Increasing Pulse Energy Research Articles

Influence of pulse energy on surface integrity of AZ31 magnesium alloy processed by femtosecond laser shock peening

Compared with traditional laser shock peening by nanosecond pulse laser, femtosecond laser shock peening (FLSP) exhibits superior flexibility and efficiency since it can be carried out directly in air without any ablative or confinement layers. In this work, FLSP was successfully performed on AZ31 magnesium alloy sheet. The influence of pulse energy on surface morphology and roughness, microstructure, surface residual stress and hardness was investigated in details. The results show that varying the laser pulse energy could effectively improve the surface integrity of the material. Regular dents are produced at the material surface with relatively low pulse energies, and the regular dent morphology disappears gradually with increasing pulse energy, suggesting an increased surface roughness. Surface microhardness and residual compressive stress are increased significantly after FLSP. The surface microhardness increases from the untreated 58.13 HV to the maximum value of about 69.58 HV at 37 μJ, by an increase of 19.7%. At 73 μJ, the residual compressive stress rises to a maximum value of approximately 59.6 MPa from an untreated 16.3 MPa. Moreover, the nano-hardness near the surface at the cross section is 1.68 GPa at 37 μJ, with an increase of 46% compared to the untreated 1.14 GPa. Particularly, twins and grain refinement layers are observed near the surface after FLSP, also indicated by the XRD analysis. The plastic deformation and grain refinement with low pulse energies contribute to the surface strengthening while the thermal effect and surface oxidation with increasing pulse energy may inhibit the enhancement of surface integrity.

Study of GHz-Burst Femtosecond Laser Micro-Punching of 4H-SiC Wafers

The micromaching of silicon carbide using femtosecond laser pulses is becoming an important field of research. High-repetition-rate sub-pulse trains, so-called pulse bursts, are a particularly promising route towards completely new process regimes. We report on the results of micro-punching n-type 4H-silicon carbide wafers using GHz pulse burst in order to systematically investigate the influence of the temporal energy distribution on laser processing. Pulse-burst experiments are performed at a laser wavelength of λ= 1030 nm using a single GHz burst containing a varying number of pulses and then compared with standard single femtosecond pulse exposures. The pulse energy is swept across the ablation threshold. For each set of parameters, the micromachining efficiency is evaluated in terms of ablation efficiency and burr characteristics. Scanning electron micrographs provide qualitative information about the machining quality. The characteristics of the laser modification are discussed in relation to an increase in the number of pulses in a burst envelope and to an increase in pulse energy. We observe that, compared to a single pulse, a GHz burst comprised of 10 lower-energy pulses leads to an increase in the ablation rate by a factor of ≤ 10.

Dissipative soliton resonance in fiber lasers

Due to the quantization of the pulse energy in fiber lasers, it is challenging to find effective ways to increase the pulse energy directly from a fiber laser oscillator. One efficient and promising technique is based on the dissipative soliton resonance (DSR) effect which is a special solution of the nonlinear propagation equation manifesting as a square-wave pulse with a constant peak power and increasing pulse energy and duration when the pumping power increases. Experimentally, DSR is favored with the use of long or ultra-long cavities and has been observed in many optical configurations with various rare-earth doped fibers. In this paper we give a comprehensive review of the DSR in fiber lasers. Theoretical background is considered as well as the most relevant experimental results.

Dissipative pure-quartic soliton resonance in an Er-doped fiber laser

Dissipative pure-quartic soliton (DPQS) that arises from the balance of positive fourth-order dispersion (FOD), nonlinearity, gain and loss has been proposed to generate high-energy pulse. Herein, we numerically investigate the generation of dissipative pure-quartic soliton resonance (DPQSR) in an Er-doped mode-locked fiber laser with all positive FOD. Our findings indicate that the formation of DPQSR depends on the reverse saturable absorption (RSA) effect of real saturable absorber. Then, the impacts of pump power, RSA coefficient and positive FOD in generating DPQSR pulse are investigated. With the increase of pulse energy, the asymmetrical temporal profile of DPQS transforms into a rectangular one. This work can be utilized to comprehend the dissipative pure-quartic soliton resonance as well as the development of an all positive FOD Er-doped fiber laser. In addition, it lays the way for the development of highly stable, high-energy pulsed lasers.

