Polarization response of the integrated optics beam combiner (IOBC). (a) In an ideal scenario with no birefringence in the IOBC, any input linear polarization state remains preserved, as shown with blue and red arrows. The electric fields combined interferometrically at the output, either Out 1 or Out 2, are identical, so no polarization-induced null degradation occurs. (b) In practice, when birefringence is present in the IOBC, input linear polarization transforms into different elliptical polarization states, as shown in Out 1 and Out 2, if it is not aligned with the waveguide or optical path’s fast or slow axis, which can cause potential null degradation.