When a disturbance acts on a fiber it induces a change in the local refractive index that influences the fiber backscattering trace. If a chirped pulse $\phi $ -OTDR setup is used to interrogate the fiber, this refractive index change appears as a local shift of the received trace, linear to the acting perturbation. However, the refractive index change influences the round trip time of all the backscattering components generated by further fiber sections as well. Due to the high sensitivity of chirped pulse $\phi $ -OTDR, the change in the round trip time of the backscattering components, which is usually negligible, may appear as a virtual perturbation in certain conditions. In this letter we derive a mathematical model for the virtual perturbation induced by a disturbance acting on the fiber, when the measurement is performed by a chirped pulse $\phi $ -OTDR. We experimentally validate the model by inducing a temperature change on a known span of fiber while monitoring its effects in a further fiber section kept at rest. The experimental results are then analyzed and compared with the theoretical ones.

中文翻译：

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啁啾脉冲∅-OTDR中干扰引起的“虚拟”扰动分析

当干扰作用在光纤上时，它会引起局部折射率的变化，从而影响光纤的反向散射轨迹。如果一个啁啾的脉冲 $\phi $ -OTDR 设置用于询问光纤，这种折射率变化表现为接收轨迹的局部偏移，与作用扰动呈线性关系。然而，折射率变化也会影响由其他光纤部分产生的所有反向散射分量的往返时间。由于啁啾脉冲的高灵敏度 $\phi $ -OTDR，反向散射分量往返时间的变化，通常可以忽略不计，在某些条件下可能会表现为虚拟扰动。在这封信中，当测量由啁啾脉冲执行时，我们推导出由作用在光纤上的扰动引起的虚拟扰动的数学模型 $\phi $ -OTDR。我们通过在已知光纤跨度上引起温度变化同时监测其在保持静止的进一步光纤部分中的影响来实验验证该模型。然后对实验结果进行分析并与理论结果进行比较。