Microstructure and properties of TiC particles planted on single crystal superalloy by electrospark discharging

The formation mechanism of the surface and cross-sectional morphology of the TiC particles planted on single crystal superalloy by the electrospark discharging process, as well as the effect of pulse energy on the microstructure and shear force of the planted particles were investigated. The results show that, the surface of the planted particles can be categorized as 3 regions: spreading region, flushing region and oxidation region. The cross sectional microstructure is formed by 3 synergistic processes: the electrospark discharging process, the particle breakage process, and electro-capillary rise process. Because of these processes, micro-cracks and merged pores are formed in the particle, which are quickly filled with the molten substrate metal, resulting in the element interdiffusion and metallurgical bonding between TiC particles and the metal substrate. With increasing pulse energy, the effect of these processes on the surface and cross-sectional morphology is changed, leading to the change of shear force.

Influence of Pulse Laser Energy on Structural and Magnetic Properties of CoFe2O4 and CoLa0.01Fe1.99O4 Thin Films

Cobalt ferrite and CoLa0.01Fe1.99O4 thin film were prepared using the pulsed laser deposition method with varying pulse laser energy. XRD analysis was used to investigate the structure of ferrite thin films. The study was expanded to include morphology and magnetic properties. The prepared films showed a strong peak at (311), which belongs to the cubic spinel cobalt ferrite phase with the presence of some peaks belonging to α-Fe2O3 as hematite phase that depressed when doping with La ion, and the crystallization improvement significantly after intensity increases by increasing the pulsed laser energy from 500 to 650 mJ. The lattice constant of Cobalt ferrite thin films increased with the increase in the laser pulse energy while the crystallite size deduced from the Williamson Hall method showed decreases with the increases of pulse laser energy. The saturation, remnant magnetization, and coercivity of prepared films were studied according to increasing pule laser energy and Lanthanum doping.

Large-mode-area soliton fiber oscillator mode-locked using NPE in an all-PM self-stabilized interferometer.

In this work, we investigate an approach to scale up the output pulse energy in an all-polarization-maintaining 17.3 MHz Yb-doped fiber oscillator via implementation of a 25 µm core-diameter large-mode-area fiber. The artificial saturable absorber is based on a Kerr-type linear self-stabilized fiber interferometer, enabling non-linear polarization rotation in polarization-maintaining fibers. Highly stable mode-locked steady states in the soliton-like operation regime are demonstrated with 170 mW average output power and a total output pulse energy of ∼10n J distributed between two output ports. An experimental parameter comparison with a reference oscillator constructed with 5.5 µm core-sized standard fiber components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity noise in the high-frequency range >100k H z.

Polariton lasing in AlGaN microring with GaN/AlGaN quantum wells

Microcavity polaritons are strongly interacting hybrid light–matter quasiparticles, which are promising for the development of novel light sources and active photonic devices. Here, we report polariton lasing in the UV spectral range in microring resonators based on GaN/AlGaN slab waveguides, with experiments carried out from 4 K up to room temperature. Stimulated polariton relaxation into multiple ring resonator modes is observed, which exhibit threshold-like dependence of the emission intensity with pulse energy. The strong exciton-photon coupling regime is confirmed by the significant reduction of the free spectral range with energy and the blueshift of the exciton–like modes with increasing pulse energy. Importantly, the exciton emission shows no broadening with power, further confirming that lasing is observed at electron–hole densities well below the Mott transition. Overall, our work paves the way toward the development of novel UV devices based on the high-speed slab waveguide polariton geometry operating up to room temperature with the potential to be integrated into complex photonic circuits.

