Laser Peak Power Research Articles

Building ultra-thin Inconel 718 sheet joints using low-frequency PLBW scheme: Weld bead quality, keyhole dynamics, and solidification characteristics

Low-frequency pulsed laser beam welding (PLBW) was employed in building 0.32 mm-thick Inconel 718 superalloy butt joints which are frequently applied in manufacturing high-temperature functional units in the aviation industry. Several process parameter combinations concerning peak laser power, welding speed, and pulse frequency were implemented to explore the macro morphology, microstructure, and mechanical properties of the welded joints. Examinations exhibit that the resulting weld beads possess high continuity at the surface with an overlap factor ranging from 0.58 to 0.73. The cross-sectional profile changes from a V-shape to an H-shape with increasing heat input. Finer grain structures composed of γ matrix and γ/Laves eutectic penetration are observed within the FZ center, as compared to that near the fusion line. Excellent performance is achieved in microhardness and tensile tests. Numerical simulation based on an integrated mathematic model reveals periodic keyhole and weld pool dynamics depending on the pulsing laser power. During a typical pulse interval, keyhole geometry collapses within a fraction of a millisecond, leading to a sharp cooling stage at ∼4 × 106 K/s, and then, the melt cools due to the thermal convection with a normal cooling rate of 105 K/s. The full growth of the adjacent weld spot results in the secondary melting zone (SMZ), wherein the columnar dendrites can be refined mainly due to the heterogeneous nucleation as compared to the primary melting zone (PMZ).

Modulation instability of incoherent beams revisited.

We examine the spatial modulation instability (MI) of a partially incoherent laser beam. We show that the P < (a/rc)2P0 criterion of beam stability, with a laser peak power P, beam radius a, correlation radius rc, and critical power of self-focusing P0, is applicable only to a limited class of MIs, viz., MIs that can be described as instabilities of a pertinent transverse correlation function found as a solution to the evolution equation, where the expectation of the four-field-product nonlinear source term is factorized as a product of the field intensity and a two-point transverse correlation function. When extended to a more general class of MIs, field evolution analysis of partially coherent beams suggests that MIs can be attenuated, but never completely suppressed. We show that spatial incoherence can lower the MI-buildup rate, thus helping avoid MI-induced beam breakup in physical settings where the MI-buildup length lMI can be kept longer than the length of the nonlinear medium L. Because the lMI > L condition sets a limitation on the field intensity rather than the laser peak power, MI-induced beam breakup can be avoided, even at laser peak powers well above the critical power of self-focusing P0.

Using femtosecond laser pulses to explore the nonlinear optical properties of a CR-39 polymer after being exposed to alpha particles

The effect of alpha particle radiation on nonlinear optical properties of poly(allyl diglycol carbonate) (CR-39) polymer has been investigated using the Z-scan technique. CR-39 polymers were exposed to alpha particles at different doses: 23.7 and 47.5 Gy. Alpha particles cause significant optical and chemical changes in the polymer CR-39. Data from UV-visible spectrometers revealed that when alpha particle doses increased, the absorption edge of polymer redshifted and the energy band gap decreased. Fourier transform infrared absorption spectra of alpha irradiation samples show a change in intensity and bond breakage in the chemical groups constituting the CR-39 polymer compared to the pristine one. With 100 fs laser pulses and 80 MHz repetition rate, the nonlinear absorption coefficient and nonlinear refractive index are measured for a pristine and irradiated polymer with different alpha doses at different excitation wavelengths and laser peak power. The samples displayed a negative nonlinear refractive index and reverse saturable absorption behavior with three photons absorption. The results reveal that the nonlinear optical properties of CR-39 were enhanced by alpha particle radiation.

