**3.1 Amplification**

There are several applications of SOAs in today and the future analogue and digital optical networks. The optical losses are compensated by optical amplifiers (OAs), it is well used in loss limited systems as post, in-line or pre-amplifier. The SOA is a semiconductor based, small size, compact, low cost, current driven device, which amplifies the incoming optical signal directly in the optical regime, without any optical/electrical conversion. Moreover, the semiconductor technology offers a wide flexibility in the choice of the gain peak wavelength by just appropriately choosing the material composition of the active layer. Another key advantage is that these devices can be integrated with other active or passive optical components to generate more complex functionalities. Finally, they are potentially cheap, thanks to the mature technology basis.
