Non-Gaussian beam profiles such as Bessel or an- nular beams enable novel approaches to modifying materials through laser-based processing. In this review paper, proper- ties, generation methods and emerging applications for non- conventional beam shapes are discussed, including Bessel, an- nular, and vortex beams. These intensity profiles have important implications in a number of technologically relevant areas includ- ing deep-hole drilling, photopolymerization and nanopatterning, and introduce a new dimension for materials optimization and fundamental studies of laser-matter interactions.

Bessel and annular beams for materials processing

A laser scanning optical system is so arranged to comprise an optical converting unit, an optical scanning unit, and an optical converging unit. Here, the optical converting unit is composed of two axicon prisms which are arranged such that apexes thereof are opposed forward or backward to each other at a predetermined distance and optical axes thereof are coincident with each other and which are made of respective materials having a same refractive index and shaped with a same apical angle. By this, the laser beam incident into the optical converting unit becomes a cylindrical ray bundle having an annular cross-sectional intensity perpendicular to the optical axis. Therefore, the cylindrical ray bundle is changed in the traveling direction by the optical scanning unit to be scanned and is then converged by the optical converging unit to become a Bessel beam having a high energy utilization factor, a high resolution and a long focal depth.

Laser scanning optical system and laser scanning optical apparatus

The imaging properties of a scanning optical system that incorporates an axicon are presented. Beam-shape characteristics including the axial distribution of the J0 beam and its control and aberration effects arising from off-axis illumination are experimentally studied. These parameters are relevant when the axiconis used in an imaging system operating in the beam-scanning mode. The J0 pattern produced by a blazed axicon transmittance grating is also presented.

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Imaging properties of axicon in a scanning optical system

The geometric law of energy conservation is utilized in evaluating the phase transmittance function for axicons with arbitrary distribution of the on-axis intensity. Several simple analytical solutions are presented, and a computer-generated holographic version of the uniform-intensity axicon is examined.

Nonparaxial design of generalized axicons

The first article by Turunen and Friberg deals with a class of fields with propagation-invariant properties such as the optical intensity distribution. Coherent and partially coherent stationary and pulsed solutions are treated in view of scalar and electromagnetic approaches. Approximations of ideal propagation-invariant fields and methods for their generation are discussed. Finally, some application areas are covered.

Propagation-Invariant Optical Fields

New optical combinations of axicons and axicons with spherical mirrors and lenses suitable for laser machining are presented. Linear and annular focusing, coaxially and radially to the laser beam, are possible. Most combinations allow continuous adjustment of exit beam parameters, focal line length, focal ring diameter, and magnification, by varying the relative position of one of the axicons. Potential new laser applications are also discussed in relation to these optical devices.

Marc Rioux

Papers

Linear, annular, and radial focusing with axicons and applications to laser machining