<title>Dynamic coherent focus for transversal resolution enhancement of OCT</title>

Abstract

One of the major problems of optical coherence tomography (OCT) is limited transversal resolution. A small focus can certainly be generated using a small confocal beam parameter. But then transversal resolution becomes severely dependent on the position of the beam focus within the object. Hence to achieve optimal transversal resolution within the whole object, the focus of the illuminating beam has to be shifted during the coherence scan throughout the object depth. To maintain coherence the optical path length of the reference beam has to be shifted by the same amount. In our experimental setup we use an optical design which makes it possible to shift the focus of the measurement beam without changing the optical path length of the reference beam. We call this technique dynamic coherent focus because a focus is created that remains coherent to the reference beam during the whole coherence scan. Using a Ronchi ruling resolution target with 200 line pairs/mm we demonstrate that the transversal resolution is better than 5 micrometer. In the cross sectional image of a human cornea in vitro epithelium and the endothelium layers are visible with a longitudinal resolution of approximately 14 micrometer and transversal resolution of better than 5 micrometers. The cellular structure of the endothelium is resolved.