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A hundred-watt chirped pulse amplification (CPA) fiber laser system based on Yb-doped rod-type photonic crystal fiber (PCF) is demonstrated. The seed was an all-fiber dispersion-managed mode-locked oscillator based on chirped fiber bragg grating (CFBG) and semiconductor saturable absorber mirror (SESAM). The oscillator delivers 1.5 ps pulses at a repetition rate of 45 MHz. The output power from the oscillator was 0.9 mW and the central wavelength was 1032 nm. After pulse stretching, pulse picking and amplification, 151 W output power with repetition rate of 1 MHz was achieved by employing a home-made 85/260 μm PCF (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences). After compression, laser pulses with duration of 616 fs and average power of 102.9 W were obtained. The single pulse energy and peak power were calculated to be 102.9 μJ and 167 MW respectively.

We report on the conceptual design of a 2 μm, ultrashort pulse laser system including power amplification based [1] on Tm-doped gain medium. We consider ceramic [2] Tm:Lu2O3 with a direct diode-pumping architectures to develop a solid-state laser technology for high-efficiency, high repetition rate and ultra-short pulse laser, with high peak power and kWscale average power.. A CW pumping (duty cycle of the diodes 100%) and a multi-pulse extraction mode [3] scheme are considered with a multipass scheme with thin disk Tm:Lu2O3 doped at 4% and with lateral (edge) [5] pumping (EPDL). Here we show the preliminary results of the modelling of a two-stage amplifier operating at a repetition rate of 1 kHz, output energy of

We investigated GHz pulse bursts ablation on metals, silicon, zirconium dioxide, soda-lime glass and sapphire for surface structuring applications with a commercial laser system providing a burst pulse frequency of 5.4 GHz and a maximum of 25 pulses per burst pulse train. The results on metal show dramatic decrease of the ablation efficiency and a reduction of the machining quality. For silicon we also observed a reduction of the ablation efficiency for GHz pulse bursts but found a strong increase for MHz pulse bursts using a 10 ps laser system. On glass an increase of the ablation rate for GHz pulse bursts was observed, however with pure machining quality indicated by crakes in the surface and boarders. Zirconium oxide was the only investigated material, where a GHz pulse bursts induces a moderate higher ablation efficiency with comparable surface qualities, however a 10% higher ablation rate was obtained with a 10 ps laser system.

Glass-to-glass welding using ultrashort pulse laser is attracting attention. However, the low processing speed and the requirement of the small air gap between glass substrates have impeded its use in industry. In our study, we achieved rapid welding of glass substrates by coaxially focusing a single femtosecond laser pulse and a continuous-wave (CW) laser with a wavelength that transmits through glass. The selective absorption of the CW laser into the excited electrons increased the processing speed by a factor of 500 compared to the conventional method, while the allowable gap increased by a factor of 4.

Laser structuring with ultrashort laser pulses of electrodes for lithium ion batteries is a promising technology to further enhance their application for energy storage in electric vehicles. We investigated the influence of laser parameters, such as average power, pulse repetition frequency and pulse number when processing electrodes of different thicknesses. Using an ultrashort pulse laser, two graphite anodes with a thickness of 136 μm and 248 μm, were structured with a dot pattern and then analyzed, with respect to their shape and size of the created structure units. Whereas the electrodes individually show similar ablation behavior, the higher coating thickness of 248 μm compared to a coating thickness of 136 μm results in deeper hole structures with a lower ablation volume when processed with the same laser parameters. Thus, the objective of creating electrodes with a three-dimensional structure and reducing material loss can be achieved more efficiently with electrodes with a higher coating thickness.

Many materials with wide bandgap, such as glasses, sapphire, and silicon carbide (SiC), have excellent optical, electrical, and mechanical properties and are used in various industrial and scientific applications. Ultrashort pulse laser processing has been attracting attention as a method of micromachining wide-bandgap materials. Because the peak intensity of ultrashort pulse laser is extremely high, its focused pulse can make wide-bandgap materials absorb its light energy via multiphoton absorption. However, it has a problem that damage occurs around the processed region. In this study, we observe the high-speed phenomena during the ultrashort pulse laser drilling of wide-bandgap materials using pump-probe imaging in combination with a high-speed camera to clarify the mechanism of damage generation. This method visualizes both the static phenomena such as the processed shape and the damage, and the dynamic phenomena such as the electron excitation and the stress wave propagation, which change with each pulse irradiation. In addition, we conduct the experiments by changing the pulse width and the material to be processed to investigate the dependence of damage generation on the processing condition. The results show that stress waves propagating inside the material during processing cause the damage, and that the damage generation pattern changes depending on the pulse width and material. This study contributes to optimizing the processing conditions to suppress the damage during the ultrashort pulse laser processing of wide-bandgap materials.

In this work, 2D piezoelectrically driven MEMS circular scanners have been designed, fabricated and tested. These mirrors own large optical apertures of 7 mm, 10 mm and 20 mm for good beam shapes. Also HR-coating layers for 515 nm and 1050 nm reaching up to 99.99% reflexion and 0.1% transmission were applied onto the mirror surface for the suitability of high power laser, where the wavelengths were specified according to the laser source development demands. Based on piezoelectric position sensing elements integrated on the MEMS mirrors a closed-loop control was developed. In this paper the design efforts, realizing circular-scanning and eliminating non-linearity during mode superposition, and fabrication efforts will be reported. Characterization results focusing on mechanical behaviors, position sensing signal, HR-coating will be also important parts of this work.

