Spot Intensity Distribution Research Articles

Generation of 351 nm UV Q-switched laser beam with controllable spatial coherence

Ultraviolet laser with controllable spatial coherence is essential for a wide range of applications. In this study, we developed a pulsed, partially spatial coherence laser source operating in the ultraviolet (UV) range through extra-cavity frequency tripling of a solid-state Nd:glass degenerate laser. The oscillating modes of the laser can be flexibly controlled by size of the spatial filter inside the laser cavity, thus modulating the spatial coherence of the UV laser. The beam pattern and focusing, speckle suppression and imaging, coherence property of the partially coherent UV laser is investigated in detail. The third-harmonic generation (THG) supports many more transverse modes than the fundamental generation and the second-harmonic generation (SHG). Compared with high coherent UV laser, low spatial coherent UV laser has better speckle suppression effect and smoother focused spot intensity distribution. The high brightness, good monochromaticity and low spatial coherence make the UV degenerate laser an attractive illumination source for inertial confinement fusion, lithography, laser processing and others.

Experimental validation and mathematical simulation for laser protection performance of light field imaging

Photoelectric imaging systems typically employ a focal plane detector structure, rendering them vulnerable to laser damage. Laser damage can severely impair or even completely deprive the information acquisition capability of photoelectric imaging systems. A laser damage protection method based on a microlens array light field imaging system is proposed to prevent photoelectric imaging systems from laser damage. The technique utilizes the light field modulation effect of the microlens array to homogenize the spot energy, thereby reducing the maximum single-pixel receiving power at the image sensor. The method’s effectiveness has been verified through numerical simulations and experimental validation. First, the laser transmission theoretical model of light field imaging is proposed. Then an experimental setup is established, and measurements are conducted to capture the spot profiles and intensity distributions on the imaging plane across various defocus distances. Finally, the impact of the propagation distance on the maximum single-pixel receiving power and suppression ratio of the light field imaging system is experimentally measured. The simulation and experimental results indicate that, with the proposed method, the energy suppression ratio can easily reach two orders of magnitude, significantly reducing the probability of laser damage in photoelectric imaging systems.

Femtosecond laser writing of infrared microlens arrays on chalcogenide glass

Sulfur-based glasses have gradually developed into a new generation of infrared optical materials with high transmittance in the mid-and long-infrared wavelengths. As2Se3, as a representative chalcogenide glass in the mid-infrared band, is commonly prepared for some infrared optical devices. In this paper, we prepare infrared microlens arrays with an area of 0.64 mm2 by femtosecond laser technology on As2Se3. In terms of morphology, the surface of the prepared microlens array has excellent morphology and smoothness. The uniform IR microlens array achieves high transparency of up to 60 % in the mid-infrared region. And there is no significant change in transmittance before and after laser processing, which has outstanding application prospects. In terms of the size of the lens, we can control the diameter and depth of microlens by adjusting the laser power and the number of pulses. In infrared imaging, the lateral magnification of the lens can be adjusted by controlling the focal length of the lens so that the size and clarity of the letter “F” can be controlled. The cross-sectional intensity distribution of the focusing spots was calculated, which demonstrates the superior uniformity of our microlenses.

Design concepts of a new imaging system for a high-intensity XUV source beam by colour centres excitation in lithium fluoride crystals

Stable colour centre production in lithium fluoride (LiF) crystals can employ as a high-spatial-resolution imaging tool for extreme ultraviolet (XUV) irradiation, as well as the possibility for images of the unfocused beam and the beam focused by a multi-layer mirror.The LiF crystal sensitivity has sufficient to impress high-contrast photo-luminescent patterns with XUV single-pulse irradiation on an area up to 40mm2. The suggested imaging technique, using LiF as a detector, can contribute to reducing the lack of sufficient knowledge for XUV beam characterization and profile featurization which can open a very wide range of XUV metrology and tomography applications.The experimental results explain the concepts of detection of high-intensity source at13.5nm using a YAG:Ce scintillator crystal embedded with a CMOS camera, additionally using LiF as a 2D high-resolution detector, and the work shows investigations outcomes and improvement procedure and analysis.The results demonstrate the potential of LiF crystals as a sub-micrometre resolution two-dimensional imaging tool for XUV irradiation applications. Moreover, the research study explains the optimization sequences of the new imaging technique that will play an important role to predict the achievable spot size, geometry, beam profile and intensity distribution, as well as the characterization complexity of XUV source features.

Study of diffractive fringes caused by tool marks for fast axis collimators fabricated by precision glass molding.

