Zhe Zhang, Min Zhou, Chao Wang, Yingying Wang · 7 authors
A new approach to conquer the thermal phase drift of an optical fiber Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. By employing a hollow-core anti-resonant fiber (HC-ARF) and optimizing the fusion splicing (includes mode field adaptation) between the lead-in single-mode fiber (SMF) and the HC-ARF, a high spectral resolution ( λ /Δ λ ≈ 3.8 × 10 4 ) optical fiber air-cavity FPI sensor with a fringe visibility higher than 7 dB is constructed. To eliminate the thermal phase drift (i.e. temperature crosstalk) of the sensor that originates from the intrinsic thermal expansion effect of the silica material of the HC-ARF, the FPI air cavity is connected to the external environments, by which the effect of air expelling from the cavity with temperature increasing can well compensate the temperature-induced cavity elongation. As a result, the thermal phase drift of the FPI is reduced to zero at a temperature range of ∼ 80–110 °C and within the temperature range of 40–80 °C, the thermal phase drift is still halved compared with the sealed FPI cavity. The nearly zero thermal phase drift of a FPI at such a temperature range has never been achieved before, to our best knowledge. As a proof of concept, a temperature-immune fiber-optic strain sensor is demonstrated. This work offers a new and efficient approach to eliminate the thermal phase drift (i.e. temperature crosstalk) of a fiber-optic device, which may significantly improve the measurement accuracy and detection limit of fiber-optic FPI sensors. Furthermore, the principle and schema can be generalized to a wide variety of fiber-optic devices.
With the advent of Internet of Things (IoT) a slow shift is happening from manual checks of capital intensive assets such as bridges, wind turbines, etc., where bolted joint is used as a main fastening method, towards unattended Structural Health Monitoring (SHM). Numerous approaches are proposed for monitoring the looseness of bolted joint assemblies, however, the majority of them can only be used for looseness detection purposes and not viable for monitoring since these approaches contain design impracticalities, not scalable or hard to implement outside of the laboratory environment. Furthermore, remote unattended monitoring is still very much in its infancy. Thus, to solve this issue, we propose a TenSense M20, a custom designed smart sensor node for continuous remote SHM of bolted joints. Complete node design is presented. Each aspect of the design is evaluated both by simulation and practical tests. Long Range (LoRa) is used as a means of wireless communication and the network can cover 3.8 km. The results show that TenSense M20 is able to precisely track the pre-tension force of a bolted joint with the approach being robust and scalable. Several transmission scenarios are analyzed and in the worst case scenario the node is estimated to last more than 5 years powered by several LiSOCl2primary batteries. Received data is securely stored in a blockchain and is easily accessible for services targeting integration with a Smart City.
An ultra-long distance distributed intrusion detecting system assisted with power amplification and sensitivity enhancement is proposed and demonstrated. First, through introducing multiple bidirectional amplifiers into the unbalanced Mach-Zehnder/Sagnac interferometer-based fiber sensing link, the sensing distance is remarkably extended, and second, the signal-to-noise ratio of this sensing system is significantly improved from less than 2 to 6-8 dB by coating the sensing fiber with organic silicone polymer. Furthermore, the high-order downtrend fitting function is adopted to implement the intrusion locating of ultralong distance sensing; the zero-padding fast Fourier transform algorithm and multiple-averaging method are jointly utilized for the improvement of the locating accuracy. Experimentally, a proof-of-concept distributed intrusion detecting system is constructed with the employment of bidirectional amplification. In particular, the ultra-long sensing distance up to 226.337 km is implemented, which is the reported longest distributed sensing system to the best of our knowledge.