Direct demodulation for alternative Shack-Hartmann alignment

Abstract

The Shack-Hartmann wavefront sensor is a widely-used device to measure the light wavefront. Cur- rently, the sensor is used as a gradient sensor, which is achieved by placing the microlens array in the plane conjugate to the deformable mirror and the aberration. The resulting spot pattern is then transformed into a gradient through the use of centroiding: the measurement of the relative movement of all spots compared to the reference image. As an alternative to the traditional alignment, alternative alignment moves the Shack-Hartmann wave- front sensor: the CCD is now optically conjugated to the deformable mirror and the aberrations, and not the gradients but the curvature of the wavefront is measured. This changes the behaviour of the Shack-Hartmann sensor: instead of the amount of spots remaining the same and the CCD region oc- cupied by them changing, the amount of spots starts to change but the CCD region occupied by them remains the same in the face of aberrations. Curvature measurements can be used directly to actuate bimorph and membrane deformable mirrors, bypassing the need for complex control schemes. As an alternative to centroiding Fourier demodulation can also be used to convert the information measured by the wavefront sensor into gradients. For large aberrations where the spots stray out of their subapertures and the centroiding algorithm breaks down this method is especially suited. In this thesis it is shown that a synergy exists between alternative alignment and Fourier demodulation. Large aberrations no longer occupy a huge region on the CCD, instead changing the amount of spots, and the movement of the spots outside of their subapertures is no longer a problem as there is no need for rigidly defined subapertures. The combined use of alternative alignment and Fourier demodulation was tested on simulations, both for square and circular apertures, and for cases with extra added Poisson noise. Finally real-life experi- ments were also conducted using circular apertures.