Large Beam Size Research Articles

Oscillation FPCB micromirror based triangulation laser rangefinder

This paper reports an electromagnetic oscillation flexible printed circuit board (FPCB) micromirror based scanning triangulation laser rangefinder (LRF). The FPCB micromirror has a large aperture (8 mm × 5.5 mm) and high flatness (radius of curvature, ∼ 15 m), that overcomes conventional MEMS micromirrors’ limitation of a small aperture size (less than 5 mm). Subsequently high power lasers with large beam sizes and good collimation can be used in micromirror based scanning LRF for better performance. As a result, the LRF in this paper achieved a larger scanning angle and longer detecting distance than those in literature. Both modelling and prototyping are presented. Three lasers (Laser 1: 2 mW; Laser 2: 20 mW; and Laser 3: 100 mW) are used to characterise the LRF. Eye-safety calculation is presented for the three lasers. Achieved performance (measurement distance and field of view (FOV)) is: with Laser 1, distance of 15–70 cm and FOV of −15° to 10°, error ≤ 4%; with Laser 2, distance of 15–130 cm and FOV of −15° to 15°, error ≤ 5%; with Laser 3, distance 15–200 cm and FOV of −15° to (5 ∼ 9°), error ≤ 5%. Fatigue test indicates 0.8 billion scanning cycles have been reached.

Single-shot diagnosing of spatiotemporal couplings in ultrashort laser pulses by spatiospectral imaging of a third-order nonlinear process.

The impact of spatiotemporal couplings (STCs) on the focal-spot intensity of ultra-intense femtosecond lasers scales up with the larger beam sizes and shorter pulse duration typically implemented to reach petawatt (PW) intensities. The repetition rate of such facilities (${ \lt }{10}\;{\rm Hz}$<10Hz) makes single-shot diagnosing of STCs mandatory. We present here a new, to the best of our knowledge, spatiotemporal diagnostic technique based on the comparison of the spatiospectral intensity of a pulse and its nonlinear third-order effect, in this case, cross-polarized wave generation (XPW), with an imaging spectrometer. The measured XPW spectrum displays specific couplings that depend on both amplitude and phase STCs of the initial pulse to be characterized. We confirm experimentally the ability of our new approach to diagnose common STCs in a laser pulse on the fly.

Propagation of Rectangular Multi-Gaussian Schell-Model Array Beams through Free Space and Non-Kolmogorov Turbulence

In this paper, rectangular multi-Gaussian Schell-model (MGSM) array beams, which consists N×D beams in rectangular symmetry, are first introduced. The analytical expressions of MGSM array beams propagating through free space and non-Kolmogorov turbulence are derived. The propagation properties, such as normalized average intensity and effective beam sizes of MGSM array beams are investigated and analyzed. It is found that the propagation properties of MGSM array beams depend on the parameters of the MGSM source and turbulence. It can also be seen that the beam size of Gaussian beams translated by MGSM array beams will become larger as the total number of terms, M, increases or coherence length, σ , decreases, and the beam in stronger non-Kolmogorov turbulence (larger α and l 0 , or smaller L 0 ) will also have a larger beam size.

Analysis of terahertz generation by beamlet superposition.

A theory of terahertz generation using a superposition of beamlets is developed. It is shown how such an arrangement produces a distortion-free tilted pulse front. We analytically show how a superposition of beamlets produces terahertz radiation with greater efficiency and spatial homogeneity compared to tilted pulse fronts generated by diffraction gratings. The advantages are particularly notable for large pump bandwidths and beam sizes, suggesting better performance in the presence of cascading effects and for high energy pumping. Closed-form expressions for terahertz spectra and transients in three spatial dimensions are derived. Conditions for obtaining performance better than conventional tilted pulse fronts and bounds for optimal pump parameters are furnished.

Magnetic Analysis of the MQXF Quadrupole for the High-Luminosity LHC

The high-luminosity upgrade of the Large Hadron Collider (HL-LHC) requires new high-field and large-aperture quadrupole magnets for the low-beta inner triplets (MQXF). The U.S. HiLumi-LHC Accelerator Upgrade and CERN are jointly developing a 150-mm aperture Nb 3 Sn magnet. Due to the large beam size and orbit displacement in the final focusing triplet, MQXF has challenging field quality targets at collision energy. Magnetic measurements have been performed both at ambient and cryogenic temperatures in the four short models that were built and tested. This paper presents the magnetic analysis, comparing field measurements with the expectations and the field quality requirements. The analysis is focused on the geometrical harmonics and iron saturation effect, including three-dimensional effects and transfer function repeatability. Persistent currents and dynamic effects are also discussed.

Improvement of Electron Probe Microanalysis of Boron Concentration in Silicate Glasses.

