Open Access
Add to BibSonomy
Abstract
In this paper, a dual-parameter sensor for the measurement of temperature and strain with a double air-gaps structure based on graded-index few mode fiber (GI-FMF) is proposed and fabricated. Two sections of GI-FMF are used: one end of them is etched with HF acid to form an air-gaps, and the other end is cut flat. Next, the single-mode fiber and these two GI-FMFs are spliced successively to form a sensor with double air-gaps structure. A four-beam interferometer is formed and the sensing principle is established. The temperature and strain response characteristics of the sensor are analyzed in the experiment. The reflection spectrum is divided into a low-frequency band affected by the air-gap and a high-frequency band affected by the compound cavity. Experimental results show that the sensor in low-frequency band is insensitive to temperature, and the strain sensitivity is 2.72pm/µɛ. In high-frequency band, the temperature sensitivity of the sensor is 10.81pm/°C, and the strain sensitivity is 1.03pm/µɛ. Simultaneous measurement of temperature and strain is achieved by solving the matrix coefficient equation. The proposed four-beam interferometric sensor has the advantages of compact structure, easy fabrication, and high sensitivity; it can be used to measure the temperature and strain simultaneously.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the advantages of wide measuring range, small size, light weight, anti-electromagnetic interference, the fiber sensors are widely used in biomedical, architectural, aerospace, instrumentation and other fields [1–3]. The physical measurement parameters of the sensor include humidity [4], temperature [5–6], strain [7–8], magnetic field [9–10], displacement [11], curvature [12–13], and refractive index (RI) [14]. The fiber multi-beam interferometric sensor has a simple structure and can be fabricated by chemical etching, laser micromachining, arc discharge and so on. The obtained air-gap and fiber cavity structure has a large refractive index difference, so that the interference fringes is greatly improved. More and more scholars have studied multi-beam interferometric sensor with air-gap in detail.
Base on the principle of multiple-beam interference, Paula A. R. Tafulo et al [15] proposed a graded-index multimode fiber Fabry-Perot(F-P) interferometer based on chemical etching and tested the temperature and strain response. The results show that the sensor has high strain sensitivity and low temperature sensitivity. In 2014, Paulo F. C. Antunes et al [16] made a cavity on the tip of the fiber and then fused it with single-mode fiber (SMF) to fabricate a F-P cavity structure, a maximum sensitivity of 2.56pm/µɛ for strain measurement was achieved. In 2016, B. Xu et al [17] fused a short segment of capillary tube to a standard SMF, and inserted a glass microsphere into the tube to form a fiber F-P interferometer. The measurement of gas pressure and temperature were performed. M. Fátima Domingues et al [18] proposed a RI sensor with a resolution of 3×10−4RIU based on the fiber cavity. In 2018, Jiajun Tian et al [19] proposed a cascade F-P sensor based on the hollow tube and SMF, the experimental results showed that the air-gap temperature sensitivity was 0.902pm/°C and the strain sensitivity was 2.97pm/µɛ, the fiber cavity temperature sensitivity was 10.45pm/°C and the strain sensitivity was 2.8pm/µɛ. Zhe Zhang et al [20] proposed a compact dual-cavity F-P interferometer sensor based on a hollow-core photonic band gap fiber. The results showed that the first F-P cavity was gas pressure sensitive but temperature insensitive, while the second F-P cavity was temperature sensitive but gas pressure insensitive. Yinggang Liu et al [21] proposed a in-fiber cavity sensor, which is machined by 193 nm excimer laser. It can realize simultaneous measurement of gas pressure and temperature. Based on the above studies, we can see that many sensors can measure a single physical quantity or dual-parameter, while the air-gap structure of multi-parameter measurement sensors is difficult to fabricate, and the sensitivity is low. We proposed a dual-parameter four-beam interferometric sensor based on graded-index few mode fiber (GI-FMF) and SMF. The manufacturing process is relatively simple, and the temperature and strain can be measured with high sensitivity simultaneously.
