Large Beam Diameter Research Articles

Single-sided laser beam welding of a dissimilar AA2024–AA7050 T-joint

Abstract In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.

Nonmechanical Laser Beam Steering Based on Polymer Polarization Gratings: Design Optimization and Demonstration

We present a wide-angle, nonmechanical laser beam steerer based on polymer polarization gratings with an optimal design approach for maximizing field-of-regard ( $F\!O\!R$ ). The steering design offers exponential scaling of the number of steering angles, called suprabinary steering. The design approach can be easily adapted for any 1-D or 2-D (e.g., symmetric or asymmetric FOR) beam steering. We simulate a system using a finite difference and ray tracing tools and fabricate coarse beam steerer with 65 $^{\circ}$ $F\!O\!R$ with $\sim$ 8 $^{\circ}$ resolution at 1550 nm. We demonstrate high optical throughput (84 $\%$ –87 $\%$ ) that can be substantially improved by optimizing substrates and electrode materials. This beam steerer can achieve very low sidelobes and supports comparatively large beam diameters paired with a very thin assembly and low beam walk-off. We also demonstrate using a certain type of LC variable retarder that the total switching time from any steering angle to another can be 1.7 ms or better.

Influence of Filler Wire Feed Rate in Laser-Arc Hybrid Welding of T-butt Joint in Shipbuilding Steel with Different Optical Setups

In this paper, a study of laser-arc hybrid welding featuring three different process fibres was conducted to build knowledge about process behaviour and discuss potential benefits for improving the weld properties. The welding parameters affect the weld geometry considerably, as an example the increase in welding speed usually decreases the penetration and a larger beam diameter usually widens the weld. The laser hybrid welding system equipped with process fibres with 200, 300 and 600μm core diameter were used to produce fillet welds. Shipbuilding steel AH36 plates with 8mm thickness were welded with Hybrid-Laser-Arc-Welding (HLAW) in inversed T configuration, the effects of the filler wire feed rate and the beam positioning distance from the joint plane were investigated. Based on the metallographic cross-sections, the effect of process parameters on the joint geometry was studied. Joints with optimized properties (full penetration, soundness, smooth transition from bead to base material) were produced with 200μm and 600μm process fibres, while fiber with 300μm core diameter produced welds with unacceptable levels of porosity.

Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa(2)S(4) pumped at 1064 nm.

We compare linear, planar ring, and rotated image singly resonant twisted rectangle (RISTRA) type nanosecond optical parametric oscillator cavities using HgGa2S4 nonlinear crystal pumped by 8ns pulses at 1064nm from a low beam quality pump source. The input-output characteristics and the output idler beam quality at 6300nm are compared for two values of the pump beam diameter presenting different cavity Fresnel numbers and magnitudes of the spatial walk-off effect due to birefringence. The RISTRA cavity ensures in all cases a circular output beam profile but is advantageous in terms of beam quality with respect to the planar ring only at a large pump beam diameter.

Single-field slice-imaging with a movable repeller: photodissociation of N₂O from a hot nozzle.

We present a new photo-fragment imaging spectrometer, which employs a movable repeller in a single field imaging geometry. This innovation offers two principal advantages. First, the optimal fields for velocity mapping can easily be achieved even using a large molecular beam diameter (5 mm); the velocity resolution (better than 1%) is sufficient to easily resolve photo-electron recoil in (2 + 1) resonant enhanced multiphoton ionization of N2 photoproducts from N2O or from molecular beam cooled N2. Second, rapid changes between spatial imaging, velocity mapping, and slice imaging are straightforward. We demonstrate this technique's utility in a re-investigation of the photodissociation of N2O. Using a hot nozzle, we observe slice images that strongly depend on nozzle temperature. Our data indicate that in our hot nozzle expansion, only pure bending vibrations--(0, v2, 0)--are populated, as vibrational excitation in pure stretching or bend-stretch combination modes are quenched via collisional near-resonant V-V energy transfer to the nearly degenerate bending states. We derive vibrationally state resolved absolute absorption cross-sections for (0, v2 ≤ 7, 0). These results agree well with previous work at lower values of v2, both experimental and theoretical. The dissociation energy of N2O with respect to the O((1)D) + N2¹Σ(g)⁺ asymptote was determined to be 3.65 ± 0.02 eV.

