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Abstract
To achieve a fiber strain sensor with a large detection range and high sensitivity, this paper proposes a wave structured fiber SPR strain sensor. When subjected to axial strain, the wave structured fiber is stretched axially, increasing the stretchability of the sensor and achieving a large detection range strain sensing. Meanwhile, axial strain reduces the longitudinal amplitude of the fiber wave structure, effectively changing the total reflection angle of the transmitted beam at the peak and valley (SPR incidence angle) to achieve high sensitivity SPR strain sensing. The experiment indicates that the strain detection range of the sensor can reach 0-1800µε, with a maximum strain sensitivity of 36.25pm/µε. The wave structured fiber SPR strain sensor designed in this article provides a new approach to improve the range and sensitivity of strain detection.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber strain sensor is widely used in engineering safety testing, wearable medical monitoring equipment, and other fields. Currently, they are mainly grating and interferometric types, which are susceptible to temperature crosstalk. Sensitivity and detection range need to be further improved. Fiber surface plasmon resonance (SPR) sensor has the advantages of low temperature crosstalk [1] and high sensitivity, but there is currently little research on fiber SPR strain sensor, and research on fiber SPR strain sensor with large detection range and high sensitivity is needed.
In 2022, a V-groove type fiber SPR strain sensor was first proposed [2]. Strain can cause the deformation of the V-groove, resulting in a change in the optical transmission mode of the sensing area behind the V-groove area, that is, a change in the SPR incidence angle, which shifts the SPR resonance valley wavelength to achieve strain sensing. The strain sensitivity is 25.09pm/µε, and the detection range is 0-600µε, which has a small strain detection range. In the same year, an S-type fiber SPR strain sensor [3] was proposed. Compared to the V-groove type fiber SPR strain sensor, the strain detection range of the sensor was doubled to 0-1200µε, but the strain sensitivity was 14.38pm/µε. Moreover, the fibers on both sides of the sensing probe structure were not in the same straight line, making it difficult to apply strain on the same axis. In 2023, an n-type fiber SPR strain sensor [4] was proposed, with a maximum strain sensitivity of 33.44 pm/µε and a detection range of 0-1000µε, achieving high sensitivity strain sensing. However, the strain detection range needs to be further improved.
The fiber strain sensor that implements the SPR principle requires strain to be able to change the total reflection angle of the sensing area beam (SPR incidence angle) to cause the SPR resonance valley wavelength to shift. By detecting the movement of the SPR resonance valley wavelength, the magnitude of the strain value can be characterized [4]. The transmitted light continuously reflects and propagates forward in cylindrical fiber, the Young's modulus of the fiber is about 70GPa. Axial strain can hardly cause deformation of the fiber, and strain cannot effectively change the total reflection angle of the transmitted light beam in the fiber, thus unable to achieve high sensitivity strain sensing. Meanwhile, due to the high Young's modulus and poor stretchability of fiber, the detection range of fiber SPR strain sensor is limited [5]. To achieve a high sensitivity fiber SPR strain sensor with a large detection range, it is necessary to construct a microstructure fiber sensing probe that can significantly change the SPR incidence angle in the sensing area. At the same time, the microstructure sensing probe needs to have greater stretchability.
In the research of stretchable electronic circuits, to solve the problem of metal wires (Young's modulus of 70-90GPa) not being significantly stretched, Gray et al. proposed wrapping micro manufactured twisted metal wires into organic silicon elastomers, greatly improving the adaptability of metal wires to linear strain [6]. The twisted metal wire utilizes the principle of spring stretching, which utilizes the net elongation of bending to adapt to strain. If the stretching principle of the spring is applied to the fiber SPR strain sensor, the strain detection range can be improved. When the spring is stretched, its pitch will increase. If the SPR incidence angle in the sensing area can be effectively adjusted, it is expected to achieve a high sensitivity fiber SPR strain sensor with a large detection range.
This article designs and manufactures a fiber SPR strain sensor with a wave structure. The use of a hydrogen oxygen fusion taper machine to fabricate wave structured optical fiber increases the elongation ability of the fiber and enhances the strain detection range of the sensor. Strain can cause deformation of the wave structure, resulting in a change in the SPR incidence angle in the sensing area and a significant shift in the SPR resonance valley wavelength, achieving highly sensitive strain sensing. The proposed wave structured fiber SPR strain sensor can be used in the field of strain sensing with high sensitivity and large detection range requirements.
