Bang Yang

With the recent advancements in large language models, high-speed optical interconnects are required between multiple servers in data centers to support model training. The stability of optical interconnects is critical to the model training. Integrated sensing and communication over fiber enable the effective utilization of existing communication fibers for vibration sensing, thereby achieving intelligent operation and maintenance of optical networks. Compared to coherent systems, vibration sensing and event recognition integrated into intensity-modulation and direct-detection (IM-DD) systems are still in the early stages, and the recognition accuracy is limited by the singular sensing source. In this paper, we propose a sensing hardware system that simultaneously monitors vibration-induced state of polarization (SOP) and phase variations for IM-DD systems. By fusing the phase-based and SOP-based sensing features using a software convolutional neural network (CNN) model, accuracy enhancement in vibration event recognition is achieved. In the hardware system, 10% of the received optical signal is tapped for sensing. This tapped portion is then split, with 90% allocated to phase-based sensing using an unbalanced Mach–Zehnder interferometer with a 3×3 coupler for fading-free phase demodulation and the remaining 10% to SOP-based sensing utilizing a polarization beam splitter (PBS) for SOP variation monitoring. Within the software CNN model for vibration recognition, the SOP-based and phase-based sensing features are concatenated along the channel dimension and adaptively fused using a channel attention mechanism. Experimental results demonstrate that the proposed scheme achieves vibration sensing within a frequency range of 100 Hz to 5 kHz, even at an extremely low received optical power (ROP) of −20dBm. Furthermore, the model with the fusion of SOP-based and phase-based features improves the average event recognition accuracy by 3% compared to using the SOP data alone and by 14.8% compared to relying solely on the phase data. The recognition accuracy improvement arises from the complementarity of phase-based and SOP-based sensing features under different vibration events. This accurate vibration recognition can effectively improve link maintenance efficiency and promises to enhance the reliability of optical networks.

In this letter, we propose a novel cascaded architecture based on frequency-domain pilot tones (FPTs) to achieve joint frequency offset estimation (FOE), carrier phase estimation (CPE) and rotation of state of polarization (RSOP) tracking in optical coherent digital subcarrier multiplexing (DSCM) systems. At the transmitter side, two FPTs are inserted at different frequencies in two polarizations respectively. At the receiver side, the power spectra of the four FPTs polarization crosstalk-induced are combined for polarization-fading-free FOE. Then, the four FPTs are employed together to cancel the effect of polarization on the FPTs’ power, achieving accurate and polarization-fading-free CPE. This avoids the estimated phase being severely impaired by noise when using a single low-power FPT for CPE. Subsequently, the FPTs are employed to estimate the Jones matrix for polarization demultiplexing. Simulation results demonstrate that the proposed scheme significantly reduces the root-mean-square error (RMSE) of phase estimation at an optical signal-to-noise ratio (OSNR) of 25 dB thanks to canceling the impact of polarization fading on CPE. Due to the accurate CPE, the proposed scheme achieves a Q-factor improvement of 0.9 dB compared to existing schemes. Experimental results further confirm that the proposed scheme reduces a 0.7 dB penalty in received optical power (ROP). The proposed cascaded carrier phase and polarization tracking scheme provides an efficient technical solution for robust carrier phase estimation and polarization tracking in future high-speed, high-order modulation optical communication systems.

The 2-micron waveband has attracted significant attention for its potential of expanding bandwidth in fiber optic communication systems from its current Tele- and Data-communication wavelength windows. However, unlike C-band equipment with mature fabrication techniques and ecosystem, devices such as modulator and photodetector at 2-micron waveband suffer from limited electronic bandwidth and pronounced nonlinearities, introducing severe inter-symbol interference (ISI) that constrains transmission capacity. In this paper, we propose an improved Volterra nonlinear equalizer (VNLE) for a high-speed intensity-modulated direct detection (IM/DD) transmission system achieving a net-bitrate beyond 100 Gbps at 2-micron waveband under severe bandwidth limitation. Experimental results from a transceiver system with an end-to-end 3-dB (20-dB) bandwidth of only <5 GHz (19 GHz) show that, the proposed improved VNLE reduces multiplication computations by 22% compared to the traditional scheme, while enhancing BER performance and receiver power sensitivity. Additionally, we achieve line-rates of 104/105.7/132.2 Gbps and the corresponding net-rates of 97.2/98.1/102.4 Gbps over 100-m solid core fiber (SCF) using PAM4, PCS-PAM8, PCS-PAM16 modulation formats, respectively, with the help of the look-up table (LUT) based nonlinear pre-distortion at the transmitter and the proposed improved VNLE at the receiver. To the best of our knowledge, these experimental results set new records for transmission capacity of single-lane demonstrations at 2-micron waveband, presenting a promising high-speed and short-reach optical interconnect scheme for this promising spectral region.

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.

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.

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.

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.

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.

We propose a simple and effective method to enhance the fidelity of radio signals in a digital-analog radio-over-fiber (DA-RoF) system by introducing nonlinearity into the transmitted signal. This nonlinearity is introduced via an arctangent function for radio signal shaping (RSS), while the corresponding tangent function is used for signal recovery. Nonlinear shaping spreads near-zero amplitude signals and compresses far-from-zero ones. After nonlinear recovery, noise in near-zero signals is suppressed, while noise in far-from-zero signals is amplified. Since the radio signal follows a Gaussian distribution, with most amplitudes near zero, the overall SNR is improved. In our experiments, we demonstrate the proposed radio signal shaping technology using a DA-RoF system based on intensity modulation with a direct detection communication system, achieving a 1.8-dB SNR gain. We achieve an SNR of 34.17 dB for QAM-1024 signals at a DA-RoF symbol rate of 104 Gbaud over 1 km of fiber. This corresponds to a CPRI-equivalent rate of 1560 Gbps and an equivalent fronthaul channel capacity of 294.85 Gbps. Our proposed scheme offers a promising solution for high-fidelity, high-capacity RoF links in Beyond 5G (B5G) and 6G networks.