Importance of rotational beam alignment in the generation of second harmonic errors in laser heterodyne interferometry

We show, using a Jones matrix analysis, that rotating the two orthogonal linearly polarized inputs of a heterodyne interferometer relative to the polarizing beam splitter axes, produces second harmonic errors in an otherwise ideal interferometer. The error undergoes two cycles as the optical pathlength difference changes from 0 to 2pi. This behaviour is distinct from previously reported periodic errors resulting from polarization leakage and frequency mixing. It is found that as the angular misalignment increases, the amplitude of the periodic errors also increases, and may reach substantial values.