High-power Laser Pulses Research Articles

Characterization of laser-induced vibration on concrete surface toward highly efficient laser remote sensing

Laser remote sensing (LRS) using irradiation of high-power laser pulses provides characteristic vibrations (CVs) which are accompanied by internal deterioration. While the principle of LRS is the same as that of the traditional hammering inspection, optimizing the irradiation parameters of laser pulses for vibrating samples is essential for effective LRS. In this study, the frequencies and magnitudes of CVs on a concrete specimen exhibiting a mock inside defect were evaluated by laser pulse irradiation and pendulum impact. When laser pulses were irradiated, the magnitude of CVs increased linearly with increasing laser pulse energy, and higher-order vibration was observed. On the other hand, an optimal spot size was indicated by the non-linear correlation between the fluence and the magnitude of CVs. To obtain an effective LRS, we propose that both high laser fluence and an optimized laser spot size are essential.

Triggering of Pressurized Gas Switches With a Class I Laser

The use of lasers to initiate the closure of high-power gas switches was initially explored soon after the development of suitable high-power pulsed lasers in the 1960s. The low jitter, excellent triggering range, and galvanic isolation provided by laser triggering have been extensively exploited in the triggering of megavolt switches in large pulsed power machines. The fact that the lasers and optical systems used in these machines are very large, complex, and expensive and have led to the perception that these are unavoidable characteristics of laser triggered gas switches, leading to a reluctance among the directed energy and compact pulsed power community to consider their use in compact pulsed power systems. The Advanced Pulsed Energy, Ionization, and Discharge Center (APERIODIC) Research Group at The University of New Mexico (UNM), Albuquerque, NM, USA, has been investigating novel triggering technologies for compact pulsed power. One of the technologies under investigation is a system we have named the microintegrated laser switch (MILS) [1]. This system holds the potential to overcome the limitations of traditional laser triggering of gas switches in compact applications. As part of the preliminary work on developing the MILS concept, UNM has collaborated with MegaWatt Lasers, Hilton Head Island, SC, USA, to conduct laser triggering experiments using a commercial off-the-shelf (COTS) erbium-doped glass passively Q-switched microlaser. This 28 mm × 9 mm laser produces 4-ns pulses at 1535 nm with an average energy per pulse of 244 μJ. One of the principal advantages of this laser is that, due to the wavelength and energy, it is a class 1 laser and can be operated without the stringent controls necessary for the lasers typically used for laser triggering.

Shock wave impact on the viability of MDA-MB-231 cells.

Shock waves are gaining interests in biological and medical applications. In this work, we investigated the mechanical characteristics of shock waves that affect cell viability. In vitro testing was conducted using the metastatic breast epithelial cell line MDA-MB-231. Shock waves were generated using a high-power pulse laser. Two different coating materials and different laser energy levels were used to vary the peak pressure, decay time, and the strength of subsequent peaks of the shock waves. Within the testing capability of the current study, it is shown that shock waves with a higher impulse led to lower cell viability, a higher detached cell ratio, and a higher cell death ratio, while shock waves with the same peak pressure could lead to different levels of cell damage. The results also showed that the detached cells had a higher cell death ratio compared to the attached cells. Moreover, a critical shock impulse of 5 Pa·s was found to cause the cell death ratio of the detached cells to exceed 50%. This work has demonstrated that, within the testing range shown here, the impulse, rather than the peak pressure, is the governing shock wave parameter for the damage of MDA-MB-231 breast cancer cells. The result suggests that a lower-pressure shock wave with a longer duration, or multiple sequential low amplitude shock waves can be applied over a duration shorter than the fundamental response period of the cells to achieve the same impact as shock waves with a high peak pressure but a short duration. The finding that cell viability is better correlated with shock impulse rather than peak pressure has potential significant implications on how shock waves should be tailored for cancer treatments, enhanced drug delivery, and diagnostic techniques to maximize efficacy while minimizing potential side effects.

Confocal Microscope-Based Laser Ablation and Regeneration Assay in Zebrafish Interneuromast Cells.