Characterization of Optical Coherence Tomography Images Acquired at Large Distances With Large-Diameter Beams

We tested the imaging capabilities for variants of a 1550-nm swept-source fiber-based optical coherence tomography system with a telecentric system incorporated at the end of its sample arm. The system was designed for in vivo imaging of burns; therefore, we acquired images from samples located at distances greater than 24 cm from the exit of the telecentric system. Each system variation had a specific combination of diameters for the reference and sample beams. In the reference arm, we used, alternately, two collimated beams with diameters of 1.5 and 14 mm, respectively. In the sample arm, we tested collimated beams with the following diameters: 1.5, 3.5, 5.7, 8.4, and 14 mm. A galvanometric mirror system scanned the collimated sample beam across the entrance pupil of the telecentric system. The sample beam exited the telecentric system parallel with its optical axis and convergent onto the sample. Depending on the collimator used in the sample arm, images were acquired with beams focused to waist diameters ranging from 40 to 240 μm. We acquired images with the sample at different locations within a ±30 mm range centered about the sample beam waist. Furthermore, we used the signal-to-noise ratio, the detected signal intensity, and the visual appearance to compare images acquired with different sample/reference beam configurations.

Ultrasound-targeted Microbubble Destruction-mediated Gene Delivery Into Canine Livers

Ultrasound (US) was applied to a targeted canine liver lobe simultaneously with injection of plasmid DNA (pDNA)/microbubble (MB) complexes into a portal vein (PV) segmental branch and occlusion of the inferior vena cava (IVC) to facilitate DNA uptake. By using a 1.1 MHz, 13 mm diameter transducer, a fivefold increase in luciferase activity was obtained at 3.3 MPa peak negative pressure (PNP) in the treated lobe. For more effective treatment of large tissue volumes in canines, a planar unfocused transducer with a large effective beam diameter (52 mm) was specifically constructed. Its apodized dual element configuration greatly reduced the near-field transaxial pressure variations, resulting in a remarkably uniform field of US exposure for the treated tissues. Together with a 15 kW capacity US amplifier, a 692-fold increase of gene expression was achieved at 2.7 MPa. Transaminase and histology analysis indicated minimal tissue damage. These experiments represent an important developmental step toward US-mediated gene delivery in large animals and clinics.

Contactless Liquid-Level Measurement With Frequency-Modulated Millimeter Wave Through Opaque Container

A novel contactless method for measuring liquid level through an opaque container is proposed. A millimeter-wave Doppler sensor is developed to “see” (i.e., sense) through a target container and measure the liquid level on the basis of the absorption of millimeter waves in liquid. One of the challenges is to accurately measure liquid level (within sub-millimeter error) despite the inherently large beam diameter of the millimeter wave due to diffraction. A piezoelectric vibrator enables accurate measurement by reflecting a limited portion of the spread beam and modulating it in frequency to distinguish it from the other portion of the beam. A prototype measurement system is fabricated and evaluated. The feasibility of our proposed method for clearly detecting an air-liquid interface concealed in an opaque container is confirmed experimentally. The nonlinearity error of the measured liquid level is within ±0.5 mm .

Statistical characteristics of the tilts of the aero-optical aberration caused by the supersonic turbulent boundary layer

The tilt of the aero-optical aberration caused by the supersonic turbulent boundary layer is obtained by analyzing the center wandering of the far-field optical pattern, and its characteristics are examined from the viewpoint of statistics. When the diameter of the beam propagating through the flow is small compared with the boundary layer thickness, the probability density function (PDF) of the tilt coefficient is centered and can be approximated with a Gaussian distribution. For a larger beam diameter, the PDF of the tilt in the spanwise direction can still be regarded as centered and Gaussian, but the tilt in the streamwise direction clearly deviates from a Gaussian distribution. The correlation of the tilts in the two directions is investigated by computing the correlation coefficient, and the results indicate that the correlation is negative and strong for a small beam diameter and becomes positive and weak for a large beam diameter. These characteristics are explained by the hairpin vortex structures in the boundary layer, which prevail and meander for a very long distance streamwise.

All-reflective, highly accurate polarization rotator for high-power short-pulse laser systems

We present the setup of a polarization rotating device and its adaption for high-power short-pulse laser systems. Compared to conventional halfwave plates, the all-reflective principle using three zero-phase shift mirrors provides a higher accuracy and a higher damage threshold. Since plan-parallel plates, e.g. these halfwave plates, generate postpulses, which could lead to the generation of prepulses during the subsequent laser chain, the presented device avoids parasitic pulses and is therefore the preferable alternative for high-contrast applications. Moreover the device is easily scalable for large beam diameters and its spectral reflectivity can be adjusted by an appropriate mirror coating to be well suited for ultra-short laser pulses.

