OLaf

Optimization of USP-Laser erosion by means of flexible beam shaping

Advantages and solutions

To generate different intensity profiles, the project used a static Diffractive Optical Element (DOE), which converts the conventional Gaussian profile into a defined top-hat distribution, and a flexible, programmable Spatial Light Modulator (SLM), which allows arbitrary energy distributions. Investigations on planar ablation have shown that especially square top-hat distributions can be used for qualitative and quantitative improvement of scanner-based ablation processes. Strong dependencies on the material and the used parameter regime were found. In the area of high ablation rates, which are caused by small pulse distances and high laser power, a further increase of the ablation rate is hardly possible by using the changed intensity profiles. However, the quality of the surface can be improved, especially with glass materials, which is shown by a reduction of the otherwise high roughness by up to 30 %. In the parameter regime of low ablation rates, which is relevant for most applications and is characterized by high pulse intervals and low power, the process efficiency could be more than doubled. By means of voltage measurements and the investigation of the interference depth damage, it could also be shown that the optical properties of the processed glass can also be improved due to the beam shaping.

In addition to surface removal, the SLM could be successfully used as a flexible beam splitter, which splits the beam into many partial beams with any arrangement. This makes it possible to use the available high laser power for the parallel processing of periodic microstructures, thus increasing the process efficiency many times over. The decisive challenge of generating individual beams with as much equal power as possible was solved by modifying the Gerchberg-Saxton algorithm. The flexible controllability of the SLM also made it possible to use it as a locally ablating intensity stamp with which three-dimensional microprofiles can be "imprinted" into the surface of the workpiece. Using specially adapted line beam profiles, which were simulated with a specially developed, one-dimensional Gerchberg-Saxton algorithm and tested in experiments, various structures such as convex cylindrical lenses, prismatic trenches or sawtooth structures could be generated. The usual slicing was omitted, so that no staircase effect occurs.

Investigations on the so-called path-time controlled ablation showed, however, that the scanner-based ablation process can also be optimized without the application of complex beam shaping technologies. In order to enable quasi-simultaneous 3D processing, a freely definable so-called tool contact zone generated by the scanner was moved over the workpiece with the aid of the axes. This geometry, which is continuously repeated by the scanner, generates a defined energy distribution whose dimensions can exceed those of the profiles generated by SLM. Through constant or function-dependent control of the axis speed, a continuously ablating 3D profile can be implemented, which is characterized by a more constant surface profile compared to conventional slicing - due to the elimination of the staircase effect. The selected shape and size of the tool contact zone is crucial to avoid heat accumulation effects and to control the spatial resolution of the process. With the help of parallel scanner and axis movement, defined three-dimensional surfaces and trenches, such as sinusoidal or V-profiles, could be calculated and practically implemented. In contrast to standard scanner processing, the possible contour length is only limited by the travel distance of the axes and therefore does not have to be interrupted by stitching points, even for large-format structures.

Target market

Pico- and femtosecond lasers are now used in almost all major production industries, e.g. for drilling, scribing, cutting or structuring. However, the use of appropriate lasers for surface processing has often not been economical up to now. The project has now provided important insights into increasing the processing speed. Depending on the application, even higher quality results can be achieved, which are characterized by lower surface roughness or material tension. The main target groups are therefore applications in which surface machining is carried out, i.e. primarily the texturing or structuring of surfaces.

Especially the precise material removal of dielectric materials places extremely high demands on the process despite the high potential of ultrashort pulses. The energy input, which is optimized by means of beam shaping, and the motion concepts developed in the project can reduce material damage and increase efficiency to an industrially viable level. The market entry for shaping UKP material removal can thus be enabled for a wide variety of applications. Since the UKP technology is in principle suitable for processing all materials, further innovative products of other material classes, such as metals or plastics, can be developed and placed on the market based on the research results for dielectrics.

The investigated processes and the system solutions generated from them enable new product ranges, among others in the fields of micro- and free-form optics, fluidic chips and sensor components. By using the SLM as a beam splitter, the laser power, which is available in ever-increasing quantities through new systems, can be used efficiently. At the same time, the parallel use of many spots in the generation of periodic structures leads to a drastic reduction in processing time. The productivity of surface texturing, e.g. to change the tribological properties of a workpiece, can be significantly reduced. In addition, the function-dependent axis movement of defined energy distributions, which can be generated by SLM or scanner, allows three-dimensional profiles with more continuous and larger profile geometries than previous standard methods allowed. The new portfolio and the possibility of placing higher quality and more cost-effective products on the market using innovative laser technology will enable the companies carrying out the work to increase their sales.

Both the UKP laser technology itself and the SLM technology used are characterized by the highest degree of flexibility. The precise, contact-free and wear-free processes for shaping and microstructuring can be programmed for a wide range of requirements and geometries and are therefore particularly suitable for prototype and small series production. Since it could be shown that the flexible SLM technology is in no way inferior to the static DOE, the research results can also facilitate the production and planning of beam-shaping elements for use in laser systems. With the help of an SLM, the calculated phase masks can first be tested in practice before they are used to produce a high-quality but expensive DOE.

The transfer of the results is done by publication at events and in journals as well as by direct contact with industrial partners. For this purpose, ifw Jena offers the following services around the process:

• Consulting
• Sample and small series production
• Further process development
• Support for process integration into the partner's plants