High performance distributed fiber sensing and application

In recent years, China is at the peak of civil engineering and infrastructure construction, and many of the world's significant projects and infrastructure construction have been completed or under constructed, such as the Hong Kong-Zhuhai-Macao bridge, the submarine optical cable project in Hainan, the Beijing-Shanghai high-speed railway project and the Qinling tunnel. While the safety and durability of major projects and infrastructure have always been the focus of social concern. Besides, China is one of the countries with frequent geological disasters in the world, and geological disasters features rich diversity, wide area, high activity frequency and great harm. Therefore, it is particularly important to apply effective structural health monitoring and diagnosis technology to major projects and infrastructure. Optical fiber sensing technology owns many advantages, such as high sensitivity, anti-electromagnetic interference, anti-corrosion and convenient layout. In particular, distributed optical fiber sensors are capable of continuous measurement over a long distance with more than one million measurement points. Brillouin optical time domain analysis (BOTDA) is a typical scheme of distributed optical fiber sensor, which can realize centimeter spatial resolution, more than 100 km sensing distance and up to kHz system sampling rate. High performance BOTDA is a promising tool for fast distributed temperature and strain measurement in structural health monitoring and other fields.

Recent research results of Dong Yongkang professor’s team, Single-shot BOTDA based on optical chirp chain probe wave for distributed ultra-fast measurement.
    The team is engaged in the basic research of distributed optical fiber sensing and has been working on BOTDA system for many years. Recently, a novel distributed ultra-fast fiber sensing technology is proposed by the team based on optical chirp chain (OCC) technology. The operation principle of the OCC-BOTDA is shown in Figure 1a. The pump is pulse modulation, while the probe is OCC modulated composed of several short optical chirp segments. Two adjacent optical short-chirp segments are cascaded by the head-to-tail cohesion. In this way, the optical pump pulse interacts with the OCC probe wave in sequence in the fiber under test (FUT). When the Brillouin gain region of the pump pulse is covered by the frequency span of the OCC probe wave [Fig. 1(b)], the BGS will be recovered along the short optical chirp segment in time domain thanks to the stimulated Brillouin scattering effect. In theory, the measurement range can be changed by adjusting the OCC frequency span, and the measurement time is only limited by the fiber length with no average.

Fig. 1  The operation principle of the OCC-BTODA

    Fig. 2 introduces a process of fast mechanical shock recorded by the proposed OCC-BOTDA system. The time evolution of the Brillouin gain spectra is shown in Figure 2a, while the demodulated strain change induced by this mechanical shock is shown in Figure 2b. The green line is original result with no averaging and the red line shows the result with 30-points moving average. As a result, a shock time of ~250 μs with a strain change ~800 με is demonstrated with the 2.5 MHz system sample rate. The sample rate has already reached the theoretical limit in the short range fiber sensing, which is capable of data collection and monitoring of explosion, chemical reaction and other rapid events. The research results are published in Nature series journal, Light: Science and Applications.

Fig.2  The time-frequency response of the mechanical shock measurement. (a) The time evolution of the Brillouin gain spectra;(b) The demodulated strain change induced by this mechanical shock

About Team
The team of Professor Dong Yongkang, affiliated to the state key laboratory of tunable laser technology of Harbin Institute of Technology, is engaged in the basic research, technology and engineering application of distributed optical fiber sensing, a series of innovative research results have been obtained and applied to many national major projects. The team published about 60 SCI papers in Light: Science and Applications, Opitcs Letters, Optics Express and other international authoritative journals, while the citations are more than 600 times. The team owns one American invention patent, more than 10 national invention patents and wins the first prize of Heilongjiang province natural science, the first prize of Shaanxi province scientific and technological progress, and the first "China optical engineering society scientific and technological innovation award". Professor Dong Yongkang, as the project leader, presided over national science instruments and equipment projects "distributed optical fiber strain monitor" (set up in 2017 with a total funding of 41 million) and more than 10 National natural science foundation and other projects. A series of distributed Brillouin fiber sensing analyzers have been developed, among which the comprehensive index of "high performance distributed Brillouin fiber temperature and strain analyzer" is the international leading level.

Wang Benzhang, Pang Chao, Zhou Dengwang, et al. Advances of key technologies in long-range distributed Brillouin optical fiber sensing[J]. Opto-Electronic Engineering, 2018, 45(9): 170484.