Target Plane Research Articles

Statistical spatial properties of a light field on a target plane

Studying the light field characteristics of a target plane is critical in controlling the laser-plasma instability (LPI), which is necessary to increase laser energy utilization and compression symmetry in high-power laser facilities. In this study, a statistical method is used to analyze the transmission light characteristics of a target plane for single and multiple beams. We reconstructed the light transmission model on the target plane and analyzed the relationships between the focal spot width and incidence angle, and speckle width and incidence angle using the autocorrelation function. In addition, the relationship between the interference pattern and incident beams is obtained by deriving the beam superposition theory. The results show that the speckle and focal spot widths are stretched in the direction in which the incident plane projects. The direction of the interference structure generated by multiple-laser beams is perpendicular to the line joining of the sub-beams, and the period is related to both wavelength and incidence angle. Experimental results are consistent with the theoretical analysis. The influence of incident beam number on the focal spot uniformity is also studied. The results are of great significance for regulating the sub-beam incidence direction, understanding light properties to further improve focal spot uniformity and suppress the LPI.

Neutron generation enhanced by a femtosecond laser irradiating on multi-channel target

A novel scheme has been proposed to enhance neutron yields, in which a multi-channel target consisting of a row of parallel micro-wires and a plane substrate is irradiated by a relativistic femtosecond laser. Two-dimensional particle-in-cell simulations show that the multi-channel target can significantly enhance the neutron yield, which is about 4 orders of magnitude greater than the plane target. Different from the case of nanowire target, we find that when the laser penetrates into the channel, the excited transverse sheath electric field can effectively accelerate the D+ ions in the transverse direction. When these energetic D+ ions move towards the nearby wire, they will collide with the bulk D+ ions to trigger D-D fusion reaction and produce neutrons, which is much more effective than the plane target case. Due to the unique trajectory of the incident D+ ions, the angular distribution of the produced neutrons is modulated from isotropic to two peaks around ±90°. Meanwhile, this enhancement and modulation is further verified in a wide range of target parameters.

Development and evaluation of a space robot prototype equipped with a cable-driven manipulator

This research focuses on developing and evaluating a free-flying space robot equipped with a cable-driven manipulator and an adaptive gripper. First, a transmission joint comprising a pair of gears and pulleys is designed and actuated by motors located in the first link of the manipulator. Afterward, based on the kinematics and dynamics models of the robot, a mission distribution control scheme is proposed for the robot to drive its end-effector to achieve the desired position and attitude for capturing space targets. Furthermore, exploiting this strategy, considerable comparison simulations between two space robots equipped with a traditional manipulator and a cable-driven arm are conducted. Wherein, the goal position and attitude are selected through a uniform sampling method. For the same capture mission, energy consumptions of the base and manipulator as well as state errors of the base are taken as cost functions for performance evaluation. Finally, a planar target capture case study using the adaptive gripper and a spatial motion test with the help of an air-bearing simulator are carried out to demonstrate the efficiency of the presented cable-driven space robot.

Image-Based Visual Servoing of Quadrotors to Arbitrary Flight Targets

Visual servoing of Unmanned Aerial Vehicles (UAVs) has achieved satisfactory performance in fixed and planar motion targets. Due to highly coupled system dynamics and the sensitivity of the target image to aircraft attitude, the problem for chasing free-flying targets remains challenging. In this paper, a vision-based algorithm is designed for controlling an UAV while tracking an intruder flying arbitrarily in 3D space. Image-based visual servoing is used to design controllers that depend directly on errors in image plane. Specifically, a virtual camera approach is adopted to decouple the UAV dynamics by compensating the pitch and yaw, and an improved image error term is proposed to reduce the impact of the UAV rotation on error signals in the process of tracking, thus a simplified control design is achieved, and the stability of the visual servo system is guaranteed. Comparison and ablation experiments in both simulated and real environments are provided to verify the effectiveness of the proposed method.