Micro/nanoscale surface patterning of zirconia ceramic is needed for surface functionalization and performance enhancement, such as improved biocompatibility of medical devices, as well as device miniaturization. Therefore, formation of laser-induced periodic surface structures (LIPSS) with periods shorter than the laser wavelength on a zirconia ceramic was carried out using an ultrashort pulsed laser. In this case, it is important to shorten the processing time required for forming the LIPSS without deteriorating the processing quality. Using the various-parameters-controlled laser processing and observation system, we optimized LIPSS formation by changing parameters such as pulse duration, repetition frequency, number of shots, and fluence.

Several conditions—including optics, work material and the environment—can influence the results of laser processing. The physical model of laser-processing phenomena is represented as a complex function; thus, the development of a prediction model using machine learning (ML) may be an effective approach. In this study, the quantity of ablation under femtosecond ultrashort pulse laser processing was predicted using an ML model. For work materials, polycrystalline diamond was used as a composite material, cold rolled steel and aluminum were used as metals, silicon was used as a semiconductor and glass was used as an insulator. A total of 340 datasets were prepared for each material. The neuralnetwork algorithm was used to develop the prediction model. We also explored the challenges from the viewpoint of materials informatics. By applying the algorithm to each material dataset, prediction models with ±20 % precision were developed. When the algorithm was applied to all material datasets, the resulting prediction models had ±40 % precision for the single materials, whereas the prediction function was far from ideal in the case of the composite material. It is necessary to tune the input datasets (particularly the physical conditions of the composite material) for the ML model.

Ultrashort pulse micromachining has found a rising number of applications in a variety of scientific and industrial fields. In order to address the growing field of applications, target materials and customer requirements, a high degree of pulse parameter flexibility and ease of integration is needed. The newest generation of the TruMicro Series 2000 delivers unique features such as fast tunable pulse duration, MHz- up to GHz-burst modes in combination with flexible Pulse on Demand and elevated average power of 100W for improved productivity scaling. Three available wavelengths (343nm, 515nm, 1030nm), an integrated hollow-core fiber interface, as well as a new advanced ultrashort pulse laser control, all combined into a new one box optomechanical design with identical interfaces and dimensions opens new paths for cutting-edge applications. The improved flexibility enables fast (

We experimentally and theoretically investigate the nonlinear frequency conversion of transparent chalcogenide optical materials using ultrashort midwave infrared laser pulses at 3.6 microns. Evidence of the structure of second through sixth harmonic generation demonstrates different levels of filamentation related to laser intensity, sample thickness, and sample position. Simulations using a (3+1)D model with experimentally measured n2 values and random quasi phase matching provide good qualitative agreement with experimental data. Together, the data suggests that focusing geometry and material structure play a significant role in harmonic generation in these materials.

Recently, ultrashort pulse laser processing has been attracting attention as a method for microprocessing SiC. However, there is a severe problem with this technique: cracks formed around the processed shape hinder precision processing. In this study, to reveal the mechanism of crack formation during the ultrashort pulse laser processing of SiC, we investigated the high-speed phenomena happening during processing. The phenomena were captured using an imaging system consisting of the pump-probe imaging method and a high-speed camera. The ultrashort pulse laser and the high-speed camera are operated at 1 kHz in synchronization, which enables us to capture the high-speed phenomena during processing that change as the process progresses. As a result, we have succeeded in capturing the damage formation process. The results show that the damage is generated and grows only near the tip of the processed hole and does not change once it is generated. As known from previous studies, stress waves propagate around the tip of the processed hole during machining, suggesting a strong correlation between stress waves and damage generation. The larger the pulse energy, the more damage is generated, due to the large stress waves generated because of the large removal volume. When the pulse width is long, the material is thermally affected, and thermal damage is thought to occur at the entrance of the processed hole.

We report on the stress induced birefringence of 10 mm BK7 cubes bonded to 15 mm x 15 mm x 5 mm aluminium coupons using ultrashort pulse laser welding, hydroxide catalysis bonding, and an optical adhesive using a standard approach used in industry. It was observed that ultrashort pulse laser welding results in a low level of stress induced birefringence within an 85% optical aperture of the 10 mm cube. These levels are suitable for use in photography and microscopy applications as defined by the relevant ISO standard for permissible stress induced birefringence limits in optics. Hydroxide catalysis bonding was shown to approach and possibly exceed this limit, however, it must be noted that the bonding area of the hydroxide catalysis samples greatly exceeds that of the welding and adhesive bonded samples by a factor of 20 and thus will likely experience grater mechanical stress from defects on the metal surface. The induced stress in the adhesively bonded samples was found to be well-confined, with the retardation falling below the maximum acceptable limits for microscopy type applications of 10 nm/cm retardation within 0.5 - 1 mm of the glass-metal interface for the majority of samples tested. The hydroxide catalysis and ultrashort pulse laser welded samples have a significant increase in retardation in the region directly above the bonded surface compared to the adhesively bonded samples. This retardation decays rapidly as a function of distance from the bonded interface. In the case of ultrashort pulse laser welded samples, optimisation of the welding parameters has been shown to control the level of retardation within the interface region, thus reducing the overall effect of the weld on the bulk optic.

