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 integrated 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.
