3. Communication chain

The purpose of a communication chain (sometimes referred to simply as a "com chain" in this article) is to transmit information from a starting point to an end point. It consists of a transmitter and a receiver. It generally uses a carrier (electromagnetic wave), which it modulates at the transmitter and demodulates at the receiver. For optical communications, the carrier is a laser wave whose spectrum can theoretically range from the ultraviolet (a few tens or hundreds of nanometers) to the far infrared (several tens of micrometers). The advent of fiber-optic communications in the terrestrial domain around 1,550 nm has led to the development of an ecosystem of sources, detectors, amplifiers and other multiplexers in this region of the optical spectrum. For optical space communications, it was decided to start from these wavelengths, in order to benefit from this ecosystem, even if some...