2. Sensor structure and simulation
2.1. Sensor structure and principle
The structure of the wave structured fiber SPR strain sensor proposed in this article is shown in Fig. 1. A wave structure is fabricated on a graded refractive index multimode fiber, and based on the typical optimization parameters of gold film thickness for fiber SPR sensor [7], a 50nm gold film is coated on the cladding of the wave structure area as the SPR strain sensing area. The left side 2cm away from the wave structure is spliced with the single-mode fiber to inject light, as shown in Fig. 1. Due to the thicker core and self-focusing effect of the graded index multimode fiber, the mode of light transmission in the wave region can be reduced and the sensitivity of the fiber SPR sensor can be improved, hence the graded multimode fiber is chosen to make the wave structure. Single mode fiber has a thinner core, which can maintain fewer transmission modes and inject light with a smaller area for graded multimode fiber. Therefore, single-mode fiber is chosen as the injection fiber. Microscopic photos of single-mode fiber end face and graded multimode fiber end face are shown in Fig. 1(a) and (b). Microscopic photos of the fiber wave structure area and beam transmission are shown in Fig. 1(c) and (d).
Fig. 1. Structure diagram of wave structured fiber SPR strain sensor. Microscopic photo of (a) the end face of the injection light single-mode fiber, (b) the sensing graded refractive index multimode fiber end face, (c) the fiber wave structure area, (d) beam transmission of fiber wave structure area.
Surface plasmon resonance (SPR) is essentially the energy transfer caused by the resonance of surface plasmon polariton (SPP) waves and evanescent waves. When the components of the surface plasmon wave and the evanescent wave vector at the total reflection point along the interface x-direction are equal, the conditions for exciting surface plasmon resonance can be obtained as follows:
The broad-spectrum light of the light source is injected from the left side of the single-mode fiber, transmitted along the single-mode fiber core, and injected from the middle of the graded multi-mode fiber core. It is modulated by the refractive index distribution of the graded multi-mode fiber core, and the beam is transmitted along the middle of the graded multi-mode fiber core. After reaching the wave structure, the transmitted light shifts with the amplitude of the wave and undergoes total reflection at the peak and valley, as shown in the photo of the beam transmission path in the wave structure area in Fig. 1(d). The evanescent wave at the total reflection point contacts with the metal film coated on the surface of the wave area, resulting in SPR effect. The beam continues to propagate to the right into the spectrometer, and the transmission spectrum generates an SPR resonance valley. When axial strain is applied to the wave structure sensing probe, the wave structure undergoes deformation, the axial period is elongated, the longitudinal amplitude decreases, and the total reflection angle at the peak and valley increases, that is, the SPR incidence angle increases. The SPR resonance valley wavelength in the transmission spectrum moves towards the shorter wavelength direction. The magnitude of the strain value can be characterized by the amount of movement of the SPR resonance valley wavelength.
2.2. Sensor simulation
In order to theoretically verify that the wave structured fiber SPR sensor can achieve strain sensing, the transmission path of the beam in the wave structured fiber is simulated using Rsoft at different strains, and the total reflection angle change at the wave peak is obtained. Using Matlab to calculate the corresponding SPR resonance valley wavelength shift for different total reflection angles to obtain the final effect of different strains on the SPR resonance valley wavelength.
Firstly, using Rsoft to simulate the beam transmission path in sensor under different strain effects, the simulation parameters are as follows: the injection area is a single-mode fiber, the light source is injected from the single-mode fiber core, and the single-mode fiber is connected to a graded refractive index multi-mode fiber to form a wave structure. The longitudinal amplitude A is 300µm, the axial period is 1800µm, and the total length of the sensing area is 4500µm. The diameter of the single-mode fiber core is 9µm, and the refractive index is 1.458. The cladding diameter is 125µm and the refractive index is 1.447. The core diameter of the graded refractive index multimode fiber is 105µm, and the refractive index type of the core is Diffused, with a maximum refractive index of 1.48. The cladding diameter is 125µm and the refractive index is 1.447. The refractive index of the external environment is 1.333, and the wavelength of the light source is 700nm. The simulation results of beam transmission are shown in Fig. 2(a), and the total reflection angle (SPR incidence angle) at the peak is 76.47 °. When 500µε is applied to the wave structured fiber SPR sensor, the axial length of the sensing area increases by 16µm, and the longitudinal amplitude A obtained from the arc length formula decreases by 3µm. The simulation results of beam transmission are shown in Fig. 2(b), and the SPR incidence angle is 77.42 °. When 1000µε is applied, the simulation results of beam transmission are shown in Fig. 2(c), and the SPR incidence angle is 78.67 °.