Quartic soliton dynamics and pattern formations in passive driven fiber resonators with different quartic dispersion regimes

Solitons are particle-like localized structures which can sustain characteristic shape in various nonlinear systems. Very recently, the novel concept of quartic soliton with remarkable energy scaling law arising from the interaction of quartic dispersion and Kerr nonlinearity, provides innovations in nonlinear optics and applications. Here we theoretically investigate the quartic soliton dynamics in passive driven fiber resonators from negative to positive quartic dispersion driving. In the pure negative quartic dispersion regimes, the typical energy-duration scaling relationship is sufficiently demonstrated. Worth to note that, we further investigate the dissipative quartic soliton formation in the positive quartic dispersion regimes, achieving remarkable pulse energy increase. Interestingly, echoing our previous work on the influence of cavity group-velocity dispersion on dissipative Faraday instability formation, pattern formation dynamics in the quartic dispersion driving fiber resonators is also explored. Our investigations might benefit the fundamental understanding of the quartic soliton dynamics.

Characterization of Multimode Soliton Self-Frequency Shift

Optical solitons in multimode fibers exhibit complex dynamics, and peculiar characteristics in terms of pulse duration and energy, which distinguish them from their single-mode counterparts. We propose a theory for Raman-induced soliton self-frequency shift in multimode fibers, that is compared against experimental data of 1-km multimode soliton propagation. Specific values of pulsewidth and energy are found, at which solitons show long-distance stability and better correspondence with the theory of self-frequency shift; those values depend on the input wavelength, but are not related to the duration of the input pulse. Raman delay is affected by a jitter, characterized by a Gaussian statistical distribution, whose standard deviation tends to stabilize to a constant value for increasing pulse energies.

Recent advance of high-energy ultrafast mode-locked oscillators based on Mamyshev mechanism with different starting modes

The nonlinear effect of fiber limits the further increase in pulse energy, and Mamyshev oscillator shows outstanding advantages in managing nonlinearity in waveguide medium, which is now associated with high peak power and high pulse energy. The potential applications of these laser sources based on Mamyshev mechanism have facilitated aggressive research and innovative ideas by researchers around the world. Here, we focus on the mode-locked principle and starting dynamics of Mamyshev oscillator. The review of Mamyshev technology is summarized from two starting modes of seed source injection and self-starting. Initial research and significant progress in this field, plus new insights and challenges of Mamyshev oscillator for ultrafast fiber laser technology are analyzed.

Narrow-Linewidth Diffraction-Limited Tapered Er-Doped Fiber Amplifier with 2 mJ Pulse Energy

The possibility to scale-up output pulse energy in diffraction-limited Er-doped fiber amplifier has been studied. It is shown that the utilization of tapered fiber design allows one to increase the pulse energy up to 2 mJ, while keeping the diffraction-limited beam quality (M2~1.4). Factors limiting the further increase in pulse energy are revealed.

Experimental study on laser peen forming of aluminium alloy 2024-T351 plate

Laser peen forming technology LPF is proposed on the basis of laser peen strengthening technology and mechanical peen forming technology. It is an advanced high-efficiency and long-life manufacturing technology for panels. This forming technology has a wide application prospect in the field of aerospace manufacturing. In the forming process, there are many factors affecting the forming process, such as laser pulse energy, shot peening times, shot peening path, material, structure and so on. At present, there are few studies on the influence of laser peen forming process parameters on the shape of parts, which can not meet the technical requirements of forming process. In this paper, based on the single curvature bending forming of al2024-T351 aluminum alloy, the elastic-plastic deformation of metal surface formed by laser shot peening is investigated. The spot morphology formed by laser shot peening is analyzed. The influence law of different forming process parameters on the forming shape of parts is established. The results show that the thickness is the main influencing factor for the curvature radius of the impact area. The radius of curvature increases with the increase of thickness, and decreases with the increase of pulse energy and coverage. Using the regression analysis method, the mathematical model of laser peen forming curvature radius is established. The predicted results are consistent with the experimental results, and the maximum relative deviation is 2.6%.

