首款4x200G硅光发射机问世!

随着云服务、AI、以及5G应用的蓬勃发展,全球流量持续爆发,因此对数据中心通信端口带宽的要求也在不断提升。在400GbE标准—IEEE (电气和电子工程师协会) 802.3bs标准完成之后,业界越来越多的目光,开始聚焦于面向下一代的800G光收发技术和标准的研究。

800G可插拔光模块MSA工作组已于2019年9月成立,并且IEEE也于近期完成了对800G及以上带宽的评估工作。基于PAM-4(4 Pulse Amplitude Modulation, 第四代脉冲幅度调制)信号格式的4×200G方案,作为面向800G光接口的潜在应用方案之一,其可以在现有400G光模块架构上通过速率升级实现系统演进,因此在数据中心光接口中极具竞争力。

因此,为满足800G光模块的要求,国际上已经出现了一些关于超高速单波200 Gb/s光发射技术的研究和报道,例如基于诸如硅、磷化铟、硅基有机聚合物、铌酸锂薄膜等材料的调制器。

其中,由于硅光技术可以与成熟的硅集成电路工艺平台兼容且适用于高密度光子集成,因此被认为是面向下一代数据中心光互连中最具前景的应用方案之一。

然而,由于硅材料中存在相对较弱且较慢的载流子色散效应,从而基于纯硅方案的电光调制器在性能方面受到限制—通常3 dB带宽仅~30 GHz左右,因此要实现800 Gb/s的高速硅光发射机仍面临很大挑战。

近日,来自国家信息光电子创新中心和中国信息通信科技集团的张红广博士和李淼峰博士,在Photonics Research 2020年第11期上(Hongguang Zhang, Miaofeng Li, Yuguang Zhang, et al. 800 Gbit/s transmission over 1 km single-mode fiber using a four-channel silicon photonic transmitter[J]. Photonics Research, 2020, 8(11): 11001776 )首次展示了一款基于硅光技术的4×200 Gb/s集成发射机。

该发射机中,通过光电共封技术将4通道的高速硅光调制器芯片和高速电驱动芯片进行集成封装。为了更好地平衡调制效率和插入损耗以及提高硅光调制器带宽,在硅光调制器中分别采用了掺杂浓度为~5×1017 cm-3的耗尽型PN结和T形结构的差分行波电极。同时,为了更好地实现与调制器行波电极的阻抗匹配,专门针对光调制器芯片对电驱动芯片进行了协同设计。

测试后,发现光调制器芯片的3 dB带宽为60 GHz,封装后的发射机3 dB带宽为40 GHz。再结合发端以及收端的离线数据信号处理技术,该发射机实现了4×120 Gb/s OOK(On-Off Keying,二进制启闭键控)信号和 4×200 Gb/s PAM-4信号的光调制。并且,4×200 Gb/s PAM-4信号在经过1 km标准单模光纤后,4个通道的误码率(衡量数据在规定时间内数据传输精确性的指标)仍然可以满足SD-FEC(soft-decision forward error correction,软判决前向纠错)技术的阈值要求。

800 Gb/s硅光集成发射机示意图

国家信息光电子创新中心的肖希博士和中国科学院半导体研究所的祁楠教授均认为,此次报道的800 Gb/s硅光发射机代表了当前硅光调制技术的最先进水平。特别是在调制速率上达到了创纪录120 Gbaud,展现出硅光技术的巨大升级潜力,可有效支撑下一代光模块向800G乃至超Tb/s持续演进。

800 Gbit/s transmission over 1 km single-mode fiber using a four-channel silicon photonic transmitter

With the booming development of cloud services, artificial intelligence (AI) and 5G applications, the explosive growth of global traffic requires higher and higher bandwidth of data center interface. As the 400GbE standard has been approved as IEEE Std 802.3bs, more and more researches are focusing on next generation of 800 Gb transceiver technology and its standard.

In September of 2019, the 800 Gb pluggable MSA group was formed. Meanwhile, IEEE also completed its bandwidth assessment regarding 800 Gb and beyond recently. As a potential solution for 800 Gb/s interface, 4×200 Gb/s transmission with four-level pulse amplitude modulation (PAM-4), which could be updated from standardized 400G-DR4 and FR4, is considered as a competitive solution for optical interconnects in the data center.

To satisfy the 800 Gb optical transceiver's demand, ultrahigh speed transmitters beyond 200 Gb/s per lane are investigated and demonstrated, based on silicon, indium phosphide (InP), silicon organic hybrid (SOH), and lithium niobate (LN) thin-film modulators.

Among them, silicon photonics (SiPh), which is compatible with mature complementary metal oxide semiconductor (CMOS) and enabling high dense optical integration, is been regarded as one of the most promising solutions to the next-generation datacom optical transceivers.

However, due to the silicon modulator's performance limitation caused by the relatively slow and week electro-optical effects in silicon, it is still challenging to realize a high-speed silicon optical transmitter which operates at 800 Gb/s.

Dr. Hongguang Zhang and Dr. Miaofeng Li, who work in National Opto-Electronics Innovation Center (NOEIC) and China Information and Communication Technology Group (CICT), recently demonstrated an 800 Gb/s SiPh ingegrated transmitter in Photonics Research (Hongguang Zhang, Miaofeng Li, Yuguang Zhang, et al. 800 Gbit/s transmission over 1 km single-mode fiber using a four-channel silicon photonic transmitter[J]. Photonics Research, 2020, 8(11): 11001776 ).

This transmitter is fabricated by co-packaging a 4-channle SiPh Mach-Zehnder modulator (MZM) chip with a broadband driver chip. To make a better balance between the modulation efficiency and insertion loss of the SiPh MZM, an depletion-mode PN junction is utilized with the optimized doping density~5×1017 cm-3. In order to improve the modulation bandwidth, a T-shape differential-driven travelling wave (TW) electrode is utilized to achieve good electro-optical velocity matching. Meanwhile, a 4-channel driver chip is co-designed with the SiPh MZM chip, to achieve the impedance matching to the TW electrode.

The measured 3-dB bandwidth for the SiPh MZM chip before and after the co-package is around 60 and 40 GHz, respectively. With the aid of off-line digital signal processing (DSP) applied at both of the transmitter and the receiver side, the high speed of 4×120 Gb/s OOK and 4×200 Gb/s PAM-4 optical modulation are achieved. Finally, the optical transmission of 4×200 Gb/s PAM-4 signal over a 1-km standard single mode fiber (SSMF) is demonstrated with the bit error rate (BER) of all four channels below the soft-decision forward error correction (SD-FEC) threshold.

Illustration of the 800 Gb/s silicon photonic transmitter

Dr. Xi Xiao from the NOEIC and CICT, and Prof. Nan Qi from the Institute of Semiconductors, Chinese Academy of Sciences (ISCAS) both believe that the presented co-designed SiPh transmitter shows the state-of-the-art performances, especially for the record high modulation speed up to 120 Gbaud, showing great potential for next-generation optical communication links toward Tb/s-scale.