Pulse Energy Research Articles

Study on the Ablation Behavior of High-Intensity Lasers in Vacuum

Laser ablation has been extensively studied by researchers due to its high precision, high efficiency processing capabilities, and wide range of application potentials. However, in a vacuum environment, due to the complexity of experimental conditions, specific application scenarios, and interdisciplinary interferences, more in-depth research on the ablation behavior of high-intensity lasers in vacuum is still insufficient. In response to such issues, experiments were conducted on titanium alloy perforation using a nanosecond laser in a vacuum environment. The variations in ablation depth and volume as functions of pulse energy, pulse number, and defocus were investigated. Both the depth and volume ablation efficiencies were calculated, and the three-dimensional morphology of the ablation holes was captured. Additionally, the ablation plume was observed to support the research conclusions. The results indicate that within the number of high-intensity laser pulses, the ablation depth per pulse can be increased by more than four times, and the average ablation volume per pulse can reach 0.97 µm3/µJ. The enhanced sputtering of molten material during the multi-pulse laser ablation process in a vacuum environment is identified as the primary factor contributing to the increased ablation efficiency. With the advancement of science and technology and the growing demand for applications, this research is crucial for the further development of fields such as space exploration and technology, advanced manufacturing technology, and basic scientific research.

Acousto-optically induced single-frequency Nd:YAG ring laser based on double corner cube retroreflectors

Using an acousto-optic modulator (AOM) to enforce the unidirectional operation of a Nd:YAG ring laser based on double corner cube retroreflectors (CCRs) was demonstrated for the first time, to the best of our knowledge. By tilting the AOM with RF applied to deviate from the Bragg angle, a different diffraction loss was introduced between the counterpropagating beams in the ring cavity. A continuous wave single-frequency laser with a maximum output power of 2.99 W was successfully obtained at 1064.5 nm, with a tunable wavelength range from 1064.3 nm to 1064.7 nm. The experimental results indicated the unidirectional operated ring cavity comprising the double CCRs performed significant tolerance to the misalignment of the cavity. By changing the RF to pulsed mode and adjusting the angle of the AOM closer to the Bragg angle, the unidirectional Q-switched operation was achieved with the increase of the diffraction loss of the oscillating light. At the repetition frequency of 110 Hz, a 0.882 mJ single-frequency pulse energy with a pulse width of 70.2 ns was obtained, corresponding to a peak power of 12.6 kW. Sngle-frequency pulses with energy of 0.761 mJ and pulse width of 72.4 ns were also obtained at the repetition frequency of 1 kHz, corresponding to a peak power of 10.5 kW.

Multimodal imaging of murine cerebrovascular dynamics induced by transcranial pulse stimulation.

Transcranial pulse stimulation (TPS) is increasingly being investigated as a promising potential treatment for Alzheimer's disease (AD). Although the safety and preliminary clinical efficacy of TPS short pulses have been supported by neuropsychological scores in treated AD patients, its fundamental mechanisms are uncharted. Herein, we used a multi-modal preclinical imaging platform combining real-time volumetric optoacoustic tomography, contrast-enhanced magnetic resonance imaging, and ex vivo immunofluorescence to comprehensively analyze structural and hemodynamic effects induced by TPS. Cohorts of healthy and AD transgenic mice were imaged during and after TPS exposure at various per-pulse energy levels. TPS enhanced the microvascular network, whereas the blood-brain barrier remained intact following the procedure. Notably, higher pulse energies were necessary to induce hemodynamic changes in AD mice, arguably due to their impacted vessels. These findings shed light on cerebrovascular dynamics induced by TPS treatment, and hence are expected to assist improving safety and therapeutic outcomes. ·Transcranial pulse stimulation (TPS) facilitates transcranial wave propagation using short pulses to avoid tissue heating. ·Preclinical multi-modal imaging combines real-time volumetric optoacoustic (OA) tomography, contrast-enhanced magnetic resonance imaging (CE-MRI), and ex vivo immunofluorescence to comprehensively analyze structural and hemodynamic effects induced by TPS. ·Blood volume enhancement in microvascular networks was reproducibly observed with real-time OA imaging during TPS stimulation. ·CE-MRI and gross pathology further confirmed that the brain architecture was maintained intact without blood-brain barrier (BBB) opening after TPS exposure, thus validating the safety of the procedure. ·Higher pulse energies were necessary to induce hemodynamic changes in AD compared to wild-type animals, arguably due to their pathological vessels.

