Abstract: The two important characteristics of semiconductor optical amplifiers(SOA),gain and polarization,are introduced completely.In the section of gain characteristic,the phenomena of gain modulation and gain saturation are described.The methods to enhance the gain of SOA are presented.In the other section,the reasons resulting in polarization sensitivity of SOA are analyzed and the way to perform SOA with low gain polarization sensitivity is discussed.At last,the phenomenon of the phase polarization sensitivity in SOA paid much attention in recent years is reported.