In this Letter, we propose a unidirectional integrated vibration sensing and communication scheme via ModeDivision Multiplexing (MDM) based on ultralow-crosstalk few-mode fibers (FMF), in which partial linearlypolarized (LP) modes are adopted for sensing while the others serve as independent communication channels. Vibration-induced phase jitters of sensing modes are retrieved by coherent detection, and cross-correlation operations are conducted for vibration localization. Based on a 41-km 4-mode fiber link, a proof-of-concept experiment is carried out, with LP01, LP02 modes for communication and LP11, LP21 modes for sensing. Experimental results show that the vibration with different intensities, locations, and frequencies up to 150 kHz can be correctly recovered, and few penalties for 32-GBaud 16QAM 2-mode multiplexed transmission are observed. This work provides a promising solution toward intelligent optical networks unidirectionally integrating highcapacity MDM data transmission with vibration sensing.

Multi-path interference (MPI) resulting from optical reflections at fiber connectors, transmitters, and receivers is a critical impairment that significantly impacts intensity modulation direct detection (IM-DD) systems. Promptly identifying and pinpointing the root cause of link failures in IM-DD systems is crucial. In this study, we present a method for estimating multi-path time delays in IM-DD systems based on power cepstrum analysis. We outline the fundamental principle of our proposed method and investigate the various factors that may affect the accuracy of the estimated results. Our results demonstrate the robustness of the proposed method against link noise and laser linewidth variations. Additionally, we assess the multi-path delay monitoring capability for systems with more than two paths, revealing that the relative amplitude strength and delay of muti-paths can be effectively monitored. Through experimental validation, we confirm the validity of our proposed method, as the results align well with theoretical and simulation outcomes. This introduces a novel efficient tool that facilitates the diagnosis and localization of failure causes in IM-DD links.

We report the first real-time mode-division-multiplexing (MDM) transmission trial over field-deployed weakly-coupled few-mode fibers (FMF). 3.6-dB Q2-factor margin are observed for 80-km LP01 /LP02 MDM transmission utilizing commercial 400G DP-QPSK OTN transceivers without any modification.

We develop and characterize the first field-deployed weakly-coupled FMF cable link with 4-mode fibers in two tubes inside a 10.4-km cable. The impact of splices on the link loss and modal crosstalk is also investigated.

We theoretically find the spatial mode dispersion is beneficial to the CD mitigation in degenerate-mode-group (DMG) IMDD transmission and experimentally demonstrate 120-GBaud 4-DMG PAM4 transmission in the C-band over 10-km weakly-coupled FMF using FFE only.

Space-division multiplexing (SDM) technology by exploring fiber cores and linearly-polarized (LP) modes in few-mode multicore fibers (FM-MCF) as spatial channels is highly expected to break the capacity bottleneck of long-haul optical fiber transmission systems. However, the utility of LP modes is seriously impeded by huge computation complexity induced by inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP) when the modal crosstalk in the fiber link is not strictly suppressed. In this paper, we propose a sparse mode-division multiplexing (MDM) scheme for weakly-coupled FM-MCF long-haul transmission, in which only a set of non-adjacent LP modes in each fiber core are selected to be active channels to significantly suppress inter-modal crosstalk. A multiple-ring-core 6-LP-mode 7-core fiber with low crosstalk is first designed and fabricated. The all-fiber spatial multiplexer and demultiplexer matched with the weakly-coupled FM-MCF are realized, which achieve low insertion loss and low crosstalk among all the spatial channels. Then, experimental recirculating-loop FM-MCF transmission system is established to verify the feasibility of the proposed sparse-MDM scheme. 205.8-Tb/s transmission over 1170-km 2-mode (LP01/LP02) 7-core weakly-coupled FM-MCF is experimentally demonstrated only utilizing 2x2 MIMO-DSP. 14 SDM x 180 wavelength-division multiplexing (WDM) channels across C-band bearing 24.5-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) signals are adopted. The proposed scheme could achieve high compatibility with conventional single-mode optical transceivers, and may pave the way for near-term long-haul SDM transmission applications.