Modeling Femtosecond Beam Propagation in Dielectric Hollow-Core Waveguides

The propagation of femtosecond pulses in guided structures is a matter of both fundamental and practical interest in nonlinear optics. In particular, hollow-core waveguides (HCWs) filled with a gas medium are fabricated and used as devices for the generation of attosecond pulses from high-order harmonics. In this process, the configuration of the laser field (intensity and phase) inside the waveguide is of crucial importance for enhancing the (well-known, low) efficiency of high-order harmonic generation (HHG). Here, we present numerical calculations which demonstrate the main features of the propagation process in fabricated HCWs. We consider a variety of experimental parameters like gas pressure, waveguide size, laser wavelength, and pulse energy and duration. In particular, the beam profile at the fiber input is found to be a sensitive parameter which influences the whole evolution of the laser field along the propagation. Our model is based on a split-step method modified to account for propagation in ionized media and is validated against experimental and theoretical data from the literature. Our results contribute to the description of the main features of beam propagation in HCWs and provide guiding directions for designing efficient configurations for HHG.

Realization of a 4 kHz, large energy single-frequency ns optical parametric oscillator by suppressing optical parametric generation

Large energy single-frequency nanosecond (ns) near-infrared light source is an essential device in the field of the remote chemical analysis based on the laser-induced breakdown spectroscopy (LIBS). In this paper, a large energy single-frequency ns 824 nm light source with high repetition rate is presented, which is generated from a seed-injection locked optical parametric oscillator (OPO). By optimizing the spot radius of the pump laser and the mode-matching between the pump laser and signal light, the optical parametric generation (OPG) process is effectively eliminated. On this basis, with the assistance of the seed-injection locking, a single-frequency 4 kHz ns 824 nm light source with an output pulse energy and a spectral width of 3.39 mJ and 32.42 MHz is obtained. For the best of our knowledge, it is the largest energy for the single-frequency ns near-infrared light source with the repetition rate of kHz level.

A 1 kV sub-nanosecond electrical pulse generated by a linear GaAs photoconductive semiconductor switch and its characterization

A 1 kV sub-nanosecond electrical pulse generated by a linear GaAs photoconductive semiconductor switch and its characterization

Efficient coupling of dual beam combined laser into micro water jet for deep processing

In order to improve the power and energy of water-jet-guided laser, this paper introduces a double beam water-jet-guided laser (DWJL) technology. Based spatially polarized light combination and temporal phase modulation, two lasers are effectively coupled into a water jet with diameter of 100 μm. The maximum output peak power reaches 100 kW and the maximum pulse energy is 4.63 mJ with an operation frequency range from 1 to 20 kHz. The coupling efficiency is as high as 90.4%. Its efficient energy utilization offers new solutions for precision cutting and processing of thick materials. Ultimately, the machining capability of high-power/high-energy DWJL was demonstrated by cutting through 7075 aluminum alloys of varying thicknesses. The cutting depth reaches 10.61 mm, corresponding to a depth-to-width ratio of the cutting gap exceeds 90:1, and the taper is only approximately 0.23°. There is no heat-affected ablation zone on the processed surface. For deep micro holes processing, a minimum radius is 248.54 μm with a depth-to-diameter ratio of 21:1. As the ongoing development of DWJL technology, this innovation is anticipated to find broader applications in aerospace, automotive manufacturing, and high-end equipment manufacturing in the future.