WETTABILITY STUDIES ON FEMTOSECOND-¬LASER-TEXTURED N-TYPE SILICON SURFACES

This present study examines the wetting behavior of N-type silicon surfaces that have been textured using a femtosecond laser. By employing three different patterns, i.e., square pillars, micro dimples, and circumferential grooves, and manipulating key femtosecond laser parameters such as laser power (ranging from 8 W to 12 W) and repetitions (ranging from 40 to 60), the wettability properties of the silicon surfaces are modified. The wettability properties of the surface were evaluated by measuring the contact angle by the sessile-drop method using distilled deionized water as a testing liquid. The textured surfaces displayed various wettability characteristics, varying from hydrophilic to hydrophobic. The hydrophobic behavior was observed on surfaces with a peak laser power of 12 W, 60 repetitions, and the lowest pitch of 160 µm. For the square pillar and micro-dimple textures, contact angles of 146° and 120°, respectively, were measured. Conversely, the circumferential grooves exhibited hydrophilic behavior with a contact angle of 20°. These results were achieved at laser powers of 10 W and 8 W, higher pitch values, and increased repetitions. The contact angle decreased with an increase in pitch and a decrease in repetitions and laser power. Based on the experimental findings, it can be concluded that the wettability of silicon surfaces can be controlled for specific applications using a single-step laser ablation technique. The desired wettability characteristics can be achieved by carefully adjusting the key femtosecond-laser parameters and geometrical features.

Evaluation of Laser Lap Weldability between the Titanium Alloy Ti-6Al-4V and Aluminum Alloy 6060-T6

This work investigates laser weldability between non-ferrous dissimilar metallic materials, specifically the aluminum 6060-T6 alloy and titanium Ti-6Al-4V. These materials are used in several engineering applications, including aerospace. In a simple lap joint configuration, these were welded with a pulsed Nd:YAG laser, with direct incidence on the titanium piece. Preheating and post-weld heating were introduced to mitigate cracking issues. Based on the primary experiments, the main variables were the peak laser power, which varied between 60 and 70%, and the number of beads (a single bead and double beads). The quality of the welds was assessed via uniaxial tensile tests, subjecting the joint to shear loading. Additionally, SEM micrographs were obtained to analyze the quality of the fusion between the dissimilar alloys. The higher strength of the welded samples achieved was 90 MPa, which is close to the reported value for the aluminum base material. A fracture occurred near the weld bead in the heat-affected zone (HAZ). The observed microporosities and cracks explain the lower value as compared to the base material. Although these were mitigated through the thermal cycle strategy employed and the shielding gas, they were not entirely avoided.

In-situ synthesis of TiC/Ti alloyed layer via pulsed Nd:YAG laser melting of Ti-6Al-4V titanium with preplaced carbon-based powder

The present research deals with fabricating an in-situ TiC/Ti alloyed layer on the Ti-6Al-4V titanium alloy through the preplacement of a mixture of graphite and nickel powders and subsequent pulsed Nd:YAG laser melting. The comprehensive effects of laser parameters, including scanning speed and energy density on the microstructural evolutions and hardness variations were investigated. It was found that the geometry of treated layers varied from 150 to 200 µm in thickness and 1200–1750 µm in width depending on the thickness of the preplaced layer and laser peak power. The promising results of X-ray diffraction patterns and scanning electron microscope observations manifested the formation of in-situ TiC and an increase in its content versus the energy density of the laser beam. The main mechanism of the in-situ formation of TiC in the matrix during laser irradiation was also discussed. The results indicated that the in-situ TiC phases had either dendritic or needle-like morphology. Laser parameters were crucial in establishing the TiC morphologies associated with temperature and chemical composition. The hardness values of the fabricated layers were directly proportional to the amount of TiC in the treated layers and varied from 1000 to 1400 HV0.3, which was nearly 3–4 times higher than that of the substrate.

Dynamic range enhancement of self-referenced spectral interferometry with extended time excursion method by the cascaded Kerr lens process.

