A slope-assisted Brillouin optical time domain reflctometry system based on grade index multi-mode fiber (GI-MMF) was presented. The coherent detection was applied in this system and the Brillouin gain spectrum (BGS) was obtained by scanning work frequency. The BGS was inhomogeneously broadened by adjusting the lateral offset between single mode fiber (SMF) and GI-MMF. The bandwidth of BGS with different lateral offset was analyzed and the BGS with bandwidth of 111MHz was achieved at lateral offset of 8 μm. For realizing slope-assisted technology, a bandpass electrical filter was added behind the balanced photo-detector to realize the function of frequency selection. The strain intensity responses with different work frequency were analyzed for maintaining significant linear relationship between strain and signal intensity. The system realized the maximum strain dynamic measurement of 3000 με with the spatial resolution of 5 m along ~1 km GI-MMF at vibrational frequency of 7.83 and 15.47 Hz. The measured error of vibrational frequency was less than 0.2 and 1.5 Hz, respectively. The obtained strain intensity responses were 0.00296 and 0.00292 mV/με, respectively. The measured strain range of this system was more than three times that of traditional systems based on SMF and could be achieved at relatively low cost. The proposed scheme has potential application prospects in large dynamic strain diagnosis.
A demodulation based on the principle of fiber Bragg grating edge filter method is used to measure impact energy of acrylic plate (PMMA) and aluminum plate in this paper. A new demodulation method based on damping attenuation principle is proposed. Fiber Bragg grating sensing system is built in the experiment. The complex vibration wave generated by impact energy is processed by using the envelope of peak value in MATLAB. The experimental results show that using the time length corresponding to the peak attenuation of 10% as the demodulation basis has a higher Goodness of fit. The linear fitness of PMMA plate and aluminium plate is 0.94 and 0.89, respectively. At the same time, it is concluded that this method can be used to preliminarily determine whether the plate is viscoelastic or not. This is a new potential demodulation method for practical impact energy detection based on fiber Bragg grating.
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