Semiconductor optical amplifiers (SOAs) are increasingly used for optical signal processing applications in all-optical integrated circuitry [1, 2]. Research on SOAs started just after the invention of semiconductor lasers in 1962 [3]. However, it was only after the 1980s that SOAs found widespread applications [4, 5]. The effectiveness of SOAs in photonic integrated circuits results from their high gain coefficient and a relatively low saturation power [6, 7]. In addition, SOAs are often used for constructing functional devices such as nonlinear optical loop mirrors [8, 9], clock-recovery circuits [10, 11], pulse-delay discriminators [12–14], and logic elements [15, 16].

A semiconductor, as its name implies, has a conductivity in between that of a conductor and an insulator. Some examples of elemental semiconductors include silicon, germanium, selenium, and tellurium. Such group-IV semiconductors have a crystal structure similar to that of diamond (a unit cell with tetrahedral geometry) and the same average number of valence electrons per atom as the atoms in diamond. Compound semiconductors can be made by combining elements from groups III and V or groups II and VI in the periodic table. Two group III–V semiconductors commonly used for making SOAs are gallium arsenide (GaAs) and indium phosphide (InP). These semiconductors enable one to manipulate properties such as conductivity by doping them with impurities, and allow the formation of the p–n junctions required for the electrical pumping of SOAs.