Beam Shaping Research Articles

Neutron and photon calculations of beam shaping assembly for BNCT based on Monte Carlo-discrete ordinates coupled method

The radiation field parameters at the beam shaping assembly (BSA) exit port are the key to boron neutron capture therapy (BNCT) treatment. However, obtaining calculation results with high efficiency and accuracy using only Monte Carlo (MC) method or discrete ordinates (SN) method is difficult. Based on NECP-MCX code and Marvin code, this paper implemented the Monte Carlo-discrete ordinates coupled method and applied this method to the calculation of the radiation field at the BSA exit port. The effect of mesh grid division, the order of quadrature set, and the bias of quadrature set on the results of the coupled calculation was studied. It was concluded that the order and bias of the quadrature set have not improved the coupled calculation results. It was found that the data library used in the SN calculation is the main cause. Therefore, the Monte Carlo-discrete ordinates coupled method can be quickly and accurately applied to the calculation of the radiation field at the BSA exit port by dividing the energy and coordinate intervals rationally and using a more suitable cross section library.

Acousto-optic femtosecond laser beam shaping for bottle-beam generation

Abstract This study explores the properties of a laser beam shaping system comprising an acousto-optic dispersive delay line (AODDL) and an acousto-optic tunable filter (AOTF) for ultrashort laser pulse shaping. The system demonstrates the AODDL’s ability to generate signals with arbitrarily positioned transmission windows and investigates the impact of spectral width on field distributions. The AOTF’s frequency scanning and simultaneous control of two and three spectral components are presented. The experimental setup utilizes femtosecond laser radiation, and the results highlight the system’s versatility for programmable spectral and spatial filtering, offering potential applications in advanced optical manipulation.

Thermal fields induced by oscillating laser beams versus non-oscillating equivalent laser beams — A numerical study

In high power laser material processing, like laser transformation hardening, laser welding or laser cladding, tailoring the laser intensity profile is a method to improve stability of the laser material process and/or improve the processing results. A method to tailor the laser intensity profile in real-time is using a Galvanometer Scanner to quickly and iteratively oscillate the focal spot of the laser beam over a predefined path. Due to the oscillating nature of this beam shaping method, the temporal heat input will differ from the temporal heat input by an equivalent non-oscillated laser intensity distribution. Mathematically this equivalent non-oscillated beam profile is the convolution of the laser intensity distribution of the oscillated focal spot and the predefined oscillation path. This work presents simulation results of laser induced thermal fields. The thermal fields induced by oscillated laser beams at different frequencies are compared with the thermal field induced by the equivalent non-oscillated beam. These comparisons show that even at the highest oscillation frequencies the oscillated and equivalent non-oscillated laser beams induce significantly different in temperature fields (e.g. up to 325 K peak temperature difference) and different cooling rates (105 K/s and 5500 K/s respectively).

Phase space framework enables a variable-scale diffraction model for coherent imaging and display

The fast algorithms in Fourier optics have invigorated multifunctional device design and advanced imaging technologies. However, the necessity for fast computations limits the widely used conventional Fourier methods, where the image plane has a fixed size at certain diffraction distances. These limitations pose challenges in intricate scaling transformations, 3D reconstructions, and full-color displays. Currently, the lack of effective solutions makes people often resort to pre-processing that compromises fidelity. In this paper, leveraging a higher-dimensional phase space method, a universal framework is proposed for customized diffraction calculation methods. Within this framework, a variable-scale diffraction computation model is established for adjusting the size of the image plane and can be operated by fast algorithms. The model’s robust variable-scale capabilities and its aberration automatic correction capability are validated for full-color holography, and high fidelity is achieved. The tomography experiments demonstrate that this model provides a superior solution for holographic 3D reconstruction. In addition, this model is applied to achieve full-color metasurface holography with near-zero crosstalk, showcasing its versatile applicability at nanoscale. Our model presents significant prospects for applications in the optics community, such as beam shaping, computer-generated holograms (CGHs), augmented reality (AR), metasurface optical elements (MOEs), and advanced holographic head-up display (HUD) systems.

