Open Access
Add to BibSonomy
Abstract
A highly sensitive fiber-tipped temperature sensor based on polydimethylsiloxane (PDMS)-filled spring Fabry-Perot (FP) cavity has been proposed and experimentally demonstrated. The spring FP cavity is first fabricated on the fiber endface by the two-photon polymerization lithography. After that, PDMS is filled into the cavity to drive the elongation of the flexible spring and thus to functionalize high-performance temperature sensing. Benefiting from the large thermal expansion coefficient of PDMS, the proposed sensor exhibits a maximal temperature sensitivity of 704.3 pm/°C with excellent operating repeatability and stability. Besides, by selecting a proper spring constant k, the FP sensitivity can be precisely adjusted in the range of 100-700 pm/°C. Thanks to the advantages of high fabrication accuracy and designable property, the proposed sensor could promote numerous usages for customizable temperature sensing.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Temperature sensing is of great importance in various fields, e.g., industrial production, clinical medicine, environment monitoring, and so on [1–3]. In the past decades, optical fiber Fabry-Perot interferometer (FPI) temperature sensors have received widespread attention due to their inherent advantages, like compact structure, electromagnetic immunity, and remote monitoring capability [4–6]. However, traditional all-fiber FPI sensors usually possess limited sensitivity ($\sim$10 pm/$^{\circ }$C) [7–9], restricting their further applications. Therefore, new types of FPI sensors based on temperature-sensitive materials including silicone-oil [10], ethanol [11], polystyrene [12], ultraviolet (UV) glue [13], polydimethylsiloxane (PDMS) [14,15] [16,1], etc. have become one of the recent research hotspots. These thermosensitive materials are usually filled into the air cavity of the FPIs, giving rise to an improved sensitivity from several hundreds of pm/$^{\circ }$C to several nm/$^{\circ }$C. However, most of the current-reported structures commonly encounter certain problems, such as low fabrication accuracy and complex reflective surface alignment [17–19]. These drawbacks severely limit the production repeatability of the sensors. Besides, the varied applications demand different temperature sensitivities and resolutions. For instance, the hyperthemia treatments require a 35 to 45 $^{\circ }$C range with a resolution of about 0.1 $^{\circ }$C [2]. And the common wearable devices possess a scope of 34-42 $^{\circ }$C with 0.01 $^{\circ }$C resolution for clinical healthcare usages [20]. Hence the easy-fabricated thermal sensors with wanted coefficients are attractive for these practical applications.
In this work, we proposed a highly sensitive fiber temperature sensor based on PDMS-filled spring-composed Fabry-Perot (FP) cavity. The spring FP cavity is manufactured directly on the fiber facet by two-photon polymerization (TPP) techniques, which ensures the great fabrication accuracy and repeatability of the spring FPI. The thermosensitive PDMS is then filled into the cavity to enhance the temperature sensitivity. Due to the adjustable springs, the proposed sensor exhibits a designable sensitivity in the range of 100-700 pm/$^{\circ }$C (a maximal value of 704.3 pm/$^{\circ }$C). In sum, the proposed spring FPI sensor is of broad potential for custom-made temperature sensing.
2. Results and discussion
2.1 Fabrication and operation principle
2.1.1 Sensors fabrication
Figure 1 presents the implementation process of the proposed sensor. Firstly, the spring FP cavity is fabricated on the endface of standard single-mode fiber (G.652D) via a commercial TPP system (PPGT2, Nanoscribe GmbH). A homemade fiber holder is adopted to overcome the alignment difficulties of fabricating 3D microstructures on waveguide facets. More detailed illustrations can be found elsewhere [21]. Figure 1(a) demonstrates the TPP manufacturing process. In brief, a drop of IP-Dip photoresist (Nanoscribe GmbH) is first deposited on the surface of the optical fiber. Then, the micro-structure is produced in a layer-by-layer manner. Finally, the structure is immersed in a bath of 2-Acetoxy-1-methoxypropane (PGMEA) for 10 minutes, and then in C$_3$H$_5$F$_9$O for another 10 minutes. After that, additional curing under ten-minute violet light (405 nm) exposure is performed to enhance the mechanical strength of the springs.
