In the past two decades Brillouin-based sensors have emerged as a newly-developed optical fiber sensing technology for distributed temperature and strain measurements. Among these, the Brillouin optical time domain reflectometer (BOTDR) has attracted more and more research attention, because of its exclusive advantages, including single-end access, simple system architecture, easy implementation and widespread field applications. It is realized mainly by injecting optical pulses into the fiber and detecting the Brillouin frequency shift (BFS), which is linearly related to the change of ambient temperature and axial strain of the sensing fiber. In this paper, the authors provide a review of new progress on performance improvement and applications of BOTDR in the last decade. Firstly, the recent advances in improving the performance of BOTDRs are summarized, such as spatial resolution, signal-to-noise ratio and measurement accuracy, measurement speed, cross sensitivity and other properties. Moreover, novel-type optical fibers bring new characteristics to optic fiber sensors, hence we introduce the different Brillouin sensing features of special fibers, mainly covering the plastic optical fiber, photonic crystal fiber, few-mode fiber and other special fibers. Additionally, we present a brief overview of BOTDR application scenarios in many industrial fields and intelligent perception, including structural health monitoring of large-range infrastructure, geological disaster prewarning and other applications. To conclude, we discuss several challenges and prospects in the future development of BOTDRs.
In Raman distributed temperature system, the key factor for performance improvement is noise suppression, which seriously affects the sensing distance and temperature accuracy. Therefore, we propose and experimentally demonstrate dynamic noise difference algorithm and wavelet transform modulus maximum (WTMM) to de-noising Raman anti-Stokes signal. Experimental results show that the sensing distance can increase from 3 km to 11.5 km and the temperature accuracy increases to 1.58 ℃ at the sensing distance of 10.4 km.
Although metal‐organic frameworks have proven to be excellent electrocatalytic materials, their application as electrode materials remains limited. The preparation of heterostructures is considered an effective method to improve catalytic activity. Herein, we describe the design and assembly of a dual‐MOF heterostructure (CoNi−ZIF‐67@Fe−MIL‐100, denoted ZIF@MIL). Specifically, we grew a layer of MIL‐100 in situ on a bimetallic ZIF‐67 surface using a solvothermal method. We demonstrate that the ZIF@MIL has remarkable OER electrocatalytic performance, requiring a low overpotential and showing a small Tafel slope, compared to pure ZIF‐67 and MIL‐100 in 1.0 m KOH. More importantly, it has excellent operational durability for 50 h at 100 mA cm−2. The high catalytic activity of ZIF@MIL can be attributed to the heterostructure that can expose more active sites, the synergistic effect between ZIF‐67 and MIL‐100, and improvement of electron transfer ability. Our work provides a new way to design and prepare dual‐MOF crystals with different structures as electrocatalysts.
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