A fiber-optic torsion sensor based on femtosecond laser inscription of straight-helix dual waveguide and a processing method thereof

Abstract

This invention discloses a fiber optic torsion sensor and its fabrication method based on femtosecond laser-etched straight-helical dual waveguides, relating to the field of fiber optic sensing technology. The sensor includes an input single-mode fiber, a coreless fiber, and an output single-mode fiber. The coreless fiber has a straight waveguide and a helical waveguide etched internally by a femtosecond laser. In use, the input light enters the coreless fiber through the input single-mode fiber and is coupled into the axial straight waveguide and helical waveguide respectively, then recouples at the output end to form a transmission interference spectrum. When the torsion to be measured is applied to the sensing area, the characteristic valleys in the transmission interference spectrum undergo wavelength shift. By tracking the amount and direction of the characteristic valley shift, the magnitude and direction of the torsion can be detected. Furthermore, the characteristic valley shift directions are opposite under clockwise and counterclockwise torsion, enabling torsion direction identification. The fiber optic torsion sensor employs a single-mode-coreless-single-mode structure, resulting in fewer fusion splices, a compact structure, and ease of packaging and repeated processing.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a fiber optic torsion sensor and its fabrication method based on femtosecond laser-written straight-helical double waveguides. Background Technology

[0002] Fiber optic sensing technology boasts advantages such as small size, light weight, resistance to electromagnetic interference, corrosion resistance, high sensitivity, and ease of long-distance transmission, and has been widely applied in fields such as structural health monitoring, aerospace, precision machinery, energy equipment, and biomedicine. Torsion, as an important mechanical deformation parameter, frequently occurs in rotating components, flexible structures, mechanical transmission systems, and engineering equipment. Therefore, highly sensitive, compact, and easily fabricated fiber optic torsion sensors have significant research value and application prospects.

[0003] Currently, research on fiber optic torsion sensors mainly focuses on fiber grating type, interferometric type, and microstructured fiber type. Fiber grating type sensors, such as those using femtosecond lasers to inscribe π phase-shift gratings or long-period fiber gratings in the fiber core, achieve torsion measurement through changes in grating birefringence or resonant wavelength drift caused by torsion. Some structures can already identify the torsion direction. For example, some studies have achieved a torsion intensity of 117.4 pm / (rad·m) by inscribing a torsion-type long-period fiber grating in a single-mode fiber. - ¹) Torsional sensitivity. Among interferometric sensors, schemes based on Mach-Zehnder interferometers (MZI) have received widespread attention. Previous studies have utilized femtosecond lasers to inscribe two parallel straight waveguides on both sides of the fiber core-cladding boundary to form an MZI, achieving ultra-high sensitivity small-angle torsion measurement through intensity demodulation. Furthermore, other studies have inscribed waveguides in the cladding of single-mode fibers while leaving gaps to improve the contrast of the interferometric spectrum. In recent years, femtosecond laser direct writing technology has become an important means of constructing novel fiber interferometric sensing structures due to its advantages such as the ability to fabricate highly free three-dimensional microstructures inside optical fibers and the elimination of the need for complex masks. Femtosecond lasers have been used to fabricate coupled waveguides and Bragg gratings in multi-core optical fibers to achieve spatial multiplexing and vector sensing of signals.

[0004] In existing technologies, some studies utilize femtosecond lasers to directly write helical waveguides within or in the cladding region of single-mode fibers, enabling coupling and interference between the original core-transmitting light and the femtosecond laser-written waveguide, thereby achieving torsion measurement. However, single-mode fibers inherently possess a pre-fabricated core, and the incident light is primarily constrained by the core. The coupling strength between the femtosecond laser-written helical waveguide and the main transmission light field is easily affected by factors such as the writing position, waveguide spacing, laser energy, and processing errors, resulting in limited interference contrast and torsion response sensitivity.

[0005] In addition, some fiber optic spiral waveguide sensing structures require the introduction of multimode fiber transition sections to enhance mode excitation and interference effects. However, such structures usually have many fusion splices, making the fabrication process relatively complex. The sensing performance is easily affected by the fusion quality, multimode interference state, and packaging method.

[0006] Therefore, it is necessary to propose a fiber optic torsion sensor and its fabrication method based on femtosecond laser writing of straight-helical dual waveguides to solve the above problems. Summary of the Invention

[0007] To address the problems existing in the prior art, the present invention solves this problem using the following technical structure.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides includes: an input single-mode fiber, a coreless fiber, and an output single-mode fiber, wherein the coreless fiber is disposed between the input single-mode fiber and the output single-mode fiber. The coreless optical fiber has a straight waveguide and a helical waveguide etched inside it by a femtosecond laser. Both the straight waveguide and the helical waveguide extend along the axial direction of the coreless optical fiber, and the straight waveguide is disposed inside the helical waveguide.

[0010] Both ends of the helical waveguide are provided with linear coupling sections, which extend along the axial direction of the coreless optical fiber.

