A Semiconductor Optical Amplifier (SOA) is essentially a laser diode (LD) with no feedback from its input and output ports and hence is also referred to as a Traveling-Wave Amplifier (TWA). Semiconductor Optical Amplifiers (SOAs) have proven to be versatile and multifunctional devices that are key building blocks for optical networks.

There are five parameters used to characterize a Semiconductor Optical Amplifier (SOA): (1) Gain (Gs); (2) Gain Bandwidth; (3) Saturation Output Power (PSAT); (4) Noise Figure (NF); (5) Polarization Dependent Gain (PDG).

A Semiconductor Optical Amplifier (SOA) should have the highest gain appropriate to the application. A wide optical bandwidth is also desirable so that the Semiconductor Optical Amplifier (SOA) can amplify a wide range of signal wavelengths. Gain saturation effects introduce undesirable distortion to the output so an ideal SOA should have very high saturation output power to achieve good linearity and to maximize its dynamic range with minimum distortion. An ideal Semiconductor Optical Amplifier (SOA) should also have a very low noise figure (the physical limit is 3dB) to minimize the amplified spontaneous emission (ASE) power at the output. Finally, an ideal Semiconductor Optical Amplifier (SOA) should have very low polarization sensitivity to minimize the gain difference between the transverse-electric (TE) and transverse-magnetic (TM) polarization states. However, an ideal Semiconductor Optical Amplifier (SOA) is impossible to realize because of the physical limitations of the various processes taking place within it.