In this paper, a dual-parameter four-beam interferometric sensor for measurement of temperature and strain with double air-gaps structure based on GI-FMF is proposed. Two sections of the GI-FMF are etched by HF acid and sequentially fused with the SMF to form a double air-gaps structure. The core and the cladding are etched simultaneously. The refractive index of the core and the cladding are different, which leads to different corrosion rates of the two, and the core has a faster corrosion rate, an air-gap can be formed. Since the reflector of the formed four-beam interferometer is located at the core, the influence of the minor deformation of the cladding on the experimental results is negligible. Due to the large core diameter, high refractive index in core, and less transmission mode of the GI-FMF, it is easier to prepare air-gap with HF corrosion. It can obtain the reflectors with high reflectivity, and reduce the interference between different modes, and improve the detection sensitivity to external physical quantities. The temperature and strain response characteristics of the sensor are analyzed in the experiment. Different sensitivity are verified in low-frequency band and high-frequency band of reflection spectrum. Simultaneous measurement of temperature and strain is achieved by solving the matrix coefficient equation.
2. Sensor production and principle
2.1 Sensor fabrication
The endface of GI-FMF are shown in Fig. 1. The GI-FMF core radius a is 9.93µm, the inner cladding radius b is 15.49µm, and the outer cladding radius c is 62.5µm. The SMF core and cladding diameter is 9 µm and 125 µm, respectively. In experiment, the GI-FMF and SMF are both fabricated by Yangtze Optical Fiber and Cable Joint Stock Limited Company, China.
The sensor with double air-gaps structure is fabricated by GI-FMF and SMF. The fabrication process of the sensor is mainly divided into six steps, which include endface cut flat, HF acid etch, cleaning, the first discharge fusion splice, tip cutting, the second discharge fusion splice. The schematic diagram for the fabrication process of the sensor is shown in Fig. 2.
Fig. 2. Fabrication process of sensor: (a)endface cut flat (b)HF acid etch (c)cleaning (d)first discharge fusion splice (e)tip cutting (f)second discharge fusion splice
In Fig. 2, the fabrication steps of the sensor are as follows: firstly, two sections of GI-FMF without coating are used and the end of them are cut flat. Secondly, the GI-FMF with one end cut flat is placed vertically into the 40% HF acid for 12 minutes at room temperature, so an air-gap is formed in the end of GI-FMF. Thirdly, the etched end of GI-FMF is put into distilled water for 5 minutes to remove the residual HF acid. Fourthly, the end of the first GI-FMF with an air-gap is fused with SMF by the fiber fusion splicer. Fifthly, another end of the first GI-FMF is cut flat with a certain length. Finally, the end of the second GI-FMF with an air-gap is fused to the flattened end of the first GI-FMF. Thereby, by adjusting the length of two sections of GI-FMF, the double air-gaps sensor with different multiple-beam interference effects can be achieved.
2.2 Sensing principle
The structure diagram of four-beam interferometric sensor with double air-gaps is shown in Fig. 3.
In Fig. 3, we can see that the sensor is composed of five reflectors formed by the front and back surfaces of two air-gaps, that are, M1, M2, M3, M4 and M5. When the beam with amplitude E0 reaches the reflector M1, the reflection and transmission phenomena are generated, and the beam will be reflected and transmitted by different reflector. The beams are reflected by M1, M2, M3, M4, M5 and then enters the SMF. The amplitude of the reflected beam is E1, E2, E3, E4 and E5, respectively. Especially, the length of second GI-FMF is quite long, about 100 mm. The amplitude E5 is relatively small and it can be ignored. Thereby, the E1, E2, E3, and E4 can be expressed as
When four reflectors are involved in the interference, three interferometers with individual air-gap or fiber cavity are formed and the corresponding optical path difference (OPD) is 2n1L1, 2n2L2, 2n3L3, respectively. Moreover, three interferometers with compound cavities are formed and the corresponding OPD is 2(n1L1+n2L2), 2(n2L2+n3L3), 2(n1L1+n2L2+n3L3), respectively. Based on the four-beam interference principle, it can be known that the larger the OPD, the denser the corresponding interference fringes, that is, the higher the interference fringe frequency. If L1=L3, the total intensity in Eq. (2) can be simplified to
Due to the thermal expansion and thermo-optic effect of the fiber cavity material, the phase difference of the two beams involving in the interference varies when the external temperature changes, which causes the resonance wavelength of the interference fringe to shift. The thermal expansion coefficient of quartz is about 5.5×10−7/°C. At room temperature and standard atmospheric pressure, the thermo-optic coefficient of air-gap C1 and C3 is about -5.6×10−7/°C, while the thermo-optic coefficient of GI-FMF cavity C2 is about 10−5∼10−6/°C. Therefore, when the interference fringe changes, the free spectral region (FSR) formed by the air-gap changes very little with temperature, which is negligible compared with the FSR formed by the fiber cavity. When the sensor is affected to external strain, the length of the air-gap and the GI-FMF cavity will change, and the change of the air-gap is more significant, which causes the resonance wavelength to shift.