Influence of Laser Power and Rotational Speed on the Surface Characteristics of Rotational Line Spot Nanosecond Laser Ablation of TC4 Titanium Alloy.

The TC4 titanium alloy is widely used in medical, aerospace, automotive, shipbuilding, and other fields due to its excellent comprehensive properties. As an advanced processing technology, laser processing can be used to improve the surface quality of TC4 titanium alloy. In the present research, a new type of rotational laser processing method was adopted, by using a beam shaper to modulate the Gaussian spot into a line spot, with uniform energy distribution. The effects of the laser power and rotational speed on the laser ablation surface of the TC4 titanium alloy were analyzed. The results reveal that the melting mechanism of the material surface gradually changes from surface over melt to surface shallow melt with the increase in the measurement radius and the surface roughness increases first, then decreases and, finally, tends to be stable. By changing the laser power, the surface roughness changes significantly with the variation in the measurement radius. Because low laser power cannot provide sufficient laser energy, the measurement radius corresponding to the surface roughness peak of the microcrack area is reduced. Under a laser power of 11 W, the surface roughness reaches its peak when the measurement radius is 600 μm, which is 200 μm lower than that of a laser power of 12 W, 13 W, and 14 W. By changing the rotational speed, the centrifugal force generated by the rotation of the specimen affects the distribution and re-condensation of the molten pool of the surface. As the rotational speed increases, the shallow pit around the pit is made shallower by the filling of the pit with molten material and the height of the bulge decreases, until it disappears. The surface oxygen content of the material increases first and then decreases with the increase in the measurement radius and gradually approaches the initial surface state. Compared with a traditional laser processing spot, the rotational line spot covers a larger processing area of 22.05 mm2. This work can be used as the research basis for rotational modulation laser polishing and has significance for guiding the innovative development of high-quality and high-efficiency laser processing technology.

Variation in Reflected Beam Shape and Pointing Accuracy Over Time and Heliostat Field Position

Heliostats are typically constructed with fixed mirror shapes, which they move to varying angles to reflect sunlight onto a receiver. While heliostat facet curvatures and canting angles are selected to maximize a chosen performance criterion, these fixed shapes inevitably cannot maintain focus throughout widely varying solar incidence angles. Further, heliostat pointing accuracy generally exhibits error due to manufacturing and installation imperfections. In this paper we present a systematic study of how reflected beam shape and pointing accuracy vary with heliostat position, time of day, and time of year. We selected nine heliostats spanning the Sandia National Laboratories National Solar Thermal Test Facility (NSTTF) heliostat field, and measured their beam shape and required pointing corrections throughout the day, for six key dates spanning a solar year. We also present ray tracing analysis to aid understanding of the observed effects. The resulting data provide insight into the nature and magnitude of heliostat focus loss at various times, and clarify the nature of the problem faced when attempting to maximize heliostat field performance, especially for high-temperature industrial applications. We conclude that fixed heliostat fields canted with an off-axis strategy will lose significant performance away from the solar noon hour to which they are tuned, and that aim point corrections can vary widely, both between independent heliostats and over time for a given heiostat. This implies that multiple measurements of each heliostat are required to determine calibration pointing corrections.

Automatic segmentation of microporous defects in composite film materials based on the improved attention U-Net module

ABSTRACT Composite film materials offer several advantages, such as high strength-to-weight ratios, flexibility, durability, or cost-effectiveness, which make them ideal for a wide range of applications across various industries. However, microporous defects in the material compromise its durability and air-tightness. Thus far, few works have been performed on inquiring non-destructive testing methods intended for the detection of microporous defects in composite film materials. In this study, the automatic detection of microporous defects is achieved by using a thermal imaging inspection system developed on the basis of a laser infrared system and the U-Net. Innovatively, an improved attention module is added to U-Net to boost its detection performance. While using a laser as a heat source, a fibre laser beam shaper is chosen as the laser beam expansion to ensure a uniform intensity distribution of the laser beam within a small heating area. Experimental results show that the proposed method can reliably detect microporous defects in the composite film materials with sizes in the order of millimeters/micrometers.

