Ultrashort Pulse Laser System Research Articles

Investigation of spatio-temporal stretching in a duplex grating compressor.

We employ the modified 6×6 matrix formalism to describe a pulsed Gaussian beam diffracted by a grating with arbitrary orientation. The matrix treatment is used to analyze the evolution of a pulsed beam propagating in a duplex grating compressor (DGC). For chirped pulse incidence, the inclination angle required in DGC setups will introduce several kinds of first-order spatio-temporal couplings (STCs). We found that temporal stretching due to spatial chirp is suppressed with enlarged beam diameter. Pulse-front tilt and residual frequency chirp in the compressed pulse will be eliminated simultaneously. Pulse with the transform-limited duration can be expected in ultra-intense and ultra-short pulse laser systems employing DGC.

A preliminary study on a tooth preparation robot

ABSTRACT This is a study of a robot-controlled ultrashort pulse laser system used for tooth preparation to overcome drawbacks of traditional manual methods, thus improving its quality, precision, and clinical efficacy. Based on the international advanced digital technologies, automation technologies, and medical laser technologies, the entire tooth preparation process was completed autonomously using the robot-controlled ultrashort pulse laser. In the study, 15 extracted human first molars were prepared automatically by the robot system. And the result accuracy was evaluated using software. Compared with the CAD data of standard tooth preparation, the overall average error, preparation depth of the natural tooth preparation, and convergence angle were 0.05–0.17 mm, 2.097 ± 0.022 mm with an error of 0.097 ± 0.022 mm, and 6.98 ± 0.35° with an error of 0.98 ± 0.35°, respectively. The feasibility and accuracy of the robot-controlled femtosecond laser technique for tooth preparation were confirmed.

Designing one-dimensional magnetized plasma photonic crystals for compensating second- and third-order dispersion effects in ultra-short pulse lasers

The dispersion compensation ability of a one-dimensional photonic crystal composed of dielectric and magnetized cold plasma materials has been theoretically investigated, based on the simple transfer matrix method in the near infrared region. Group delay, group velocity dispersion and third-order dispersion of the considered multilayer structure are deduced, and the propagating pulse shape correction in terms of external magnetic field is investigated for TE mode at normal incidence. The numerical simulations show that, for initially chirped ultra-short pulses, the external magnetic field controls both the pulse width and ripple. It is found that with the increasing of static magnetic field applied on the plasma layers, the pulse propagating through the structure reaches its minimum duration at shorter lengths of the crystal. The results of this study may be utilized to develop dispersion engineered ultra-short pulse laser systems. Moreover, the outcomes may also be useful in designing compact size tunable pulse compressors and pulse shapers.

Investigation of heat accumulation effects during deep hole percussion drilling by high power ultrashort pulsed laser radiation

In the last few years, commercially available ultrashort pulsed (usp) laser systems have reached average powers of several 100 W, which makes them also interesting for enhancing traditional applications. For example, laser drilling, a conventionally melt-dominated process, would benefit from the advantages of an usp ablation process. Due to the small processing area in laser drilling, substantial heat accumulates already at a few Watts of average power. This heat accumulation creates melt but also increases the mean ablation rate at least 1 order of magnitude and could lead to a productive drilling process. In this study, the increase of the mean ablation rate in percussion drilling due to heat accumulation is examined for various metals and sample thicknesses for high average powers of up to 300 W and a pulse energy in the milliJoule range. Those investigations have not yet been performed in such detail. It is shown that by doubling the pulse energy the drilling time can be decreased by 2 orders of magnitude due to heat accumulation. This behavior is valid for various metals like steel or aluminum, despite their varying material parameters. By analyzing the influence of different repetition rates and focal diameters, it is shown that the predominant parameter to characterize the heat accumulation is the average power. No significant difference is observed if the average power starting from 20 W is set up by a high pulse energy or repetition rate. Stainless steel has a different behavior compared to the other investigated metals when the pulse duration is changed from 2 to 20 ps, which is caused by a modified plasma generation. For stainless steel, the drilling time is found to be describable with one empirical formula for the entire range of examined average power and sample thickness.

