Any temperature change or strain acting on a section of the fiber induces a local variation of the refractive index. If the fiber is monitored by a chirped pulse φ-OTDR system, the variation of the refractive index causes a local shift of the backscattering trace and produces a change in the round trip time of the light coming from any further position of the fiber. While usually negligible, due to the high sensitivity of the chirped pulse φ-OTDR, in extreme occasions the distributed round trip time change may appear in the measurement as an undesired “virtual” perturbation. In this paper, we discuss and experimentally validate a mathematical model to account for (and eventually correct) the “virtual” perturbation.
Chirped pulse phase-sensitive optical time-domain reflectometry (chirped pulse Φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high sensitivity and linearity, but typically with spatial resolution limited to ten meters to ensure proper processing and signal-to-noise ratio (SNR). In this paper, we propose a method to increase the spatial resolution of a chirped pulse Φ-OTDR without reducing the pulse width. The improvement is achieved by adding an optical carrier to the input chirped pulse and by applying digital filtering to the measured backscatter traces. Experimental results validate the method, demonstrating a 10-fold resolution improvement with minimum impact on the measurement SNR.
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