This book will highlight the motivation for coherent optics in access and introduce digital coherent optical system in detail, including advanced modulation formats, architecture of modulation and detection, and DSP flow for both transmitter and receiver. This book will also demonstrate potential approaches to re-design and re-engineer the digital coherent concept from long-haul and metro solutions to the access network, leveraging reduction in complexity and cost as well as the benefits of capacity increases and operational improvements. This book will illustrate the details on optimization of the digital, optical, and electrical complexity and standardization and interoperability.