Towards a versatile point-of-care system combining femtosecond laser generated microfluidic channels and direct laser written microneedle arrays

Microneedle-based microfluidic systems have a great potential to become well-accepted medical devices for simple, accurate, and painless drug delivery and lab-on-a-chip diagnostics. In this work, we report on a novel hybrid approach combining femtosecond direct laser written microneedles with femtosecond laser generated microfluidic channels providing an important step towards versatile medical point-of-care systems. Hollow microneedle arrays are fabricated by a laser system designed for two-photon polymerization applications. Compression tests of two different types of truncated cone-shaped microneedle arrays prepared from OrmoComp® give information about the microneedle mechanical strength, and the results are compared to skin insertion forces. Three-dimensional microchannels are directly created inside PMMA bulk material by an ultrashort pulse laser system with vertical channels having adjustable cross-sectional areas, which allow attaching of microneedles to the microfluidic system. A comprehensive parameter study varying pulse duration and repetition rate is performed on two-photon polymerization to identify an optimal laser power range for fabricating microneedles using the same pulse duration and repetition rate as for microchannels. This addresses the advantage of a single laser system process that overcomes complex fabrication methods. A proof of concept flow test with a rhodamine B dye solution in distilled water demonstrates that the combination of microneedles and microchannels qualifies for microfluidic injection and extraction applications.

Acoustic emission-based characterization of focal position during ultra-short pulse laser ablation

Microstructures were ablated using an ultra-short pulse laser system in order to investigate the influence of focal position on the surface topography. In addition, acoustic emissions measured by a piezoelectric sensor adapted to the AISI 4140 workpiece were analyzed and correlated with the focal position and the resulting surface topography. Frequency ranges sensitive to variations of the z-axis position were determined by STFT analysis. Subsequently, significant signal components were processed to enable an inference about the focal position. The hypothesis of assessing the focal position in-process based on acoustic emissions to ensure high precision during laser ablation could be confirmed.

X-RAY EMISSION FROM MATERIALS PROCESSING LASERS.

The emission of laser induced X-rays from materials processing ultra-short pulsed laser systems was measured. The absolute spectral photon fluence was determined using a thermoluminescence detector based few-channel spectrometer. The spectra at 10 cm from the laser focus were in the energy region between 2 and 25 keV with mean energies of ~4–6 keV (when weighted by fluence or directional dose equivalent) and up to 13 keV (when weighted by ambient dose equivalent). The operational quantities, ′(0.07), ′(3) and *(10), were determined to be in the order of 1600–7300, 16–71 and 1–4 mSv per hour processing time, respectively, depending on the material and condition of the workpiece. The dose contribution due to photons above 30 keV was for all quantities negligible, i.e. below 10−3.

Microstructural characterization and quantitative analysis of the interfacial carbides in Al(Si)/diamond composites

The existence of interfacial carbides is a well-known phenomenon in Al/diamond composites, although quantitative analyses are not described so far. The control of the formation of interfacial carbides while processing Al(Si)/diamond composites is of vital interest as a degradation of thermophysical properties appears upon excessive formation. Analytical quantification was performed by GC–MS measurements of gaseous species released upon dissolving the matrix and interfacial reaction products in aqueous NaOH solutions and the CH4/N2 ratio of the evolving reaction gases can be used for quantification. Although the formation of interfacial carbides is significantly suppressed by adding Si to Al, also a decline in composite thermal conductivity is observed in particular with increasing contact time between the liquid metal and the diamond particles during gas pressure infiltration. Furthermore, surface termination of diamond particles positively affects composite thermal conductivity as oxygenated diamond surfaces will result in an increase in composite thermal conductivity compared to hydrogenated ones. In order to understand the mechanisms responsible for all impacts on the thermal conductivity and thermal conductance behaviour, the metal/diamond interface was electrochemical etched and characterized by SEM. Selected specimens were also cut by an ultrashort pulsed laser system to characterize interfacial layers at the virgin cross section in the reactive system Al/diamond.

