Advanced Modulations of Optical Interconnections for Mega-Datacenters

Recently, we are experiencing a soar of the traffic needs for the data networks with the paradigm shifting of industry and the upgrade of the information infrastructure. It places a bandwidth-hunger of the data interconnections in almost all scenarios, particularly for ones between and inside large-scale datacenters. Besides the needs for data-rate, the design and deployment of datacenter interconnects are limited by other equally important requirements, e.g. power consumptions, costs, latencies, footprint, lifetime and reliability. Thus, the current simple modulation scheme, i.e. non-return-to-zero (NRZ), lags behind expectations for the future development. For this reason, advanced modulation schemes and physical configurations have been proposed in the past few years.
This thesis summaries a three-year Ph.D. research on the advanced modulations for the large datacenter interconnections, using 850 nm verticalcavity surface-emitting lasers (VCSELs). The majority covers the application of multi-dimensional coded modulations in the transmission systems with intensity-modulated direct-detection (IM/DD) schemes. It presents the benefits of both multi-level signals and multi-subcarrier systems. The work starts with the design of four and eight-dimensional multilevel modulation formats, and further simplifications regarding the complexity of the modulation and demodulation, as well as the decision process. Then, an eight-dimensional eight-level format, i.e. BB8, with equivalent 2 bit/symbol, is theoretically proposed and experimentally demonstrated. It is further extended to a family of eight-dimensional modulation formats, named as E8Flex-m. The family based on the optimal eight-dimensional
lattices. It provides a smooth transition of the bit-rate with a finer granularity of 0.125 bit/symbol. Based on the similar designing philosophy, we demonstrated a 24-dimensional pulse amplitude modulation (PAM), by combining the temporally adjacent symbols, based on the densest lattice in 24-D space, i.e. Leech lattice. Formats with equivalent 2 bits are designed and demonstrated. It shows that the performance of the 24-D PAM outperforms its 8-D counterpart, as well as the conventional PAMs in a 100 Gbit/s system. Further, we extend such format into a bit-rate flexible version by using the cutting and scaling techniques on the hyperspace. To enhance the performance, we combine the above mentioned rateflexible hyperspace formats with the multi-subcarrier techniques, such as
discrete multitone (DMT) and carrier-less amplitude phase modulations (CAP). We demonstrate a world record 120 Gbit/s DMT using 850 nm multi-mode VCSELs over 100 m MMFs and four-dimensional formats. Based on that, we demonstrate the transmission with a serial of fourdimensional formats loaded with general benefits in data-rates. A further application of the eight-dimensional formats on DMT transmissions is investigated, showing a series of benefits in the bit-rate increase, the reach extension, and the enhancement of the tolerance for the thermal and insertion loss. Meanwhile, the experiments of the 24-D rate-flexible formats loaded for the multi-band CAP transmission are demonstrated. It shows the further improvement of the performances, including the reach, thermal
tolerance and the insensitivity of the insertion loss. Some other advanced modulation schemes related with this three-year research are briefly introduced, such as the mode-selective launching and the optical compensation techniques for the mode-division multiplexing of optical angular momenta. In summary, this thesis presents various alternative modulation schemes, opening new possibilities and feasibilities of multi-dimensional coded modulations in future products, improving the state of the art techniques, extending the understanding of the benefits gained for the next generation optical data links used inside and between mega-datacenters, and hopefully bringing them closer to the actual deployments.