Search

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.

With the fast development of ultrashort pulse laser sources, the classic Gaussian beam has become a bottleneck for faster and more efficient micromachining of transparent materials. Its relatively short focal zone is not suitable for efficient microchannel creation in various glasses, therefore the Bessel beam is a great alternative for that, because of its elongated focal line. One of the most common optical elements for such beam generation is an axicon-conical lens with a sharp tip at the center that can be either refractive or diffractive. However, these beam generation methods have disadvantages, such as the complex manufacturing of an ideal surface of axicons. There also exist alternative ways of producing nondiffracting beams by geometrical phase elements. Such optical elements consist of polarization-sensitive volume nanogratings, which can have unique and complex patterns for the manipulation of incoming light. This work aims to familiarize with the formation and control of invariant, Bessel type beams by using geometric phase elements. The results of numerical simulation and experimental data are presented where controllable phase shifts are induced in axicon mask, therefore new and unique invariant beams are generated. The main results and conclusions: 1. Numerical simulations of beam propagation behind phase-shifted diffractive elements have similar properties to the Bessel beam. Beam propagation simulations provided in this work are particularly close to experimental results, small deviations could be caused by optical schemes or elements irregularities. 2. Demonstrated phase-shifted axicons produced as geometric phase elements, which could be reliable for various invariant beam formation, due to high threshold and efficiency (93 - 94 %), also it is a good alternative for complex phase mask processing. 3. Geometric phase elements could be reliable for invariant beam formation and high power laser micromachining: was demonstrated borosilicate glass modification by using two different invariant beams. One of them was used for controllable microcracks formation in the transparent material and producing laser dicing technology with high diced wall quality, 0.2 μm roughness was achieved.