The effects of birefringence on performances of SOA-based interferometer devices are analyzed theoretically. The necessity of fabricating SOA whose gain and induced phase shift are both independent on polarization state is proposed. The calculations for a SOA-based Mach-Zehnder interferometer wavelength-converter indicate that the extinction ratio will decrease and become very sensitive to the polarization state of input light if there is birefringence in SOAs, the extinction ratio variation is higher than 10 dB. A scheme is proposed to eliminate these effects, and the possibility of making a SOA with both gain and induced phase shift insensitive to polarization is analyzed.