High Pulse Repetition Rate Research Articles

A method to resolve thermal and electronic contributions to the nolinear optical response

This paper presents the study and implementation of a modified Z-scan technique using a chopper to change the thermal load of the sample, while keeping the input peak irradiance constant. It was verified numerically and experimentally that by changing the frequency of the chopper and keeping the power of the laser beam constant, it is possible to vary the thermal load on the sample. This technique is illustrated under the study of the nonlinear optical (NLO) properties of nucleated Au–Pt metallic nanoparticles (NPs) by ion implantation. The nonlinear optical response was studied using the Z-scan. technique using a Ti:Sapphire laser with femtosecond pulses at a wavelength of 800 nm and a repetition rate of 76 MHz. By scanning the sample using the Z-scan technique it was It is possible to determine the refractive and absorbing contributions to the nonlinear response, in this case as a consequence of a high pulse repetition rate, cumulative pulse-to-pulse thermal effects occurred. Because of this, a modification of the Z-scan technique allowed to separate the resolution of the electronic and thermal contributions on the sample.

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers.

439 MHz, 94 fs, low-threshold mode-locked all fiber ring laser

Pulsed fiber lasers with high repetition rates play a crucial role in applications such as high-efficiency processing and bio-optical imaging. However, many ring fiber mode-locked lasers primarily focus on shortening the cavity length to achieve high repetition rate pulse output. Attention is often directed towards the gain fiber and cavity structure, with less consideration given to the challenge of increasing mode-locking threshold power. In this study, we present a high-repetition-rate annular all-fiber laser system with dispersion management. Through strategic adjustment of the intracavity dispersion away from the near-zero dispersion region and careful control of the intracavity gain strength, we have successfully lowered the mode-locking threshold power. This approach can generate pulses with a repetition rate of 439.71 MHz and a mode-locking threshold power of approximately 600 mW. Following pulse compression, the achieved pulse width is 94.8 fs. Its time-bandwidth product is 0.368. To our knowledge, this represents the highest fundamental repetition rate currently attained using a ring all-fiber resonator, accompanied by the lowest mode-locking threshold. Our work introduces a ring fiber laser system characterized by a low mode-locking threshold and a high repetition rate, offering a valuable method for achieving higher fundamental repetition rate pulses.

Efficient Ablation, further GHz Burst Polishing, and Surface Texturing by Ultrafast Laser

A large variety of ultrafast laser‐matter interaction regimes and different processed surface finishing qualities of stainless steel can be achieved by varying the laser processing parameters. The optimization of the laser fluence to the most efficient ablation also leads to the lowest surface roughness of the ablated material. High laser fluence together with a high pulse repetition rate leads to heat accumulation, which results in the formation of microstructures with various morphology and scales. The use of GHz bursts allows a rough stainless‐steel surface to be smoothed and even polished. In this work, the fast and high‐quality milling of 2.5D cavities is demonstrated. Laser beam spot optimization is used to increase the throughput of the high‐power femtosecond laser (67.8 W), resulting in an ablation rate of 13 mm3 min−1. The complex cavities are produced by laser milling and cutting in layers. The formation of surface structures on stainless steel because of laser irradiation has been demonstrated. The possibilities of further GHz burst polishing are examined on previously laser‐milled stainless steel using with lowest possible surface roughness. The ability to polish the surface with microstructures below the original roughness is demonstrated by bursts of ultrashort pulses with a repetition rate in the GHz range. It has been demonstrated that mold milling in stainless steel designed for LED diffusers can be fabricated using a modern femtosecond laser source together with beam size optimization technique for efficient ablation with low surface roughness, layer‐by‐layer milling techniques, and GHz burst polishing techniques.

High repetition-rate pulse shaping of a spectrally broadened Yb femtosecond laser

We demonstrate compression and shaping of few cycle pulses from a high average power ytterbium laser system. The pulses from a commercial 20 W, 100 kHz Yb laser system are spectrally broadened in two-stages using cascaded, gas-filled, stretched hollow-core fibers and then compressed and shaped in an acousto-optic modulator-based pulse-shaper. The pulse-shaper allows for compression, characterization, and shaping all in one system, producing ∼10 fs pulses with 30 μJ of energy.