Figure 1(a) also gives a scanning electron microscopy (SEM) image of the fabricated fiber-tipped spring FPI. Two parallel surfaces, including the fiber facet and the upper circular plane, constitute the FP cavity, where a triple-spring connects the two surfaces. Four key parameters of $R$, $L$, $t$, and $w$ define the geometries of the spring FPI, in which the former three are designated as 15 µm, 56 µm, and 4 µm, respectively. Note that $w$ is finely adjusted in the range of 4-14 µm (step: 2 µm) in order to create a set of spring constant $k$. Previously, we systematically investigated $k$ values with respect to the spring structures. The detailed information can be examined in our previous work [22] and Table S1. As can be seen from the SEM image, the FPI shows a smooth surface and well-defined structural appearance, indicating the high-quality TPP printing techniques.
Following the TPP-enabled fabrications, a plastic ring sleeve is employed to encapsulate the fragile spring. As shown in Fig. 1(b), the fiber-tipped FPI is first inserted into the ring sleeve. Then, a drop of PDMS, prepared by mixing 1 ml PDMS with 0.1 ml curing agent, is injected into the sealed sleeve. Since the PDMS is rather viscous, the sensor is further transferred into a vacuum oven for about 30 min to ensure that the PDMS completely infills the cavity. At last, the PDMS is fully cured at 80$^{\circ }$C for over 2 hours.
Fig. 1. (a) Schematic diagrams of the spring FPI fabricated on the fiber endface. On the bottom right, an SEM image presents the spring FPI with geometrical parameters of $R$, $L$, $t$, and $w$. (b) Encapsulation process of the PDMS-filled FPI sensor.
2.1.2 Experimental setup and operation principle
Figure 2(a) presents the experiment setup for temperature sensing. The optical path mainly consists of a tunable laser (TSL-510, Santec), an optical powermeter (MPM-210, Santec), and a fiber coupler. The sensor is placed inside a sealed electric drying oven, where the temperature is tuned in the range of 30-50$^{\circ }$C. For each experiment, the reflection spectra are recorded after the oven is held at the targeted degrees for at least 20 minutes.
To better understand the operation principle of the sensor, a diagram illustrating the interference pattern is displayed in Fig. 2(a). The reflected intensities of the two cavity surfaces are marked as $I_{1}$ and $I_{2}$, respectively. Here the reflections originating from the cavity’s outer interface are ignored, since the layer thickness is much smaller than the cavity length (i.e., $L^{\prime }$ $\ll$ $L$). Then, the interference pattern $I$ can be expressed as:
where $n$, $L$, $\lambda$, and $\phi _{0}$ is the refractive index of the cavity medium, cavity length, wavelength, and the initial phase of the FPI, respectively. The phase condition at the dip position of the fringe satisfies the following equation. where $m$ is an integer and $\lambda _{m}$ represents the dip wavelength of $m^{th}$ order fringe. According to Eq. (2), the temperature sensitivity $S$ (see Fig. 2(b)) can be expressed as [13]:Fig. 2. (a) Schematic diagrams of the experimental setup and the working principle of the FPI sensor. (b) The typical wavelength shift of two interference spectra obtained from the proposed sensor at varied temperatures. (c) Schematic diagrams showing small-k and large-k induced PDMS-modified spring stretching.
The relationship between the change of optical length ($nL$) and temperature can be further illustrated.
Combining Eq. (3) with Eq. (4), the temperature sensitivity $S$ can be evaluated as:
Since the cavity is sealed with PDMS, here $dn/(ndT)$ represents the thermal-optic coefficient of PDMS. And $dL/(LdT)$ is the variation of unit cavity length versus temperature, which is co-decided by the thermal expansion coefficient of PDMS and the spring constant $k$. Figure 2(c) describes the PDMS-filled small-$k$ and large-$k$ imposed cavity length changes under a fixed temperature variation (i.e., $\Delta T = T_{2} - T_{1}$). In general, the more flexible spring with a decreased $k$ can be more easily stretched by PDMS (i.e., $\Delta L_{1} > \Delta L_{2}$), thus leading to a larger thermal sensitivity. Therefore, the targeted sensitivity can be achieved by precisely sculpturing $k$ value.
