Flat-top Laser Research Articles

Inferring thermal conductivity of a dual-layer woven thermal protection system using cross-validation

Thermal response modeling is performed for the Heatshield for Extreme Entry Environment Technology (HEEET), a dual layer woven thermal protection system. The 3-D material response code, Icarus, is used for modeling the multi-dimensional thermal response of HEEET in the Laser Hardened Materials Environmental Laboratory, where a flat-top fiber laser is used to generate radiative heating representative of atmospheric entry. Discrepancies between simulation predictions and experimental measurements are suggestive of several uncertainties in the magnitude and spatial distribution of the incident laser radiative boundary condition, as well as the material properties, particularly for the thermal conductivity. By reconstructing the magnitude and spatial distribution of the applied radiative heating, the measurements are used to infer the conductivity of HEEET through constrained optimization and cross-validation. The inference shows that a reduction in thermal conductivity from nominal values is necessary for simulations to achieve improved agreement with experimental data.

Wide-fan-angle Flat-top Linear Laser Beam Generated by Long-pitch Diffraction Gratings

Wide-fan-angle Flat-top Linear Laser Beam Generated by Long-pitch Diffraction Gratings

Residual stress distribution and surface geometry of medical Ti13Nb13Zr alloy treated by laser shock peening with flat-top laser beam

The metallic implants made of Ti13Nb13Zr alloy are usually used under the action of the cyclic loads conditions and fatigue failure occurs occasionally. Laser shock peening (LSP) is an innovative surface treatment process, which has been used to improve the fatigue life of metallic materials. The present research was performed on the Ti13Nb13Zr alloy subject to laser modification in order to determine the effects on the residual stress distribution and surface geometry. A neodymium doped yttrium lithium fluoride (Nd: YLF) pulse laser with a flat-top beam was applied at three different overlapping rates (10%, 30%, 50%), laser pulse energies (5J, 6J, 7J), and impact times (1 impact, 2 impacts, 3 impacts), respectively. The residual stresses were determined by X-ray diffractometer with sin 2ψ method. Firstly, the 30% overlap was chosen by comparing the residual stresses induced by LSP with different overlapping rates. Then, the effects of laser energies and impact times on surface residual stresses, in-depth residual stresses, surface deformation, and surface roughness were investigated and compared. The results showed that the residual stresses, surface deformation, and surface roughness increase with the increasing laser energies and impact times. The effected layer depth of residual stress is about 1058 μm with the 6J laser energy.

Defects and Morphology Contribution to Photoluminescence of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Nanostructured by Femtosecond Laser Pulses

Despite basic optoelectronic properties of metal-organic perovskites are quite robust to defects, thelatter affect performance of related photovoltaic devices made of such promising materials. Recently,direct femtosecond projection lithography appeared as prospective tool for high-performing nondestructivenano- and microstructuring of perovskite films. Here, we study in details how defects affectthe photoluminescence properties of CH3NH3PbI3 hybrid perovskite films patterned with single- andmulti-pulse irradiation with flat-top femtosecond laser pulses. Scanning electron, wide-field multiphoton,photoluminescence and laser confocal microscopies being combined with photoluminescencedecay measurements are carried out for these studies. The obtained experimental results are analyzedwith a model for carriers kinetics allowed to determine contributions from radiative and non-radiative processes.

Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo.

We measure the electrical resistivity of hcp iron up to ∼170 GPa and ∼3000 K using a four-probe van der Pauw method coupled with homogeneous flattop laser heating in a DAC, and compute its electrical and thermal conductivity by first-principles molecular dynamics including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp iron increases almost linearly with temperature, and is consistent with our computations. The results constrain the resistivity and thermal conductivity of hcp iron to ∼80±5 μΩ cm and ∼100±10 W m^{-1} K^{-1}, respectively, at conditions near the core-mantle boundary. Our results indicate an adiabatic heat flow of ∼10±1 TW out of the core, supporting a present-day geodynamo driven by thermal and compositional convection.

