Advanced Coherent Systems for High-Performance Fiber-Optic Communications and Distributed Acoustic Sensing

This thesis explores phase retrieval schemes and algorithms that leverage phase information in two areas: coherent fiber-optic communications and distributed fiber-optic acoustic sensing (DAS). In both fields, phase retrieval plays a critical role. In coherent optical communications, it enables the use of higher-order modulation formats and the compensation of propagation-related impairments. In DAS systems, phase retrieval is essential for extracting information about acoustic vibration from the phase variations of backscattered light. This thesis introduces novel phase-retrieving schemes for coherent optical communications, aimed at reducing the complexity of conventional coherent receivers. It demonstrates the use of neural networks (NNs) to perform phase retrieval in coherent optical communication, achieving reduced computational and hardware complexity compared to state-of-the-art phase retrieval schemes. Additionally, this thesis investigates the combined use of optical frequency combs (OFCs) and space-division multiplexing (SDM) fibers to enable cost-effective, high-capacity networks by exploiting phase-coherent carriers. It presents the first study of shared carrier phase recovery across both wavelength and spatial domains, showing that 6 master WDM channels can provide phase information for over 470 slave channels across the entire C-band, achieving a capacity greater than 40 Tb/s per core. This thesis also demonstrates the widest OFC regeneration from a transmitted seed lightwave, with the transmitter and regenerated comb spanning 134 nm and supporting over 650 carriers. In the context of high-capacity unrepeatered transmission, this thesis reports a record capacity of 16.1 Tb/s over a 363 km C-band link without the use of remote amplifiers. This was achieved by careful link design, optimization of the launch signal power, and high-sensitivity phase retrieval using coherent detection. The thesis also extends DAS to multi-mode and multi-core fibers, leveraging their spatial diversity for enhanced performance. It introduces a novel phase-coherent averaging technique, employing both spatial and longitudinal diversity to improve the signal-to-noise ratio without significantly degrading spatial resolution. Additionally, a new DAS system based on digital self-heterodyne detection is proposed for phase noise cancellation in phase-sensitive optical time-domain reflectometry. In conclusion, the research presented in this thesis advances the development of phase-retrieving coherent receivers, high-capacity unrepeatered systems, simplified OFC-based SDM networks, and DAS systems. It presents novel approaches for high-speed communications and DAS, and discusses the potential integration of DAS with coherent optical communications.