Transforming Growth Factor-β1 Regulates Cdk5 Activity in Primary Sensory Neurons

In addition to many important roles for Cdk5 in brain development and synaptic function, we reported previously that Cdk5 regulates inflammatory pain signaling, partly through phosphorylation of transient receptor potential vanilloid 1 (TRPV1), an important Na(+)/Ca(2+) channel expressed in primary nociceptive afferent nerves. Because TGF-β regulates inflammatory processes and its receptor is expressed in TRPV1-positive afferents, we studied the cross-talk between these two pathways in sensory neurons during experimental peripheral inflammation. We demonstrate that TGF-β1 increases transcription and protein levels of the Cdk5 co-activator p35 through ERK1/2, resulting in an increase in Cdk5 activity in rat B104 neuroblastoma cells. Additionally, TGF-β1 enhances the capsaicin-induced Ca(2+) influx in cultured primary neurons from dorsal root ganglia (DRG). Importantly, Cdk5 activity was reduced in the trigeminal ganglia and DRG of 14-day-old TGF-β1 knock-out mice, resulting in reduced Cdk5-dependent phosphorylation of TRPV1. The decreased Cdk5 activity is associated with attenuated thermal hyperalgesia in TGF-β1 receptor conditional knock-out mice, where TGF-β signaling is significantly reduced in trigeminal ganglia and DRG. Collectively, our results indicate that active cross-talk between the TGF-β and Cdk5 pathways contributes to inflammatory pain signaling.

Electron acceleration by tightly focused radially polarized few-cycle laser pulses

Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.

Selective photothermolysis to target sebaceous glands: Theoretical estimation of parameters and preliminary results using a free electron laser

The success of permanent laser hair removal suggests that selective photothermolysis (SP) of sebaceous glands, another part of hair follicles, may also have merit. About 30% of sebum consists of fats with copious CH(2) bond content. SP was studied in vitro, using free electron laser (FEL) pulses at an infrared CH(2) vibrational absorption wavelength band. Absorption spectra of natural and artificially prepared sebum were measured from 200 to 3,000 nm, to determine wavelengths potentially able to target sebaceous glands. The Jefferson National Accelerator superconducting FEL was used to measure photothermal excitation of aqueous gels, artificial sebum, pig skin, human scalp, and forehead skin (sebaceous sites). In vitro skin samples were exposed to FEL pulses from 1,620 to 1,720 nm, spot diameter 7-9.5 mm with exposure through a cold 4°C sapphire window in contact with the skin. Exposed and control tissue samples were stained using H&E, and nitroblue tetrazolium chloride staining (NBTC) was used to detect thermal denaturation. Natural and artificial sebum both had absorption peaks near 1,210, 1,728, 1,760, 2,306 and 2,346 nm. Laser-induced heating of artificial sebum was approximately twice that of water at 1,710 and 1,720 nm, and about 1.5× higher in human sebaceous glands than in water. Thermal camera imaging showed transient focal heating near sebaceous hair follicles. Histologically, skin samples exposed to ~1,700 nm, ~100-125 milliseconds pulses showed evidence of selective thermal damage to sebaceous glands. Sebaceous glands were positive for NBTC staining, without evidence of selective loss in samples exposed to the laser. Epidermis was undamaged in all samples. SP of sebaceous glands appears to be feasible. Potentially, optical pulses at ~1,720 or ~1,210 nm delivered with large beam diameter and appropriate skin cooling in approximately 0.1 seconds may provide an alternative treatment for acne.

Direct photonic production: towards high speed additive manufacturing of individualized goods

World market competition currently boosts trends like mass customization and open innovation which result in a demand for highly individualized products at costs matching or beating those of mass production. This work focus on the resolution of the production related dilemma between scale and scope, e.g. either the low-cost production of high quantities or the high-end and thus cost-intensive low-volume production of individualized goods. One of the areas of greatest potential for the resolution of this dilemma are rapid manufacturing (RM) technologies due to their almost infinite geometrical variability and freedom of design without the need for part-specific tooling. Selective Laser Melting (SLM) is one of the RM technologies that additionally provides series identical mechanical properties without the need for downstream sintering processes, etc. However, the state-of-the-art process and cost efficiency is not yet suited for series production. In order to improve this efficiency and enable SLM to enter series production it is indispensable to increase the build rate significantly by means of increased laser power and larger beam diameters. To exploit this potential, a new generation production machine including a kW laser and an optical multi-beam system is developed and experimental results and real life components are shown.