Self-referenced spectral interferometry with extended time excursion (SRSI-ETE) is a powerful method for single-shot characterization of the temporal contrast of a high peak power laser, which has high temporal resolution but a low dynamic range. Here, a temporal contrast reduction method is proposed that uses the cascaded Kerr lens process in two thin glass plates. Combined with the SRSI-ETE method, the measurement dynamic range of the method is increased about two orders of magnitude while having a 20fs temporal resolution and a 40ps time window in single shot.

From design to realization of high diffraction efficiency Littrow configuration diffraction gratings based on multilayer dielectric mirrors patterned with electron beam lithography

Diffraction efficiency (DE) and laser-induced damage threshold of Chirped pulse amplification (CPA) system diffraction gratings (DGs) are one of the main limiting factors for the anticipated progress of high peak power lasers. Optimization of a high reflectivity dielectric multilayer ZrO2/SiO2 stack mirror with a DG etched in the top SiO2 layer was carried out employing the Rigorous coupled wave analysis method. Two families of solutions possessing DE > 0.99 over the bandwidth of the 1030 nm wavelength ultrashort laser and withstanding different fabrication margins were identified for the Littrow configuration. DGs were produced and verified experimentally. Structures permitting variation in both the depth of the DG and filling factor without loss in DE were obtained for the 690 nm thick SiO2 layer. While the 770 nm thick layer permitted separation of the two parameters requiring either rigorous control of the DG depth or filling factor. DG realization technology was proposed employing electron beam lithography and inductively coupled plasma reactive ion etching. Measurements of the fabricated structures demonstrated 0.91 DE for the first diffraction order over the 60 nm bandwidth and under a 6° mounting error from the Littrow configuration. Spectral angular reflectance DE simulations and respective measurements indicated quantitative agreement. The work provides an integrated overview of high DE periodic structure numerical optimization and manufacturing roadmap which could be applied for the high-peak-power laser CPA system development.

Fast laser field reconstruction method based on a Gerchberg–Saxton algorithm with mode decomposition

Knowledge of the electric field of femtosecond, high intensity laser pulses is of paramount importance to study the interaction of this class of lasers with matter. A hybrid method to reconstruct the laser field from fluence measurements in the transverse plane at multiple positions along the propagation axis is presented, combining a Hermite–Gauss mode decomposition (MD) and elements of the Gerchberg–Saxton algorithm (GSA). The proposed GSA-MD takes into account the pointing instabilities of high intensity laser systems by tuning the centers of the HG modes. Furthermore, it quickly builds a field description by progressively increasing the number of modes and thus the accuracy of the field reconstruction. The results of field reconstruction using the GSA-MD are shown to be in excellent agreement with experimental measurements from two different high peak power laser facilities.

High-power xenon lamp-pumped Er:YAP pulse laser operated in free-running and acousto-optical Q-switching modes

We demonstrate a high-energy and high-power pulse laser on a xenon lamp-pumped Er:YAP crystal. The laser performance and thermal focal lengths under different working frequencies are discussed. The results show that the thermal lens effect is gradually aggravated with the increase of working frequencies, and even working at 100 Hz, a single pulse energy of 234 mJ can be achieved. A maximum average power of 41.5 W is achieved with a working frequency of 20 Hz and slope efficiency of 2.82%. This output power is much higher than other xenon lamp-pumped erbium laser devices. A Q-switched laser is demonstrated by using the TeO2 crystal, the maximum output energies of 11.5 mJ and 3.5 mJ are obtained at 50 and 100 Hz, the corresponding peak powers are 93.4 kW and 17.2 kW, respectively. The laser wavelengths and beam quality factors are also characterized in the free-running and Q-switched modes. A higher pulse energy and peak power laser could be achieved further by improving the damage threshold of TeO2 acousto-optical Q-switching. All the experimental results illustrate that the xenon lamp-pumped Er:YAP laser is a promising candidate for high-power and high-frequency mid-infrared laser devices.

Nonlinear Imaging Histopathology: A Pipeline to Correlate Gold-Standard Hematoxylin and Eosin Staining With Modern Nonlinear Microscopy.

