Open Access
Add to BibSonomy
Abstract
This study proposes a highly sensitive and stable optical fiber probe based on Vernier effect for high temperature measurement (up to 1000 °C), utilizing photonic crystal fiber (PCF)–based Fabry-Perot interferometers (FPIs). The cascaded FPIs are fabricated by fusion splicing a section of polarization maintaining PCF to a lead-in single-mode fiber, and then a section of temperature-insensitive hollow core PCF is spliced between the PMPCF and a multi-mode fiber. The shift of the spectral envelope is monitored to measure the temperature variation. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C when operating from room temperature to 1000 °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73%. The proposed probe has great application prospects due to compactness, high sensitivity and low cost.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber high temperature sensors (OFHTSs) have been applied for harsh conditions in various industries, such as oil exploration, high power electrical systems monitoring, power plant monitoring and tunnel fire alarms, owing to the intrinsic merits: high sensitivity, compactness, robustness, immunity to electromagnetic interference, the capability of remote sensing and the possibility of multiplexing [1].
OFHTSs have attracted growing interest and different types have been proposed in recent years. Femtosecond laser inscribed fiber Bragg gratings (FBGs) were utilized for high temperature sensing [2–4]. However, the fabrication process is complex. CO2 laser written long period fiber gratings (LPFGs) were proposed for temperature monitoring, but they have large crosstalk of bend and strain, and the sizes are not compact [5–7]. As an alternative, interferometer structures were introduced for high temperature measurement, such as multipath Michelson interferometer (MI) [8], all fiber Mach-Zehnder interferometer (MZI) [9], hollow core fiber based interferometer [10,11], Fabry-Perot interferometer (FPI) [12–15] and hybrid FPIs [16,18]. However, MI is not compact enough. As MZI and hollow core fiber based interferometers work in transmission, they would not be available in narrow space. The aforementioned FPIs are not easy to fabricate as tapering, micro-machining or special splicing are required. In addition, the sensitivities of OFHTSs based on FBGs, LPFGs or interferometers are required to be further improved (usually dozens of pm/ °C) [1–18].
Recently, sensors based on cascaded FPIs utilizing Vernier effect were presented to improve the sensitivity [19–22]. A section of simplified hollow-core fiber was fusion spliced between two single-mode fibers (SMFs) to form cascaded FPIs for high temperature measurement [19]. However, the end face of the SMF working as the third mirror is exposed to ambient environment, the sensor can not work in the scenarios with dusts or liquids, such as tunnels, steam pipes and oil tubes. Vernier effect was reported to enhance the gas refractive index sensitivity [20]. A cylinder-type fiber optic Vernier probe was demonstrated for low temperature measurement [21]. Two intrinsic FPIs were inscribed in a standard SMF by a femtosecond laser for temperature sensing while the sensor is sensitive to strain and the fabrication is difficult [22].
In this paper, we propose a highly sensitive and compact high temperature Vernier probe (HTVP) utilizing photonic crystal fiber (PCF) based FPIs and Vernier effect. The HTVP consists of a lead-in SMF, a polarization maintaining PCF (PMPCF), a hollow core PCF (HCPCF) and a multi-mode fiber (MMF). The HTVP is fabricated simply by fusion splicing the lead-in SMF with three sections of different fibers in line. All end faces of the cascaded FPIs are inside the fiber, which are immune to dusts and liquids. The pure silica core of the PMPCF works as the sensing element. The shift of the spectral envelope is monitored to measure the temperature variation. The HTVP is tested from room temperature to 1000 °C and the viability for high temperature measurement is demonstrated. Moreover, the stability is investigated.
2. Principle
The schematic structure of our proposed HTVP is illustrated in Fig. 1(a). Three sections of different fibers are fusion spliced with a lead-in SMF to fabricate the cascaded FPIs. Meanwhile, the other end of the MMF is cut to form a rough end face to prevent light reflection. The cross views of the PMPCF (NKT PM-1550-01) and the HCPCF (NKT HC-1550-02) are shown in Figs. 1(b) and 1(c). The core and cladding diameters of the PMPCF are 6.3/4.4 µm and 125 µm. The core, PCF region and cladding diameters of the HCPCF are 10 µm, 70 µm and 120 µm, respectively. The SMF and MMF are standard, the cladding diameters are 125 µm, the core diameters are 8.2 µm and 50 µm, respectively. Owing to the thin core of PMPCF and two large air holes, the light can be reflected easily at the interface between the SMF and PMPCF.
