**2. Past history and basic concept of reflective SOA**

In this section, the fundamental properties and the main concepts of SOA and RSOA are introduced in a simple way. We discuss the past history and the evolution of RSOA with the development of optical communication systems. Then, we detail the physics involved in SOA and RSOA. A multi-section approach is chosen to model the device behaviour under static conditions. The aim of this section is not to propose a complex model but to underline and understand the different mechanisms in RSOA devices.
