Elliptical Beam Profile Research Articles

Tailored laser beam shapes for welding of copper using green laser radiation

AbstractThe rapid development of laser beam sources and adapted welding technologies in recent years lead to an increased use of laser welding techniques in automated production nowadays. Especially its precision and local energy input are key features for joining applications in electric vehicle components, where joints have to meet both mechanical and electrical requirements as current-carrying connections. However, the copper materials used are difficult to weld due to their physical properties, making a stable process with fewest seam imperfections only feasible within a limited process window. Recently available beam sources emitting visible laser radiation have proven to overcome the low absorptivity at process start, but spattering is still a prone defect significantly affecting process efficiency and quality. Literature approaches for modifying the energy input point to laser beam shaping as a method for reducing process imperfections, which, however, has not been extensively researched in copper processing using green laser radiation.Thus, this study investigates the influence of a shaped intensity profile for visible laser radiation created with a reflective diffractive optical element in laser beam welding with laser powers up to 3 kW. A characterization of the process dynamics is performed by use of high-speed imaging, and metallographic analysis is used to elaborate benefits of the applied beam shapes. With beam shaping, an enlarged heat conduction welding regime and an advantageous seam shape are found. Furthermore, a decrease in spatter formation during deep penetration welding is detected for the elliptical beam profile, which correlates with an oscillation movement of the capillary.

Lines-based rotating beam profiling algorithm using pixel ionization chamber with dead pixels

In a pencil beam scanning proton therapy system, critical beam parameters, including the spot size, spot center position, and tilt angle, are monitored for security by beam profile detectors such as two-dimensional (2D) pixel-segmented ionization chambers. A method, named lines-based beam profiling (LBBP), is proposed to identify the five geometric parameters μx,μy,σx,σy,θ of rotating elliptical beam profiles using several lines (rows and columns) in the 2D detector data. The method provides solid results even in the presence of dead pixels, which can be located using a wavelet analysis. Simulation experiments were used to define the number of lines used by the LBBP method and to characterize its performance as a function of the signal-to-noise (SNR). For SNR greater than 30, the root mean square errors (RMSEs) of the beam spot size, spot center position, and rotation angle obtained with the LBBP method are less than 0.16 mm, 0.05 mm, and 0.78 degrees, respectively. The LBBP method was applied to the data collected by a pixel chamber illuminated by an X-ray source, demonstrating the capability of identifying dead pixels and providing more accurate measurements of the beam spot size and center position than the standard Gaussian fitting.

Burn time and combustion regime of laser-ignited single iron particle

An improved particle generator based on electrodynamic powder fluidization is proposed and constructed for investigating single metal particle’s combustion. The designed setup is able to generate a single metal particle moving upward with a well controlled velocity and trajectory and ignite it at near-uniform conditions by an infrared laser beam with flattened elliptical beam profile. Mechanically sieved narrow fractions of spherical iron particles with mean sizes in the range of around 26–54 μm were used in experiments. Particles burned in O2/N2 mixtures with oxygen content varying from 21% to 36%. Particle’s trajectories, velocities, and arbitrary radiant intensities were measured by taking images with a high-speed camera and processing them with an in-house developed data processing program. Two characteristic times associated with particle combustion were measured: 1) total duration of high-temperature phase (ttot) and 2) time to the maximum brightness (tmax). The results show that ttot and tmax can be described by a dn-law with 1.57≲n≲1.72 and 1.46≲n≲1.60, respectively. The effect of oxygen concentration on ttot,tmax, and tdec=ttot−tmax was analyzed for selected particle sizes of 30, 40, and 50 μm. It was found that tmax∝(1/XO2)n with 1.04≲n≲1.18 is almost linearly proportional to 1/XO2, while tdec shows a very weak dependency on the oxygen concentration at 26%–36%. This can be explained by the idea that the overall combustion process of iron is controlled by first external and then internal diffusion of oxygen owing to the saturation of oxygen on the particle surface.

Generation of a high-resolution 3D-printed freeform collimator for VCSEL-based 3D-depth sensing

This Letter discusses the generation of 3D-printed micro-optics to obtain the desired beam profile from a multimode vertical-cavity surface-emitting laser (VCSEL) with a significantly reduced divergence angle via the usage of high-resolution two-photon polymerization. Due to the low cost and compact packaging, the VCSEL array is a novel light source for structured-light projection. Particularly for long-distance 3D sensing applications, a greatly reduced divergence angle ensures that a good signal with a sufficiently large number of photons can be recorded, and the projected illumination spots do not overlap. Therefore, exact laser beam characterization and appropriate physical modeling are required in accurate production of an optimal collimator lens. Furthermore, elliptical beam profiles with different orientations can solve the correspondence problem and improve the post-processing speed and robustness in structured light. To generate this special type of beam profile and verify the optical design process, this Letter describes thoroughly the optical prototyping process starting from the beam characterization, the optical design to the production of the two-photon polymerized optics, and its validation. The test of the beam profile and divergence confirm a good match of the produced optics with the physical optical simulation in Zemax. The collimator transforms the input laser beam divergence angle of 324 mrad to an output angle of 20 mrad only.