Hair cells are mechanosensory cells that mediate the sense of hearing. These cells do not regenerate after damage in humans, but they are naturally replenished in non-mammalian vertebrates such as zebrafish. The zebrafish lateral line system is a useful model for characterizing sensory hair cell regeneration. The lateral line is comprised of hair cell-containing organs called neuromasts, which are linked together by a string of interneuromast cells (INMCs). INMCs act as progenitor cells that give rise to new neuromasts during development. INMCs can repair gaps in the lateral line system created by cell death. A method is described here for selective INMC ablation using a conventional laser-scanning confocal microscope and transgenic fish that express green fluorescent protein in INMCs. Time-lapse microscopy is then used to monitor INMC regeneration and determine the rate of gap closure. This represents an accessible protocol for cell ablation that does not require specialized equipment, such as a high-powered pulsed ultraviolet laser. The ablation protocol may serve broader interests, as it could be useful for the ablation of additional cell types, employing a tool set that is already available to many users. This technique will further enable the characterization of INMC regeneration under different conditions and from different genetic backgrounds, which will advance the understanding of sensory progenitor cell regeneration.

Plasma modulator for high-power intense lasers

A type of plasma-based optical modulator is proposed for the generation of broadband high-power laser pulses. Compared with normal optical components, plasma-based optical components can sustain much higher laser intensities. Here we illustrate via theory and simulation that a high-power sub-relativistic laser pulse can be self-modulated to a broad bandwidth over 100% after it passes through a tenuous plasma. In this scheme, the self-modulation of the incident picoseconds sub-relativistic pulse is realized via stimulated Raman forward rescattering in the quasi-linear regime, where the stimulated Raman backscattering is heavily dampened. The optimal laser and plasma parameters for this self-modulation have been identified. For a laser with asub-relativistic intensity of I ∼ 1017W/cm2, the time scale for the development of self-modulation is around 103 light periods when stimulated Raman forward scattering has been fully developed. Consequently, the spatial scale required for such a self-modulation is in the order of millimeters. For a tenuous plasma, the energy conversion efficiency of this self-modulation is around 90%. Theoretical predictions are verified by both one-dimensional and two-dimensional particle-in-cell simulations.

Length Dependence on Broadband Microwave Emission From Laser-Generated Plasmas

A high-power ultrashort laser pulse focused in air generates a plasma that radiates broadband electromagnetic waves. The transient current source responsible for the radiation remains an open area of study. To better understand how the laser drives currents in the plasma, we investigate the dependence of radiation at microwave frequencies on the length of the plasma column. Radiation patterns of the microwaves are consistent with currents that flow longitudinally (in the direction of laser propagation). We measure the angular dependence of the microwave frequency spectrum to determine the microwave radiation source characteristics. We also model the plasma evolution using a PIC code to provide initial conditions for a finite-difference time domain (FDTD) simulation of the plasma currents responsible for the microwaves. The experiments and simulations show that the plasma length determines the shape of the radiation pattern, and also weakly influences the microwave frequency content. Our research demonstrates that the radiation mechanism is coupled with variation in the current along the entire plasma length.

High-intensity THz pulse generation by TW laser radiation in ionized gas and nonlinear crystals

Intense electromagnetic fields can be obtained now in THz frequency range. THz pulses with electric field intensity above 1 MV/cm are now requested for strong-field applications like particle acceleration, short electron bunches measurements, controlling the state of the matter. The advantage of a THz pulse in comparison with the visible frequency range is the almost non-oscillating field that interacts with charged particles more efficiently. The advantage over a direct field is the ability to remotely focus this radiation in space and in time. Except for the huge free-electron lasers facilities, the only way to get THz pulses of MV/cm intensity is the conversion of high-power femtosecond laser pulses. We compare two ways of generating intense THz pulses using the radiation of TW and GW laser systems operating at 800 nm. They are two-color filamentation in gas and optical rectification in nonlinear crystals. The prospects for scaling of THz generation methods for a petawatt laser are discussed.