[0011] The helical waveguide includes a constant radius helical segment and two gradually decreasing radius helical segments respectively disposed at both ends of the constant radius helical segment. The two ends of the constant radius helical segment are respectively connected to the two gradually decreasing radius helical segments, and the two gradually decreasing radius helical segments are respectively connected to two straight coupling segments. The spiral radius of the gradually increasing spiral segment increases sequentially from the side furthest from the constant radius spiral segment to the side closest to the constant radius spiral segment, while the spiral radius of the constant radius spiral segment remains unchanged.

[0012] The length of the gradient radius helical segment along the axial direction of the coreless optical fiber is 1.5 mm to 2.5 mm; The spiral radius of the gradually changing radius spiral segment gradually changes from 20 μm to 50 μm from the side away from the constant radius spiral segment to the side closer to the constant radius spiral segment.

[0013] The length of the constant radius helical segment along the axial direction of the coreless optical fiber is 5.5 mm to 6.5 mm; The helical radius of the constant radius helical segment is 50 μm.

[0014] The length of the linear coupling segment along the axial direction of the coreless optical fiber is 0.4 to 0.6 mm.

[0015] The input single-mode fiber and the output single-mode fiber are respectively fused together at both ends of the coreless fiber 3.

[0016] A method for fabricating an optical fiber torsion sensor, used to fabricate the aforementioned optical fiber torsion sensor based on femtosecond laser-written straight-helical double waveguide, the method comprising: The input single-mode fiber and the output single-mode fiber are fused to both ends of the coreless fiber to form a single-mode-coreless-single-mode structure. The single-mode-coreless-single-mode structure is fixed on a three-dimensional platform, and the coreless optical fiber is placed in the focusing area of ​​a femtosecond laser. The femtosecond laser inscribes straight waveguides and helical waveguides inside the coreless optical fiber.

[0017] When the femtosecond laser inscribes straight waveguides and helical waveguides inside the coreless optical fiber, the center wavelength of the femtosecond laser is 510-520nm and the repetition frequency is 180-190kHz.

[0018] When the femtosecond laser writes straight waveguides and helical waveguides inside the coreless optical fiber, the single pulse energy of the femtosecond laser is set to 240 nJ and the writing rate is 0.05 mm / s.

[0019] The following beneficial effects can be achieved by using the structure described above in this invention: In use, the straight waveguide serves as the reference arm, and the helical waveguide as the sensing arm. Together, they form an embedded Mach-Zehnder interference structure within the fiber, capable of converting torsion into a clear wavelength shift. The input light enters the coreless fiber via the input single-mode fiber, and is then coupled into the axial straight waveguide and helical waveguide respectively. At the output end, they are recoupled to form a transmission interference spectrum. When the torsion to be measured is applied to the sensing area, the characteristic valleys in the transmission interference spectrum experience wavelength shifts. By tracking the amount and direction of these characteristic valley shifts, the magnitude and direction of the torsion are detected. In this application, the direction of rotation observed from the rotating end along the fiber axial direction towards the fixed end is used as the criterion for determining the direction of torsion. Rotation of the rotating end clockwise along this observation direction is defined as clockwise torsion, and rotation of the rotating end counterclockwise is defined as counterclockwise torsion. Under clockwise and counterclockwise twisting, the drift direction of the characteristic valley is opposite, which can realize the identification of the twisting direction; and the above-mentioned fiber twisting sensor adopts a single-mode-coreless-single-mode structure with fewer fusion splice nodes, compact structure, easy packaging and repeated processing, and direct writing of straight waveguides and helical waveguides inside the coreless fiber by femtosecond laser, the three-dimensional path is controllable and no external microstructure assembly is required. This invention uses coreless optical fiber as the waveguide writing substrate, avoiding the limitations imposed by pre-fabricated fiber cores on the optical field distribution and writing position. After the incident light enters the coreless optical fiber, it propagates in the form of a larger mode field, enabling more efficient coupling with the straight waveguide and helical waveguide written by the femtosecond laser. The straight waveguide (reference arm) and the helical waveguide (sensing arm) together constitute an embedded Mach-Zehnder interference structure, which is beneficial for forming a stable interference output and improving the discriminability of the interference spectrum. The phase change caused by torsion can be clearly converted into a significant wavelength shift of the characteristic valley in the transmission spectrum. Experimental data show that under clockwise / counterclockwise torsion, the wavelength shift of the characteristic valley has good linearity (goodness of fit R² is greater than 0.97), and the sensitivity reaches approximately 1.23 nm / (rad / m) and -0.99 nm / (rad / m), respectively, indicating that the sensor has good linear response and bidirectional torsion recognition capability.

[0020] This invention adopts a three-segment basic structure of single-mode-coreless-single-mode (SNS), with the entire sensor containing only two welding nodes (input end and output end) and very few welding interfaces. This compact structure significantly reduces welding loss and performance uncertainty introduced by multiple nodes, while facilitating standardized packaging and batch repetitive processing, which is beneficial for engineering promotion.