Infrared Camera Geometric Calibration: A Review and a Precise Thermal Radiation Checkerboard Target

Different infrared (IR) planar geometric calibration targets have been developed over the years that exploit a well-established and flexible optical camera geometric calibration procedure following the pinhole approximation. This geometric calibration is typically neglected in IR cameras, due to the relatively low resolution of thermal images and the complex IR targets needed for the geometric calibration in comparison to the optical targets. In this study, a thorough literature review of numerous IR camera geometric calibration targets, along with their respective outcomes, were summarized and leveraged to deliver a practical checkerboard target for less experienced end users, while offering the lowest reprojection errors. It was concluded that the fabrication of high emissivity contrast and precise square points of intersection within a checkerboard pattern extends the accuracy of capturing these control points in a thermal image for an optimized IR camera geometric calibration. Accordingly, two simple planar checkerboard targets were fabricated using laser engraving and ultraviolet (UV) printing technologies on a polished stainless steel (SS304) plate. The UV-printed checkerboard target on a polished metallic alloy delivered the lowest mean reprojection error (MRE) of 0.057 pixels and the lowest root mean square error (RMSE) of reprojection of 0.063 pixels, with a standard deviation lower than 0.003 pixels. The UV-printed design offers better accuracy than any other checkerboard calibration target, and comparable results to the best prominent circular pattern results reported in the literature.

Self-supervised learning for single-pixel imaging via dual-domain constraints.

Deep-learning-augmented single-pixel imaging (SPI) provides an efficient solution for target compressive sensing. However, the conventional supervised strategy suffers from laborious training and poor generalization. In this Letter, we report a self-supervised learning method for SPI reconstruction. It introduces dual-domain constraints to integrate the SPI physics model into a neural network. Specifically, in addition to the traditional measurement constraint, an extra transformation constraint is employed to ensure target plane consistency. The transformation constraint uses the invariance of reversible transformation to impose an implicit prior, which avoids the non-uniqueness of measurement constraint. A series of experiments validate that the reported technique realizes self-supervised reconstruction in various complex scenes without any paired data, ground truth, or pre-trained prior. It can tackle the underdetermined degradation and noise, with ∼3.7-dB improvement on the PSNR index compared with the existing method.

Numerical simulation of laser-produced plasma expansion on a droplet surface

In this study, a numerical model of the plasma expansion on a droplet surface based on the initial plasma method was proposed. The initial plasma was obtained through the pressure inlet boundary condition, and the effect of ambient pressure on the initial plasma and adiabatic expansion of the plasma on the droplet surface, including the effect on the velocity and temperature distribution, were investigated. The simulation results showed that the ambient pressure decreased, leading to an increase in the expansion rate and temperature, and therefore a larger plasma size was formed. Plasma expansion creates a backward driving force and eventually envelops the entire droplet, indicating a significant difference compared to planar targets.

Parametric scatterer extraction method for space‐target inverse synthetic aperture radar image CLEAN

AbstractOwing to the time variance of the inverse synthetic aperture radar (ISAR) imaging plane of space targets, scatterers with large heights relative to the imaging plane migrate through several Doppler cells. This defocussing in the azimuth direction of each scatterer cannot be compensated via the traditional migration through the resolution cell (MTRC) compensation method because the height of each scatterer is quite different and independently distributed in the ISAR image. In this study, a parametric azimuth‐defocussing model‐based CLEAN method is proposed for the space‐target ISAR image. First, high‐order defocus parameters and their influences on the image of space targets were analysed at different arc segments. Subsequently, a mathematical azimuth‐defocussing expression of the ISAR image scatterers was derived after MTRC and azimuth high‐order phase compensation, and an approximate defocus expression based on the Fresnel integral was provided for computational efficiency. This model can estimate the complex amplitude, coordinates and defocus parameters of each scatterer in the image domain by applying a sequential quadratic programing algorithm. Finally, the validity and stability of the proposed algorithm for the isolated and coupled scatterer extraction were verified at different signal‐to‐noise ratios, and different defocus parameters were verified through simulations.

Multi-frequency acoustic hologram generation with a physics-enhanced deep neural network

Here, a physics-enhanced multi-frequency acoustic hologram deep neural network (PhysNet_MFAH) method is proposed for designing multi-frequency acoustic holograms, which is built by incorporating multiple physical models that represent the physical processes of acoustic waves propagation for a set of design frequencies into a deep neural network. It is demonstrated that one needs only to feed a set of frequency-specific target patterns into the network, the proposed PhysNet_MFAH method can automatically, accurately, and rapidly generate a high-quality multi-frequency acoustic hologram for holographic rendering of different target acoustic fields in the same or distinct regions of the target plane when driven at different frequencies. Remarkably, it is also demonstrated that the proposed PhysNet_MFAH method can achieve a higher quality of the reconstructed acoustic intensity fields than the existing optimization methods IASA and DS for designing multi-frequency acoustic holograms at a relatively fast-computational speed. Furthermore, the performance dependencies of the proposed PhysNet_MFAH method on different design parameters are established, which provide insight into the performance of the reconstructed acoustic intensity fields when subject to different design conditions of the proposed PhysNet_MFAH method. We believe that the proposed PhysNet_MFAH method can facilitate many potential applications of acoustic holograms, ranging from dynamic particle manipulation to volumetric display.

