Progress on broadband control and deformable mirror tolerances in a 2-DM system

The Stroke Minimization algorithm developed at the Princeton High Contrast Imaging Laboratory has proven symmetric dark hole generation using minimal stroke on two deformable mirrors (DM) in series. We extend the concept of minimizing DM actuation to achieve symmetric dark holes in broadband light. This requires simultaneously correcting both amplitude and phase aberrations over the bandwidth. Here we address the relationship of amplitude and phase aberrations with wavelength and the implication for wavefront control and design tolerances. This drives the number of deformable mirrors, their locations in the optical path, and design constraints on the deformable mirrors. Broadband suppression is achieved experimentally by using three wavelengths to define the bandwidth of correction in the optimization problem. This windowed approach to Stroke Minimization makes the optimization in wavelength tractable and allows for estimation only at a single wavelength which reduces the number of exposures required for correction. The output of the estimation algorithm is extended to the higher and lower wavelengths by establishing a functional relationship of the image plane electric field in wavelength. The accuracy of the functional relationship will ultimately bound the achievable bandwidth, therefore as a metric these results are also compared to estimating each wavelength separately.