Characteristics of a laser beam spot focused by a binary diffractive axicon

Bessel beams formed by an axicon are nondiffracting beams. Such beams are used in the field of laser machining because they have several desirable characteristics, including a micron-sized focal spot and a deep focal depth. Conical lenses are generally employed for producing Bessel beams. A binary diffractive axicon was made by electron beam fabrication; the first-order diffracted beam from this axicon has a similar optical field to that of a refracted beam from a conical lens. The characteristics of a laser beam spot focused by the axicon were investigated. We demonstrated theoretically and experimentally that the peak intensity on the optical axis oscillated with a larger amplitude as the spot size decreased when the beam spot was observed in the vicinity of the axicon. This phenomenon was well explained by taking higher-order diffraction into account. We also investigated the wavelength dependence of the spot size focused by the axicon, and found that the spot size was constant.