Light beams generated by oblate-tip axicon

Abstract

The attention is focused on the real shape of the tip of the axicon, which is not sharp but rather oblate. The imperfect axicon with rounded tip approximated by a hyperboloid is considered, and the properties of the beam generated in far field behind such an axicon are analysed theoretically and experimentally. It has been demonstrated that if the axicon tip deviates in its apex from the ideal sharp tip in the range of tens of micrometers, the transversal intensity distribution is strongly oscillatory. Meanwhile, the ring width (area, within which normalised intensity is larger than 0.5) is smaller as compared with the case of ideal axicon. These oscillations result from the interference of parts of the incoming beam propagating through the round, lens-like axicon tip and the conical surface surrounding the tip. It is shown that the periodicity of oscillations depends on the parameter of bluntness: if this parameter increases the periodicity of oscillations (as well as the radius of the center of the light ring) decreases, and their amplitude increases. The method for determination of the bluntness of real axicon is proposed and tested. Obtained results can be useful for correction of characteristics of conventional axicons.