Summary
Researchers led by Li Jian and Professor Zhang Mingjiang at Taiyuan University of Technology in China proposed a new Raman distributed fiber-optic temperature sensing technique. The method improves the spatial resolution of Raman distributed temperature sensors and can achieve centimeter-level spatial resolution over kilometer-scale sensing distances. These results represent the best spatial resolution reported so far for long-range Raman distributed fiber sensing. The work was published as "Physics and Applications of Raman Distributed Fiber Sensing" and "Chaotic Raman Distributed Fiber Sensing" in Light: Science & Applications.
Background and Applications
Distributed fiber-optic temperature sensing is an effective monitoring tool with important applications in military equipment, deep-sea and deep-earth exploration, polar scientific expeditions, large hydraulic projects, smart grid monitoring, and other fields. High spatial resolution distributed fiber sensing can support deep-earth studies, deep-sea resource exploration, and in-situ ocean observation.
Such techniques are also valuable for cable condition monitoring in smart grids and for failure and disaster analysis. High spatial resolution distributed fiber instruments can play a significant role in polar research, for example by providing efficient in-situ monitoring of Antarctic ice shelves and ocean temperature.
Challenges
Previous experiments by the group showed that when the length of the fiber under test is shorter than the system spatial resolution, measured temperatures can deviate significantly from the actual environmental temperature. In addition, modal dispersion in multimode fiber can degrade the achievable spatial resolution at the fiber end to several meters, which severely limits the practical application of Raman distributed fiber sensing.
Existing approaches to address these issues include pulse-coded modulation on single-mode fiber, narrow pulse-width demodulation, and the use of specialty fibers. However, for long-range Raman distributed sensing over distances greater than 10 km, limitations imposed by source pulse width and time-of-flight distributed sensing principles typically restrict the best spatial resolution to around 1 m.
When the length of the fiber region affected by a localized event is smaller than the system spatial resolution, the temperature change associated with that region is masked by environmental temperature noise within the resolution length, making it difficult to detect localized temperature features. This impedes early detection and mitigation of related incidents. Therefore, further improving the spatial resolution of Raman distributed fiber sensing over long distances is a critical challenge.
Approach: Chaotic Laser Sensing Signal
Chaotic lasers, a special output regime of lasers, exhibit wide spectral bandwidth, noise-like characteristics, and large intensity fluctuations. Using a chaotic laser as the sensing signal can offer advantages for improving the spatial resolution of long-range distributed fiber sensing. Inspired by this, the team combined chaotic sensing signals with Raman distributed fiber sensing to develop the new Raman distributed fiber temperature sensing technique based on wideband chaotic laser sources. This approach aims to overcome the limitations faced by the field.
Image source: Light: Science & Applications
Further Work
The team has also conducted full-chain research on new distributed fiber sensing theory and methods, key technologies and devices, and engineering instruments and applications. Through this line of research, they have achieved long-range, high spatial resolution distributed strain and temperature sensing, addressing safety monitoring and disaster early warning challenges for major infrastructure such as transportation tunnels and energy transmission networks.
