High-energy Laser Research Articles

Demonstration of a metastable argon laser of 10 W by transverse pumping.

Optically pumped metastable rare gas lasers have been extensively investigated as promising high-energy lasers. These systems employ discharge-excited metastable inert gases as the lasing medium. Following optical pumping, a buffer gas, typically helium, is introduced to facilitate a collisional population transfer to the p2[1/2]1 level, thereby establishing population inversion. To date, laser outputs in the watt level have been demonstrated. However, further power scaling crucially depends on the ability to stably generate high metastable densities at elevated pressures approaching atmospheric conditions. In this Letter, we report a pulsed discharge technique based on a peaking capacitor rapid discharge circuit, which is capable of producing metastable particle densities exceeding 1014 cm-3 at pressures up to 900 mbar. By employing this discharge approach in conjunction with transverse optical pumping, we have realized a maximum output power of 12.5 W from a semiconductor-pumped metastable argon laser system.

Study on the modification and mechanism of N-Mo co-doping for U-2.0Nb alloy surface by pulsed laser

The surface of a U-2.0Nb alloy pre-coated with a Mo layer was successfully modified by pulsed laser in N2 gas, resulting in an N-Mo co-doped layer. Evaluation of this layer at 363 K in a water vapor environment demonstrated its exceptional corrosion resistance for the U-2.0Nb substrate. Analysis using XRD and XPS revealed that the layer comprises uranium nitrides (UN and UO2-xNy) and uranium alloys (U, Nb, Mo), whose chemical stability and distribution are crucial for significantly enhancing corrosion resistance. This study also confirms that high-energy pulsed laser modification induces the sequential formation of reactants, favoring those with higher melting points. This technology shows promise for developing high melting point, ablation-resistant layers and enhancing the corrosion resistance of advanced nuclear fuels like UN/UC/UC1-xNx, thereby broadening the application scope of pulsed laser modifications.

The Role of Bone Edema in Plantar Fasciitis Treated with Temperature-Controlled High-Energy Adjustable Multi-Mode Emission Laser (THEAL) and Exercise: A Prospective Randomized Clinical Trial.

Plantar fasciitis is one of the most common causes of foot pain; in 35% of cases, it is also associated with bone edema of the heel. The aim of this study was to investigate the relationship between bone edema and the outcomes of temperature-controlled high-energy adjustable multi-mode emission laser (THEAL) and/or exercises in patients with plantar fasciitis. A prospective randomized clinical trial was designed, in which 48 patients suffering from plantar fasciitis, with or without bone edema, were treated with temperature-controlled high-energy adjustable multi-mode emission laser and exercises (the laser group) or with exercises only (the control group). The patients were evaluated at recruitment (T0) and at 2 (T1) and 6 months (T2), monitoring pain (with the Visual Analogue Scale), functionality (with the Foot Function Index), perception of improvement (with the Roles and Maudsley Score), and fascia thickness (with ultrasound examination). In both groups, there was a significant improvement in pain, functional recovery, perception of remission, and a reduction in plantar fascia thickness at T1 and T2. The laser group presented statistically better values at T2 for the Roles and Maudsley Score (z: 2.21; 0.027). The regression analysis showed that a greater reduction in fascia thickness occurred in the laser group (p-value: 0.047). In conclusion, the two conservative treatments were effective in patients suffering from plantar fasciitis, even in the presence of bone edema, but with lesser results.

