Stitching interferometry: how and why it works

Laser MegaJoule and other large optical systems require the metrology of large components. MegaJoule's tilted amplifying slabs, for example, have a diagonal size of approximately 800 millimetres, thus requiring a (very) large interferometer. Large interferometers, 600 millimetres diameter and above, are expensive. This is due, in part, to the large collimator and reference plate required. These parts, being large and heavy, also generate mechanical stability problems. Stitching interferometry is a method by which large optical components are analysed using a standard "small" interferometer. This result is obtained by taking multiple overlapping images of the large component, and stitching these "subapertures" together. Previous papers have dealt with the subject(1,2) Obviously, stitching must bring some of error. But which error, and how much ? In this paper, we start with a graphical explanation of the stitching principle. We then briefly introduce the Power Spectral Density (PSD), and explain its relation to the lateral scale of defects. We can then move on to the main part, which is the analysis of the errors that subaperture stitching generates in interferograms. We show that the PSD is well suited to the description of such errors. Indeed, one unique number certainly does not convey the amount of information to be found in a complex 2D image. We conclude that the PSD, the practical usefulness of which is materialised by its inclusion in international standards (ISO 10110 - part 8), finds another use in the characterisation of stitching interferometry. The development of the stitching interferometer was performed under contract from CEA-DAM (CEA-LV), as part of the Laser MegaJoule development.