In the ever-growing world of optical telecommunications. Semiconductor Optical Amplifiers (SOAs) are called to play an increasingly important role. Their attractive performances both in the linear and non-linear regime along with their reduced cost and small size compared to Erbium-doped Fiber Amplifiers (EDFAs) render them extremely appealing for metro and area optical networks, where cost and size are of utmost importance. We have carried out a comprehensive study of several amplifiers (from standard and gain clamped to new amplifiers with distributed electrodes for non-uniform current injection) with novel configurations by means of two independent experimental setups providing complementary information on both the static carrier distribution along the active laver of the devices (spatially resolved spontaneous emission collection and analysis) and their transient response (cross gain modulation with 150fs of temporal resolution). One of these new configurations, called Optical Speed-up at Transparency (OSAT), was proposed by our group in 2000 and consists of a high power assist MN' beam injected near the amplifier transparency. As we shall demonstrate, this scheme copiously enhances the performances of SOAs on all critical issues: acceleration of the response of the amplifier reducing its gain recovery time, larger saturation output power allowing for an extended operation in a linear regime and reduced noise figure. One striking outcome of the OSAT principle further revealing its full potential emerges when applied to a gain clamped amplifier. These devices exhibit improved amplification stability through the presence of an internal lasing mode offset with respect to gain maximum but suffer from relaxation oscillations arising from the coupling between the internal lasing mode and carrier density just like semiconductor lasers do. These disturbing oscillations fade away once the assist light is ted into the amplifier. entailing an ultrafast (13ps of record gain recovery time) and highly stable device. Our experimental findings regarding carrier distribution and dynamic behaviour are sustained by a powerful simulation code solving the position dependent rate equation for the carrier density coupled to the propagation equations for all optical signals present within the device including amplified spontaneous emission. The future of semiconductor lasers and amplifiers has been enlightened by the appearance of brand-new devices based on Quantum Dot structures. We have carried numerical and experimental study of the inherent relaxation oscillations in the time-domain in a two-state lasing QD laser revealing an extremely slow modulation response with a period of the order of ins. Although this low dimensionality technology is still tender, it definitely holds the key to the evolution of ultimate all-optical telecommunications networks.