An analytical model that utilizes the minimum mean square error (MMSE) equalization matrix is proposed to accurately estimate mode-dependent loss (MDL)-induced signal-to-noise ratio (SNR) penalties with substantially reduced computational cost. The method achieves an estimation error within 0.2 dB compared with conventional digital signal processing (DSP)-based estimation, while providing significant speed-ups for strong-coupling space-division multiplexing (SDM) optical systems. An $\alpha$ correction factor is incorporated to account for non-uniform distributed noise loading, further enhancing robustness under realistic network conditions. Numerical validations over diverse system configurations confirm the effectiveness of the proposed method, demonstrating its potential for real-time optimization and planning in dynamic SDM optical networks.

We demonstrate an ultrahigh-resolution all-fiber spectrometer achieving spectral measurements of signals with independent wavelengths separated by merely 48 femtometers. This is enabled by a weakly-coupled few-mode fiber with multiple-ring core structure to suppressed modal coupling.

Traditional nonlinear compensation techniques often involve complex models that introduce significant computational overhead, particularly in high-speed, high-capacity optical communication systems. To address this challenge, we propose a low-complexity nonlinear compensation method based on a multi-task neural network (MT-NN), combined with a complexity-aware mean square error (MSE) and partial grid search optimization for coherent optical communication systems. The proposed framework exploits shared network weights to simultaneously process multiple symbols, thereby reducing redundant computations while maintaining compensation accuracy. Additionally, transfer learning (TL) is incorporated to further enhance training efficiency. Experimental results demonstrate that the MT-NN-based approach effectively lowers computational complexity across diverse optical transmission scenarios without compromising system performance. Compared to a conventional single-task neural network (ST-NN), our method achieves a superior trade-off between accuracy and efficiency. This work provides a promising solution for practical, low-complexity nonlinear compensation in next-generation optical communication systems.

We propose multi-task learning based NN equalization for nonlinearity compensation in coherent optical system. Compared with conventional NN equalization, 30% complexity reduction is achieved in an 800-Gb/s PDM-16QAM system with 5 output symbols.

We for the first time experimentally analyze the interaction between intramodal XPM and ILMD effects in weakly-coupled FMF, and prove that the ILMD could be a major factor for effectively reducing the intramodal XPM impairments.

We propose an SDM-priority SDM-WDM transmission scheme with simplified optical transceiver structure, for which a record throughput of 5.27 peta-bit/s transmission over 55-km weakly-coupled 10-mode 7-core fiber is experimentally demonstrated with up to 4×4 MIMO-DSP.

We experimentally demonstrate high-capacity MDM self-homodyne coherent transmission over 30-km weakly-coupled 10-mode fiber with specially designed multiple-ring-core profile, achieving a total throughput of 130.6 Tb/s with 9 information-bearing modes carrying 16-λ 120-GBaud PCS 64-QAM signals.

Self-homodyne coherent transmission has recently received extensive investigation as a coherent lite candidate for high-speed short-reach optical networks. In this Letter, we propose a weakly coupled mode-division-multiplexing (MDM) self-homodyne coherent scheme using a multiple-ring-core few-mode fiber, in which one of the modes transmits a self-homodyne local oscillator (LO) and the rest are utilized for carrying signals. Multiple rings of index perturbations in the fiber core are applied to achieve low modal crosstalk, allowing the signals and the remote LO to be transmitted independently. We experimentally demonstrate a 7.2-Tb/s (5.64-Tb/s net rate) self-homodyne coherent transmission with an 800-Gb/s data rate for each of the nine information-bearing modes formatted in 80-GBaud probabilistic constellation-shaped 64-quadrature-amplitude modulation. To the best of our knowledge, this is the first experimental demonstration of an MDM self-homodyne coherent transmission with up to 10 spatial modes. The proposed scheme may pave the way for future high-capacity data center interconnections.

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.