Multilayered Ru/TiO[formula omitted] hyperbolic material for nonlinear optics

In this work we present the design, fabrication and study of the optical properties of hyperbolic materials based on multilayered metal–dielectric Ru/TiO2 structures. The samples were fabricated using the Atomic Layer Deposition technique, and their structure was designed to have an Epsilon Near Zero point near 800 nm to match the laser wavelength employed to study their response. The linear optical properties were studied using spectrophotometry and spectral ellipsometry. The nonlinear response was studied as a function of incident irradiance using the z-scan technique with fs pulses as a function of wavelength throughout the visible, allowing the determination of the refractive and absorptive contributions to the nonlinear response. Given that a high pulse repetition rate was employed, cumulative pulse to pulse thermal effects can be present, because of this, a modification of the z-scan technique allowed the resolution of the electronic and thermal contributions to the response.

Self-Shielding of X-ray Emission from Ultrafast Laser Processing Due to Geometrical Changes of the Interaction Zone.

Materials processing with ultrashort laser pulses is one of the most important approaches when it comes to machining with very high accuracy. High pulse repetition rates and high average laser power can be used to attain high productivity. By tightly focusing the laser beam, the irradiances on the workpiece can exceed 1013 W/cm2, and thus cause usually unwanted X-ray emission. Pulsed laser processing of micro holes exhibits two typical features: a gradual increase in the irradiated surface within the hole and, with this, a decrease in the local irradiance. This and the shielding by the surrounding material diminishes the amount of ionizing radiation emitted from the process; therefore, both effects lead to a reduction in the potential X-ray exposure of an operator or any nearby person. The present study was performed to quantify this self-shielding of the X-ray emission from laser-drilled micro holes. Percussion drilling in standard air atmosphere was investigated using a laser with a wavelength of 800 nm a pulse duration of 1 ps, a repetition rate of 1 kHz, and with irradiances of up to 1.1·1014 W/cm. The X-ray emission was measured by means of a spectrometer. In addition to the experimental results, we present a model to predict the expected X-ray emission at different angles to the surface. These calculations are based on raytracing simulations to obtain the local irradiance, from which the local X-ray emission inside the holes can be calculated. It was found that the X-ray exposure measured in the surroundings strongly depends on the geometry of the hole and the measuring direction, as predicted by the theoretical model.

Ultrashort pulse laser micro processing strategies for fast beam deflection with acousto-optical deflectors

Acousto-optical deflectors (AOD) enable laser pulse-synchronous beam deflection up to the MHz range, which can deviate from the usual path control and raster scan strategies. Where the small working field of the AODs is not sufficient, a combination with a galvo scanner or an axis system can be used. Laser micromachining with modern ultrashort pulse lasers with a high pulse repetition rate is increasingly faced with the challenge of applying the laser power to the workpiece without too much heat remaining locally in the surface. AOD-based beam deflection can help here by quickly distributing the laser pulses on the surface, which is equivalent to a locally reduced laser pulse repetition rate without reducing the laser’s repetition rate. We provide an overview of the technology and present examples of innovative machining strategies resulting from the use of acousto-optical laser beam deflection.

Acoustic process monitoring during the structuring of the diffusion media for fuel cells with Ultrashort Laser Pulses

Inline monitoring is already an essential part of laser material processing. Due to the ongoing technological improvements as well as the development of new sensor technology, new fields of application emerge. In this regard, optical microphones are a promising alternative to conventional microphones as the membrane-free measuring principle allows them to cover a significantly wider frequency spectrum and, thus, enable new possibilities for process observation. Simultaneously to the development in sensor technology, ultrashort-pulse laser beam sources are being further commercialized, which results in a gradual enlargement of their fields of application due to the improved economic efficiency. In order to increase the productivity of ultrashort-pulse laser beam sources, high pulse repetition rates in the upper kilohertz or even in the megahertz region are often used. Together with the emitted pulses exhibiting durations in the picosecond or femtosecond range, the inline process observation is challenging. In this work, the diffusion media for polymer electrolyte membrane fuel cells, consisting of a carbon black particle-based microporous layer and a carbon fiber-based substrate layer, were structured with a laser beam source emitting pulses in the picosecond range. An optical microphone was used for process monitoring in combination with dedicated data processing methods. It was shown that acoustic data can be used to determine the focus position as well as the material transitions during ultrashort-pulsed laser material processing. Additionally, information about the ablation volume and the presence of surface defects can be drawn from the data.

