Ring-shaped Beam Research Articles

Influence of adjustable ring modes on molten pool behavior and microstructure of Al–Si coated boron steel in fiber laser welding

The influence of adjustable ring modes on the phase transition and microstructural evolution in conduction mode laser welding of Al–Si coated boron steel was investigated. Distinct geometric variations and molten pool behavior were observed through a ring-shaped beam effect. The ring beam introduced by ring-beam-mode (RBM) and dual-beam-mode (DBM) laser welding inhibited Al segregation into the fusion zone (FZ). The ring beam caused the Al molten particles to be driven towards the outer boundaries of the weld pool. A ring-beam-mode (RBM) and a dual-beam-mode (DBM) laser welding inhibited Al segregation into the FZ, resulting from a minimal temperature gradient and extension of the solidification time. Phase transition calculations demonstrated that a mount of residual ferrite within the FZ is significantly correlated with the Al concentration at specific temperature points, indicating a predominant role of a central-beam-mode (CBM) FZ in δ-ferrite formation. The RBM and DBM effectively suppressed Al segregation into the FZ, leading to a reduction in ferrite formation and the development of a refined microstructure characterized by a uniform and high density of geometrically necessary dislocations. Electron backscattered diffraction (EBSD) analysis revealed a higher proportion of low-angle grain boundaries in the RBM and DBM, attributed to the unique thermal distribution induced by the ring beam. These findings highlight the critical role of beam mode in tailoring the microstructure and properties of laser welds in Al–Si coated boron steel.

Random Raman Lasing in Diode-Pumped Multi-Mode Graded-Index Fiber with Femtosecond Laser-Inscribed Random Refractive Index Structures of Various Shapes

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

Revealing the influence of ring-shaped beam profiles in high-speed laser beam microwelding by synchrotron x-ray imaging

Laser beam microwelding is a precise technique for joining miniature metal components with high feed rates, which is crucial for productivity. However, high feed rates provoke humping formation—periodic beadlike protuberances along the weld seam—that compromise weld integrity. While humping has been associated with the keyhole transition from a narrow to an elongated shape using standard laser intensity distributions (e.g., Gaussian, top-hat), the impact of complex beam profiles, like ring-shaped intensity distributions, remains less understood. In this work, the influence of core-only, ring-only, and superimposed core-ring intensity distributions on humping formation during laser beam microwelding is investigated by means of synchrotron x-ray imaging. Single-track experiments on stainless steel (1.4404) at 1000 mm/s reveal that the keyhole geometry shifts from deep and narrow with core-only power input to shallow and elongated with ring-only power input. Using a superimposed core-ring intensity distribution (Pc = 300 W, Pr = 600 W) results in a U-shaped capillary and the reduction of the humping amplitude by nearly 80% (from 45.61 μm with core-only to 10.29 μm). The additional laser power comes with the tripling of the melt pool width (from 81 μm with core-only to 263 μm) likely decreasing the melt flow velocity. The reduced variability of the capillary length present for the superimposed intensity distribution further indicates a stabilized evaporation behavior. This work provides valuable insights into mitigating humping formation during laser beam microwelding of stainless steel at elevated feed rates using core-ring intensity distributions.

Laser powder bed fusion of pure copper using ring-shaped beam profiles

Additive manufacturing of copper using laser powder bed fusion (PBF-LB/M) enables the production of highly complex components. However, processing of copper by means of near-infrared laser radiation is challenging due to its absorptivity of only 5%–20%. Using a keyhole welding process with a Gaussian intensity distribution increases the absorptivity up to 53% due to multireflection. This enables the production of components with a density larger than 99.5% and electrical conductivity larger than 90% of the International Annealed Copper Standard (IACS), but this type of welding leads to keyhole porosity due to keyhole instabilities. One way of counteracting is the use of a heat conduction welding process. However, due to the Gaussian intensity distribution, it is not possible to supply sufficient energy to eliminate lack-of-fusion porosity and concurrently avoid the formation of a keyhole. Ring-shaped beam profiles have proven their advantages in stabilizing the PBF-LB/M process with a tendency toward higher laser power, but pure copper has not yet been processed in this way. Therefore, this study investigates the potential of three ring-shaped beam profiles to produce specimens with a density of more than 99.5% and their respective electrical conductivity using a laser power of up to 1300 W. In order to understand the underlying welding process, the weld geometry of single-tracks is analyzed. Specimens with a density of up to 99.77% and an electrical conductivity of up to 101.62% IACS are produced, whereby the material properties and welding regime depend on the selected ring-shaped beam profile.

