Phase-shifting interferometry with a reflective diffraction grating for adaptive optics

A phase-shifting interferometry method to measure the wavefront reflected from a deformable mirror is described. Usually, an adaptive optics system encompasses a deformable mirror and a Shack-Hartmann wavefront sensor as the main components. However, an interferometric method is suited for calibration and traceability purposes of such measurements. In this regard, we developed a system able to characterize the wavefront in situ with both a Shack- Hartmann sensor and a phase-shifting interferometer. The method encompasses a reflective diffraction grating, where the plus-first diffraction order reflected off the grating was incident on a deformable mirror. This beam was then reflected back to the grating where it interfered with the incident beam and the interference pattern was collected on the minusfirst order. As with the four-step phase-shifting interferometry method, four interference patterns were recorded and processed to extract the wavefront information. The wavefront sensor recorded the wavefront directly reflected from the deformable mirror, via a beam splitter.