Solid State Pulsed Power Modulator With High Repetition Rate and Short Pulse Width for High-Speed Pulsed Lasers

This paper proposes a modular pulsed power modulator (PPM) with a short pulse width and high repetition rate for pulsed lasers. The presented PPM is based on the modular structure to use semiconductor switches with low voltage ratings since these switches have fast switching speed which is an essential requirement to generate short pulses. In addition, to make a short gate signal which is also needed to generate short pulses, a gate driver circuit which applies positive and negative voltages asymmetrically is proposed. Moreover, a heatsink structure to minimize the parasitic capacitances is presented for the high repetition rate. Furthermore, a DC-DC converter for the modular PPM is developed, which connects transformers in parallel at the primary side to achieve voltage balance without any compensation circuit. Experimental results from a 5 kV PPM with the pulse width below 20 ns and repetition rate of 1 MHz are presented to verify the proposed works.

Characteristics of high-repetition-rate bipolar pulse DBD under various electrical conditions in atmospheric-pressure air

Numerous studies have been conducted on pulse dielectric barrier discharge (DBD) because it can produce powerful discharges uniformly at atmospheric pressure with a fast rise time. Although much research has been conducted on pulse DBD below 10 kHz, relatively little has been conducted on pulse DBD at high pulse repetition rates (PRRs). Therefore, in this study, the ozone generation and discharge characteristics of bipolar pulse DBD in atmospheric-pressure air at a high PRR of 10 kHz or above were investigated. According to the results of this study, with the exception of electron temperature, most discharge characteristics need for practical applications—like transfer charge, electron density, and discharge uniformity—improved as the voltage and duty ratio increased at high PRR. On the contrary, increasing the PRR exhibited trade-off features like low electron temperature, low discharge uniformity, and a high number of discharges per unit time. Ozone generation demonstrated good results at high voltage, appropriate PRR, and low duty ratio, but applying suitable electrical conditions is crucial considering ozone generation speed and power consumption. The findings of this study will be very beneficial for high-PRR pulse DBD applications that require quick and effective processing. Additionally, they will be useful for researching the characteristics of pulse DBD at high PRR.

Therapeutic perspectives of high pulse repetition rate electroporation

Electroporation, a technique that uses electrical pulses to temporarily or permanently destabilize cell membranes, is increasingly used in cancer treatment, gene therapy, and cardiac tissue ablation. Although the technique is efficient, patients report discomfort and pain. Current strategies that aim to minimize pain and muscle contraction rely on the use of pharmacological agents. Nevertheless, technical improvements might be a valuable tool to minimize adverse events, which occur during the application of standard electroporation protocols. One recent technological strategy involves the use of high pulse repetition rate. The emerging technique, also referred as “high frequency” electroporation, employs short (micro to nanosecond) mono or bipolar pulses at repetition rate ranging from a few kHz to a few MHz. This review provides an overview of the historical background of electric field use and its development in therapies over time. With the aim to understand the rationale for novel electroporation protocols development, we briefly describe the physiological background of neuromuscular stimulation and pain caused by exposure to pulsed electric fields. Then, we summarize the current knowledge on electroporation protocols based on high pulse repetition rates. The advantages and limitations of these protocols are described from the perspective of their therapeutic application.

Laser processing of silicon with GHz burst pumped third harmonics for precise microfabrication.