Spatial light modulator based laser microfabrication of volume optics inside solar modules.

Ultrashort pulse laser systems enable new approaches of material processing and manufacturing with enhanced precision and productivity. Time- and cost-effectiveness in the context of the industrialization of ultrashort laser pulse processes require an improvement of processing speed, which is of key importance for strengthening industrial photonics based manufacturing and extending its field of applications. This article presents results on improving the speed of a laser process by parallelization for creating light deflecting volume optics. Diffractive optical elements are fabricated directly inside the encapsulant of solar modules by utilizing a spatial light modulator based parallel laser microfabrication method. The fabricated volume optical elements effectively deflect light away from front side electrodes and significantly reduce the corresponding optical losses.

Influence of laser spot size and shape on ablation efficiency using ultrashort pulse laser system

Ultra-short pulsed laser systems provide high accuracy and high-quality materials processing for various kinds of applications. However, one of the highest industrial needs is to identify most efficient processes to achieve high throughput. In terms of ultra-short pulsed laser processing of metals, the specific removal rate (ablated volume per time and average power) shows a maximum value i.e. there exists an optimum fluence where the ablation process is most efficient. In this study the influence of spot size and shape (Gaussian and Top Hat) on the ablation efficiency on AISI 304 (1.4301) steel grade was examined using 10 ps and 350 fs laser pulses at wavelengths of 1064 nm and 1030 nm. The investigation revealed a significant drop of the specific removal rate with increasing Gaussian spot size for both pulse duration. While this behaviour is less pronounced for a Top Hat, showing just 8% decrease of specific removal rate for spot sizes bigger than 40 µm.

Simulation of the dynamics of heat accumulation during in-volume modification with ultrashort laser pulses

Working with ultrashort pulsed (USP) laser systems with high repetition rates ≥ 100 kHz, heat accumulation effects evolve and subsequently the absorption becomes linear. As a consequence, the process dynamics change significantly. Structuring glasses with those USP Lasers leads to melted lines that are used for waveguides, welding or selective laser-induced etching whose resulting qualities are strongly depending of the structuring results and given smoothness. We perform simulations during in-volume modification processes, by recalculating the absorption behavior in between the iteration steps of heat accumulation. The simulation results provide explanations for smoothness or non-smoothness of modifications comparable to experiments in glass.

First results with the novel petawatt laser acceleration facility in Dresden

We report on first commissioning results of the DRACO Petawatt ultra-short pulse laser system implemented at the ELBE center for high power radiation sources of Helmholtz-Zentrum Dresden-Rossendorf. Key parameters of the laser system essential for efficient and reproducible performance of plasma accelerators are presented and discussed with the demonstration of 40 MeV proton acceleration under TNSA conditions as well as peaked electron spectra with unprecedented bunch charge in the 0.5 nC range.

Unstable and Multiple Pulsing Can Be Invisible to Ultrashort Pulse Measurement Techniques

Multiple pulsing occurs in most ultrashort-pulse laser systems when pumped at excessively high powers, and small fluctuations in pump power in certain regimes can cause unusual variations in the temporal separations of sub-pulses. Unfortunately, the ability of modern intensity-and-phase pulse measurement techniques to measure such unstable multi-pulsing has not been studied. Here we report calculations and simulations finding that allowing variations in just the relative phase of a satellite pulse causes the second pulse to completely disappear from a spectral interferometry for direct electric field reconstruction (SPIDER) measurement. We find that, although neither frequency-resolved optical gating (FROG) nor autocorrelation can determine the precise properties of satellite pulses due to the presence of instability, they always succeed in, at least, seeing the satellite pulses. Also, additional post-processing of the measured FROG trace can determine the correct approximate relative height of the satellite pulse and definitively indicate the presence of unstable multiple-pulsing.

