pulsecoding technology can effectively improve the performance of the phase-sensitive optical time domain reflectometry (Phi-OTDR) system, but the cross-correlation decoding for mass data along a long sensing distanc...
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pulsecoding technology can effectively improve the performance of the phase-sensitive optical time domain reflectometry (Phi-OTDR) system, but the cross-correlation decoding for mass data along a long sensing distance brings a lengthy extra time cost. In this paper, a pre-frequency-shift method is proposed for the lightweight demodulation in the randomnumbercoding Phi-OTDR system. By using the first-order sideband generated by the phase modulation, this method can effectively reduce the frequency of the beating signal, thus reducing the requirement of the data sampling rate. The principle of the cross-correlation decoding algorithm based on sequence interpolation is studied, and the extra computation caused by the cross-correlation operation is analyzed theoretically. In the experiment, the validity of the pre-frequency-shift scheme is verified, and the sampling rate of the system is reduced from 500 MSps to 100 MSps. Comparative experiments using conventional down-sampling techniques with sampling rates of 250 MSps, 125 MSps, and 60 MSps validate the superiority of the proposed method. Finally, the vibrations of different frequencies are successfully located at 62.24 km with a spatial resolution of 5 m. At this distance, the algorithmic time cost of the proposed method is only 13.28 % of that of the traditional scheme.
In this paper, a synchronous cascade scheme using randomnumber modulation is proposed for signal-to-noise ratio (SNR) enhancement in phase-sensitive optical time-domain reflectometry (tp-OTDR) systems. The SNR gain i...
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In this paper, a synchronous cascade scheme using randomnumber modulation is proposed for signal-to-noise ratio (SNR) enhancement in phase-sensitive optical time-domain reflectometry (tp-OTDR) systems. The SNR gain is obtained by the randomnumber codes, the effects of the pulse extinction ratio and the rising edge on coding gain are investigated, and the coding quality is improved by the synchronous cascade modulation (SCM) structure. The theoretical simulation and experimental results show that the scheme can improve the pulse extinction ratio and shorten the pulse rising edge, so as to optimize the coding gain and enhance the SNR. Compared with the traditional coding modulation scheme, the extinction ratio is improved by 21.9 dB, the rising edge is shortened from 10.12 ns to 3.31 ns, and the coding gain is improved by 1.1 dB. In the equivalent experiments, the results along a 42.240 km fiber under test (FUT) show that the phase signal demodulated is more stable under a static environment, and the SNR of the demodulated vibration signal is increased by 7.82 dB. Benefitting from the SNR enhancement, the sensing distance can be extended to 62.440 km with a 4.4 m spatial resolution, and the SNR of the demodulated signal under 100 Hz vibration is up to 28.4 dB.
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