A unified calibration method of 3D laser profile measurement with different of laser-line lengths

PurposeExisting calibration methods mainly focus on the camera laser-plane calibration of a single laser-line length, which is not convenient and cannot guarantee the consistency of the results when several three-dimensional (3D) scanners are involved. Thus, this study aims to provide a unified step for different laser-line length calibration requirements for laser profile measurement (LPM) systems.Design/methodology/approach3D LPM is the process of converting physical objects into 3D digital models, wherein camera laser-plane calibration is critical for ensuring system precision. However, conventional calibration methods for 3D LPM typically use a calibration target to calibrate the system for a single laser-line length, which needs multiple calibration patterns and makes the procedure complicated. In this paper, a unified calibration method was proposed to automatically calibrate the camera laser-plane parameters for the LPM systems with different laser-line lengths. The authors designed an elaborate planar calibration target with different-sized rings that mounted on a motorized linear platform to calculate the laser-plane parameters of the LPM systems. Then, the camera coordinates of the control points are obtained using the intersection line between the laser line and the planar target. With a new proposed error correction model, the errors caused by hardware assembly can be corrected. To validate the proposed method, three LPM devices with different laser-line lengths are used to verify the proposed system. Experimental results show that the proposed method can calibrate the LPM systems with different laser-line lengths conveniently with standard steps.FindingsThe repeatability and accuracy of the proposed calibration prototypes were evaluated with high-precision workpieces. The experiments have shown that the proposed method is highly adaptive and can automatically calibrate the LPM system with different laser-line lengths with high accuracy.Research limitations/implicationsIn the repeatability experiments, there were errors in the measured heights of the test workpieces, and this is because the laser emitter had the best working distance and laser-line length.Practical implicationsBy using this proposed method and device, the calibration of the 3D scanning laser device can be done in an automatic way.Social implicationsThe calibration efficiency of a laser camera device is increased.Originality/valueThe authors proposed a unified calibration method for LPM systems with different laser-line lengths that consist of a motorized linear joint and a calibration target with elaborately designed ring patterns; the authors realized the automatic parameter calibration.

High-speed speckle averaging for phase-only beam shaping in laser materials processing

A phase-only spatial light modulator (SLM) is a powerful tool to shape arbitrary intensity distributions and finds application in various research fields and industry. However, since phase-only beam shaping cannot control the full complex light field, a speckle pattern overlays the shaped target intensity and strongly impairs the result. Several methods have been developed to diminish speckle noise, all of which have their advantages and disadvantages. Usually they offer a compromise between quality and efficiency. We present a method for phase-only beam shaping which achieves near-perfect and highly efficient speckle averaging with a liquid crystal on silicon SLM based on a single phase mask. A galvanometer scanner scans the laser beam over the active area of the SLM to achieve high-speed averaging. The applied angle resulting from the scanning movement is compensated by a proper optical setup to maintain a static target image with speckle averaging out. As an additional benefit of the scanning movement, artifacts from light not diffracted by the SLM disappear in the target plane. The setup is designed for high-energy laser beams as intermediate foci are avoided at all optical components. Besides optical averaging on camera images, we validate our method with ablation experiments where we make a profound analysis of the involved system parameters and the achieved quality. Thereby, it becomes apparent that the proposed method not only stands out by the high-quality results but also by its simplicity in implementation.