Developing very strong texture in a nickel-based superalloy by selective laser melting with an ultra-high power and flat-top laser beam

Selective laser melting (SLM) is an important metal additive manufacturing method for manufacturing functional parts with high precision. However, the effective control of texture is very challenging for SLM, due to the use of very small heat focal point, fast scan speed, and the resulted complex thermal history. In this study, we hypothesize that the proper control of laser energy distribution using an ultra-high laser power will generate strong texture in SLMed materials. Inconel 718 superalloy is adopted as the work material, and three types of laser beams, i.e., 200 W Gaussian beam, 2 kW Gaussian beam, and 6 kW square-shaped flat-top beam, are adopted for comparison. It is found that laser beam size and energy distribution plays an important role in the solidification of metal powders. The use of ultra-high energy large-area flat-top laser beam drastically changes the grain morphology to a wide and planar geometry. Grains are elongated to millimeter-scale along the build direction, and the grain growth becomes less competitive. The grain size increases by over 150%, as compared to the case with the 200 W Gaussian laser beam. The very strong texture along the 〈100〉 crystalline direction can be obtained by using the ultra-high-energy flat-top beam thanks to a unified heat gradient direction. The local crystal misorientation and residual strain can be effectively reduced, as compared to the case with the regular 200 W Gaussian beam.

Flat-top laser beams over an extended range

Designs based on single diffractive-optical-elements for obtaining flat-top laser intensity distributions that remain constant over a long range during free-space propagation are presented. Flat-top beams with different orders n exhibit a different range of propagation. For various working distances z, the resulting flat-top beam yields a different depth of focus. By controlling spectral properties of laser distributions, it is possible to maintain invariant flat-top intensity distributions for relatively long propagation distances.

Simple intra-cavity beam shaping for generating a shape-invariant flat-top laser beam

For laser users, the usual Gaussian beam (GB) may not always be the best intensity profile. Often, a preferred option is a Flat-Top (FT) beam which is generally obtained by using diffractive optics in a reshaping operation. In this case, the resulting FT profile occurs only in the focal plane of the focusing lens, and its vicinity. We consider here the generation of a shape-invariant FT laser beam resulting from the incoherent weighted (50%-50% in power) mixing of LG00 (GB) and LG01 (doughnut) beams. For that we consider the insertion inside the laser cavity of a diaphragm and an absorbing ring, and optimising their sizes makes possible the simultaneous oscillation on LG00 and LG01 modes allowing the generation of a shape-invariant laser beam.

Analysis of element content in cement by Gaussian and flattop laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an elemental qualitative and quantitative analytical technique. To improve the stability and accuracy of LIBS in the determination of elemental content in cement, the Gaussian laser beam was transformed into a homogeneous flattop laser beam by a diffractive optical element. Analytical performance was evaluated by comparing their ablation crater morphology and three parameters (R2, SD, and LOD). The flattop LIBS obtained more constant ablation, more stable spectral intensity, better correlation calibration curve and lower LOD. The results demonstrated that the flattop LIBS has great potential for industrial applications.

Supercontinuum generation by femtosecond flat-top laser pulses in fused silica

In this work we report the numerical results on the filamentation and supercontinuum (SC) generation of femtosecond flat-top pulses with different steep edges propagating in fused silica. Results show that the filamentation process is determined mainly by the self-steepening effect and ionization, and the SC generation is influenced mainly by the self-steepening effect. The filamentation onset and termination is determined by the leading edge and the tailing edge, respectively. The steeper the leading edge is, the earlier the filamentation onset is; the steeper the tailing edge is, the earlier the filamentation termination is. The cutoff wavelength in the blue-side region of the SC has no obvious difference for the pulses with different leading and tailing edges. But the spectral intensity of the blue extension is much stronger for the pulses with the steeper leading edges. The stronger spectral intensity results from the formation of more steep descending edges during filamentation.

Simulation and Experimental Study on Residual Stress Distribution in Titanium Alloy Treated by Laser Shock Peening with Flat-Top and Gaussian Laser Beams.

The residual stress introduced by laser shock peening (LSP) is one of the most important factors in improving metallic fatigue life. The shock wave pressure has considerable influence on residual stress distribution, which is affected by the distribution of laser energy. In this work, a titanium alloy is treated by LSP with flat-top and Gaussian laser beams, and the effects of spatial energy distribution on residual stress are investigated. Firstly, a 3D finite element model (FEM) is developed to predict residual stress with different spatial energy distribution, and the predicted residual stress is validated by experimental data. Secondly, three kinds of pulse energies, 3 J, 4 J and 5 J, are chosen to study the difference of residual stress introduced by flat-top and Gaussian laser beams. Lastly, the effect mechanism of spatial energy distribution on residual stress is revealed.