Femtosecond Laser Direct Writing of Antireflection Microstructures on the Front and Back Sides of a GaSe Crystal

The development of antireflection coatings is crucially important to improve the performance of various photonic devices, for example, to increase the efficiency of harmonic generators based on high-refractive index crystals with significant Fresnel losses. A promising technique for the reducing of radiation reflection is to change the refractive index by fabrication of antireflection microstructures (ARM) on the surface. This paper presents the results of ARM direct writing on the surfaces of a nonlinear GaSe crystal (of ε modification, according to Raman and photoluminescence spectroscopy data) using fs laser radiation and a multiples approach. An increase in transmission from 65% to 80% for an ARM fabricated on one side of the crystal and up to 94% for ARMs fabricated on both sides is demonstrated. The increase in transmission with the increasing pulse energy, as well as with an increase in the number of pulses used for the formation of a single crater, is shown. The experimental results of ARM transmission of GaSe are in qualitative agreement with the simulation results based on the measured profiles and morphology of the ARM structures.

Experimental study on the fluctuation–magnification effect of chirped volume Bragg grating compressor in high-energy, ultrafast, thin-disk chirped pulse amplification system

The fluctuation–magnification effect on the peak intensity of a laser pulse caused by the nonlinear Kerr effect in the chirped volume Bragg grating (CVBG) compressor is investigated experimentally for a high-energy, thin-disk, chirped pulse amplification system. The nonlinear Kerr effect occurs at the blue end, and the accumulated nonlinear phase shift (B-integral) in the CVBG rises with the increase of laser pulse energy. Experiments show that small fluctuations in peak power of uncompressed pulses cause increasing of the temporal fluctuation and spatial fluctuation due to high Kerr-nonlinearity in the CVBG when B-integral is larger than π. Thus the initial fluctuation would be magnified by the CVBG compressor.

Experimental analysis on CPA-free thin-disk multipass amplifiers operated in a helium-rich atmosphere

We present an experimental investigation on the benefits of helium as an atmospheric gas in CPA-free thin-disk multipass amplifiers (TDMPAs) for the amplification to average powers exceeding 1 kW and pulse peak powers reaching 5 GW. Both the performance of the amplifier and the properties of the amplified sub-400 fs laser pulses centred at a wavelength of 1030 nm are compared for different helium concentrations in air, outlining and quantifying the benefits of a helium-rich atmosphere. The amplification of 100 µJ pulses in an atmosphere with 60% helium instead of air led to a maximum increase in efficiency from 24% to 29%. This translated into an increase of average output power and pulse energy of 34 W (i.e +19%) and 0.34 mJ (i.e. +19%) respectively. At the same time an improvement of the beam quality from M2 = 1.18 to M2 = 1.14 was achieved. For the amplification of 10 µJ pulses to over 1 kW of average power an atmosphere with 33% helium led to an improved beam pointing stability by a factor of 2. Moreover, the beam propagation factor M2 improved by 0.1, and the power stability improved by approximately 10%.

Investigation of Focusing Properties on Astigmatic Gaussian Beams in Nonlinear Medium.

Ultra-short laser filamentation has been intensively studied due to its unique optical properties for applications in the field of remote sensing and detection. Although significant progress has been made, the quality of the laser beam still suffers from various optical aberrations during long-range transmission. Astigmatism is a typical off-axis aberration that is often encountered in the off-axis optical systems. An effective method needs to be proposed to suppress the astigmatism of the beam during filamentation. Herein, we numerically investigated the impact of the nonlinear effects on the focusing properties of the astigmatic Gaussian beams in air and obtained similar results in the experiment. As the single pulse energy increases, the maximum on-axis intensity gradually shifted from the sagittal focus to the tangential focus and the foci moved forward simultaneously. Moreover, the astigmatism could be suppressed effectively with the enhancement of the nonlinear effects, that is, the astigmatic difference and the degree of beam distortion were both reduced. Through this approach, the acoustic intensity of the filament (located at the tangential focal point) increased by a factor of 22.8. Our work paves a solid step toward the practical applications of the astigmatism beam as the nonlinear lidar.