Controlling melt pool shape, microstructure and residual stress in additively manufactured metals using modified laser beam profiles

Laser powder bed fusion has proven to be an effective additive manufacturing technology for the manufacture of complex metal components. However, the local thermal history associated with Gaussian beam, raster scan processes produces heterogeneous and spatially non-uniform microstructures that differ from those produced from conventional manufacturing and often lack optimized mechanical properties. Steep thermal gradients and high cooling rates produce large thermal strains driving residual stress fields that can negatively affect the dimensional accuracy of the as-built component. Here, we present experimental and simulation methods for controlling microstructure and residual stress through tailored laser beam profiles. Elliptical and Bessel beam profiles are shown to produce more equiaxed microstructures as compared to those of Gaussian beams, while distributed diode-based illumination profiles allow for reduced residual stresses. These experimental results are supported by high-fidelity powder-scale simulation models coupled to the cellular automata and thermomechanical models that account for macroscale residual stress.

Twisted elliptical multi-Gaussian Schell-model beams and their propagation properties.

We introduce a new kind of partially coherent source whose cross-spectral density (CSD) function is described as the incoherent superposition of elliptical twisted Gaussian Schell-model sources with different beam widths and transverse coherence widths, named twisted elliptical multi-Gaussian Schell-model (TEMGSM) beams. Analytical expression for the CSD function propagating through a paraxial ABCD optical system is derived with the help of the generalized Collins formula. Our results show that the TEMGSM beam is capable of generating a flat-topped elliptical beam profile in the far field, and the beam spot during propagation exhibits clockwise/anti-clockwise rotation with respect to its propagation axis. In addition, the analytical expressions for the orbital angular momentum (OAM) and the propagation factor are also derived by means of the Wigner distribution function. The influences of the twisted factor and the beam index on the OAM and the propagation factor are studied and discussed in detail.

Mitigating the relativistic laser beam filamentation via an elliptical beam profile.

It is shown that the filamentation instability of relativistically intense laser pulses in plasmas can be mitigated in the case where the laser beam has an elliptically distributed beam profile. A high-power elliptical Gaussian laser beam would break up into a regular filamentation pattern-in contrast to the randomly distributed filaments of a circularly distributed laser beam-and much more laser power would be concentrated in the central region. A highly elliptically distributed laser beam experiences anisotropic self-focusing and diffraction processes in the plasma channel ensuring that the unstable diffractive rings of the circular case cannot be produced. The azimuthal modulational instability is thereby suppressed. These findings are verified by three-dimensional particle-in-cell simulations.

Theory of terahertz generation by optical rectification using tilted-pulse-fronts.

A model for terahertz (THz) generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for in two spatial dimensions (2-D) (i) the spatio-temporal variations of the optical pump pulse imparted by the tilted-pulse-front setup, (ii) the nonlinear coupled interaction of THz and optical radiation, (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of its spectral broadening due to THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. Furthermore, it is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs in optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation are discussed to guide the development of such sources.

Random source generating far field with elliptical flat-topped beam profile

Circular and rectangular multi-Gaussian Schell-model (MGSM) sources which generate far fields with circular and rectangular flat-topped beam profiles were introduced just recently (Sahin and Korotkova 2012 Opt. Lett. 37 2970; Korotkova 2014 Opt. Lett. 39 64). In this paper, a random source named an elliptical MGSM source is introduced. An analytical expression for the propagation factor of an elliptical MGSM beam is derived. Furthermore, an analytical propagation formula for an elliptical MGSM beam passing through a stigmatic ABCD optical system is derived, and its propagation properties in free space are studied. It is interesting to find that an elliptical MGSM source generates a far field with an elliptical flat-topped beam profile, being qualitatively different from that of circular and rectangular MGSM sources. The ellipticity and the flatness of the elliptical flat-topped beam profile in the far field are determined by the initial coherence widths and the beam index, respectively.

Elliptical Laguerre-Gaussian correlated Schell-model beam.

A new kind of partially coherent beam with non-conventional correlation function named elliptical Laguerre-Gaussian correlated Schell-model (LGCSM) beam is introduced. Analytical propagation formula for an elliptical LGCSM beam passing through a stigmatic ABCD optical system is derived. The elliptical LGCSM beam exhibits unique features on propagation, e.g., its intensity in the far field (or in the focal plane) displays an elliptical ring-shaped beam profile, being qualitatively different from the circular ring-shaped beam profile of the circular LGCSM beam. Furthermore, we carry out experimental generation of an elliptical LGCSM beam with controllable ellipticity, and measure its focusing properties. Our experimental results are consistent with the theoretical predictions. The elliptical LGCSM beam will be useful in atomic optics.

