Quantum key distribution (QKD) generates symmetric keys between two remote parties and guarantees the keys are not accessible to any third party. Wavelength-division multiplexing between QKD and classical optical communications by sharing the existing fiber-optics infrastructure is highly desired in order to reduce the cost of QKD applications. However, comparing to the light for classical transmission, quantum signals are very weak and easily affected by impairments from classical light, such as the spontaneous Raman-scattering effect. Here, by selecting an optimal wavelength of quantum signals, we significantly reduce the influence of the Raman-scattering effect. In addition, through coherent optical communication technology, we achieve high-speed classical data transmission with relatively low launch powers, thereby further reducing the impairments from classical light. As a result, we realize the multiplexing and long-distance copropagation of QKD and terabit classical data transmission up to 80 km. The data capacity is two orders of magnitude larger than the existing results. Our demonstration verifies the feasibility of QKD and classical communication to share the resources of backbone fiber links and thus taking the utility of QKD a great step forward.

Artificial-intelligence-generated content has driven explosive data traffic growth in data-center interconnects. Traditional direct detection solutions struggle with limited spectral efficiency and distance, prompting the shift to coherent optics for cost-sensitive short-reach links. One specific challenge is integrating low-cost lasers while overcoming severe phase noise on high-order modulation formats. Here, we propose a residual carrier modulation scheme for precise and efficient carrier frequency and phase recovery. The residual optical carrier can continuously track phase fluctuations without redundancy compared with discrete time-domain pilots, and address the digital-to-analog convertor resolution reduction issue of frequency-domain digital pilots. In proof-of-concept experiments, we transmit a net 1-Tb/s probabilistic-shaped 256-ary quadrature amplitude modulated (PS-256-QAM) signal using a 3 MHz distributed feedback (DFB) laser. Our scheme improves bitrate by 41% compared to conventional time-domain pilots, achieving a record laser linewidth sum and symbol duration product of 6.89 × 10⁻⁵. This approach supports MHz linewidth DFB lasers in low-cost coherent optical communications.

Fiber Kerr nonlinearity is a fundamental limitation to the achievable capacity of long-distance optical fiber communication. Digital back-propagation (DBP) is a primary methodology to mitigate both linear and nonlinear impairments by solving the inverse-propagating nonlinear Schrödinger equation (NLSE), which requires detailed link information. Recently, the paradigms based on neural network (NN) were proposed to mitigate nonlinear transmission impairments in optical communication systems. However, almost all neural network-based equalization schemes yield high computation complexity, which prevents the practical implementation in commercial transmission systems. In this paper, we propose a center-oriented long short-term memory network (Co-LSTM) incorporating a simplified mode with a recycling mechanism in the equalization operation, which can mitigate fiber nonlinearity in coherent optical communication systems with ultralow complexity. To validate the proposed methodology, we carry out an experiment of ten-channel wavelength division multiplexing (WDM) transmission over 1600 km standard single-mode fiber (SSMF) with 64 Gbaud polarization-division-multiplexed 16-ary quadrature amplitude modulation (16-QAM) signals. A 0.51 dB Q ² factor gain is observed with the Co-LSTM equalization, which is comparable to that of DBP. The complexity of the Co-LSTM equalization is only 5.2% of that of the conventional bi-directional LSTM, and 28.4% of that of the DBP method with a single step per span. In principle, the complexity of the Co-LSTM with a simplified mode is almost independent of the transmission distance, which shows an essential benefit over the DBP method that determined by the optical signal evolution along the fiber link. The proposed Co-LSTM methodology presents an attractive approach for low complexity nonlinearity mitigation with neural networks.

In this work, based on a conventional silicon photonic travelling-wave Mach-Zehnder modulator (MZM) with a 3-dB bandwidth of 22.5GHz, we experimentally demonstrate ultra-high speed optical interconnects with Nyquist shaped pulse amplitude modulation (PAM) signals. For bandwidth-limited systems, a two-tap digital post filter is employed to suppress the equalization-enhanced high frequency noise. The post filter induced inter-symbol interference (ISI) is subsequently eliminated by maximum likelihood sequence detection (MLSD). Enabled by the post filter and MLSD, we achieve 200Gb/s (80Gbaud) PAM-6 signal direct detection (DD) transmission over 1km standard single-mode fiber (SSMF) with a bit-error rate (BER) below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5×10 $^{-2}$ . For PAM-4 format, we successfully transmit 192Gb/s (96Gbaud) and 176Gb/s (88Gbaud) signals over 1km and 2km SSMF, respectively, with BERs lower than the 20% HD-FEC threshold. For PAM-8 format, 192Gb/s (64Gbaud) and 168Gb/s (56Gbaud) signals are generated at back-to-back (BTB) and transmitted over 1km SSMF, respectively. To our best knowledge, we achieve the highest single lane bitrates ever reported for single polarization PAM-4/6/8 signal generation and DD transmission with all-silicon MZM. The flexible receiver-side digital signal processing (DSP) can significantly enhance the performance of silicon MZM, which provides a promising solution for future single lane 200G data-center interconnects.

We demonstrate $1.728\mathrm{Tb}/\mathrm{s}(16\times 108\mathrm{Gb}/\mathrm{s})$ direct-detection wavelength division multiplexing (WDM) transmission over 80 km standard single mode fiber (SSMF) with Nyquist 64-ary quadrature amplitude modulation (64-QAM) and half-cycle subcarrier modulation. Each channel carries single sideband 18 GBaud 64-QAM signal and the channel spacing is 27 GHz. Considering 20% soft-decision forward error correction and frame redundancy, a net spectral efficiency record of 3.25 b/s/Hz is achieved for 100 G single polarization direct-detection WDM transmission.

We report the experimental demonstration of single wavelength terabit free-space intensity modulation direct detection (IM-DD) system employing both orbital angular momentum (OAM) multiplexing and polarization division multiplexing (PDM). In our experiment, 12 OAM modes with two orthogonal polarization states are used to generate 24 channels for transmission. Each channel carries 30 Gbaud Nyquist PAM-4 signal. Therefore an aggregate gross capacity record of 1.44 Tb/s (12 × 2 × 30 × 2 Gb/s) is acheived with a modulation efficiency of 48 bits/symbol. After 0.8m free-space transmission, the bit error rates (BERs) of all the channels are below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10⁻². After applying the decision directed recursive least square (DD-RLS) based filter and post filter, the BERs of two polarizations can be reduced from 5.3 × 10⁻³ and 7.3 × 10⁻³ to 2.2 × 10⁻³ and 3.4 × 10⁻³, respectively.