Tom Häfner

Influences on incubation in ps laser micromachining of steel alloys

Efficient and accurate generation of micro holes and feature geometries is one object of investigation in ultrashort pulse laser processing. Different analytical models exist to describe the relationship between energy input and removed volume. These models relate to the ablation threshold and the energy penetration depth representing material-dependent parameters. Both parameters are influenced by incubation. Against this background, incubation effects depending on the number of pulses applied on two steel alloys are presented in the paper. The ablation threshold and the energy penetration depth are analyzed by the zero-damage method and an analytical model based on Beers law describing the crater depth. The pulse frequency is chosen to fP = 100 Hz to investigate incubation excluding the influence of temporal effects between subsequent pulses. The used ultrashort pulsed laser has a pulse duration of 10 ps and a wavelength of 1064 nm. Craters are generated with defined pulse numbers between N = 1 and N =...

Speckle reduction techniques in holographic beam shaping for accurate and efficient picosecond laser structuring

Holographic beam shaping using a spatial light modulator (SLM) provides flexible adaptation of the intensity profile in laser material processing. This dynamic beam shaping is advantageous regarding the adaptation of accurate and efficient ultrashort laser based material ablation processes. However, speckles occur due to the pixelated display of the SLM and consequently discretized phase shifts. Speckles reduce the quality of a shaped intensity profile and the accuracy of generated microfeatures and therefore have to be suppressed. Against this background, selected speckle reduction techniques are applied, modified, and evaluated regarding the quality of a desired top-hat intensity profile. This beam shape is relevant for the generation of friction influencing microfeatures. Holograms are calculated by the iterative Fourier Transformation algorithm. The criteria for top-hat evaluation such as flatness, speckle contrast, and edge steepness are applied according to DIN EN ISO 13694. Furthermore, the effects of speckles on a defined microfeature geometry generated in a steel alloy are presented. The quality and the ablation efficiency including the diffraction efficiency of the SLM are evaluated and compared to conventional micromachining with the Gaussian intensity profile. The speckle reduction techniques of deterministic shift-averaging and time-averaging which is based on averaging of the reconstruction of different independently calculated holograms result in a high flatness factor and high quality of material removal. The number of holograms is determined, which is necessary to generate microfeatures of sufficient accuracy and low roughness. In contrast, stochastic shift-averaging leads to intensity profiles with higher speckle contrast and microfeatures with higher roughness. These averaging techniques limit the processing speed of microstructuring due to numerous hologram variations at low switching frequencies of the SLM. Therefore, an additional method is applied. Sufficient speckle reduction is achieved for a single hologram. As a result, defined microfeatures can be generated by an averaging of the reconstruction of different holograms, which enables higher ablation efficiency for microstructuring.Holographic beam shaping using a spatial light modulator (SLM) provides flexible adaptation of the intensity profile in laser material processing. This dynamic beam shaping is advantageous regarding the adaptation of accurate and efficient ultrashort laser based material ablation processes. However, speckles occur due to the pixelated display of the SLM and consequently discretized phase shifts. Speckles reduce the quality of a shaped intensity profile and the accuracy of generated microfeatures and therefore have to be suppressed. Against this background, selected speckle reduction techniques are applied, modified, and evaluated regarding the quality of a desired top-hat intensity profile. This beam shape is relevant for the generation of friction influencing microfeatures. Holograms are calculated by the iterative Fourier Transformation algorithm. The criteria for top-hat evaluation such as flatness, speckle contrast, and edge steepness are applied according to DIN EN ISO 13694. Furthermore, the effects...

High-speed pump-probe imaging of ultrashort pulsed laser cutting of polymers

Ultrashort pulsed laser processing is an effective technology for high-precision cutting, ablation, and drilling of almost all types of material, with low thermal input to the substrate. Polymers which are usually transparent for the laser pulses can be efficiently processed. The material is heated very fast resulting in direct evaporation of the irradiated material volume. On one hand, incubation effects occur due to changed surface roughness, voids, and chemical changes of the material, leading to a variation of energy absorption. On the other hand, if a high repetition rate and high pulse energy are applied to achieve high productivity at polymer cutting, heat accumulation occurs leading to melting and heat affected zones. An estimation of the pulse number dependent material modification and heat input which leads to these effects is not easily accessible for quantitative measurements. Furthermore, numerous process parameters influence the interaction making process analysis and optimization extensive. A versatile tool towards improved knowledge on ultrashort pulsed processes is pump-probe imaging. By using this, effects on a very short timescale such as nonlinear laser absorption, propagation of material waves and plasma generation can be visualized. The recording device in the setup which is applied for the analysis of polymer cutting and drilling in this paper is a high-speed camera with frame rates of up to 50 kHz. This enables recording video sequences contrary to taking a single picture. The authors utilized this to enable the observation of effects of heat accumulation which evolves after a large amount of pulses. Simultaneously, the authors were able to maintain the stated advantages of the pump-probe configuration. Therefore, under variation of several process parameters such as pulse repetition rate, pulse energy, and lateral pulse to pulse separation video sequences were recorded. By a variation of the pump-probe delays of 0 up to 5 ns, material wave propagation can be identified and optimized for evaluation. The points of origin of the observed waves are considered as the regions of highest nonlinear energy deposition; thus, the deposited energy distribution can be estimated. As every single pulse up to a pulse number of 1000 pulses is imaged, the transition from high penetration depth of the first incident pulses to strong surface absorption for an increasing pulse number due to incubation can be shown. Furthermore, strongly absorbing spots in the bulk volume caused by impurities and material modification lead to vapor generation under high pressure. Finally, the confined pressure is assumed to be a main reason for crack formation.Ultrashort pulsed laser processing is an effective technology for high-precision cutting, ablation, and drilling of almost all types of material, with low thermal input to the substrate. Polymers which are usually transparent for the laser pulses can be efficiently processed. The material is heated very fast resulting in direct evaporation of the irradiated material volume. On one hand, incubation effects occur due to changed surface roughness, voids, and chemical changes of the material, leading to a variation of energy absorption. On the other hand, if a high repetition rate and high pulse energy are applied to achieve high productivity at polymer cutting, heat accumulation occurs leading to melting and heat affected zones. An estimation of the pulse number dependent material modification and heat input which leads to these effects is not easily accessible for quantitative measurements. Furthermore, numerous process parameters influence the interaction making process analysis and optimization extensive....