Spatially sculpted laser scissors for study of DNA damage and repair

We present a simple and efficient method for controlled linear induction of DNA damage in live cells. By passing a pulsed laser beam through a cylindrical lens prior to expansion, an elongated elliptical beam profile is created with the ability to expose controlled linear patterns while keeping the beam and the sample stationary. The length and orientation of the beam at the sample plane were reliably controlled by an adjustable aperture and rotation of the cylindrical lens, respectively. Localized immunostaining by the DNA double strand break (DSB) markers phosphorylated H2AX (gamma H2AX) and Nbs1 in the nuclei of HeLa cells exposed to the "line scissors" was shown via confocal imaging. The line scissors method proved more efficient than the scanning mirror and scanning stage methods at induction of DNA DSB damage with the added benefit of having a greater potential for high throughput applications.

SU-FF-T-168: Dose Variations From Passively Scattered Proton Beams Due to Changes in Beam Size and Orientation

Purpose: For passively scattered proton beam delivery, day-to-day fluctuations in beam size extracted from the proton accelerator and changes to the orientation of an elliptical beam profile may cause variations in the delivered dose. The purpose of this study was to determine the magnitude of the variations in the dose delivered due to changes in size and orientation of the beam that is injected into the treatment nozzle. Method and Materials: A model of a clinical passive scattering treatment nozzle was developed using the MCNPX Monte Carlo code. This model was based on original manufacturer design documents. Monte Carlo calculations were used to study changes of the beam's in-air energy distribution and spatial fluence profiles at isocenter, as well as, the central axis depth dose profiles in water resulting from changes in the incident beam size and orientation. Results: No significant changes in central axis dose, the in-air lateral fluence profile, or the energy distribution at isocenter were observed due to changes in the orientation of the elliptical incident beam resulting from rotation of the treatment gantry. However, our results indicated that the energy profile of the beam hardens with increasing beam size. Also, for large increases in beam size, a decrease in the 90% proximal to 90% distal width of the SOBP was observed. Conclusions: Our findings indicate that the dose distribution is not sensitive to changes in the size and orientation of an incident elliptical proton beam that results from either rotation of the gantry or small day-to-day fluctuations of the beam size. However, for large increases (outside of clinical treatment limits) of the incident beam profile, the width of the delivered SOBP decreases.

Four-wave parametric oscillation in sodium vapor by electromagnetically induced diffraction

We have observed a novel type of parametric oscillation in sodium atomic vapor where four off-axis signal waves simultaneously build up under resonant and counterpropagating pump beams with elliptical beam profiles. The four waves, two of them Stokes shifted and the other two anti-Stokes shifted, have similar output powers of up to 10 mW with a conversion efficiency of 30% and are parametrically coupled by electromagnetically induced diffraction.

Laser-Wakefield Acceleration of Monoenergetic Electron Beams in the First Plasma-Wave Period

Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations ctau >or= lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing ctau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration <25 fs. This interpretation is supported by 3D particle-in-cell simulations.

Minimization of acquisition time in short-range free-space optical communication.

We consider short-range (1-3-km) free-space optical communication between moving parties when covertness is the overriding system performance requirement. To maximize covertness, it is critical to minimize the time required for the acquisition phase, during which the party initiating contact must conduct a broad-field scan and so risks revealing his position. Assuming an elliptical Gaussian beam profile, we show how to optimize the beam divergence angles, scan speed, and design of the raster scan pattern so as to minimize acquisition time. In this optimization, several constraints are considered, including signal-to-noise ratio, required for accurate bearing detection and reliable decoding; limited receiver bandwidth; limited scanner speed; and beam divergence as limited by the scanner mirror dimensions. The effects of atmospheric turbulence are also discussed.

Elliptical-focusing effect on parametric oscillation and downconversion

A theoretical study is presented that enables the calculation of the optimal focusing parameters in a singly resonant optical parametric oscillator and difference-frequency generation with elliptical foci. The effect of diffraction, phase matching, beam walk-off, and resonator astigmatism are included in the model. The results show that an optimal focusing configuration is obtained with unequal confocal beam parameters and that an elliptical beam profile can substantially increase the conversion efficiency when beam walk-off is present in the crystal.

Blue Laser Devices Using Frequency-Doubled Laser Diodes.

Modulatable, stable, and highly efficient small-size blue laser sources are recently considered very promising for high-density optical data storage, laser printer, and so on. A frequency-dou-bled AlGaAs laser diode using a monolithic KNbO3 resonator is expected to be more practical be-cause of its high SH power. We developed compact blue laser devices by this method. These devices, with size of 50×30×20mm3, have an SH output above 10mW with relative intensity noise of - 136dB/Hz and slightly elliptical Gaussian beam profile. We also found a new techni-que for direct modulation of blue radiation by using the EO-effect of KNbO3 crystal.