Helical drilling and helical cutting using a rotating Dove-prism is a highly advanced manufacturing technology for high-precision laser micro processing. This optical approach is often combined with an ultra-short pulsed laser source to enable the process capability of almost all materials due to the vapor dominated ablation process. The optical system shall be designed and coated for the use of multiple wavelengths based on the wavelength- dependent absorption behavior of different materials. The designed length of the Dove-prism is a function of the wavelength. Therefore the use of more than one wavelength results in an average solution. Moreover the manufacturing deviations in length and angle have to be taken into account. The collimated deviations from the ideal state result in a misalignment causing the optical path to take the shape of a Limacon of Pascal. This effect needs to be minimized to a maximum deviation between the two circles of 1µm. Additional optical elements and degrees of freedom in adjusting the position and angle of the dove prism relatively to the rotation axis are needed. To solve this issue the balancing holder has been expanded to hold four optical elements that each can be rotated around an axis that either lies in a horizontal plane or perpendicular to this. The optical elements are fixed after adjustment in a way that they do not perform any movement even under the load of high speed rotation of up to 15.000RPM. This paper will present the adjusting process for three independent systems to show the overall tolerance that is needed to achieve accuracies of at least 1µm.
Helical drilling promises high quality laser drilling geometries together with a controllable shape (taper, diameter) in multiple materials. Latest research has shown that the use of a short pulsed laser source and an ultra-short pulsed laser source increase those benefits while the process time is increased. However reproductively and stability still remain an issue on these application due to the complex optical alignment and the dynamic beam rotation. The research presented covers a full fractional experimental parameter study of the controllable parameters such as laser power, optical angle and optical offset. Manufacturing deviations and process capability are derived. In addition to that a Taguchi experimental setting with a noise array (temperature, focus drift, change in process gas pressure) has been carried out. The results of five-level factor plan L25(5^9) with 9 parameters (6 control parameters and 3 noise parameters) is analyzed using ANOVA and Taguchi Robust Design. The outcome is used in an optimization process to identify any parameter combinations that result in decreased deviations from the quality remark (e.g. entrance diameter). Afterwards the control parameter are used to adjust the quality remark to the targeted value. It is shown that this optimized parameter set offers higher stability and therefore an increased process capability.
High-precision laser micro machining gains more importance in industrial applications every month. Optical systems like the helical optics offer highest quality together with controllable and adjustable drilling geometry, thus as taper angle, aspect ratio and heat effected zone. The helical optics is based on a rotating Dove-prism which is mounted in a hollow shaft engine together with other optical elements like wedge prisms and plane plates. Although the achieved quality can be interpreted as extremely high the low process efficiency is a main reason that this manufacturing technology has only limited demand within the industrial market. The objective of the research studies presented in this paper is to dramatically increase process efficiency as well as process flexibility. During the last years, the average power of commercial ultra-short pulsed laser sources has increased significantly. The efficient utilization of the high average laser power in the field of material processing requires an effective distribution of the laser power onto the work piece. One approach to increase the efficiency is the application of beam splitting devices to enable parallel processing. Multi beam processing is used to parallelize the fabrication of periodic structures as most application only require a partial amount of the emitted ultra-short pulsed laser power. In order to achieve highest flexibility while using multi beam processing the single beams are diverted and re-guided in a way that enables the opportunity to process with each partial beam on locally apart probes or semimanufactures.
High-precision micro laser drilling with high aspect ratios requires laser imaging effects such as optical double rotation. Optical double rotation is an effect where the laser beam is guided through any optical elements with a total amount of reflections that remains uneven. Those optical elements need to be mounted in a rotary stage that spins the elements with a certain velocity. In an ideal case the optical axis is identically with the rotational axis. Few optical elements such as the Dove-prism show the effect that the beam is rotated in itself while it is moving on a helical path. That offers an independency of the beam profile. However the Dove-prism alone can not be adjusted in a way that the two axis match. This is based on geometrical errors of the Dove-prism due to manufacturing technologies. Certain deviation in length and angle lead to a helical error. Additional optical elements can compensate this effect. Alignment that only takes place in one 2D plane (e.g. the focal plane) leads most likely to a cross-over of both axes (x-alignment) in that one plane. In order to match both axes the alignment needs to be done at least in two 2D planes. That requires the opportunity to both influence the optical angle and the optical position (parallel shift) in both planes. The highly complex optical alignment method as well as the mechanical storage of the optical elements will be shown in this paper.
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