Fig. 2. The simulation results of the beam transmission path in the sensor under the strain of (a) 0µε, (b) 500µε, (c) 1000µε.
Based on the obtained SPR incidence angles under different strains, the SPR resonance valley movement under different strains was calculated using Matlab. The simulation parameters are as follows: the refractive index of the graded multimode fiber cladding is 1.447, the refractive index of the external environment medium is 1.333, the thickness of the gold film is 50nm. The SPR incidence angles of sensor under different strains are obtained from Fig. 2, and the simulation results are shown in Fig. 3. It can be seen that when strain is applied to the wave structured fiber SPR sensor, the wave structure undergoes deformation, and the total reflection angle at the peak of the sensing area increases, causing the SPR resonance valley wavelength to shift towards the short wavelength direction. The amount of movement of the SPR resonance valley wavelength can characterize the magnitude of strain.
3. Sensor fabrication and experimental setup
3.1. Sensor fabrication
The fabrication process of the wave structured fiber SPR strain sensor is as follows: the coating layer is peeled off from the middle part of the graded refractive index multimode fiber (GI105/125-24/250, YOFC), with a stripping area length of 2cm. The area where the coating layer is peeled off is placed directly below the flame head of the fiber melting cone machine (COUPLER), and both ends of the fiber are fixed on the rotating fixture of the cone machine. The left end fixture is adjusted to be H higher than the right end fixture, as shown in Fig. 4(a). Using a hydrogen and oxygen flame head with an inner diameter of 10mm, with a hydrogen flow rate of 180SCCM and an oxygen flow rate of 180SCCM, and a flame head height of 3cm, after igniting the hydrogen and oxygen flame, an S-shaped structure is formed by tilting the fiber cable from high left to low right under heating and gravity, as shown in Fig. 4(b). Restore the height of the left end rotating fixture of the cone pulling machine, keep it level with the right end fixture, adjust the S-shaped fiber to be placed parallel below the hydrogen oxygen flame head, set the speed of the electric rotating fixture on the right side of the fiber to 12000µm/s, start the heating of the hydrogen oxygen flame and the rotation of the right fixture for spiral wave processing with the processing time of T, and fabricate a wave structured fiber as shown in Fig. 4(c). By controlling the height difference H between the left and right fixtures of the fiber, the longitudinal amplitude of the fabricated fiber wave structure can be controlled. By controlling the rotational heating time T, the axial period and total length of the wave area of the fabricated fiber wave structure can be controlled. When H is taken as 600µm and T is taken as 2 seconds, the longitudinal amplitude A of the fiber wave structure produced is 300µm, the axial period is 1800µm, and the total length of the wave zone is 4500µm. After the fabrication of the wave structure is completed, cut off the fiber 2cm on the left side of the wave structure, facing the fusion spliced single-mode optical fiber (SMF-28e, Corning) on the left side, as shown in Fig. 4(d). Cover the other areas except for the wave structure with a capillary glass tube, insert the left and right ends of the probe into the bare fiber adapter of the rotating coating clamping device and fix it. After starting the rotating clamping device, place it into the plasma sputtering instrument (ETD-650MS, YLBT), and deposit a 50nm gold film on the wave structure area, as shown in Fig. 4(e). Finally, a layer of UV cured adhesive (NOA133, Norland) with a refractive index of 1.333 is coated on the surface of the gold film in the wave area as the external environmental medium and protective layer for the wave SPR sensor, as shown in Fig. 4(f). The fabrication cost of this sensor is relatively low, and wave structured fiber can be made using only a fiber fused biconical taper machine. The fiber uses common single-mode fiber and graded multimode fiber.
Fig. 4. Fabrication process diagram of wave structured fiber SPR sensor. (a) Left high and right low clamping and heating, (b) rotating heating S-shaped structure, (c) fabricating a graded multimode fiber wave structure, (d) splicing the injection single-mode fiber (e) coating 50nm gold film, (f) coating UV curing adhesive.
3.2. Strain testing experimental device
Build a strain testing experimental setup, as shown in Fig. 5. The broad-spectrum light emitted by the light source (HL-2000, Ocean Optics) is injected from the left side of the single-mode fiber and transmitted to the wave structure area of the graded multimode fiber. Total reflection occurs at the peaks and valleys, and evanescent waves penetrate the gold film in the wave area to undergo SPR. The signal light that undergoes SPR effect continues to be transmitted along the graded multimode fiber to the spectrometer (USB2000+, Ocean Optics). The spectrometer collects the SPR spectrum and transmits it to the computer for data analysis. The wave structured fiber SPR strain sensor is placed on the lifting platform, with strain stretching tables fixed on both sides. The left and right ends of the sensor are fixed on the strain stretching table fixture, and the strain loading test can be achieved by controlling the left and right movement of the stretching table through the strain control console.
