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.

Intensity-modulated direct-detection (IM/DD) optical systems are most widely employed in short-reach optical interconnects due to their simple structure and cost-effectiveness. However, IM/DD systems face mixed linear and nonlinear channel impairments, mainly induced by the combination of square-law detection and chromatic dispersion, as well as the utilization of low-cost non-ideal transceivers. To solve this issue, recent years have witnessed a growing trend of introducing machine learning technologies such as neural networks (NNs) into IM/DD systems for channel equalization. NNs usually present better system performance than traditional approaches, and various types of NNs have been investigated. Despite the excellent system performance, the associated high computational complexity is a major drawback that hinders the practical application of NN-based equalizers. This paper focuses on the performance and complexity trade-off of NNs employed in IM/DD systems, presenting a systematic review of the current status of NN-based equalizers as well as a number of effective complexity reduction approaches. The future trends of leveraging advanced NN in IM/DD links are also discussed.

Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations. The NN connections are either directly cutoff or represented by a proper number of quantization bits by weight partitioning, leading to a hybrid compressed sparse network that computes much faster and consumes less hardware resources. The proposed approach is verified in a 50-Gb/s 25-km pulse amplitude modulation (PAM)-4 IM/DD link using a directly modulated laser (DML) in the C-band. Compared with the traditional fully connected NN-based equalizer operated with standard floating-point arithmetic, about 80% memory can be saved at a minimum network size without degrading the system performance. Quantization is also shown to be more suitable to over-parameterized NN-based equalizers compared with NNs selected at a minimum size.

Clock recovery (CR) algorithms that support higher baud rates and advanced modulation formats are crucial for short-distance optical interconnections, and it is desirable to push CR to operate at baud rate with minimal computing resources and power. In this Letter, we proposed a hardware-efficient and multiplication operation-free baud-rate timing error detector (TED) as a solution to meet these demands. Our approach involves employing both the absolute value of samples and the nonlinear sign operation to emphasize the clock tone, which is deteriorated by severe bandwidth limitation in Nyquist and faster than Nyquist (FTN) systems. Through experimental investigations based on a transceiver system with a 3 dB bandwidth of 30 GHz, the proposed baud-rate TED exhibits excellent performance. The proposed scheme successfully achieves clock synchronization of the received signals with the transmitted signals, including 50 GBaud PAM4/8, 80 GBaud PAM4, and up to 120 GBaud PAM4 FTN signals. To the best of our knowledge, the CR based on the proposed baud-rate TED is the most optimal solution for ultrahigh-speed short-reach IM/DD transmission, comprehensively considering the timing jitter, bit error rate (BER), and implementation complexity.

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.