Aspheric cylindrical lenses, including fast axis collimators (FACs), are commonly used to collimate laser beams in the fast axis direction. Precision glass molding (PGM) is applied in the production of these optical lenses due to its high accuracy and efficiency. However, the profile errors and surface topography transferred from the mold reduce the optical performance of aspheric cylindrical lenses. In this paper, the surface errors of a FAC fabricated by combining ultraprecision diamond cutting and precision glass molding are analyzed. An optical simulation model is then established to qualitatively analyze the effects of tool marks on the optical defects, and the numerical calculations are carried out to determine the relative intensity distribution of light spots. Experiments are conducted to verify the theoretical results, which prove that the tool marks cause diffractive fringes and that the geometric parameters of the tool marks that are caused by cutting conditions affect the distribution of the fringe line defects. Finally, the critical conditions to eliminate diffractive fringes and improve the optical performance of the FAC are determined based on the experimental results.

Traceable x-ray focal spot reconstruction by circular edge analysis: from sub-microfocus to mesofocus

The size and shape of the focal spot, i.e. the area where x-rays are generated in an x-ray tube, is a key figure of merit of a radiographic imaging system, because it governs the achievable resolution and, if the shape is irregular, can introduce anisotropic blur in the radiographs. Size and shape are mainly determined by the diameter of the accelerated electron-beam and electron interactions with the target material. Since focal spot parameters change over time, due to target wear and drift in the electron optics, traceable methods to monitor the focal spot size and shape are required. Here, we present a method to map the 2D intensity distributions of focal spots, ranging from sub-microfocus (0.1 µm) to mesofocus (>50 μm). The method evaluates radial profiles of a circular aperture and applies filtered back projection to reconstruct the focal spot intensity distribution. Special emphasis is placed on traceability and influence factors, such as image noise and aperture geometry. The method was validated over a focal spot range from 0.3 µm to 200 µm. Experimental results were compared to complementary methods, using line pair gauges (JIMA) for microfocus (1 µm–10 µm), e-beam characterisation for sub-microfocus (0.3 µm–1.0 µm), and pinhole camera imaging for mesofocus spots (50 µm–200 µm). Good agreement between the focal spot reconstruction and the complementary methods was achieved. Limitations due to edge penetration, caused by the finite thickness of the circular aperture gauge, were experimentally verified. From these findings, we derived recommendations for an improved gauge geometry, regarding its thickness and the aperture diameters. The method perfectly complements the well-established pinhole imaging, which is limited to focal spots larger than ∼50 µm.

Understanding of adopting flat-top laser in laser powder bed fusion processed Inconel 718 alloy: simulation of single-track scanning and experiment

Laser beam intensity profile used in laser powder bed fusion (LPBF) process is generally Gaussian, which may not be the best to deliver site-specific microstructures. In this study, a flat-top laser with large-size laser spot and uniform intensity distribution is adopted in the LPBF process with Inconel 718 powder. Comparison investigations are conducted to the standard Gaussian laser. Firstly, the parameters for the flat-top laser are optimized by multi-physics simulation of single tracks. Then, the thermodynamic behaviors and morphologies of the molten pool are observed combined with experiments. A cubic sample is also fabricated utilizing double-laser system to reveal the different characteristics of the resultant microstructures. The results show that the flat-top laser can work at a relatively wider range of laser power without depression, which often occurs in the case of Gaussian laser. The molten pool induced by the flat-top laser has large width-depth ratio with a flat bottom boundary, and more moderate flow in the half bottom. These features combined with the temperature and flow fields of the flat-top laser provide advantageous conditions for directional epitaxial growth of columnar grains during multi-layer fabrication. The epitaxial growth is retained and can extends to 4 mm from the single-crystal baseplate. This demonstrates great potential of LPBF with the adopted flat-top laser to be applied in fabricating or repairing components with directional crystal structures or single-crystal structures.

Multispot optics for beam shaping of high-power single-mode and multimode lasers

The performance of various laser technologies, such as welding, laser powder bed fusion (LPBF), brazing, cladding, and sheet metal cutting, based on the use of high-power multimode fiber lasers, fiber-coupled solid-state, and diode lasers, can be improved using the patent pending beam-shaping optics providing optimal energy distributions by splitting the laser beam into several separate spots in the working plane and variable energy sharing between these spots. Various patterns, such as square, line, and rhombus, consisting of four or nine separate spots, are expected to eliminate or reduce spatter and to realize optimum temperature distributions in the melt pool and stabilizing the processes in the welding of tailored blanks, copper and aluminum parts in the production of batteries, zinc-coated steel, cladding, and LPBF. Because multimode lasers have a comparably low spatial coherence characterized by large beam parameter products or beam quality (M²) values, it is difficult to control the intensity distribution by methods other than imaging the fiber end with a collimator and a focusing objective. The proposed solution is a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis using special optical components and creating a working spot as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize common spot intensity distributions for particular applications. To ensure reliable operation with multi-kW lasers, the refractive optical components of the multispot devices are implemented from athermal optical materials characterized by insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The article presents descriptions of multispot optics and examples of intensity profile measurements and application results, while the reduction in spattering was observed using multispot laser welding. It is concluded that the melt pool flows homogenize when applying several laser spots compared to a single spot. The possibility of tailoring melt pool dimensions in LPBF was shown.