Based on the results of Refs. [23–24], we can obtain that the longer the length of the air-gap, the higher the transmission loss and the smaller the fringe visibility of the reflection spectrum. The graded-index fiber(GIF) has a focusing period, the fringe visibility can reaches a maximum at a quarter period. Therefore, the length of the selected GI-FMF will affect the change of fringe visibility. According to the Eq. (3), when L1=L3=25.45µm and L2=400µm, the simulation interference fringe of four-beam interferometric sensor with double air-gaps structure is shown in Fig. 4.
Fig. 4. Simulation interference fringe of four-beam interferometric sensor with double air-gaps structure
When the external temperature rises from 0°C to 100°C, the strain increases from 0µɛ to 4000µɛ, and the simulation results of the reflection spectrum is shown in Fig. 5.
In Fig. 5, it can be seen that when the temperature rises, the high-frequency band in reflection spectrum shifts to the long wavelength direction, which is a red-shift phenomenon. When the strain increases, the low-frequency band and high-frequency band in reflection spectrum are all red-shift. In this article, the low-frequency band is formed by the reflected light interference of air-gap with small optical path difference, the lower envelope of the spectrum is taken as its low-frequency spectrum. The high frequency band is formed by the reflected light interference of fiber cavity or composite cavity with large optical path difference, the thin interference fringe of the spectrum is taken as its high frequency spectrum.
3. Experiment and analysis
3.1 Temperature experiment
With reference to the structure in Fig. 3, we fabricated the four-beam interferometric sensor with double air-gaps, which the L1=L3=25.45µm and L2=400µm. The photomicrograph and reflection spectrum of the sensor are shown in Fig. 6.
Fig. 6. The four-beam interferometric sensor with double air-gap structure: (a)photomicrograph, (b)reflection spectrum
The fast Fourier transform(FFT) is performed on the reflection spectrum in Fig. 6, and the spatial frequency spectrum is obtained as shown in Fig. 7.
In Fig. 7, there are four distinct spatial frequency peaks, which are labeled as peak1, peak2, peak3 and peak4. According to the relationship between spatial frequencies and air-gap or fiber cavity of different lengths, it can be known that peak1 corresponds to individual air-gap C1 with OPD of 2n1L1, peak2 corresponds to individual GI-FMF cavity C2 with OPD of 2n2L2, peak3 corresponds to compound cavity of C1 and C2 with OPD of 2(n1L1+n2L2), peak4 corresponds to compound cavity of C1, C3 and C2 with OPD of 2(2n1L1+n2L2). The reflectivity of each reflector in the sensor is low, and the intensity of the light after multiple reflections is getting smaller and smaller. This causes the amplitude of the high-frequency band in the reflection spectrum to become smaller and smaller. The amplitude of peak4 is the smallest, that is, E4 is small. Therefore, the reflection spectrum of the proposed sensor is formed by four-beam interference. The amplitude difference between different high-frequency bands is small, and the OPD of the corresponding air-gap or fiber cavity also has little difference, that is, their FSR contribution to the spectral intensity is almost similar, it is difficult to distinguish, so we can think the high-frequency band in the spectrum is the result of the interaction of several cavities.
The experimental setup for testing the temperature characteristics of the sensor is shown in Fig. 8, which is composed of the broadband optical source (ASE, a wavelength range of 1520∼1610 nm), optical spectrum analyzer (OSA, AQ6375), optical circulator (OC, a center wavelength of 1550 nm) and OVEN (WHL-30B, a resolution of 0.1°C). The ASE source is manufactured by Shanghai Yongmao Photoelectric Technology Corporation, China. The OSA is manufactured by Yokogawa Electric Corporation, Japan.
