Theory and design of a hybrid wave-front sensor for adaptive optics

Wave-front sensors (WFS) are used in adaptive optics (AO) systems to measure real-time aberrations caused by the atmosphere. The design of a WFS is driven by three desirable parameters: linearity, high sensitivity, and a wide dynamic range. The benefit of the Hybrid WFS (HyWFS) is its wide dynamic range and high sensitivity combined with only a minimally non-linear reconstruction. This is achieved by combining a Shack-Hartmann WFS (SHWFS), which has wide dynamic range, and a Pyramid WFS (PyWFS), which has high sensitivity, into a single system. The first half of the system will resemble a typical PyWFS while the second half includes a lenslet array, typical of a SHWFS. This system will focus portions of the pupils created by the pyramid prism to points on the detector, creating an image of four spot arrays. The output of the HyWFS system has been analyzed in simulations using reconstructors based from slope measurements, raw intensity values, and a combination of the two. These simulations show that, when the applied wave-front error is small, the system benefits from using the PyWFS reconstruction. For larger applied wave-front error the HyWFS can switch to the SHWFS reconstruction, which allows for larger dynamic range than the PyWFS. With this ability to shift, the HyWFS is able to reduce the residual wave-front error by preserving the range and sensitivity without requiring PyWFS modulation or reconstruction.