Frequency response enhancement for long-range φ-OTDR system by additive random sampling and nonlinear frequency modulation

We reported a new method based on additive random sampling (ARS) and non-linear frequency modulation (NLFM) to enhance the frequency response of long-range phase-sensitive optical time-domain reflectometry (φ-OTDR) system. Using the NLFM interrogation pulse, the side-lobe suppression ratio (SLSR) and signal-to-noise ratio (SNR) of the demodulated traces are improved, and phase signal detection with less than 3m spatial resolution is achieved over 50 km sensing range. By modulating the NLFM laser pulse intervals, we realize the ARS and the uniform sampling alternately for every sensing point of the long interrogation fiber, and therefore the frequency domain aliasing is avoided. We test the proposed system by detected a 20 kHz harmonic signal, and this signal is well identified and reconstructed over 50 km sensing range.