Es wurden Untersuchungen zur gezielten Erzeugung und Nutzung von Mikrostrukturen beim Micro Cladding durchgeführt. Die notwendigen Prozessbedingungen dafür wurden gefunden. Die Baurate konnte um den Faktor 5 erhöht werden. Eine weitere Skalierung der Baurate auf das Doppelte durch Erhöhung der Scangeschwindigkeit konnte ebenfalls gezeigt werden. Die gezielte Mikrostrukturierung ist geeignet, um 2,5 D Mikrostrukturen mit hohem Aspektverhältnis in hoher Geschwindigkeit zu erzeugen.

Using ultra-short laser pulses for micro structuring or drilling applications reduces the thermal influence to the surrounding material. The best achievable beam profile equals a Gaussian beam. Drilling with this beam profile results in cylindrical holes. To vary the shape of the holes, the beam can either be scanned or – for single pulse and percussion drilling – manipulated by masks or lenses. A high flexible method for beam shaping can be realized by using a deformable mirror. This mirror contains a piezo-electric ceramic, which can be deformed by an electric potential. By separating the ceramic into independent controllable segments, the shape of the surface can be varied individually. Due to the closed surface of the mirror, there is no loss of intensity due to diffraction. The mirror deformation is controlled by Zernike polynomials and results e.g. in a lens behavior. In this study a deformable mirror was used to generate e.g. slits in thin steel foils by percussion drilling using ultra-short laser pulses. The influence of the cylindrical deformation to the laser beam and the resulting geometry of the generated holes was studied. It was demonstrated that due to the high update rate up to 150 Hz the mirror surface can be varied in each scan cycle, which results in a high flexible drilling process.

The increasing average power of ultra-short pulse laser systems also increases the demand of new processing strategies to optimize the volume ablation rate. High fluences do not increase the volume ablation rate linearly, but support the formation of substructures and the roughness of the surface. The optimal processing fluence for high ablation rates can be calculated based on material parameters like optical penetration depth and ablation threshold fluence. It is known that the optimal ablation rate depends on the pulse energy and the irradiated area. In this study a deformable mirror is used to change shape and dimension of the laser beam. Due to a closed surface and a thin dielectric coating, the transformation efficiency is better than 99 %. The ablation results are analyzed with respect to the ablation efficiency, roughness. The ablation behavior of the generated beam shapes show comparable results to the results investigated by circular beams.

Using ultra-short laser pulses for the generation of microstructures results in a high flexible tool for free form geometries in the micro range. Increasing laser power and repetition rates increase as well the demand of high flexible and efficient process strategies. To increase the ablation efficiency the optimal fluency can be determined, which is a material specific value. By varying the beam shape, the ablation efficiency can be enhanced. In this study a deformable mirror was used to vary the beam shape. This mirror is built by combining a piezo-electric ceramic and a mirror substrate. The ceramic is divided into several segments, which can be controlled independently. This results in a high flexible deformable mirror which influences the beam shape and can be used to vary the spot size or generate line geometries. The ablation efficiency and roughness of small generated cavities were analyzed in this study as well as the dimensions of the cavity. This can be used to optimize process strategies to combine high volume ablation and fine detail generation.

During laser material processing with high laser beam intensities, a laser-induced vapor formation can occur. Due to the shockwave behavior of the vapor plume and the associated rapid cooling, a significant particle formation can be initiated by nucleation. The laser radiation interacts with the particles which can result in a dynamic change of the intensity distribution on the surface. Especially in the field of laser remote processing, the attenuation of laser radiation by nanoparticles can influence the process stability and reduce the processing quality. The presented work is focused on the particle formation at a height of 10 mm above the material surface during the laser welding of stainless steel with a fiber laser. The laser beam intensity on the surface was varied between 1.3 and 5.1 MW/cm2. Transmission electron microscopy images of the nanoparticles and high speed images of the vapor propagation in the ambient atmosphere were analyzed. The attenuation of a probe beam in the vapor plume was evaluated in dependence on the wavelength. The results indicate a linear connection between the laser beam power and the particle formation rate.

In laser micro processing there is a demand of shaped beams to improve the ablation results. Beam shaping is used e.g. for generating isolation grooves in solar cells. In addition the variation of the beam profile allows modifications of the surface properties of the processed material. These modifications can result in surface functionalization e.g. hydrophobic behavior. Phase changing optical elements are used to influence the wave-front and vary the beam shape. For high-power laser applications these elements have to be compliant with large pulse energies or high average power. A deformable mirror is one of the most suitable tools for the application with high power lasers, because of the absence of intensity losses due to diffraction. In this study a unimorph deformable mirror was used for beam-shaping during a picosecond laser ablation on metals and dielectrics. A piezoelectric disc behind the deformable mirror is separated into 35 segments, which can be individually driven by a voltage from -100 V up to 250 V. This feature allows complex deformation of this mirror which results in an individual variation of the spot geometry. The generation of different focus geometries, e.g. elliptical or line geometries, were analyzed. For this purpose on one hand the intensity profile and the beam propagation and on the other hand the influence to the surface modifications were studied. Another field of application for deformable mirrors is the variation of the focus position. By using additional optical components a controllable focus shift of 5 mm with a step size down to 25 μm was realized. This feature was required to keep a continuous spot size on the ablation surface. The influence of this defined defocusing during the ablation process was analyzed and compared to a focus shift realized by a motorized translation axis. It could be shown that beam-shaping based on a deformable mirror is a precise method for intensity variations of the laser spot and focus shifting without loss of intensity.