Stabilising mechanism of cathode jet and droplet transfer in hybrid-laser–GMAW-based directed energy deposition of titanium alloy

ABSTRACT A hybrid laser–GMAW-based directed energy deposition (DED) was developed to stabilise the cathode jet and droplet transfer in DED of Ti alloy. High-speed photography, spectroscopic diagnostic, theoretical calculation, and numerical simulation were employed to investigate the stabilising mechanisms. At a high laser energy density, the highest temperature of the molten pool was located in the laser-irradiated zone, generating a stable cathode jet. Ti atoms easily ionised into Ti ions at high temperatures (>8200 K); therefore, the cathode jet was primarily composed of Ti ions. The laser-induced metal vapour attracted and contracted the arc, resulting in an increase in the arc temperature, and strong evaporation near the wire during the peak-current stage. Therefore, the distribution areas of Ti I and Ti II were enlarged. Introducing the laser inhibited the obstructive effect of the cathode jet on the droplet, thereby enhancing the stability of droplet transfer.

Mechanism Analysis of Rock Vitrification by Strip Laser Scanning.

The process of casing the wellbore in oil and gas drilling consumes a significant amount of time and economic resources. High-energy laser rock fracturing, as an efficient and cost-effective new approach, holds the potential to create a glass-like casing by irradiating the rocks as an alternative to traditional casing. The mechanism behind the vitrification of rocks using laser irradiation, a key factor in achieving glassified casings, remains to be studied. This paper, based on experiments involving scanning sandstone with a line laser, investigates the mechanism of rock vitrification using numerical simulations and X-ray diffractometers. The results demonstrate that the sandstone surface is transformed into glass after laser scanning, with multiple scans and the application of high-speed airflow helping to reduce the formation of bubbles and other phenomena. Furthermore, the speed of laser scanning showed a negative correlation with the laser ablation depth, glass thickness, temperature diffusion rate, and temperature gradient. Based on these findings, a groundbreaking method is proposed for creating high-quality glass by moving the laser to scan the rocks multiple times, offering insights for research into laser-manufactured wellbore casings. Furthermore, this approach holds promising prospects for enhancing and embellishing the exterior of structures and for in situ environmental modifications on planetary surfaces and beyond.

Generation of high-energy self-mode-locked pulses in a Tm-doped fiber laser

The incorporation of a material-based or artificial saturable absorber into a fiber laser cavity imposes a limitation on energy enhancement owing to its low damage threshold and high environmental sensitivity. To address this issue, one promising alternative approach is the utilization of the self-mode-locking technique. Here, we present a robust self-mode-locked Tm-doped fiber laser with high pulse energy emission. A simple and compact fiber laser structure is realized by utilizing a section of a Tm-doped fiber, serving both as a gain medium and a saturable absorber. Thus, the operational stability is enhanced, especially under high-energy conditions. Furthermore, the realization of high-energy pulses is accomplished through the integration of dispersion management technique. Experimental results reveal that the maximum single-pulse energy increases from 34.8 pJ to 120.2 nJ as the round-trip group delay dispersion decreases from −0.43 to −12.40 ps2. The proposed self-mode-locked Tm-doped fiber laser under high-energy operation exhibits remarkable performance. Our results provide a simple approach to obtaining a mid-infrared laser source with high pulse energy and hold significant potential for advancing high-energy laser systems.

5.3 W/265 μJ Mid-IR All-Fiber Er3+:ZBLAN Gain-Switched Laser Based on Dielectric Fiber Mirror and Fiber-Tip Protection

We report a 2.8 μm all-fiber high-power and high-energy gain-switched Er3+:ZBLAN laser based on dielectric fiber mirror and fiber-tip protection. The fiber pigtail mirror, specifically designed for dichroic operation (i.e., anti-reflection at 976 nm pump wavelength and high-reflection around 2.8 μm laser wavelength), shows high damage density of >10 MW/cm2. An anti-reflection protective film is coated on the input tip of Er3+:ZBLAN fiber and an AlF3 endcap is spliced to the output tip of Er3+:ZBLAN fiber for mitigating the fiber-tip photodegradation and high-power catastrophic failure at 2.8 μm. The compact all-fiber cavity is formed by efficiently connecting the Er3+:ZBLAN fiber with dielectric fiber mirror using the standard FC/PC fiber adaptor. When the 976 nm pump operates in pulsed regime, the all-fiber mid-infrared gain-switched laser can be attained with two states of single-pulse and pulse-burst output. The extracted maximum pulse energy is 4.8 μJ in the single-pulse state, and the shortest pulse width is 426 ns. The pulse-burst mode can generate a maximum average power of 5.291 W and burst energy of 264.55 μJ. This work may offer a promising way to realize the low-cost, all-fiber, high-power and high-energy gain-switched laser at MIR wavelengths.