Heterogenous angular spectrum approach based holograms for trans-skull focused ultrasound therapy

Focused Ultrasound (FUS) therapy in the brain entails accounting for strong distortions in the transmitted wavefields introduced by the skull. While these aberrations are compensated by MR-guided FUS (MRgFUS) phased array-based systems by adjusting the phase and amplitude of individual transducer elements, their cost and reliance on MRI may limit their widespread adoption. This work presents phase-only acoustic holograms coupled to a single transducer element as a safe, effective, and affordable alternative. We used the Heterogenous Angular Spectrum Approach (HASA) to propagate between the transducer and target plane(s) and applied automated differentiation for gradient computation to iteratively optimize the phase of the holographic lens. To test our design approach, we employed experimentally validated k-wave simulation. Our results show a 9 dB increase in the peak signal-to-noise ratio (PSNR) of the target acoustic field at 1 MHz as compared to the case where aberrations are uncorrected. Furthermore, we addressed the effect of frequency and aperture on the quality of hologram construction. We showed the feasibility of implementing it in a clinical-scale FUS system targeting deep (>6 cm) brain structures. In summary, the combination of HASA with automated differentiation is an efficient and accurate tool for hologram design through heterogeneous media.

Quantitative biomechanics using reverberant optical coherence elastography

In this presentation, I will introduce and discuss recent developments in our effort for quantitative biomechanics of tissues and organs using a multifocal acoustic radiation force source (that combines an ultrasound transducer and a 3D-printed acoustic lens) in reverberant optical coherence elastography (Rev-OCE). An array of plano-concave acoustic lenses, each with an 11.8 mm aperture diameter, were used to spatially distribute the acoustic energy generated by a 1 MHz planar ultrasound transducer, producing multiple focal spots on a target plane. These focal spots generate reverberant shear wave fields detected by the Optical Coherence Tomography (OCT) system. The effectiveness of the multifocal Rev-OCE system in probing mechanical properties with high resolution is demonstrated in layered gelatin phantoms, tissues ex vivo, and whole mouse embryos in vivo.

Nonlinear evolution of the Weibel instability with relativistic laser pulses

The Weibel instability is investigated using relativistic intense short laser pulses. A relativistic short laser pulse can generate a sub-relativistic high-density collisionless plasma. By irradiating double parallel planar targets with two relativistic laser pulses, sub-relativistic collisionless counterstreaming plasmas are created. Since the growth rate of the Weibel instability is proportional to the plasma density and velocity, the spatial and temporal scales of the Weibel instability can be much smaller than that from nanosecond large laser facilities. Recent theoretical and numerical studies have revealed that astrophysical collisionless shocks in sub-relativistic regimes in the absence and presence of an ambient magnetic field play essential roles in cosmic ray acceleration. With experimental verification in mind, we discuss the possible experimental models on the Weibel instability with intense short laser pulses. In order to show the experimental feasibility, we perform 2D particle-in-cell simulations in the absence of an external magnetic field as the first step and discuss the optimum conditions to realize the nonlinear evolutions of the Weibel instability in laboratories.

Calibration method of line-structured light sensors based on a hinge-connected target with arbitrary pinch angles.

Line-structured light 3D measurement is often used for 3D contour reconstruction of objects in complex industrial environments, where light plane calibration is a key step. In this paper, we propose a calibration method for a line-structured optical system based on a hinge-connected double-checkerboards stereo target. First, the target is moved randomly in multiple positions at any angle within the camera measurement space. Then, by acquiring any one image of the target with line-structured light, the 3D coordinates of the light stripes feature points are solved with the help of the external parameter matrix of the target plane and the camera coordinate system. Finally, the coordinate point cloud is denoised and used to quadratically fit the light plane. Compared with the traditional line-structured measurement system, the proposed method can acquire two calibration images at once; thus, only one image of line-structured light is needed to complete the light plane calibration. There is no strict requirement for the target pinch angle and placement, which improve system calibration speed with high accuracy. The experimental results show that the maximum RMS error of this method is 0.075mm, and the operation is simpler and more effective to meet the technical requirements of industrial 3D measurement.

Freeform reflector light source used for space traceable spectral radiance calibration on the solar reflected band.

For purpose of improving the accuracy of in-orbit radiometric calibration of Chinese Space-based Radiometric Benchmark (CSRB) reference payload on the reflected solar band and reduce resource consumption, this paper proposed a freeform reflector radiometric calibration light source system based on the beam shaping characteristics of the freeform surface. The design method of initial structure discretization based on Chebyshev points was used to design and solve the freeform surface, and the feasibility of the design method was verified by optical simulation. The designed freeform surface is machined and tested, and the test results show that the surface roughness root mean square (RMS) of the freeform reflector is 0.61 μm, which indicates that the continuity of the machined surface is good. The optical characteristics of the calibration light source system are measured, the results show that the irradiance uniformity and radiance uniformity are better than 98% in the effective illumination area of 100 mm × 100 mm on the target plane. The constructed freeform reflector calibration light source system can meet the requirements of large area, high uniformity and light weight for onboard calibration of the payload of the radiometric benchmark, improving the measurement accuracy of spectral radiance on the reflected solar band.

