Fiber lasers of high-order spatial modes are advantageous in a number of applications. We experimentally demonstrate a transverse mode-selective few-mode Brillouin fiber laser using a pair of mode-selective photonic lanterns as spatial mode filters. The three LP lasing modes were generated based on both intra- and inter-modal stimulated Brillouin scattering. Their slope efficiencies, optical spectra, mode profiles, and linewidths were characterized for both cases. Based on the ring-cavity configuration, distributed temperature and strain sensing with 1-m spatial resolution is also demonstrated. The Brillouin dynamic grating was generated in the LP01 mode and high-order modes were used as probes. The feasibility of simultaneous temperature and strain sensing has also been investigated.
The paper introduces the DAS system based on advanced phase-sensitive optical time domain reflectometry (φ-OTDR) with fading noise suppressed. Besides, the DAS system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) and its improved systems are introduced. The application of DAS in railway perimeter security is introduced at the end of the paper.
The frequency division multiplexing (FDM) technique is firstly introduced into a direct-detection phase-sensitive OTDR to improve the distributed fiber acoustic sensing performance by using a frequency step sweeping laser source and a dual probe pulse scheme. By using FDM technique, a 40 kHz sampling rate to vibration is realized with a 10 km measurement range, which implies the tradeoff between the frequency response and the measurement range is broken. In experiment, a 6 kHz vibration is successfully measured.
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