Femtosecond laser processing has proved to be a valuable tool for various microfabrication applications. In order to further increase the quality and efficiency of femtosecond laser processing, processing with GHz burst mode lasers has gained attention in recent years, where packets of high-repetition rate pulses are used instead of single pulses at the fundamental repetition rate. However, the use of burst-pulses has mainly been limited to the fundamental wavelength of powerful regenerative amplifier systems, often near 1 micrometer wavelength. In this study, we explore the characteristics and potential benefits of further wavelength conversion of burst-pulses emitted at the near-infrared to the ultraviolet region via direct third-harmonic generation. We construct an in-line process evaluation setup with a chromatic confocal sensor, and evaluate the ablation characteristics of the burst-pumped and non-burst processing of silicon. We observe that burst-mode processing has significantly reduced surface roughness and debris, resulting in high-quality laser processing. To demonstrate the utility of such burst-pumped UV processing, we show the successful milling of a spherical structure enabled by in-line surface profile feedback, while similar processing with non-burst conditions did not work. We believe such results show the strong potential of burst laser sources for use in accurate microfabrication of structures with micrometer-scale resolution.

Auto-setting multi-soliton temporal spacing in a fiber laser by a hybrid GA-PSO algorithm.

Multi-soliton operation in fiber lasers is a promising platform for the investigation of soliton interaction dynamics and high repetition-rate pulse. However, owing to the complex interaction process, precisely manipulating the temporal spacing of multiple solitons in a fiber laser is still challenging. Herein, we propose an automatic way to control the temporal spacing of multi-soliton operation in an ultrafast fiber laser by a hybrid genetic algorithm-particle swarm optimization (GA-PSO) algorithm. Relying on the intelligent adjustment of the electronic polarization controller (EPC), the on-demand temporal spacing of the double solitons can be effectively achieved. In particular, the harmonic mode locking with equal temporal spacing of double solitons is also obtained. Our approach provides a promising way to explore nonlinear soliton dynamics in optical systems and optimize the performance of ultrafast fiber lasers.

High repetition rate step acousto-optic Q-switched laser with three nanosecond equal-interval and amplitude pulses

A compact, high repetition rate pulse group laser with three nanosecond equal-interval and equal-amplitude sub-pulses has been demonstrated in this paper. Under three-step nanosecond continuous acousto-optic control, a laser pulse group containing three nanosecond interval sub-pulses can be realized. The three sub-pulses have a pulse width of 15 ns, 23 ns, and 25 ns and minimum pulse intervals of 119 ns at a repetition rate of 30 kHz. The corresponding optical-optical conversion efficiency is 29.2 %.

EXtra-Xwiz: A Tool to Streamline Serial Femtosecond Crystallography Workflows at European XFEL

X-ray free electron lasers deliver photon pulses that are bright enough to observe diffraction from extremely small crystals at a time scale that outruns their destruction. As crystals are continuously replaced, this technique is termed serial femtosecond crystallography (SFX). Due to its high pulse repetition rate, the European XFEL enables the collection of rich and extensive data sets, which are suited to study various scientific problems, including ultra-fast processes. The enormous data rate, data complexity, and the nature of the pixelized multimodular area detectors at the European XFEL pose severe challenges to users. To streamline the analysis of the SFX data, we developed the semiautomated pipeline EXtra-Xwiz around the established CrystFEL program suite, thereby processing diffraction patterns on detector frames into structure factors. Here we present EXtra-Xwiz, and we introduce its architecture and use by means of a tutorial. Future plans for its development and expansion are also discussed.

Q-switched vortex waveguide laser generation based on LNOI thin films with implanted Ag nanoparticles.

Lithium-niobate-on-insulator (LNOI) thin films have gained significant attention in integrated photonics due to their exceptional crystal properties and wide range of applications. In this paper, we propose a novel approach to realize a Q-switched vortex waveguide laser by incorporating integrated lithium niobate thin films with embedded silver nanoparticles (Ag:LNOI) as a saturable absorber. The saturable absorption characteristics of Ag:LNOI are investigated using a home-made Z-scan system. Additionally, we integrate Ag:LNOI as a saturable absorber into a Nd:YAG "ear-like" cladding waveguide platform, which is prepared via femtosecond laser direct writing. By combining this setup with helical phase plates for phase modulation in the resonator, we successfully achieve a passive Q-switched vortex laser with a high repetition rate and narrow pulse duration in the near-infrared region. This work demonstrates the potential applications of LNOI thin films towards on-chip integration of vortex waveguide laser sources.

Effects of Nanosecond Pulsed Electric Field (nsPEF) on a Multicellular Spheroid Tumor Model: Influence of Pulse Duration, Pulse Repetition Rate, Absorbed Energy, and Temperature.