The recent developments in high-speed laser machining combine high-average power lasers with ultrafast beam deflection systems.1 However, when using ultrashort pulses at megahertz pulse repetition frequencies the incoming laser beam may interact with the plasma/particle ablation plume that is due to the very short time intervals between the impinging megahertz-repetitive pulses. Thereby, the subsequently irradiated pulses will either be reflected, absorbed and/or scattered by the still existent plasma/particle plume as induced by the previously irradiated pulse(s). This shielding of the incoming laser beam is adversely affecting material ablation and can potentially be avoided when the beam is ultrafast moving in front of the emerging plasma/particle plume. For the real-time analysis of such fairly unknown processing regimes, a pump-probe shadowgraph imaging technique was used to visualize laser materials interaction in high-speed ultrashort pulse laser machining. The pump laser source supplied a beam of λ=1030 nm, ƬH =600 fs and 48MHz maximum pulse repetition frequency for material ablation. A diode laser system provided the probe beam of λ=688 nm and ƬH =13 ns. The pump and the probe beam could be electronically delayed within a time frame of up to 10 µs. Sequences of shadowgraphs will be presented to visualize plasma/ablation plume expansion for providing unique insights into high-pulse repetition frequency ultrashort pulse laser materials interactions at ultrafast laser beam movements above 250m/s. The shadowgraphs reveal that the form of the emerging plasma/particle plumes is influenced by the laser beam moving speed. The plumes vary in their density and structure from clearly distinguishable at highest speed, through smoothly detached to be turbulent. In addition, it is shown on the shadowgraphs material ejection takes place at the time when the ultrafast scanned laser beam moved away from the initial irradiation area.

Self-organized structures like cavities and LIPSS are often used to generate specials surface functionalities with ultrashort pulsed laser machining. For a better understanding of the formation of cavities and LIPSS we performed a layer by layer surface visualizations on single crystal iron with different specific lattice orientations. The experiments indicate that formation and grow rate of cavities depends on the orientation of the crystal lattice. Based on this information we have been able to predict the cavity formation for given fluence on an electrical steel sheet with non-oriented grains with an accuracy better than 90% by analyzing the lattice orientations of the grains by a previous EBSD measurement.

As a result of laser ablation, a surface roughness Sa between 0.4 μm and 2μm is obtained on 3D structures that have to be polished afterwards to meet customer requirements. For that reason a laser polishing process using ultrashort pulse laser sources is investigated. Applying the polishing process immediately after laser structuring in the same setting simplifies the process chain and saves both time and money. The results reveal an improvement of the surface roughness from an initial grinded surface with 0.4 μm to 0.2 μm by ultrashort pulse laser polishing. The productivity measured by the area that can be processed per time (polishing rate) is with 12.15 cm2/min one order of magnitude higher than state of the art laser polishing using nanosecond pulsed lasers.

We investigate the nonlinear optical properties of transparent optical materials using ultrashort midwave infrared laser pulses between 3 and 4 microns. Random quasi-phase matching in polycrystalline materials generates multiple frequency harmonics of both odd and even orders throughout the transmission window of the target. We also investigate single crystal and amorphous materials and demonstrate a range of frequency conversion and pulse broadening. Simulations using a nonlinear polarization model enhanced with ionization and experimentally measured n2 values provide good qualitative agreement with experimental data.

We present a new fabrication method to realize smooth structures in lithium niobate. Therefore a detailed study on laser micro-polishing using ultrashort laser pulses is carried out by separating the effects of spatial pulse overlap and temporal pulse overlap. The adventage of this approach is the smoothing of the processed area by a simultaneous ablation. That will allow ablation depths between 1 µm and 4 µm with rms-roughness values of ≈20 nm.

We present a laser beam source with average output powers of up to 15 W and pulse energies of up to 50 μJ at a wavelength of 3.4 μm based on a frequency converted ultrashort pulse laser. The laser beam source consists of a commercially available ultrashort pulse laser with a pulse duration of 10 ps and a downstream multi-stage optical parametric frequency converter based on large-aperture PPLN crystals. Different output powers, pulse energies, conversion efficiencies and beam qualities are achieved depending on the design of the multi-stage setup. In addition, we present some results of processing experiments on different polymer films obtained with the laser beam source presented here.

Changes in the polarization properties of a spatially and spectrally partially coherent stochastic electromagnetic Gaussian Schell-model (EGSM) ultrashort pulse laser beam propagating through the atmospheric turbulence are investigated. We derive analytic equations for the spectral degree of polarization and the polarization angle in terms of the extended Huygens-Fresnel principle and elements of the 2×2 cross-spectral density function matrix of the electric field. Within the framework of the Tatarskii model of the turbulent atmosphere, which taking the inner scale of the turbulent eddies into consideration, the dependence of along the z-axis and off the z-axis the spectral degree of polarization and the polarization angle of a stochastic EGSM ultrashort pulse laser beam on the parameters of the source including spatial coherence length and temporal coherence length are stressed and illustrated numerically. Results show that the spectral degree of polarization and the polarization angle of the EGSM ultrashort pulse laser beam propagating through the atmospheric turbulence are determined by the parameters of the source. Our results have potential applications in atmospheric remote sensing and ground-to-satellite optical communications.

In this paper, we have developed a chirped-pulse Er-doped all-fiber ultrashort pulse (USP) laser suitable for highresolution Raman distributed temperature sensor (RDTS) system application. Chirped-pulse regime which is determined by the positive net-cavity dispersion of +0.12 ps2 allows us to increase the energy of the pulses for effective signal-to-noise ratio in receiving system. It also helps to avoid the influence most of nonlinear effects due to relatively long pulses duration of ~24.6 ps. The average power of the pulses is estimated to be ~1 mW from master oscillator, and currently increased up to ~15 mW by power amplifier. A relatively low repetition rate of ~ 7.925 MHz with signal-to-noise ratio ~ 69 dB was achieved using the resonator length of ~25.6 m. To characterize short-term stability of the obtained regime we have also measured the relative intensity noise of the laser, which is < -107 dBc/Hz in the range of 3 Hz - 1 000 kHz.