[0021] This invention utilizes femtosecond laser direct writing technology to complete the three-dimensional writing of straight waveguides and helical waveguides inside a coreless optical fiber in one go. It is a fully fiber-embedded processing method that does not require any external microstructure assembly or complex masks. The processing path, waveguide position, and geometric parameters of the femtosecond laser can all be precisely controlled by software, which has a high degree of processing freedom and controllability. This ensures the consistency of the waveguide structure and avoids the reliability risks brought by external components.

[0022] This invention identifies the torsion direction by tracking the wavelength drift direction of characteristic valleys in the transmission interference spectrum: when twisted clockwise, the characteristic valley drifts towards the long wavelength direction, and when twisted counterclockwise, the characteristic valley drifts towards the short wavelength direction. The direction is clearly distinguished and the physical mechanism is well understood. Wavelength demodulation is used instead of intensity demodulation, so the wavelength drift is less affected by fluctuations in light source power and changes in optical path loss, and it has good anti-interference ability and stability.

[0023] The straight-helical dual-waveguide interference structure of this invention is specifically optimized for torsional parameters. It is suitable for both precise measurement of static torsion and real-time monitoring of dynamic torsion. The sensor is small in size, lightweight, and resistant to electromagnetic interference. It can be widely used in various scenarios such as rotating component monitoring, flexible structure deformation analysis, mechanical transmission system condition assessment, aerospace structural health monitoring, and precision instrument micro-torsion measurement, and has broad application prospects. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the present invention.

[0025] Figure 2 This is a schematic diagram of the torsion testing system of the present invention; Figure 3 This is the transmission spectrum of the present invention under clockwise rotation; Figure 4 This is a linear fitting diagram of the characteristic valley wavelength and torsion rate under clockwise torsion according to the present invention; Figure 5 This is the transmission spectrum of the present invention under counterclockwise twisting; Figure 6 This is a linear fitting diagram of the characteristic valley wavelength and torsion rate under counterclockwise torsion according to the present invention.

[0026] In the diagram: 1. Input single-mode fiber; 2. Input fusion splice interface; 3. Coreless fiber; 4. Output fusion splice interface; 5. Output single-mode fiber; 6. Linear coupling section; 7. Gradually decreasing radius helical section; 8. Constant radius helical section; 9. Straight waveguide. Detailed Implementation

[0027] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0028] It should be noted that the terms "comprising" and "having" and any variations thereof in the specification, claims and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such process, method, product or device.

[0029] Reference Appendix Figure 1The fiber optic torsion sensor based on a femtosecond laser-etched straight-helical dual waveguide includes: an input single-mode fiber 1, a coreless fiber 3, and an output single-mode fiber 5. The coreless fiber 3 is disposed between the input single-mode fiber 1 and the output single-mode fiber 5. A straight waveguide 9 and a helical waveguide are etched inside the coreless fiber 3 using a femtosecond laser. Both the straight waveguide 9 and the helical waveguide extend along the axial direction of the coreless fiber 3. The straight waveguide 9 is located inside the helical waveguide. In use, the straight waveguide 9 serves as a reference arm, and the helical waveguide serves as a sensing arm. Together, they form an embedded Mach-Zehnder interference structure within the fiber, which can convert the torsion effect into a clear wavelength shift. The input light enters through the input single-mode fiber 1. After the coreless fiber 3 is coupled into the axial straight waveguide 9 and the helical waveguide respectively, it is recoupled at the output end to form a transmission interference spectrum. After the torsion to be measured is applied to the sensing area, the characteristic valley in the transmission interference spectrum undergoes wavelength drift. By tracking the amount and direction of the characteristic valley drift, the magnitude and direction of the torsion can be detected. Moreover, the drift direction of the characteristic valley is opposite under clockwise and counterclockwise torsion, so the torsion direction can be identified. Furthermore, the above-mentioned fiber torsion sensor adopts a single-mode-coreless-single-mode structure with fewer fusion nodes, a compact structure, and is easy to package and repeat. The straight waveguide 9 and the helical waveguide are directly inscribed inside the coreless fiber 3 by femtosecond laser, and the three-dimensional path is controllable, without the need for external microstructure assembly.