130 μJ linear-polarized single-frequency 12-μm-core Er/Yb co-doped fiber amplifier based on pre-shaped seed pulse

<sec>Stimulated Brillouin scattering (SBS) is the major barrier in the process of energy scaling for pulsed single-frequency fiber master oscillator power amplifier (MOPA). Due to gain saturation effect, the laser pulse profile will be gradually distorted with the increase of pump power, which induces steep leading edge and narrower width for the amplified pulses. The resulting laser peak power would increase rapidly and thus the SBS threshold is reached earlier to limit the amplification of pulse energy.</sec><sec>A method to obtain high-energy pulsed single-frequency laser by pulse pre-shaping is demonstrated in this work. By designing the leading edge of the triangular pulse, optimizing its rising trend and the duration of the low-intensity rising part, the pulse width compression phenomenon caused by gain saturation is alleviated effectively. Thereafter, the laser peak power increase process can be retarded to reach the SBS threshold so that higher energy can be amplified for the pulsed single-frequency fiber laser. In the experiment, when the seed pulse is optimized to be a triangular pulse with a low-intensity rising edge of 401 ns and a pulse width of 520 ns, a linear-polarized pulse single-frequency fiber laser of 130.9 μJ is obtained in a 12-μm-core Er/Yb co-doped polarization-maintaining fiber. The pulse width is broadened to 608 ns at the maximum energy. When it is compared with the triangular pulse seed with a rapidly rising leading edge, its maximum energy is increased by about 25%. The optical signal-to-noise ratio and polarization extinction ratio are measured to be 42 dB and 16 dB at the maximum pulse energy, respectively. The corresponding spectral linewidth measured by a delayed self-heterodyne system is 542 kHz. Higher pulse energy can be anticipated by further optimizing the pulse profile and using large-mode-are gain fibers.</sec>

Facile control of giant green-emission in multifunctional ZnO quantum dots produced in a single-step process: femtosecond pulse ablation.

Controlling both UV and visible emissions in ZnO quantum dots (QDs) poses a significant challenge due to the inherent introduction of defects during the growth process. We have refined the photoluminescence (PL) emission characteristics of ZnO QDs through a single-step, reagent-free femtosecond pulsed laser ablation in liquid (fs-PLAL) technique. The ratio of the near band edge (NBE) to deep-level emission (DLE), which determines the shape of the QDs' optical emission spectrum, is precisely controlled by the ablation laser pulse parameters-namely, pulse energy and temporal duration. Having established our ability to control the optical properties, we have investigated the mechanisms and physics involved in controlling optical emission. The key highlight of the work is that ablation with a fs-pulse induces substantial defect states without altering the particle size, with the extent of the effect being dependent on the pulse energy and pulse duration. The spectroscopic techniques inducing Raman spectroscopy, excitation power dependent PL and transient PL study provided deep insight into the PL emission properties of these similarly sized QDs. The improved DLE in these laser-ablated QDs is explained by a surface-recombination-layer approximation process employing steady-state and transient PL. Moreover, we have demonstrated the applicability of green emission for pH sensing within a linear range of 7-10 and highlight the inherent antibacterial properties of these QDs.

Effects of laser wavelength and pulse energy on the evaporation behavior of TiN coatings in atom probe tomography: A multi-instrument study

Effects of laser wavelength and pulse energy on the evaporation behavior of TiN coatings in atom probe tomography: A multi-instrument study

Pulse-by-pulse transient thermal deformation in crystal optics under high-repetition-rate FEL.

Time-domain modeling of the thermal deformation of crystal optics can help define acceptable operational ranges across the pulse-energy repetition-rate phase space. In this paper, we have studied the transient thermal deformation of a water-cooled diamond crystal for a cavity-based X-ray free-electron laser (CBXFEL), either an X-ray free-electron laser oscillator (XFELO) or a regenerative amplifier X-ray free-electron laser (RAFEL), by numerical simulations including finite-element analysis and advanced data processing. Pulse-by-pulse transient thermal deformation of a 50 µm-thick diamond crystal has been performed with X-ray pulse repetition rates between 50 kHz and 1 MHz. Results for temperature and thermal deformation have been compared with the results of transient analysis using a continuous wave (CW) power loading. Temperature and thermal deformation results from pulse-by-pulse transient analysis vary with time about the results for the CW case for the same average power. The variation amplitude increases with pulse energy and decreases with repetition rate. When the repetition rate increases to infinity, both temperature and thermal deformation converge to the results for the CW case. Two critical time scales for the operation of crystal optics in a CBXFEL are (1) first-turn time, i.e. the time for the XFEL pulse to complete the first turn around the cavity so that the crystal sees the recirculated XFEL pulse, and (2) period-end time, i.e.the time that the next electron bunch arrives for the amplification, so that the crystal outcouples the amplified FEL power. For the same average power, simulation results show that the crystal thermal deformation seen by the XFEL beam decreases with repetition rate at the first-turn time of a 300 m-long cavity and increases with repetition rate at the period-end time. For the wavefront preservation requirement of the crystal optics, a pulse-energy versus repetition-rate phase space has been established. The upper bounds of the pulse energy at both first-turn and period-end times decreases with repetition rate, especially at the period-end time. The upper bound of the thermal deformation of the crystal at the period-end time for any repetition frequency can be estimated from the CW case. For a water-cooled diamond crystal of dimension 5 mm × 5 mm × 0.05 mm, the time to reach a quasi steady-state is about 50 ms for temperature and 50 µs for thermal deformation.