Numerical Simulation Analysis of Erbium-Doped Yttrium Aluminum Garnet Laser-Activated Irrigation in Root Canal Therapy

Objectives: 1. To evaluate the velocity, pressure, and force in the root canal system during LAI. 2. To explore the application of numerical simulation analysis in root canal therapy studies. Materials and methods: A simplified model of laser-activated irrigation (LAI) was established. The volume of fluid (VOF) model was used to solve the Navier-Stokes equation and standard k-ε turbulent model was used as turbulent model. The axial maximum velocity and axial force at 1 mm, 4 mm, and 16 mm away from apex, as well as the pressure at apex in10 000-time steps were calculated. Results: The peak pressure and axial force increased with laser energy. At 1 and 4 mm away from apex, the axial maximum velocity increased with laser energy below 35 mJ, and slightly decreased from 35 mJ to 50 mJ. Conclusions: 1. The pressure and axial maximum force at the apex will increase when the laser pulse energy increases, whereas the maximum axial velocity will increase from 10 mJ to 35 mJ and then decrease, which indicates that an optimal energy exists in clinical work. 2. VOF models are suitable for numerical simulation analysis in root canal therapy

Efficient Strategy for Investigating the Third-Order Nonlinear Optical (NLO) Properties of Solid-State Coordination Polymers.

The investigation of third-order nonlinear optical (NLO) properties of coordination polymers (CPs) based on solid samples is very difficult but is crucial for practical applications. Herein, we show a method for preparing high optical quality CP films in a polymer matrix to study the third-order NLO performance of solid-state CPs. Two novel azobenzene-based CPs, [CdL(DMAc)(H2O)]n (1) and {[CuL(4,4'-azobpy)]·3H2O}n (2) (H2L = 5-((4-(phenyldiazenyl)phenoxy)methyl)isophthalic acid), were selected as study subjects. The corresponding microcrystals with a grain size of around 3 μm were doped into poly(vinyl alcohol) (PVA), forming CP films (1-MC/PVA and 2-MC/PVA). 1-MC/PVA and 2-MC/PVA exhibit NLO absorption switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) with increasing pulse energy. Moreover, their NLO properties can also be efficiently modulated by photostimulation energy due to the trans → cis isomerization of an azobenzene moiety. The density functional theory (DFT) results show that the narrower the band gap between the conduction band minimum and the valence band maximum, the denser the electron density distribution in the central mental and coordination atoms, which is beneficial to exhibit better third-order NLO performance. This work provides a feasible method for the wider practical application of solid materials with excellent third-order NLO performance.

Influence of Pulse Energy and Defocus Amount on the Mechanism and Surface Characteristics of Femtosecond Laser Polishing of SiC Ceramics.

SiC ceramics have excellent comprehensive properties and are typical hard and brittle materials that are difficult to process and are widely used in many fields. Laser polishing technology has developed into a new surface processing technology, and femtosecond laser polishing has become an important method for the precision machining of hard and brittle materials. In this paper, SiC ceramics were ablated and polished by infrared femtosecond laser, the laser ablation threshold of SiC ceramics was calculated and the influence of pulse energy and defocus amount on the surface morphology, surface roughness, polishing depth and oxidation degree of femtosecond laser polishing of SiC ceramics were investigated. The results show that when the laser repetition frequency f = 175 kHz, wavelength λ = 1064 nm and ablation time t = 9 s, the laser ablation threshold of SiC ceramics is 0.355 J/cm2. With the increase in pulse energy, the surface roughness first decreased and then increased, and the polishing depth showed an overall upward trend. The change of defocus amount will lead to the change of the laser spot diameter. With the increase of the defocus amount, the laser spot irradiated on the workpiece surface becomes larger, and the laser energy density decreases, which results in the decrease of the laser ablation ability and polishing depth and the increase of the polished surface roughness. Periodic nano-ripple structures appeared on the laser-induced surface. Through Energy Dispersive Spectrometer (EDS) elemental analysis, it was found that there was an oxidation phenomenon in SiC ceramics polished by femtosecond laser in an air environment, and the change of pulse energy and defocus amount had insignificant effects on the degree of oxidation.