Flexible and fast calibration method for uni-directional multi-line structured light system

Multi-line Shift Profilometry (MLSP) is widely used to measure the high-reflectivity object by projecting high-contrast binary stripe image. However, in the calibration of a MLSP system, the lens distortion of the projector makes the most existing calibration methods fail in obtaining high accurate result. Therefore, to avoid the influence of lens distortion, we propose a novel flexible and fast calibration method for MLSP, which requiring only uni-directional patterns. In this method, for each bending light plane, lens distortion is avoided by establishing a polynomial model between three-dimensional (3D) coordinates and camera sub-pixel coordinates. Because the position of calibration planar targets is placed freely without any constraint, this method is flexible and convenient to operate. Experimental results demonstrate that the proposed method achieves high-precision 3D reconstruction.

3-D analysis of electron beam shape at target plane through parametric variation of an indirectly heated cathode-based electron gun

In this paper, a detailed 3-D analysis of the electron beam shape at the target plane of high power (60 kW, 60 kV, 1 A) 2700-bent indirectly heated cathode-based electron gun has been successfully carried out. This work will help analyse individual electron beam trajectories in the actual 3-D plane to ensure optimum electron beam bending and focusing at the target plane. Moreover, due to the presence of an additional electrode in an indirectly heated cathode-based electron gun i.e., a filament which is resistively heated and an indirectly heated solid cathode (heated through electron bombardment), this analysis becomes even more imperative. The work carried out in this paper will ensure better performance, characterization, and optimization of the 2700-bent indirectly heated cathode-based electron gun. Rigorous simulation work has been carried out by performing 3-D electrostatic, electromagnetic, and particle tracking simulations using CST Studio Particle Tracking Module. Adaptive mesh refinement has been utilized to ensure high accuracy of the results obtained. An optimum filament-solid cathode interspacing was found, using strip-shaped filament by providing a higher negative potential to the grid cup. It led to a well-focused 2700-bent electron beam, exhibiting minimum electron beam emittance at the target plane.

Enhanced electron acceleration by high-intensity lasers in extended (confined) preplasma in cone targets

We report on experimental results from a high-intensity laser interaction with cone targets that increase the number (×3) and temperature (×3) of the measured hot electrons over a traditional planar target. This increase is caused by a substantial increase in the plasma density within the cone target geometry, which was induced by 17 ± 9 mJ prepulse that arrived 1.5 ns prior to the main high intensity (>1019 W/cm2). Three-dimensional hydrodynamic simulations are conducted using hydra which show that the cone targets create substantially longer and denser plasma than planar targets due to the geometric confinement of the expanding plasma. The density within the cone is a several hundred-micron plasma “shelf” with a density of approximately 1020 ne/cc. The hydra simulated plasma densities are used as the initial conditions for two-dimensional particle-in-cell simulations using EPOCH. These simulations show that the main acceleration mechanism is direct-laser-acceleration, with close agreement between experimentally measured and simulated electron temperatures. Further analysis is conducted to investigate the acceleration of the electrons within the long plasma generated within a compound parabolic concentrator by the prepulse.

Enhanced proton acceleration from laser interaction with curved surface nanowire targets

A novel curved surface nanowire target is proposed to improve the cutoff energy of accelerated protons via target normal sheath acceleration. The interaction of a laser of intensity $1.37\times 10^{20}\ {\rm W}\ {\rm cm}^{-2}$ with a curved surface nanowire target is studied by two-dimensional particle-in-cell simulations. The numerical results indicate that the sheath electric field at the target rear side is significantly enhanced by this simple target design, compared with using the planar nanowire target. The transverse motion of hot electrons is effectively confined and the energy density of electrons is naturally increased. A series of simulations with various target parameters is carried out to investigate the performance of this novel target. This tailored target may provide implications for generating high-quality proton beams in experiments.