Effects of Laser Irradiation on Caries Prevention

Introduction: Although dental caries is a preventable disease, it is still common and remains a public health problem. One of the potentially effective preventive measures is the use of lasers. The purpose of this study was to review the studies about use of laser irradiation on the inhibition of carious lesions and the effectiveness of different commercial laser type (Nd:YAG, CO2 , and Argon). Method: A literature search included journal databases, existing systematic reviews, and studies identified by content experts. Studies meeting inclusion criteria were assessed for quality. Results: Some studies have demonstrated the potential preventive effect of laser irradiation on sound enamel; the effect of irradiation on white spot lesions is still unclear. The effects of irradiating demineralized enamel surfaces coupled with the use of topical fluoride application is also still unclear and further research needs to be conducted in this area. Laser irradiation alone can significantly enhance acid resistance of sound enamel surfaces and prevent caries progression. Conclusion: Combined use of topical fluoride application and laser irradiation on sound enamel surfaces has provided the best protection against caries initiation and progression. Laser irradiation was of limited value in managing incipient carious lesions. Argon laser use may be easier clinically due to its large and visible beam diameter, which allows for irradiation of the whole surface of the tooth instead of

Mechanical Performance of Large Cantilevered CFT Beam-Column Joint in Jiangyin Magic Cube Time Square

The leading building in Magic Cube Time Square in Jiangyin is a rare large-cantilevered structure in China, with the standard floors overhanging 9m outwards. Concrete filled tube columns in large diameter and steel beams are applied in the peripheral part of the building frame. In this research, the full-scale test is conducted to investigate the mechanical performance of the concrete-filled beam-column joint. Self-balanced loading frame with dimensions of 15m×10m×10m is built due to the complexity of the joint, the greatness of load and the difficulty of loading condition. Meanwhile, the finite element model is generated to inspect the performance of the beam-column joint under the test loads for verification. Through comparisons of test results and FEM results, good mechanical performance and reasonable load transfer mechanism of the joint are presented in the paper. It is verified that the joint is reliable under 1.3 times of the design load. The effect of ring stiffeners is also studied in this paper.

Short-wavelength soft-x-ray laser pumped in double-pulse single-beam non-normal incidence

We demonstrated a $7.36$ nm Ni-like samarium soft-x-ray laser, pumped by $36$ J of a neodymium:glass chirped-pulse amplification laser. Double-pulse single-beam non-normal-incidence pumping was applied for efficient soft-x-ray laser generation. In this case, the applied technique included a single-optic focusing geometry for large beam diameters, a single-pass grating compressor, traveling-wave tuning capability, and an optimized high-energy laser double pulse. This scheme has the potential for even shorter-wavelength soft-x-ray laser pumping.

Individualized production by means of high power Selective Laser Melting

Selective Laser Melting is one of the rapid manufacturing technologies that enable the production of complex shaped individual parts with series-identical mechanical properties without the need for part-specific tooling or downstream sintering processes, etc. However, the state-of-the-art process and cost efficiency is not yet suited for series production. In order to improve this efficiency and enable SLM to enter series production it is indispensable to increase the build rate significantly by means of increased laser power and larger beam diameters. Hence, a new prototype machine tool including a kW laser and a multi-beam system is developed and first experimental results are shown.

Multi-pulse oscillation and instabilities in microchip self-Q-switched transverse-mode laser

Multi-transverse-mode competition, coupling induced instabilities and multi-pulse or satellite pulse oscillations were investigated experimentally and theoretically in laser-diode-pumped Cr,Nd:YAG self-Q-switched microchip lasers under large pump beam diameter. The different transverse modes have great effects on the laser pulse temporal characteristics such as pulse profile, pulse width, instability of peak power and repetition rate jitter. Multi-transverse-mode, multi-pulse oscillation and periodical pulsation were observed by varying the pump beam diameter.The effect of transverse modes on the instability and multi-pulse oscillation were studied by modified coupled rate equations by taking into account the transverse-mode competition of inversion population under different pump conditions. The numerically simulated results are in good agreement with the experimental results. These results show that the multi-pulse oscillation and instability in the pulse train were attributed to different transverse mode coupling and competition. The peak power instabilities and pulse repetition rate jitter of the laser pulses due to transverse mode coupling were also investigated.

On continuous-line acoustic array behavior in a laser Doppler sound pressure measurement system.

Continuous-line acoustic array behavior in a laser doppler anemometry (LDA) based sound pressure measurement system is considered. Traditional LDA ultrasound measurement systems intentionally minimize the laser beam intersection or interference fringe volume to better achieve point measurements of the sound field. The large laser beam diameters due to beam spreading in remote sensing applications from space or airborne platforms make this approach impractical. Since the sound pressure signal is integrated over the interference fringe volume, continuous-line acoustic array behavior is expected to be observed proportional to the length of the laser light path through the interference fringe volume. This behavior is analyzed from a theoretical perspective and an experimental setup to test for continuous-line acoustic array behavior in a LDA sound pressure measurement system is presented.