| Abstract [eng] |
Invariant Bessel beams have greatly attracted interest in transparent material microprocessing applications. The small central peak and elongated focal region of zeroth order Bessel beam provides advantages in machining high aspect ratio volumetric media modifications. One of the mostly used optical elements for such beam generation is an axicon. However, the beam properties critically depend on quality of the optical element itself and any physical irregularities cause in worsening of the beam shaping performance. There are alternative ways of producing Bessel beams, e.g. diffractive optical elements (DOEs) that overcome fabrication problems of glass-made axicons. In this study, the geometric phase optical elements (GPOEs) are demonstrated as high damage threshold polarization sensitive DOEs with addition custom phase profile modifications in order to show high level capabilities of producing novel Bessel type beams with fanciful intensity patterns suitable for various micromachining applications: asymmetrical central core, multiple peaks with selective distances. Using numerical modeling and experimental verification, high quality performance of GPOEs with practical transparent material micromachining applications is shown. The aim of this work is to investigate Bessel type beam generation using phase shifted axicons, by applying numerical simulations and comparing experimental data; also, to test beam generation quality of these geometric phase elements by inducing material modifications inside transparent glasses and applying it in practical laser micromachining applications. The main results and conclusions of this work: The demonstration of geometric phase optical elements as modified axicons was shown in order to create new, fanciful Bessel type beams and comparing its generation quality with numerical simulations and experimental results obtained with spatial light modulator provides with a new way for high quality beam shaping optical elements. Practical application of laser micromachining was also demonstrated: with a quarter phase shifted axicon asymmetric Bessel beam is created and such properties are applicable in stealth laser dicing, cutting of thin transparent materials with side quality of Ra~ 0.5 µm. Other, double intensity peak Bessel type beams are generated and demonstration with additional laser induced chemical wet etching is shown in order to create desirable micro channels in thin glass. Thin channels of 1:50 width/depth ratio are created in 0.5 mm Schott D263T glass. In comparison with other types of axicons, the GPOEs are a great choice in high quality Bessel type beam shaping with additional level of freedom for creating it as free from optical elements suitable for specific micromachining applications. |
