FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber and its manufacturing method

Abstract

This invention discloses an FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber and its fabrication method. The sensor comprises an input single-mode fiber, multiple segments of coreless optical fiber, and an output single-mode fiber connected sequentially. The input single-mode fiber is fused together with the first segment of the coreless fiber in a misaligned manner, and the next segment of the coreless fiber is fused together with the previous segment in a misaligned manner, with the misalignment direction of the next segment opposite to that of the previous segment, but the misalignment distance being the same. The output single-mode fiber and the last segment of the coreless fiber are fused together axially. The central axes of the input and output single-mode fibers are collinear, and the cores of the input and output single-mode fibers are connected by a photolithographically written straight waveguide. A fiber Bragg grating is then optically etched along the straight waveguide. This invention uses femtosecond lasers to fabricate the FBG on the misaligned fiber structure, increasing the structure's strain response and making the strain sensitivity of the FBG on this structure higher than that of an FBG fabricated on ordinary silica optical fiber.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensor technology, and in particular to a fiber Bragg grating (FBG) strain sensor based on misaligned fusion spliced ​​coreless optical fiber and its manufacturing method. Background Technology

[0002] Fiber Bragg gratings (FBGs) are passive filtering devices that use a specific method to periodically modulate the refractive index of an optical fiber core along its axial direction. Their peak-position detection method effectively improves the sensitivity of sensing and measurement systems. With the widespread application of femtosecond laser technology in materials microfabrication, point-by-point direct writing of FBGs using femtosecond lasers has gradually become the main method for FBG fabrication due to its speed and stability. The femtosecond laser interacts with a transparent medium, inducing a change in the refractive index of the fiber core through nonlinear effects to write the grating. This method does not require the optical fiber to have strong photosensitivity, and the written grating exhibits high temperature stability, narrow linewidth, and large refractive index modulation characteristics.

[0003] In 1978, K. Hill et al. fabricated the first fiber grating (FBG) by forming periodic refractive index modulation regions along the axial direction of the germanium-doped fiber core using a 488nm argon-ion laser interferometric standing-wave method. However, this interferometric standing-wave method required a high-performance laser source, resulting in low fabrication efficiency and poor spectral characteristics of the fabricated FBG. In 1999, Kondo et al. first proposed femtosecond laser point-by-point writing technology, ushering in a new era for FBG writing. FBG writing was no longer limited by a phase plate; different wavelength FBGs could be fabricated simply by adjusting the platform's movement speed and the laser frequency. In 2004, Martinez et al. fabricated FBGs with first, second, and third-order resonances in the C-band on ordinary single-mode fiber. In 2008, M. Aslund et al. fabricated FBG structures in single-mode fiber core materials using an 800nm ​​femtosecond laser point-by-point writing method. Through continuous research, innovation, and accumulation, femtosecond laser technology has broken through the limitations of ultraviolet laser writing technology, successfully writing grating structures in various types of optical fibers. This has diversified the structural types of FBGs, providing a wider range of choices for practical applications. In 2015, Jiang Nuan et al. etched the cladding of single-mode fibers and wrote FBGs in these fibers with different cladding diameters, studying the relationship between the cladding diameter and the strain sensitivity of the FBG. Experimental results showed that as the cladding diameter decreased from 125 μm to 57 μm, the strain sensitivity increased from 0.578 pm / με to 2.57 pm / με. In 2017, Yang Tingting et al. successfully fabricated FBGs inside multimode fibers based on single-to-multimode single-fiber structures and studied the effect of high temperature on their strain sensitivity. Experiments showed that the strain sensitivity could reach 1.66 pm / με at 700℃. In 2021, S. Sridhar et al. coated MoS2 onto a fiber core with the cladding removed, leaving only the fiber core with the FBG. This successfully and significantly improved the strain sensitivity of the FBG. However, the sensor strength was extremely low due to the severe damage to the fiber structure.

[0004] In summary, various methods have been attempted to improve the strain sensitivity of FBGs to date, but most of the methods that have achieved significant improvements have high prerequisite requirements or complex operations, which are not conducive to fabrication and will seriously affect the stability and strength of the fiber structure, thus hindering practical applications.