Measuring the orbital angular momentum of high-power laser pulses

In this article, we showcase the experimental results of methods to produce and characterize orbital angular momentum (OAM) carrying high-power lasers. The OAM pulses were produced on the ASTRA laser of the Central Laser Facility using a continuous spiral phase plate. Three different characterization methods were then used to measure the OAM content of the beam. The methods that were used were a cylindrical lens diagnostic, an interferometric diagnostic, and a projective diagnostic. We further discuss the relative advantages and disadvantages of each method in the context of high-power laser experiments.

Remote Detection of Uranium Using Self-Focusing Intense Femtosecond Laser Pulses

Optical measurement techniques can address certain important challenges associated with nuclear safety and security. Detection of uranium over long distances presents one such challenge that is difficult to realize with traditional ionizing radiation detection, but may benefit from the use of techniques based on intense femtosecond laser pulses. When a high-power laser pulse propagates in air, it experiences collapse and confinement into filaments over an extended distance even without external focusing. In our experiments, we varied the initial pulse chirp to optimize the emission signal from the laser-produced uranium plasma at an extended distance. While the ablation efficiency of filaments formed by self-focusing is known to be significantly lower when compared to filaments produced by external focusing, we show that filaments formed by self-focusing can still generate luminous spectroscopic signatures of uranium detectable within seconds over a 10-m range. The intensity of uranium emission varies periodically with laser chirp, which is attributed to the interplay among self-focusing, defocusing, and multi-filament fragmentation along the beam propagation axis. Grouping of multi-filaments incident on target is found to be correlated with the uranium emission intensity. The results show promise towards long-range detection, advancing the diagnostics and analytical capabilities in ultrafast laser-based spectroscopy of high-Z elements.

High-power DUV picosecond pulse laser with a gain-switched-LD-seeded MOPA and large CLBO crystal.

We report the generation of a 266 nm deep ultraviolet (DUV) picosecond pulse with an average output power of 14 W by the fourth-harmonic generation (FHG) from two consecutive frequency-doubling stages of a 1064 nm pulse based on a gain-switched-laser-diode (LD)-seeded hybrid fiber/solid-state master oscillator and power amplifier (MOPA) system. Through the gain-switched operation of a narrow-spectral-linewidth distributed-feedback laser diode and by using a Yb-doped fiber and a two-stage ${\rm Nd}:{{\rm YVO}_4}$Nd:YVO4 solid-state amplifier, we achieved an average power of 46.5 W near the Fourier transform limit for a 13 ps pulse with a repetition rate of 200 kHz. The narrow linewidth pulse characteristics enabled highly efficient frequency conversion, and the efficiency of conversion from 532 to 266 nm was 54%, and from 1064 to 266 nm was 31%. The beam quality factor ${M^2}$M2 of the generated DUV pulse was below 1.2. The highly efficient FHG process resulted in appeasing thermal stress caused by nonlinear absorption in the crystal, and more than 5000 h of continuous operation were achieved without any power down or beam profile degradation.

Two-stage nonlinear compression of high-power femtosecond laser pulses

Two-stage compression of laser pulses with a power of 250 TW is experimentally realised by broadening their spectrum during self-phase modulation in fused silica and subsequent dispersion compensation upon reflection from chirping mirrors. A five-fold decrease in the duration is demonstrated, from 75 to 15 fs, with a B-integral value of about 5 at each stage. It is possible to avoid small-scale self-focusing due to self-filtering of the laser beam during free propagation in vacuum. With optimal parameters of the dispersive mirror, the pulse can be compressed to a duration of less than 5 fs.

Efficient absorption of laser light by nano-porous materials with well-controlled structure

The absorption of high-power nanosecond laser pulses in a porous matter is investigated through theoretical and numerical calculations. The effects of structural properties of the porous target such as size of pores and thickness of solid elements on the laser absorption are investigated. The time and space-dependent absorption coefficient of the laser created partially homogenized plasma is used in a plasma hydrodynamic code to reproduce the laser absorption and plasma formation processes in a porous matter. It is observed that the structural characteristics of the porous material can be optimized to significantly increase the laser energy absorption. For porous targets with pore sizes in the range 30–60 nm a value around 1000 nm could be desirable for the wall thickness to increase the laser absorption efficiency to more than 90%. The results can be advantageous in production of efficient laser absorber targets which are desirable in advanced applications such as inertial confinement fusion and laser-plasma x-ray sources.