Fabrication of a turbid optofluidic phantom device with tunable μa and μ's to simulate cutaneous vascular perfusion.

Microfluidic devices are oftenly used to calibrate the imaging reconstruction, because they simulate the morphology of microvasculature. However, for lack of optical properties in microfluidics, the functional recovery of oximetry information cannot be verified. In this work, we describe the fabrication of a novel turbid optofluidic tissue phantom. It is designed to mimic the vascular perfusion and the turbid nature of cutaneous tissue. This phantom contains an interior hollow microfluidic structure with a diameter of ϕave = 50 μm. The microfluidic structure includes the geometry of an inlet, a river-like assay and an outlet. This structure can be perfused by hemoglobin solution to mimic the cutaneous micro-circulation. The multiple-layered phantom matrices exhibit the representative optical parameters of human skin cutis, namely the absorption coefficient μa and the reduced scattering coefficient . The geometry of the generated microfluidic structure is investigated by using Spectral-Domain Optical Coherence Tomography. This optofluidic phantom bridges the gap between tissue equivalent phantoms and Lab-On-Chip devices. Perspectively, this device can be used to calibrate a variety of optical angiographic imaging approaches.

Nonlinear absorption measurements of intraocular lens polymers by integrating Z-scan

An ultrashort pulsed laser processing knowledge of the amount and distribution of the absorbed energy in the workpiece is essential. For transparent dielectrics, energy input by ultrashort laser pulses occurs by nonlinear absorption in the bulk material. Even at high intensities, the penetration depth is high and for most cases decays with an unknown function. Additionally, nonlinear absorptivity dramatically changes depending on the number of incident pulses due to incubation of the workpiece. This is usually attributed to chemical modifications or generated voids inside the workpiece. Particularly, polymers are strongly influenced by incubation preventing full understanding of ablation at industrial processes. Especially, the Polymethylmethacrylate (PMMA) copolymer analyzed in this paper is of high interest for ophthalmic applications and requires high processing quality and efficiency. To gain detailed insight into interaction of polymers with multiple subsequent picosecond laser pulses, investigations...

Friction Adjustment within Dry Deep Drawing by Locally Laser Textured Tool Surfaces

In deep drawing processes sustainability can be increased and processing steps can be omitted by abolishing any lubricants. Tailored tools with locally textured surfaces offer a possibility to compensate higher strains caused by the changed tribological system. Ultrashort pulsed laser machining is an advantageous approach to generate surface features. Thus, very hard and brittle materials can be processed inducing negligible heat affected zones so that the surrounding tool material keeps its initial properties. The material-dependent process parameters for efficient picosecond laser structuring are applied. The effects of features with single feature sizes in the range of 100 µm to 500 µm on friction of the tribological pairing are presented. The dependencies of the friction coefficient on the properties of the micro features - the geometry, the shape, the density and the orientation - are investigated by using a ring-on-disc-tribometer. The ring representing the tool is made out of the cold work steel 1.2379. The zinc-coated deep-drawing steel DC04 is used as disc respectively workpiece material. During the ring-on-disc-tests a constant contact pressure of 2.1 MPa and a mean sliding velocity of 100 mm/s are applied. To obtain the significant influences of micro features on friction, screening tests by varying the parameters according to the Shainin method are carried out. Because of the stochastically occurring high wear observed in reference experiments a changed methodology of ring-on-disc-tests is proposed. Applying this method the effects of textured ring surfaces on friction coefficient of steel-zinc-sliding are evaluated and compared to untextured rings. The latter are tested non-lubricated as well as with lubrication. The screening tests show that the feature orientation is the significant parameter influencing the friction. Selecting this parameter together with the feature density the friction coefficient can be adjusted with regard to untextured surfaces.