Flexible conjugate adaptive optics with a refractive wavefront modulator

Diffraction-limited imaging in microscopy is only possible if different layers within the objective's working distance all have an uniform refractive index. However, in many practical imaging problems the samples are inhomogeneous in optical density, and refraction through them introduces field-dependent wavefront aberrations. On the image plane these are manifested in varying degrees of resolution and contrast degradation across the field-of-view. In pupil adaptive optics (AO), where a wavefront modulator is accommodated at the objective's pupil-plane, a single correction profile is applied for all fields, and the correctable field-of-view (FoV) is limited by the isoplanatic patch. In the alternative configuration of conjugate adaptive optics, where the corrective element and the dominant aberrating layer within the sample sit at optically conjugate planes, effective correction across the entire FoV is possible in principle. This configuration, however, is relatively difficult to implement with deformable mirrors for wavefront modulation, since several folded optical paths have to be constructed. In this work, we present the design and evaluation of a completely in-line conjugate-AO system based on a refractive wavefront modulator (DELTA7 from Phaseform GmbH, Germany). We evaluate the performance of the system on a Zeiss Axiovert inverted microscope by imaging fluorescent beads through custom-fabricated phase plates.