Bipolar-Coding $\Phi$-OTDR with Interference Fading Elimination and Frequency Drift Compensation

Optical-pulse-coding (OPC) technique can break the trade-offs between key parameters of distributed fiber-optic sensing systems, therefore it has received great attention in recent years. OPC Φ-OTDR has achieved significant performance improvements over conventional Φ-OTDR; however, scan-rate, interference fading and frequency drift remain critical issues for this type of sensing systems. In this article, for the first time bipolar Golay coding Φ-OTDR with heterodyne detection is proposed and realized to address the scan-rate issue, the laser frequency drift is compensated by a novel real-time compensation method, and the interference fading is eliminated by incorporating the spectrum extraction and remix method. Theoretical analysis is described in detail, and the benefits provided by bipolar code are experimentally demonstrated in 10 km sensing range with 0.92m spatial resolution, achieving 7.1 dB SNR improvement and halving measurement time compared with unipolar code case. By addressing frequency drift and fading issues, the distributed sensing capability of OPC Φ-OTDR becomes available, at a reasonable expense of enlarged spatial resolution (6 m). It should be noted that, such a bipolar scheme can also be applied in other coding schemes, opening up more possibilities for OPC in Φ-OTDR.