This book explores the evolution of optical communication systems into intelligent, quantum-ready networks that will underpin future global connectivity. It unifies coherent optics, Ethernet over DWDM, SDN control, machine learning, and quantum communication within a single architectural framework. Readers are guided from physical-layer fundamentals through autonomous network operation and quantum-safe security. The text introduces quantum networking, post-quantum cryptography, and digital twins as key enablers of next-generation infrastructure. Together, these concepts define the roadmap toward the Quantum-Optical Internet.