Generating microstructures using ultra-short laser pulses reducing the thermal influence to the work piece is a rapidly growing technology field. The physical connections for material laser interactions with short laser pulses are often too complex for industrial processes. Scientific results for a single pulse ablation can hardly be transferred to industrial applications for volume ablation. Finding a suitable parameter setting is a challenge because of the numerous process influences, such as pulse duration, repetition frequency or scanning parameters. An automated inline process control supports the analysis of material reactions to different laser parameter settings. A database that includes these reactions can be used to identify essential process variables. These variables can be used to generate a laser process sequence for the ablation of a structure based on a CAD generated model. In addition this database assists to analyze possible errors in the manufacturing process before the processing starts. These errors may be differences between a CAD generated model and the laser fabricated work piece and may result in inaccurate edge structures or incorrect ablation depth. Recurring measurements as taking pictures of the treated surface or analyzing the ablation depth and roughness offer a high potential for an automated process. The Laser Center of the University of Applied Sciences Muenster (LFM) presents a system design that involves a picosecond laser (8 ps, 800 kHz repetition rate) with scanning optics for the generation of microstructures, as well as microscopes (field view 2 to 10 mm) and chromatic sensors (z-resolution 0.2 to 1 µm) for the surface inspection of the generated microstructures. This machine design is used to generate a database including changeable laser settings and its material reactions such as ablation depth and roughness. Using algorithm for database evaluation it is possible to identify process influences which lead e.g. to a defined roughness and ablation depth. Combining this database with a CAD designed microstructure it is possible to evolve a process sequence for the laser ablation process. It can be shown that the information of the database helps to predict the deviation of the processed structure to the designed one. Using this feature the design of microstructures can be changed to reduce these deviations before processing, which decrease waste of material and developing time of the structure.

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.

The interaction between the vapor plume and the incident laser radiation affects remote laser welding. Relating to laser systems with an emitted wavelength around 1μm, a significant loss mechanism can be traced back to the extinction by laser-induced particle formation. Due to the tight coupling between the particle formation and the evaporation rate inside the keyhole, the particle formation shows a strong dependence on the keyhole geometry and thus on process parameters (e.g. feed rate and laser beam power). In order to verify the relationship between particle formation and process parameters, the beam of a broadband LED was guided through the vapor plume during the welding processes with a fiber laser. The attenuated probe beam was analyzed in dependence on the wavelength. In addition, the propagation of the vapor plume was investigated by using high speed imaging.

Laser remote processing is used in a wide field of industrial applications. Among other things, it is characterized by flexible beam guidance in combination with high processing velocities. But in most cases process gas support in the interaction zone is omitted. Consequently, interaction mechanism between the vapor plume and the incident laser radiation can dynamically affect the process stability. Referring to remote welding with high brilliant laser sources having a wavelength around 1 lam, the interaction between the incident laser radiation and formed particles plays an important role. The presented work shows results of the investigation of the laser-induced particle formation during the laser welding of stainless steel with a 2 kW fiber laser under remote conditions. It is therefore concentrated on the dynamical behavior of the laser-induced particle formation and the dependence of the particle formation on the laser beam power. TEM images of formed particles were analyzed. In addition, the radiation of a LED was directed through the vapor plume. On the one hand, the dynamic of the attenuation was considered. On the other hand, the Rayleigh approximation was used in order to evaluate the detected signals.

Laser cut quality is strongly influenced by the cutting process parameters. In this study, 5 mm mild steel is cut with a high brightness 2.5 kW diode laser. The process parameters are optimized using systematic design of experiment (DOE) to obtain burr free laser cuts with minimal cut edge surface roughness. Varied factors include cutting speed, gas pressure, focus position, nozzle diameter and nozzle distance. The goal of this work is to relate these factors to burr formations and cut edge surface roughness. The relationship is generated and validated with help of a mathematical model, which is then used to predict and minimize burr heights and cut edge surface roughness.