Imaging of Dental Hard Tissue Surfaces Prepared by an Ultrashort Pulsed Laser System (USPL)

The aim of this study was to compare surface structures of laser-irradiated dental hard tissues using confocal (CFM), atomic force (AFM), and scanning electron microscopy (SEM). The general potential of the AFM in analyzing laser-irradiated surfaces was determined in this context. Specimens of human enamel and dentin were irradiated using an 8.6 W Nd:YVO4 laser with a pulse duration of 8 ps, λ Center=1,064 nm, and a pulse repetition rate of 500 kHz. Surface topology of irradiated areas (1 mm2) was investigated using AFM, CFM, and SEM. Surface roughness R z was measured only with the AFM and the CFM. For non-irradiated enamel and dentin surfaces, roughnesses for CFM and AFM are in the nanometer range. However, major differences in roughness were determined for laser-prepared surfaces. For enamel, R z (CFM)=2.33 μm is much higher compared with R z (AFM)=0.09 μm; in the case of dentin, R z (CFM)=5.35 μm is also much higher compared with R z (AFM)=0.093 μm. Information regarding structural properties of surfaces needs real dimensions, particularly for use in dentistry. In this respect, AFM technology provides no additional results that lead to a significant improvement.

Erratum to: Ablation of carious dental tissue using an ultrashort pulsed laser (USPL) system.

Erratum to: Ablation of carious dental tissue using an ultrashort pulsed laser (USPL) system.

Fracture toughness of ultrashort pulse-bonded fused silica

We determined the bond interface strength of ultrashort pulse laser-welded fused silica for different processing parameters. To this end, we used a high repetition rate ultrashort pulse laser system to inscribe parallel welding lines with a specific V-shaped design into optically contacted fused silica samples. Afterward, we applied a micro-chevron test to measure the fracture toughness and surface energy of the laser-inscribed welding seams. We analyzed the influence of different processing parameters such as laser repetition rate and line separation on the fracture toughness and fracture surface energy. Welding the entire surface a fracture toughness of \(0.71\,\hbox {MPa}\,\hbox {m}^{1/2}\), about 90 % of the pristine bulk material \(({\approx } 0.8\,\hbox {MPa}\,\hbox {m}^{1/2})\), is obtained.

ELBE Center for High-Power Radiation Sources

In the ELBE Center for High-Power Radiation Sources, the superconducting linear electron accelerator ELBE, serving two free electron lasers, sources for intense coherent THz radiation, mono-energetic positrons, electrons, γ-rays, a neutron time-of-flight system as well as two synchronized ultra-short pulsed Petawatt laser systems are collocated. The characteristics of these beams make the ELBE center a unique research instrument for a variety of external users in fields ranging from material science over nuclear physics to cancer research, as well as scientists of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

Beam Shaping in Ultra-short Pulse Laser Processing for Enhancing the Ablation Efficiency

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.

Generation of spectrally stable 6.5-fs visible pulses via filamentation in krypton

We produced $5\text{-}{\rm\mu}\text{J}$ , 6.5-fs visible pulses at a repetition rate of 1 kHz using filamentation in a gas cell filled with krypton followed by spectral selection and phase compensation by a combination of dielectric mirrors. The visible pulses have a smooth spectrum from 520 to 650 nm with a shot-to-shot stability in each spectral component of better than 2% (standard deviation). This pulse compression scheme is simple and robust, and can be easily integrated into intense ultrashort-pulse laser systems.

Self-compression in a solid fiber to 24 MW peak power with few-cycle pulses at 2 μm wavelength.

We report on the experimental realization of a compact, fiber-based, ultrashort-pulse laser system in the 2 μm wavelength region delivering 24 fs pulse duration with 24 MW pulse peak power and 24.6 W average power. This performance level has been enabled by the favorable quadratic wavelength-dependence of the self-focusing limit, which has been experimentally verified to be at approximately 24 MW for circular polarization in a solid-core fused-silica fiber operated at a wavelength around 2 μm. The anomalous dispersion in this wavelength region allows for a simultaneous nonlinear spectral broadening and temporal pulse compression. This makes an additional compression stage redundant and facilitates a very simple and power-scalable approach. Simulations that include both the nonlinear pulse evolution and the transverse optical Kerr effect support the experimental results.