There are three reflective mirrors marked M1, M2 and M3 in the sensor head. The FPI made of M1 and M2 is named FPI-1, the FPI made of M2 and M3 is named FPI-2, and the FPI made of M1 and M3 is named FPI-3. The input light is reflected on the fiber end faces and the normalized reflection intensity can be modeled with three-beam optical interference equation [20]:
The dips in the wavelength spectrum of single FPI-1 meet the condition 4πn1L1/λ=(2m + 1)π (m is an integer), the dip wavelength can be described as:
The temperature sensitivity of single FPI-1 can be expressed as [18]:The FSRs of FPI-1 and FPI-2 are designed similar to produce Vernier effect and a large envelope emerges in the spectrum. As the FSR of FPI-3 is much smaller than FPI-1 and FPI-2, it only changes the shapes of the high-frequency fringes of FPI-1 and does not affect the envelope. Substituting Eq. (1) into dI/dλ=0, the envelope function can be derived as:
The temperature sensitivity magnification factor M, originated by the Vernier effect in HTVP, is written as [21]:
3. Simulation and sensor fabrication
3.1 Simulation
According to Eq. (1), assuming that R1=0.03, R2=0.03, R3=0.03, k1=0.5, k2=0.7, L1=392.6 µm, L2=538.3 µm, n2=1.0, the simulated spectra when n1=1.45 and 1.451 are plotted in Fig. 2 in the wavelength range from 1400 nm to 1540 nm. It can be seen from Fig. 2 that the FSR of the simulated envelope is 36.83 nm with the center-wavelength of 1512 nm. The selected dip in the envelope is 1458.1 nm, it is red shifted to 1476.62 nm when the temperature induced RI change is 0.001, consequently the wavelength shift of the envelope is 18.52 nm. The calculated single FPI-1 wavelength shift is 1.0 nm, consequently the simulated amplification factor is 18.52.
3.2 Sensor fabrication
The fabrication process of HTVP is quite simple, only fusion splicing and cleaving are required. In addition, a CO2 laser fusion splicer (Fujikura, LZM-100) is applied to avoid large collapses in the PCF region. The main parameters of the fusion splicer used for sensor fabrication are list in Table 1. Firstly, the PMPCF is fusion spliced with a lead-in SMF. Then the PMPCF is cleaved to form a fiber tip with the length of L1. Secondly, the HCPCF is fusion spliced with a MMF. Then the HCPCF is cleaved to the length of about n1L1 with the help of a long focal length microscope. Thirdly, the aforementioned two parts are fusion spliced together. Finally, the free end of the MMF is cut roughly.
Table 1. Main Parameters of the CO2 laser fusion splicer.
Three HTVPs named HTVP-1, HTVP-2 and HTVP-3 are fabricated and shown in Figs. 3(a), 3(c) and 3(e). It shows that the cavity lengths L1 and L2 of HTVP-1, HTVP-2 and HTVP-3 are 489.4 µm and 654.4 µm, 392.6 µm and 538.3 µm, and 407.4 µm and 577.6 µm, respectively.
Fig. 3. (a) Microscope image and (b) measured spectrum of HTVP-1, (c) microscope image and (d) measured spectrum of HTVP-2, and (e) microscope image and (f) measured spectrum of HTVP-3.
The probes are connected to an ultra-wide-band light source (UWBLS, Golight) and an optical spectrum analyzer (OSA, Yokogawa AQ6370C) by an optical fiber circulator (OFC). The reflection spectra of HTVP-1, HTVP-2 and HTVP-3 under 24 °C are plotted in Figs. 3(b), 3(d) and 3(f). The measured FSREs of HTVP-1, HTVP-2 and HTVP-3 are 18.00 nm, 35.12 nm, and 89.44 nm with the center-wavelengths of 1470 nm, 1496 nm and 1454 nm, respectively. The theoretical FSREs of HTVP-1, HTVP-2 and HTVP-3 are 19.66 nm, 36.83 nm and 81.20 nm with the center-wavelengths of 1472 nm, 1512 nm and 1456 nm, respectively. The theoretical results are consistent with the measured FSREs.
