Efficient upstream data transmission by joint scheme of dispersion compensation and electro-modulation
Graphic description: Figure a shows the eye diagrams of the transmitted NRZ and PAM-4 signal. Figure b shows the picture of the experimental set up. Figure c shows the eye diagrams of the received Duobinary and PAM-4 signal. Figure d shows a schematic of passive optical network..
With the worldwide applications of emerging multi-media services and mobile fronthaul, the user's bandwidth demand is constantly increasing. So the passive optical network (PON) with 100 Gb/s system capacity supporting 25 Gb/s data rate per wavelength is actively discussed and studied. In order to achieve 25 Gb/s data rate per wavelength for upstream signals, modulation formats, optical devices and the technical solutions supporting burst-mode transmission while overcoming accumulated chromatic dispersion become the main factors to be considered. How to achieve better performance with reduced system cost is a critical problem for 100 Gb/s PON in upstream links.
A scheme of partial chromatic dispersion compensation combined with advanced modulation formats is proposed to solve the above-mentioned problems by Professor Lilin Yi's group, from State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University. By employing low cost optical devices, upstream 0-40 km transmission of 25 Gb/s per wavelength has been achieved. The research result is reported in Chinese Optics Letters Volume 15, No. 2, 2017 (Honglin Ji, et al., Upstream Dispersion Management of 25 Gb/s Duobinary and PAM-4 Signals to Support 0-40 km Differential Reach).
In order to achieve 25 Gb/s data rate per wavelength for upstream signals, they use 10 GHz electro-modulation laser (EML) based electrical duo-binary (EDB) and quaternary level pulse amplitude modulation (PAM-4) format with optical dispersion compensation (ODC) in optical line terminal (OLT) side. To overcome the accumulated chromatic dispersion while considering the positive and negative dispersion tolerance of EDB or PAM-4, partial dispersion compensation method is employed to support 0-40 km differential reach. And the EDB-based modulation scheme achieves the highest uplink power budget of 39.5 dB so far. Therefore, EML based EDB format in ONU side combined with ODC in OLT will be an attractive solution for the upstream data transmission in symmetric 100 G-PONs.
Professor Weisheng Hu from the research group believes that the proposed partial dispersion compensation method combined with advanced modulation formats can achieve higher transmission data rate, higher power budget with low cost, which will facilitate the 100G-PON standardization and deployment.
The following works will mainly focus on the deep analysis of the system performance and the system stability in the long-term measurement.
双管齐下，使数据上行传输更快捷
图片说明：图a是发射端传输信号NRZ和PAM-4的眼图，图b是系统传输的实物图，图c是用户接收到的信号Duo-binary和PAM-4的眼图，图d是用户接入的无源光网络示意图。
随着新兴多媒体业务的发展和移动回传的广泛应用，用户对网络带宽的需求在不断地增加。因此，具有100 Gb/s系统容量、支持单通道25 Gb/s传输速率的光接入网（PON）正在被广泛地讨论和研究。为了实现上行单波长25 Gb/s的传输速率，调制码型的选择、光器件的选择、以及在克服光纤累积色散的同时支持上行突发模式接收的技术方案的选择等，都是该系统需要考虑的主要因素。如何在取得较高系统性能的同时降低系统的成本，是实现上行100 Gb/s系统容量的关键。
上海交通大学区域光纤通信网与新型光通信系统国家重点实验室的义理林教授课题组提出了高阶调制码型结合部分色散补偿的方案，解决了100 G PON系统中上行传输所存在的上述问题，由此，使用低成本的光器件便可实现上行单波长25 Gb/s传输速率，并支持0-40 km高性能光接入。相关研究成果发表在Chinese Optics Letters 2017年第2期上（Honglin Ji, et al., Upstream Dispersion Management of 25 Gb/s Duobinary and PAM-4 Signals to Support 0-40 km Differential Reach）。
为了支持上行单通道25 Gb/s的传输速率，该工作采用低成本、低带宽的10 GHz电吸收调制激光器（EML）结合电双二进制（EDB）或四电平的脉冲幅度调制（PAM-4）码型的技术方案。为了克服光纤传输的累积色散，同时考虑到EDB或PAM-4对光纤正色散和负色散的容忍性，该课题组首次提出了以部分色散补偿的方式实现0-40 km的差分距离传输。基于EDB调制的技术方案取得了到目前为止最高的上行功率预算39.5 dB。因此，在用户侧使用EML结合EDB调制格式，并在局端使用部分色散补偿的方式将是实现对称100 G-PON系统的一个很有吸引力的技术解决方案。
该课题组的胡卫生教授认为，该研究提出的基于部分色散补偿的方式结合低成本的光器件能实现较高的传输速率并取得较高的功率预算，推动了100 G-PON的标准化工作，对100 G-PON的实际部署具有一定的意义。
后续工作主要是对提出的技术方案的性能进行深入的分析，研究在长时间工作的情况下，该系统传输性能的稳定性。