Within the last ten years, ultrashort pulsed lasers have developed from complex scientific devices to reliable industrial tools for various applications. MOPA systems with fiber amplifiers offer an excellent combination of small size, reliable design and flexible parameters. However, their power and more specifically pulse energy levels could not reach the high level of other laser designs based on disk or rod amplifiers. In the latest development generation of TRUMPF’s TruMicro 2000 series, we present an all-fiber ultrashort pulse laser with a maximum pulse energy of 100 µJ based on industrial grade technologies. The high energy level allows for efficient frequency conversion to a wavelength of 515 nm which is beneficial for numerous applications. Furthermore, the linear amplifier concept combined with intelligent electronics and software allow for flexible adaption of parameters: The laser is equipped with a sophisticated pulse-on-demand function that allows variable pulse picking from the seed frequency of 50 MHz. This bears the challenge of fluctuating pulse ensures which is overcome by a combination of measures that ensure constant pulse energies even with variable pulse distances. As a result, the laser pulses can be synchronized to external trigger signals with a time jitter as low as 30 ns. Additionally, the TruMicro 2000 offers a flexible burst mode up to 8 pulses in one burst and the pulse duration can be changed continuously from femto- to picoseconds. With a newly patented technology, the full sweep from 300 fs to 20 ps is done in less than 500 ms without any influence on beam parameters.

The high peak power of ultrashort laser pulses enables the processing of transparent materials by inducing absorption nonlinearly. There are already a variety of applications in the field based on volume or surface absorption. Spatial beam shaping offers high potential, for example by applying Bessel-like beams for single pulse full thickness modification in cutting applications. Temporal shaping the pulse or applying bursts of pulses adapted in amplitude and interval is a further option to localize and dose the energy deposition. An alternative option for scaling is processing at elevated repetition rates. This typically results in accumulation effects, often not desired, sometimes useful or even necessary for several applications. Learning about the complex interplay of the effects relevant for ultrashort pulse laser processing of transparent materials is crucial for the development of advanced industrial applications. Pump-probe diagnostics have proven to be a powerful tool for analyzing the laser matter interaction of spatially shaped beams with high temporal resolution. By extending this to broader range of temporal parameters of the pump, including flexible burst operation, combined with unlimited delay range of the probe and integrated optional polarization microscopy, high speed camera and observation during translation of the workpiece, the setup is suitable to analyze effects on different temporal and spatial scales in a single setup. The potential of this modular experimental system is demonstrated by analyzing multi pulse focusing of Gaussian and Bessel-like beams into glass.

The use of self-organized micro/nanostructured surfaces formed using femtosecond laser surface processing (FLSP) techniques has become a promising area of research for enhancing surface properties of metals, with many applications including enhancing heat transfer. In this work, we demonstrate advantages of the use of dual-pulse versus single-pulse FLSP techniques to produce self-organized micro/nanostructures on copper. With the dual-pulse technique, the femtosecond pulses out of the laser (spaced 1 ms apart) are split into pulse pairs spaced < 1 ns apart and are focused collinear on the sample surface. Single-pulse FLSP techniques have been widely used to produce self-organized “mound-like” structures on a wide range of metals including a number of stainless steel alloys, aluminum, nickel, titanium, and recently on copper. Due to its high thermal conductivity, copper is used in many critical heat transfer applications and micro/nanostructured copper surfaces are desired to further improve heat transfer characteristics. Using single-pulse (pulses spaced 1 ms apart) FLSP techniques, self-organized microstructure formation on copper requires much higher pulse fluence than is commonly used for producing microstructures on other metals, which results in instabilities during laser processing (non-uniform surfaces), low processing efficiency, and limitations on the control of the types of structures produced. In this paper, we report results that demonstrate that the dual-pulse FLSP technique can be used to produce microstructures on copper more efficiently than using single-pulse FLSP, with better control of the surface structures produced. Cross-sectional subsurface microstructure analysis is also presented for single-pulse versus dual-pulse FLSP functionalized copper surfaces.

The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel, aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 1013 W/cm2 , X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.

SURVICE [S17-095008/DOTC-17-01-INIT0086]; Air Force Office of Scientific Research (AFOSR) [FA9550-16-1-0069]; AFOSR multidisciplinary research program of the university research initiative (MURI) [FA9550-16-1-0013]; AFOSR [FA9550-16-1-0121]

High-power pulsed lasers are widely used in scientific and industrial applications, and control of irradiance distribution using beam shaping optics is important to increase efficiency of laser techniques. A popular approach of building optical systems is use of a refractive beam shaper and imaging optics afterwards to create resulting spot of required intensity profile and size in a working plane. Important feature of imaging optical systems is creating the image after the paraxial focal plane – this means focusing of a high energy pulse before the image plane and probable air break down disturbing the intensity distribution in image and reducing performance of entire system. This problem becomes especially actual with modern ultrashort pulse lasers which pulse energy is constantly rising. Specific requirements to beam shaping optics in these laser systems are providing variable irradiance distributions, saving of beam consistency and flatness of phase front, capability to work with TEM00 and multimode lasers, resistance to high peak power radiation. Among various refractive and diffractive beam shaping techniques only refractive field mapping beam shapers like piShaper meet these requirements. The operational principle of these devices presumes almost lossless transformation of laser beam irradiance from Gaussian to flat-top, super-Gauss or inverse-Gauss through controlled wavefront manipulation inside a beam shaper using lenses with smooth optical surfaces. To overcome the problem of eventual air break down by imaging after a beam shaper it is suggested to apply optical systems with field lenses, which main function is changing the focusing conditions without change of the system transverse magnification. This approach allows to “move” the point of pulse energy concentration from the image zone and increase the focus spot size in order to reduce probability of air break down or avoid it at all. This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts of their applying in optical systems of high-power lasers. Examples of real implementations and experimental results will be presented as well.