[0005] Therefore, those skilled in the art are dedicated to developing an FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber and its manufacturing method, which effectively improves the strain sensitivity of FBG on silica optical fiber, is easy to manufacture, and has a stable and highly repeatable structure. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is how to enhance the strain sensitivity of FBG. FBG is fabricated on a misaligned fiber structure by femtosecond laser, thereby increasing the structure's response to strain and making the strain sensitivity of FBG on this structure higher than that of FBG fabricated on ordinary quartz fiber.

[0007] To achieve the above objectives, this invention provides an FBG strain sensor based on staggered fusion spliced ​​coreless fiber, comprising an input single-mode fiber, multiple segments of coreless fiber, and an output single-mode fiber connected in sequence; the input single-mode fiber is fused together with the first segment of coreless fiber in a staggered manner, and the next segment of coreless fiber is fused together with the previous segment of coreless fiber in a staggered manner, with the staggering direction of the next segment being opposite to that of the previous segment, and the staggering distance being the same; the output single-mode fiber and the last segment of coreless fiber are fused together axially, the central axes of the input single-mode fiber and the output single-mode fiber are on the same straight line, and the cores of the input single-mode fiber and the output single-mode fiber are connected by a photolithographically written straight waveguide, along which a fiber Bragg grating is optically etched.

[0008] Furthermore, the cladding diameter of both the input single-mode fiber and the output single-mode fiber is 125 μm, and the core diameter of both the input single-mode fiber and the output single-mode fiber is 4 to 10 μm; the diameter of each of the multiple coreless fiber segments is 125 μm.

[0009] Furthermore, the multiple coreless optical fiber segments are an even number of segments between 2 and 20; each coreless optical fiber segment has the same length, ranging from 200 to 2000 μm; and the misalignment distance is between 4 and 25 μm.

[0010] Furthermore, the width of the straight waveguide is 4 to 16 μm; the period of the fiber Bragg grating is 0.3 to 3 μm.

[0011] This invention also provides a method for manufacturing an FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber, comprising the following steps:

[0012] Step 1: Fabrication of the coreless fiber misalignment structure: First, the single-mode fiber with the coating removed and the coreless fiber are fused together in opposite directions in a misaligned manner. The coreless fiber is cut to the selected length. Using the central axis of the single-mode fiber as a reference, each subsequent segment of the coreless fiber is fused together with the same distance offset vertically. Finally, another single-mode fiber is fused onto the last segment of the coreless fiber, aligned with the central axis.

[0013] Step 2: Femtosecond laser writing of the straight waveguide: Fix the misaligned structure on a three-dimensional precision displacement platform, immerse the fiber in refractive index matching liquid, and then calibrate the fiber in the horizontal and vertical directions; ensure that the single-mode fiber cores on both sides are on the same straight line, on the same horizontal plane, and at the same distance from the femtosecond laser lens; set the parameters for writing the straight waveguide on the femtosecond laser control software, and write the straight waveguide using the direct writing technology, connecting the fiber cores of the single-mode fibers at both ends;

[0014] Step 3: Femtosecond laser writing of fiber Bragg gratings: Set the various parameters for grating writing, and use direct writing technology to write dot-shaped gratings on the fabricated straight waveguide.

[0015] Furthermore, in step 1, the cladding diameter of the single-mode fiber is 125 μm, the core diameter is 4 to 10 μm, and the diameter of the coreless fiber is 125 μm.

[0016] Furthermore, in step 1, the number of segments of the coreless optical fiber is an even number between 2 and 20; the length of each segment of the coreless optical fiber is the same, ranging from 200 to 2000 μm; and the misalignment distance is between 4 and 25 μm.

[0017] Furthermore, in step 2, the refractive index of the refractive index matching liquid is 1.448; when writing the straight waveguide, the femtosecond laser pulse energy is 1.2 μJ, the frequency is 1 kHz, the platform moving speed is 30 μm / s, and the width of the straight waveguide is 4 to 16 μm.

[0018] Furthermore, in step 3, when writing the fiber Bragg grating, the femtosecond laser pulse energy is 0.3 μJ, the frequency is 100 Hz, the platform moving speed is 107 μm / s, and the period of the fiber Bragg grating is 0.3 to 3 μm.

[0019] Furthermore, the fabrication method for the straight waveguide is a stacked straight-line fabrication method. Specifically, each fabrication is of the same length and connects the two ends of the single-mode fiber core and penetrates a short distance into the core to increase the light coupled into the modulation region. The spacing between each fabrication is 0.25 to 1 μm, and a total of 16 fabrications are made. The width of the completed modulation region is 4 to 16 μm.

