Components and Subsystems

End‐to‐end wavelength division multiplexing (WDM) transmission consists of a transmitter, a transmission channel, and a receiver. The transmission channel in all relevant WDM applications consists of a silica single‐mode fiber (SMF). This chapter describes the different SMF types, and discusses the several functional stages of WDM transmitters and receivers. It focuses on the photodetectors (photodiodes) and the associated active electronics. In high‐speed WDM transmission, almost exclusively photodiodes are used as photodetectors. The chapter considers the commonly used PIN photodiodes and avalanche photodiodes (APDs). In order to increase the transmission distance or to allow other transmission impairments, forward error correction (FEC) can be applied to the signal. FEC allows the operation of the optical transmission at a higher BER, while maintaining payload data integrity. Electronic distortion compensation (e.g., CD compensation) is an alternative to analog optical compensation.