We present an investigation of the polarization dependence of THz air-biased coherent detection (ABCD) on the polarization directions of fundamental waves. The intensity, ellipticity, and polarization angle of the THz-field-induced second-harmonic are affected by the air plasma birefringence and can be predicted by cross-phase modulation theory. Due to the plasma birefringence, the polarization state of THz waves cannot be determined by the polarization state of the second harmonic with the traditional polarization measurement method. It would be inaccurate to measure arbitrary THz-wave polarization in a single scan with THz ABCD when the birefringence effect is significant.
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