Breathers in Mode-Locked Lasers Based on Saturable Absorbers

Breathing pulses, as a unique nonlinear pulse phenomenon, play a key role in laser performance optimization, nonlinear optical processes, and complex signal transmission. Unlike stable solitons, the energy of breathing pulses fluctuates periodically over time, exhibiting periodic changes in both pulse frequency and amplitude. Through appropriate nonlinear effects, lasers can generate stable breathing pulses, achieving a mode-locked state that demonstrates a periodic "breathing" pattern. Based on this, a fiber laser incorporating a saturable absorber as the mode-locking element was designed and built, and stable breathing states were successfully observed at lower pump power levels. High-speed detection techniques and time-stretched dispersive Fourier transform (TS-DFT) technology were used to time-amplify and spectrally analyze the rapid pulses, while monitoring the evolution of the breathing pulse in both time and frequency domains. Experimental results indicate that changes in pump power significantly affect the periodic modulation induced by additional oscillations, thereby controlling the breathing ratio and ultimately leading to the formation of a stable soliton. When the pump power is between 470 <i>mW</i> and 480 <i>mW</i>, the formation of the breathing pulse was first observed, with a breathing ratio of up to 4.5. As the pump power increases, the breathing effect gradually diminishes, and at 510 <i>mW</i>, it completely disappears, with the breathing ratio dropping to 1.<br>These results confirm the critical role of pump power in controlling the breathing pulse state and its transition, demonstrating the potential for controlling pump power in ultrafast laser technology and nonlinear optics. The breathing pulse phenomenon, as a periodic pulse behavior, reflects the complex dynamical characteristics between nonlinear optical effects and cavity parameters. When combined with the natural synchronization system formed between the breathing frequency and the cavity frequency (determined by the cavity length), the periodic change of the breathing pulse becomes a crucial factor for controlling laser output. By adjusting parameters such as the laser’s nonlinearity and dissipation, the characteristics of the breathing pulse and breathing ratio can be precisely controlled, thus achieving precise control of the laser output. The periodic oscillatory characteristics of the breathing pulse inside the laser cavity lead to the non-uniform distribution of pulses, a feature that demonstrates enormous potential in pulse shaping, ultrashort pulse generation, and precise frequency comb control. Additionally, the presence of the breathing pulse may impact the stability and energy conversion efficiency of the laser, offering new perspectives for laser design and optimization.

Measurement of Laser Pulse Energy and Duration Through Air Induced Breakdown Under Electric Field Action

Measurement of Laser Pulse Energy and Duration Through Air Induced Breakdown Under Electric Field Action

Generation of Dark Pulses in Mode‐Locked Erbium‐Doped Fiber Ring Laser Operating in L‐Band Region Using Sc2O3 Absorber

