A heterodyne interferometer constructed in an integrated optics and its metrological evaluation of a picometre-order periodic error

It is known that the periodic error of heterodyne interferometers limits the accuracy of measurement to nanometre order. It is mainly derived from frequency mixing in the interferometer optics and this can be resolved using the configuration with two spatially separated beams. However, there remains a periodic error associated with parasitic reflections. Recently, we have developed a differential single-path heterodyne interferometer with two spatially separated beams, which was constructed within a single small quasi-cubic optical unit to minimize the parasitic reflections. In this work, the residual periodic error of this interferometer was evaluated with two evaluation methods, 'the amplitude method' and 'the subtraction method', and their results were compared. By applying uncertainty estimation procedures to the results, these two methods were shown to be highly consistent, confirming the reliability of our evaluation results. The error in λ/2 period of this interferometer was 7.6 pm.