New design paradigms for long-haul optical fiber communication systems
Keywords: Electrical Engineering.

Long-haul optical fiber communication systems form the backbone of worldwide connectivity. In particular, submarine fiber systems today carry 99% of all data traffic between the continents, with continuing rapid growth. Increases in system reach and capacity have been enabled by new technologies as well as design approaches, among the chief examples the erbium-doped fiber amplifier (EDFA), allowing wideband optical amplification of wavelength-division-multiplexed signals, and coherent detection. Each cable uses hundreds of in-line EDFAs offering several THz of bandwidth and near-quantum-limited noise performance. The capacity of these systems is now limited by the electrical power that can be delivered to the amplifiers. Addressing this power limitation requires a new design paradigm, space-division multiplexing (SDM), which uses many parallel spatial channels, each carrying less data and less optical power, a regime in which the amplifiers are typically less efficient. Interestingly, the utility of these power-limited photonic systems is increased by operating the photonic devices less efficiently. Future growth will also rely on the ability of new paradigms employing integrated SDM approaches to scale up the number of spatial paths for lightwave communication beyond standard single-mode fibers. State-of-the-art power-limited long-haul systems have already begun to use the concept of SDM implemented as parallel single-mode fibers. On the other hand, as yet unrealized potential lies in a closer integration of communication paths or spatial channels, encompassing multicore and multimode fiber approaches, to increase the spatial information density and enable compact scaling of components. This integration is most promising in designing the amplifiers which compensate span losses periodically along a long-haul link, as higher power and cost efficiency could result from sharing power between spatial channels in multicore or multimode EDFAs. Furthermore, the strong coupling regime of multicore or multimode SDM transmission could ameliorate typical optical channel impairments due to fiber Kerr nonlinearity and mode-dependent loss or gain. In this thesis, we study the parallel single-mode SDM system paradigm before investigating strongly-coupled multimode SDM. We begin by presenting extensive measurements and modeling of power efficiency in a single-mode long-haul optical fiber transmission testbed whose design was optimized considering detailed amplifier physics. Our results demonstrate the potential for optimized power-limited SDM submarine systems to achieve Pb/s cable capacities. Next, motivated by the potential of wideband multimode and coupled-core multicore optical amplifiers, we present a unified, comprehensive treatment of the effect of polarization-dependent gain and mode-dependent gain on the capacity of strongly coupled ultra-long-haul SDM transmission systems, using simulations of a multisection model. We quantify the requirements for optical amplifiers given tolerable capacity losses, considering linear equalizers implemented in typical multiple-input multiple-output receivers. We then propose an amplifier subsystem design based on optimized multimode fiber amplifiers meeting long-haul system requirements, enabled by wavelength- and mode-selective pump couplers, towards efficient, integrated amplification for long-haul SDM systems. We also discuss challenges for multimode transmission. Finally, we shift to studying the optimization of signal design for inter-data center links that may use Kramers-Kronig receivers, which have been proposed as an intermediary between the standard direct detection and standard coherent detection paradigms of short- and long-haul optical links, respectively. By the end, we cover the full range of link lengths in long-haul optical communications, from ultra-long-haul submarine cables over ~10000 km down to campus-size links of up to ~100 km