[0030] like Figure 1 As shown, both ends of the helical waveguide are provided with linear coupling sections 6, which extend along the axial direction of the coreless fiber 3. After the optical signal enters the coreless fiber from the input single-mode fiber through the splice, it needs to efficiently "enter" the helical path from the coreless fiber. The linear coupling section acts as a "gentle entry point," allowing the extended optical field in the coreless fiber to gradually couple into the refractive index modified waveguide formed by femtosecond laser writing, reducing coupling loss caused by mode mismatch and abrupt path changes, while improving the stability of the interference spectrum. Furthermore, after the optical signal passes through the helical path, it needs to efficiently "return" and couple into the output single-mode fiber. The linear coupling section avoids the severe scattering and loss caused by the optical signal being abruptly refracted directly from the helical tip. The helical waveguide includes a constant radius helical section 8 and two gradually decreasing radius helical sections 7 respectively located at both ends of the constant radius helical section 8. The two ends of the spiral waveguide are connected to two gradually decreasing radius spiral segments 7, which are respectively connected to two straight coupling segments 6. The spiral radius of the gradually decreasing radius spiral segment 7 increases gradually from the side away from the constant radius spiral segment 8 to the side closer to the constant radius spiral segment 8, while the spiral radius of the constant radius spiral segment 8 remains unchanged. The spiral waveguide with gradually decreasing radius spiral segments 7 and constant radius spiral segments 8 optimizes the coupling efficiency, completes the smooth transition between the straight coupling segment 6 and the constant radius spiral segment 8, ensures the stability of the optical field, is beneficial to input and output coupling, and can form a stable main torsional response region.

[0031] Further optimization involves adapting the dimensions of the linear coupling segment 6, the gradient radius spiral segment 7, and the constant radius spiral segment 8 to actual usage requirements and effects. In this embodiment, the length of the linear coupling segment 6 along the axial direction of the coreless fiber 3 is 0.4 to 0.6 mm (e.g., 0.4 mm, 0.5 mm, or 0.6 mm, preferably 0.5 mm in this embodiment); the length of the gradient radius spiral segment 7 along the axial direction of the coreless fiber 3 is 1.5 mm to 2.5 mm (e.g., 1.5 mm, 2 mm, or 2.5 mm, preferably 2 mm in this embodiment); the spiral radius of the gradient radius spiral segment 7 gradually increases from 20 μm to 50 μm from the side away from the constant radius spiral segment 8 to the side closer to the constant radius spiral segment 8; the length of the constant radius spiral segment 8 along the axial direction of the coreless fiber 3 is 5.5 mm to 6.5 mm (e.g., 5.5 mm, 6 mm, or 6.5 mm, preferably 6 mm in this embodiment); and the spiral radius of the constant radius spiral segment 8 is 50 μm.

[0032] Further optimization involves fusion splicing the input single-mode fiber 1 and output single-mode fiber 5 at both ends of the coreless fiber 3, forming an input-end fusion interface 2 and an output-end fusion interface 4. Specifically, a fiber optic cleaver is used to cut the end faces of the input single-mode fiber 1 (with the coating removed), the coreless fiber 3, and the output single-mode fiber 5 flat to reduce insertion loss at the fusion interfaces. After wiping the end faces with alcohol wipes to ensure cleanliness, the fibers are placed sequentially in the fusion splicer's fixture. First, one end of the input single-mode fiber 1 and the coreless fiber 3 are aligned, and a suitable discharge intensity and time are selected for fusion splicing to form the input-end fusion interface 2. Then, the other end of the coreless fiber 3 is fused to the output single-mode fiber 5 in the same way to form the output-end fusion interface 4. During the fusion process, it is crucial to ensure appropriate discharge parameters to avoid excessive discharge that could cause melting and deformation of the fiber end faces, affecting the optical path quality. After fusion splicing, a single-mode-coreless-single-mode (SNS) basic structure is obtained. The entire sensor contains only two fusion nodes, resulting in a compact structure.

[0033] This application also discloses a method for fabricating an optical fiber torsion sensor, used to fabricate the aforementioned optical fiber torsion sensor based on femtosecond laser-written straight-helical double waveguide, the method comprising: The input single-mode fiber 1 and the output single-mode fiber 5 are fused to the two ends of the coreless fiber 3 to form a single-mode-coreless-single-mode structure. The single-mode-coreless-single-mode structure is fixed on a three-dimensional platform, and the coreless fiber 3 is placed in the femtosecond laser focusing area. The femtosecond laser inscribes a straight waveguide 9 and a helical waveguide inside the coreless fiber 3.

[0034] The specific method is as follows: I. Material Preparation In this embodiment, the input single-mode fiber 1, the coreless fiber 3, and the output single-mode fiber 5 are all commercially available standard single-mode fibers and commercially available standard coreless fibers. Their core diameter, cladding diameter, etc., are selected adaptively according to actual needs. The diameter, refractive index, etc. of the coreless fiber 3 are selected adaptively according to actual needs.

