In this paper, integrated scheme of enhanced vibration-sensing and digital subcarrier multiplexing (DSCM) communication based on frequency-domain pilot tones (FPTs) is proposed and demonstrated experimentally. In the scheme, FPTs are employed for multiple receiver digital signal processing (DSP) modules including frequency offset estimation (FOE), carrier phase estimation (CPE) and polarization demultiplexing. Meanwhile, with the recovered phase after CPE, the vibration-induced phase is extracted for sensing with band-pass filter within a frequency range of 7 kHz to 50 kHz. Thanks to two FPTs, dynamic FOE performance is enhanced under the frequency drift of 100 kHz ECLs compared to the conventional FOE methods, such as Max-Fast Fourier Transform (Max-FFT) and Viterbi-Viterbi (VV) algorithm. This enhancement can effectively suppress the background noise of the recovered phase after CPE, which inherently can subsequently improve the vibration sensing ability at a low frequency. In the experiment, sensing signal-to-noise ratio (SSNR) at 10 kHz is improved by 10 dB with the aid of FPTs. As a result, the vibration of frequency down to 10 kHz is successfully detected based on forward phase-based sensing in integrated DSCM communication system using commercial 100 kHz ECLs. The proposed integrated solution is compatible with the existing optical networks employing commercial ECLs to support intelligent operation and maintenance.

In this Letter, we propose and experimentally validate a high-fidelity and adaptive forward-phase-based vibration sensing using a Wiener filter (WF). In commercial coherent digital subcarrier multiplexing (DSCM) systems under external cavity lasers (ECLs), frequency-domain pilot tones (FPTs) in subcarrier intervals are employed for dynamic frequency offset estimation (FOE), carrier phase estimation (CPE), and polarization demultiplexing. The phase estimated by the CPE module is processed with the WF to achieve high-fidelity extraction of the vibration-induced phase. Compared to traditional bandpass filter (BPF)-based phase extraction, the WF method effectively suppresses sensing background noise caused by laser phase noise, improving sensing signal-to-noise ratio (SSNR). Furthermore, without knowing vibration frequency and waveform information, the WF method can adaptively detect various vibration waveforms, including amplitude-modulated continuous wave (AMCW) and frequency-modulated continuous wave (FMCW). In experiments, for 10 kHz sinusoidal vibration detection, the WF method demonstrated an 8 dB SSNR improvement compared to the BPF method. The experiments further verified that both AMCW and FMCW vibrations can be detected adaptively with the WF method using the same filter parameters as for sinusoidal vibrations.

Digital subcarrier multiplexing (DSCM) systems offer flexibility and software configurability, making them promising for point-to-multipoint (P2MP) communications. Meanwhile, DSCM systems exhibit enhanced communication damage tolerance and capabilities for damage monitoring and compensation. These capabilities include tolerance to equalized enhanced phase noise (EEPN) and high-speed Polarization State (SOP) tracking. The subcarrier intervals in DSCM systems naturally allow the insertion of pilot or training sequences. This facilitates enhanced communication functionality and integrated sensing capabilities. This review article summarizes the principles and schemes of integrated communication and sensing in DSCM systems. It analyzes performance in communication enhancement and sensing integration. Finally, it discusses unresolved technical challenges and future technological prospects.

With the rapid advancement of artificial intelligence (AI) technologies, the stability of optical interconnects in data centers has become increasingly important. Vibration sensing integrated in optical interconnect systems is conducive to identifying external disturbances in optical interconnects and achieving intelligent operation and maintenance. This paper proposes an easy-integration vibration-sensing scheme based on the state of polarization (SOP) of the fiber link. This scheme combines photonic technology with low-complexity digital signal processing (DSP) to detect link vibrations, ensuring full compatibility with intensity-modulation direct-detection (IM/DD) optical interconnect systems while minimizing additional complexity. Experiments show that our proposed scheme effectively detects SOP variations across a wide frequency range (0.5 Hz to 159 kHz). Based on the sensing system, a recognition scheme leveraging the Gramian angular field analysis and convolutional neural network (CNN) is proposed to recognize four types of vibration events simulated by a robotic arm, achieving a classification accuracy of 98%. Furthermore, experimental results confirm that the sensing system can detect SOP variations even under conditions of extremely low received optical power (ROP), where the communication system becomes inoperative. The proposed scheme enables robust event detection with minimal hardware overhead, which is suitable for real-world deployment in pluggable optical modules.

This study firstly proposes and experimentally demonstrates a phase demodulation method with high sensitivity for interferometric fiber sensors (IFSs) on the basis of the fast Fourier transform of a wavenumber domain. The phase information of IFSs triggered by changes in environmental parameters is obtained by calculating the initial phase variation of a specific Fourier-transformed spatial frequency. Theoretically, phase sensitivity can be improved by n times when the optical path difference (OPD) of a spatial frequency peak is increased by a multiple ( n times) of the OPD of other spatial frequency domain peaks. To verify the method experimentally, this study designed a large laterally offset spliced sensor formed by common single-mode fibers on the basis of mode interference. The sensing characteristics of temperature and strain in each set of two-beam interference are analyzed simultaneously by calculating the phase sensitivities of the frequency domain peaks. Furthermore, the highest reported temperature and strain sensitivities of 0.0795 rad/°C and −0.0088 rad/μϵ, respectively, with good repeatability are realized.

In this letter, we propose a flexible vibration-induced phase extraction scheme based on a three-pass Kalman smoother (KS) with scalar linear time-invariant model for integrated communication and sensing systems under commercial external-cavity lasers (ECLs). By differentiating the phase estimated by the carrier phase estimation (CPE) module, the laser phase noise is transformed into Gaussian noise. Then, the three-pass KS is used to estimate the differentiated vibration-induced phase. In the three-pass KS, the forward filter pass is used to obtain the posteriori estimates, the backward filter pass is employed to obtain the priori estimates, and the smoother pass is utilized to combine both posteriori and priori estimates. The vibration-induced phase can be obtained by integrating the estimated differentiated vibration-induced phase. In experiments, compared to conventional vibration-induced phase extraction scheme based on band-pass filter (BPF), the KS-based scheme achieves a 1.5dB sensing signal-to-noise ratio (SSNR) enhancement at 10 kHz without requiring vibration frequency or waveform information. Furthermore, the KS-based scheme enables frequency-modulated continuous wave (FMCW) vibration detection in the range of 200 kHz to 1 MHz with the same smoother parameters used for sinusoidal vibration detection, highlighting the flexibility of the KS-based scheme. This flexible vibration-induced phase extraction scheme provides a feasible solution for intelligent sensing in future optical networks.