Hematoxylin and eosin (H&E) staining, the century-old technique, has been the gold standard tool for pathologists to detect anomalies in tissues and diseases such as cancer. H&E staining is a cumbersome, time-consuming process that delays and wastes precious minutes during an intraoperative diagnosis. However, even in the modern era, real-time label-free imaging techniques such as simultaneous label-free autofluorescence multiharmonic (SLAM) microscopy have delivered several more layers of information to characterize a tissue with high precision. Still, they have yet to translate to the clinic. The slow translation rate can be attributed to the lack of direct comparisons between the old and new techniques. Our approach to solving this problem is to: 1) reduce dimensions by pre-sectioning the tissue in 500 μm slices, and 2) produce fiducial laser markings which appear in both SLAM and histological imaging. High peak-power femtosecond laser pulses enable ablation in a controlled and contained manner. We perform laser marking on a grid of points encompassing the SLAM region of interest. We optimize laser power, numerical aperture, and timing to produce axially extended marking, hence multilayered fiducial markers, with minimal damage to the surrounding tissues. We performed this co-registration over an area of 3 × 3 mm2 of freshly excised mouse kidney and intestine, followed by standard H&E staining. Reduced dimensionality and the use of laser markings provided a comparison of the old and new techniques, giving a wealth of correlative information and elevating the potential of translating nonlinear microscopy to the clinic for rapid pathological assessment.

Filamentation instability in the interaction of high-order Hermite cosh Gaussian laser beam and inhomogeneous plasmas

Based on high order paraxial theory and WKB approximation, filamentation instability in the interaction of high-order Hermite cosh Gaussian laser beam (HchG) and inhomogeneous plasma is studied. By changing the initial light intensity parameters and the non-uniformity of plasma density, the effect of different mode indices beams propagating in uniform and non-uniform plasmas on self-focusing filamentation structures is investigated. The results show that the HchG laser beam exhibits obvious self-focusing and filamentation phenomena along propagation distance in inhomogeneous plasma under the effect of the ponderomotive force. It is found that the mode index of HchG and plasma inhomogeneity have significant effects on filamentation instability, the higher order mode indices of HchG laser beam can strengthen filamentation instability, and plasma inhomogeneity also can enhance filamentation instability. The results provide a useful theoretical basis for effectively regulating the filamentation instability, increasing the peak laser power and designing narrower pulse width lasers.

Pulsed Laser Spot Welding Thermal-Shock-Induced Microcracking of Inconel 718 Thin Sheet Alloy.

This paper investigates the change in solidification microcrack susceptibility under the influence of thermal-shock-induced effects for pulsed laser spot welding molten pools with different waveforms, powers, frequencies, and pulse widths. During the welding process, the temperature of the molten pool under the effect of thermal shock changes sharply, triggering pressure waves, creating cavities in the molten pool paste area, and forming crack sources during solidification. The microstructure near the cracks was analyzed using a SEM (scanning electron microscope) and EDS (electronic differential system), and it was found that bias precipitation occurred during the rapid solidification of the melt pool, and a large amount of Nb elements were enriched in the interdendritic and grain boundaries, which eventually formed a liquid film with a low melting point, known as a Laves phase. When cavities appear in the liquid film, the chance of crack source formation is further increased. Using a slow rise and slow fall waveform is good for reducing cracks; reducing the peak laser power to 1000 w is good for reducing cracks in the solder joint; increasing the pulse width to 20 ms reduces the degree of crack damage; reducing the pulse frequency to 10 hz reduces the degree of crack damage.

