Figure 2.

Lissajous trajectory of the data sets measured at two photodetectors, when the optical path-length difference (OPD) changes by 2$\pi$ in Figure 1. $a$ and $b$ are the AC amplitudes of the interference signals, and $L_a$ and $L_b$ are the semi-major axis and semi-minor axis of the ellipse; $\alpha$ is the rotating angle.