Long-range displacement measurements using sideband modulation method

In this work, the concept for a long-range displacement measurement with a possibility of up to 100 mu m with sub-nanometre uncertainty is introduced. In the proposed method, the displacements are calculated by use of frequency change measurements of the stabilized laser using a differential Fabry-Perot interferometer (FPI) and a sideband modulation technique. In the work, an overall displacement measurement of up to 4 mu m was performed with a combined uncertainty of 0.3 nm, in which the fractional part of the displacement was obtained by use of the sideband frequency technique, whereas the integer part of the displacement was calculated by the fringe counting of the FPI resonances. The suitability of the proposed method for long-range measurements up to 100 mu m with sub-nanometre (0.6 nm) combined uncertainties is discussed.