Study on the element segregation and Laves phase formation in the laser metal deposited IN718 superalloy by flat top laser and gaussian distribution laser

The element segregation, Laves phase formation, and mechanical properties of the laser metal deposited IN718 superalloy by the flat top laser beam (FTLB) and gaussian distribution laser beam (GDLB) are studied. It is found that the Laves phase formation in the gaussian distribution laser deposited IN718 (GDLD-IN718) is substantially higher than that in the flat top laser deposited IN718 (FTLD-IN718). The higher production of the Laves phase in the GDLD-IN718 contributes to the higher microhardness and lower tensile strength (about 20% reduction) of the as-deposited IN718 than that of the FTLD-IN718. The element redistribution behavior in the laser rapid solidification under both of the lasers are also studied through the finite element simulation. The results show that the severe laser energy concentration in the center of the GDLB produces higher molten pool temperature, lower horizontal thermal gradient to vertical thermal gradient ratio (GX/GZ) of the solid-liquid interface. These typical thermal characteristic of the GDLB generated molten pool eventually results in a lower redistribution coefficient of the alloying elements and as a result, the interdendritic element segregation and Laves phase formation are dramatically improved in the GDLD-IN718. The present comparative study proves that the FTLB is more superior for the laser additive manufacturing than that of the GDLB.

Pattern analysis of 532- and 1,064-nm picosecond-domain laser-induced immediate tissue reactions in ex vivo pigmented micropig skin

Optical pulses from picosecond lasers can be delivered to the skin as single, flat-top beams or fractionated beams using a beam splitter or microlens array (MLA). In this study, picosecond neodymium:yttrium aluminum garnet laser treatment using a single flat-top beam and an MLA-type beam at the wavelengths of 532 nm and 1,064 nm were delivered on ex vivo genotype-regulated, pigmented micropig skin. Skin specimens were obtained immediately after treatment and microscopically analyzed. Single flat-top beam treatment at a wavelength of 532 nm and a fluence of 0.05-J/cm2 reduced melanin pigments in epidermal keratinocytes and melanocytes, compared to untreated controls. Additionally, 0.1 J/cm2- and 1.3 J/cm2-fluenced laser treatment at 532 nm elicited noticeable vacuolation of keratinocytes and melanocytes within all epidermal layers. Single flat-top beam picosecond laser treatment at a wavelength of 1,064 nm and a fluence of 0.18 J/cm2 also reduced melanin pigments in keratinocytes and melanocytes. Treatment at 1,064-nm and fluences of 1.4 J/cm2 and 2.8 J/cm2 generated increasing degrees of vacuolated keratinocytes and melanocytes. Meanwhile, 532- and 1,064-nm MLA-type, picosecond laser treatment elicited fractionated zones of laser-induced micro-vacuolization in the epidermis and dermis. Therein, the sizes and degrees of tissue reactions differed according to wavelength, fluence, and distance between the microlens and skin.

Focusability of laser pulses at petawatt transport intensities in thin-film compression

Thin-film compression (TFC) and the focusability of high-power laser pulses after self-phase modulation in thin films at transport intensities (∼1 TW/cm2) for petawatt laser systems is demonstrated. High-energy (∼296 mJ) laser pulses are compressed from ∼55 fs to ∼31 fs. Additionally, the focusability of high-power (∼45–55 TW) flat-top laser pulses after spectral broadening in thin films is found to be largely maintained, showing only modest decreases in the energy contained in the central part of the focal spot. In light of these findings, TFC offers a method for moving toward single-cycle pulse durations at significantly higher energies than those found at present, and if beam instabilities can be mitigated, maybe even higher intensities.

Generation of shaped four-point pattern from splitting a flat-top laser beam

We report on the generation of shaped four-point pattern from splitting a flat-top laser beam using a diffractive optical element (DOE). It is very common to use a DOE to realize laser beam shaping and many algorithms, like Gerchber G-S axton (G-S) algorithm, have been studied. We employ the well-established modified G-S algorithm to design shaped four-point pattern with a novel approach, making use of input intensity interception and flat-top target setting to split a flat-top laser beam in a nearly smallest sampling number range. We demonstrate that the restricted condition of the approach is dependent on sampling number of output beam and the number range is discussed. The element of eight-level phase modulation is fabricated. In the experiment, the shaped four-point pattern is detected on the focal plane.

First Octahedral Spherical Hohlraum Energetics Experiment at the SGIII Laser Facility.

