Developed distributed acoustic sensing (DAS) makes it possible to simultaneously sense and communicate via a single fiber link. In a long-haul optical fiber link, relay amplifiers are required to simultaneously amplify forward communication signals and Rayleigh backscattering (RBS) signals. However, cascading relay amplifiers progressively accumulate and amplify amplified spontaneous emission (ASE) noise, elevating the DAS system’s intensity noise and thereby limiting the sensing and communication distance. To this end, we conduct research on the design of multi-span relay amplification scheme in an optical fiber link. A noise model of cascaded relay amplification is established, and the influence of the relay amplification scheme on the signal-to-noise ratio (SNR) of RBS signals is theoretically analyzed. Experiments are performed, and these results confirm the validity of the theoretical analysis. We focus on analyzing the impact of key parameters of relay amplification on the noise performance of the DAS system, excluding hardware design of the relay amplifier.

By establishing a noise model for cascaded bidirectional erbium-doped fiber amplifiers (EDFAs), we investigate factors influencing the SNR (R_SN) of RBS signals in DAS systems, including EDFA gain (G), nonlinear threshold (P_th), single span length (L), and the number of cascaded spans (M). The critical threshold for determining whether a DAS system can acquire sensing signals is when the R_SN drops to 1. Based on this condition, it is essential to comprehensively optimize system parameters such as pump power (P_p), pulse width (T_p), the correlation coefficient of ASE noise (2nsphνΔν), gain coefficients (G_a, G_b), single span length (L), and the number of cascaded spans (M) to achieve the best sensing and communication performance. Finally, a DAS system with a cascaded amplified link is constructed to experimentally validate the effectiveness of the noise model.

For optical fiber links, each additional relay amplifier contributes to a cumulative increase in the DAS system’s noise. To maintain the SNR, it is necessary to shorten the single span length (L) and reduce the EDFA gain (G). Specifically, it can be implemented by referring to Eq. (7). Additionally, optimizing the DAS system’s probe pulse can enhance the RBS signals strength while mitigating excess noise introduced by nonlinear effects such as modulation instability (MI) and stimulated Brillouin scattering (SBS). This approach involves precisely controlling the probe pulse’s peak power and pulse width, and extending the linear frequency sweep range to exceed the fiber’s Brillouin gain bandwidth.

We conduct a comprehensive study on the design of multi-span relay amplification for integrated sensing-communication fiber links. By establishing a noise model for cascaded bidirectional EDFAs, the effects of EDFA gain (G), ASE noise (P_ASE), single span length (L), and the number of cascaded spans (M) on the SNR of RBS signals are investigated. Theoretical and experimental results show that this relay scheme supports a maximum single span length of 108 km. Accounting for optical nonlinear noise, we further elucidated the relationship between EDFA gain constraints and probe pulse characteristics (peak power, pulse width, and linear-frequency sweep range). This study provides a solid theoretical foundation for optimizing multi-span relay amplification in integrated sensing-communication fiber links, with significant practical relevance for fiber infrastructure monitoring and security applications.