High-Spatial-Resolution Coherent Doppler Lidar With Pseudo-Random Phase-Shift Keying

A high-spatial-resolution coherent Doppler wind lidar (CDWL) incorporating pseudo-random phase-shift keying (PR-PSK) modulation is proposed and demonstrated. The pseudo-random phase coding technique can enhance the spatial resolution without broadening the backscattered spectrum but suffers from the non-instantaneous transition edges caused by the limited bandwidth. In this work, ideal binary PSK modulation is achieved by exploiting the zero crossing of the Mach-Zehnder modulator’s (MZM) transfer function, thus greatly reducing the bandwidth requirement. The amplitude shaping and phase modulation of the pulses are naturally aligned as they are realized together in one MZM. Meanwhile, a real-time bias monitoring scheme for the MZM is proposed to facilitate engineering applications. At laser peak power of 300 W, continuous radial wind profile measurement in 800 m is demonstrated, with spatial/temporal resolution of 3 m/0.5 s.

High peak power and energy scaling in the mid-IR chirped-pulse oscillator-amplifier laser systems.

The paper introduces a new route towards the ultrafast high laser peak power and energy scaling in a hybrid mid-IR chirped pulse oscillator-amplifier (CPO-CPA) system, without sacrificing neither the pulse duration nor energy. The method is based on using a CPO as a seed source allowing the beneficial implementation of a dissipative soliton (DS) energy scaling approach, coupled with a universal CPA technique. The key is avoiding a destructive nonlinearity in the final stages of an amplifier and compressor elements by using a chirped high-fidelity pulse from CPO. Our main intention is to realize this approach in a Cr2+:ZnS-based CPO as a source of energy-scalable DSs with well-controllable phase characteristics for a single-pass Cr2+:ZnS amplifier. A qualitative comparison of experimental and theoretical results provides a road map for the development and energy scaling of the hybrid CPO-CPA laser systems, without compromising pulse duration. The suggested technique opens up a route towards extremely intense ultra-short pulses and frequency combs from the multi-pass CPO-CPA laser systems that are particularly interesting for real-life applications in the mid-IR spectral range from 1 to 20 μm.

Modelling and optimization of laser welding of Al2024 aluminium alloy

PurposeAluminium alloys can be used as lightweight and high-strength materials in combination with the technology of laser beam welding, an efficient joining method, in the manufacturing of automotive parts. The purposes of this paper are to conduct laser welding experiments with Al2024 in the lap joint configuration, model the laser welding process parameters of Al2024 alloys and use propounded models to optimize the process parameters.Design/methodology/approachLaser welding of Al2024 alloy has been conducted in the lap joint configuration. Then, the influences of explanatory variables (laser peak power, scanning speed and frequency) on outcome variables (weld width [WW], throat length [TL] and breaking load [BL]) have been investigated with Poisson regression analysis of the data set derived from experimentation. Thereafter, a multi-objective genetic algorithm (MOGA) has been used using MATLAB to find the optimum solutions. The effects of various input process parameters on the responses have also been analysed using response surface plots.FindingsThe promulgated statistical models, derived with Poisson regression analysis, are evinced to be well-fit ones using the analysis of deviance approach. Pareto fronts have been used to demonstrate the optimization results, and the maximized load-bearing capacity is computed to be 1,263 N, whereas the compromised WW and TL are 714 µm and 760 µm, respectively.Originality/valueThis work of conducting laser welding of lap joint of Al2024 alloy incorporating the Taguchi method and optimizing the input process parameters with the promulgated statistical models proffers a neoteric perspective that can be useful to the manufacturing industry.

Multistage Smoothing Compressor for Multistep Pulse Compressors

Ultrahigh peak-power lasers are important scientific tools for frontier laser physics research, in which both the peak power improvement and operating safety are very important. Based on spatial-chirp-induced beam smoothing in both the near field and far field, a multistage-smoothing-based multistep pulse compressor (MS-MPC) is proposed here to further improve safety and operating convenience. In the MS-MPC, beam smoothing is not simply executed in the pre-compressor or main compressor but is separated into multiple stages. As a result, important and expensive optics are directly protected in every stage. The prism-pair-based pre-compressor induces a small spatial chirp, making it both easier to achieve than the previous multistep pulse compressor and sufficient to protect the first grating directly. Furthermore, the asymmetric four-grating compressor, which serves as the main compressor, induces a spatial chirp that further smooths the laser beam, protecting the last grating. In this way, a 10 s to 100 s petawatt laser pulse can be compressed with a single laser beam using the currently available optics. Additionally, an extra beam-smoothing stage can be added before the main amplifier to safeguard the largest amplification crystal from damage. The MS-MPC can be easily integrated into all existing PW laser facilities to improve their potential compressed pulse energy and operational safety.