4. Strain sensing experiment and performance optimization
4.1. Strain sensing experiment
Place the wave structured fiber SPR strain sensor with a longitudinal amplitude A of 300µm fabricated in the previous text into the strain measurement experimental device for strain sensing experiments. Adjust the strain control console to apply strain, and collect SPR spectra every 100µε increases (the distance between the fiber clamps of the two strain stretching tables is 24cm, and every 24µm increases in distance increases by 100µε). The strain sensing experimental results are shown in Fig. 6(a). It can be seen that within the strain detection range of 0-1400µε, as the strain increases, the SPR resonance valley wavelength moves in a regular manner towards the short wavelength direction. Linear fitting is performed on the SPR resonance valley wavelength and the applied strain, as shown in Fig. 6(b). As the applied strain increases from 0µε to 1400µε, the SPR resonance valley wavelength shifts from 694.44nm to 659.06nm, totaling 35.38nm, with a strain sensitivity of 26.04pm/µε.
Fig. 6. Experimental results of strain sensing using a wave structured fiber SPR strain sensor with a longitudinal amplitude of A = 300µm. (a) SPR resonance spectrum, (b) fitting results of SPR resonance valley wavelength and strain.
4.2. Sensor performance optimization
When the longitudinal amplitude A of the wave structure fiber SPR strain sensor is different, the total reflection angle of the beam at the wave peak is different, corresponding to different initial SPR resonance angles, which will affect the working band, sensitivity, and detection range of the sensor. To study the effect of longitudinal amplitude on the performance of strain sensors, wave structured fiber SPR strain sensors with A of 250µm and 350µm were fabricated, and their beam transmission and strain sensing performance were studied. A comparative analysis was conducted with the previously tested fiber SPR strain sensor with A of 300µm.
(1) Observation of wave structured fiber transmission beam with different longitudinal amplitudes. fabricate wave structured fiber probes with longitudinal amplitudes A of 250µm, 300µm, and 350µm (without gold film coating on the cladding surface), place the probes on the microscope stage, inject 532nm laser, add eosin solution dropwise, and observe the transmission optical path of the probes at the wave structure. The micrograph is shown in Fig. 7. It can be clearly seen that as the longitudinal amplitude A increases, the total reflection angle at the peak and valley decreases, that is, the initial SPR incidence angle decreases (a decrease in SPR incidence angle will cause the SPR working band to move towards longer wavelength and increase sensing sensitivity).
Fig. 7. Microscopic photos of wave structured fiber transmission paths with the longitudinal amplitude of (a) A = 250µm, (b) A = 300µm, (c) A = 350µm.
(2) Performance testing of wave structured fiber SPR strain sensors with different longitudinal amplitudes. Place wave structured fiber SPR strain sensors with longitudinal amplitudes A of 250µm and 350µm into the strain measurement experimental device for strain sensing experiments. Figure 8(a) shows the experimental results of the sensor with A = 250µm. As the applied strain increases from 0 to 1200µε (beyond this strain range, the SPR resonance valley no longer shifts significantly), the SPR resonance valley wavelength shifts from 633.33nm to 616.16nm, totaling 17.17nm, with a strain sensitivity of 14.71pm/µε. Figure 8(b) shows the experimental results of the sensor with A = 350µm. As the applied strain increases from 0 to 1800µε, the SPR resonance valley wavelength shifts from 729.64nm to 664.93nm, totaling 64.71nm, with a strain sensitivity of 36.25pm/µε. Figure 9 shows the linear fitting results of SPR resonance valley wavelength and applied strain for wave structured fiber SPR strain sensors with longitudinal amplitudes A of 250µm, 300µm, and 350µm, respectively.
Fig. 8. Experimental results of strain sensing using wave structured fiber SPR strain sensors with the longitudinal amplitude of (a) A = 250µm, (b) A = 350µm.
Fig. 9. SPR resonance valley wavelength and strain fitting results of wave structured fiber SPR strain sensors with different longitudinal amplitudes A.
