Mr. Zhenhua Xu Profile

Zhenhua Xu

at Guilin Univ. of Electronic Technology

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Proceedings Article | 10 April 2023 Paper

A simple method of measuring vibration frequency based on φ-OTDR

Zesen Zhang, Jieqia Chen, Zhenhua Xu, Zujun Qin

Proceedings Volume 12614, 126140K (2023) https://doi.org/10.1117/12.2672638

KEYWORDS: Signal detection, Ferroelectric materials, Signal processing, Pulse signals, Digital signal processing, Bragg cells, Repetition frequency, Reflectometry

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Distributed vibration sensors by amplitude-demodulated phase sensitive optical time-domain reflectometers (φ-OTDR) have been widely used in many applications like safety monitoring of large-scale infrastructures and fence security. In order to locate vibrations, every two Rayleigh backscattering traces (RBTs) spaced apart by a certain interval (i.e., differential step k) are traditionally subtracted over a certain measurement time, and then all the subtractions are summed together and used to detect external vibration events. This method is denoted as amplitude-based accumulative differential method (ADM). Yet, the ADM has been demonstrated that the superimposing differential signal shows highly dependent on k which is closely related to vibration frequency (i.e., fs). Inappropriate k values would fail to locate the vibration. It is thus possible to envisage measuring f_s by taking advantage of this strong dependence of the ADM on f_s. In this manuscript, a simple method of estimating f_s is proposed by measuring the relationship between the ADM signal and k. The validity of this method is demonstrated by our experiment, in which a vibration event induced by a sinusoidal-driven PZT is detected.

Proceedings Article | 1 November 2021 Paper

Mode-conversion in few-mode fibers based on PC-FWM

Longtao Yang, Jiamin Li, Cong Chen, Zhenhua Xu, Zujun Qin, Jianyuan Li

Proceedings Volume 12057, 120572C (2021) https://doi.org/10.1117/12.2605857

KEYWORDS: Optical fibers, Phase matching, Signal processing, Refractive index, Phase conjugation

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Four-wave mixing (FWM) in few-mode fibers (FMFs) has been extensively investigated to develop mode-related alloptical signal processing, such as wavelength conversion, parametric amplification and mode conversion. Compared to the FWM processes in single-mode fibers, intermodal FWM in FMFs shows more flexible phase-matching condition by tailoring the modal dispersion of each optical mode. Generally, there are two mainly different types of FWM processes, namely, Bragg scattering (BS) and phase conjugation (PC). In this paper, we focus our interest on the PC-FWM in both graded-index (GI) and step-index (SI) FMFs to probe mode conversion. In the PC-FWM, the energy transfers from pump modes to both signal and idler waves. From the point of phase matching, the modal dispersions of the two FMFs is firstly optimized by genetic algorithm (GA) to design optimal core radius and core-cladding refractive difference. We then investigate the effect of the small deviations of these two parameters from their optimal values upon the phase mismatch. Numerical results show that both SI and GI fibers are able to convert the LP₀₁ mode to the LP₀₂ mode with the phase matching condition of the SI fiber being more sensitive to the changes of fiber parameters. In addition, we analyze the dependence of mode conversion performance (bandwidth and efficiency) on fiber length and pump level. It is shown that the 3dB-bandwidth increases with the pump power in the PC-FWM, which can be attributed to the nonlinear phase shift induced by the high pump power compensate for the linear phase mismatch.

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