This document discusses optical fiber communication systems across five units. Unit I introduces optical fiber communication systems and basic optical concepts including modes, fiber materials and fabrication techniques. Unit II covers transmission characteristics such as attenuation, dispersion, and fiber properties. Unit III focuses on optical sources like LEDs and lasers as well as detectors. Unit IV examines optical receivers, measurements, coupling, and digital signal transmission. Finally, Unit V reviews system design, passive and active components, networks, and interfaces.