Experimental and theoretical study on the shear behaviors of an innovative precast UHPC composite beams reinforced with inverted T-shaped steel

The innovative precast composite beam consisted of reinforced UHPC beam and inner inverted T-steel shape (forming ITSRU composite beam) is proposed for the marine structures serviced in harsh environment and subjected to complicated loading conditions. The novel composite beam possesses the advantages of lightweight, high strength and ductility, enhanced durability. To study the shear behaviors of ITSRU beams, a shear test program consisted of seven composite beams were conducted to study the effects of shear span-to-depth ratio, steel fiber volume fraction, stirrup ratio and inverted T-steel web thickness. UHPC used in this paper possess compressive strength more than 125 MPa, ultimate tensile strength more than 8.9 MPa and ultimate tensile strain more than 2256με. The inverted T-steel and steel rebar used in this paper have grades of Q355 and HRB400, respectively. A segmented loading control method was used in the shear test, in which a forced-controlled with a rate of 20 kN/min was used before the beam yield and the displacement control was implemented at a rate of 0.5 mm/min after reaching the yield load. The test results revealed that the ITSRU beams exhibited admirable deformation capacity and considerable residual shear capacity even for the composite beam with shear span-to-depth ratio of 1.0. The increasing of stirrup ratio, steel fiber volume fraction and inverted T-steel web thickness could improve the shear resistance and the ductility as well as suppress the appearance of the diagonal shear critical cracks. Based on the test results, an analytical shear model considering the contributions of UHPC in shear compression region, stirrups, steel fibers and inverted steel web was developed to predicted the shear resistance of ITSRU beams. The interaction mechanism of the different components in the ITSRU beams was considered in the analytical shear model. The accuracy of the developed calculation method was verified by the test results.

Wave Optical Modeling of the SEM Column From Source to Specimen.

Probe formation in scanning electron microscope (SEM) is often reduced to objective lens action modeling based on a point-spread function or Fourier transforms. In this study, we present the first complete wave optical modeling of the whole SEM column based on plane-by-plane propagation of the electron beam wavefunction without simplifying the optical system. We identify the challenges in plane-by-plane beam propagation and show how sampling limitations produce aliased results. Through a careful selection and combination of propagators, we have developed a general wave optical propagation method that is able to overcome the aliasing problem to achieve the appropriate probe widths. Using a two-step propagator, we show that it is possible to model the electron beam distribution throughout the column from the virtual source plane to the specimen plane. We also show that our results from the wave optical simulations are consistent with the geometrical theory of probe formation. Finally, as a direct application of this method, we demonstrated that the combined effect of aberrations in the condenser lens and the probe forming objective lens cannot be accurately represented using only the objective lens. Designing beam shaping experiments and studying the effect of partial coherence can be some novel applications.

Two-photon polymerization-based fabrication of millimeter-sized precision Fresnel optics

Two-photon polymerization (2PP) is known to be the most precise and highest resolution additive manufacturing process for printing optics, but its applicability is restricted to a few applications due to the limited size of printable objects and low throughput. The presented work is intended to demonstrate the performance of printing millimeter-scale optics by implementing appropriate stitching methods into a setup that combines a Galvo scanner and translational axes. In this work, specifically, Fresnel axicons with a diameter of 3.5 mm are manufactured by 2PP to substantiate the applicability of the process. Manufacturing Fresnel optics instead of volumetric optics allows for attaining acceptable process times with durations of tens of hours highlighting the appeal of 2PP for rapid prototyping in optics. The suitability of the Fresnel axicons for beam shaping is confirmed through illumination with a laser beam. The resulting ring-shaped intensity distribution in the far field behind the Fresnel axicon is captured using a beam profiler. Furthermore, the influence of different stitching parameters on the resulting intensity distribution is investigated. The experimental results are validated by simulations, where the intensity distribution in the far field behind an axicon was calculated by Fourier transformation. Simulations were carried out to discuss the effect of manufacturing errors on the far field intensity distribution.

