PHASE-SHIFTING METHODS FOR INTERFEROMETERS USING LASER-DIODE FREQUENCY-MODULATION

Laser diode interferometers are of particular interest for metrologica l applications since the optical-frequency dependence on the injection current can be used to produce modulations in the interferometric pha se. By this way, the optical path difference can be evaluated accurate ly without being modulated. However, injection current variations indu ce output intensity changes which prevent the direct application of ph ase-shifting algorithms. In this paper, we show that a corrected fring e pattern can be reconstructed, by considering pairs of output intensi ties with a relative phase-shift multiple of 2 pi. Subsequently, the a ssumption of constant background intensity and fringe visibility is sa tisfied and any classical phase-shifting algorithm can be applied. Onl y larger sets of phase-shifted intensities need be considered. The pro posed method is validated by the application to experimental data.