High-resolution chirped-pulse φ-otdr by means of sub-bands processing

Conventional chirped-pulse (CP) phase-sensitive optical time-domain reflectometry (CP φ-OTDR) allows the interrogation of tens of kilometers of optical fiber with high accuracies of millikelvin or nanostrain. With respect to standard coherent-detection φ-OTDR, it shows increased robustness to coherent fading and allows a linear and quantitative monitoring of the perturbations acting on the fiber. Its spatial resolution, however, remains a critical parameter and new techniques allowing its improvement without reducing significantly other performances (or increasing the setup complexity) are constantly being researched. In this paper, we propose a method to improve the spatial resolution of CP φ-OTDR without reducing the input pulse width, by means of sub-bands processing. The method is based on adding an optical carrier to the input pulse. Using digital filtering, the spectrum of the fiber backscatter can be split into multiple sub-bands. Each of these sub-bands corresponds to the fiber response generated by a short optical pulse, chirped over a smaller frequency range. This way each sub-band results in φ-OTDR measurements with high spatial resolution, but with a reduced SNR. A dedicated post-processing methodology is proposed to mitigate the SNR reduction obtained from each sub-band, while securing high-resolution measurements of the perturbations acting on the fiber. Experimental results demonstrate the possibility of achieving CP φ-OTDR measurements with a 15-fold spatial resolution improvement over the conventional CP analysis, at the expense of an SNR reduction lower than a factor 2.