Interferometric calibration of piezocapacitive displacement actuators

A calibration set-up based on optical heterodyne interferometry with sub-nanometric resolution for displacement measurements is described. For minimising the effects of thermal drifts and periodic non-linearity of the interferometer, the optical path difference is changed in steps of integer lambda for a full scan loop of the actuator. Accurate h-steps are obtained by means of a computer-based loop control of the out-of-phase between the reference and the measuring signal of the apparatus. A combined standard uncertainty of about 0,7 nm + 0,7 . 10(-4) L was estimated for the measured displacements of the actuator, corrected by a quadratic polynomial fit. The actuator has then been used to test the non-linearity of the differential interferometer. A sinusoidal non-linearity of amplitude 1,5 nm and period h was observed with an optimised optical set-up.