Enhancement of extended depth of field by digital focusing
With the use of a Bessel-like illumination beam produced by an axicon lens an extended depth of field can be achieved. The resulting illumination pattern exhibits a strong central lobe extending over 300um along the optical axis without diffraction. Here we present a fast digital focusing algorithm to correct for remaining out of focus artifacts, manifest in sidelobes for out of focus scatterers, due to the decoupled Gaussian detection. The waist of the detection in the focal plane is 2.1um, with a Rayleigh-range of 35um. The FDOCT data is Fourier transformed in the lateral spatial directions. Passing from the spectral (K) to the spatial (Z) domain, the correction term is implemented in a chirped z- transform algorithm, performing a resampling in the K domain. Finally the data is transformed back in the lateral direction to spatial coordinates. The illumination scheme provides a tightly focused spot over the whole axial extent of the illumination needle, providing the high signal to noise ratio necessary to correct for the diffraction effects of the detection. Signals from scatterers multiple Rayleigh distances away from the focal plane are recovered with minimal loss in resolution. With this digital focusing algorithm the performance of the physically extended depth of field is further enhanced, presenting a uniform high lateral resolution of 1.3um, corresponding to an effective numerical aperture of NA=0.24, over more than 300um in the axial direction.
Record created on 2008-02-26, modified on 2016-08-08