Propagation properties of the partially coherent radially polarized twisted beam through optical system in turbulence atmosphere

In this paper, the transmission properties of the partially coherent radially polarized twisted (PCRPT) beam propagating in the turbulence atmosphere are investigated. The analytical formulas for the components of the cross-spectral density matrix for the PCRPT beam passing through the optical system in the turbulent atmosphere are developed using the Collins integral and aperture function. Research results indicate that modifying the variable parameters and dimensions of the optical system can control the near-field and far-field distributions of the beam, while providing a more flexible choice of receivers for the PCRPT beam in the receiving plane. By utilizing the Cassegrain reflector system and adjusting the optical system parameters, it is possible to achieve collimated transmission of the PCRPT beam and significantly enhance the beam transmission efficiency in turbulent atmospheric conditions. The derivation process and the research results presented in this paper can be expanded to analyze the application of optical systems to control high-dimensional beam field variations. The envisioned utilization of the results obtained from this research investigation pertains to the fields of beam shaping and optical communications.

Wide cone beam CT dosimetry: Analysis of results and uncertainty evaluation

ObjectiveTo investigate the consistency of the recommended methodology, we analyzed the dosimetric results obtained for each setup beam shaping filter and tube voltage and evaluated the uncertainty associated with the full dose measurement process. MethodsA 300-mm PTIC was used to validate the results obtained with the 100-mm PTIC. Considering the other dosimetric parts, a cylindrical 160-mm diameter PMMA phantom and a cylindrical 320-mm diameter PMMA phantom were also used in the experimental protocol. ResultsFor the lowest and highest tube voltages available, the CTDIair,160 values obtained by 1-step dosimetry with the 300-mm PTIC were greater than the respective values obtained by 2- and 3-step dosimetry with the 100-mm PTIC. ConclusionThis study established that careful positioning of the 100-mm PTIC in 2 or 3 steps, as well as proper execution of the other dosimetric parts recommended by the IAEA, represents a validated approach within up to 20% uncertainty for wide cone beam CT dosimetry.

Workpiece surface laser regulation in laser-induced plasma electrolyte jet machining: Position, shape, and energy

Laser-induced plasma electrolyte jet machining is a new hybrid machining method. This machining method utilizes the plasma impact effect and plasma to increase the inter-pole conductivity for material removal, with low laser energy loss, high material removal rate (MRR), and good surface quality. The advantages of laser machining and electrolyte jet machining are fully utilized. However, it is difficult to avoid refraction of the laser in the electrolyte column, which affects the efficient coupling of the laser to the electrolyte jet machining. In this paper, to improve the localization and MRR, a study is carried out to address the problem of energy utilization efficiency in laser-induced plasma electrolyte jet machining. A compensation model for the refraction of the laser beam in the electrolyte is proposed. Compensation improves the shape and positional accuracy of the laser beam coupled to the electrolyte beam during aluminum alloy machining. Compared before and after compensation, the MRR was improved by 30.2 %, and the groove localization was improved by 37.4 %. Realization of high energy coupling for laser electrolyte jet hybrid machining.

Realization of equivalent multi-level DOEs by the stack of two few-level DOEs using phase dividing

Multi-level DOEs are always desired for the wider application. However, they require more accurate fabrication and expensive production costs. This paper presents an improved stacked DOEs method. Based on the phase distribution of t-level DOEs, a phase dividing method is used to divide the phase of t-level DOEs into m and n-level DOEs (t = n × m), the equivalent t-level DOEs can be realized by the stack of m and n-level DOEs. A 16-level beam shaping DOE is taken as an example to explore the method. The 4 and 4-level (or 2 and 8-level) stacked DOEs show the CV of 3.69% and diffractive efficiency of 98.11%, which is similar to 3.58% and 98.11% of a 16-level DOE. In addition, the tolerance of the stack method is also analyzed in horizontal displacement, vertical displacement, and angular deviation of stacked DOEs. The proposed stacked DOEs method can eliminate the need to directly manufacture multi-level DOEs with more steps, thus providing the possibility of reducing cost and fabricating difficulty.

