This paper reviews the history of coherent optical com-munications, the principle of coherent receivers, quantum-mechanical characteristics of coherent detection, multi-level modulation formats, and the newly developed digital coher-ent receiver, including the details of. This paper reviews the history of coherent optical com-munications, the principle of coherent receivers, quantum-mechanical characteristics of coherent detection, multi-level modulation formats, and the newly developed digital coher-ent receiver, including the details of. In fiber-optic communication systems, it is crucial for operators to accurately monitor various physical parameters along optical links to fully leverage the potential transmission capacity and conduct fault analysis. Digital longitudinal monitoring (DLM) has been intensively studied for its. Polarization-multiplexed quadrature-amplitude modulation maximizes spectral efficiency and power efficiency, by utilizing all four available DOF, the two field quadratures in the two polarizations. Dual-polarization homodyne or heterodyne downconversion are linear processes that can fully recover. Optical Fiber Communication (OFC) revolutionizes modern telecommunications, enabling rapid data transfer across long distances with minimal signal loss. This comprehensive review explores OFC's historical evolution, core principles, components, and versatile applications. It traces OFC's. Abstract—The recently developed digital coherent receiver en-ables us to employ a variety of spectrally efficient modulation for-mats such as M-ary phase-shift keying and quadrature-amplitude modulation.