[0020] The beneficial effects of this invention are as follows:

[0021] (1) The basic structure of this invention is simple, requiring only single-mode optical fiber and coreless optical fiber;

[0022] (2) This invention effectively improves the strain sensitivity of FBG on quartz optical fiber through simple structural transformation.

[0023] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the manufacturing process of a preferred embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram of an FBG structure according to a preferred embodiment of the present invention;

[0026] Figure 3 This is an FBG microscope image of a preferred embodiment of the present invention;

[0027] Figure 4 These are transmission spectrum images of the structure at different manufacturing stages of a preferred embodiment of the present invention;

[0028] Figure 5 This is a strain response spectrum of FBG according to a preferred embodiment of the present invention;

[0029] Figure 6 This is a fitted diagram of the FBG strain response according to a preferred embodiment of the present invention.

[0030] Among them, 1-single-mode fiber, 2-coreless fiber, 3-femtosecond laser. Detailed Implementation

[0031] The preferred embodiments of the present invention are described below with reference to the accompanying drawings to make the technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0032] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings.

[0033] Example

[0034] This embodiment provides an FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber and its manufacturing method, including the following steps:

[0035] Step 1: Fabrication of the coreless fiber misalignment structure

[0036] Figure 1The diagram shows the fabrication process of the misaligned structure. The specific fabrication method is as follows: S1, First, the single-mode fiber 1 (with the coating removed) and the coreless fiber 2 are spliced ​​together in a misaligned manner using discharge fusion. S2, The coreless fiber 2 is cut to a selected length L. S3, Using the central axis of the single-mode fiber 1 as a reference, each subsequent segment of the coreless fiber 2 is spliced ​​together with the same vertical misalignment distance Δx. S4, Finally, another single-mode fiber 1 is spliced ​​together with the central axis and spliced ​​onto the last segment of the coreless fiber 2.

[0037] Step 2: Femtosecond laser writing of straight waveguide

[0038] The misaligned structure was fixed on a three-dimensional precision displacement platform. A refractive index matching liquid (1.448) was dripped around the optical fiber to immerse it, and then the fiber was calibrated horizontally and vertically. The two single-mode fiber cores were aligned on the same straight line and horizontal plane, maintaining the same distance from the femtosecond laser's lens. The parameters for writing the straight waveguide were set in the femtosecond laser control software, and the straight waveguide was written using direct writing technology, connecting the two single-mode fiber cores. During the straight waveguide writing process, the femtosecond laser pulse energy was 1.2 μJ, the frequency was 1 kHz, the platform movement speed was 30 μm / s, and the width of the straight waveguide was 8 μm.

[0039] Step 3: Femtosecond laser inscription of FBG

[0040] By setting the parameters for grating writing, a dot-shaped grating is written onto the fabricated straight waveguide using a direct-write technique. The completed grating structure is shown below. Figure 2 As shown, the grating microscope image is as follows Figure 3 As shown. During FBG writing, the femtosecond laser pulse energy was 0.3 μJ, the frequency was 100 Hz, the platform moving speed was 107 μm / s, and the FBG period Λ was 1.07 μm.

[0041] In this embodiment, single-mode fiber 1 and coreless fiber 2 are made of quartz fiber.

[0042] In this embodiment, the single-mode fiber 1 has a cladding diameter of 125 μm and a core diameter of 8.2 μm, and the coreless fiber 2 has a diameter of 125 μm.

[0043] In this embodiment, the fiber splice misalignment distance Δx is 12μm, and the length L of each coreless fiber segment is 1000μm, for a total of 6 coreless fiber segments.

[0044] This embodiment utilizes a staggered coreless fiber structure and femtosecond laser micromachining to enhance the strain response of a fiber-to-glass (FBG) on a silica fiber. The staggered fusion splicing method increases the strain level under stress, resulting in a structure with higher strain sensitivity compared to FBGs fabricated on ordinary silica fibers. The sensor's basic structure consists of two single-mode fiber segments and multiple staggered coreless fiber segments between them. Light propagates in the fundamental mode in the single-mode segment; upon reaching the boundary between the single-mode and coreless segments, a series of higher-order modes are excited in the coreless fiber.