Bridgman growth and characterizations of nonlinear optical crystal La3Ga5.5Nb0.5O14

La3Ga5.5Nb0.5O14 (LGN) crystal is a promising material for optical parametric chirped-pulse amplification (OPCPA) with high laser damage threshold and wide transmission range which is able to achieve high peak power laser. LGN with a diameter of 25 mm was successfully grown by the vertical Bridgman method for the first time and a series of characterizations were performed. The 1.7 µm absorption peak in transmittance spectra of LGN crystal which was caused by the Czochralski technique can be effectively eliminated by the vertical Bridgman method. LGN crystal possesses a high transmittance of 81.3 % in the range of 0.294 ∼ 7.500 μm. The experimental optical band gap of LGN is 4.16 eV which is positively correlated with laser damage threshold. The experimental laser damage threshold of LGN was determined to 1.416 GW/cm2 by Nd:YAG. The outstanding thermal properties of LGN indicate that it possesses thermal stability in high energy laser field. This work demonstrates that LGN crystal holds great potential for OPCPA applications and the vertical Bridgman method is proved to be a effective and excellent technique to grow high quality LGN crystal.

2.09 µm high-gain, high-energy sub-nanosecond laser amplification system and its application to a millijoule-level mid-infrared ZnGeP2 optical parametric generator.

In this Letter, we report for the first time to our knowledge a 2 mJ-level 2.09 µm Ho:YAG regenerative amplifier (RA) seeded by the first-stage Ho-doped fiber (HDF) preamplifier of a gain-switched laser diode (GSLD). After the single-pass power amplifier (SPPA), the output of a 2.09 µm pulse laser with 1 kHz, 570 ps, and >10 mJ was achieved. The overall gain of the whole amplifier system was greater than 90 dB, providing a novel, stable, and reliable sub-nanosecond (sub-ns) pump source operating at a pulse repetition frequency (PRF) of 1 kHz for an optical parametric generator (OPG) based on ZnGeP2 (ZGP). Specifically, for the ZGP OPG structure, a maximum pulse energy of 1.82 mJ at 3-5 µm had been achieved with an injected pump pulse energy of 5.47 mJ, corresponding to a slope efficiency of 39.5% and an optical-to-optical conversion efficiency (OOCE) of 33.27%.

Pulse compression via cascaded SBS and α-Al2O3 nanosheet enhanced LIB

The leading-edge based stimulated Brillouin scattering (SBS) pulse compression technology is increasingly being recognized as a significant advancement for the generation of high-energy short-pulse lasers. Trailing-edge based laser-induced breakdown (LIB) compression is anticipated to be an effective supplement to SBS, helping to overcome its compression limitations. In this study, we integrated SBS and LIB to generate a high-power, short-pulse laser, thereby surpassing the compression constraints of SBS. Furthermore, we enhanced LIB by doping the LIB medium with α-Al2O3 nanosheets obtained through molten salt synthesis. As a result, we have successfully compressed the pulse duration from 8ns to 241ps, achieving relatively symmetrical leading and trailing edges. Moreover, compared to pure water, we obtained a higher quality far-field light spot.

