Coherent optics is expanding beyond traditional long-haul networks into metro, data center interconnect, fiber access and even space-based satellite communications, driven by AI workloads and bandwidth demand. tion assisted by digital signal processing (DSP). The objective of this tutorial chapter is to briefly review the operating principles of state-of-the-art ong-haul coherent optical communications systems. Due to limitations in space, it focuses mainly on coherent optical systems usin major. Short-reach transmission systems traditionally utilize intensity modulation (IM) at the transmitter and direct detection (DD) at the receiver due to their cost-effectiveness, small footprint, and low power consumption. It traces OFC's. The higher receiver sensitivity and enhanced tuning ability theoretically provided by coherent techniques offer the prospect of significantly improving upon the performance of present direct intensity detection single-mode optical fiber systems.
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