Evolution of femtosecond laser-induced periodic structures formed on amorphous silicon surface

The laser-induced periodic surface structures (LIPSS) with period close to the femtosecond laser radiation wavelength (1.25 μm) were formed on amorphous hydrogenated silicon films (a-Si:H). The ripple ridges direction relative to laser radiation polarization was changed from perpendicular to parallel with increasing number of pulses from 50 to 1000. The experimentally observed LIPSS evolution is in a good agreement with the model proposed by J.E. Sipe where so-called efficacy factor depends on the real and imaginary parts of the dielectric constant and defines the LIPSS wave vector on the irradiated surface. In turn, the dielectric constant complex value is varied due to concentration change for the nonequilibrium electrons excited by different number of high-power femtosecond laser pulses. According to the theoretical modelling, the excited nonequilibrium electron concentration required for turning the LIPSS direction is equal to 8.2·1021 cm−3.

Application of high power pulsed nanosecond fibre lasers in processing ultra-thin aluminium foils

This study is directed at developing a micro-scale laser scoring process and understanding the laser-material interaction in ultra-thin aluminium foils (160 μm thickness). The research is carried out by the latest generation of nanosecond pulsed fibre lasers manufactured by SPI Lasers. Presented comparative analysis of 70 W and 120 W laser types gives important insights for laser functionality and can provide recommendations for scalability of the process and quality assurance. High average power lasers were used to meet the requirement of high processing speed in aluminium foils. Scoring in aluminium foils was analysed using the fundamental laser-material interaction parameters that fully determine the way the material responds to the laser energy independent of the laser system. This approach permits transferring the results from the micro-scale laser process between various laser systems. Energy density and laser power density calculated for considered beam spot area at various frequencies were found to be the parameters that determine the scoring ablation depth in the range of 40 μm–60 μm. Microscopic examination of the ablation depth and score quality was carried out by an LED illumination 3D microscope. The SEM micrographs indicated minor growth of material debris from the ejected material.

Reliability of adaptive optical system elements for high-power pulsed laser installations

Reliability of adaptive optical system elements for high-power pulsed laser installations

Interference-based laser-induced micro-plasma ablation of glass

AbstractGlass is one of the most important technical surfaces for numerous applications in automotive, optical, and consumer industries. In addition, by producing textured surfaces with periodic features in the micrometre range, new functions can be created. Although laser-based methods have shown to be capable to produce structured materials in a wide amount of materials, due to its transparency large bandgap dielectrics can be only processed in a controlled manner by employing high-power ultra-short pulsed lasers, thus limiting the employable laser sources. In this article, an interference-based method for the texturing of soda-lime glass using a 15 ns pulsed (1 kHz repetition rate) infrared (1053 nm) laser is proposed, which allows fabricating different periodic patterns with micrometre resolution. This method consists on irradiating a metallic absorber (stainless steel) put in direct contact with the glass sample and inducing locally an etching process on the backside of the glass. Then, the produced plasma at the interference maxima positions leads to the local fabrication of well-defined periodic line-like and dot-like surface patterns. The produced patterns are characterised using white light interferometry and scanning electron microscopy.

