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It is well known that the interaction of a material with a laser beam depends on the wavelength of the incident beam, the pulse duration, the repetition rate, the shape of the beam, and the properties of the material itself. In majority of laser micromachining applications Gaussian mode is most common and versatile. Since the processing of transparent material requires laser beams that would form long and narrow plasma channels, an alternative to Gaussian beam was sought. One of the most prominent examples would be Bessel beams of different order. Bessel beams are notorious for characteristic of having diffraction-less focal zone, which is quite longer than the Rayleigh range of a Gaussian beam. Thus, Bessel beams exhibit a greater depth of field than Gaussian beams. Bessel beams of the zeroth order have intensity maxima in the center, and higher order Bessel beams have dark central core - intensity minima, which is sorounded by alternate bright and dark rings. In addition, the higher order Bessel beams also have a hellical phase front, which leads to quite interesting properties like carrying orbital angular momentum. Different variations of nondiffracting Bessel-like beams having unique transverse intensity distributions can be obtained by superimposing several higher order Bessel beams. The purpose of this work is to generate superimposed higher-order Bessel beams of different topologies and spatial frequencies. We start with numerical simulations for superimposed beams, which are followed by experimental tests forming mentioned beams using spatial light modulator to confirm our theoretical predictions. Lastly, we inscribe a beam forming mask inside fused silica sample to create a geometrical phase element required for beam shaping and investigate the ability of superimposed beams to induce surface modifications in various glasses. The main results and conclusions are: 1. Numerical simulations for superimposed beams has been performed successfully. Several cases of superposition were investigated, where in each case interfering higher order Bessel beams possessed different parameters (topological charges and Bessel cone angles). During such superpositions, vortex Bessel beams having unique transverse intensity distributions can be obtained. 2. In the assembled optical setup, the interference of two higher order Bessel beams were realized. A close overlap between theoretical and experimental results was found. However, small distortions in the transverse intensity distributions of the experimentally realized beams were observed. 3. Phase profile of the chosen superimposed Bessel beam was directly inscribed into geometrical phase element. The intensity distribution of composite beam thus formed slightly differs from numerically simulated one. 4. Superimposed Bessel beam generated with the help of geometrical phase elemet was used to induce modifications on the surface of fused silica. From the photos can be clearly seen that overall form of induced modifications fully corresponds to the intensity distribution of composite beam formed by geometrical phase element. |
