Full length articleSensitivity-enhanced temperature sensor based on encapsulated S-taper fiber Modal interferometer
Introduction
High-sensitivity temperature sensing measurement plays an important role in many fields, such as environmental science, biomedical and manufacturing industry. Compared to conventional electrical sensors, fiber-optic temperature sensors have been extensively studied owning to their intrinsic advantages of light weight, capability for remote sensing, and immunity to electromagnetic interference. In recent years, various types of fiber-based sensors have been demonstrated that enable temperature measurement, for example, fiber Bragg gratings [1], [2], [3], [4], various configurations of modal interferometers [5], [6], [7], [8], [9], [10], [11], and new techniques such as surface plasmon resonance (SPR) [12], [13]. Among these configurations, inline Mach-Zehnder interferometers (MZIs) have attracted a lot of research attentions due to their compact structure and ease of fabrication. In most MZIs designs, however, since both the core and cladding modes involved in the interference mainly propagate in the same material silica, the relative difference of the thermo-optic coefficient is small, resulting in a low sensitivity of tens of pm/°C [9], [10], or hundreds of picometers per degree [11] when using some special fibers.
One way to enhance the sensitivity for an MZI sensor is to create large difference in thermo-optical coefficient between the fiber core and cladding. There are several ways to achieve this. One can use special fiber, for example, few-mode multicore fiber [14], or artificially create such conditions by selectively infiltrating liquids with high thermo-optic coefficient [15], [16]. Although the sensitivity can reach 40–55 nm/°C, they suffer from narrow measurement range. Moreover, these configurations usually require micromachining using high power femtosecond lasers.
Considering fabrication complexity and cost, infiltrating liquids into a capillary is an effective method to realize a simple and compact structure with high sensitivity and moderate measurement range. Yang et al. reported sealing an S-shape fiber taper with refractive index (RI) liquid inside a silica capillary, and obtained a sensitivity of 1.403 nm/°C [17]. Xue et al. showed an improved sensitivity of −3.88 nm/°C by filling isopropanol in an optical microfiber taper-based MZI [18]. More recently, Jiang et al. developed an optical microfiber coupler with RI liquid encapsulated in Teflon capillary, with a sensitivity of −5.3 nm/°C reported [19]. A technical disadvantage for these microfiber couplers is that the waist diameter is limited to only a few microns, which makes the device fragile and brings challenge for packaging. Moreover, the papers mentioned above did not explain their results theoretically in aspect of the interference modes.
In this paper, we proposed and demonstrated a compact in-line fiber-optic temperature sensor based on multimode interference. We carefully optimized several parameters for achieving high sensitivity: the length and waist of the single mode S-taper, the material of the capillary, and the filling liquid. The experimental results showed that the highest temperature sensitivity of −15.66 nm/°C with high linearity (R2 = 0.993) was obtained for as S-taper encapsulated with isopropanol in a Teflon capillary, with a temperature range of 28–32 °C. In a larger range of 25.1–35 °C, the sensitivity was −11.88 nm/°C. This is at least an enhancement of compared with other capillary infiltrated temperature sensors reported [17], [18], [19]. In addition, we also studied the underline mechanism for achieving such high sensitivity. Our numerical simulation showed there existed a critical cladding mode for a single mode fiber. When the cladding mode involved in the interference with the core mode is close to the critical mode, the sensor exhibits the highest sensitivity. In addition, given the interference cladding mode, the direction for the wavelength shift when temperature varies can be determined. The calculated results were consistent with our experimental observation. The merits of compactness, robustness, and ease to production make the proposed temperature sensor a strong candidate for high sensitivity near room temperature measurement in applications including industrial production and environmental monitoring.
Section snippets
Sensor principle and theory
The proposed temperature sensor consists of two parts: the main part was an S-shape tapered SMF, as illustrated in Fig. 1(a-b). The taper was then encapsulated with RI liquid inside a Teflon capillary. Unlike normal tapers, two curvatures were created in one S-taper to form the MZI. At the first bend, multiple cladding modes are excited. After travelling a certain distance along the waist, these cladding modes were coupled back into the fiber core and combine with the core mode at the second
Sensor fabrication
The S-taper was fabricated by a commercial fusion splicer (Fujikura 80S) using two SMFs. We started off by separating the two fibers with a ~62 μm offset in the vertical direction. During splicing, the fibers were stretched outwards to create the tapering shape. A number of tapering parameters have been tried to optimize the shape of the taper, such as tapering speed, tapering length, arc current, and arc duration. Our results show that the shape of the S-taper is most sensitive to two
Experimental results and discussion
The experimental setup for the temperature sensing test was illustrated in Fig. 3. The sealed sensor was placed inside a column oven with accuracy of 0.1 °C. We used a broadband source (BBS, YSL, 900–1700 nm) as the input light source, which passed through the proposed sensor and was received by an optical spectrum analyzer (OSA, YOKOGAWA AQ6370B).
Conclusions
In summary, we have demonstrated the simple design for an inline fiber interferometer as a highly sensitive temperature sensor based on encapsulated S-taper fiber filled with liquids. The multi-interference model explains the underlying physics behind the high sensitivity. We found that there existed a critical cladding mode, dependent on the external RI. When the excited cladding mode involved in the interference with the core mode was close to the critical mode, the sensor was likely to
CRediT authorship contribution statement
Jianwen Ma: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft. Shun Wu: Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Haihao Cheng: Software, Validation. Xuemei Yang: Software, Validation. Shun Wang: Validation, Writing - review & editing. Peixiang Lu: Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by grants from Natural Science Foundation of China (NSFC) (61805182, 11804258).
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