Focus-extension by depth-encoded synthetic aperture in Optical Coherence Tomography.

We present a novel method to extend the depth-of-focus of Optical Coherence Tomography (OCT). OCT is an interferometric imaging technique that provides depth-resolved scattering information. The axial resolution in OCT is provided by the coherence gate and is invariant over the full image depth. The lateral resolution is determined by the beam parameters such as wavelength and numerical aperture. The Rayleigh range determines the depth range over which the lateral resolution can be maintained. The lateral resolution is often sacrificed to maintain relatively long Rayleigh range. In this study, we propose to use a depth-encoded synthetic aperture detection scheme to extend the depth range over which a sharp focus can be maintained beyond the Rayleigh range. An annular phase plate is inserted into the light path in the sample arm, which gives rise to three separate images in a single B-scan, corresponding to three different optical path length encoded apertures. These three images are coherently summed after phase-manipulation to reconstruct a new image with a lateral resolution that is maintained over a five times larger depth range.