High sensitivity curvature sensor based on sandwiched multimode fiber mach-zehnder interferometer

Abstract

The application relates to the technical field of sensors, in particular to a high-sensitivity curvature sensor based on a sandwiched multimode fiber Mach-Zehnder interferometer. The high-sensitivity curvature sensor comprises a graded multimode fiber, a step multimode fiber and a single-mode fiber, the two ends of the graded multimode fiber are provided with the step multimode fibers with different fiber core diameters, one end of the single step multimode fiber is connected with one end of the graded multimode fiber, and the other end is provided with the single-mode fiber connected therewith, the outer sides of the graded multimode fiber, the step multimode fiber and the single-mode fiber are provided with claddings, and the outer diameters of the claddings of the graded multimode fiber, the step multimode fiber and the single-mode fiber are the same. The technical scheme is used to provide an optical fiber curvature sensor with lower temperature cross-sensitivity, higher curvature sensitivity and more compact size.

Description

Technical Field

[0001] This invention relates to the field of sensor technology, and in particular to a high-sensitivity curvature sensor based on a sandwich multimode fiber Mach-Zehnder interferometer. Background Technology

[0002] Fiber optic sensors possess advantages such as corrosion resistance, electromagnetic interference resistance, small size, and strong remote sensing capabilities. In recent decades, fiber optic sensors based on temperature, curvature, strain, biomedical, and physiological parameters have been developed. Among these parameters, curvature is crucial because accurate curvature detection is essential for a wide range of industrial applications, such as health monitoring of bridges and roads, measurement of mechanical bending angles, and medical applications. To date, different types of fiber optic curvature sensors have been reported. In 2019, Sun proposed a large-range curvature sensor based on a micro / nano fiber optic probe with a cone waist diameter of approximately 5 μm, ranging from 8.73 to 11.82 m. -1 -700pm / m -1 Maximum curvature sensitivity measurement. Yuan reported a Mach-Zehnder interferometer based on ring-core fiber, in the range of 1.3856–3.6661 m. -1 Within this range, the curvature sensitivity is -3.68 nm / m. -1 Susana reported a single-mode multimode single-mode (SMS) fiber structure using a 40mm multimode fiber (MMF) segment, in the range of 1.1–1.42m. -1 The curvature sensitivity tested within the range reached 8.7 nm / m. -1 However, the aforementioned curvature sensors are all based on wavelength demodulation methods, which require very expensive demodulation equipment and have limited response time. Furthermore, wavelength is easily affected by temperature, which introduces unavoidable measurement errors into curvature sensing. To address these issues, Liu proposed a curvature sensor fabricated by etching fiber Bragg gratings into multi-core optical fibers, capable of operating in the 0-0.7m range. -1 Within the curvature range, the maximum sensitivity is 15.9 dB / m. -1 Zhang et al. proposed a curvature sensor based on a seven-core fiber (7CF) and a Bragg grating, with a curvature range of 0-1m, using ultraviolet light exposure. -1 Its curvature sensitivity is 7.27 dB / m. -1 W. Cui et al. reported a bending sensor that tapers an FBG (fiber grating) from 0 to 1.4 m when the fiber grating waist diameter is 57 μm. -1 Within the curvature range, the maximum detection sensitivity is 0.1196 dB / m. -1However, these structures still have some drawbacks, such as relatively low curvature sensitivity, narrow curvature range, or complex manufacturing. Therefore, designing a fiber optic curvature sensor with low temperature cross-sensitivity, high curvature sensitivity, and a more compact size is of great significance for practical curvature sensing.

