Johannes Heberle

Electro-optic and acousto-optic laser beam scanners

Electro-optical deflectors (EOD) and acousto-optical deflectors (AOD) are based on deflection of laser light within a solid state medium. As they do not contain any moving parts, they yield advantages compared to mechanical scanners which are conventionally used for laser beam deflection. Even for arbitrary scan paths high feed rates can be achieved. In this work the principles of operation and characteristic properties of EOD and AOD are presented. Additionally, a comparison to mirror based mechanical deflectors regarding deflection angles, speed and accuracy is made in terms of resolvable spots and the rate of resolvable spots. Especially, the latter one is up to one order of magnitude higher for EOD and AOD systems compared to conventional systems. Further characteristic properties such as response time, damage threshold, efficiency and beam distortions are discussed. Solid state laser beam deflectors are usually characterized by small deflection angles but high angular deflection velocities. As mechanical deflectors exhibit opposite properties an arrangement of a mechanical scanner combined with a solid state deflector provides a solution with the benefits of both systems. As ultrashort pulsed lasers with average power above 100 W and repetition rates in the MHz range have been available for several years this approach can be applied to fully exploit their capabilities. Thereby, pulse overlap can be reduced and by this means heat affected zones are prevented to provide proper processing results.

Four-dimensional light shaping: manipulating ultrafast spatiotemporal foci in space and time

The spectral dispersion of ultrashort pulses allows the simultaneous focusing of light in both space and time, which creates so-called spatiotemporal foci. Such space–time coupling may be combined with the existing holographic techniques to give a further dimension of control when generating focal light fields. In the present study, it is shown that a phase-only hologram placed in the pupil plane of an objective and illuminated by a spatially chirped ultrashort pulse can be used to generate three-dimensional arrays of spatio-temporally focused spots. By exploiting the pulse front tilt generated at focus when applying simultaneous spatial and temporal focusing (SSTF), it is possible to overlap neighboring foci in time to create a smooth intensity distribution. The resulting light field displays a high level of axial confinement, with experimental demonstrations given through two-photon microscopy and the non-linear laser fabrication of glass.

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 Apparatus for Observation of Transitional Effects in Ultrafast Laser Micromachining Processes

A pump-probe experimental approach has been shown to be a very efficient tool for the observation and analysis of various laser matter interaction effects. In those setups, synchronized laser pulses are used to create an event (pump) and to simultaneously observe it (probe). In general, the physical effects that can be investigated with such an apparatus are restricted by the temporal resolution of the probe pulse and the observation window. The latter can be greatly extended by adjusting the pump-probe time delay under the assumption that the interaction process remains fairly reproducible. Unfortunately, this assumption becomes invalid in the case of high-repetition-rate ultrafast laser material processing, where the irradiation history strongly affects the ongoing interaction process. In this contribution, the authors present an extension of the pump-probe setup that allows to investigate transitional and dynamic effects present during ultrafast laser machining performed at high pulse repetition frequencies.

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....

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.

Simulation of energy buildups in solid-state regenerative amplifiers for 2-μm emitting lasers

A numerical model for solid-state regenerative amplifiers is presented, which is able to precisely simulate the quantitative energy buildup of stretched femtosecond pulses over passed roundtrips in the cavity. In detail, this model is experimentally validated with a Ti:Sapphire regenerative amplifier. Additionally, the simulation of a Ho:YAG based regenerative amplifier is conducted and compared to experimental data from literature. Furthermore, a bifurcation study of the investigated Ho:YAG system is performed, which leads to the identification of stable and instable operation regimes. The presented numerical model exhibits a well agreement to the experimental results from the Ti:Sapphire regenerative amplifier. Also, the gained pulse energy from the Ho:YAG system could be approximated closely, while the mismatch is explained with the monochromatic calculation of pulse amplification. Since the model is applicable to other solid-state gain media, it allows for the efficient design of future amplification systems based on regenerative amplification.

Parallelized SSTF-overlapping foci in space and time (Conference Presentation)

Simultaneous spatio-temporal focusing describes the technique of spectrally dispersing an ultrashort pulse to increase it’s duration, before recombining all the spectral components to reform the short pulse at the focus of a lens. This has several advantages for optical applications using ultrashort pulses, enabling a focus with large lateral extent but displaying much greater axial confinement than with conventional focussing. Meanwhile non-linear propagation inside samples is also minimised. We extend the technique to generate multiple SSTF focal spots via holography. In contrast to previous implementations, a phase hologram is placed in the ultrafast beam while it is spatially chirped. Thus, we show that it is possible to create multiple focal spots formed by SSTF arranged in three dimensions. Demonstrations are given for ultrashort pulse laser machining on the surface and inside glass. Furthermore, due to the strong pulse front tilt exhibited by an SSTF spot, we show how multiple spots can be stitched together in space and time to generate extended intensity distributions. Spatially overlapped regions can be overlapped in time using the property of pulse front tilt such that holographic arrays of SSTF spots can be created with apparently no spatial separation.