Quasi–Nondiffractive Beams for OCT–Visualization: Theoretical and Experimental Investigations

Abstract

Optical coherence tomography (OCT) is a method for imaging the internal structure of biological tissue in vivo with micron resolution. OCT has been recognized as an extremely promising tool for the diagnosis of pathological changes in biological tissue (Chauhan et al., 2001; Fercher et al., 2003). The coherence–domain range in OCT is performed by using a Michelson interferometer. By measuring singly back scattered light as a function of depth, OCT has the potential to image the structure of tissue with a high resolution and sensitivity. It is of importance for a diagnostics of structures inside a cornea, a cornea edema, for a quantitative imaging of the optic disc in glaucoma, for an evaluation of retinal thickness, etc. in ophthalmology. The high axial resolution of OCT is realized by the use of a broadband light source whereas the lateral resolution is determined by the numerical aperture (NA) of the focusing lens. Although a high NA of a conventional focusing lens in the sample arm of the imaging optical system enables high lateral resolution imaging, but low axial resolution. A low NA is required to achieve a large depth of focus, but in this case we deal with the low lateral resolution. The improvement of the axial–lateral resolution is a very important problem in imaging OCT-systems. Furthermore, a restriction of an imaging method is closely connected with a problem of tissue light–scattering: it leads to the decreasing in the focal depth of a probing beam and an image involves a “noisy”–speckle field superimposed on imaged structures as a result of scattering by a volume medium.