Phase-shifting methods for interferometers using laser-diode frequency-modulation

Laser diode interferometers are of particular interest for metrological applications since the optical-frequency dependence on the injection current can be used to produce modulations in the interferometric phase. By this way, the optical path difference can be evaluated accurately without being modulated. However, injection current variations induce output intensity changes which prevent the direct application of phase-shifting algorithms. In this paper, we show that a corrected fringe pattern can be reconstructed, by considering pairs of output intensities with a relative phase-shift multiple of 2 pi. Subsequently, the assumption of constant background intensity and fringe visibility is satisfied and any classical phase-shifting algorithm can be applied. Only larger sets of phase-shifted intensities need be considered. The proposed method is validated by the application to experimental data.