Wavelength-tolerant generation of Bessel-Gaussian beams using vortex phase plates

With their nearly non-diffracting and self-healing nature, Bessel-Gaussian beams (BGBs) are attractive for many applications ranging from free-space communications to nonlinear optics. BGBs can successfully be generated on background laser beams by imprinting and subsequently annihilating multiply charged optical vortices followed by focusing the resulting ring-shaped beam with a thin lens. For high-power applications optical vortices are preferentially created by spiral phase plates because of their high damage threshold. These are fabricated to realize an azimuthal change of the accumulated phase of a multiple of 2π at a predetermined wavelength. This raises the expectation that the use of spiral phase plates for the generation of BGBs is limited to the design wavelength and therefore not applicable to broadband applications involving short-pulse lasers. In this paper we present experimental data showing that this limitation can be overcome in a broad spectral range around the design wavelength. Experimental cross-sections of the BGBs for several off-design wavelengths are found in a good quantitative agreement with the theoretical Bessel functions at distances up to 540 cm after the focus of the lens.

Optical Halo: A Proof of Concept for a New Broadband Microrheology Tool.

Microrheology, the study of material flow at micron scales, has advanced significantly since Robert Brown's discovery of Brownian motion in 1827. Mason and Weitz's seminal work in 1995 established the foundation for microrheology techniques, enabling the measurement of viscoelastic properties of complex fluids using light-scattering particles. However, existing techniques face limitations in exploring very slow dynamics, crucial for understanding biological systems. Here, we present a proof of concept for a novel microrheology technique called "Optical Halo", which utilises a ring-shaped Bessel beam created by optical tweezers to overcome existing limitations. Through numerical simulations and theoretical analysis, we demonstrate the efficacy of the Optical Halo in probing viscoelastic properties across a wide frequency range, including low-frequency regimes inaccessible to conventional methods. This innovative approach holds promise for elucidating the mechanical behaviour of complex biological fluids.

The impact of ring-shaped laser beam on dissimilar welding of Al-Cu thin sheets for battery tab-to-busbar connection: Microstructural, mechanical and electrical characteristics

Al-Cu dissimilar laser welding has gained substantial attention in the electric vehicle battery manufacturing, while challenged by the formation of brittle Al-Cu intermetallic compounds (IMCs). This study investigated the impact of beam shaping technology on the characteristics of Al-Cu dissimilar laser welds by using a coaxial core and ring dual beam laser system. The investigation assessed multi-level weld characteristics including weld geometry, IMC formation, mechanical and electrical properties. Results showed that the dual laser beam can effectively avoid imperfections observed with single beam, such as full penetration related to core-only beam and surface discontinuity defects associated with ring-only beam, respectively. For the core-only weld, extensive upward migration of Cu promoted the formation of brittle CuAl2 and Cu9Al4/Cu3Al2, leading to a considerable reduction in joint strength and a sharp increase in the electrical resistance. For the ring-only weld, the insufficient bonding area between Al and Cu sheet, leads to a significant degradation of joint strength, although a minimal electrical resistance was identified. An optimized power splitting was determined at 40% for the core beam and 60% for the ring beam, which allows the minimal formation of Al-Cu IMCs and consequently, the improved mechanical strength and reduced electrical resistance.

Laser beam shaping using a photoinduced azopolymer droplet-based mask.

