Ultra-short pulse laser beam shaping for microstructuring using a deformable mirror

Abstract

Using ultra-short laser pulses to treat material with low thermal influences is a rapidly growing technology. Structuring using Gaussian beams generates grooves and influences the geometry of the generated pattern. For special applications e.g. in solar industries a top-hat intensity profile is being used to generate isolation grooves, which leads to higher scanning speed. These profiles are generated by using diffractive optical elements (DOE) and exists only in the focal plane for which the optical element was designed for. A higher flexibility for beam shaping can be achieved by using spatial light modulators (SLM), but its transmission efficiency is lower than a DOE and due to diffraction there is an additional loss of intensity.To generate one single beam shaped laser pulse with low loss of intensity, a deformable mirror can be integrated. These mirrors are built by combining a piezo-ceramic to a mirror substrate. By pattering the piezo-ceramic into independent controllable segments the mirror surface can be deformed individually. The deformed surface influences the incident wave front and results e.g. in a beam displacement or a variation of the focus position. The Laser Center of the University of Applied Sciences Muenster (LFM) demonstrates how a deformable mirror can be integrated into an ultra-short pulse laser system for micro-structuring. An ABCD-matrix method is used to calculate focus variations, step sizes and the limitations of an optical system including a deformable mirror and an f-theta lens. To reduce limitations and to adjust the focus variation additional optics are used. An optical system was built which can vary the focus in an area of 5 mm. It can be shown, that the focus diameter can be kept constantly, which is important to get a reliable process independent to the surface height. The ablation results of microstructures generated with a focus shift realized by the deformable mirror are compared to a focus shift realized by a motorized translation axis. Another influence to the wave front is the variation of an astigmatism, which includes two different focus positions along the beam propagation. It is possible to vary the distance between these focus positions by deforming the mirror. The influence of astigmatic beams were studied due to the ablation depth. It is shown that a deformable mirror is a high flexible and fast method for beam shaping of laser pulses.Using ultra-short laser pulses to treat material with low thermal influences is a rapidly growing technology. Structuring using Gaussian beams generates grooves and influences the geometry of the generated pattern. For special applications e.g. in solar industries a top-hat intensity profile is being used to generate isolation grooves, which leads to higher scanning speed. These profiles are generated by using diffractive optical elements (DOE) and exists only in the focal plane for which the optical element was designed for. A higher flexibility for beam shaping can be achieved by using spatial light modulators (SLM), but its transmission efficiency is lower than a DOE and due to diffraction there is an additional loss of intensity.To generate one single beam shaped laser pulse with low loss of intensity, a deformable mirror can be integrated. These mirrors are built by combining a piezo-ceramic to a mirror substrate. By pattering the piezo-ceramic into independent controllable segments the mirror surfac...