Characteristics of the beam smoothing using the combination of induced spatial incoherence and continuous phase plate

Beam smoothing effect of induced spatial incoherence combined with continuous phase plate is reported. General theoretical models that can be used to describe the spatiotemporal characteristics of the focal region in beam smoothing is proposed. The focal region evolution using induced spatial incoherence is studied and compared with that of using smoothing by spectral dispersion. The result shows that, when using induced spatial incoherence combined with continuous phase plate for beam smoothing, the focal region spatiotemporal evolution is chaotic and random, and the size of speckle on spacetime is determined by the coherence time of the laser. The comparison shows that the smoothing speed of induced spatial incoherence is higher. Focal spot time-resolved spectrum and focal spot intensity distribution using induced spatial incoherence are measured experimentally. The results show that the beam smoothing method has an excellent ability to obtain the desired low coherence in focal region, and can completely suppress the middle-high frequency nonuniformity of the focal spot.

Optimal Laguerre-Gaussian modes for high-intensity optical vortices.

With increasing interest in using orbital angular momentum (OAM) modes in high-power laser systems, accurate mathematical descriptions of the high-intensity modes at focus are required for realistic modeling. In this work, we derive various high-intensity orbital angular momentum focal spot intensity distributions generated by Gaussian, super-Gaussian, and ideal flat-top beams common to high-power laser systems. These intensity distributions are then approximated using fitted Laguerre-Gaussian basis functions as a practical way for describing high-power OAM beams in theoretical and numerical models.

Modelling of the focal spot intensity distribution and the off-focal spot radiation in kilovoltage x-ray tubes for imaging

X-ray tubes for medical applications typically generate x-rays by accelerating electrons, emitted from a cathode, with an interelectrode electric field, towards an anode target. X-rays are not emitted from one point, but from an irregularly shaped area on the anode, the focal spot. Focal spot intensity distributions and off-focal radiation negatively affect the imaging spatial resolution and broadens the beam penumbra. In this study, a Monte Carlo simulation model of an x-ray tube was developed to evaluate the spectral and spatial characteristics of off-focal radiation for multiple photon energies. Slit camera measurements were used to determine the horizontal and vertical intensity profiles of the small and the large focal spot of a diagnostic x-ray tube. First, electron beamlet weighting factors were obtained via an iterative optimization method to represent both focal spot sizes. These weighting factors were then used to extract off-focal spot radiation characteristics for the small and large focal spot sizes at 80, 100, and 120 kV. Finally, 120 kV simulations of a steel sphere (d = 4 mm) were performed to investigate image blurring with a point source, the small focal spot, and the large focal spot. The magnitude of off-focal radiation strongly depends on the anode size and the electric field coverage, and only minimally on the tube potential and the primary focal spot size. In conclusion, an x-ray tube Monte Carlo simulation model was developed to simulate focal spot intensity distributions and to evaluate off-focal radiation characteristics at several energies. This model can be further employed to investigate focal spot correction methods and to improve cone-beam CT image quality.

Anti-cat-eye effect imaging technique based on the light-field imaging technique

Based on previous anti-cat-eye effect imaging techniques sacrificing too much imaging quality to achieve substantial retroreflection reduction, an anti-cat-eye effect imaging technique based on light-field imaging is proposed. Relevant studies have been carried out regarding the mechanism and effectiveness of both antilaser reconnaissance and blinding for this technique. By applying the Fresnel–Kirchhoff diffraction theory and defining the microlens array as a superposition of a series of microlens units, the retroreflection formation of the light-field imaging system is theoretically modeled. Based on the physical model, the influences of defocusing on the intensity distributions of spots on both the light-field detector and observation planes are further studied. The results show that, compared with a conventional system with the defocus invariant and flexible reconstruction properties, the light-field imaging system not only increases the interference and blinding thresholds by nearly one order of magnitude but also reduces both the retroreflection maximum intensity and the echo-detector receiving power by more than one order of magnitude, which sufficiently manifests the superior antilaser reconnaissance and blinding performances of the light-field imaging system.