[0003] In fact, various SMS fiber structures based on multimode interference have attracted widespread attention. In MMFs, when a light field enters the MMF, specific MMF eigenmodes can be excited. Different modes propagate along different paths and interfere with each other. A significant advantage of SMS fiber structures based on multimode interference is their superior performance and simple fabrication process. However, to date, research on SMS fiber structures has mainly focused on step-index multimode fiber (SIMMF) rather than graded-index multimode fiber (GIMMF). GIMMF is commonly used in mode field adapters and communications to suppress dispersion between modes. Some reports have also documented the application of GIMMF-based SMS structures in sensing fields (such as refractive index and temperature); however, in these studies, the GIMMF lengths are relatively long, typically exceeding 40 cm, thus limiting their sensing applications in small spaces. Summary of the Invention

[0004] To address the shortcomings of the aforementioned technologies, the present invention aims to provide a high-sensitivity curvature sensor based on a sandwich multimode fiber Mach-Zehnder interferometer, thereby proposing a fiber curvature sensor with lower temperature cross-sensitivity, higher curvature sensitivity, and more compact size.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A high-sensitivity curvature sensor based on a sandwich multimode fiber Mach-Zehnder interferometer includes a graded multimode fiber, a step multimode fiber, and a single-mode fiber. Both ends of the graded multimode fiber are provided with step multimode fibers with different core diameters. One end of each step multimode fiber is connected to one end of the graded multimode fiber, and the other end is provided with a single-mode fiber connected to it. The outer sides of the graded multimode fiber, the step multimode fiber, and the single-mode fiber are all provided with cladding, and the outer diameter of the cladding of the graded multimode fiber, the step multimode fiber, and the single-mode fiber is the same.

[0007] The working principle of this technical solution is as follows:

[0008] When light is transmitted through the single-mode fiber (SMF) at the input end to SIMMF1 (referring to a SIMMF set at one end), due to mode field mismatch, a portion of the optical power is coupled to the core mode of the GIMMF, and the remainder is coupled to the cladding mode. The light in the core of the GIMMF moves along a trajectory similar to a sine curve and then is transmitted to SIMMF2. Some of the cladding modes of the GIMMF are coupled to the guided mode of SIMMF2 and interfere with the core mode of the GIMMF, and finally are coupled to the fundamental mode of the SMF.

[0009] Further specified, the core diameters of the single-mode fiber, graded multimode fiber, and step-index multimode fiber are 8.3 μm, 50 μm, and 105 μm, respectively.

[0010] Furthermore, the outer diameter of the cladding is 125 μm.

[0011] Further specified, the length of the step-index multimode fiber is 1 mm.

[0012] Furthermore, the length of the graded multimode fiber is less than 10 mm.

[0013] Furthermore, the graded multimode fiber supports LP01 to LP05 mode transmission.

[0014] The technical effects achieved by this technical solution are as follows:

[0015] (1) In this technical solution, an ultra-high sensitivity and temperature-independent curvature fiber sensor, when the GIMMF length is sufficiently short (≤10mm), the intensity of the interference valley is extremely sensitive to the applied bending, but the wavelength shift is negligible, which is the opposite of temperature. Therefore, temperature-independent curvature sensing can be achieved simply by tracking the intensity change of the interference valley; (2) The lengths of GIMMF and SIMMF are only 1 mm, and the total length of the sensor can be as short as 3 mm, in the range of 0-2.36 m. -1 Within a small curvature range, the maximum curvature sensitivity can reach -78.75 dB / m. -1 (3) The different interference valleys of this structure exhibit different curvature sensitivities, providing more options for curvature sensing in practical applications; (4) Due to the small size, ease of manufacturing, good reproducibility and low cost of this sensor, it has broad application prospects in various packaging of high-precision bending-related engineering applications, such as wearable devices, small vibration signal detection and tactile sensing. Attached Figure Description

[0016] Figure 1 (a) Schematic diagram of a sandwich multimode fiber Mach-Zehnder interferometer based on SIMMF-GIMMF-SIMMF fiber structure; (b) Refractive index distribution diagram of GIMMF and SIMMF;

[0017] Figure 2 LP in GIMMF with a core diameter of 50 μm 01～05 The model field distribution diagram;

[0018] Figure 3 Simulation results of light transmission in direct SIMMF-GIMMF-SIMMF;

[0019] Figure 4 (a)L GIMMF Simulation results of the effect of SIMMF length on sensor spectrum when = 1mm; (b) at L SIMMF Simulation results of the effect of GIMMF length on sensor spectrum when GIMMF is 1 mm.