4. Experimental setup and results
4.1 Experimental setup
The illustration of experimental setup for high temperature measurement is shown in Fig. 4. The fabricated three HTVPs are placed in a tubular furnace (MTI GSL-1100X) sensing from room temperature (24 °C) to 1000 °C. The UWBLS is used as light source, and the reflection spectra are captured by the OSA under different temperatures.
Moreover, in order to investigate the tested temperature sensitivity magnification factor M, one single FPI-1 is tested from 24 °C to 600 °C, which is fabricated by fusion splicing a PMPCF of 387 µm to a lead-in SMF.
4.2 Temperature response
The spectral responses of the single FPI-1 at different temperatures are plotted in Fig. 5(a), it is observed that the dips are red shifted. The dip wavelengths of 1334.16 nm, 1363.96 nm and 1396.6 nm are selected to investigate the temperature sensitivity. The dip wavelength shifts versus the temperatures are shown in Fig. 5(b). The wavelength shifts of the three dips are linear to the temperatures (dip 1: 99.66%, dip 2: 99.27% and dip 3: 99.53%). The measured temperature sensitivities of dip 1, dip 2 and dip 3 are 11.79 pm/ °C, 12.08 pm/ °C and 12.48 pm/ °C, respectively. As αT=8.3×10−6 °C−1 and βT=5.5×10−7 °C−1 [18], the theoretical sensitivities are 11.81 pm/ °C, 12.07 pm/ °C and 12.36 pm/ °C by using Eq. (3). The measured sensitivities agree with the theoretical calculations. The experimental results show that the sensitivity of the single FPI-1 is only related to the selected dip wavelength.
Fig. 5. (a) Measured spectra of the single FPI-1 under temperatures from 24 °C to 600 °C, and (b) the dip wavelength shifts versus the temperatures.
The reflection spectra of HTVP-1, HTVP-2 and HTVP-3 under temperatures from 24 °C to 1000 °C are shown in Figs. 6(a), 6(c) and 6(e), respectively. The dip wavelength shifts of the spectral envelope versus the temperatures are plotted in Figs. 6(b), 6(d) and 6(f). Figure 6(b) shows that the temperature sensitivities are 158.02 pm/ °C at the dip of 1341.88 nm, 165.43 pm/ °C at the dip of 1429.28 nm, and 173.43 pm/ °C at the dip of 1466.92 nm, respectively. The values of the linearity are 99.53%, 99.50% and 99.38%. Figure 6(d) shows that the temperature sensitivities are 230.53 pm/ °C at the dip of 1335.8 nm, 227.06 pm/ °C at the dip of 1365.52 nm and 228.12 pm/ °C at the dip of 1396.16 nm, respectively. The values of the linearity are 99.10%, 99.23% and 99.50%. Figure 6(f) shows that the temperature sensitivities are 524.08 pm/ °C at the dip of 1312.72 nm and 535.16 pm/ °C at the dip of 1291.16 nm, respectively. The values of the linearity are 99.03% and 99.73%.
Fig. 6. Spectra of (a) HTVP-1, (c) HTVP-2 and (e)HTVP-3 under temperatures from 24 °C to 1000 °C, the shifts of the spectral envelope of (b) HTVP-1, (d) HTVP-2 and (f) HTVP-3 versus the temperatures.
As the sensitivity of single FPI-1 is only related to the dip wavelength, the measured temperature sensitivity magnification factors of Vernier effect are 13.31, 13.08 and 13.36 for HTVP-1, 19.50, 18.79 and 18.46 for HTVP-2, and 45.11 and 46.83 for HTVP-3, respectively. The measured magnification factors are consistent with the simulated values of 12.86 (HTVP-1), 18.52 (HTVP-2) and 45.1 (HTVP-3).
4.3 Stability
It is anticipated to have a stable performance owning to the pure silica core of the PMPCF that is utilized as the sensing element. HTVP-2 was selected to investigate the stability. The probe is maintained for 6 hours in the temperature of 1000 °C. The reflection spectra were saved by the OSA every hour and are plotted in Fig. 7(a). The wavelengths of dip 1, dip 2 and dip 3 are linear fitted, and the residuals are shown in Fig. 7(b), the residual sum of squares (RSS) are 0.06669, 0.09851 and 0.10874 for the dips. The experimental results show that the proposed probe has a good stability in high temperature.
Fig. 7. (a) Spectra of HTVP-2 under 1000 °C in the duration of 6 hours and (b) residual of dip wavelength.