2.2 Experimental results and discussion
The temperature responses of the air-filled and PDMS-filled spring FP cavities have been first investigated in the range of 30 $^{\circ }$C to 50 $^{\circ }$C (step: 5 $^{\circ }$C). The spring used here is made with parameters of $R$=15 µm, $L$=56 µm, $w$=4 µm, and $t$=4 µm, producing an experimentally determined $k$ value of 9.9 µN/µm [22]. As shown in Fig. 3(a) & (b), the reflection spectra of the air-filled spring FPI exhibit a slight redshift, leading to a temperature sensitivity of 16.3 pm/$^{\circ }$C ($R^{2}$=0.9836). This value is about five times smaller than the pillar-supported FPI in our previous work [21]. The significant difference can be ascribed to the varied structure-imposed thermal expansions. Compared to the solid pillars, the relatively low expansion of spring results in a smaller cavity length change and thus generates a much lower sensitivity.
Fig. 3. The reflection spectra shift of the air-filled spring FP cavity (a) and the PDMS-filled one (c) as the temperature increases from 30 $^{\circ }$C to 50 $^{\circ }$C. Corresponding linear fitting of temperature sensitivity is depicted in (b) and (d), respectively. Here the spring constant of $k$ is estimated as 9.9 µm/µN using geometric parameters of $R$=15 µm, $L$=56 µm, $w$=4 µm, and $t$=4 µm, respectively.
However, when the spring FP cavity is packed with PDMS, a pronounced spectral redshift can be found as presented in Fig. 3(c). With a good linearity ($R^{2}$=0.9985), a temperature sensitivity of 704.3 pm/$^{\circ }$C is obtained, being about 43 times greater than the air FPI. This remarkable sensitivity promotion is partly due to the high thermal expansion coefficient of PDMS (9.6 $\times$ $10^{-4}$ $K^{-1}$) [18], which is about an order of magnitude higher than IP-Dip (8 $\pm$ 0.5 $\times$ $10^{-5}$ $K^{-1}$) [23]. The expansion of PDMS within the spring imposes a larger extension of the FP cavity and thus enhances the sensor performance.
To evaluate the temperature-sensing repeatability, cycle tests by increasing and decreasing environmental temperature (step size: 5$^{\circ }$C) are conducted as data shown in Fig. 4(a). The sensitivities of the heating and cooling process correspond to 737.5 pm/$^{\circ }$C and 703.0 pm/$^{\circ }$C, respectively, both highlighting excellent linearity ($R^{2}$ > 0.999). In addition, three-round temperature sensing tests by increasing the temperature from 30$^{\circ }$C to 50$^{\circ }$C have also been carried out. The data in Fig. 4(b) show similar temperature sensitivities (737.5 pm/$^{\circ }$C, 704.3 pm/$^{\circ }$C, and 693.1 pm/$^{\circ }$C) with a low standard deviation of 23.1 pm/$^{\circ }$C. The slight coefficient variations can be attributed to multiple factors including sensing hysteresis. Although at least 15 minutes are applied after the targeted degree is arrived. The oven-induced temperature variations could also lead to measurement errors and sensitivity changes. Besides, the wavelength shifts for the heating and cooling process between 30 $^{\circ }$C and 50 $^{\circ }$C in three continuous cycle tests have been analyzed in Supplement 1, SI. The standard deviations of the dip wavelength at two temperatures are calculated as 0.38 nm and 0.11 nm, respectively, demonstrating outstanding sensing repeatability. To examine the long-term sensing stability, the spring FPI is further placed in the electric drying oven for three hours test (see Supplement 1, Fig. S2). Under two temperature configurations (i.e., 30$^{\circ }$C and 50$^{\circ }$C), the standard deviations of the dip wavelength are both less than 70 pm. Such small spectral variations confirm the sensing stability of our spring thermal sensor.
Fig. 4. (a) The temperature-sensing characteristics curve for the heating and cooling process. (b) Three-round temperature sensing tests by increasing the temperature from 30 $^{\circ }$C to 50 $^{\circ }$C.