Convolutional neural network is investigated for equalization and decoding in partial response shaped intensity modulation direct detection systems for the first time. Without the channel state information as needed in the MLSE equalizer, 260-Gb/s PR PAM4 transmission can be successfully achieved.

Weakly coupled mode-division-multiplexing (MDM) systems based on intensity modulation and direct detection (IM-DD) are a good candidate for further improving the capacity of short-reach optical interconnections. However, restrained by the modal crosstalk of the transmission link and the reception of degenerate mode groups (DMGs) utilizing bandwidth-limited multimode photodetectors (PDs), high-speed MDM IM-DD has encountered a capacity bottleneck. In this Letter, we investigate a high-speed weakly coupled MDM IM-DD transmission system utilizing a degenerate mode diversity receiver scheme adopting high-bandwidth single-mode PDs over a multiple-ring-core (MRC) few-mode fiber (FMF) and a low-crosstalk mode multiplexer/demultiplexer (MUX/DMUX). An MDM IM-DD transmission with four DMGs and eight wavelengths is experimentally demonstrated with 112-GBaud four-level pulse-amplitude modulation (PAM4) and probabilistically shaped PAM8 per lane over 200-m weakly coupled MRC-FMF. To the best of our knowledge, this is the first experimental demonstration of the MDM IM-DD transmission system with up to 112-GBaud baud rate and beyond 6.4-Tb/s net rate. Meanwhile, the experimental results show that the proposed MDM IM-DD transmission link has a superior performance only adopting a low-complexity feedforward equalizer, making it a promising candidate for high-speed optical interconnections.

The understanding of nonlinear propagation effects in low-crosstalk few-mode fiber is crucial for a weakly coupled mode-division multiplexed system. In this Letter, we report the first, to the best of our knowledge, experimental verification of the advantage of intramodal dispersion on mitigating intramodal cross-phase modulation in a weakly coupled few-mode fiber transmission. The experimental system is established over a 70-km multiple-ring-core few-mode fiber accommodating 6 linearly polarized modes, based on which the influences of intramodal cross-phase modulation on transmission performances of each linearly polarized mode are evaluated. Experimental results show that the intramodal cross-phase modulation of degenerate linearly polarized modes with much larger intramodal dispersion values are significantly weaker than those of non-degenerate linearly polarized modes, in which the maximum suppression of intramodal cross-phase modulation noise is up to 9.7 dB. We believe that this work would be beneficial to practical applications of weakly coupled mode-division multiplexing technologies.

We propose uniform/non-uniform quantized Volterra and V-DFE receivers for hardware-efficient nonlinear equalization in IM/DD systems. Compared with conventional floating-point-based approaches, experimental results show that the required receiver memory can be reduced by more than half while maintaining acceptable system performance.

Directly modulated lasers (DMLs) have been implemented in short-reach optical networks as an intensity modulation and direct detection (IM-DD) scheme due to their high output power, small footprint, and low-cost fabrication. However, the attainable data rate of C-band DML-based IM-DD systems faces limitations imposed by the DML bandwidth, power fading due to chromatic dispersion, and nonlinear impairments resulting from the interplay between frequency chirp and chromatic dispersion. In this paper, we propose a joint sparse Volterra nonlinear equalization (SVNE) and gradient-descent noise whitening (GD-NW) digital signal processing technique to effectively mitigate the overall impairments. Through experimental verification, we achieve, to the best of our knowledge, the highest net data rates of 142.3/143/134.6/102.8/73.7/61.4 Gb/s over 0/5/10/20/40/60 km standard single-mode fiber for C-band DML-based IM-DD links, with the help of the proposed technique. Compared with previous works, our approach leads to an increase in transmission capacity by at least 10%. Furthermore, we conduct a comprehensive investigation into the achieved net data rate considering various factors, including modulation format, peak-to-peak voltage of the electrical output, utilization of the electrical amplifier, transmission reaches and different application scenarios, DML's chirp and extinction ratio, and the roll-off factor of the pulse shaping filter. Detailed guidelines are provided for performance-oriented efficient operations and measurements of DML-based IM-DD links.