Zhou Honghang, Li Yan, Lu Dan, Yue Lei, Gao Chao, Liu Yuyang, Hao Ruibin, Zhao Zhixi, Li Wei, Qiu Jifang, Hong Xiaobin, Guo Hongxiang, Zuo Yong, Wu Jian
Opt Express. 2019 Apr 15;27(8):11385-11395. doi: 10.1364/OE.27.011385.
With the rapid development of cloud services, data-center applications and the Internet of Things, short-reach communications have attracted much more attention in recent years. 4-level pulse amplitude modulation (PAM4) is a promising modulation format to provide both high data rate and relatively low cost for short-reach optical interconnects. In this paper, a joint clock recovery and feed-forward equalization algorithm (CR-FFE) is proposed to simultaneously eliminate the inter-symbol interference (ISI) and track large sampling clock offset (SCO) in PAM4 transmission. The algorithm estimates timing error according to the difference between two tap coefficients of fractionally spaced equalizers, thus solving the problem of incompatible prerequisites between clock recovery and channel equalization. A 10GHz directly modulated laser (DML) based 50-Gbit/s PAM4 transmission experiment is implemented to investigate the performance of the proposed algorithm. Experimental results show that the proposed CR-FFE algorithm can resist SCO up to 1000 ppm after 40 km standard single-mode fiber (SSMF) transmission under the 2x10 SD-FEC BER threshold, which is dramatically improved comparing with that of 20 ppm in traditional CR cascaded by FFE algorithm.
随着云服务、数据中心应用和物联网的快速发展，近年来短距离通信受到了更多关注。4级脉冲幅度调制（PAM4）是一种很有前景的调制格式，可为短距离光互连提供高数据速率和相对较低的成本。本文提出了一种联合时钟恢复和前馈均衡算法（CR-FFE），以同时消除PAM4传输中的符号间干扰（ISI）并跟踪大采样时钟偏移（SCO）。该算法根据分数间隔均衡器的两个抽头系数之间的差异来估计定时误差，从而解决了时钟恢复和信道均衡之间前提条件不兼容的问题。开展了基于10GHz直接调制激光器（DML）的50Gbit/s PAM4传输实验，以研究该算法的性能。实验结果表明，在2x10 SD-FEC误码率阈值下，所提出的CR-FFE算法在40km标准单模光纤（SSMF）传输后可抵抗高达1000ppm的SCO，与传统的由FFE算法级联的CR算法的20ppm相比有显著提高。
