Interference Fading Free φ-OTDR Using Dual Polarization Multi-Subcarrier LFM Signals With MIMO in Fractional Domain

We have proposed and experimentally validated a novel approach to suppress interference fading in phase-sensitive optical time-domain reflectometry (φ-OTDR) by employing dual-polarization orthogonal frequency division modulation (OFDM) signals within the fractional Fourier domain. The fractional OFDM signals for orthogonal polarization states consist of multi-subcarriers that are linear frequency modulation (LFM) signals with identical sweep bandwidths but different initial frequencies. By combining fractional Fourier transform (FrFT) algorithm with multiple input multiple output (MIMO) processing, each subcarrier with specific frequency modulation (FM) slope and overlapped sweep bandwidth can be not only compressed but also distinguished, even when affected by polarization evolution. Consequently, the complementary backscattering signals extracted can be utilized to concurrently eliminate interference fading and polarization fading after the synthesis by MIMO-FrFT. The proof-of-concept experiment has demonstrated that this approach can reduce intensity fluctuations from over 70 dB to 15 dB. The lowest intensity of the synthesized trace is 6.7 dB higher than the noise floor, resulting in fading free performance. The fading-free φ-OTDR is experimentally verified through dynamic measurements under single-frequency or sweep-frequency vibrations, attaining a sensing distance of 10 km and a sub-meter spatial resolution of 50 cm. Moreover, the block sliding window FrFT scanning is proposed to reduce complexity consumption of data processing, leading to 99.2% decrease in multiplication compared to FrFT scanning sample by sample. The sweep bandwidths of LFM signals for orthogonal polarization states completely overlap, resulting in ultra-high spectrum efficiency.