In this study, an optical setup based on two phase-only spatial light modulators is presented, capable to shape two parallel laser beams. Each of the light modulators creates different and complementary Gaussian multi-spot distributions in the focal plane. By the polarization based combination of two differently shaped beams, a multi-spot beam profile with higher multi-spot density than for a single spatial light modulator setup can be obtained without speckles to appear. Beam shaping results are characterized by means of a beam profile camera and compared to a single spatial light modulator setup. Beam shapes generated by the presented setup are applied to ultrashort pulse laser ablation of metals. The potential of the presented optics is discussed regarding ground roughness and waviness of the ablated structure.

The ongoing increase of peak and average power of diode lasers makes the consideration of thermo-optical effects due to absorption in the optical system necessary. Otherwise, this local increase in temperature might lead to a significant reduction of the entire functionality or a complete failure. Here, we consider this issue in volume Bragg gratings (VBG) as they are typically used for wavelength stabilization of the laser diodes. Conventionally, these gratings are fabricated very effectively by inscription using ultraviolet light into special photosensitive glasses. However, the use of these special glasses might compromise the transmission and, thus, limit the application. As an alternative, we inscribe VBGs in pure fused silica using ultrashort laser pulses and the phase mask scanning technique. Applying these gratings for external stabilization of high power diode lasers already demonstrated an outstanding performance in sense of spectral drift due to load change compared to conventional VBGs. Here, we present investigations to further optimize the residual absorption of the gratings especially in the NIR. Apart from analyzing the influence of the processing parameters on the residual absorption, we also studied the possibility for a thermal post-processing to anneal absorbing defects. The resulting gratings exhibit a residual absorption close to the intrinsic absorption of fused silica, making them ideally suited for high power applications. Further applications in ultrashort pulse laser systems for pulse stretching and compression are discussed.

We investigate the influence of different laser parameters on Grain-Oriented electrical silicon steels (GO SiFe). The advanced use of ultra-short laser pulses for scribing steel sheets with magnetic domains refinement and loss reduction purpose is proposed. The GO steel samples are commercial ones, commonly used in transformer cores. We compare laser impact on domains refinement obtained by thermal effects (with CW and long laser pulses), with thermal-free ablation produced by an ultrashort source. The laser scribing conditions (power or pulse energy, speed and pattern geometry, size and orientation) are optimized to have minimal deformation on material surface, while maximizing the influence and benefits on the magnetic properties. The steels used for these experiments have got one fixed composition (Silicon Iron alloy with 3% Silicon), the sheets used have got one thickness of 0.23 mm, very big grains size around 3 mm in average and one preferred lamination direction. The effects of the laser on steel samples are studied with different techniques, in order to show the impact on the surface topography, the metal microstructure and on the scalar magnetic properties. The confocal microscopy is used to probe the surface topography. The MFM (Magnetic Force Microscopy) is used to analyze in detail the formation of magnetic domains in the vicinity of laser patterns at the nanoscopic scale. The MOKE microscopy (Magneto-Optic Kerr Effect) is used to get images of magnetic domains at the microscopic scale. The SST (Single Sheet Tester) and the Epstein frame are used to measure the static and dynamic behavior and the power losses with two or one directional flux at the macroscopic scale. The motivation is to find the best laser treatment and patterns to improve the scalar magnetic properties and to reduce the energy consumption of electrical devices made with this steel.

Deep ultraviolet (DUV) femtosecond laser repair of Cr binary and phase-shift photomasks is routine and well established over decades of practice. As Moore’s law progresses into sub-10 nm nodes, there is a necessary diversification of lithography technologies which can similarly benefit from the high-throughput, non-contact, contaminate-selective capabilities of ultrashort pulsed laser repair. These alternative lithography masks include extreme ultraviolet (EUV) TaN reflective and DUV SiN-based photomasks. Additionally, parametrically systematic studies are shown with intent to find the limits of selective, sub-resolution, removal of simulated soft defects in various patterns on DUV photomasks.

Surface functionality is an increasing and crucial factor for the success and acceptance of a product. Through structured surfaces products can gain additional functions. Microstructures enable, for example, friction reduction in combustion engines or optimize the efficiency of LED-based illumination systems. Furthermore, in the area of consumer products optical and haptic functions determine essentially the product quality. The most common way to create surface functionality in mass production are replication processes via structured mold tools. In industrial environment nanosecond pulsed laser sources are established due to their high throughput. Ultrafast laser processing still have shown their advantages characterized by highly precise ablation with minimal thermal influence. Nevertheless, the achieved productivity is still too low for industrial applications in many cases due to the low ablation rate of ultrashort pulse laser processing. To overcome the above mentioned drawback, we investigate ultrashort pulsed laser processing using pulse bursts including pauses to avoid heat accumulation and several scaling strategies like increasing the repetition frequency and enlarging the spot size. The experiments are processed with a pulse duration of 900 fs or 2 ps for productivity reasons. Using average powers of more than 70 W an ablation rate of up to 13 mm3/min is achieved. Not only productivity benefits from burst processing with pauses, but also the surface quality gets significantly improved. The roughness of an ablated surface processed with conventional bursts for example amounts Rα = 5.19 μm while the same burst configuration including a pause produces a surface roughness of Rα = 0.51 μm.