ABSTRACTBy incorporating scandium oxide (Sc2O3) powder into a PVA film, a Sc2O3 saturable absorber (SA) was successfully developed, boasting a modulation depth of 25%. When integrated into a ring erbium‐doped fiber laser, the Sc2O3‐SA facilitated the production of mode‐locked dark pulses featuring multi‐wavelength emission, leveraging its saturable absorbing ability along with the nonlinear properties inherent in the Sc2O3 thin film and the long erbium‐doped fiber. Mode‐locking operation was achieved within a pumping range of 209.3–239.8 mW, resulting in a peak pulse energy of 0.71 nJ. Operating at 1597.8 nm, the mode‐locked laser demonstrated a repetition rate of 10.42 MHz and a pulse width of 23 ns. Domain‐wall dark pulse operation was demonstrated in the L‐band region, achieving a relative stability with a signal‐to‐noise ratio of 55.6 dB, utilizing a Sc2O3 thin film absorber and an 8‐meter‐long active fiber. These findings underscore the promising potential of Sc2O3 to be used in pulsed laser systems, particularly due to its exceptional nonlinear optical properties.

Enhancement of harmonic generation by an intense driving laser with high-order waveguide modes in a high-pressure gas-filled hollow waveguide

High-order harmonics have been widely used as reliable tabletop coherent radiation sources recently, but their applications have often been limited by the available pulse energy. Here, we report that by using an overdriven intense laser in a long waveguide with high-pressure gas, phase matching can be achieved in three distinct “regimes”. In the third regime, favorable phase matching is achieved at near-axis positions to enhance harmonic yields. Our results are supported by a full theoretical analysis, and we demonstrate that coupling of the driving laser with the high-order waveguide modes (instead of the fundamental mode used in most prior experiments) is responsible for achieving phase matching. Furthermore, we establish that this phase matching (and harmonic enhancement) is robust, and a scaling relation is derived for the necessary waveguide and gas parameters, allowing our predictions to be tested immediately in any laboratory today.

Multi-watt picosecond 1.7 µm Tm-doped fiber laser amplification system

We report on a multi-watt, high-repetition-rate picosecond 1.7 µm Tm-doped fiber (TDF) laser amplification system. The seed oscillator is a figure-9 passively mode-locked TDF laser, which delivers a pulse train with a center wavelength of 1738nm and a fundamental repetition rate of ∼85 MHz. After a pre-amplifier and two stages of TDF amplifiers, the output power can be amplified to 5.2 W at a pump power of 10 W, corresponding to a slope efficiency of 52.1%. The output pulse duration is 33.87 ps and the pulse energy is 61 nJ. These results demonstrated that it is an effective method for achieving high-power ultrafast fiber laser source at 1.7 µm waveband, which would be a promising candidate for diverse applications such as polymer welding, bioimaging, mid-infrared laser generation and medical applications.

Application and Output Performance Comparison of Janus and Traditional Transition Metal Chalcogenides in Ytterbium-Doped Fiber Lasers.

Janus transition metal disulfide (TMD) monolayers have two distinct carbon surfaces that break the inherent ground external mirror symmetry. When compared to traditional TMD materials, Janus TMDs not only inherit the advantages of traditional TMDs but also have new characteristics that are different from those of traditional TMDs. This paper describes the development of a stable passive Q-switched ytterbium-doped fiber laser (YDFL) with operating wavelengths of 1032.9 and 1030.6 nm using two saturated absorbing materials: tantalum sulfide (TaSSe) and tantalum disulfide (TaS2). Our experimental results show that TaSSe, as a saturable absorber (SA), can generate a higher single-pulse energy and withstand higher pump power, and the single maximum pulse energy can reach 108.81 nJ. In the TaS2-SA Q-switched YDFL, increasing pump power from 180 to 330 mW results in a minimum pulse width of 3.18 μs. The maximum pulse energy is 50.68 nJ. This study showed that Janus TMD TaSSe has superior optical properties compared to traditional TMD TaS2, indicating that it has great potential for use in fiber laser development.