Cellular response upon nsPEF exposure depends on different parameters, such as pulse number and duration, the intensity of the electric field, pulse repetition rate (PRR), pulsing buffer composition, absorbed energy, and local temperature increase. Therefore, a deep insight into the impact of such parameters on cellular response is paramount to adaptively optimize nsPEF treatment. Herein, we examined the effects of nsPEF ≤ 10 ns on long-term cellular viability and growth as a function of pulse duration (2-10 ns), PRR (20 and 200 Hz), cumulative time duration (1-5 µs), and absorbed electrical energy density (up to 81 mJ/mm3 in sucrose-containing low-conductivity buffer and up to 700 mJ/mm3 in high-conductivity HBSS buffer). Our results show that the effectiveness of nsPEFs in ablating 3D-grown cancer cells depends on the medium to which the cells are exposed and the PRR. When a medium with low-conductivity is used, the pulses do not result in cell ablation. Conversely, when the same pulse parameters are applied in a high-conductivity HBSS buffer and high PRRs are applied, the local temperature rises and yields either cell sensitization to nsPEFs or thermal damage.

Inactivation of bacteria using different Histotripsy regimes: Toward the treatment of abscesses

Infected abscesses are defined as localized collections of infected purulent material delineated by a capsule composed of granulation and connective tissue. They can affect any part of the body. Current treatment typically involves antibiotics with either long-term catheter drainage or incision and washout. Given the ubiquitous nature of bacteria and the serious threat of multi-drug resistant strains, abscesses have become a persistent global healthcare problem. Using histotripsy, a potential new noninvasive treatment modality, we have evaluated bacterial inactivation of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) microbes. We have determined the peak negative pressure threshold for inactivation of E. coli in solution for cavitation histotripsy (low cycles, high pulse repetition rate) and boiling histotripsy (high cycles, high pulse repetition rate) for different treatment times. The effect of transducer frequency on bacterial inactivation has also been evaluated. The optimal regimen was used to evaluate the S. aureus inactivation. Bacterial inactivation has also been evaluated in an abscess phantom model. Custom transducers have been developed and evaluated in an animal model for abscesses. [Work supported in part by NIH NIBIB #R01EB019365 and NIAID #R01AR080120.]

Nanosecond laser grooving of water-immersed silicon carbide assisted by high intensity focused ultrasound (HIFU)

Silicon carbide (SiC) is a very important semiconductor and ceramic material with many applications. However, its micromachining is difficult and it is still challenging to achieve a good combination of high quality, productivity and low cost. The corresponding author previously proposed the “ultrasound-assisted water-confined laser micromachining” (UWLM) process, a new machining technology utilizing in-situ ultrasound to improve laser machining of a water-covered workpiece. The author's previous studies show that UWLM based upon a high intensity focused ultrasound (HIFU) transducer is able to produce a relatively high machining rate and quality for metals. Thus, it is natural to guess the HIFU-based UWLM process might also possibly help provide a good solution for SiC micromachining. However, SiC have properties very different from metals. An experimental study is critically needed to confirm whether or not HIFU-based UWLM can still produce a high machining rate and quality for SiC, and overcome additional challenge(s) arising (if any). However, such a study is seldom reported to the authors' knowledge. This paper reports a study on the micro grooving of SiC via the HIFU-based UWLM process. With the conditions in this study, it has been found that UWLM can produce a much better apparent groove quality (i.e., much less recast material and/or debris re-deposition) than the laser machining process in the ambient air, and a much higher machining depth per pulse than the laser machining process in water with no ultrasound. The likely fundamental mechanism is the in-situ cleaning effect due to ultrasound-induced cavitation flow and pressure waves in water. The laser machining process in water with or without ultrasound can produce cracks at relatively low pulse repetition rates (PRRs). Time-resolved pressure measurements in water have been performed to help analyze stress sources causing the cracks. The crack problem appears to be effectively solved by using a relatively high PRR of 5 kHz. One possible major reason is expected to be the inter-pulse thermal accumulation effect at the high PRR, improving workpiece fracture toughness and/or reducing temperature gradients.