High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scatter-ing
Due to the fluctuations of optical properties, the scattering of light occurs in the optical fiber. The scattering waves can be classified into three major categories, known as the Rayleigh scattering, Brillouin scattering and Raman scattering. The Rayleigh scattering is a typical quasi-elastic scattering resulting from the non-propagating density fluctuations, while the Brillouin scattering is the light scattering from the density waves (i.e. acoustic phonons), adding a Doppler shift to its backscattering wave. In recent twenty years, great advances in devices and schemes have promoted the improvements on sensor performances, especially in the aspects of range, spatial resolution, accuracy and dynamic measurements. A variety of valuable information such as the location, temperature, strain, and vibration frequency become available. For the strain measurements, the phase-sensitive optical time-domain reflectometry (φ-OTDR) and Brillouin optical time domain analysis (BOTDA) are typical and optimal representatives, due to the high performance and simple implementation.
The φ-OTDR is a good candidate for distributed dynamic strain sensing, due to its high sensitivity and fast measurement, which has already been widely used in intrusion monitoring and geophysical exploration etc. For the frequency scanning based φ-OTDR, the phase change manifests itself as a shift of the intensity distribution, and the correlation between the reference and measured spectra is employed for relative strain demodulation. However, the frequency-scanning φ-OTDR is more suitable for high-sensitivity relative strain measurements rather than absolute strain demodulation. Because the absolute strain is obtained by accumulating the relative strain change between two continuous measurements, it is required that the monitoring process must be made continuously and start with no strain applied. Moreover, the measurement errors are introduced and then accumulated during the data processing, which degrades the measurement accuracy. Fortunately, the BOTDA allows for the absolute strain demodulation with only one measurement, however it is not able to realize nano-strain measurements due to its low strain resolution. Based on the above analysis, both the φ-OTDR and BOTDA sensors are able to realize the dynamic strain measurements, but the intended applications are different for these two sensors. They are complementary schemes to extend the functions and explore new applications. The article is entitled “High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering” and published in Opto-Electronic Advances Issue 12 2020.
The research group of Prof. Yongkang Dong from Harbin Institute of Technology has proposed a novel distributed dynamic fiber sensor, which is able to simultaneously measure the φ-OTDR and BOTDA signals by using the same set of frequency-scanning optical pulses. In order to obtain the frequency-scanning optical pulses, the frequency of the optical wave is scanned from v1 to vN enabled by the frequency-agile technique, and then the optical wave is pulse modulated in each frequency duration. Next, these pulses enter into the fiber under test in sequence, and only one pulse travels in the fiber under test (FUT) to avoid the signal overlapping. The probe wave for BOTDA sensing is injected from the other end of the sensing fiber, which travels in the same direction as the Rayleigh scattering signal. Therefore, the Rayleigh scattering of each pulse can be collected to map Rayleigh spectrum, and meanwhile the pulses can interact with the probe wave to reconstruct the Brillouin gain spectrum (BGS). Besides, a fiber Bragg grating (FBG) is used to separate the Rayleigh and Brillouin signals according to the frequency difference between these two signals.
In the experiment, a vibration was applied to the FUT by changing the motorized positioning system between two positions periodically, and the relative displacement was 1 μm, corresponding to 500 nε. Moreover, a fixed large strain was applied in advance, and two groups of experiments were made with the same vibration under different absolute strains. As shown in Fig. 1(a), the Brillouin demodulation results of these two groups were provided, while the fiber without strain was used as a reference. It could be seen that the Brillouin signals showed the absolute strain change between two groups of measurements, but the sub-micro strain vibration was failed to be measured due to the relatively low strain resolution. As shown in Fig. 1(b) and Fig. 1(c), a 9.9 Hz vibration with a peak-to-peak value of 500 nε was obtained, but these two dynamic strain results were similar only containing the information of relative strain variations. By combining the Rayleigh and Brillouin measurement results, the dynamic strain changed in the range from 296.45 to 296.95 με periodically. Similarly, the dynamic strain of Group2 varied between 554.55 με and 555.05 με. It could be seen that the proposed sensor was able to provide a high-sensitivity dynamic absolute strain results, by combining the Rayleigh and Brillouin information together. Besides, the Rayleigh and Brillouin signals were obtained simultaneously using the same set of frequency-scanning pulses, which did not increase the complexity of the proposed sensor. In conclusion, the proposed sensor allows for dynamic absolute strain measurements with a high sensitivity, thus opening a door for new possibilities which are yet to be explored.
Fig. 1 Two groups of vibration measurements by integrating the Rayleigh and Brillouin strain information: (a) the dynamic strain measurement results of two groups using the BOTDA, (b) and (c) the dynamic strain measurement results of the group 1 and group 2 employing the φ-OTDR, respectively.
About The Group
The research group of Prof. Yongkang Dong is affiliated with the National Key Laboratory of Tunable Laser Technology of HIT. The research interests of the group include optical fiber sensing, microwave photonics, nonlinear optics, gas sensing, LIDAR and structural health monitoring. The group has more than 40 faculties, postdoctoral and graduate students, and has undertaken more than 10 projects, such as National major scientific instrument and equipment development project "Distributed Optical Fiber Strain Analyzer", National Natural Science Foundation of China, and other special projects. In recent years, the group has published more than 90, some of which are published in the Light: Science and Applications, Optica, Photonics Research, Opitcs Letters, called Express and other impactful international journals, and these articles have been cited more than 1000 times. The group owns one international invention patents, more than 20 national invention patents, four book chapter and makes more than 30 special reports on the international conference. Moreover, the group wins the first prize of Heilongjiang province natural science, Shanxi province science and technology progress award and the first "optical engineering society of China Science and technology innovation award". In 2015, the group found the RealPhotonics, a state-level high-tech enterprise, which focuses on the product development and engineering application of distributed fiber sensing technology, among which, the comprehensive index of "high-performance distributed Brillouin fiber temperature and strain analyzer" is taking an international lead. The distributed optical fiber sensors can realize the space continuous measurement over a long distance, and the monitoring measuring points can reach up to a million, which has incomparable advantages in the aspect of large-capacity sensing compared with traditional point sensors. The measured parameters include strain and temperature, which can be used for health monitoring of large infrastructure such as oil and gas pipelines, high voltage power lines, high-speed railways and large Bridges, as well as monitoring and warning of geological hazards such as landslides and road subsidence.
Wang B Z, Ba D X, Chu Q, Qiu L Q, Zhou D W et al. High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering. Opto-Electron Adv 3, 200013 (2020).