The first octahedral spherical hohlraum energetics experiment is accomplished at the SGIII laser facility. For the first time, the 32 laser beams are injected into the octahedral spherical hohlraum through six laser entrance holes. Two techniques are used to diagnose the radiation field of the octahedral spherical hohlraum in order to obtain comprehensive experimental data. The radiation flux streaming out of laser entrance holes is measured by six flat-response x-ray detectors (FXRDs) and four M-band x-ray detectors, which are placed at different locations of the SGIII target chamber. The radiation temperature is derived from the measured flux of FXRD by using the blackbody assumption. The peak radiation temperature inside hohlraum is determined by the shock wave technique. The experimental results show that the octahedral spherical hohlraum radiation temperature is in the range of 170-182eV with drive laser energies of 71kJ to 84kJ. The radiation temperature inside the hohlraum determined by the shock wave technique is about 175eV at 71kJ. For the flat-top laser pulse of 3ns, the conversion efficiency of gas-filled octahedral spherical hohlraum from laser into soft x rays is about 80% according to the two-dimensional numerical simulation.

Alternate analytic formulation of optical force on a dielectric sphere in the ray optics limit

In the past, the optical force on a micrometer-sized dielectric sphere in a single-beam gradient laser trap was formulated by considering 2D distribution of rays for a plane-wave excitation. However, laser beams usually have a Gaussian transverse intensity profile, which, upon tight focusing, leads to a 3D optical trap. Here, we systematically formulate a generalized ray/geometric optics formalism for estimating force (and potential) for both flat-top and Gaussian laser beams using 2D distribution of light rays as well as 3D distribution of light cones. We also compare our method with the exact Mie theory. In addition, we present a detailed discussion on the nature of force (and potential) considering the optical Kerr effect under high-repetition-rate ultrafast pulsed excitation.

Charge production studies from [formula omitted] photocathodes in a normal conducting RF gun

This work discusses the behavior of electron bunch charge produced in an L-band normal conducting radio frequency gun from Cs2Te photocathodes illuminated with ps-long UV laser pulses and presumed homogeneous flattop laser transverse distribution. The measured charge shows the expected linear dependence in the quantum efficiency limited emission regime at low laser pulse energies. At higher laser pulse energy, the measured charge in the space charge limited emission regime should saturate, assuming an ideal homogeneous flattop laser transverse distribution. However, this behavior is not observed experimentally. Instead of saturating, the measured charge continues to increase with laser pulse energy, albeit with much weaker dependence than in the quantum efficiency limited emission regime. Simulations with the space charge particle tracking code ASTRA show that the charge saturates as expected using a homogeneous flattop laser transverse distribution. The discrepancy between simulations and measured excess charge may be attributed to the presence of unintentional Gaussian-like decaying radial halo beyond the core of the otherwise presumed homogeneous flattop core. The rate of increase of the measured charge at high laser pulse energies seems to be proportional to the amount of halo despite charge saturation in the core of the transverse laser radial profile. By utilizing core + halo particle distributions based on measured radial laser profiles, ASTRA simulations and semi-analytical emission models reproduce the behavior of the measured charge for a wide range of RF gun and laser operational parameters within the measurement uncertainties.

Spectral shaping for suppressing stimulated-Raman-scattering in a fiber laser.

We report on the effects of spectral shaping of the output coupler fiber Bragg grating (OC-FBG) in a Yb-doped fiber laser on the laser emission spectrum for the purpose of inhabiting stimulated Raman scattering (SRS). The lasers with four different OC-FBGs were built and characterized. We found that the laser with a multiple reflection peak chirped-moiré OC-FBG produced a broad laser emission linewidth, which, in turn, led to about 100 times lower the SRS emission as compared with the laser with a 0.19 nm Gaussian-shaped OC-FBG. A nearly flat-top laser output spectrum was obtained from the lasers with a triangle-shaped and chirped-moiré OC-FBG, respectively.

A semi-analytical solution for 2-D axisymmetric modeling of repetitive pulse laser heating with body absorption

In the forthcoming paper (Chen et al., 2017), we presented an analytical solution for two-dimensional (2-D) modeling of repetitive pulse laser heating with surface absorption using the method of separation of variables combining with Laplace transform. But in practice, it often confronts the body absorption for the laser heating. Unfortunately, the above method due to the surface absorption is no longer suitable to the body absorption. In this paper, a semi-analytical solution for 2-D axisymmetric modeling of repetitive pulse laser heating with body absorption is obtained using the integral transform method. The method is validated by comparing with the results of existing finite element method. Temperature distributions for different repetitive pulses heating Silicon are modeled, and some physical mechanisms of repetitive pulses heating are analyzed. Results show that: repetitive pulse laser heating has obvious temperature intermittent cumulation effect; in the thin layer of material surface vicinity, temperature rises due to absorption of irradiated energy dominates over the conduction energy transport from surface to vicinity, and as the depth increases, the absorption decreases, whereas the conduction enhances; the shape of radial distribution of temperature is similar to that of the radial distribution of laser intensity for both Gaussian and flat top laser.