[0020] Figure 5 Curvature from 0 to 2.4m -1 (a) Simulation results of intensity change of Dip 1; (b) Simulation results of intensity change of Dip 2;

[0021] Figure 6 The manufacturing process of MMF–GIMMF–MMF optical fiber structure;

[0022] Figure 7 Interference spectra of SIMMF-GIMMF-SIMMF fiber structures with different GIMMF lengths: (a) 1, 3, 6, 8, 10 and 20 mm; (b) 3, 5 and 7 cm; Spatial spectra of sensors with different GIMMF lengths: (c) 1, 3, 8 and 10 mm; (d) 3, 5 and 7 cm.

[0023] Figure 8 Experimental apparatus for measuring curvature;

[0024] Figure 9 (a)L GIMMF Transmission spectrum of sensor with diameter = 1 mm; curvature response of (b~f)Dip1~Dip3;

[0025] Figure 10 .L GIMMF Temperature characteristics of a 1 mm SIMMF-GIMMF-SIMMF fiber structure: (a) spectral evolution; (b) wavelength drift and intensity variation.

[0026] Figure 11 Curvature response of sandwich multimode fiber Mach-Zehnder interferometers with different GIMMF lengths: (a) 1.8 mm; (b) 3 mm; (c) 6 mm; (d) 8 mm; (e) Functional relationship between intensity and curvature of Dip A on the first day and the thirtieth day.

[0027] Figure 12Curvature response of sandwich multimode fiber Mach-Zehnder interferometers with different GIMMF lengths in the wavelength range of 1500–1650 nm: (a) 4 mm; (b) 8 mm. Detailed Implementation

[0028] In this study, the SIMMF and GIMMF used in designing the sandwich multimode fiber Mach-Zehnder interferometer were from Yangtze Optical Fibre and Cable Co., Ltd. The refractive index distribution of the multimode fiber is expressed as:

[0029]

[0030] Where n0 is the refractive index of the core axis, NA is the numerical aperture of the optical fiber, a is the radius of the core, r is the radial distance, and q is the attenuation index of the refractive index.

[0031] The structure of a sandwich multimode fiber Mach-Zehnder interferometer is as follows: Figure 1 As shown in (a), the sensor was fabricated by sandwiching a GIMMF between two 1 mm long SIMMF segments, with the SIMMF introduced and extracted via an input-SMF and an output-SMF. The core diameters of the SMF, GIMMF, and SIMMF are 8.3 μm, 50 μm, and 105 μm, respectively, and their cladding diameters are the same (125 μm). The refractive index distribution of the SIMMF (q = ∞) and the GIMMF used in this study (q = 2) are shown in the figure. Figure 1 As shown in (b). The basic principle of this sensor can be explained as follows: Figure 1 As shown in (a), when light is emitted to SIMMF1 through the single-mode fiber at the input end, due to mode field mismatch, a portion of the optical power is coupled to the core mode of GIMMF, and the remainder is coupled to the cladding mode. The light in the GIMMF core moves along a trajectory similar to a sine curve, as... Figure 1 As shown by the yellow dashed line in (a), the signal is then transmitted to SIMMF2, where some cladding modes of GIMMF are coupled to the guided modes of SIMMF2, interfering with the core modes of GIMMF, and finally coupled to the fundamental mode of SMF. A GIMMF with a core diameter of 50 μm can support LP. 01～05 Mode transmission, its mode field distribution is as follows Figure 2 As shown. The interference pattern of this structure can be simply understood as the interaction between the core mode and the main cladding mode of the GIMMF. Interference effects may also exist between different core modes or different cladding modes of the GIMMF, but its spectral free range (FSR) is large, and within a limited spectral range, it only plays a role in modulating the main interference pattern. Therefore, the transmission power of the SIMMF-GIMMF-SIMMF structure can be obtained by the following formula:

[0032]

[0033] Where I is the output light intensity, I core and I clad These refer to the strengths of the core mold and cladding mold of GIMMF, respectively. and λ represents the effective refractive index of the core mode and cladding mode of the GIMMF, respectively; L is the length of the GIMMF, and λ is the wavelength.