The dewetting of an azopolymer droplet, followed by the photostructuration of the evaporated droplet, is employed to create an amplitude mask. This straightforward process yields a large area featuring periodic micro- and nanostructures. The resulting pattern is utilized to generate a nondiffracting beam. Starting with a Gaussian beam illuminating the amplitude mask, the critical aspect is the production of a bright, ring-shaped beam with a high radius-to-width ratio and symmetric central laser spots, each with the same intensity. This alternative approach to shaping a laser beam is demonstrated as a rapid and cost-effective fabrication technique.

The effect of in-source spatial beam shaping on the laser welding of e-mobility metals and alloys

Electromobility applications require several welded connections using demanding materials often in dissimilar combinations. Copper, aluminium, or steel alloys are laser welded for energy storage and traction related components. On the one hand, high power fiber laser sources provide in-source beam shaping solutions able to modify the irradiance profile towards ring-shaped beams. On the other hand, research focused on the effect of the beam shapes on the melting mechanisms and process quality is still in progress. This work studies the effect of different beam profiles on AISI301LN, AA6082 and pure Cu with a 5 kW fiber laser. Linear trends of power over penetration depth as a function of speed confirms the validity of employing the lumped heat capacity model for ring-shaped beams. Moreover, the specific melting fluence is observed to exhibit an exponential decaying trend with the proportion of power allocated in the fiber core, irrespective of tested material.

Mitigating spatters in keyhole-mode laser welding by superimposing additional ring-shaped beam

Mitigating spatters in keyhole-mode laser welding by superimposing additional ring-shaped beam

Structural transformation of Cu-coated multi-walled carbon nanotubes subjected to high-energy laser irradiation

The Cu coating on carbon nanotubes (CNTs) with low infrared laser absorptivity was applied to provide a protective layer against the structural damage caused by laser irradiation in this paper. The effect of Cu coating on the structural evolution of multi-walled carbon nanotubes (MWCNTs) under a ring-shaped high-energy laser beam was studied. As the laser power increases, the morphology of 35 wt.% Cu-coated MWCNTs (35Cu-CNTs) samples transforms from discrete fine powder particles to caking structure with a roughened surface. A 45 wt.% Cu coating effectively provides structural protection for MWCNTs against laser-induced damage at a laser power of 600 W. During laser irradiation, the Cu coating melts, detaches from the surface of MWCNTs, and subsequently solidifies into a mixture of nearly spherical particles. Without the protection of Cu coating, the MWCNTs bundles are fragmented into nanoscale segments in varying sizes. These segments undergo a transformation into amorphous carbon particles as a result of the rapid heating and cooling process induced by the high-energy laser irradiation. The thermal impact of the high-energy laser beam plays a crucial role in determining the structural transformation of MWCNTs. The coalescence and spheroidization of amorphous carbon particles can be induced by the reduction of surface energy.

Effect of a ring-shaped laser beam on the weldability of aluminum-to-hilumin for battery tab connectors

Advances in laser beam shaping technologies are being studied and are considered beneficial in many aspects of dissimilar metal joining, which include reducing intermetallic compounds (IMCs), optimizing weld pool profiles, and controlling porosity and spatters. This paper utilizes a coaxial ring and core dual beam laser and aims to study the impact of the power ratios between core and ring beams on the weldability of 1100 aluminum alloy to hilumin (steel). High-resolution electron microscopy was performed in the cross sections of the weld pools to quantify the melt pool composition and subsequent IMC formation and weld defects (cracking and cavitation). Lap-shear mechanical testing and electrical resistivity testing were also carried out. Results showed that the optimal power ratio for lap-shear strength was 0.4 (intermediate core and ring) due to the reduction in the Fe-rich liquid into the upper weld region. As a result, this produced a smaller interface between the Fe-rich region and Al, thus reducing the formation of the most detrimental IMC (e.g., Fe2Al5). Conversely, a power ratio of 0.2 (core-dominant) was found beneficial for reducing electrical resistance due to a reduced total IMC volume.