Investigation of the entrapped micro bubble defect as a high-energy laser damage precursor in fused silica optics

Investigation of the entrapped micro bubble defect as a high-energy laser damage precursor in fused silica optics

Depth localization of subsurface defects by optical dark-field confocal microscopy

Abstract Subsurface defects (SSD) in optical components pose a significant challenge for enhancing the power density of high-energy laser devices. This study investigated the issue of systematic deviation between the measured and actual depths of SSD when employing optical dark-field confocal microscopy for three-dimensional measurements, which is attributed to refractive index disparities between the sample and the observation environment. This paper introduces geometric and diffraction optical models for correcting errors in the SSD depth, along with a calculation method for determining the correction coefficient. By comparing the experimental data and model simulations, a linear relationship between the measured and actual depths was identified with linearity errors below 2.5% and a minimum of 0.67%. The correction coefficients derived from the optical diffraction model are in good agreement with those obtained experimentally. These findings offer valuable insights for calculating SSD depth correction coefficients across various scenarios and requirements to ensure precise measurements.

Removal of micro-nano particles based on water-assisted enhanced plasma shock wave

Foreign micro/nanoparticles can compromise the production and performance of semiconductors, high-energy laser systems, and other technologies. Owing to the strong adhesion (in GPa) of these particles to the surfaces of these devices, removing them using traditional methods such as ultrasound is difficult. Laser plasmas at high temperatures and pressures can effectively clean the micro/nanoparticles. Compared with traditional air media, laser plasma generates stronger shock waves and water flow erosion underwater, which can effectively clean these particles. In this study, we analysed the particle removal process under the action of laser plasma in water dielectrics, primarily the distribution characteristics of particles under spatial phase transition, final removal efficiency, removal thresholds, and optimal removal regions of particles with different sizes, as well as the corresponding academic analysis based on thermodynamic effects. We used a silicon wafer that was subjected to a high-energy laser beam to generate plasma shock waves underwater, which were used to remove micro- and nano-sized impurities. We found that the shock wave pressure of the plasma and water flow gradually decreased from the centre to the outside, and the corresponding action region could be divided into three areas, namely, 0–30°, 30–45°, and 45–50°, with particle removal rates of 96.93 %, 75.52 %, and 36.60 %, respectively. For particles in the 0–30° and 30–45° regions, the maximum removal rate corresponded to particle sizes ranging from 50–200 nm and greater than 200 nm, respectively. However, in the 45–60° region, the pressure of the shock wave was lower than the fragmentation threshold and, hence, had the worst particle removal effect owing to the lowest pressure. We believe that these findings will provide guidance for the subsequent development of particle removal technologies using underwater laser plasma shock waves.

Influence of the Glycidyl Azide Polymer on the Energy Release of Aluminum Sub-Micron Particles under Ultrafast Heating Rates Stimulated by Electric Explosion and Solid Laser

Glycidyl azide polymer (GAP)-coated sub-micron aluminum (sub-mAl@GAP) particles exhibit higher heat release than their uncoated counterparts under low heating rates. However, their application in explosives has been hindered due to a lack of understanding of their energy release characteristics under heating rates of detonation levels. To address this problem, the energy release performances of sub-mAl@GAP particles under ultrafast heating rates stimulated by an electric explosion of wire and high-energy laser were studied. The results showed that the reaction of sub-mAl@GAP particles was more violent than that of an uncoated counterpart under an electric explosion stimulus. Additionally, the reaction time of the former was 0.4 ms shorter than that of the latter. In addition, the propagations of shock waves of the sub-mAl@GAP and sub-mAl were analyzed. The propagation distances of shock waves of the sub-mAl@GAP were all longer than those of sub-mAl under laser fluences of 0.5 J/cm2, 1.2 J/cm2, and 2.4 J/cm2. The distance difference gradually increased with the decrease in the laser fluence. Under a laser fluence of 0.5 J/cm2, the velocity and distance differences of the sub-mAl@GAP and sub-mAl were both the largest due to the energy contribution from the GAP. In conclusion, the fast decomposition rate of the GAP and its energy contribution would benefit the energy release of sub-mAl under ultrafast heating rates.

