LaRochelle, "Multiparameter sensorbased on stimulated Brillouin scattering in inverse-parabolic graded-indexfiber," Opt Lett 41, 1138-1141 (2016).; Reference 4: BW Wang, L.Wang, N.Guo, ZYZhao, CYYu, and C.Lu, "Deep neural networks assisted BOTDA for simultaneous temperature and strain measurement with enhanced accuracy," Opt Express 27, 2530-2543 (2019).).However, the reported optical fibers have complex refractive index distributions and small side peaks in the Brillouin gain spectrum, which increases sensing errors (Reference 5: N. Lalam, H. Bhatta, XG Sun, P. Lu, P. Ohodnicki, MP Buric, and R. Wright, "Multi-parameter distributed fiber optic sensing using double-Brillouin peak fiber in Brillouin optical time domain analysis," Opt Express 31, 36590-36602 (2023).). In addition, these optical fibers often have weak resistance to bending losses. It is challenging to use a simple optical fiber structure to excite equal-power dual Brillouin gain peaks while having strong resistance to bending.

[0003] Therefore, in order to improve the peak power ratio and thus achieve equal-power double Brillouin peaks, and to enhance the resistance to bending loss of optical fiber, the present invention evolves the ring core to the circular core, studies the changes in photoacoustic interaction, and accurately extracts the simple M-type optical fiber structure corresponding to the equal-power double Brillouin gain peak from the change in the Brillouin gain spectrum from a single peak to a double peak and then back to a single peak. The power ratio of the double peak is 1:1, and the peak spacing is 139MHz. At the same time, the optical fiber also exhibits excellent resistance to bending loss. The optical fiber can provide a competitive choice for distributed dual-parameter sensing, such as dual physical parameters such as temperature and strain, temperature and curvature. Summary of the invention

[0004] The present invention cleverly combines the characteristics of ordinary ring optical fiber and circular optical fiber in series. In the process of evolving the ring core to the circular core, the optical and acoustic mode characteristics and the evolution of the Brillouin gain spectrum are studied based on the photoacoustic interaction theory, and a simple M-type optical fiber structure is proposed that can achieve equal-power double Brillouin gain peaks and resist bending losses.

[0005] 1. Specific content of the present invention

[0006] Double-peak Brillouin gain spectrum is crucial for dual-parameter distributed sensing to overcome the cross-sensitivity problem between parameters. Bending loss seriously affects the accuracy and signal-to-noise ratio of distributed sensing. It is challenging to simultaneously achieve equal-power double-peak Brillouin gain spectrum and excellent resistance to bending loss using a simple optical fiber structure.

[0007] (1) The present invention evolves from a ring core to a round core optical fiber, such as Figure 1, optimize and design a simple M-type fiber structure that can achieve equal-power double-peak Brillouin gain spectrum. The refractive index of the pure silica cladding is 1.444, the refractive index of the germanium-doped ring core region is 1.4552, and the refractive index of the center (n c ) increases from 1.444 to 1.4552 in steps of 0.0002. The outer diameter and inner diameter of the cladding, the toroidal core are 150 μm, 18.8 μm and 11.6 μm respectively.

[0008] (2) In the process of the evolution from the ring core to the circular core, the refractive index of the fundamental optical mode gradually increases, while the effective mode area shows a slow increase first and then a rapid decrease. The changes in the acoustic frequencies of multiple acoustic modes show a unique change trend, and n is determined. c Threshold.

[0009] (3) The photoacoustic interaction theory provides a basis for studying the evolution of the Brillouin gain spectrum. The photoacoustic interaction coupling area, coupling efficiency and Brillouin gain variation are studied, and a simple structure that can achieve an equal-power double-peak Brillouin gain spectrum is determined in the evolution of the Brillouin gain spectrum.

[0010] (4) By comparing the designed M-type optical fiber with other optical fibers, the excellent bending loss resistance of the designed M-type optical fiber is highlighted.

[0011] 2. The advantages of the present invention are as follows:

[0012] (1) The present invention uses a simple optical fiber structure to achieve equal-power double Brillouin gain peaks, increase the intensity of the secondary peak, and improve the peak ratio of the double peak (main-secondary peak) to 1:1.

[0013] (2) The simple M-type optical fiber structure adopted in the present invention has excellent resistance to bending loss.

[0014] 3. The principles of the present invention are as follows:

[0015] (1) The refractive index distribution in the optical fiber must present at least two step-types to excite multiple acoustic modes, so that the basic optical mode can be photoacoustically coupled with multiple acoustic modes. Only when two strong photoacoustic couplings occur and the acoustic frequencies of the two acoustic modes have a reasonable frequency difference, can two Brillouin gain peaks be shown in the Brillouin gain spectrum. In the evolution of the ring core to the circular core, the photoacoustic coupling efficiency of the two basic optical modes caused by the overlap of two strong photoacoustics is equal to that of the two acoustic modes, and the frequency difference of the acoustic modes involved in the strong coupling is reasonable, so an equal-power double-peak Brillouin gain spectrum will be excited in a simple M-type optical fiber.

[0016] (2) M-type optical fiber has some characteristics of ring-core optical fiber, including a relatively narrow optical mode distribution, which makes it have excellent resistance to bending loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] (1) Figure 1 This is a schematic diagram of the evolution from a ring core to a round core fiber. This method is used to optimize the design of a simple M-type fiber structure that can achieve an equal-power double-peak Brillouin gain spectrum. The refractive index of the pure silica cladding is 1.444, the refractive index of the germanium-doped ring core region is 1.4552, and the refractive index of the center (n c ) increases from 1.444 to 1.4552 in steps of 0.0002. The outer diameter and inner diameter of the cladding, the toroidal core are 150 μm, 18.8 μm and 11.6 μm respectively.

