Neural networks (NNs) are widely employed as effective equalizers in intensity-modulated direct-detection (IM/DD) links due to their excellent ability in dealing with nonlinear channel impairments. However, the complexity concern impedes the real-time application of NN-based receivers. To address this issue, we propose mixed-precision quantization of recurrent NN (RNN)-based equalizers in a 100-Gb/s 15-km C-band IM/DD system, which saves about 73.3% and 22.4% memory compared with traditional floating-point-based and fixed-precision quantized RNN. A simple and effective neuron clustering approach is proposed to realize mixed-precision quantization of RNN without degrading system performance.
The rapid growth of modern Internet applications demands ever-increasing transmission capacity and reduced latency in optical interconnect systems utilizing intensity modulation and direct detection (IM/DD). However, the intrinsic limitations of silica-based standard single-mode fiber (SMF) will ultimately be insufficient to meet these escalating demands. The nested antiresonant nodeless fiber (NANF), a newly designed hollow-core fiber, has garnered significant attention as a potential solution to these challenges. In this paper, we simultaneously address the issues of capacity and latency in a self-developed 20 km NANF-based four-level pulse amplitude modulation (PAM-4) IM/DD system. To mitigate the chromatic dispersion-induced power fading encountered in NANF, we implement effective compensation and fast equalization based on Tomlinson-Harashima precoding (THP) and a modified feedforward equalization with parallel multi-output (MFFE). Additionally, we measure the propagation latency in NANF using high-time-resolution entangled photon pairs. Experimental results demonstrate an achievement of beyond 200 Gb/s/λ in a PAM-4 IM/DD system operating in C-band over 20 km NANF transmission. Furthermore, the fast-computational equalizer reduces the processing time by 64.6% compared to conventional FFE. In terms of latency, the NANF demonstrates a 31.72% reduction in time delay compared to SMF, offering a viable pathway for future high-capacity and low-latency fiber-optic systems.
Direct detection system is expected to possess the phase and polarization diversity in order to achieve high spectral efficiency and fiber impairment compensation such as chromatic dispersion and polarization rotation. In this Letter, we theoretically extend the concept of the proposed Jones-space field recovery (JSFR) to include a dynamic polarization rotation matrix and experimentally demonstrate the rapid polarization state tracking ability of the JSFR receiver based on a 3 × 3 optical coupler. Under a rotation of the state of polarization at a rate of 1 Mrad/s, we successfully transmit 59-GBd dual-polarization 16-ary quadrature-amplitude-modulation signals over an 80-km standard single-mode fiber based on a decision-directed least mean square (DD-LMS) or a recursive least square (DD-RLS), with a bit-error rate below the 14% hard-decision forward error correction threshold of 1 × 10-2. The experimental results indicate that the legacy polarization tracking algorithms designed for coherent optical communication are also applicable for this direct detection scheme. To our best knowledge, this work demonstrates the first polarization rotation-tolerant direct detection system with phase and polarization diversity, providing a low-cost and high-speed solution for short-reach communications.
We propose the generalized carrier assisted differential detection schemes with reduced hardware complexity, which requires the same number of photodiodes and ADCs as the coherent homodyne counterpart but without the narrow-linewidth local oscillators. The performance of the simplified receiver is evaluated by 60-Gbaud 16-QAM OFDM signals.
We propose a complementary polarization-diversity coherent receiver (C-PDCR) based on complementary polarization detection. The proposed C-PDCR features rapid polarization tracking for remote LO using electronic DSP. The robustness is verified by a 1.08-Tb/s dual-polarization PCS-256QAM signal with up to 314-Krad/s polarization tracking speed.
A simplified short-blocklength variable-to-fixed distribution matching algorithm is proposed for probabilistic shaping. Compared with the original V2F DMLP method, the computational cost is reduced to 0.41 operations per bit, which is 15 times less.
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