Closed-loop multiconjugate adaptive optics for microscopy

When imaging a sample, inhomogeneities in refractive index cause blur in the image and decrease resolution. Adaptive optics (AO) is a technique that can correct for the resulting aberrations. The most common implementation of AO uses a single deformable mirror that is conjugate to the pupil. A single pupil-conjugate corrective device provides correction over a limited field of view owing to field-dependent aberrations. To overcome this limitation, an additional specimen-conjugate deformable mirror can be used. However, adding a second reflective correction device significantly increases system complexity. We have developed a closed-loop multiconjugate AO system for field-dependent aberration correction in a confocal fluorescence microscope. A 140-actuator deformable mirror is used in the pupil plane with a custom 37-element transmissive deformable phase plate inserted in a sample-conjugate plane. Both devices are calibrated and controlled in closed-loop using a Shack-Hartmann sensor in combination with an integral control law. The sensor consists of an EMCCD and lenslet array with a 500 μm pitch and a 47 mm focal length. Results from a Drosophila ovary and HeLa cells are presented.