Solutes in water affect the primary cavitation bubble generated by a pulsed erbium-doped yttrium aluminium garnet laser

Laser-activated irrigation (LAI) of root canal systems depends on the generation of cavitation bubbles in the endodontic irrigant. Physical studies thus far focused on pulse energy, pulse length, frequency, and fiber tip shape, mostly in plain water. This study investigated the effect of endodontically relevant molecules (sodium hypochlorite (NaOCl), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), and their combination) in water on physical properties of the resulting solution, and their impact on primary cavitation bubble features. A commercially available 3% NaOCl irrigant was used, as well as an etidronate powder (Dual Rinse HEDP) to be admixed. Physical parameters (density, surface tension, and viscosity) of these solutions were assessed, including HEDP effects in an ascending concentration series of up to 20%. Primary cavitation bubble features (dimensional and temporal) in conjunction with a pulsed erbium-doped yttrium aluminium garnet (Er: YAG) laser equipped with a flat or conical fiber tip were studied in these liquids using a high-speed camera. Solutes increased the solution’s density, surface tension, and viscosity, with an almost linear response to HEDP dosage (Pearson correlation coefficient > 0.95). This reduced the speed of the primary cavitation bubble, and increased its size and lifetime. Increased HEDP concentrations had a pronounced effect on the shape of bubbles generated at the flat tip. NaOCl and HEDP alter the physical properties of water, which, in turn, affect its cavitation behavior.

Photo-physical mechanism of near-IR femtosecond laser-induced refractive-index change in PMMA

Micromodification in bulk undoped polymethylmethacrylate (PMMA) by single focused (numerical aperture (NA) = 0.25), 1030-nm 250-fs laser pump pulses was explored by pump self-transmittance; optical, 3D-scanning confocal photoluminescence (PL); Raman micro-spectroscopy; and optical polarimetric and interferometric microscopy. Starting from the threshold pulse energy Eth = 0.4 ± 0.1 μJ (peak laser intensity Ith ≈ 8 TW/cm2), visible bright micro-voxels emerged inside PMMA at the 100 ÷ 300-μm depth, with their PL-acquired dimensions increasing versus pulse energy. Optical phase change was interferometrically measured in the voxels at the 532-nm wavelength, exhibiting versus the pulse energy the isotropic refractive index increase Δn = +(4 ÷ 10) × 10−4, and a new 1640-cm−1 peak of C=C vibrations emerged in the Raman spectra. Pump self-transmittance measurements demonstrated the predominating eight-photon absorption (excited energy level ≈ 9.7 eV, coefficient β8 ≈ 3 × 10−5 cm13/TW7) at the sub-threshold I < Ith, implying photoionization of the PMMA chains (the ionization potential of MMA molecule ≈ 9.7 eV). At higher peak intensities I > Ith, inverse brems-strahlung absorption (coefficient ∼103cm−1) of near-critical micro-plasma (density >5 × 1020 cm−3) predominates over the multi-photon PMMA absorption, providing the bulk energy density >6 × 102 J/cm3 and the temperature rise ΔT > 2.2 × 102 K, which are sufficient for PMMA (de)polymerization near the equilibrium bulk temperature TP ≈ 220°C. These results uncover the quantitative mechanism of fs-laser modification of PMMA, justifying the previous qualitative findings and enabling controllable energy deposition during fs-laser PMMA micromachining of diverse functional applications.

Evaluation of Aortic Diseases Using Four-Dimensional Flow Magnetic Resonance Imaging.

The complex hemodynamic environment within the aortic lumen plays a crucial role in the progression of aortic diseases such as aneurysms and dissections. Traditional imaging modalities often fail to provide comprehensive flow dynamics that are essential for precise risk assessment and timely intervention. The advent of time-resolved, three-dimensional (3D) phase-contrast magnetic resonance imaging (4D flow MRI) has revolutionized the evaluation of aortic diseases by allowing a detailed visualizations of flow patterns and quantification of hemodynamic parameters. This review explores the utility of 4D flow MRI in the assessment of thoracic aortic diseases, highlighting the key hemodynamic parameters, including flow velocity, wall shear stress, oscillatory shear index, relative residence time, vortex, turbulent kinetic energy, flow displacement, pulse wave velocity, aortic distensibility, energy loss, and stasis. We elucidate the significant findings of studies utilizing 4D flow MRI in the context of aortic aneurysms and dissections, highlighting its role in enhancing our understanding of disease mechanisms and improving clinical outcomes. This review underscores the potential of 4D flow MRI to refine risk stratification and guide therapeutic decisions, ultimately contributing to better management of aortic diseases.