Long distance Brillouin optical time domain reflectometer (BOTDR) sensing system based on bypass remote optical pumped amplifier, Erbium-doped fiber and backward Raman amplification, is achieved 253km sensing distance with 40m spatial resolution.
This paper proposes a Brillouin and Rayleigh fusion system for multi-parameter monitoring of power OPGW cables. Temperature, strain, and vibration measurements are important indicators for safe cable operation of OPGW cables. Temperature monitoring helps detect ice-coating, mountain fire, and lightning. Strain measures cable stress, fiber core safety, ice-coating, and fatigue damage caused by wind-induced vibrations. Vibration monitoring is useful for ice covering and wind vibration. The proposed system overcomes limitations such as nonlinear effects and limited sensing distance. By using a fixed delay of 500ns, mutual position calibration of temperature/strain and vibration measurements is achieved. The system uses a two-wavelength distributed optical fiber fusion scheme with dual light sources, ensuring no interference or degradation due to index conflicts.
In this paper, we propose a high fidelity phase-sensitive optical time-domain reflectometer (Φ-OTDR) system based on compact and flexible multi-frequency probe pulse modulation. Single-frequency continuous light is multi-frequencymodulated by a broadband acousto-optic modulator (AOM) loaded with multiple microwave signals of different frequencies, finally, multiple Rayleigh backscattered (RBS) optical signals with different intensity distributions can be obtained by a single acquisition, and the most accurate signal is always selected for phase reconstruction to achievethehigh fidelity Φ-OTDR system, which has the advantages of compact structure, precise control of phase delay, flexible and controllable frequency components, and no sacrificing response bandwidth and spatial resolution, etc. In the experiment Φ-OTDR system, we simultaneously modulate the multi-frequency probe pulse light with a width of 100nsand three non-equidistant frequencies, and inject it into a 2km sensing fiber. The RBS light signal multiplexing results show that the probability of interference fading effect in the system is reduced from 17.541% to 1.123%. And the high-fidelity phase information of a 100Hz simulated vibration signal was extracted on a 3km sensing fiber, corresponding to a strain value of about 11.9nε.
In this work, a high-sensitivity distributed lateral pressure sensor based on Brillouin Optical Time-Domain Reflectometry (BOTDR) technique is proposed to enhance the pressure sensitivity using a double-coated Single-Mode Fiber (SMF). Firstly, a pressure-induced strain model is developed to analyze the enhancement mechanism of pressure sensitivity by Brillouin Frequency Shift (BFS). With increasing of outer coating radius, or with decreasing of the Young’s modulus and Poisson’s ratio of outer coating, the lateral pressure sensitivity increases. By using a double-coated SMF whose outer coating radius is 1500 μm, a high sensitivity of -3.51 MHz/MPa in the range of 0-30 MPa is achieved, which is about five times higher than that of single-coated SMF. At a spatial resolution of 1.5 m, the maximum measurement error of the proposed pressure sensing system is less than 0.09 MPa.
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.
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