[0045] By modulating the refractive index of a coreless fiber using a femtosecond laser, the refractive index of the modulated region is made greater than that of the coreless fiber itself. This allows most of the light to propagate within the modulation region. Utilizing this property, a straight waveguide connecting two single-mode segments is fabricated. This allows light to still propagate along the modulation region in a misaligned coreless fiber, thereby reducing or even eliminating the excitation of higher-order modes; that is, most light still propagates in the fundamental mode. This forms the basis for FBG (Flash-in-the-Fiber) fabrication.

[0046] FBG is a simple and common type of fiber optic grating. Its refractive index modulation depth and grating period are usually constant. When incident light passes through an FBG, wavelengths that satisfy the Bragg condition are reflected back to the incident end, while other wavelengths continue to propagate through the fiber grating. The reflected wavelength can be expressed as:

[0047] λ=2n eff Λ / m

[0048] In the formula n eff Λ is the effective refractive index of the fiber core, Λ is the period of the grating, and m is the grating order. When the grating is subjected to stress changes, its effective refractive index and period will change accordingly, thereby shifting the center wavelength.

[0049] This embodiment utilizes a femtosecond laser point-by-point writing method to fabricate an FBG (Fast-Filled Grating). A femtosecond laser 3 is used as the light source, and a precision displacement platform controls the movement of the optical fiber to write the grating point by point. By controlling the platform's movement speed and the laser's frequency, gratings of arbitrary periods can be written. The above equation can be rewritten as:

[0050] λ=2n eff v / (mf)

[0051] In the formula, v is the platform's moving speed, and f is the laser's frequency. The specific method for femtosecond inscription is as follows:

[0052] 1. Fix the fabricated misaligned structure on a three-dimensional precision displacement platform, and immerse the optical fiber in a refractive index matching liquid (1.448) that is similar to the refractive index of the coreless optical fiber. Then, calibrate the structure in the horizontal and vertical directions so that the single-mode fiber cores at both ends are on the same straight line and are always parallel and equidistant from the lens.

[0053] 2. Set the parameters for fabricating the straight waveguide in the software, and use a stacked straight-line writing method for waveguide fabrication. Specifically, each writing stroke is set to have a consistent length, connecting the two ends of the single-mode fiber core and penetrating a short distance into the core to increase the light coupling into the modulation region. The spacing between each writing stroke is 0.5 μm, and a total of 16 strokes are performed, resulting in a modulation region width of 8 μm. The femtosecond laser pulse energy is 1.2 μJ, the frequency is 1 kHz, and the platform movement speed is 30 μm / s during straight waveguide fabrication. The transmission spectrum of the completed fabrication is as follows: Figure 4 As shown, multimode interference is greatly reduced, and the overall transmission tends to be single-mode.

[0054] 3. Fabricate the FBG at the center of the prepared straight waveguide. The FBG is fabricated using a point-by-point writing method. During writing, the femtosecond laser pulse energy is 0.3 μJ, the frequency is 100 Hz, and the platform movement speed is 107 μm / s. At this time, the FBG period Λ is 1.07 μm. The completed structure is shown below. Figure 3 As shown, the transmission spectrum is as follows Figure 4 As shown, its center wavelength is 1558.6 nm.

[0055] The measurement process of strain sensing:

[0056] The strain measurement system consists of a set of fiber optic clamps, a high-precision electrical displacement platform, an electrical displacement controller, a broadband light source, and a spectrometer. The fiber optic structure is fixed straight using clamps, with one end of the clamp stationary. The high-precision electrical displacement platform and controller move the other end of the clamp outwards, applying stress to the fiber optic structure. Figure 5 As shown, the strain changes by 240 με each time, and the strain range of 0–1440 με is achieved by continuously adjusting the displacement platform.

[0057] This embodiment achieves high-resolution strain measurement within the strain range of 0–1440 με by detecting the shift in the center wavelength of the FBG. Figure 6 As shown, after one fitting, the measurement sensitivity of this sensor reaches 1.53 pm / με.

[0058] This invention features a compact structure, high repeatability, and a simple basic structure, requiring only single-mode fiber and coreless fiber. Through simple structural transformation, it successfully improves the strain sensitivity of FBG prepared by femtosecond laser on quartz fiber. It has important application value in fields such as construction engineering, power industry, and aerospace, and provides new ideas for improving the strain detection performance of sensors and other related fields.