Influence of ring-shaped beam profiles on spatter characteristics in laser-based powder bed fusion of metals

Despite the maturity of laser-based powder bed fusion of metals (PBF-LB/M), some barriers prevent the manufacturing process from fully being established in the industry. One drawback is spatter formation, which is disadvantageous to PBF-LB/M for three main reasons. First, adhering spatter can initiate coater collision, resulting in process failure. Second, large adhering spatter may cause lack-of-fusion defects as they require more energy to remelt sufficiently compared to unprocessed powder. Furthermore, big nonadhering spatter cannot be recycled as powder. The recycling of small spatter particles potentially results in degraded material properties. Ring-shaped beam profiles have been established for deep penetration welding to reduce spatter formation. Investigations on ring-shaped beam profiles in PBF-LB/M focus on improving productivity and process robustness. Qualitative spatter reduction in PBF-LB/M using ring-shaped beam profiles has also been reported. This publication quantitatively examines the influence of ring-shaped beam profiles on spatter formation in PBF-LB/M. Image processing algorithms of on-axis high-speed images are utilized for spatter detection and tracking. A self-developed spatter segmentation is used to determine the spatter size. A Laplacian of Gaussian filter is combined with a Kalman tracker to count and track the spatter. The results show that spatter formation is highly influenced by the beam profile and the chosen process parameters. Considering the melt track width, ring-shaped beam profiles could reduce the number of spatter per fused area. High numbers of spatter are generated when parameter sets result in balling. Moreover, spatter velocity is primarily dependent on the introduced dimensionless enthalpy.

Reduction of depolarization loss in a Tm-doped sesquioxide ceramic laser using a ring-shaped pump beam.

We report on a simple method for reduction of the depolarization loss in an end-pumped Tm:Y2O3 ceramic laser by using a near-field ring-shaped pump beam. Initially, we theoretically derive the expression of the depolarization loss in a bulk laser end-pumped with a near-field flat-top-hat or ring-shaped beam, where a significant reduction of depolarization loss in the latter case is presented. Experimental verification is thereafter carried out with a Tm:Y2O3 ceramic laser employing these two different pump configurations. It shows that the experimentally measured depolarization losses are close to the simulated values; the loss in the case of the annular-beam pump is almost 18 times lower than that with a quasi-top-hat beam at a same absorption pump power of 7.4 W. This work, as a proof-of-principle study, indicates that depolarization loss in the end-pumped bulk lasers can be significantly reduced simply by using a ring-shaped pump beam.

Gouy phase of Bessel-Gaussian beams: theory vs. experiment.

It is well-known that the wave of a freely propagating Gaussian beam experiences an additional π phase shift compared to a plane wave. This phase shift, known as the Gouy phase, has significant consequences in, e.g., nonlinear optics, since the nonlinear processes require high peak intensity and phase matching of the focused beams. Hence, determining and controlling the Gouy phase is crucial in many fields of modern optics and photonics. Here, we develop an analytical model for the Gouy phase of long-range Bessel-Gaussian beams obtained by annihilating highly charged optical vortices. The model accounts for the influence of the relevant experimental parameters (topological charge, radius-to-width ratio of the initial ring-shaped beam, and focal length of the Fourier-transforming lens). We find an evolution of the Gouy phase varying nearly linearly with propagation distance and confirm this result experimentally.

Spatial quantization of exciton-polariton condensates in optically induced traps

We study the formation of exciton-polariton condensates in potlike traps created by optical pumping in a planar microcavity with embedded quantum wells. The trap is formed by a repulsive reservoir of incoherent excitons excited by a ring-shaped nonresonant laser beam. Polariton condensates confined in a trapping potential are subject to spatial confinement leading to energy quantization. We reveal experimentally the discrete spectrum of polariton eigenstates in an optical trap that can be characterized by a pair of quantum numbers, azimuthal and radial quantum numbers, that correspond to the number of nodes of a condensate wave function in the corresponding directions. The occupation numbers of the eigenstates of a polariton condensate are determined by the overlap integral of the condensate wave function and the exciton reservoir spatial density distribution. The nonresonant pumping scheme enables engineering the shape and size of the trap, that allows to selectively excite specific superpositions of the eigenstates of a polariton condensate in each experiment. We demonstrate both single- and multiple-mode polariton lasing in an optical trap.

