The seamless integration of communication and sensing has become a crucial factor in redesigning optical network architectures. This integration enables multi-dimensional sensing capabilities while supporting the intelligent evolution of information infrastructure, addressing contemporary network demands. The demand for ultra-low-frequency (ULF) distributed acoustic sensing (DAS) continues to grow in essential applications such as geophysical exploration, structural health monitoring, and marine seismic monitoring. However, current fiber optics integrated sensing and communication (ISAC) systems encounter substantial challenges, including limited sensitivity to low-frequency signals and suboptimal spectral resource utilization due to the separation of communication and sensing functions in conventional systems. To address these challenges, this paper introduces a novel co-wavelength-channel ISAC system architecture based on frequency division multiplexing (FDM). The system achieves concurrent high-capacity communication and ULF sensing by integrating a digital subcarrier multiplexing (DSM) signal for communication and a linear frequency-modulated pulse signal for sensing within the same wavelength channel.

We investigate the performance of proposed ISAC system for ULF DAS and high-capacity communication using standard single-mode fiber (SSMF), which is widely deployed in existing optical networks. At the transmitter side, DSM and sensing signals are generated independently but share a common laser source. The signals are then combined in the optical domain through FDM, leveraging the flexible spectrum allocation capability of the DSM signal. The FDM scheme used in the proposed system is classified into two configurations, depending on the placement of the sensing signal relative to the DSM signal. As for in-band and out-of-band FDM, the sensing signal is placed within and outside the frequency band of DSM signal, respectively. Meanwhile, a careful guard-band allocation is required to minimize the interference between communication and sensing signals. In this study, the sensing and communication performance is investigated between in-band and out-of-band FDM schemes. After transmission over the fiber, the forward-propagated ISAC signal is received using a coherent receiver. The received signal undergoes digital bandpass filtering to extract the communication signal, which is then equalized to recover the transmitted data. Simultaneously, the backscattered ISAC signal, which carries the sensing information, is processed. An optical filter is first applied to remove the DSM signal, followed by either direct detection or coherent detection to reconstruct the vibration information imposed on the fiber. The sensing performance comparison between direct detection and coherent detection is also studied under the condition of ULF sensing. Finally, we examine the bit error ratio (BER) performance of DSM signals after transmission over a 38 km SSMF link, and the sensing sensitivity utilizing a piezoelectric transducer (PZT) at the end of SSMF to generate an ULF acoustic wave.

The proposed ISAC system effectively demonstrates both high-capacity optical transmission and ULF DAS capabilities over a 38 km SSMF link. Experimental results indicate that while the out-of-band FDM scheme delivers a higher distributed sensing signal-to-noise ratio (SNR) by minimizing interactions between communication and sensing signals (Fig. 9), its increased device bandwidth requirements result in a lower communication Q-factor (Fig. 10). The in-band FDM scheme achieves a more optimal balance between spectral utilization and communication performance. Experimental findings confirm that the direct detection scheme surpasses coherent detection in low-frequency vibration sensing (Fig. 11). This advantage is particularly significant in applications requiring high sensitivity to ULF signals, such as seismic monitoring and structural health diagnostics. Through optimization of spectral positioning and guard-band allocation between communication and sensing signals, and implementation of direct detection with autocorrelation demodulation, the proposed ISAC system achieves ULF sensing while maintaining high communication capacity. Based on dense wavelength division multiplexing (DWDM) technique, the transmission of 96-channnel dual polarization 16-state quadrature amplitude modulation (DP-16QAM) DSM signals reaches the BER threshold of 20% soft-decision forward error correction coding (SD-FEC). The system maximizes communication capacity by inserting guard-band only within one wavelength channel while maintaining full utilization of remaining channels for high-speed data transmission, achieving an aggregated communication capacity of 34.44 Tbit/s (Fig. 12). The ULF sensing capability is validated through vibration experiments using a PZT. The sensing system demonstrates a sensitivity of 5.49 nε/Hz@0.1 Hz with a spatial resolution of 20 m (Fig. 13).

We demonstrate the feasibility of integrating high-capacity optical communication with ULF DAS using SSMF. By leveraging the flexible spectrum allocation of DSM signals, the high-speed DSM signal and the linear frequency-modulated pulse sensing signal are frequency-division multiplexed within the same wavelength channel. After optimizing the spectral positioning and guard-band allocation between communication and sensing signals, together with utilizing the direct detection and autocorrelation demodulation for sensing signals, we successfully achieve a sensitivity of 5.49nε/Hz @0.1 Hz with a spatial resolution of 20 m, together with a transmission capacity record of 34.44 Tbit/s over 38 km SSMF. These results validate the feasibility of enhancing existing SSMF communication networks with sensing capabilities for ULF monitoring, paving the way for the intelligent evolution of optical networks.