When light is focussed by a lens or a spherical mirror we assume that we can focus down the light into a very small spot, unfortunately can we not achieve such a small spot, because of imperfection of the lens surface and curvature. By detecting the wavefront of light we can see how much imperfection we are adding to the light and can try to reshape the wavefront by using a deformable mirror to compensate such imperfections. This technique is called adaptive optics and it consists of two main components, the wavefront detector and the wavefront corrector.
The wavefront detection can be a camera with a lenslet array in front of it, this lenslet array will create multiple foci on the camera and by using the orientation of the foci the wavefront can be reconstructed. The wavefront corrector or deformable mirror is a mirror, which can change its surface to compensate for the measured wavefront deformation and trying to regain a small and clean focus spot.
Adaptive optics is used in many different regimes, such as astronomy and their telesopes or microscopes to detect very small samples. In optical coherence tomography its main purpose is to create a nice and small focus at the back of the eye. The human eye consists as well of an lens, which is unfortunately not perfect and makes it very difficult to create a small focus, by trying to compensate for these imperfections we can see smaller things, like the structures that enables us to see and distinguish colours.
The effect of adaptive optics is shown with a 50 micrometre spaced grid. The lines of the grid appear much sharper and thinner when using the wavefront correction (right images) compared to without correction (left image).
Written by: Felix FLEISCHHAUER