Extended wavefront reconstruction for quadri-wave lateral shearing interferometry

Quadriwave lateral shearing interferometry (QWLSI) is a well-defined and robust wavefront sensing technique used in various fields such as laser beam qualification, lens or surface testing, bio-imaging, and other advanced optical systems. Despite its strengths, a notable limitation is its inability to measure the edges of the optics because of the non-interfering region in the shearing structure. We present a newly developed method for obtaining extended-aperture wavefront information using QWLSI. The proposed method involves advanced algorithms that analyze the interference patterns from QWLSI using the inverse shearing equation to create four sub-aperture wavefronts up to the edge. These wavefronts are stitched to reconstruct the input wavefront. This technique was successfully applied in testing of a 1.2 m diameter aspheric mirror using QWLSI with infrared laser at 10.6 μm wavelength. The measurement error outside the interferogram area was less than 0.2 μm rms compared to that of the commercially visible interferometer. This error is sufficiently small, supporting the application of this method to the measurement of surfaces during the grinding and early polishing steps. This feature is particularly useful for the large optics, such as mirrors for Giant Magellan Telescope and Extreme Large Telescope.