Absolute distance measurement

Precision Experiments

Most of the length measuring interferometers are working incrementally. Thus beam interruptions are causing errors in distance measurement. This interruptions might happen accidentally by parts moving through the beam or on purpose when the distance to different mirrors shall be measured. To afford distance measurements with possible beam interruptions an absolute length measurement is needed.

This shall be reached by white-light interferometry. The setup is based on a Michelson interferometer and uses a red LED as short coherent light source. Due to the short coherence an interferogram is only visible where both interfering beams have passed the same distance. In one path the beam is reflected by a mirror, which is placed on a micrometer stage. In the second path the beam is reflected by a blazed grating, which works in this case as a stepped mirror. This means that different parts of the beam reflected by different steps of the grating have travelled different distances. By tilting the grating continuous optical distances in a range from 10 to 17 mm are reached. The grating is imaged onto a detector (CCD) under Scheimpflugs law .

In the post-processing first of all the column with the interferogram has to be found. This is done by subtraction of the mean and following summation of the squares for each column. With this technique we get the maximum values only for periodic signals like the interferogram. To find the centre of the interferogram the Hilbert transformation with following phase evaluation is used. To get an absolute distance between the beam splitter and the movable mirror this setup needs to be calibrated. Therefore the position of the micrometer stage is measured once by the Michelson white-light interferometer and at the same time with an incrementally measuring interferometer by Sios. First calibration results and following measurements have shown a repeatability of 700 nm up to 1500 nm.

Braunschweig IGSM
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