Drilling micrometer and sub-micrometer holes with a laser beam is an important technological challenge that can be solved with a feedback loop detection system. In this master's thesis, we have developed a detection system that allows us to control perforation of a thin metal foil and installed it into a laser system with a nanosecond fiber laser. We set up a laser system containing a nanosecond fiber laser (λ = 1060 nm) , scanning head an optics (the beam diameter in focus equals 13 um). The experiments were performed on stainless steel foil with a thickness of 28 µm and titanium with a thickness of 32 µm. The open-loop system for micro-holes drilling, was transformed into the closed-loop system by using a photodiode, an aspherical lens and a control circuit, which we designed and developed within this thesis. The detection system works by collecting light on the photodiode, thereby detecting a laser-induced breakdown, while interrupting further laser pulses for the drilled hole. With the detection system, we found the optimal parameters for the production of holes in a stainless steel foil in range of 1.3 µm ± 0.5 µm and showed that they can be reproduced. The effect of polarization on the shape of the output hole was investigated and the output diameter controlled by delaying a certain number of laser pulses, after detecting a brakdown through the material. An optical microscope, with a five hundred times magnification, was used to analyze the samples, followed by characterization with a scanning electron microscope (SEM), where the holes were measured. Finally, we checked the minimum distance between the holes to produce the largest array of perforated holes. We found that the drilling time of one hole was 0.3 ms, while it takes 6 min 48 s for a matrix of 40000 holes per cm2. The exit diameter of the smallest hole we were able to drill equalled 0.8 um.
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