Wavefront sensor noise reduction and dynamic range expansion by means of optical image intensification

Abstract

A Hartmann wavefront sensor measures optical wavefront aberrations by using a lens array to break the input sample into subapertures. The lens in each subaperture produces an image whose centroid position depends on the average phase front tilt in the subaperture. The simplest measurement implementation, a quad cell detector, is limited in tilt dynamic range to 1 wave because the (diffraction-limited) image has a radius equivalent to 1 wave of tilt. For greater tilts, the image is off-null and no positional information is available. Dynamic range can be increased by defocusing the spot but only at the expense of increased centroid variance. However, if the spot image is optically intensified before being detected, the intensified image can be defocused without increasing the centroid variance. A theoretical derivation of sensor performance with and without intensification is given. Results of simulations and laboratory experiment are presented showing good agreement with theory. The use of this technique may greatly extend the capabilities of wavefront sensor systems at modest increase in cost or complexity.