The generation of microstructures by ultrashort pulse laser irradiation is - depending on process parameters and the applied material - often accompanied with the creation of substructures like ripples or micro canals on the ablation ground. This side effect can be used to create local topographic modifications on a microscopic scale which can change functional properties of the surface. The combination of micro structuring and functionalisation within one production step can only be successful if the interaction mechanisms are well known. In this study the options to modify the wetting behavior on stainless steel, Al2O3 ceramic and PMMA plastic were analyzed. Therefore the contact angles of water drops on picoseconds-laser-produced samples were measured by a self-made measuring system. Test measurements offered post-process effects on surfaces of steel and ceramic. On those substrates the final contact angle adjusts after several hours up to days. In total with this technique contact angles between 5° and 160° could be realized, depending on the material. This allows generation of hydrophilic up to super-hydrophobic effects on precise defined areas. The combination technique offers novel options particularly for micro fluidic. Some produced samples for “Lab-on-a-Chip- Systems” should demonstrate that.

New developments and characteristics of high brilliant laser sources have led to new applications in the field of laser remote processing. Due to high particle formation rates within the vapor plume, a significant influence of the interaction between laser radiation and nanoparticles on the process may occur. The presented work shows results of the investigation of the dynamical formation of nanoparticles within the vapor plume during the welding of stainless steel with a 2 kW Multi-Mode fiber laser under laser remote conditions. The particle size distribution is measured by the evaluation of TEM-images, whereas, the plasma temperature and particle density are analyzed in dependence of the irradiation time.

The use of picosecond lasers for microstructuring, especially in the combination with scanner optics, leads to undesired effects with increasing ablation depths. The cavity edges slope to a degree ranging between 50° and 85°, depending on the material. With highly reflective substrates, ditches of up to 20% of their total depth can be formed on its ground structure. In certain materials also diverse substructures such as holes, canals, or grooves can be developed. These could impact the precision of the ablation geometry partially. A systematic study of the specific ablation characteristics is needed to achieve a defined depth of the structure. Considering a huge number of influential parameters, an automation of such measurements would be meaningful. For a study of eight different materials (high-alloy steels, copper, titanium, aluminum, PMMA, Al2O3 ceramics, silicon and fused quartz), an industrial ps-laser coupled with a chromatic sensor for distance measurement was used. Hence a direct acquisition of the generated structures as well as an automatic evaluation of the parameters is possible. Furthermore an online quality control and a local post processing can be implemented. In this way the generation of complex structures with a higher precision is possible.

A characteristic of the laser ablation process of metals with high brilliant radiation is a significant formation of nanoparticles within the vapour plume. Due to the interaction between the incoming radiation and the particles, the intensity is attenuated at the workpiece. This leads to a decrease of the ablation rate and the related ablation velocity. In order to verify the influence of the interaction between nanoparticles and high brilliant radiation on the ablation process, experimental and theoretical results of the laser ablation process of stainless steel with a single mode fiber laser are presented. For an ablation process with an intensity of 1.9×108W/cm², it is shown that the average particle size is 9nm. Within a further time-dependent analysis of the correlation between particle formation and ablation velocity, the beam of a test laser is directed through the vapour plume and the angle-dependent scattered radiation is detected. The results point out that a decrease of the ablation velocity corresponds with an increasing of the particle density within the ablation plume.

Nanometer sized particles are formed within the vapor plume during the ablation of metal with laser radiation. Thereby, the particle formation rate depends strongly on the used intensity of the laser source. High brilliant laser sources have the ability to generate intensities higher than 1⋅108W/cm2 during cw operation. Due to the widespread use of high brilliant laser sources in research and industrial applications, it is important to investigate the influence of particle formation on the ablation process. Therefore, the presented work is focused on the particle formation during the ablation process of stainless steel with a single-mode fiber laser. Results of experimental work are shown and analyzed. In the experimental work a probe laser beam is directed through the ablation plume and the scattered intensity is analyzed. TEM images of particles show an average particle size of 9 nm at an intensity of 1.92⋅108Wcm2.

High-speed laser micro-perforation (“on-the-fly” technique) represents a processing technique for the time-efficient and cost-effective production of microsieves. In order to increase the perforation rate this process is researched with use of a brilliant laser source. These laser sources combine ample output power with an excellent beam quality. The high perforation rate [<50 000 drillings/s, laser source: YLR-300SM (IPG)], which can be achieved with this technique makes it an attractive option for the creation of large drilling matrices and drilling widths of less than 15 μm. The described experimental work is carried out to validate a theoretical analysis of the maximum obtainable perforation rate and its main influencing factors. It was found that Mie-scattering represents a crucial factor for the whole process with respect to the application of a brilliant laser source.

In contrast to most publications and developments concerning frequency doubling in this research neither a pulsed nor a single-frequency laser source is used. Instead of these lasers the SH is generated with a 200 W cw fiber laser with 1.6 nm bandwidth as the fundamental source in order to extend its application possibilities in laser macro machining. In experimental studies 3.7 W of frequency doubled radiation (@543 nm) at 3.2 % optic-optical conversion efficiency are generated. The lattice structure of KTP for SHG@1086 nm is analyzed using the Laue-method and additionally the temperature, crystal length and angular dependences of the frequency doubled output power are investigated.