Picometer wavefront sensing using the phase-contrast technique

Precision wavefront sensing and interferometry are essential in many fields of industry and fundamental research. Characterization of semiconductor devices, optics in lithography systems, and biologic features of living cells all require measurement resolution at the nanometer level. The field of high-contrast imaging in space-based astronomy has pushed wavefront sensing requirements to a new regime with current and future concepts requiring sensitivity on the order of 10 pm. Techniques to achieve this level of precision have been demonstrated, but require large, expensive instrumentation with custom light sources, and therefore do not provide a solution for in-space operation. Here we demonstrate experimentally the ability to detect picometer-level wavefront errors at spatial frequencies limited only by the pixel count of the sampling detector using a simple, inexpensive method. The system is based on the Zernike wavefront sensor (ZWFS) that implements the phase-contrast technique whereby the DC portion of an optical wavefront is phase-shifted with respect to its higher spatial frequency components. In our demonstration, a highly repeatable deformable mirror is used to introduce phase variations into an optical path. We readily sense 60 pm RMS changes in wavefront errors with the ZWFS system with measurement repeatability on the order of 0.6 pm. This technique is an enabling technology for future astronomy missions; however, there are widespread applications to many other fields requiring high-precision interferometry.