Ghost noise in single-fiber bidirectional transmission links and its suppression approaches

Single-fiber bidirectional transmission (Bidi transmission) enables efficient reuse of existing global communication cables for sensing, yet environmental disturbances induce link noise that fundamentally limits long-distance sensing performance. Despite extensive research efforts dedicated to Bidi transmission link noise suppression, existing methods fail to eliminate link noise, forcing the abandonment of the Bidi transmission scheme in large-scale deployments. Here, we uncover that the origin of this residual noise is the "ghost noise," which can be decomposed into polarization cross-coupling noise (PCN) and calculation cross-coupling noise (CCN). To address this ghost noise, we develop a polarization-traversal interrogation (PTI) technique integrated with pseudo-random binary sequence (PRBS) phase modulation. This hybrid approach achieves zero noise increase over a 7 km Bidi transmission link and realizes 94.8% background noise reduction in a 10 km Bidi transmission link. This breakthrough extends the low-noise Bidi analog transmission distance by six times, holding significant implications for the construction of sensing networks through global communication infrastructure.