In Fig. 8, the light from the ASE source enters the sensor by the OC, and the reflected interference signal enters the OSA. In this experiment, the sensor is placed in the OVEN, the temperature raised from 30°C to 90°C, and spectral data is recorded every 10°C. The temperature experimental results of the sensor in different frequency bands are shown in Fig. 9.
In Fig. 9, the reflection spectrum in the low-frequency band hardly changes with the increase of temperature, so the individual air-gap is not sensitive to temperature. In high-frequency band, the reflection spectrum shifts to the long wavelength direction with the increase of temperature, indicating that the compound cavity is sensitive to temperature. The main reason is that the GI-FMF cavity is sensitive to temperature.
The temperature fitting reults in high-frequency band is shown in Fig. 10. It can be known that the temperature sensitivity of the sensor is 10.81pm/°C, the linearity is 0.9979.
3.2 Strain experiment
The experimental setup for testing the strain characteristics of the sensor is shown in Fig. 11. We fix the two ends of the sensor on platforms to make the sensor suspend in the air. The distance between the two fixed points is S. The sensor is axially displaced by adjusting one side of the platform. As the displacement increases, the strain applied to the sensor increases.
In Fig. 11, the axial strain ɛ can be expressed as
where $\Delta S$ is the moving distance of the platform in the axial direction. S = 87 mm. When the platform moves 50µm each time, and the corresponding spectral data is recorded. The measurement range of the strain is 0∼3448µɛ.The strain experimental results of the sensor in different frequency bands are shown in Fig. 12.
It can be clearly seen from Fig. 12 that as the strain increase, the spectra of the low-frequency band and the high-frequency band both show red-shift, which is consistent with the theoretical results. The strain fitting results in low-frequency band and high-frequency band are shown in Fig. 13.
In Fig. 13, the strain sensitivity in low-frequency band and high-frequency band is 2.72pm/µɛ and 1.03pm/µɛ, the linearity is 0.9979 and 0.9993, respectively. The strain sensitivity in low-frequency band is higher than that in high-frequency band, which is mainly due to the change of air-gap or fiber cavity length cauesd the OPD changing. The OPD of compound cavity is much larger than that of air-gap. For the same optical path change, the change of the air-gap is much more obvious than that of the compound cavity, so the wavelength shift in high-frequency band formed by the compound cavity is relatively small.
In addition, the simultaneous measurement of strain and temperature can be achieved by solving the matrix coefficient equation. By analyzing the high-frequency and low-frequency bands in reflection spectra, the double-parameter responses of the air-gap and compound cavity can be obtained. The experimental results show that the change of temperature and strain causes the shift of the reflection spectrum. When temperature and strain change simultaneously, the spectral shift $\Delta \lambda$ generated by the individual cavity can be expressed as
where ${k_1}$and ${k_2}$ are the temperature and strain sensitivity of the air-gap or fiber cavity, $\Delta T$ and $\Delta \varepsilon$ are the the variation quantity of temperature and strain, respectively. Therefore, the spectral shift generated by the two different cavities can be expressed asFor example, in the proposed four-beam interferometric sensor with double air-gaps structure, the air-gap is not sensitive to temperature, and the temperature sensitivity of the compound cavity is 10.81pm/°C, that is, k11=0, k12=10.81. In 0∼3448µɛ, the strain sensitivity of the air-gap and compound cavity is 2.72pm/µɛ and 1.03pm/µɛ, that is, k21=2.72, k22=1.03. We substitute these parameters into Eq. (7), and the matrix equation of temperature and strain of the four-beam interferometric sensor can be obtained as
In experiment, some associated errors may be caused mainly by the following factors.
- (1) Errors resulting from the accuracy of temperature, strain measurement and wavelength selection are main factors which lead to the observed deviation. Error can be reduced by using high precision instruments. These measurement errors also can be decreased by testing multiple sensors repeatedly.
- (2) In the fabrication process of air-gap, the size and shape of the air-gap are influenced by HF acid concentration. We use the same steps to prepare the sensor, there will be a minor difference between them, but the trend of spectrum is consistent. The processing error is inevitable in the preparation of air-gap.
