This chapter outlines the key technologies required for long-distance optical fiber communication systems. In the optical transmitter, the semiconductor laser driven by the electrical signal converts it to an optical signal. Optical modulators may also be used for generating optical signals. The transmitted optical signal is attenuated by losses in the optical fiber, and the optical repeater restores the optical signal level to its original state before sending it to the next optical fiber section. Optical repeaters include regeneration repeaters, which reproduce optical signals through optical-electrical-optical conversion, and optical amplifier repeaters, which amplify optical signals directly in the optical domain. On the receiver side, the transmitted optical signal is converted into an electrical signal by the photodetector, and the TDM demultiplexing circuit demultiplexes to the low-speed electrical signals, transmitting the information to the destination.