Phase-shifting interferometry by wave amplitude modulation: General case

A phase shifter for phase-shifting interferometry by amplitude modulation is presented. Based on the interference of three beams with one of them used as probe and the other two used as references, resulting from their combination a reference beam, it can be modeled as the interference of two beams. We show that if an arbitrary phase difference different from mπ between the two references is considered to be constant; the resulting beam can be modulated in amplitude and phase by varying amplitudes-only from the references, which implies a change in the contrast and a phase-shift in the interferogram, simultaneously. In order to keep a constant contrast while the phase-step is varying as it is required for phase-shifting interferometry, being that a condition for phase-only modulation, we show that the amplitudes seen as ordered pairs in a coordinate system must obey to the equation of an ellipse. A theoretical model and some numerical simulations will be shown.

► Phase shifting by modulating two reference beams in an interference of three.
► The three-beam interference can be modeled as an interference of two.
► The additional phase term and one of the beams are composed by the two references.
► To keep a constant visibility, these references obey to the equation of an ellipse.