- (3) In the theoretical analysis, the reflectors of the four-beam interferometer are considered to be ideally parallel. In the practical fabrication process, the reflector of the air-gap formed by corrosion is not perfect, which will cause a certain error in experiment.
4. Conclusion
In this paper, an all-fiber dual-parameter four-beam interferometric sensor based on the double air-gaps structure is proposed. Two sections of the GI-FMF are etched by HF acid to form the air-gap structure, and fused with SMF to form the double air-gaps structure. The formation of the reflection spectrum is analyzed in principle, and the effects of temperature and strain on the sensor are simulated. The response characteristics of the temperature and strain for this sensor are tested experimentally. The results show that this four-beam interferometric sensor is not sensitive to temperature in low-frequency band, and the temperature sensitivity in high-frequency band is up to 10.81pm/oC in 30∼90°C. The strain sensitivity in low-frequency band and high-frequency band is 2.72pm/µɛ and 1.03pm/µɛ in 0∼3448µɛ. This sensor has the advantages of compact structure, convenient fabrication, high sensitivity, and can realize simultaneous measurement of temperature and strain.
Funding
National Natural Science Foundation of China (61575170, 61605168); the Key Basic Research Program of Hebei Province (17961701D); the State Scholarship Fund of China (201708130199).
Acknowledgments
The authors thank the National Natural Science Foundation of China and the key basic research program of Hebei province in China. X. Fu also thank the China Scholarship Council for support.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. H. Murayama, D. Wada, and H. Igawa, “Structural health monitoring by using fiber-optic distributed strain sensors with high spatial resolution,” Photonic Sens. 3(4), 355–376 (2013). [CrossRef]
2. A. Wada, S. Tanaka, and N. Takahashi, “Fast interrogation using sinusoidally current modulated laser diodes for fiber Fabry-Perot interferometric sensor consisting of fiber Bragg gratings,” Opt. Rev. 25(5), 533–539 (2018). [CrossRef]
3. T. T. Wang, Y. X. Ge, H. B. Ni, J. H. Chang, J. H. Zhang, and W. Ke, “Miniature fiber pressure sensor based on an in-fiber confocal cavity,” Optik 171, 869–875 (2018). [CrossRef]
4. N. Liu, M. L. Hu, H. Sun, T. T. Gang, Z. H. Yang, Q. Z. Rong, and X. G. Qiao, “A fiber-optic refractometer for humidity measurements using an in-fiber Mach-Zehnder interferometer,” Opt. Commun. 367, 1–5 (2016). [CrossRef]
5. C. X. Yue, H. Ding, W. Ding, and C. W. Xu, “Weakly-coupled multicore optical fiber taper-based high-temperature sensor,” Sens. Actuators, A 280, 139–144 (2018). [CrossRef]
6. Y. Zhao, L. Cai, and X. G. Li, “In-fiber modal interferometer for simultaneous measurement of curvature and temperature based on hollow core fiber,” Opt. Laser Technol. 92, 138–141 (2017). [CrossRef]
7. Y. Sun, D. M. Liu, P. Lu, Q. Z. Sun, W. Yang, S. Wang, L. Liu, and J. S. Zhang, “Dual-parameters optical fiber sensor with enhanced resolution usingtwisted MMF based on SMS structure,” IEEE Sens. J. 17(10), 3045–3051 (2017). [CrossRef]
8. B. Yin, Y. Li, Z. B. Liu, S. C. Feng, Y. L. Bai, Y. Xu, and S. S. Jian, “Investigation on a compact in-line multimode-single-mode-multimode fiber structure,” Opt. Laser Technol. 80, 16–21 (2016). [CrossRef]