Experimental demonstration of sequential excitation scheme for H− laser assisted charge exchange

Charge exchange injection is the standard mechanism used to accumulate short, intense pulses of proton beams from an ${\mathrm{H}}^{\ensuremath{-}}$ injector into a synchrotron. Historically, this process has relied on injection foils to remove the two electrons from the ${\mathrm{H}}^{\ensuremath{-}}$, a technique that has been successfully employed for beam powers up to 1.4 MW. However, such foils are known to sublimate beyond a threshold beam power density, requiring the development of another stripping technology that does not have the same limitation. This work reports on the experimental development of laser assisted charge exchange (LACE) as a mechanism for replacing the foils. In the present work, a method of laser assisted charge exchange that is scalable to full duty factor operation is experimentally demonstrated. The method, termed ``sequential resonance excitation,'' relies on a two-step quantum excitation of the electron from the ground state to the excited state. Compared with previously reported LACE experiments that utilized a single-step excitation, this method significantly reduces the required peak laser power, allowing for scalability to millisecond-long ${\mathrm{H}}^{\ensuremath{-}}$ pulses with conventional laser technology.

Tunable High‐Field Terahertz Radiation from Plasma Channels

AbstractTunable broadband terahertz (THz) sources with power at the gigawatt level are desirable for many applications. A scheme to generate such THz emission by off‐axially injecting a weakly relativistic ultrashort laser into a parabolic plasma channel is presented. By utilizing two‐dimensional particle‐in‐cell simulation, it is demonstrated that there are two major physical mechanisms involved, i.e., linear mode conversion from laser wakefields and the electromagnetic waveguide mode excitation inside the plasma channel. The two radiation modes can lead to linearly polarized and radially polarized THz emissions, respectively, with distinct frequency spectra and spatial distributions. It is found that they predominate alternatively with the change of the plasma channel length. For a given plasma channel, one can switch the radiation modes by adjusting the injection position and the injection angle of the laser pulse. In particular, the radiation mode of the linear mode conversion can produce THz pulses with the peak amplitude of sub‐GV cm−1 with the energy conversion efficiency ≈10−3, even though the peak power of the incident laser is just at the terawatt level. The scheme provides a powerful THz source with tunable intensity, spatial distributions, spectra, and polarizations by simply adjusting the injection conditions of incident laser pulses.

In situ thermal characterisation and filamentary modification in Polymethylpentene

Polymethylpentene (PMP) polymers can be used to make advanced optics, especially for midinfrared (MIR) ultrafast optics. In this study, an optical parametric converter pumped by a Ti sapphire femtosecond laser was used to investigate the polymer. The thermal effect generated by loosely focused femtosecond near mid-infrared (MIR) pulses in bulky materials is observed in real-time, and the polymer exhibits thermal stability and conformability. To avoid strong linear absorption and to increase the nonlinear damage threshold, the spectral region for ultrafast applications is 1310 nm (0.95 eV)–1350 nm (0.92 eV). For the process in region VI, there is a sign of stronger coupling for short-wavelength photons, and the pulse duration is recommended to be lower than 160 fs. In the ultraviolet (UV) region, filamentary writing using a low-focused beam, the laser peak power was selected in the range of 2.2 MW–9.2 MW. High-density bulky gratings were written. The refractive-index modulation was significant, and the polymer exhibited highly nonlinear features. This study provides insights into the applications and production of PMP polymers in laser optics and laser engineering.