4.4 Discussion
The experimental results show that the temperature response of the proposed HTVP is linear in a wide temperature range from 24 °C to 1000 °C, in our opinion, the reason is that pure silica core not germanium doped core is applied as the sensing element. As FSR of the envelope and spectral shift are limited by the wavelength range of the light source, the sensitivity can not be enhanced infinitely. However, the sensitivity magnification factor can be enhanced if a broader band light source is available.
The sensors are judged as compact if the sensing element size is several millimeters or smaller. Otherwise, they will be regarded as not compact. Compared to the performance of reported OFHTSs in literature listed in Table 2, our proposed HTVP has the advantages of high sensitivity and compactness.
Table 2. Comparison between our HTVP and the reported OFHTSs.
5. Conclusion
In summary, a highly sensitive and compact HTVP for high temperature measurement utilizing PCF based FPIs is proposed and demonstrated. The probe consists of a lead-in SMF and three sections of different fibers. The shift of the spectral envelope is monitored for temperature measurement. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73% in a wide temperature range. The RSS of the dip wavelength of the envelope is as low as 0.067. In addition, the probe is immune to dusts and liquids. The proposed probe has promising application prospects due to high sensitivity, good stability and low cost.
Funding
National Natural Science Foundation of China (51627804); Provincial Key R&D Program of Anhui (1804a0802214); National Basic Research Program of China (973 Program) (2016YFC0301902).
Disclosures
The authors declare no conflicts of interest.
References
1. X. R. Liao, H. F. Chen, and D. N. Wang, “Ultracompact optical fiber sensor for refractive index and high-temperature measurement,” J. Lightwave Technol. 32(14), 2531–2535 (2014). [CrossRef]
2. C. M. Jewart, Q. Wang, J. Canning, D. Grobnic, S. J. Mihailov, and K. P. Chen, “Ultrafast femtosecond-laser-induced fiber Bragg gratings in air-hole microstructured fibers for high-temperature pressure sensing,” Opt. Lett. 35(9), 1443–1445 (2010). [CrossRef]
3. T. Habisreuther, T. Elsmann, A. Graf, and M. A. Schmidt, “High-temperature strain sensing using sapphire fibers with inscribed first-order Bragg gratings,” IEEE Photonics J. 8(3), 1–8 (2016). [CrossRef]
4. Q. Guo, Y. S. Yu, Z. M. Zheng, C. Chen, P. L. Wang, Z. N. Tian, Y. Zhao, X. Y. Ming, Q. D. Chen, H. Yang, and H. B. Sun, “Femtosecond laser inscribed sapphire fiber Bragg grating for high temperature and strain sensing,” IEEE Trans. Nanotechnol. 18, 208–211 (2019). [CrossRef]
5. Y. J. Rao, D. W. Duan, Y. E. Fan, T. Ke, and M. Xu, “High-temperature annealing behaviors of CO2 laser pulse-induced long-period fiber grating in a photonic crystal fiber,” J. Lightwave Technol. 28(10), 1530–1535 (2010). [CrossRef]
6. B. Wang, W. G. Zhang, Z. Y. Bai, L. Wang, L. Y. Zhang, Q. Zhou, L. Chen, and T. Y. Yan, “CO2-laser-induced long period fiber gratings in few mode fibers,” IEEE Photonics Technol. Lett. 27(2), 145–148 (2015). [CrossRef]
7. X. R. Jin, C. T. Sun, S. J. Duan, W. L. Liu, G. A. Li, S. Zhang, X. D. Chen, L. Zhao, C. L. Lu, X. H. Yang, T. Geng, W. M. Sun, and L. B. Yuan, “High strain sensitivity temperature sensor based on a secondary modulated tapered long period fiber grating,” IEEE Photonics J. 11(1), 1–8 (2019). [CrossRef]