Dynamic change process of a recast layer in Nd:YAG millisecond laser trepan drilling

Laser drilling has extensive applications in modern industry. To improve the quality of microholes drilled by a high-power millisecond pulsed laser, the laser trepan drilling technique was studied in depth in this paper. Processing parameters, such as trepanning speed (TS) and rotation number (RN), were experimentally studied for their influence on a recast layer (RL), and the detailed change process of the RL in laser repairing stage was also analyzed. It was discovered that the RL thickness would decrease with lower TS and larger RN, but micro edge cracks would appear at the hole entrance if the two parameters exceeded some appropriate ranges. The reduction of the RL during laser trepanning was neither linear nor uniform; it was a dynamic change process. The minimal RL obtained in this study was about 1 µm at the hole entrance. The difference in laser trepanning diameter has no influence on the RL. The mechanism of RL reduction in laser trepan drilling is that the recast material may intermittently be remelted by the rotating laser beam, and once it is remelted, it can be pushed downward by the vaporization-induced recoil pressure and assist gas. This process can continue only if the absorbed energy can support and keep the molten material in liquid state; therefore, the thickness of the RL is inversely proportional to the duration of the remelted material remaining in liquid state per unit length. The theoretical analysis is consistent with the experimental results.

High-contrast, fast chemical imaging by coherent Raman scattering using a self-synchronized two-colour fibre laser

Coherent Raman scattering (CRS) microscopy is widely recognized as a powerful tool for tackling biomedical problems based on its chemically specific label-free contrast, high spatial and spectral resolution, and high sensitivity. However, the clinical translation of CRS imaging technologies has long been hindered by traditional solid-state lasers with environmentally sensitive operations and large footprints. Ultrafast fibre lasers can potentially overcome these shortcomings but have not yet been fully exploited for CRS imaging, as previous implementations have suffered from high intensity noise, a narrow tuning range and low power, resulting in low image qualities and slow imaging speeds. Here, we present a novel high-power self-synchronized two-colour pulsed fibre laser that achieves excellent performance in terms of intensity stability (improved by 50 dB), timing jitter (24.3 fs), average power fluctuation (<0.5%), modulation depth (>20 dB) and pulse width variation (<1.8%) over an extended wavenumber range (2700–3550 cm−1). The versatility of the laser source enables, for the first time, high-contrast, fast CRS imaging without complicated noise reduction via balanced detection schemes. These capabilities are demonstrated in this work by imaging a wide range of species such as living human cells and mouse arterial tissues and performing multimodal nonlinear imaging of mouse tail, kidney and brain tissue sections by utilizing second-harmonic generation and two-photon excited fluorescence, which provides multiple optical contrast mechanisms simultaneously and maximizes the gathered information content for biological visualization and medical diagnosis. This work also establishes a general scenario for remodelling existing lasers into synchronized two-colour lasers and thus promotes a wider popularization and application of CRS imaging technologies.

Interference microscopy study of the preplasma formed on an iron target surface exposed to high-power femtosecond laser pulses

The characteristic scale of spread of the plasma formed on the surface of a bulk iron target irradiated by a femtosecond laser pulse with an intensity of 1016 W cm−2 is measured by time-resolved interference microscopy using femtosecond pulses emitted by a Cr : forsterite laser system with an intensity contrast of 107. The chosen technique is demonstrated to be efficient in such measurements. It is shown experimentally that, as a result of laser pulse impact, the displacement of a plasma layer with a density exceeding critical does not exceed 30 nm.

Development of shock-dynamics study with synchrotron-based time-resolved X-ray diffraction using an Nd:glass laser system.

The combination of high-power laser and synchrotron X-ray pulses allows us to observe material responses under shock compression and release states at the crystal structure on a nanosecond time scale. A higher-power Nd:glass laser system for laser shock experiments was installed as a shock driving source at the NW14A beamline of PF-AR, KEK, Japan. It had a maximum pulse energy of 16 J, a pulse duration of 12 ns and a flat-top intensity profile on the target position. The shock-induced deformation dynamics of polycrystalline aluminium was investigated using synchrotron-based time-resolved X-ray diffraction (XRD) under laser-induced shock. The shock pressure reached up to about 17 GPa with a strain rate of at least 4.6 × 107 s-1 and remained there for nanoseconds. The plastic deformation caused by the shock-wave loading led to crystallite fragmentation. The preferred orientation of the polycrystalline aluminium remained essentially unchanged during the shock compression and release processes in this strain rate. The newly established time-resolved XRD experimental system can provide useful information for understanding the complex dynamic compression and release behaviors.