Effects of process parameters on the surface characteristics of laser powder bed fusion printed parts: machine learning predictions with random forest and support vector regression

Laser powder bed fusion (L-PBF) fuses metallic powder using a high-energy laser beam, forming parts layer by layer. This technique offers flexibility and design freedom in metal additive manufacturing (MAM). However, achieving the desired surface quality remains challenging and impacts functionality and reliability. L-PBF process parameters significantly influence surface roughness. Identifying the most critical factors among numerous parameters is essential for improving quality. This study examines the effects of key process parameters on the surface roughness of AlSi10Mg, a widely used aluminum alloy in high-tech industries, fabricated by L-PBF. Part orientation, laser power, scanning speed, and layer thickness were identified as crucial parameters via cause-and-effect analysis. To systematically examine their effects, the Taguchi method was employed within the framework of the design of experiment (DoE). Experimental results and statistical analysis revealed that laser power, scanning speed, and layer thickness significantly influence surface roughness parameters: arithmetic mean (Ra) and root mean square (Rq). Main effect plots and energy density analyses confirmed their impact on surface quality. Microscopic investigations identified surface flaws such as spattering, balling, and porosity contributing to poor quality. Given the complex interplay between parameters and surface quality, accurately predicting their effects is challenging. To address this, machine learning models, specifically random forest regression (RFR) and support vector regression (SVR), were used to predict the effects on surface roughness. The RFR model’s R2 values for predicting Ra and Rq are 97% and 85%, while the SVR model’s predictions are 85% and 66%, respectively. Evaluation metrics demonstrated that the RFR model outperformed SVR in predicting surface roughness.

Modeling and preparation of additive-manufactured laser ceramics

ABSTRACT Additive manufacturing (AM) presents promising prospects for the production of laser ceramics, particularly those with composite structures. However, achieving uniform density within these ceramics is a formidable challenge, primarily due to the intrinsic limitations of uneven forces within the mold. This paper introduces the Density Spatiotemporal Evolution (DSE) model, a novel predictive tool designed to map the density distribution in laser ceramics throughout the AM process. The DSE model elucidates that ceramic density increases in proximity to the indenter and mold wall and reveals an oscillatory pattern in particle density during pressing. These insights have been corroborated through experimental measurements and simulation analyses. Leveraging the DSE model and a Simulated Annealing (SA) algorithm, an innovative AM process is proposed. The pressure exerted to each layer of ceramics is gradient distributed, i.e. 18, 18.36, 19.31, 22.08, 33.02 MPa. Preliminary experimental results indicate a significant enhancement in density uniformity, with an improvement of up to 45% in the optimized ceramics. The advancement promises to yield more homogeneous ceramic materials, potentially accelerating the progress of high-energy laser technology.

Optical transformation characteristic of silicone resin matrix composite coatings under high-energy laser ablation

The easy destruction of most conventional materials by high-energy lasers is a great concern. Anti-laser coatings with ablative heat absorption capabilities are effective in preventing laser damage. However, their extremely low reflectivity implies a high energy absorption rate, which can result in rapid failure. In this study, silicone resin matrix composite coatings were designed and prepared by incorporating ceramic modification components to realize desired optical transformation characteristics. The modification effects on ZrC/SiC, ZrSi2, TiC/SiC, and TiSi2 were specifically studied with a high-energy continuous-wave laser (1080 nm). The results showed that oxidation reactions during laser ablation play a significant role in phase change and optical transformation. The element analysis proved that the composition rapidly changed from carbide and silicide to oxide, leading to the formation of oxide layer with relatively high reflectivity in laser ablation region. In particularly, oxidation and ablation of ZrSi2 facilitated the exposure, gathering, and sintering of ZrO2, which realized an increase in reflection from 6.2 % to 34.9 % during laser ablation (500 W/cm2, 60 s). The optical transformation characteristics were compared with the evolution of diffuse reflection intensity and its corresponding differential curve, which indicates that ZrSi2 exhibited the best performance.

