A high-sensitivity and fast distributed optical fiber pressure sensor based on optical frequency-agile phase-sensitive optical time-domain reflectometry (φ-OTDR) technique is proposed, and the standard single-mode fiber (SMF) is used as sensing fiber to demonstrate its pressure sensing performance. Two electro-optic modulators (EOMs) are connected in series in the same optical path and modulated by an arbitrary waveform generator (AWG) to generate a set of frequency agile pulses. The duration of single frequency pulse is 250 ns, and the interval between the two sets of frequency-agile pulses sequence is 5 μs. The injection locking technique is used to filter and amplify the frequency-agile pulses with a higher extinction ratio, and make the peak power of the frequency-agile pulses more stable. The measurement results show that in the pressure range of 0 to 1.8 MPa, and the pressure sensitivity of Rayleigh scattering spectrum frequency shift difference of the standard SMF is 702.5 MHz/MPa, which is about 1000 times that of the standard SMF based on BOTDA method. With an average of 64 times, the single pressure measurement time is only 1.92 ms.
A high resolution distributed dynamic strain sensing has been proposed and experimentally demonstrated based on the combination of Brillouin and Rayleigh scattering. The proposed scheme employs the same set of frequency-scanning optical pulses modulated through the frequency-agile technique for fast measurements. The Brillouin optical time domain analyzer (BOTDA) technology is used to provide absolute measurement benchmarks, while the phase-sensitive optical time domain reflectometer (φ-OTDR) technology is used to capture relative strain changes in details. Two groups of 100 Hz vibrations with different amplitude (300 nε and 250 nε) have been measured under two different absolute strains (1173.9 με and 525.3 Με), which allows for dynamic absolute strain measurement with a high resolution of 8.4 nε.
A dynamic distributed Brillouin optical fiber pressure sensor based on frequency agility technology is proposed, and the performance of dynamic and static pressure sensing is experimentally demonstrated. A set of frequency-agile pump pulse sequences with single pulse duration of 250 ns are generated by an arbitrary waveform generator. The interval of the frequency sweep pulse sequence is 30 μs, the maximum repetition frequency is 18.2 kHz, and the average is 64 times. Double-coated single-mode fiber (SMF) is used as the sensing fiber to enhance the pressure sensitivity of Brillouin frequency shift (BFS), which the outer coating diameter is 3000 μm. The BFS pressure sensitivity of -3.32 MHz/MPa is achieved in the pressure range of 0-24 MPa, which is about 4.5 times that of SMF. The measurement time of the proposed optical fiber pressure sensing system is only 3.52 ms. Furthermore, the dynamic pressure measurement experiment is carried out, and the continuous measurement of the dynamic range of 6-0 MPa is achieved, and the dynamic distributed pressure measurement ability of the sensing system is verified.
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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