[0034] According to equation (2), the resonant wavelength (λ) diP The free spectral range (FSR) can be expressed as:

[0035]

[0036]

[0037] Δn eff This represents the effective refractive index difference between the GIMMF core mold and the cladding mold.

[0038] For a straight GIMMF, the refractive index distribution of the cross-section is symmetrical along the fiber axis. For curvature sensing, the equivalent refractive index of the outward convex portion of the curved fiber is higher than that of the inward convex portion, and the mode field of the transmission mode shifts towards the outward convex portion. After conformal mapping, the refractive index distribution of the curved GIMMF cross-section can be equivalent to that of a straight GIMMF:

[0039]

[0040] Where n0 is the refractive index distribution of the GIMMF before bending, C represents the curvature value, and x is the coordinate. The positive x-axis is perpendicular to the core axis of the curved GIMMF, with the center of the core serving as the origin and pointing towards the convex direction of the GIMMF.

[0041] According to the references, the normalized extinction ratio (ER) can be expressed as:

[0042]

[0043] Where I co and I cl Let Ico and Icl represent the intensity in the fiber core and the intensity of certain cladding modes, respectively. Ico is equal to the double integral of the electromagnetic field distribution of the core mode or cladding mode with the electromagnetic field distribution of the input field. According to equation (5), bending will change the refractive index distribution of the multimode fiber cross-section, which will change the electromagnetic field distribution of the transmission mode. Simultaneously, Ico and Icl will also change, and the transmitted energy will be converted between different modes. Clearly, changes in Ico and Icl will manifest as an increase or decrease in the interference spectrum ER, providing the possibility of intensity demodulation for curvature measurement.

[0044] For temperature sensing, the resonant wavelength can be expressed as:

[0045]

[0046] β represents the difference in thermo-optical coefficients between the core and cladding layers, and α is the coefficient of thermal expansion. It can be seen that the relationship between temperature and resonant wavelength depends on 2L0β / (2m+1), which is a positive constant. Therefore, the interference tilt angle redshifts with increasing temperature.

[0047] To prove the above analysis, such as Figure 3 As shown, the propagation of light in a straight SIMMF-GIMMF-SIMMF was simulated using Rsoft and the beam scaling function. The free-space wavelength was set to 1550 nm. Other important simulation parameters are shown in Table 1 (below). Figure 3 It can be seen that energy transfer exists in both the core and cladding layers of the GIMMF. When SIMMF2 is reached, the core mode and cladding mode are coupled together to the fundamental mode of the output SMF. Therefore, the interference tilt angle produced by the interference between the GIMMF core mode and cladding mode can be observed in the transmission spectrum.

[0048] Table 1 Structural parameters of SIMMF-GIMMF-SIMMF

[0049]

[0050] The effects of SIMMF and GIMMF lengths on the sensor spectrum were investigated using the Rsoft beam propagation method. Figure 4 As shown in (a), the length of GIMMF is fixed at L. GIMMF =1mm, the length of SIMMF was set to 0, 1, 2, 3 and 4mm respectively. This shows that the length of SIMMF has a significant impact on the FSR of the spectrum. As the length of SIMMF increases, the FSR tends to decrease, but it has little effect on the fringe visibility. However, when L SIMMF At 0 mm, there is no interference spectrum because the GIMMF cladding mode is not excited in the absence of SIMMF. Considering the visibility of the stripe pattern and the total length of the SIMMF-GIMMF-SIMMF fiber sensor, a 1 mm long SIMMF was chosen to construct the sensor in the following simulation. Figure 4 (b) shows the effect of GIMMF length on the interference spectrum, indicating that when the SIMMF length (L) SIMMF When ) = 1 mm, the FSR decreases as the GIMMF length increases from 0 to 5 mm. In particular, without GIMMF (L GIMMFThe fringe visibility of a Mach-Zehnder interferometer with a diameter of 0 mm is very weak, essentially constituting a typical SMS fiber structure. This is due to the minimal overlap of mode fields between the core modes of the SIMMF in the SMF core region. Based on the above simulation results and theoretical analysis, it can be concluded that, compared to the traditional single-mode-multimode-single-mode fiber structure (SMS), the SIMMF-GIMMF-SIMMF fiber structure with appropriate structural parameters exhibits higher spectral contrast and a more compact size. These characteristics make the proposed SIMMF-GIMMF-SIMMF fiber structure highly suitable for sensing applications in small spaces, such as bending, vibration, acceleration, or strain.