Design of an optical system for generating ring-shaped laser beam

Design of an optical system for generating ring-shaped laser beam

Circular array transducer based-photoacoustic/ultrasonic endoscopic imaging with tunable ring-beam excitation

Photoacoustic/ultrasound endoscopic imaging is regarded as an effective method to achieve accurate detection of intestinal disease by offering both the functional and structural information, simultaneously. Compared to the conventional endoscopy with single transducer and laser spot for signal detection and optical excitation, photoacoustic/ultrasound endoscopic probe using circular array transducer and ring-shaped laser beam avoids the instability brought by the mechanical scanning point-to-point, offering the dual-modality imaging with high accuracy and efficiency. Meanwhile, considering the complex morphological environments of intestinal tracts in clinics, developing the probe having sufficient wide imaging distance range is especially important. In this work, we develop a compact circular photoacoustic/ultrasonic endoscopic probe, using the group of fiber, lens and home-made axicon, to generate relatively concentrated ring-shaped laser beam for 360° excitation with high efficiency. Furthermore, the laser ring size can be tuned conveniently by changing the fiber-lens distance to ensure the potential applicability of the probe in various and complex morphological environments of intestines. Phantom experimental results demonstrate imaging distance range wide enough to cover from 12 mm to 30 mm. In addition, the accessibility of the photoacoustic signals of molecular probes in ex vivo experiments at the tissue depth of 7 mm using excitation energy of 5 mJ has also been demonstrated, showing a high optical excitation efficiency of the probe.

Ranging accuracy improvement by using a spiral phase plate in a time-of-flight underwater lidar system

We report on an investigation of spatial coherence filtering using a spiral phase plate (SPP) to reduce the amount of scattered light collected in an underwater lidar. This approach exploits the difference in spatial coherence between target-reflected light and scattered light as a means of separating and filtering out the multiple-scattered clutters. An underwater target was illuminated by a Gaussian beam, and both the target-reflected light and scattered light passed through an SPP. The spatially coherent target-reflected component formed a ring-shaped vortex beam, while the spatially incoherent scattered component was unaffected and remained a centrally stronger distribution obeying the Mie scattering law. A mask was placed behind the SPP which allowed only the light on the vortex ring to pass through. Therefore, an all-optical spatial coherence filter composed of an SPP and a mask provides an effective way to reject undesired scattered light without affecting the target-reflected light. A numerical simulation based on Zemax software was carried out and showed that the approach reduces the power of received scattered light without changing the power of received target-reflected light. Experimental results showed that the approach reduced the temporal broadening and excessive delay of the returned pulse caused by forward scattering, therefore reducing the ranging error. Moreover, using a vortex beam with a bigger topological charge improved the ranging accuracy more obviously, especially in turbid water. When the attenuation length was 12.5, by using a 40th-order SPP, the ranging error was reduced from 18 cm to 5.3 cm with a diffuse target.

Injection of electron beams into two laser wakefields and generation of electron rings.

Mutual injection of electron beams into two laser plasma wakefields was observed experimentally when driving laser pulses interfered in plasma at a small crossing angle and were slightly relatively delayed, approximately by one pulse duration. Particle-in-cell simulations revealed that the mutual injection was sensitive to the spatial overlap of the laser pulses, which therefore could be used to control the mutual injection. The dual synchronized, femtosecond electron beams are potentially useful for pump-probe experiments in ultrafast science. In addition, out-of-axis ring-shaped electron beams were detected in both experiments and simulations.