[0035] II. Forming an SNS structure by fiber optic fusion splicing The end faces of the input single-mode fiber 1, coreless fiber 3, and output single-mode fiber 5 (with the coating removed) were cut flat using a fiber cleaver (Fujikura CT-30) to reduce insertion loss at the splice interface. After wiping the end faces with alcohol wipes to ensure cleanliness, the fibers were placed sequentially into the fixture of the fusion splicer (Fujikura FSM-80S). First, one end of the input single-mode fiber 1 and the coreless fiber 3 were aligned, and a suitable discharge intensity and time were selected for splicing to form the input splice interface 2. Subsequently, the other end of the coreless fiber 3 was spliced ​​to the output single-mode fiber 5 in the same manner to form the output splice interface 4. During the splicing process, it is necessary to ensure that the discharge parameters are moderate to avoid excessive discharge that could cause melting and deformation of the fiber end faces, affecting the optical path quality. After the splicing is completed, a basic single-mode-coreless-single-mode (SNS) structure is obtained. The entire sensor contains only two splice nodes, resulting in a compact structure.

[0036] III. Femtosecond laser-written waveguide The fused SNS structure is fixed on a three-dimensional precision displacement platform, and the coreless fiber 3 is placed in the focusing area of ​​the femtosecond laser processing system. This embodiment uses a femtosecond laser with a center wavelength of 517 nm and a repetition rate of 188 kHz. After being focused by a 60x oil immersion objective (numerical aperture NA = 1.42), the laser is precisely focused into the coreless fiber 3. During processing, a broadband light source is connected to the input single-mode fiber 1, and a spectrometer is connected to the output single-mode fiber 5 to monitor changes in the transmission spectrum in real time during the writing process and to assist in adjusting the writing parameters.

[0037] Before formally writing the waveguide, the objective lens height is adjusted using a CCD observation system to clearly observe the upper and lower boundaries of the coreless fiber 3 in the field of view. The center position and axial length of the coreless fiber 3 are then precisely calibrated using a three-dimensional platform. The specific steps for femtosecond laser writing of the straight waveguide 9 and the helical waveguide are as follows: (a) Writing a straight waveguide A straight scanning path is set in the central region of the coreless fiber 3 along its axial direction (i.e., the direction of light propagation, defined as the X-axis). The single-pulse energy of the femtosecond laser is set to 240 nJ, and the writing rate is set to 0.05 mm / s. Under these parameters, the nonlinear absorption in the laser focal region causes a local increase in the refractive index of the coreless fiber 3 material, forming a permanent waveguide structure. The diameter of the straight waveguide 9 is approximately 5~8 μm, and its length is the total length of the coreless fiber 3 (approximately 11 mm in this embodiment, including the total length of the gradient region and the constant region). The straight waveguide 9 is located inside the helical waveguide and serves as the reference arm of the interferometer.

[0038] (ii) Characterizing the helical waveguide A spiral scanning path is established along the axis of the coreless optical fiber 3, centered on the straight waveguide 9. The spiral waveguide consists of a straight coupling segment 6, a gradually varying radius spiral segment 7, and a constant radius spiral segment 8, with the following specific dimensions: Linear coupling segment 6: Located at both ends of the helical waveguide, extending 0.5 mm along the axial direction of the coreless fiber 3. The function of this segment is to enable the input light to be effectively coupled from the coreless fiber 3 into the helical waveguide, and to recouple the light in the helical waveguide back to the coreless fiber 3 at the output end.

[0039] Graded-radius helical segment 7: Located between the straight-line coupling segment 6 and the constant-radius helical segment 8, its length along the axis of the coreless fiber 3 is 2 mm. Its helical radius gradually increases from 20 μm to 50 μm from the side furthest from the constant-radius helical segment 8 towards the side closer to it. This graded design facilitates a low-loss transition between the straight-line and constant-radius segments for both input and output light, reducing coupling loss caused by mode mismatch.

[0040] Constant radius helical segment 8: Located between the two gradually decreasing radius helical segments 7, it has a length of 6 mm along the axial direction of the coreless fiber 3, and the helical radius remains constant at 50 μm. This segment is the main torsional response region of the sensor, and the constant radius ensures the linearity and consistency of the optical path change of the helical waveguide under torsion.

[0041] The femtosecond laser scans point-by-point along the aforementioned path, completing the writing of the helical waveguide under the same laser parameters. During the processing, the polarization direction of the laser can be set perpendicular to the scanning direction to facilitate obtaining a waveguide structure with high cross-sectional roundness. The principle of femtosecond laser writing lies in the fact that when a high-energy-density femtosecond pulse is focused inside a transparent medium, permanent refractive index modification is generated in the focal region through multiphoton ionization and avalanche ionization mechanisms, forming a light-guiding channel.