[0051] Considering both stripe visibility and the total length of the sensor, SIMMFs and GIMMFs with a length of 1 mm were selected to construct the SIMMF-GIMMF-SIMMF structure in the subsequent simulation, fabrication, and experiments. This was done to study L... SIMMF =L GIMMF The curvature sensing characteristics of a sensor with a diameter of 1 mm were simulated in Rsoft by changing the RI distribution of the GIMMF cross section according to equation (5). Figure 4 (a) Curvature response of Dip 1 (1292.05 nm) and Dip 2 (1422 nm) in (black line). The results are as follows... Figure 5 As shown, in the range of 0 to 2.4m -1 Within the range, the intensity of Dip 1 increases monotonically by 10.4 dB, while the intensity of Dip 2 decreases by 18 dB, indicating that microbending will cause energy to be converted in different transmission modes, which is manifested as an increase in the extinction ratio of some interference peaks and a decrease in the extinction ratio of some interference peaks in the interference spectrum.

[0052] The manufacturing process of the SIMMF-GIMMF-SIMMF sensor is as follows: Figure 6As shown in (a–c). First, the fiber coating was stripped, and its end was carefully cut using a precision fiber cleaver to achieve a flat end face. Then, the SMF and SIMMF were spliced ​​using a fiber splicer (KL-300T). Next, the splicing point of the SMF-SIMMF was vertically aligned with the cleaver and fixed on the fiber precision cleaver platform. Then, the rotation controller of the displacement stage was rotated to move the SMF-SIMMF outward by 1 mm and cut the fiber. This process was repeated twice to produce two SMF-SIMMFs (1-mm). Subsequently, the GIMMF and SMF-SIMMF (1-mm) were fused together, and the length of the GIMMF was precisely controlled in the same way. Finally, the fabricated SMF-SIMMF-GIMMF was fused with another SMF-SIMMF (1-mm) to form a SIMMF-GIMMF-SIMMF structure. In the experiment, several SIMMF-GIMMF-SIMMF structures with GIMMF lengths of 1–7 cm were fabricated. Then, the effect of the GIMMF length (1–7 cm) on the interference spectrum was studied, as follows. Figure 7 As shown in (a) and (b), the FSR decreases with increasing GIMMF length, consistent with simulation results. However, when the GIMMF length exceeds 10 mm, fringe visibility shows a significant attenuation, likely due to the decrease in cladding mode energy with increasing transmission distance. Furthermore, as... Figure 7 As shown in (b), when the length of the GIMMF is relatively long (>2cm), the envelope (dashed line) can be clearly observed in the transmission spectrum of the GIMMF. This envelope is extracted from the dense interference spectrum (solid line) by a low-pass filter.

