Performance of Bandwidth Extension Techniques for High-Speed Short-Range IM/DD Links

In recent years, advanced modulation formats for short-range high-speed communications have been increasingly discussed to achieve the transmission rates needed to satisfy the future demand of broadband services. A key parameter in such applications is the use of low-cost optical components, which mainly means the use of intensity modulation (IM) and direct detection (DD) schemes. In order to exploit the available bandwidth offered by the latest optical components, high-speed data converters (DAC and ADC) are required. However, the available bandwidth of DACs and ADCs is generally smaller, than that of their optical counterparts, resulting in a capacity bottleneck. This issue can be addressed by bandwidth extension techniques, which enable the full use of the optical components. In this paper, two high-speed systems are presented and experimentally analyzed for an optical link based on IM/DD, utilizing a bandwidth extension technique based on electrical up/down-conversion and passive signal combining. The experiment for the first system uses two independent subsignals, with a total bandwidth beyond 40 GHz and enables transmission rates up to 180 Gb/s. Different aspects of the system are highlighted including general performance, transmission distance, use of different modulation formats, and the influence of important components. For the second system, the experiment consists of the generation of a spectrally continuous pulse amplitude modulation signal with the help of two subsignals and its transmission over an optical IM/DD link. The challenges and requirements for the hardware and software are discussed and the impact of power and phase mismatches between both subsignals is investigated as well.