Correction of defocused images in full-field optical coherence tomography using digital holography

Abstract

We propose the digital holographic technique that can mathematically reconstruct the distorted two dimensional en-face images obtained with full-field optical coherence tomography (FF-OCT). As a powerful biomedical imaging modality, FF-OCT provides inner microstructure images of a biological sample noninvasively but with a submicron depth resolution. The main advantage of the FF-OCT over other OCT techniques is that, it requires only depth scanning (C-scan) without any transverse mechanical scanning (B-scan). In a FF-OCT system based on a Michelson interferometer, not only the length of the reference arm should be matched with the length to the imaging plane in the sample arms, but also the focal plane of the system should be matched with the imaging plane. When the sample has a very high refractive index than the surrounding medium, in which the reference mirror is immersed, the mismatch between the imaging plane and the focusing plane becomes a severe problem and results in degradation of OCT image. In this study, we confirm the existence of the imaging and focusing plane mismatch problem in the FF-OCT system, and propose the method that can retrieve the focused image from a defocused image with the help of digital holography. One of the major advantages of the proposed technique is that it does not require any mechanical movement for refocusing. Only numerical calculation based on the Fresnel diffraction theory is enough. The performance is demonstrated with the image of the USAF resolution target. The image of the chromium coated pattern on the target was blurred with the existence of the glass substrate, when the OCT image was taken through the substrate. The blurred image was digitally corrected to get on the focused clear image of the pattern.