To explore the resonance mechanism of nanocrystalline FeCoNi flake composites, characterization of the flakes was investigated. The structural and magnetic properties of flakes manifestly tailored the features of microwave permeability spectrum of flake composites, relating to the physical effects of nanograins. The exchange mode with a few modifications was used to explain the origin of composite microwave performance, and the computed results were close to the experiment. It is believed that the exchange energy, dominating in the microwave resonance of spherical nanoparticles, plays an equally important role in nanocrystalline flakes.
We propose a phase-sensitive optical time-domain reflectometry (Φ-OTDR) scheme with counterpumping fiber Brillouin amplification (FBA). High-sensitivity perturbation detection over 100 km is experimentally demonstrated as an example. FBA significantly enhances the probe pulse signal, especially at the second half of the sensing fiber, with only 6.4 dBm pump power. It is confirmed that its amplification efficiency is much higher than 28.0 dBm counterpumping fiber Raman amplification. The FBA Φ-OTDR scheme demonstrated in this work can also be incorporated into other distributed fiber-optic sensing systems for extension of sensing distance or enhancement of sensing signal level.
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