8. L. Duan, P. Zhang, M. Tang, R. X. Wang, Z. Y. Zhao, S. N. Fu, L. Gan, B. P. Zhu, W. J. Tong, D. M. Liu, and P. P. Shum, “Heterogeneous all-solid multicore fiber based multipath Michelson interferometer for high temperature sensing,” Opt. Express 24(18), 20210–20218 (2016). [CrossRef]
9. X. W. Hu, X. Shen, J. J. Wu, J. G. Peng, L. Y. Yang, J. Y. Li, H. Q. Li, and N. L. Dai, “All fiber M-Z interferometer for high temperature sensing based on a hetero-structured cladding solid-core photonic bandgap fiber,” Opt. Express 24(19), 21693–21699 (2016). [CrossRef]
10. Z. Zhang, C. R. Liao, J. Tang, Y. Wang, Z. Y. Bai, Z. Y. Li, K. K. Guo, M. Deng, S. Q. Cao, and Y. P. Wang, “Hollow-core-fiber-based interferometer for high-temperature measurements,” IEEE Photonics J. 9(2), 1–9 (2017). [CrossRef]
11. D. J. Liu, Q. Wu, C. Mei, J. H. Yuan, X. Xin, A. K. Mallik, F. F. Wei, W. Han, R. Kumar, C. X. Yu, S. P. Wan, X. D. He, B. Liu, G. D. Peng, Y. Semenova, and G. Farrell, “Hollow core fiber based interferometer for high-temperature (1000 °C) measurement,” J. Lightwave Technol. 36(9), 1583–1590 (2018). [CrossRef]
12. J. L. Kou, J. Feng, L. Ye, F. Xu, and Y. Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Express 18(13), 14245–14250 (2010). [CrossRef]
13. Z. S. Chen, S. S. Xiong, S. C. Gao, H. Zhang, L. Wan, X. C. Huang, B. S. Huang, Y. H. Feng, W. P. Liu, and Z. H. Li, “High-temperature sensor based on Fabry-Perot interferometer in microfiber tip,” Sensors 18(2), 202 (2018). [CrossRef]
14. H. H. Yu, Y. Wang, J. Ma, Z. Zheng, Z. Z. Luo, and Y. Zheng, “Fabry-Perot interferometric high-temperature sensing up to 1200 ◦C based on a silica glass photonic crystal fiber,” Sensors 18(1), 273 (2018). [CrossRef]
15. C. Zhu, Y. Y. Zhuang, B. H. Zhang, M. Roman, P. P. Wang, and J. Huang, “A miniaturized optical fiber tip high-temperature sensor based on concave-shaped Fabry-Perot cavity,” IEEE Photonics Technol. Lett. 31(1), 35–38 (2019). [CrossRef]
16. A. Zhou, B. Y. Qin, Z. Zhu, Y. X. Zhang, Z. H. Liu, J. Yang, and L. B. Yuan, “Hybrid structured fiber-optic Fabry-Perot interferometer for simultaneous measurement of strain and temperature,” Opt. Lett. 39(18), 5267–5270 (2014). [CrossRef]
17. Y. F. Wu, Y. D. Zhang, J. Wu, and P. Yuan, “Fiber-optic hybrid-structured Fabry-Perot interferometer based on large lateral offset splicing for simultaneous measurement of strain and temperature,” J. Lightwave Technol. 35(19), 4311–4315 (2017). [CrossRef]
18. P. C. Chen and X. W. Shu, “Refractive-index-modified-dot Fabry-Perot fiber probe fabricated by femtosecond laser for high-temperature sensing,” Opt. Express 26(5), 5292–5299 (2018). [CrossRef]
19. P. Zhang, M. Tang, F. Gao, B. P. Zhu, Z. Y. Zhao, L. Duan, S. N. Fu, J. Ouyang, H. F. Wei, P. P. Shum, and D. M. Liu, “Simplified hollow-core fiber-based Fabry-Perot interferometer with modified Vernier effect for highly sensitive high-temperature measurement,” IEEE Photonics J. 7(1), 1–10 (2015). [CrossRef]
20. M. R. Quan, J. J. Tian, and Y. Yao, “Ultra-high sensitivity Fabry-Perot interferometer gas refractive index fiber sensor based on photonic crystal fiber and Vernier effect,” Opt. Lett. 40(21), 4891–4894 (2015). [CrossRef]
21. L. X. Kong, Y. X. Zhang, W. G. Zhang, Y. S. Zhang, L. Yu, T. Y. Yan, and P. C. Geng, “Cylinder-type fiber-optic Vernier probe based on cascaded Fabry-Perot interferometers with a controlled FSR ratio,” Appl. Opt. 57(18), 5043–5047 (2018). [CrossRef]
22. T. Paixao, F. Araujo, and P. Antunes, “Highly sensitive fiber optic temperature and strain sensor based on an intrinsic Fabry-Perot interferometer fabricated by a femtosecond laser,” Opt. Lett. 44(19), 4833–4836 (2019). [CrossRef]
