Quasi-Distributed Acoustic Sensing Based on Orthogonal Codes and Empirical Mode Decomposition

As an important branch of phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR), quasi-distributed acoustic sensing (QDAS) has attracted lots of attention due to its unique advantages. Varieties of scenarios require the capability of detecting signals in different frequency ranges. However, the influence of low-frequency noises limits the ability to sense low-frequency perturbations; sensing bandwidth is limited by the round-trip repetition in the QDAS system. In this article, a high performance QDAS scheme based on orthogonal codes and empirical mode decomposition (EMD) algorithm is proposed. EMD is utilized to extract and eliminate the low-frequency noises including frequency drift of laser, realizing sensing signals at the level of Hz. Importantly, to break the trade-off, massive orthogonal codes on the same carrier (OCSC) are generated and used to multiplex sensing channels, resulting in a significant enhancement in spectral efficiency. In addition, as a kind of optical pulse coding (OPC), OCSC can improve the signal to noise ratio (SNR) for long-distance sensing requirement without sacrificing spatial resolution. As a result, the influence of low-frequency noises is first eliminated in OPC QDAS system, and the sensing bandwidth is enlarged by 20 times compared with conventional single-pulse interrogation scheme. The frequency-band detection range of 1 Hz–10 kHz is realized at the far end of 99.4 km fiber, without consuming extra frequency-domain resources and distributed amplification; the noise level is about $33 {p}\varepsilon /\sqrt {\text {Hz}}$ in the range of 20 Hz-10 kHz, with 10 m spatial resolution.