For astronomy, the search of exo-planets and spectral analysis of galaxies and stars with ground based telescopes is an ongoing topic especially with the future planned telescopes with even larger mirror diameters. But for the ground based observation the atmosphere is a limiting factor. Besides of air fluctuations also spectral noise is influencing the observation. A forest of emission lines by OH relaxations that are orders of magnitudes stronger than the stars and galaxies appear in the night sky. These lines vary in intensity but are fixed in wavelength. Therefore, fiber Bragg gratings (FBG) are a perfect tool for the suppression of these emission lines. FBGs provide a high filter quality by filtering out magnitudes of intensity in a narrow wavelength window of below 0.5nm bandwidth. Because fibers are already widely used in telescopes to deliver light into spectrographs, the FBGs could be inscribed directly into the fibers. But for astronomy mostly multimode fibers are used where FBGs do not work as needed because of the different propagation constants of higher modes. The solution is the transition to single mode fibers. In terms of compactness and robustness a multicore fiber would be the optimal solution. But the homogeneous modification of a multicore fiber is a challenging task. We report on the ultrashort pulse laser inscription of FBGs into a multicore fiber consisting of 7 cores. Furthermore, investigations on the homogeneity of the inscribed modifications as well as the spectral properties are presented. I would like to participate in the Student Competition.

Additive manufacturing gained increasing interest during the last decade due to the potential of creating 3D devices featuring nearly any desired geometry. One of the most widely used methods is the so-called powder bed method. In general, conventional cw and pulsed laser sources operating around 1030 nm and CO2 lasers at 10.6 μm are usually applied. Among other materials like polymers, these systems are feasible for several metals, alloys and even ceramics, but easily reach their limitation at a wide range of other materials, regarding required absorption and intensity. In order to overcome these limits, ultrashort pulse laser systems are one approach. Due to the increased peak power and ultrashort interaction times within the femtosecond and picosecond time range, materials with extraordinary high melting points, increased heat conductivity or new composites with tailored specifications are coming into reach. Moreover, based on the nonlinear absorption effect, also transparent materials can be processed. Here, we present the selective laser melting of pure copper using ultrashort laser pulses. This work involves a comparative study using 500 fs pulses at processing wavelengths of 515 nm and 1030 nm. The repetition rate of the applied laser system was varied within the MHz range in order to exploit heat accumulation. By using the ultrashort interaction times and tailoring the repetition rate, the induced melt pool can be significantly optimized yielding robust copper parts revealing thin-wall structures in the range below 100 μm.

This paper discusses latest results obtained in high-pulse repetition frequency micro hole percussion drilling of metal foils ranging between 25 μm and 100 μm thickness. In the investigations a high-repetition frequency, high average power ultrashort pulse laser source was applied, providing a near-infrared laser beam of 1.03 μm wavelength, 87.5 W maximum average laser power (at the processing plane), 51.48 MHz maximum pulse repetition frequency, 650 fs pulse duration, and 30 μm focal spot size. In the experiments, the process parameters pulse energy, pulse repetition frequency, and the pulse number were varied thus in order to evaluate their influence on both micro hole geometry (hole diameter, roundness), and drilling quality (thermal modification, melting residues). The minimum pulse number required to drill through the metal foils was detected by an in-house developed pulse counter. A significant lower pulse number to drill through the material was observed when irradiating ultrashort pulses of megahertz pulse repetition rates. On the one hand, this effect might be induced by thermal accumulation thus increasing the substrate temperature which, in turn, enhances the laser beam absorption or rather lowers the energy consumption needed for evaporation. On the other hand, pulse interactions with the still existing plasma at these very short pulse intervals cannot be neglected, so a higher amount of material might be ejected assisted by the higher plasma pressure. As another detrimental effect for megahertz pulses, a significantly lower processing quality was obtained that was due to intensive melting affected by heat accumulation, as the melt re-solidified within and/or surrounding the drilling holes. Surprisingly, by irradiating pulses at 20 MHz and above, the melt was completely ejected out from the drilling holes as the drilling efficiency was even the highest. Finally, based on the findings of this study, optimum parameter settings are presented with regard to highest machining quality and drilling throughput.

This paper presents results of poly(L-lactide) and poly(L-lactide)/hydroxyapatite composite cutting optimization using 2nd harmonic of femtosecond fiber laser. There are several limitations regarding the use of femtosecond lasers for processing of heat-sensitive medical grade polymers such as poly(L-lactide) (PLLA). Improper use of ultrashort pulse laser may lead to heat load into surrounding material causing its melting and crack formation when excessive energy is deposited and pulse repetition frequency is too high. The optimization of laser parameters is necessary not only because of heat but also due to reflection and scattering of incident light in high aspect ratio V-shaped groove resulting in decrease of ablation rate with an increasing number of repetitions. This problem is especially important in case of polymer foils thicker than 200 μm and the beam spot size which is typically around 15 - 30 μm in commercial systems. In this work we present threshold fluence and ablation rates for three types of material: amorphous PLLA, crystalline PLLA and PLLA/hydroxyapatite composite. For those materials in form of thick foils (∼400 μm) we performed cutting optimization. A significant improvement of cutting efficiency in case of thick foils was made by applying a method of multiple, overlapped cuts.

