We investigated the focusing properties of a radially polarized multi gaussian beam through an annular high Numerical Aperture (NA) objective lens based on vector diffraction theory. It is observed that our proposed system can generate sub wavelength focal spot segment with long focal depth (26λ) is achieved. This kind of non diffracting focal segments is called as dark channel, which may have application in atom trapping, nanolithography, trapping and manipulating particles.