Exploration of laser ablation protection for HfC coatings on C/C composites above 2900K

Enhancing the laser ablation resistance of C/C composites is of paramount importance for their prospective application as thermal protection materials, given the rapid advancements in high-energy continuous laser technology. Herein, HfC coating was prepared by chemical vapor deposition (CVD) on the surface of C/C composites to resist the irradiation of high-energy continuous laser. The melting occurred with the formation of oxide products, and these molten phases accumulated at the edge of the ablation pit after laser ablation with two different powers. Furthermore, many cracks were formed around the ablation pit due to the formation of phase transformation stress during ablation. With the increase of laser power, the surface temperature and damage area of the coating increased, and the melting of oxide scale caused by ultra-high temperature was the main reason for the failure of coating. This work could be helpful to provide a theoretical basis for the design and application of laser protective coatings.

The use of low-level lasers in the treatment of oral lichen planus (OLP) – a review of the literature

A laser is a device that emits radiation in the visible light, ultraviolet or infrared range, using the phenomenon of forced emission. The name is an acronym for Light Amplification by Stimulated Emission of Radiation. The main characteristics of a laser beam include: monochromaticity, coherence, parallelism and intensity. By power, we can divide lasers into 2 groups: high-energy (hard) lasers with emission power above 1 W and low-energy (<1 W) lasers. Lasers are used in medicine, industry, military technology, telecommunications, surveying and construction, among other fields. Lichen planus is a chronic disease of the mucous membranes and skin with a fairly common occurrence, characteristic clinical picture and unclear etiology. The therapy of lichen planus of the mucous membranes, especially its oral form (OLP), is still a major problem despite continuous medical advances. To date, a fully effective treatment regimen has not been developed. Current methods are mainly based on pharmacotherapy. With advances in technology, the search for alternative forms of treatment has begun. In recent years, an increasing number of reports have suggested the validity of using Low Level Laser Therapy (LLLT) to treat oral lichen planus. The use of this treatment may reduce the incidence of side effects observed with traditional corticosteroid (CS) therapy, sulfones or immunosuppressants.

Copper foil microforming through underwater laser oblique impact

Laser-induced cavitation impact forming is a high-speed forming process that employs a high-energy laser to produce cavitation in a liquid and act on the workpiece. When the laser is irradiated vertically to the workpiece surface through the guide hole, oxidation occurs on the produced workpiece's surface, affecting the surface quality. Laser-induced cavitation oblique impact workpiece formation is proposed in this paper. The surface quality of the workpiece created at various angles is investigated by adjusting the angle between the laser and the guide hole. A finite element model (FEM) was employed to simulate the temperature change and collapse speed of laser-induced cavitation, and a hydrophone was utilized to measure the impact pressure of laser-induced cavitation under various guide holes. The variations in forming depth and surface quality of the workpiece caused by laser-induced cavitation oblique impact were investigated experimentally. The experimental results show that as the angle between the laser and the guide hole increases from 0° to 90°, the width of the oxidized area on the surface of the formed micro-groove decreases from 513 μm to 0 μm, the forming depth of the micro-groove decreases from 80 μm to 20 μm, and the surface roughness of formed parts decreases from 2.93 μm to 0.56 μm. The oxidation of the formed workpiece surface during the oblique laser impact is primarily caused by the laser irradiation, according to the analysis of the surface oxidation mechanism of the formed micro-grooves. When the angle between the laser and the guide hole is 60°, all of the heat emitted by the laser is focused to the guide hole's inner surface. The angle between the laser and the pilot hole was set to 60° in the experiment, and the micro-groove structure was generated by three impacts, with the thickness distribution of the formed groove investigated. The results demonstrate that the largest thinning of the micro-groove cross-section occurs at the mold's entry and the bottom of the micro-groove, with thinning rates of 20 % and 15 %, respectively.