| Abstract [eng] |
Conventional material processing methods are being replaced by modern laser micromachining, which uses Gaussian beam. Gaussian beam has huge peak energy, calculation of beam parameters are not convoluted, but on the other hand, energy in Gaussian beam tail is not sufficient for material ablation, thus, material is heated. Using top-hat intensity distribution beam might reduce heat affected zone, improve micromachining quality. Top-hat beam can be shaped by using various optical designs, one of them is using binary phase plate, which was used in this research. This research is significant because using theoretical models top-hat beam was designed to be cost-effective and easy to make in the laboratory environment. The aim of this research was theoretically design binary phase mask and evaluate top-hat shaped beam. During research, phase mask design in MATLAB coding environment was developed and theoretical top-hat beam demonstrated. Furthermore, the experiment was conducted using the same parameters as in the theoretical models, theoretical and experiment results were compared. As a result, it was discovered that it is possible to form quasi top-hat beam using binary phase plate with square aperture, which delays Gaussian beam tail by π compared to the central part of beam. The quasi top-hat beam is observed about 7 mm in beam propagation direction or 18 % of Rayleigh length. Experiment results were comparable with theoretical data by using diffraction theory (Fresnel approximation), errors were caused due to small phase mask aperture dimensions mismatch and Gaussian beam asymmetry or beam width fluctuation also beam phase front is spherical. It was detected, that laser does not emit beam with planar phase front, it is lightly spherical, thus optical elements used in this experiment has influence on beam phase front form and its sphericality. Wavefront phase pattern was formed with resemblance to mathematical model after thorough optimisation, measured phase pattern change as phase mask was rotated also phase mask glass input to wavefront distortion was measured. To conclude, this beam shaping method requires prolonged tuning, although result can be lost with minor disturbance in optical system. |