At last, we focus on sensitivity tuning by a variety of springs $k$. Occupying six different $k$ sensors (see labels), Fig. 5(a) plots the dip wavelength shift as temperature increases. Great linear fitting with an R-square superior to 0.99 can be noticed for every $k$ value. The sensitivity dataset is further analyzed in Fig. 5(b). A simple linear fitting curve ($R^{2}$=0.962) with a slope of -5.8 (pm/$^{\circ }$C)/(µN/µm) can illustrate the correlation between temperature sensitivities and the $k$ values. This relationship indicates that our sensitivity can be finely adjusted in a wide range of 100-700 pm/$^{\circ }$C. Such a sensitivity-designable sensor would benefit real applications such as biomedical signal monitoring [2,20].
Fig. 5. (a) Sensitivity benchmarking of six sensors composed of diversed $k$ springs. (b) The relationship between temperature sensitivities and the $k$ values.
The performance comparisons between the proposed sensor and other reported schemes have been listed in Table 1. Note that our sensitivity is an order higher than that of traditional all-fiber sensors (about 10 pm/$^{\circ }$C). Compared to other material-infilled FPIs (e.g., silicone-oil, ethanol, and UV glue), our sensor also reach an identical sensitivity level. Although there exist more sensitive structures (e.g., PDMS-filled silica capillary tube (SCT) and microfiber FPI), our spring FPI exhibits attractive advantages including high precision fabrication, easy waveguide integration, and appreciable customizability. Therefore, we believe that our spring-composed FPI sensor has great potential in temperature sensing.
Table 1. Comparison of sensing performance for various temperature sensors.
3. Conclusion
In this work, a fiber-tipped PDMS-filled spring FPI has been proposed and experimentally demonstrated for temperature sensing. The sensor mainly consists of a micro-spring FP cavity, that is first manufactured through TPP techniques and subsequently filled by temperature-sensitive PDMS. The sensing performance of the fiber FPI has been fully explored in the range of 30-50$^{\circ }$C, yielding a maximal sensitivity of 704.3 pm/$^{\circ }$C. The following experiments confirm its outstanding repeatability and stability. Moreover, we showcase that the thermal sensitivity can be well adjusted in a scope of 100-700 pm/$^{\circ }$C by choosing a proper spring constant of $k$. All in all, we anticipate that the proposed fiber FPI would profit tremendous usages for customizable temperature sensing [2,20].
Funding
National Natural Science Foundation of China (62005224).
Acknowledgement
This work is supported by the National Natural Science Foundation of China. We thank Westlake Center for Micro/Nano Fabrication for the facility support and technical assistance.
Disclosures
The authors declare no conflict 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.
Supplemental document
See Supplement 1 for supporting content.
References
1. R. Pan, M. Liu, Y. Bian, T. Xu, W. Yang, Y. Yang, J. Wang, X. Mu, and L. Bi, “High-sensitive temperature sensor with parallel pdms-filled fpis based on dual vernier effect,” Opt. Commun. 518, 128284 (2022). [CrossRef]
2. P. Roriz, S. Silva, O. Frazao, and S. Novais, “Optical fiber temperature sensors and their biomedical applications,” Sensors 20(7), 2113 (2020). [CrossRef]
3. H. Gao, H. Hu, Y. Zhao, J. Li, M. Lei, and Y. Zhang, “Highly-sensitive optical fiber temperature sensors based on pdms/silica hybrid fiber structures,” Sens. Actuators, A 284, 22–27 (2018). [CrossRef]
4. X. Cui, H. Zhang, and D. Wang, “Parallel structured optical fiber in-line fabry–perot interferometers for high temperature sensing,” Opt. Lett. 45(3), 726–729 (2020). [CrossRef]