Generation of extremely strong accelerating electric field by focusing radially polarized THz pulses with a paraboloid ring

Efficient acceleration of electrically charged particles by focused electromagnetic pulses requires a strong accelerating electric field as well as a sufficiently large interaction region. Single-cycle terahertz pulses are promising candidates for such applications due to their advantageous wavelength and available peak electric field at the MV/cm level. We propose and characterize a focusing optics for a possible electron vacuum accelerator. This consists of a reflaxicon for beam shaping and a ring-like segment of an on-axis parabolic mirror for tight focusing of the radially polarized THz pulse in order to reach a strong accelerating electric field. The electric field distribution in the focal region was determined by the Stratton–Chu vector diffraction method. Semi-analytical and purely analytical formulae were also derived, simplifying the computation procedure. By focusing radially polarized single-cycle terahertz pulses with 3mJ pulse energy, calculations predict accelerating electric field component in the order of 100MV/cm, well suitable for particle acceleration.

Rapid surface patterning to strengthen adhesive bonding of carbon fiber reinforced polymer by spatial shaping femtosecond laser

Carbon fiber reinforced polymer (CFRP) have become gradually important in the aerospace industry due to their outstanding strength-to-weight ratio. However, traditional mechanical surface treatment methods are challenging to apply to CFRP because of their anisotropic and nonhomogeneous properties. Femtosecond laser offers unique advantages for surface treatment, as it allows processing with very low thermal load due to the extremely short interaction time. This study investigates the effect of different surface structures resulting from surface treatment using a femtosecond laser on adhesive properties of CFRP. The experimental results show that: 1) beam shaping can be realized by using the plano-convex cylindrical mirror, which improves the quality of laser processing and greatly improves processing efficiency. It only takes 20 s to complete the laser processing of a 1 cm*1 cm area, which increases the work efficiency by 49 times. 2) pre-bonding surface treatment significantly enhances the tensile shear strength of the single lap joint, and the shear strength of samples with low spatial frequency LIPSS (LSFL) (14.57 ± 1.58 MPa) is 2.96 times higher than that of the untreated sample (US) (4.92 ± 1.34 MPa). 3) LSFL structure exhibits the best results, because the surface of CFRP with LSFL exhibits a relatively higher surface polarity and surface energy. This study provides an efficient, high-precision, and low-damage surface treatment method for preparing CFRP for adhesive bonding, which may promote the application of femtosecond laser technology in difficult-to-process composite materials and provide new methods and technical support for its application in aerospace, vehicle manufacturing, and other fields.

Technical note: Phantom-based evaluation of CBCT dose calculation accuracy for use in adaptive radiotherapy.

High-quality 3D-anatomy of the day is needed for treatment plan adaptation in radiotherapy. For online x-ray-based CBCT workflows, one approach is to create a synthetic CT or to utilize a fan-beam CT with corresponding registrations. The former potentially introduces uncertainties in the dose calculation if deformable image registration is used. The latter can introduce burden and complexity to the process, the facility, and thepatient. Using the CBCT of the day, acquired on the treatment device, for direct dose calculation and plan adaptation can overcome these limitations. This study aims to assess the accuracy of the calculated dose on the CBCT scans acquired on a Halcyon linear accelerator equipped withHyperSight. HyperSight's new CBCT reconstruction algorithm includes improvements in scatter correction, HU calibration of the imager, and beam shape adaptation. Furthermore, HyperSight introduced a new x-ray detector. To show the effect of the implemented improvements, gamma comparisons of 2%/2mm, 2%/1mm, and 1%/1mm were made between the dose distribution in phantoms calculated on the CBCT reconstructions and the simulation CT scans, considering this the standard of care. The resulting gamma passing rates were compared to those obtained with the Halcyon 3.0 reconstruction and hardware without HyperSight's technologies. Various anatomical phantoms for dosimetric evaluations on brain, head and neck, lung, breast, and prostate cases have been used in thisstudy. The overall results demonstrated that HyperSight outperformed the Halcyon 3.0 version. Based on the gamma analysis, the calculated dose using HyperSight was closer to the CT scan-based doses than the calculated dose using iCBCT Halcyon 3.0 for most cases. Over all plans and gamma criteria, Halcyon 3.0 achieved an average passing rate of 92.9%, whereas HyperSight achieved 98.1%. Using HyperSight CBCT images for direct dose calculation, for example, in (online) plan adaptation, seems feasible for the investigatedcases.