[0042] IV. Key Points of Parameter Optimization The writing parameters have a significant impact on waveguide quality and interference spectral contrast. In this embodiment, when a femtosecond laser writes the straight waveguide 9 and the helical waveguide inside the coreless fiber 3, the center wavelength of the femtosecond laser is 510-520nm (e.g., 510nm, 515nm, 517nm, or 520nm), and the repetition frequency is 180-190kHz (e.g., 180kHz, 184kHz, 188kHz, or 190kHz); the single-pulse energy of the femtosecond laser is set to 230-250nJ (e.g., 230nJ, 240nJ, or 250nJ), and the writing rate is 0.04-0.06mm / s (e.g., 0.04mm / s, 0.05mm / s, or 0.06mm / s). In this embodiment, the preferred center wavelength is 517nm, the single-pulse energy is 240nJ, the writing rate is 0.05mm / s, and the repetition frequency is 188kHz, based on the following considerations: Single-pulse energy and repetition frequency: If the energy is too low, sufficient refractive index modulation cannot be formed in the coreless fiber; if the energy is too high, it may cause material damage or excessively wide waveguides and multimode transmission. A repetition frequency of 188kHz combined with a single-pulse energy of 240nJ can maintain high processing efficiency while ensuring a moderate heat accumulation effect between pulses, forming a waveguide cross-section with a uniform refractive index distribution.

[0043] The writing rate of 0.05 mm / s ensures sufficient net flux within each laser pulse irradiation area, resulting in adequate and continuous refractive index modulation. Too high a rate may lead to discontinuous modulation of the waveguide, while too low a rate may cause over-processing and excessive waveguide width.

[0044] Focusing objective: A 60x oil immersion objective (NA=1.42) can be used to obtain a small focused spot size (about 0.5~1μm), which is beneficial for accurately locating the waveguide position inside a coreless optical fiber with a diameter of only 125μm, and ensuring the symmetry of the waveguide cross section and reducing birefringence.

[0045] When the femtosecond laser is used to inscribe the straight waveguide 9 and the helical waveguide inside the coreless optical fiber 3, the center wavelength of the femtosecond laser is 517nm and the repetition frequency is 188kHz.

[0046] When the femtosecond laser is used to write the straight waveguide 9 and the helical waveguide inside the coreless fiber 3, the single pulse energy of the femtosecond laser is set to 240 nJ, the writing rate is 0.05 mm / s, the focusing objective is a 60x oil immersion objective, and the numerical aperture NA=1.42.

[0047] This application also discloses a testing method, such as Figures 2-6As shown, the fiber optic torsion sensor described above was tested. During testing, the sensor was installed in a torsion measuring device, with one end fixed and the other end subjected to clockwise or counterclockwise torsion via a rotating clamp. A broadband light source was input to the sensor, and a spectral analyzer acquired the output transmission spectrum. FFT analysis was performed on the acquired transmission spectrum, and the bandpass range was selected for filtering based on the principal spatial frequency. The center wavelengths of the three characteristic valleys were tracked. Experimental results show that, under clockwise torsion, the wavelengths of the three characteristic valleys shift towards longer wavelengths with increasing torsion rate, corresponding to sensitivities of approximately 1.230, 1.228, and 1.226 nm / (rad / m); under counterclockwise torsion, the three characteristic valleys shift towards shorter wavelengths, corresponding to sensitivities of approximately -0.989, -0.990, and -0.989 nm / (rad / m).

[0048] Furthermore, Figure 3 The graph shows the transmission spectrum obtained from the clockwise torsion test, where the horizontal axis represents wavelength in nm and the vertical axis represents transmitted light intensity in dB. The different colored curves in the graph represent the transmission interference spectra collected by the sensor at different torsion rates of 0 rad / m, 0.873 rad / m, 1.745 rad / m, 2.618 rad / m, 3.49 rad / m, 4.363 rad / m, 5.236 rad / m, 6.109 rad / m, and 6.981 rad / m, respectively. Figure 3 The labels dip1, dip2, and dip3 indicate three characteristic valleys selected from the transmission interference spectrum. The red arrows show the wavelength shift direction of these valleys as the clockwise torsion rate increases. Figure 3 It can be seen that under clockwise torsion, all three characteristic valleys move towards the longer wavelength direction, and the spectral curves corresponding to each torsion ratio still maintain clear interference fringes, indicating that the straight waveguide and helical waveguide of this application can form a stable interference output and can realize clockwise torsion detection through characteristic valley drift.

[0049] Furthermore, Figure 4 This is a linear fitting graph showing the relationship between the wavelength of the characteristic valley and the torsion rate under clockwise torsion. The horizontal axis represents the torsion rate in rad / m, and the vertical axis represents the center wavelength of the characteristic valley in nm. The black squares, red circles, and blue triangles represent the measured center wavelengths of dip1, dip2, and dip3 under different torsion rates, respectively. The black, red, and blue solid lines represent the linear fitting curves for the corresponding characteristic valleys. The slope in the graph is the fitting slope, R² is the goodness of fit, and the red ellipse is used to mark example data points on the fitting curve. Figure 4It can be seen that under clockwise torsion, the wavelengths of dip1, dip2 and dip3 all increase approximately linearly with the increase of the torsion rate, with corresponding sensitivities of approximately 1.230 nm / (rad / m), 1.228 nm / (rad / m) and 1.226 nm / (rad / m), respectively. The goodness of fit is greater than 0.97, indicating that the sensor of this application has a good linear response in clockwise torsion measurement.