5. Y. Hu, H. Wei, Z. Ma, L. Zhang, F. Pang, and T. Wang, “Microbubble-based optical fiber fabry-perot sensor for simultaneous high-pressure and high-temperature sensing,” Opt. Express 30(19), 33639–33651 (2022). [CrossRef]
6. X. Fu, R. Ran, Q. Li, R. Zhang, D. Li, G. Fu, W. Jin, W. Bi, Y. Qi, and Q. Hu, “A sensitivity-enhanced temperature sensor with end-coated pdms in few mode fiber based on vernier effect,” Opt. Commun. 497, 127173 (2021). [CrossRef]
7. C. Zhu, Y. Zhuang, B. Zhang, R. Muhammad, 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]
8. L. Zhao, Y. Zhang, Y. Chen, and J. Wang, “Composite cavity fiber tip fabry-perot interferometer for high temperature sensing,” Opt. Fiber Technol. 50, 31–35 (2019). [CrossRef]
9. Z. Chen, S. Xiong, S. Gao, H. Zhang, L. Wan, X. Huang, B. Huang, Y. Feng, W. Liu, and Z. Li, “High-temperature sensor based on fabry-perot interferometer in microfiber tip,” Sensors 18(2), 202 (2018). [CrossRef]
10. X. Zhu, H. Chen, C. Jiang, X. Guo, H. Zhang, P. Wang, and S. Sun, “In-fiber fabry-perot temperature sensor using silicone-oil-filled the quartz capillary tube,” Opt. Fiber Technol. 71, 102937 (2022). [CrossRef]
11. K. Cao, Y. Liu, and S. Qu, “Compact fiber biocompatible temperature sensor based on a hermetically-sealed liquid-filling structure,” Opt. Express 25(24), 29597–29604 (2017). [CrossRef]
12. T. T. Salunkhe, D. J. Lee, H. K. Lee, H. W. Choi, S. J. Park, and I. T. Kim, “Enhancing temperature sensitivity of the fabry–perot interferometer sensor with optimization of the coating thickness of polystyrene,” Sensors 20(3), 794 (2020). [CrossRef]
13. B. Li, Y. Liu, X. Song, H. Fu, Z. Jia, and H. Gao, “High-sensitivity temperature sensor based on ultraviolet glue-filled silica capillary tube,” J. Modern Opt. 67(16), 1327–1333 (2020). [CrossRef]
14. Z. Liu, B. Zhao, Y. Zhang, Y. Zhang, C. Sha, J. Yang, and L. Yuan, “Optical fiber temperature sensor based on fabry-perot interferometer with photopolymer material,” Sens. Actuators, A 347, 113894 (2022). [CrossRef]
15. Y. Liu, L. Huang, J. Dong, B. Li, and X. Song, “High sensitivity fiber-optic temperature sensor based on pdms glue-filled capillary,” Opt. Fiber Technol. 67, 102699 (2021). [CrossRef]
16. Z.-K. Yin, R.-Q. Lv, J.-Y. Zhang, J.-B. Zhou, C. Liu, M.-X. Zhang, and J.-X. Yang, “Optical fiber temperature sensor based on pdms-filled air-cavity fabry-perot structure,” IEEE Trans. Instrum. Meas. 72, 1–6 (2023). [CrossRef]
17. Z. Li, Y. Zhang, C. Ren, Z. Sui, and J. Li, “A high sensitivity temperature sensing probe based on microfiber fabry-perot interference,” Sensors 19(8), 1819 (2019). [CrossRef]
18. M.-Q. Chen, Y. Zhao, F. Xia, Y. Peng, and R.-J. Tong, “High sensitivity temperature sensor based on fiber air-microbubble fabry-perot interferometer with pdms-filled hollow-core fiber,” Sens. Actuators, A 275, 60–66 (2018). [CrossRef]
19. J. Li, Z. Li, J. Yang, Y. Zhang, and C. Ren, “Microfiber fabry-perot interferometer used as a temperature sensor and an optical modulator,” Opt. Laser Technol. 129, 106296 (2020). [CrossRef]
20. X. Wang, Z. Liu, and T. Zhang, “Flexible sensing electronics for wearable/attachable health monitoring,” Small 13, 1602790 (2017). [CrossRef]
21. S. Cao, X. Shang, H. Yu, L. Shi, L. Zhang, N. Wang, and M. Qiu, “Two-photon direct laser writing of micro fabry-perot cavity on single-mode fiber for refractive index sensing,” Opt. Express 30(14), 25536–25543 (2022). [CrossRef]
22. X. Shang, N. Wang, Z. Wang, H. Jiang, Y. Jia, N. Zhou, and M. Qiu, “Customizable and highly sensitive 3d micro-springs produced by two-photon polymerizations with improved post-treatment processes,” Appl. Phys. Lett. 120(17), 171107 (2022). [CrossRef]
23. J. Qu, M. Kadic, A. Naber, and M. Wegener, “Micro-structured two-component 3d metamaterials with negative thermal-expansion coefficient from positive constituents,” Sci. Rep. 7(1), 40643 (2017). [CrossRef]
