Alignment of master and sample in comparative digital holography

Abstract

A comparative digital holography system suitable for shape and deformation comparisons between master and sample objects with rough surfaces is described. The innovative aspect of comparative digital holography is the illumination of the sample by the conjugated wavefront of the master, as a type of coherent mask, using a liquid crystal display (LCD). The resulting interferogram indicates directly the shape or the deformation differences between the master and sample. As it is not necessary that both objects to be compared are located at the same place for this technique, remote shape or deformation comparison between a master and a sample is possible. A current research topic is the precise alignment of the sample and the reconstructed master wavefront so that the resulting phase map only contains information of the differences in shape or deformation. The reconstructed master wavefront can be adjusted digitally to correctly illuminate the sample object, by introducing an artificial phase-shift. This phase-shift is induced by the LCD, and offers also the possibility of calibrating precisely the set-up. The value for the phase-shift is obtained by a comparison of the resulting interferogram with a database containing fringes from simulations of misalignments between master and sample objects. Using the iterative algorithm described here, the correction of the sample position can be controlled by an automatic adaptation of the coherent mask.

Remote laboratories for optical metrology: from the lab to the cloud

“…Using the SLM, misalignment between the master and the sample can be compensated for by a corresponding phase shift of the reconstructed master wave front. 63 A comparison of Fig. 7(c) with 7(d), the result of conventional holographic contouring, shows the advantage of comparative digital holography for shape comparison: only the difference in shape of two nominally identical objects 9 Remote Laboratory for Digital Holographic Microscopy In this section, we describe the functional implementation of the architecture focusing mainly on the setup of the holographic system for digital holographic microscopy and the configuration of the remote controlling (i.e., the "rig" in Fig.…”