9. X. Z. Ding, H. Z. Yang, X. G. Qiao, P. Zhang, O. Tian, Q. Z. Rong, N. A. M. Nazal, K. S. Lim, and H. Ahmad, “Mach-Zehnder interferometric magnetic field sensor based on a photonic crystal fiber and magnetic fluid,” Appl. Opt. 57(9), 2050–2056 (2018). [CrossRef]
10. Y. Luo, X. Q. Lei, F. Q. Shi, and B. J. Peng, “A novel optical fiber magnetic field sensor based on Mach-Zehnder interferometer integrated with magnetic fluid,” Optik 174, 252–258 (2018). [CrossRef]
11. K. Tian, G. Farrell, X. F. Wang, E. Lewis, and P. F. Wang, “Highly sensitive displacement sensor based on composite interference established within a balloon-shaped bent multimode fiber structure,” Appl. Opt. 57(32), 9662–9668 (2018). [CrossRef]
12. F. Xia, Y. Zhao, and M. Q. Chen, “Optimization of Mach-Zehnder interferometer with cascaded up-tapers and application for curvature sensing,” Sens. Actuators, A 263, 140–146 (2017). [CrossRef]
13. Y. Zhao, F. Xia, and M. Q. Chen, “Curvature sensor based on Mach-Zehnder interferometer with vase-shaped tapers,” Sens. Actuators, A 265, 275–279 (2017). [CrossRef]
14. T. T. Wang, Y. X. Ge, J. H. Chang, and M. Wang, “Wavelength-interrogation Fabry-Perot Refractive Index Sensor Based on a Sealed In-fiber Cavity,” IEEE Photonics Technol. Lett. 28(1), 3–6 (2016). [CrossRef]
15. P. A. R. Tafulo, P. A. S. Jorge, J. L. Santos, F. M. Araújo, and O. Frazão, “Intrinsic Fabry-Pérot Cavity Sensor Based on Etched Multimode Graded Index Fiber for Strain and Temperature Measurement,” IEEE Sens. J. 12(1), 8–12 (2012). [CrossRef]
16. P. F. C. Antunes, M. F. F. Domingues, N. J. Alberto, and P. S. André, “Optical Fiber Microcavity Strain Sensors Produced by the Catastrophic Fuse Effect,” IEEE Photonics Technol. Lett. 26(1), 78–81 (2014). [CrossRef]
17. B. Xu, Y. M. Liu, D. N. Wang, and J. Q. Li, “Fiber Fabry–Perot Interferometer for Measurement of Gas Pressure and Temperature,” J. Lightwave Technol. 34(21), 4920–4925 (2016). [CrossRef]
18. M. F. Domingues, P. Antunes, N. Alberto, R. Frias, R. A. S. Ferreira, and P. André, “Cost effective refractive index sensor based on optical fiber micro cavities produced by the catastrophic fuse effect,” Measurement 77, 265–268 (2016). [CrossRef]
19. J. J. Tian, Y. Z. Jiao, S. B. Ji, X. L. Dong, and Y. Yao, “Cascaded-cavity Fabry-Perot interferometer for simultaneous measurement of temperature and strain with cross-sensitivity compensation,” Opt. Commun. 412, 121–126 (2018). [CrossRef]
20. Z. Zhang, J. He, B. Du, F. C. Zhang, K. K. Guo, and Y. P. Wang, “Measurement of high pressure and high temperature using a dual-cavity Fabry–Perot interferometer created in cascade hollow-core fibers,” Opt. Lett. 43(24), 6009–6012 (2018). [CrossRef]
21. Y. G. Liu, T. Zhang, Y. X. Wang, D. Q. Yang, X. Liu, H. W. Fu, and Z. N. Jia, “Simultaneous measurement of gas pressure and temperature with integrated optical fiber FPI sensor based on in-fiber micro-cavity and fiber-tip,” Opt. Fiber Technol. 46, 77–82 (2018). [CrossRef]
22. W. Zhang, Y. G. Liu, T. Zhang, X. Liu, H. W. Fu, and Z. A. Jia, “Dual micro-holes-based in-fiber Fabry-Perot interferometer sensor,” Acta Phys. Sin. 67(20), 204203 (2018). [CrossRef]
23. X. Z. Xu, J. He, M. X. Hou, S. H. Liu, Z. Y. Bai, Y. Wang, C. G. Liao, Z. B. Ouyang, and Y. P. Wang, “A Miniature Fiber Collimator for Highly Sensitive Bend Measurements,” J. Lightwave Technol. 36(14), 2827–2833 (2018). [CrossRef]
24. F. C. Zhang, X. Z. Xu, J. He, B. Du, and Y. P. Wang, “Highly sensitive temperature sensor based on a polymer-infiltrated Mach–Zehnder interferometer created in graded index fiber,” Opt. Lett. 44(10), 2466–2469 (2019). [CrossRef]