Today ultrashort pulse (USP) fiber lasers are in great demand in a frequency metrology field, THz pulse spectroscopy, optical communication, quantum optics application, etc. Therefore mode-locked (ML) fiber lasers have been extensively investigated over the last decade due the number of scientific, medical and industrial applications. It should be noted, that USP fiber lasers can be treated as an ideal platform to expand future applications due to the complex ML nonlinear dynamics in a laser resonator. Up to now a series of novel ML regimes have been investigated e.g. self-similar pulses, noise-like pulses, multi-bound solitons and soliton rain generation. Recently, we have used a highly nonlinear germanosilicate fiber (with germanium oxides concentration in the core ~ 50 mol. %) inside the resonator for more reliable and robust launching of passive mode-locking based on the nonlinear polarization evolution effect in fibers. In this work we have measured promising and stable ML regimes such as stretched pulses, soliton rain and multi-bound solitons formed in a highly-nonlinear ring laser and obtained by intracavity group velocity dispersion (GVD) variation in slightly negative region. As a result, we have obtained the low noise ultrashort pulse generation with duration < 250 fs (more than 20 bound pulses when obtained multi-bound soliton generation with intertemporal width ~ 5 ps) at a repetition rate ~ 11.3 MHz (with signal-to-noise ratio at fundamental frequency > 59 dB) and relative intensity noise

This paper uses the dual-phase-lag model to study the ablation characteristics of femtosecond laser processing for nanometer-sized ceramic films. In ultrafast process and ultrasmall size where the two lags occur, a dual-phase-lag can be applied to analyse the ablation characteristics of femtosecond laser processing for materials. In this work, the ablation rates of nanometer-sized lead zirconate titanate (PZT) ceramics are investigated using a dual-phase-lag and the model is solved by Laplace transform method. The results obtained from this work are validated by the available experimental data. The effects of material thermal properties on the ablation characteristics of femtosecond laser processing for ceramics are also discussed.

We investigate the tight focusing of radially polarized ultrashort pulse laser beam. It is found that pulse delay phenomenon occurs near the focus. This is, near the focus, the photon travels slower than the speed of light.

Ultrashort pulsed laser sources offer new possibilities in precise and efficient material processing. Deep understanding of the fundamental laser-material interaction aspects is of great importance. We report on pump-probe reflectometric investigations of the ablation process on molybdenum over the complete temporal process range from the pulse impact to the final steady state. The ablation process can roughly be separated in three sections. In the first tens of picoseconds mainly the optical material properties are changed without significant material motion. Between 50 ps and a few ns the irradiated material is bulging in a spallation or phase explosion process. The actual ablation by material ejection is observed at delay times greater than 20 ns. The transient reflectivity during and in the first tens of ps after the laser irradiation in conjunction with the transient absorption influences decisively the laser-matter interaction for example when working with longer pulse durations or double pulse sequences. Direct measurements of the absorption properties by ultrafast time-resolved ellipsometry at fluences close to the ablation threshold fluence are missing to date. In this paper, pump-probe ellipsometric measurements on molybdenum – complementing the pump-probe reflectometric measurements – are presented showing ultrafast changes of the complex refractive index N = n – i k including additional information on the absorption. The imaginary part k is reduced already after 10 ps by 50% representing an increase of the optical penetration depth by a reduction of the material density. These extensive investigations pave the road towards a better understanding of pulse duration dependent laser ablation efficiency, double or burst mode laser ablation and lattice modifications in the first ps after the laser pulse impact.

We report on ultrashort pulse induced welding of fused silica without previously optical contacting of the samples. We used a TruMicro 2020 by TRUMPF delivering bursts of intense ultrashort laser pulses with an individual pulse energy of up to 10 μJ, an intraburst pulse separation of 20 ns and a burst repetition rate of 200 kHz. With this setup we could realize a large pool of molten material with a length of up to 450 μm and a diameter of around 160 μm. If the laser focus is placed near the surface of a glass sample the low viscosity of the hot material induces bulging of the surface and ejection of the molten material. This molten material can be used to fill a gap of up to 3 μm between fused silica samples. We also determined the breaking strength with a three-point bending test. The determined maximal value of 73 MPa is equivalent to 85% of the stability of the pristine bulk material.

Ultrashort pulse laser ablation has become an important tool for material processing. Adding liquids to the process can be beneficial for a reduced debris and heat affected zone width. Another application is the production of ligand-free nanoparticles. By measuring the ablation rate of iron for femtosecond pulsed laser ablation in different solvents and solvent-mixtures, the influence of the solvent properties on the ablation process is studied. The ablation efficiency is quantified by measuring the ablation rate in dependency of the fluence from 0.05 J/cm2 up to 5 J/cm2 in water-ethanol and water-acetone mixtures which are varied in 25 % steps. The ablation rate is significantly influenced by the solvent-mixtures.

New ultrashort pulse laser systems exhibit an ever increasing performance which includes shorter pulses and higher pulse energies. Optical components used in these systems are facing increasing requirements regarding their durability, and therefore understanding of the damage mechanism is crucial. In the ultra-short pulse regime electron ionization processes control the damage mechanisms. For the single wavelength, single pulse regime the Keldysh [1] and the Drude model [2] allow a quantitative description of these ionization processes. However, in this model, the electrical field is restricted to a single wavelength, and therefore it cannot be applied in the case of irradiation with two pulses at different wavelengths. As frequency conversion is becoming more common in ultra-short pulse applications, further research is needed in this field to predict the damage resistance of optical components. We investigate the damage behavior of high reflective mirrors made of different metal oxide materials under simultaneous exposure to ultra-short pulses at the wavelengths 387.5 nm and 775 nm, respectively.