[0053] In order to analyze Figure 7 The detailed features of these interference fringes in (a) and (b) are obtained by using the FFT method to obtain their corresponding spatial spectra, such as... Figure 7 As shown in (c) and (d). From Figure 7 (c) It can be seen that when the GIMMF length is less than 10mm, there is only one main frequency, and it increases with the increase of the GIMMF length. However, for sensors with GIMMF lengths of 3, 5, and 7cm, Figure 7 (d) indicates that, in addition to a dominant frequency in the high-frequency region, new frequency components have appeared in the low-frequency region. To study the interference modes corresponding to these new frequency components, we will... GIMMF Mach-Zehnder interferometers with sandwiched multimode fibers of 1 mm and 3 cm were immersed in glycerol (RI = 1.4746), and their spectra were recorded for comparison. The results were obtained by... Figure 7 (a) and Figure 7As shown by the orange line in (b). Because the RI of glycerol is greater than that of the cladding, higher-order cladding modes in GIMMF may transform into radiation modes and disappear, but this has little effect on the transmission of lower-order cladding modes. From Figure 7 (a) It can be seen that, compared to the spectrum in air, L GIMMF The 1mm sensor exhibits only overall spectral attenuation in glycerol, indicating interference between the core mode and lower-order cladding modes. However, Figure 7 (b) L GIMMF The spectral envelope of the 3cm sensor disappears, while the dense interference spectrum remains. Therefore, we can conclude that... Figure 7 (d) The frequency components in the high-frequency region are caused by interference between the core mode and the low-order cladding mode of the GIMMF, which is related to Figure 7 The dense interference spectrum in (b) corresponds to this. Figure 7 (d) The dominant frequency in the low-to-mid frequency region is caused by interference between low-order and high-order cladding modes of GIMMF, which has an impact on... Figure 7 (b) The dense interferometric spectrum plays a modulation role, which is reasonable because the cumulative phase difference between the low-order and high-order cladding modes of GIMMF increases with L. GIMMF The dynamic range of the sensor increases with increasing length, and interference fringes can only be observed within a limited spectral range (e.g., 1250–1650 nm) when the length exceeds a certain value (typically > 2 cm). However, in practical applications, excessively small FSR and fringe visibility reduce the sensor's dynamic range. Therefore, considering these two main factors, the following experiments primarily focus on structural parameters L. SIMMF =L GIMMF The SIMMF-GIMMF-SIMMF fiber optic sensor has a diameter of 1 mm, and the total length of the sensor is only 3 mm.

[0054] Curvature experimental apparatus such as Figure 8 As shown, light emitted from a broadband light source (BBS, 1250-1650nm) passes through a SIMMF-GIMMF-SIMMF structure, which is fixed in the middle of a precision displacement stage by two magnets. Then, an OSA (600-1700nm) spectrometer with a resolution of 0.02nm records its interference spectrum. One displacement stage is fixed, and the distance between the two fixed points can be adjusted by a rotation controller of the other displacement stage. The curvature of the sensor is adjusted in this way, and the curvature values ​​corresponding to different displacements of the displacement stage are approximately:

[0055]

[0056] Where c is the curvature value, R is the bending radius, L is the length between the two fixed points when the fiber is straight, and x is the single displacement step increment of the displacement platform. In the experiment, L and x were set to 60 mm and 1 μm, respectively. Two L values ​​were used in the test. SIMMF =L GIMMF =1mm SIMMF-GIMMF-SIMMF sensors, their interference spectra are as follows Figure 9 As shown in (a).

[0057] from Figure 9 (a) It can be seen that, after considering optical path loss, the interference spectra of the two identical sensors fabricated using the above method are basically consistent with the simulation results. The differences between them may be due to the fact that material dispersion and sensor length measurement errors were not considered in the simulation. Figure 9 (bf) shows the curvature response of three different interference valleys (Dip1, Dip2, and Dip3). For Dip1, the curvature response is 0–2.36 m. -1 Within the curvature range, the intensity of the interference valley increases significantly with increasing curvature, while the wavelength shift is very small. Figure 9 The blue line in (d) plots the relationship between Dip1 intensity and curvature, indicating that it has a non-linear response to curvature. The fitted curve can be represented as:

[0058] y = 1.58x 3 -1.19x 2 +1.986x-53.3 (9)

[0059] Where y represents intensity, measured in decibels (dB). Dip1 is more sensitive to intensity over high curvature ranges, and its linear range is 1.599–2.36 m. -1 At that time, the maximum sensitivity reached 17.6 dB / m -1 Dip 1's wavelength-based curvature sensitivity is only 0.8 nm / m. -1 ( Figure 9 (red line in (d)).