5. Discussion
Through simulation analysis and experimental testing, this paper has achieved a fiber SPR strain sensor with a large detection range and high sensitivity. The maximum strain detection range is 0-1800µε, and the maximum strain sensitivity reaches 36.25pm/µε. When strain is applied to the wave structured fiber SPR strain sensor, the wave structured fiber undergoes deformation (axial period elongation and longitudinal amplitude reduction) to adapt to the strain, and the stretchability of the wave structure increases the strain detection range. Meanwhile, the deformation of the wave structure causes a significant change in the SPR incidence angle, and the wavelength of the SPR resonance valley shifts significantly, achieving highly sensitive strain sensing. We conducted research on grating type, interferometric type, and SPR type fiber strain sensor in recent years, and compared them with our study in terms of strain detection range, strain sensitivity and temperature crosstalk. The results are shown in Table 1. Compared with other fiber strain sensor, the wave structured fiber SPR strain sensor proposed in this paper can achieve high sensitivity strain sensing and has a larger strain detection range. Meanwhile, it has lower temperature crosstalk, as the temperature sensitivity of fiber SPR sensor depends on the thermal optical coefficient of the outer coating material of the sensor probe. This article uses UV cured adhesive with a refractive index of 1.33 to coat the outer side of the sensing probe, with a temperature sensitivity of 0.864nm/°C. Therefore, the temperature crosstalk of the wave structured optical fiber SPR strain sensor can be calculated to be 0.024µε/°C. If it is necessary to further reduce temperature crosstalk, a material with a smaller thermal optical coefficient can be chosen to be wrapped on the outside of the sensor probe. Alternatively, temperature compensation algorithms can be used to calculate the strain sensitivity in the absence of temperature crosstalk [8,9].
Table 1. Performance comparison results of fiber strain sensor
The proposed wave structured fiber SPR strain sensor excites the cladding mode at the peak and valley of the fiber wave structure, resulting in optical loss. Therefore, the optical loss was measured in this article, and the optical power loss at the sensor output end was 28.05%. But the sensor adopts wavelength modulation and detects the shift of SPR resonance valley wavelength to achieve sensing, so the transmission optical power loss has little effect on the experimental results, and the spectrometer can also respond well after the loss. The proposed sensor has a larger longitudinal amplitude A, and the working wavelength of the SPR resonance valley wavelength is closer to the long wavelength direction. The strain sensitivity is higher, and the strain detection range is larger. The comparison of experimental test data is shown in Table 2. The larger the longitudinal amplitude A, the smaller the corresponding initial SPR incidence angle, resulting in redshift in the working band and increased sensitivity. When the wave structure of the sensor is stretched to the point where applying strain cannot effectively change the shape of the wave structure, the SPR incidence angle no longer increases, and the SPR resonance valley no longer moves, reaching the maximum detection range of the wave structure. Therefore, the larger the longitudinal amplitude A, the larger the range of effective stretching, and the larger the strain detection range of the sensor (due to the limitations of the fiber taper machine used in this article, it is difficult to fabricate the fiber wave structure with longitudinal amplitude A greater than 350µm.) In the future, wave structured sensing probe with different longitudinal amplitudes and periods can be designed according to application requirements, changing their detection range and sensitivity.
Table 2. Performance comparison results of wave structured fiber SPR strain sensors with different longitudinal amplitudes A
6. Conclusion
This article proposes a large detection range and high sensitivity fiber SPR strain sensing based on wave structure. Through simulation and experimental verification, the sensor can effectively achieve strain sensing. The influence of different longitudinal amplitudes of wave structures on the strain detection range and sensitivity was studied. It was found that when the longitudinal amplitude A is 350µm, the maximum strain detection range is 0-1800µε, and the maximum strain sensitivity reaches 36.25pm/µε. This sensor has a large strain detection range and high sensitivity, which has great application prospects in wearable human health monitoring equipment, mechanical engineering health detection, and other fields.
Funding
Chongqing Talent Project (cstc2024ycjh-bgzxm0057, cstc2024ycjh-bgzxm0227); Chongqing Natural Science Foundation (CSTB2023NSCQ-MSX0020); Doctor's Direct Train research project in Wanzhou Chongqing (wzstc20230413); Chongqing Three Gorges Medical College Project (XJ2023000703); Foundation of Intelligent Ecotourism Subject Group of Chongqing Three Gorges University (zhlv20221008); Open Project Program of Key Laboratories of Sensing and Application of Intelligent Optoelectronic System in Sichuan Provincial Universities (ZNGD2207, ZNGD2208); Open Project Program of Chongqing Key Laboratory of Development and Utilization of Genuine Medicinal Materials in Three Gorges Reservoir Area (KFKT2022005); Fundamental Research Funds for Chongqing Three Gorges University of China (19ZDPY08); Science and Technology Project Affiliated to the Education Department of Chongqing Municipality (KJZD-M202201201); National Natural Science Foundation of China (61705025).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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