We report on a Yb:YAG laser amplifier for ultrashort pulse applications at kW-class average power. The laser uses two large-aperture, disk-type gain elements fabricated from composite ceramic YAG material, and a multi-pass extraction architecture to obtain high gain in a chirped-pulse amplification system. The disks are edge-pumped, thus allowing for reduced doping of the host material with laser ions, which translates to lower lasing threshold and lower heat dissipation in the Yb:YAG material. The latter makes it possible to amplify a near diffraction-limited seed without significant thermo-optical distortions. This work presents results of testing the laser amplifier with relay optics and passive polarization switching configured for energy extraction with up to 40 passes through the disks. Applications for the ultrashort pulse laser amplifier include producing a laser-induced plasma channel, laser material ablation, and laser acceleration of atomic particles.

We report on the investigation of a novel ceramic Tm:Lu2O3 amplifier lasing at around 2-μm and offering efficient generation of high-energy pulses with high-peak power at high repletion rate, high efficiency, and with near-diffraction-limited beam quality (BQ). The amplifier has a bandwidth of over 300 nm, which offers broad tunability. The bandwidth also supports generation of ultrashort pulses in the femtosecond regime. The “2-for-1” pump architecture of the Tm ion enables high optical-to-optical efficiency while pumping at around 800 nm. High thermal conductivity of the Lu2O3 host supports operation at high-average power. The ceramic nature of the Lu2O3 host overcomes the scalability limits of single crystal sesquioxides.

We report on a Yb:YAG laser amplifier for ultrashort pulse applications at kW-class average power. The laser uses two large-aperture, disk-type gain elements fabricated from composite ceramic YAG material, and a multi-pass extraction architecture to obtain high gain in a chirped-pulse amplification system. The disks are edge-pumped, thus allowing for reduced doping of host material with laser ions, which translates to lower lasing threshold and lower heat dissipation in the Yb:YAG material. The latter makes it possible to amplify a near diffraction-limited seed without significant thermo-optical distortions. This work presents results of testing the laser amplifier with relay optics configured for energy extraction with up to 40 passes through the disks. Applications for the ultrashort pulse laser amplifier include producing laser-induced plasma channel, laser material ablation, and laser acceleration of atomic particles.

Interaction of ultrashort laser pulses with transparent materials is a powerful technique of modification of material properties for various technological applications. The physics behind laser-induced modification phenomenon is rich and still far from complete understanding. We present an overview of our models developed to describe processes induced by ultrashort laser pulses inside and on the surface of bulk glass. The most sophisticated model consists of two parts. The first part solves Maxwell’s equations supplemented by the rate and hydrodynamics equations for free electrons. The model resolves spatiotemporal dynamics of free-electron population and yields the absorbed energy map. The latter serves as an initial condition for thermoelastoplastic simulations of material redistribution. The simulations performed for a wide range of irradiation conditions have allowed to clarify timescales at which modification occurs after single laser pulses. Simulations of spectrum of laser light scattered by laser-generated plasma revealed considerable blueshifting which increases with pulse energy. To gain insight into temperature evolution of a glass material under the surface irradiation conditions, we employ a model based on the rate equation describing free electron generation coupled with the energy equations for electrons and lattice. Swift heating of electron and lattice subsystems to extremely high temperatures at fs timescale has been found at laser fluences exceeding the threshold fluence by 2-3 times that can result in efficient bremsstrahlung emission from the irradiation spot. The mechanisms of glass ablation with ultrashort laser pulses are discussed by comparing with the experimental data. Finally, a model is outlined, developed for multi-pulse irradiation regimes, which enables gaining insight into the roles of defects and heat accumulation.

The class of ultra-short-pulse (USP) laser sources are used, whenever high precession and high quality material processing is demanded. These laser sources deliver pulse duration in the range of ps to fs and are characterized with high peak intensities leading to a direct vaporization of the material with a minimum thermal damage. With the availability of industrial laser source with an average power of up to 1000W, the main challenge consist of the effective energy distribution and disposition. Using lasers with high repetition rates in the MHz region can cause thermal issues like overheating, melt production and low ablation quality. In this paper, we will discuss different approaches for multibeam processing for utilization of high pulse energies. The combination of diffractive optics and conventional galvometer scanner can be used for high throughput laser ablation, but are limited in the optical qualities. We will show which applications can benefit from this hybrid optic and which improvements in productivity are expected. In addition, the optical limitations of the system will be compiled, in order to evaluate the suitability of this approach for any given application.

In this paper, high-throughput ultrashort pulse laser machining is investigated on various industrial grade metals (Aluminium, Copper, Stainless steel) and Al2O3 ceramic at unprecedented processing speeds. This is achieved by using a high pulse repetition frequency picosecond laser with maximum average output power of 270 W in conjunction with a unique, in-house developed two-axis polygon scanner. Initially, different concepts of polygon scanners are engineered and tested to find out the optimal architecture for ultrafast and precision laser beam scanning. Remarkable 1,000 m/s scan speed is achieved on the substrate, and thanks to the resulting low pulse overlap, thermal accumulation and plasma absorption effects are avoided at up to 20 MHz pulse repetition frequencies. In order to identify optimum processing conditions for efficient high-average power laser machining, the depths of cavities produced under varied parameter settings are analyzed and, from the results obtained, the characteristic removal values are specified. The maximum removal rate is achieved as high as 27.8 mm3/min for Aluminium, 21.4 mm3/min for Copper, 15.3 mm3/min for Stainless steel and 129.1 mm3/min for Al2O3 when full available laser power is irradiated at optimum pulse repetition frequency.