Interferometric calibration of microdisplacement actuators

The interferometer set-up in use at IMGC for the calibration of microdisplacement actuators is driven by a computer-based loop control of the out-of-phase between the reference and the measuring signal of the interferometer. In such a way, the displacements are changed in steps of lambda of the optical path difference (OPD) and the periodic non-linearity error of the heterodyne interferometer is minimised. Nevertheless, with a double-pass optical configuration a few sampling points are available for the calibration of small displacements in the sub-wavelength range. The non-linearity has a period of lambda in terms of optical path change and is due to optical mixing of the two beam frequency components in the two arms of the interferometer. Some methods have been proposed to. compensate the optical non-linearity. An approach based on fast phase-shifting to zero-fill the phase difference of the heterodyne beat signals at each subwavelength displacement to be measured, is discussed in this contribution. Use is made of a transverse electro-optic modulator for phase shifting the path of the measuring beam of the heterodyne interferometer. The preliminary results show a significant reduction of the non-linearity in the full sub-wavelength range.