Due to its high sensitivity and fast measurement, the phase-sensitive optical time-domain reflectometer (φ-OTDR) is a good candidate for distributed dynamic strain sensing and has been widely used in intrusion monitoring, geophysical exploration, and other fields. In the case of frequency scan-based φ-OTDR, the phase change is expressed as a shift in the intensity distribution. The correlation between the reference and measured spectra is used for relative strain demodulation, which imposes a limitation on the absolute strain demodulation for continuous measurements. The Brillouin optical time domain analysis (BOTDA) fortunately enables the demodulation of the absolute strain in just one measurement. A combination of φ-OTDR and BOTDA is proposed and demonstrated in this work by using the same set of frequency-scanned optical pulses, and also introducing a frequency-agile technique for fast measurements. Measurements of 9.9 Hz vibrations were made at two different absolute strains (296.7 με and 554.8 με) with a strain range of 500 nε, made possible by integrating Rayleigh and Brillouin information. The submicron strain vibrations were demonstrated by φ-OTDR signals with a high sensitivity of 6.8 nε, while the absolute strains were measured by BOTDA signals with an accuracy of 5.4 με. Thus, the proposed sensor allows dynamic absolute strain measurements with high sensitivity, thus opening the door to new possibilities yet to be explored.
Brillouin-based optical fiber sensors have been developed over the past few years and played a significant role for distributed temperature and strain measurements, which include many properties such as high measurement accuracy, large measurement range and environmental suitability. Among these sensors, Brillouin optical fiber sensors via optical chirp chain (OCC) become an ideal choice for ultrafast distributed measurement to distinguish quick-changing events in practical applications. This paper begins with the introduction of the concept and the generation schemes of OCC, the spectra distortion characteristic of OCC Brillouin signal are analyzed. The efforts towards such OCC based Brillouin optical fiber sensing for ultrafast measurement are reviewed here as well, with the OCC based Brillouin optical time domain analysis and the OCC based Brillouin optical time domain reflectometry, which give distinguished performance for dynamic measurement, long distance measurement and one end access measurement. Meanwhile, the consequent future challenges of OCC based distributed sensing are discussed and presented, such as as high spatial resolution and high accuracy measurement.
For several years, Brillouin-based optical fiber sensors plays an important roles in the fields of distributed temperature and strain measurements in the real world. Among these sensors, the optical chirp chain (OCC) based Brillouin optical fiber sensor is a good candidate to realize ultrafast distributed sensing, which is of great importance to distinguish quickchanging events in practical applications. In this paper, the principle of OCC and the OCC based Brillouin optical timedomain analysis (BOTDA) sensing are introduced. In OCC-BOTDA, there are three types of spectral distortions, i.e. the back end distortion, the frequency lag of main peak and the frequency saltation distortion, which are influenced by the transient stimulated Brillouin scattering, are verified in the simulation and experiment in this paper and attributed to the rapid frequency sweeping.
In view of the limitations of the traditional Brillouin optical time domain analysis (BOTDA) system such as low sampling rate, large transmission and storage space, a fast BOTDA scheme based on compressed sensing technology has been proposed to realize the random frequency sampling of Brillouin gain spectrum (BGS). The proposed scheme uses a data-adaptive sparse base obtained by the principle component analysis algorithm to realize the sparse representation of Brillouin spectrum. Then, it can be reconstructed successfully with orthogonal matching-pursuit algorithm. Compared with the traditional uniform spectrum sampling with a step size of 4 MHz, the proposed compressed sampling scheme can recover the BGS using 30% of the frequency. With fewer sampling frequencies, compressed sensing technology can improve the sensing performance of traditional fast BOTDA, including increasing the sampling rate by 3.3 times and reducing the amount of data storage by 70%.
A distributed optical fiber pressure sensor based on Brillouin scattering technique is proposed and experimentally demonstrated, where double-layer polymer coatings are used on the single-mode fibers (SMF) to improve the Brillouin frequency shift (BFS) pressure sensitivity. The single-coated and double-coated fibers are designed to demonstrate their BFS pressure sensitivity, where the outer coating radius of double-coated fibers are 450 μm, 1000 μm and 1500 μm, respectively. Experimental results show that the BFS pressure sensitivity are -0.74 MHz/MPa, -1.61 MHz/MPa, -2.59 MHz/MPa and -3.51 MHz/MPa in the pressure range of 0-30 MPa for above four kinds of fibers. According to the experimental results, it can be concluded that the BFS pressure sensitivity can be improved with increasing of outer coating radius or decreasing of outer coating Young's modulus and Poisson's ratio. The maximum BFS pressure sensitivity is measured about 5 times higher than single-coated SMF, it is of great significance for SMF to be used in practical distributed pressure measurement.
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