Generation of Bessel type beams via phase shifted axicons encoded in geometrical phase elements

Abstract

Zeroth order Bessel beams are widely used in laser micromachining of transparent materials. The small diameter of central core and elongated focus enables to generate high aspect ratio voids. The simplest way to generate this beam is to induce a conical shape phase with an axicon. However, the quality of the axicon tip is very crucial to generate smooth Bessel beams since it is known that a blunt axicon tip induces large intensity modulation in propagation direction. Alternative Bessel beam generation method is to use a Diffractive Optical Elements (DOEs) that do not suffer from previously mentioned problem. In this work we demonstrate generation of a zeroth order Bessel beam with Geometric Phase Optical Elements (GPOEs) (manufactured by Workshop of Photonics) acting as a diffractive beam shaping element. Having absolute control of induced beam phase, we have modified mask phase so that half of it had additional phase shift or spatial transposition resulting in creation of fanciful induced beam phase patterns. With the use of laser beam propagation numerical modeling we show that these new phase masks can create various beam transverse intensity patterns such as asymmetrical central core, generation of multiple peaks or even large rings that are highly demanded for various laser micromachining applications. We have chosen couple of most perspective beam shapes and manufactured GPOEs to generate them. The experimentally generated beams were compared to numerical simulations. As the GPOEs are able to work with high power pulses we have also investigated induced transparent material modifications.