[0050] Furthermore, Figure 5 The image shows the transmission spectrum obtained from the counterclockwise torsion test, where the horizontal axis represents wavelength in nm and the vertical axis represents transmitted light intensity in dB; the different colored curves represent the transmission interference spectra collected by the sensor under different counterclockwise torsion rates. Figure 5 The labels dip1, dip2, and dip3 also represent the three characteristic valleys used for demodulation. The red arrows indicate the wavelength shift direction of these characteristic valleys as the counterclockwise torsion rate increases. Figure 5 It can be seen that under the counterclockwise torsion, the three characteristic valleys move as a whole towards the shortwave direction, and their drift direction is the same as that of the three characteristic valleys. Figure 3 The clockwise rotation is the opposite, so the direction of rotation can be distinguished by the drift direction of the characteristic valley.

[0051] Furthermore, Figure 6 This is a linear fitting plot of the characteristic valley wavelength and torsion rate under counterclockwise torsion. The horizontal axis represents the torsion rate in rad / m, and the vertical axis represents the center wavelength of the characteristic valley in nm. The black squares, red circles, and blue triangles represent the measured center wavelengths of dip1, dip2, and dip3, respectively. The black, red, and blue solid lines represent the corresponding linear fitting curves. The slope in the figure represents the counterclockwise torsion sensitivity, R² represents the goodness of fit, and the red ellipse is used to mark example data points on the fitted curve. Figure 6 It can be seen that under counterclockwise twisting, the wavelengths of dip1, dip2, and dip3 decrease approximately linearly with increasing twist rate, corresponding to sensitivities of approximately -0.989 nm / (rad / m), -0.990 nm / (rad / m), and -0.989 nm / (rad / m), respectively. The negative sign indicates that the characteristic valley shifts towards shorter wavelengths. Combined with... Figures 3 to 6 It is understood that the sensor of this application can not only determine the magnitude of the torsion by the wavelength drift, but also identify clockwise and counterclockwise torsion by the direction of the drift.

[0052] This application can be applied to, but is not limited to, the following scenarios: Application scenarios I. Torsional Monitoring of Mechanical Transmission Systems The fiber optic torsion sensor of this invention is attached to the surface of a mechanical transmission shaft or embedded in a transmission component. When the transmission bearing undergoes torsional deformation due to torque, the helical waveguide in the coreless fiber 3 of the sensor undergoes a helical geometric change, causing a characteristic valley wavelength drift. By monitoring the wavelength change of the characteristic valley in real time, real-time torsional state information of the transmission shaft can be obtained, providing data support for equipment operating status assessment and fault early warning. Due to the sensor's anti-electromagnetic interference characteristics, it is suitable for online torque monitoring in strong electromagnetic environments such as motors and generators.

[0053] II. Deformation Analysis of Flexible Structures Sensors are encapsulated in flexible materials or attached to the surface of flexible structures to monitor the deformation response of structures under combined bending and torsional loads. In the aerospace field, they can be used to monitor torsional deformation of structures such as wings and control surfaces; in the civil engineering field, they can be used to monitor torsional vibration of bridge cables and tall structures.

[0054] III. Precision Instrument Micro-torsion Measurement Due to its small size and high sensitivity, the sensor is suitable for measuring minute torsional angles in precision instruments, such as testing the torsional characteristics of MEMS devices, calibrating the angle of a precision turntable, and detecting weak torsional signals in torsion balance experiments. Wavelength demodulation reduces the impact of light source power fluctuations on the measurement results, which is beneficial for stable monitoring.

[0055] In summary, this invention uses coreless optical fiber as the waveguide writing substrate, reducing the constraint of the pre-fabricated fiber core on the optical field distribution. After the incident light enters the coreless optical fiber, it propagates in the form of a larger mode field, which can couple more efficiently with the straight waveguide and helical waveguide written by the femtosecond laser. The straight waveguide (reference arm) and the helical waveguide (sensing arm) together form an embedded Mach-Zehnder interference structure, which is beneficial to forming a stable interference output and improving the discriminability of the interference spectrum. The phase change caused by torsion can be clearly converted into a significant wavelength shift of the characteristic valley in the transmission spectrum. Experimental data show that under clockwise / counterclockwise torsion, the wavelength shift of the characteristic valley has good linearity (goodness of fit R² is greater than 0.97), and the sensitivity reaches about 1.23 nm / (rad / m) and -0.99 nm / (rad / m), respectively, indicating that the sensor can achieve bidirectional torsion detection with good linearity.

[0056] This invention adopts a three-segment basic structure of single-mode-coreless-single-mode (SNS), with the entire sensor containing only two welding nodes (input end and output end) and very few welding interfaces. This compact structure significantly reduces welding loss and performance uncertainty introduced by multiple nodes, while facilitating standardized packaging and batch repetitive processing, which is beneficial for engineering promotion.