[0060] Similarly, as curvature increases, Dip 2 and Dip 3 also primarily show intensity changes in the interference valleys, with relatively small wavelength shifts. Therefore, the sensor is more suitable for intensity demodulation during curvature sensing. Figure 9 As shown in (e) and (f), the strengths of Dip2 and Dip3 are in the range of 0–2.36 m. -1 It decreases monotonically within the curvature range. Its linear range is 2.16–2.36 m. -1 The corresponding curvature sensitivity can reach -78.75dB / m. -1 and -74.03dB / m -1 The overall intensity trends of Dip1 and Dip2 with curvature are respectively... Figure 5 The simulation results in (a) and (b) are consistent. These results indicate that different interference valleys have different intensity-based curvature sensitivities. This provides more options for curvature sensing applications; in practical applications, we can select the interference valley with the best performance as needed. Compared to Dip 2 and Dip 3, Dip 1 is more suitable for applications where curvature sensing resolution is not critical, such as curvature-sensing fiber optic wearable breathing bands. Conversely, for Dip 2 and Dip 3, a properly designed packaging structure can stabilize the sensor to a specific curvature radius, enabling intensity-modulated ultrasensitive curvature sensing (e.g., -78.75 dB / m). -1 It has greater potential in monitoring small external fluctuations, such as small vibration signals, pulse signals, or tactile sensations.

[0061] In practical applications, the temperature characteristics of the sensor are also very important. Therefore, L was also studied. SIMMF =L GIMMF Temperature response of a 1mm thick sandwich multimode fiber Mach-Zehnder interferometer. The sensor is fixed to a temperature controller with a resolution of ±0.1℃, and data is recorded 10 minutes after the temperature controller reaches the preset temperature. Figure 10 As shown in (a), as expected, Dip 1 redshifts with increasing temperature, and the relationships between its wavelength, intensity, and temperature are as follows: Figure 10 (b) shows the solid and dashed lines. Figure 10 (b) shows that Dip 1 achieves a temperature sensitivity of 102 pm / ℃ within the range of 40–130℃, while the intensity fluctuation is only ±0.2457 dB. This small intensity variation is typically caused by fluctuations in the light source and can be eliminated using differential compensation methods. Dip 2 and Dip 3 exhibit the same temperature response as Dip 1. However, since the interference valley intensity of the SIMMF-GIMMF-SIMMF sensor is independent of temperature, tracking only 0–2.36 m... -1 The change in the intensity of the interference valley within the curvature range can enable temperature-independent micro-bending sensing.

[0062] To verify L GIMMF The general law governing the spectrum variation of the SIMMF-GIMMF-SIMMF sensor with bending at ≤10mm was investigated, and further research was conducted on the bending at L... SIMMF Under the condition of 1 mm, L GIMMF Sensors with diameters of 1.8, 3, 4, 6, and 8 mm correspond to total lengths of only 3.8, 5, 6, 8, and 10 mm for the constructed SIMMF-GIMMF-SIMMF sensors. Some interference valleys were randomly selected for verification. Experimental results are shown below. Figure 11As shown in (ad), this indicates that within such a small range of curvature, the intensity of the interference valley changes significantly with increasing curvature, while the shift in interference wavelength is not obvious. These results are consistent with... Figure 9 (c) L GIMMF The results are consistent with those of the SIMMF-GIMMF-SIMMF sensor with a diameter of 1 mm, indicating that L GIMMF The spectrum evolution of SIMMF-GIMMF-SIMMF sensors ≤10mm exhibits the same regularity with bending. Furthermore, to evaluate the stability of the proposed sensor, L... GIMMF The 1.8mm SIMMF-GIMMF-SIMMF sensor was placed for 30 days, and the bending sensing experiment was repeated. The results are as follows. Figure 11 As shown in (e), the sensor's bending response exhibits good reproducibility within the range of 1.2–1.696 m. -1 Within the linear range, the intensity-based curvature sensitivity was 21.34 dB / m in the first and second tests, respectively. -1 and 20.98dB / m -1 Therefore, this sensor exhibits good stability.

