The spectral properties of the Rayleigh backscattered traces measured by a phase-sensitive optical time-domain reflectometer (φOTDR) with direct detection are theoretically and experimentally analyzed. The spectrum of the measured φOTDR signal is found to be strictly dependent on the spectral shape of the probing optical pulse. Furthermore, the visibility, spatial resolution, fading rate, and correlation spectrum of the traces are analyzed using different detection bandwidths. Results point out that the quality of φOTDR traces and target spatial resolution are secured only if the electrical bandwidth of the photodetector is broad enough to cover at least 80% of the total power contained in the electrical spectral density function of the measured trace. This means that in the case of using direct detection of the Rayleigh backscattered light induced by rectangular-shaped optical pulses, the minimum bandwidth required for a proper detection of the traces is equal to the reciprocal of the pulse temporal width (which is larger than the pulse spectral width). Although the theoretical analysis and numerical simulations are here experimentally validated for rectangular and sinc-shaped optical pulses, the results and methodology presented in this article can be applied to optimize the direct-detection bandwidth of φOTDR sensors using any optical pulse shape.

中文翻译：

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具有直接检测的相敏光时域反射计中信号的光谱特性

从理论上和实验上分析了由具有直接检测的相敏光时域反射计 (φOTDR) 测量的瑞利背散射迹线的光谱特性。发现测得的 φOTDR 信号的频谱严格依赖于探测光脉冲的频谱形状。此外，使用不同的检测带宽分析轨迹的可见性、空间分辨率、衰落率和相关谱。结果指出，只有当光电探测器的电带宽足够宽以覆盖所测迹线的电谱密度函数中包含的总功率的至少 80% 时，才能确保 φOTDR 迹线的质量和目标空间分辨率。这意味着在直接检测矩形光脉冲引起的瑞利背向散射光的情况下，正确检测轨迹所需的最小带宽等于脉冲时间宽度的倒数（大于脉冲谱宽度）。尽管本文对矩形和正弦形光脉冲的理论分析和数值模拟进行了实验验证，但本文中介绍的结果和方法可用于优化使用任何光脉冲形状的 φOTDR 传感器的直接检测带宽。