[0057] This invention utilizes femtosecond laser direct writing technology to complete the three-dimensional writing of straight waveguides and helical waveguides inside a coreless optical fiber in one go. It is a fully fiber-embedded processing method that does not require any external microstructure assembly or complex masks. The processing path, waveguide position, and geometric parameters of the femtosecond laser can all be precisely controlled by software, which has a high degree of processing freedom and controllability. This ensures the consistency of the waveguide structure and avoids the reliability risks brought by external components.

[0058] This invention identifies the torsion direction by tracking the wavelength drift direction of characteristic valleys in the transmission interference spectrum: when twisted clockwise, the characteristic valley drifts towards the long wavelength direction, and when twisted counterclockwise, the characteristic valley drifts towards the short wavelength direction. The direction is clearly distinguished and the physical mechanism is well understood. Wavelength demodulation is used instead of intensity demodulation, so the wavelength drift is less affected by fluctuations in light source power and changes in optical path loss, and it has good anti-interference ability and stability.

[0059] The straight-helical dual-waveguide interference structure of this invention is specifically optimized for torsional parameters. It is suitable for both precise measurement of static torsion and real-time monitoring of dynamic torsion. The sensor is small in size, lightweight, and resistant to electromagnetic interference. It can be widely used in various scenarios such as rotating component monitoring, flexible structure deformation analysis, mechanical transmission system condition assessment, aerospace structural health monitoring, and precision instrument micro-torsion measurement, and has broad application prospects.

[0060] The above are merely preferred embodiments of this application, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides, characterized in that, include: The single-mode fiber (1), the coreless fiber (3), and the single-mode fiber (5) are provided, with the coreless fiber (3) disposed between the single-mode fiber (1) and the single-mode fiber (5). The coreless optical fiber (3) has a straight waveguide (9) and a helical waveguide inside it by femtosecond laser inscription. Both the straight waveguide (9) and the helical waveguide extend along the axial direction of the coreless optical fiber (3), and the straight waveguide (9) is disposed inside the helical waveguide.

2. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 1, characterized in that: Both ends of the spiral waveguide are provided with linear coupling sections (6), which extend along the axial direction of the coreless optical fiber (3).

3. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 2, characterized in that: The helical waveguide includes a constant radius helical segment (8) and two gradually decreasing radius helical segments (7) respectively disposed at both ends of the constant radius helical segment (8). The two ends of the constant radius helical segment (8) are respectively connected to the two gradually decreasing radius helical segments (7), and the two gradually decreasing radius helical segments (7) are respectively connected to two straight coupling segments (6). The spiral radius of the gradually increasing spiral segment (7) increases sequentially from the side away from the constant radius spiral segment (8) to the side closer to the constant radius spiral segment (8), while the spiral radius of the constant radius spiral segment (8) remains unchanged.

4. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 3, characterized in that: The length of the tapered radius helical segment (7) in the axial direction of the coreless optical fiber (3) is 1.5 mm to 2.5 mm; The spiral radius of the gradually changing radius spiral segment (7) gradually changes from 20 μm to 50 μm from the side away from the constant radius spiral segment (8) to the side closer to the constant radius spiral segment (8).

5. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 4, characterized in that: The length of the constant radius helical segment (8) in the axial direction of the coreless optical fiber (3) is 5.5 mm to 6.5 mm; The helical radius of the constant radius helical segment (8) is 50 μm.

6. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 5, characterized in that: The length of the linear coupling segment (6) in the axial direction of the coreless optical fiber (3) is 0.4 to 0.6 mm.

7. The fiber optic torsion sensor based on femtosecond laser-written straight-helical dual waveguides according to claim 1, characterized in that: The input single-mode fiber (1) and the output single-mode fiber (5) are respectively fused together at both ends of the coreless fiber (3).

8. A method for fabricating an optical fiber torsion sensor, used to fabricate the optical fiber torsion sensor based on femtosecond laser-written straight-helical double waveguide as described in claim 1, characterized in that: The methods include: The input single-mode fiber (1) and the output single-mode fiber (5) are fused to both ends of the coreless fiber (3) to form a single-mode-coreless-single-mode structure. The single-mode-coreless-single-mode structure is fixed on a three-dimensional platform, and the coreless fiber (3) is placed in the femtosecond laser focusing area. The femtosecond laser inscribes a straight waveguide (9) and a helical waveguide inside the coreless fiber (3).

9. A method for fabricating an optical fiber torsion sensor according to claim 8, characterized in that: When the femtosecond laser inscribes the straight waveguide (9) and the spiral waveguide inside the coreless optical fiber (3), the center wavelength of the femtosecond laser is 510-520nm and the repetition frequency is 180-190kHz.

10. A method for fabricating an optical fiber torsion sensor according to claim 9, characterized in that: When the femtosecond laser writes the straight waveguide (9) and the helical waveguide inside the coreless optical fiber (3), the single pulse energy of the femtosecond laser is set to 230-250nJ and the writing rate is 0.04-0.06mm / s.

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