[0063] at last, Figure 12 (a) and (b) are L GIMMF =4mm and 8mm SIMMF-GIMMF-SIMMF sensors in 0-1.9m -1 The spectral evolution within the curvature range can also be observed. Energy conversion between different interference valleys caused by the curvature can also be observed, which is related to L... GIMMF A sensor with a diameter of 1 mm is similar to ( Figure 9 (b)). These are reasonable because when L GIMMF When the bending is relatively short (≤10 mm), interference mainly occurs in the mandrel and low-order cladding modes. When the bending is within a small range (e.g., 0-2.4 m), the interference is more pronounced. -1 At that time, leakage of low-order cladding modes is negligible. According to equation (6) and the law of conservation of energy, the redistribution of intensity among different low-order cladding modes will lead to a decrease in the ER of some interference patterns, while an increase in others. Furthermore, with Figure 9 The results in (df) are the same. Figure 12 The different interference valleys in (a) and (b) demonstrate different intensity-based curvature sensitivities and linear measurement ranges. Therefore, in practical applications, we can select the interference valley with the best performance according to our needs.

[0064] An ultra-sensitive and temperature-independent curvature fiber optic sensor was designed. This sensor is fabricated by sandwiching a short segment of GIMMF (giant intermodal fiber optic cable) between two 1mm long SIMMF (simulated intermodal fiber optic cables), with the SIMMFs introduced and extracted via single-mode fiber. When the GIMMF length is sufficiently short (≤10mm), the intensity of the interference valley of the sensor is extremely sensitive to applied bending, but the wavelength shift is negligible, which is the opposite of temperature. Therefore, temperature-independent curvature sensing can be achieved simply by tracking the intensity changes of the interference valley. With the GIMMF and SIMMFs being only 1mm long, the total length of the sensor is as short as 3mm, within the range of 0-2.36m. -1 Within a small curvature range, the maximum curvature sensitivity can reach -78.75 dB / m. -1 This is significantly higher than that of most current fiber optic curvature sensors. Furthermore, different interference valleys in this structure exhibit varying curvature sensitivities, providing more options for curvature sensing in practical applications. Due to its small size, ease of manufacture, good reproducibility, and low cost, this sensor has broad application prospects in various packaged high-precision bending-related engineering applications, such as wearable devices, small vibration signal detection, and tactile sensing.

Claims

1. High sensitivity curvature sensor based on a sandwiched multimode fiber Mach-Zehnder interferometer, characterized in that, The application relates to a multi-mode fiber Mach-Zehnder interferometer, which comprises a graded multi-mode fiber, step multi-mode fibers with different core diameters connected to both ends of the graded multi-mode fiber, and a single-mode fiber connected to one end of the step multi-mode fiber, wherein the outer diameter of the cladding of the graded multi-mode fiber, the step multi-mode fibers and the single-mode fiber is the same. The core diameters of the single-mode fiber, the graded multi-mode fiber and the step multi-mode fiber are 8.3 microns, 50 microns and 105 microns respectively. The length of the graded multi-mode fiber is 1 mm. The length of the step multi-mode fiber is 1 mm. The total length of the high-sensitivity curvature sensor of the multi-mode fiber Mach-Zehnder interferometer is 3 mm.

2. The high-sensitivity curvature sensor based on a sandwiched multimode fiber Mach-Zehnder interferometer according to claim 1, characterized in that, The outer diameter of the cladding is 125 microns.

3. The high-sensitivity curvature sensor based on a sandwiched multimode fiber Mach-Zehnder interferometer according to claim 1, characterized in that, The graded multimode fiber core supports LP 01～05 modes transmission.

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