PyMOE: Mask design and modeling for micro optical elements and flat optics

We introduce a new open-source software package written in Python to design and model micro optical elements, such as diffractive lenses, holograms, as well as other components within the broad area of flat optics, and generate their corresponding (production-ready) lithography mask files. To this aim, the package provides functions to design a multitude of kinoform lenses, phase masks and holograms, but is versatile and the user can implement any arbitrary numerical or analytical optical component designs. For validating the designs, this package provides scalar diffraction propagation to simulate optical field propagation in different regimes covering near- and far-field regions (Fresnel, Fraunhofer and Rayleigh-Sommerfeld). Particularly, by implementing Rayleigh-Sommerfeld propagation, we demonstrate accurate field propagation within near- and far-field ranges, providing versatility and accuracy. Importantly, the package allows to directly export production-ready multilevel/binary lithography mask files of the designed optical components. Additionally, metasurface masks can conveniently be generated for any user-defined meta-element library given as input. Finally, the software package capabilities are illustrated with examples of mask design and modeling of diffractive lenses, holograms, and metasurfaces susceptible of being fabricated via lithography techniques. Beyond lithography, the package can also straightforwardly be used in other applications requiring mask generation, such as beam shaping, optical trapping and digital holography.

Beam-shaping in laser-based powder bed fusion of metals: A computational analysis of point-ring intensity profiles

Laser-based powder bed fusion has the potential to enable manufacturing of local material properties and locally varying alloys. One promising approach under research is shaping of the applied laser beam. The current commercial standard using a Gaussian laser beam profile is associated with a high energy intensity peak and strong gradients. A potential alternative beam shape is the point-ring profile where laser energy is distributed between a central spot and an outer ring. We conduct a numerical study of beam shape impact on melt-pool shapes, surface temperatures, evaporation and flow dynamics of single-melt-tracks. To that end, we apply a Weakly Compressible Smoothed Particle Hydrodynamics solver coupled with a ray-tracing laser scheme. A point-ring profile is compared to a standard Gaussian profile. The profiles are studied at two different beam sizes to find the influence of beam size on the shape effect. We find good agreement between numerical results and experimental data for both narrow and deep, as well as wide and shallow melt-tracks. The simulation results show that beam size and thus energy density influence the effect beam-shaping has on the melt-pool dynamics. Both the distribution of evaporation and surface tension forces over the melt-pool surface are found to depend on the used beam shape. The study highlights the importance of distinguishing between size and shape effects when investigating laser beam shapes. Both energy density and its distribution need to be tuned for the desired melt-pool behavior.

Conceptional design of photoneutralization test system for negative ion-based neutral beam injection

Neutral beam injection is one of the effective heating methods in the field of magnetic confinement fusion, and neutralization is the most crucial link in the case of negative ions. To further increase the neutral beam injection power, improve the long pulse operation capability, and optimize the efficiency of the NNBI system, further research and verification about the neutralization mode are needed. Theoretically, photoneutralization can achieve more than 90% neutralization efficiency. However, maintaining stable operation of the megawatt laser cavity over extended periods poses corresponding challenges. Additionally, the cost associated with laser target surpasses the benefit gained from increased neutralization efficiency, leading to its lack of practical application thus far. This paper proposes a solution to these issues by designing a single-channel, multi-fold photoneutralization verification system utilizing the CRAFT NNBI one-quarter and one-half size negative source test equipment. An outline of the system's test and diagnostics approach is provided. Key parameters such as laser target thickness, negative ion energy, beam shape and efficiency of the photoneutralization system are numerically calculated. Combined with the experimental data of the negative source test platform, theoretical calculations show that the neutralization efficiency can achieve 63% with the system efficiency exceeding 40%. Even by increasing the incident laser power or the number of reflections, neutralization efficiency can be increased to 95%, with a simultaneous increase in system efficiency to 60%. Maintaining efficiency while increasing incident laser power could reduce the number of reflections to approximately ten, reaching an acceptable threshold. However, this adjustment will increase the irradiation density of a single mirror from 660W/mm2 increases to 3000W/mm2. This paper methodically designs a practical laser neutralization verification platform, which is expected to substantially improve the neutralization efficiency, and facilitate practical application and validation.