Enantioselective optical trapping of chiral nanoparticles by tightly focused vector beams

Two enantiomers (mirror images) can show drastically different behaviors, resulting in the enantiomers’ identification and separation being in high demand in biomedical research and industry. Here, we introduce an optical approach in which, by using a tightly focused vector beam with radially varied polarizations, we realize the selective trapping of both enantiomeric forms. Numerical results show that such a focused field exhibits bifocal spot intensity distribution and can simultaneously stably trap one enantiomer in one focal spot and the other enantiomer in the other spot in three dimensions, achieving an effective separation of the chiral entities. The trapping distance and position of the enantiomeric pairs can be changed by separately varying the magnitude and sign of the polarization topological charge of the vector beam. And the difference in trapping potentials of the particles with different chirality provides a further identification of the chirality. Our theory indicates that the enantiomers’ identification and separation can be mediated by the same incident beam, providing a possible route to detect, separate, and manipulate chiral objects at nanometer scales.

Wavefront coded light-field imaging system to achieve substantial retroreflection reduction and anti-laser blinding property

To achieve a substantial retroreflection reduction accompanied with a superior anti-laser blinding property without suffering significant degradations in imaging quality, we design a novel imaging system based on the wavefront coded light-field rendering. Relevant studies have been carried out around its mechanism in both anti-laser reconnaissance and blinding. According to the Fresnel-Kirchhoff diffraction theory, the laser propagation through the composite imaging system is theoretically modeled. By comparison among the conventional, the wavefront coding, the light-field, and the composite imaging systems with different defocus parameters and phase modulation coefficients, the spot profiles and intensity distributions on both the imaging and observation planes are further studied, according to which the optimization principle is proposed. The results show that compared with the conventional one, the composite imaging system could not only increase the interference and blinding thresholds by nearly one order of magnitude but also attenuate the retroreflection peak intensity and the echo-detector receiving power by even more than one order of magnitude, achieving the superior properties of anti-laser reconnaissance and blinding.

Intermittent plasticity in individual grains: A study using high energy x-ray diffraction.

Long-standing evidence suggests that plasticity in metals may proceed in an intermittent fashion. While the documentation of intermittency in plastically deforming materials has been achieved in several experimental settings, efforts to draw connections from dislocation motion and structure development to stress relaxation have been limited, especially in the bulk of deforming polycrystals. This work uses high energy x-ray diffraction measurements to build these links by characterizing plastic deformation events inside individual deforming grains in both the titanium alloy, Ti-7Al, and the magnesium alloy, AZ31. This analysis is performed by combining macroscopic stress relaxation data, complete grain stress states found using far-field high energy diffraction microscopy, and rapid x-ray diffraction spot measurements made using a Mixed-Mode Pixel Array Detector. Changes in the dislocation content within the deforming grains are monitored using the evolution of the full 3-D shapes of the diffraction spot intensity distributions in reciprocal space. The results for the Ti-7Al alloy show the presence of large stress fluctuations in contrast to AZ31, which shows a lesser degree of intermittent plastic flow.

Low-coherece laser based lens array beam smoothing techique

The experimental study of laser-driven material state equation puts forward extremely high requirements for the uniformity and stability of the target spot intensity distribution, and these two characteristics greatly determine the accuracy and repeatability of the experimental results. In this paper, a beam smoothing scheme combining diffraction-weakened lens array (LA) with induced spatial incoherent (ISI) technique based on low-coherence laser is proposed to solve the problems, that is, the uniformity and stability of the target spot intensity distribution in the material state equation experiments driven with narrow-band coherent laser. The super-Gaussian soft aperture used in our scheme can improve the intensity fluctuation caused by the hard-edge diffraction of the lens elements, and the temporal smoothing technique, ISI, can reduce the interference effect between the lens array elements. The speckle patterns of target spot, which are caused by interference between beamlets and determine the high nonuniformity, will randomly reconstruct after each coherent time. The high-frequency components are further smoothed by the time-average effect. In broadband high-power laser devices, ISI can be combined with LA by making the lens elements with different thickness values. This scheme can enhance the focal spot uniformity and improve the tolerance of the system to the wavefront phase distortion. The influence of wavefront phase distortion on target surface uniformity and stability are analyzed. The simulation results show that this smoothing scheme significantly reduces the target spot nonuniformity, improves the tolerance of random wavefront phase distortion, and presents a uniform and stable target spot intensity distribution. The nonuniformity of target spot will be reduced to about 10% after 10 ps, and about 3% after 100 ps. In addition, statistical analysis shows that the peak-to-valley value and the nonuniformity of the target spot intensity distribution are strongly correlated with the gradient of root-mean-square of the wavefront phase distortion. Using this method, the tolerance range of the wavefront phase distortion can be given according to the requirements of the experiments, which has reference value for designing and optimizing the laser driver parameters in the state equation experiment.

