Chirp-pulse pair φOTDR.

We proposed a novel, to the best of our knowledge, chirp-pulse pair phase-sensitive optical time-domain reflectometry (CPP-φOTDR) technique, enhanced by an adaptive filtering algorithm. This technique utilizes a pair of chirp pulses: one with a low chirp rate and another with a high chirp rate, with their Rayleigh backscattering (RBS) processed through a low-pass (LP) electrical filter. The adaptive filtering algorithm effectively preserves the extensive measurement range afforded by the high chirp rate pulse while enhancing the sensitivity provided by the low chirp rate pulse. Consequently, the CPP-φOTDR enables vibration measurements over a wide dynamic range and broad frequency bandwidth without incurring additional acquisition costs. In the experiments, we employed a chirp-pulse pair featuring bandwidths of 500 MHz and 8 GHz, utilizing only the receiver's 500 MHz bandwidth to retrieve the vibrational signal. The dynamic range of the CPP-φOTDR was enhanced by 25.1 dB, with the assistance of adaptive filtering of the acoustic waveform demodulated with RBS of the chirp-pulse pair. The proposed method could be utilized for monitoring the ocean in marine science and for analyzing seismic waves in geophysics.