[0059] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A FBG strain sensor based on a dislocation fusion coreless optical fiber, characterized in that, The system comprises an input single-mode fiber, multiple coreless fiber segments, and an output single-mode fiber connected in sequence. The input single-mode fiber is fused together with the first coreless fiber segment in a staggered manner, and the next coreless fiber segment is fused together with the previous coreless fiber segment in a staggered manner, with the staggering direction of the next segment being opposite to that of the previous segment, and the staggering distance being the same. The output single-mode fiber and the last coreless fiber segment are fused together axially. The central axes of the input single-mode fiber and the output single-mode fiber are on the same straight line. The cores of the input single-mode fiber and the output single-mode fiber are connected by a photolithographically written straight waveguide, and a fiber Bragg grating is optically etched along the straight waveguide.

2. The FBG strain sensor based on a misaligned fusion spliced coreless optical fiber according to claim 1, wherein, The cladding diameter of both the input single-mode fiber and the output single-mode fiber is 125 μm, and the core diameter of both the input single-mode fiber and the output single-mode fiber is 4 to 10 μm; the diameter of each of the multiple coreless fiber segments is 125 μm.

3. The FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 1, characterized in that, The multiple coreless fiber segments are an even number of segments between 2 and 20; each coreless fiber segment has the same length, ranging from 200 to 2000 μm; and the misalignment distance is between 4 and 25 μm.

4. The FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 1, characterized in that, The width of the straight waveguide is 4 to 16 μm; the period of the fiber Bragg grating is 0.3 to 3 μm.

5. A method for manufacturing an FBG strain sensor based on staggered fusion spliced ​​coreless optical fiber, characterized in that, Includes the following steps: Step 1: Fabrication of the coreless fiber misalignment structure: First, the single-mode fiber with the coating removed and the coreless fiber are fused together in opposite directions in a misaligned manner. The coreless fiber is cut to the selected length. Using the central axis of the single-mode fiber or the previous coreless fiber as a reference, each subsequent coreless fiber segment is fused together with the same distance offset vertically. The misalignment direction of the next segment is opposite to that of the previous segment. Finally, another single-mode fiber is aligned with the central axis and fused onto the last coreless fiber segment. Step 2: Femtosecond laser writing of straight waveguide: Fix the coreless fiber misalignment structure on a three-dimensional precision displacement platform, immerse the fiber in refractive index matching liquid, and then calibrate the fiber in the horizontal and vertical directions; ensure that the single-mode fiber cores on both sides are on the same straight line, on the same horizontal plane, and at the same distance from the femtosecond laser lens; set the parameters for writing the straight waveguide on the femtosecond laser control software, and write the straight waveguide using direct writing technology, connecting the fiber cores of the single-mode fibers at both ends; Step 3: Femtosecond laser writing of fiber Bragg gratings: Set the various parameters for grating writing, and use direct writing technology to write dot-shaped gratings on the fabricated straight waveguide.

6. The manufacturing method of the FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 5, characterized in that, The single-mode fiber in step 1 has a cladding diameter of 125 μm and a core diameter of 4 to 10 μm, while the coreless fiber has a diameter of 125 μm.

7. The manufacturing method of the FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 5, characterized in that, The number of segments of the coreless optical fiber in step 1 is an even number between 2 and 20; the length of each segment of the coreless optical fiber is the same, ranging from 200 to 2000 μm; and the misalignment distance is between 4 and 25 μm.

8. The manufacturing method of the FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 5, characterized in that, The refractive index of the refractive index matching liquid in step 2 is 1.448; when writing the straight waveguide, the femtosecond laser pulse energy is 1.2 μJ, the frequency is 1 kHz, the platform moving speed is 30 μm / s, and the width of the straight waveguide is 4 to 16 μm.

9. The manufacturing method of the FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 5, characterized in that, In step 3, when writing the fiber Bragg grating, the femtosecond laser pulse energy is 0.3 μJ, the frequency is 100 Hz, the platform moving speed is 107 μm / s, and the period of the fiber Bragg grating is 0.3 to 3 μm.

10. The manufacturing method of the FBG strain sensor based on misaligned fusion spliced ​​coreless optical fiber as described in claim 8, characterized in that, The fabrication method for the straight waveguide is a stacked straight-line writing method. Specifically, each writing is of the same length and connects the two ends of the single-mode fiber core and penetrates a short distance into the fiber core to increase the light coupled into the modulation region. The spacing between each writing is 0.25 to 1 μm, and a total of 16 writings are performed. The width of the completed modulation region is 4 to 16 μm.

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