Eggshell spotting pattern is related to hatching asynchrony, hematocrit value and growth of nestling great titsParus major

Eggshell pigmentation may signal the quality of the egg, that of the female and the environment, and thus nestling development may be related to this egg trait. However, so far, few studies have investigated the relationship between eggshell pigmentation and nestling development. Our aim was to study in a partial cross‐fostering experiment whether the protoporphyrin‐based eggshell pigmentation (spot intensity and spot distribution) showed any significant associations with several traits related to the development of nestling great titsParus major. We found that nestlings from clutches with darker spotted eggshells hatched more synchronously, had higher hematocrit value and shorter wings. Females under adverse environmental circumstances were previously shown to lay darker‐spotted eggs, and these females probably delayed the start of incubation due to energetic constraints, resulting in a more synchronous hatching. The shorter wings of fledglings originating from clutches with darker‐spotted eggs may be caused by the lower general quality of their mothers, mediated through either genetic factors or early maternal effects. Darker spotted eggs may contain different levels of some biomolecules than paler spotted eggs, which could have an effect on nestling metabolism and activity, resulting in an elevated hematocrit value. More aggregated eggshell spotting was related to faster bone growth and slightly longer fledging tarsus length. Eggs with aggregated spotting may have thinner eggshells that could lead to hatchlings with smaller bones due to the lower availability of calcium during development. Nestlings may increase bone growth rate to compensate for their smaller initial size. Overall, our results indirectly suggest lower fitness for nestlings hatched from darker spotted eggs and for nestlings hatched from eggs with more aggregated spotting distribution due to the possible long‐term costs of synchronous hatching, shorter wings, and accelerated tarsus growth rate.

Propagation characters of multi-Gaussian beam with large eccentric displacement in collisionless plasma: Higher order paraxial theory

The multiple coherent identical Gaussian beams with symmetry distribution are abbreviated as multi-Gaussian beam, of which the propagation characters in collisionless plasma are studied with the WKB method and higher order paraxial theory. The initial beam profile presents the flat top like or hollow like distribution when the eccentric displacement is large enough. Based on the derived nonlinear propagation equations, the initial condition analyses are performed, and the impact of eccentric displacement on the free propagation effect and ponderomotive nonlinearity is thoroughly discussed. The propagation characters of dimensionless beam width parameter, spot intensity, and spatial distribution of plasma electron density are presented. Results show that the initial spot intensity is expanding outwards along the propagation and the ring structure of electron evacuation in plasmas is generated.

Axial multifocal binary-phase zone plate for high numerical aperture focusing

Binary-phase zone plates (BPZPs) are proposed for generating two foci along the optical axis of a high numerical aperture objective. The radii of BPZPs are given by an analytical expression, in which the axial shifting distance of two foci is included. By changing the axial shifting distance, a set of BPZPs can be designed for achieving tunable two foci in the optical axial direction of the objective. Theoretical analysis and simulation results prove that the tunable axial two foci can be realized successfully, and furthermore, the intensity distribution of focal spots with special shapes (optical needle and optical bubble) can also be created by using BPZPs with suitable axial shifting distance. In addition, axial multifocal spots array can also be realized by superposition of multiple BPZPs with different axial shifting distances.

Centroid shift analysis of microlens array detector in interference imaging system

Most CCD imaging detectors integrated microlens arrays (MLAs) to increase fill factor and sensitivity. However, they also introduce spot calibration issues with the inconsistency of spot geometry center and intensity distribution center. We setup theoretical and experimental models to research the problem of centroid shifting. According to the Seidel and Zernike coefficients of the optical model, we analyze main aberrations of microlens. In “Chief Ray” and “Centroid” reference frames, centroid shift numerical value is calculated with Geometric Ensquared Energy (GEE). Based on pentaprism test for 8.4m mirror segment, we conduct spot imaging experiment in interference system. Spots images are obtained, and two-dimensional centroid algorithm processing is performed on them to get the analog experiment values of centroid movements. The results show that the MLA placed in KAI‐16000 imaging detector causes the spot centroid to move. When there is a 14° (or −14°) angle of incident ray, the shifting values are about 1.46μm in simulation and 2.18μm in experiment. Our research makes a contribution to the compensation of calibrated error in metrology technology. We also prove that a significant portion of the shift comes from the low order aberration of microlens.