[0018] Figure 2 (a) is the effective refractive index and effective mode area of ​​the simulated fundamental optical mode (LP01) as the central refractive index (n c )Added change graph. Figure 2 (b) is the evolution of optical mode distribution. c = 1.4526, the effective mode area decreases rapidly and linearly, and the optical mode distribution is similar to a flat disk, such as Figure 2 For comparison, the solid line and dotted line represent n c =1.4524 and 1.4528.

[0019] Figure 3 (ad) are the frequency evolutions of the fundamental acoustic mode (L01) and three higher-order acoustic modes (L02, L03, L04), respectively.

[0020] Figure 4 (a) is the photoacoustic coupling efficiency, in n c =1.4526, the fiber structure can show a reasonable equal photoacoustic coupling efficiency. Figure 4 (b) is the equal-power double-peak Brillouin gain spectrum excited in the designed M-type fiber structure.

[0021] Figure 5 The figure compares the calculated bending loss of the designed M-type optical fiber with other reported optical fibers. DETAILED DESCRIPTION

[0022] 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:

[0023] In order to achieve equal-power dual Brillouin gain peaks, the invention evolves from a ring core to a round core and adopts an M-type optical fiber, such as Figure 1 As shown. Central refractive index n c The refractive index of the cladding gradually increases to the refractive index of the core. This process shows the evolution from a ring core to a circular core. cTo explore the changes in the fundamental optical and acoustic modes, as well as the photoacoustic interaction, is important for obtaining an optimized Brillouin enhancement peak with double equal power.

[0024] Figure 2 (a) shows that the optical effective refractive index gradually increases with the increase of nc. Unlike the effective refractive index, the effective mode area of ​​the fundamental optical mode shows a slow increase first and then a rapid decrease, and c =1.4526, the optical mode distribution becomes flat, and the mode area is -3.259×10 4 μm 2 The linear rate decreases rapidly, such as Figure 2 (b).

[0025] Figure 3 The frequency evolution of the four acoustic modes is shown. By comparison, the L01, L02 and L03 acoustic modes change significantly at the thresholds nc = 1.4544, 1.4538 and 1.452, which also shows that as the order of the acoustic mode increases, n c The threshold is getting smaller and smaller. And, as the acoustic mode order increases, below the threshold, the blue shift of the acoustic frequency changes from two consecutive increases to a single increase, and the final frequency change sign changes to a red shift. When it exceeds n c After the threshold, the frequency decrease rate becomes faster and faster with the increase of the acoustic mode order. These acoustic frequency change behaviors may be attributed to the fact that the effective sound velocity of the higher-order acoustic mode decreases much faster than that of the fundamental acoustic mode and is more unstable. In addition, the higher-order acoustic mode is more distributed in the center of the optical fiber with low refractive index and high sound velocity.

[0026] Figure 4 (a) shows the evolution of coupling efficiency and finds the intersection of multiple acousto-optic interaction curves. The first intersection is determined to be the best according to the frequency interval. The fundamental optical mode and the L01 and L03 acoustic modes have strong photoacoustic coupling, and the corresponding n c =1.4526. Figure 4 (b) shows the Brillouin gain spectrum under the optimal fiber structure, and the peak-to-peak ratio of the two Brillouin gains is 1:1.

[0027] Figure 5 The fiber bending loss curve is shown. By comparing with other optical fibers, the designed M-type simple optical fiber structure with equal peak Brillouin gain spectrum has excellent anti-bending loss ability, which is comparable to the anti-bending ability of ordinary ring optical fiber.

[0028] The present invention proposes a simple M-type optical fiber structure that can achieve equal-power dual Brillouin gain peaks and resist bending loss. Through the theory of photoacoustic interaction, the characteristics of optical mode and acoustic mode are studied, and photoacoustic coupling is studied to determine the optical fiber structure that can achieve the goal. This provides a competitive option for dual-parameter distributed sensing.

Claims

1. A simple M-type optical fiber structure that can achieve equal-power dual Brillouin gain peaks and resist bending loss, characterized in that: The standard ring core fiber evolves to round core fiber, corresponding to the central refractive index (n c ) increases from the low refractive index of the cladding to the high refractive index of the core. The optical fiber structure is M-type, with a cladding diameter of 150μm, an outer ring diameter of the annular region of 18.8μm, and an inner ring diameter of 11.6μm.

2. The central refractive index according to claim 1, characterized in that: The fiber center refractive index changes from 1.444 to 1.4552 with a step size of 0.0002. During this evolution, the fiber exhibits a simple M-type structure.

3. According to the equal power dual Brillouin gain peak described in claim 1, it is characterized in that: The Brillouin gain spectrum generated by the acousto-optic interaction in the optical fiber has two gain peaks of equal power. The peak-to-peak ratio of the two gain peaks is 1:1, and the peak spacing is about 139MHz.

4. The optical fiber structure and Brillouin gain spectrum evolution according to claims 1, 2 and 3, characterized in that: As the ring core evolves to a circular core, the excited Brillouin gain spectrum also shows a transformation from a single peak to a double peak and then back to a single peak. The effective refractive index, effective area and acoustic mode frequency of the optical mode involved in the acousto-optic interaction show a unique change trend. The specific M-type fiber structure is determined by the equal-power double Brillouin gain peaks shown in the spectrum evolution.

5. The bending loss resistance according to claim 1, characterized in that: Compared with the reported optical fibers used for sensing, the designed M-type optical fiber has good bending loss resistance.

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