A tunable optical transceiver is a device that shares similarities in both operation and physical appearance with fixed transceivers. The basic principle of an optical transmitter involves the modulation of a light source, such as a laser or light-emitting diode (LED), to encode the. In optical transmission systems, there are three key elements: the transmitter (laser and modulator), the photodetector, and the optical transmission medium (the fiber). Typically, the detector is characterized by a level of sensitivity to impinging optical power. Therefore, we begin this chapter by reviewing the fundamentals of digital communications, including principles of modulation, channel modeling, and detection.