The determination of low boron concentrations in silicate glasses by electron probe microanalysis (EPMA) remains a significant challenge. The internal interferences from the diffraction crystal, i.e. the Mo-B4C large d-spacing layered synthetic microstructure crystal, can be thoroughly diminished by using an optimized differential mode of pulse height analysis (PHA). Although potential high-order spectral interferences from Ca, Fe, and Mn on the BKα peak can be significantly reduced by using an optimized differential mode of PHA, a quantitative calibration of the interferences is required to obtain accurate boron concentrations in silicate glasses that contain these elements. Furthermore, the first-order spectral interference from ClL-lines is so strong that they hinder reliable EPMA of boron concentrations in Cl-bearing silicate glasses. Our tests also indicate that, due to the strongly curved background shape on the high-energy side of BKα, an exponential regression is better than linear regression for estimating the on-peak background intensity based on measured off-peak background intensities. We propose that an optimal analytical setting for low boron concentrations in silicate glasses (≥0.2 wt% B2O3) would best involve a proper boron-rich glass standard, a low accelerating voltage, a high beam current, a large beam size, and a differential mode of PHA.

Visualizing choriocapillaris using swept-source optical coherence tomography angiography with various probe beam sizes.

Imaging choriocapillaris (CC) is a long-term challenge for commercial OCT angiography (OCTA) systems due to limited transverse resolution. Effects of transverse resolution on the visualization of a CC microvascular network are explored and demonstrated in this paper. We use three probe beams with sizes of ~1.12 mm, ~2.51 mm and ~3.50 mm at the pupil plane, which deliver an estimated transverse resolution at the retina of 17.5 µm, 8.8 µm and 7.0 µm, respectively, to investigate the ability of OCTA to resolve the CC capillary vessels. The complex optical microangiography algorithm is applied to extract blood flow in the CC slab. Mean retinal pigment epithelium (RPE) to CC (RPE-CC) distance, mean CC inter-vascular spacing and the magnitude in the radially-averaged power spectrum are quantified. We demonstrate that a clearer CC lobular capillary network is resolved in the angiograms provided by a larger beam size. The image contrast of the CC angiogram with a large beam size of 3.50 mm is 114% higher than that with a small beam size of 1.12 mm. While the measurements of the mean RPE-CC distance and CC inter-vascular spacing are almost consistent regardless of the beam sizes, they are more reliable and stable with the larger beam size of 3.50 mm. We conclude that the beam size is a key parameter for CC angiography if the purpose of the investigation is to visualize the individual CC capillaries.

Investigation on fracture processes of concrete

Series of geometrically similar concrete beam of small, medium and large beam sizes has been tested under the action of monotonic and fatigue loading. Digital image correlation technique has been utilized to understand the fracture processes and crack growth behavior in concrete. The tip of effective crack in concrete beams under static and fatigue loading has been calculated through DIC analysis. Further, existence of size effect has been observed when the results of geometrically similar concrete beams were compared.

Acoustic Field of Phased-Array Ultrasound Transducer with the Focus/Foci Shifting

High-intensity focused ultrasound (HIFU) is becoming popular in the treatment of solid tumors because of its non-invasiveness with few complications. The acoustic field is of importance in evaluating the safe focus shifting distance and determining the treatment plan. The propagation of finite-amplitude acoustic wave from a 331-element HIFU phased-array with focus steering along and transverse to the transducer axis and 4-foci shifting on the focal plane was simulated using the angular spectrum approach (ASA) with a marching second-order operator-splitting scheme. In addition, the acoustic field produced by a truncated asymmetric transesophageal HIFU annular array was also simulated, and the effects of driving frequency and the number of concentric rings were investigated. Because of the nonlinear effects, the peak negative pressure is lower than that of peak positive pressure at the main lobe but has a larger beam size. However, the peak positive and negative pressures at the grating lobe are quite similar. The effects of the focus shifting distances on the main and grating lobe (both acoustic pressure and − 6 dB beam size) were evaluated. With the focus shifting axially away from the transducer surface, the main lobe has decreased acoustic pressure by ~ 1.9 fold and increased beam size by ~ 4.5 fold while the grating lobe has the increased acoustic pressure by ~ 1.8 fold. The focus shifting laterally leads to the reduced pressure at the main lobe by ~ 1.4 fold but the slight decrease at the grating lobe by ~ 1.1 fold. In comparison, the shifting of multi-foci has similar influences on the main lobe but increases the pressure at the grating lobe. Driving frequency of annular array is found to have greater influences on the peak pressure and beam size. Our algorithm can simulate the acoustic field of phased-array in arbitrary shape and optimize the transducer design, and the focus shifting distance and strategy should be selected appropriately for the safe HIFU exposure.

Electron probe microanalysis of Fe2+/ΣFe ratios in calcic and sodic-calcic amphibole and biotite using the flank method

We present an analytical approach to determine the Fe2+/ΣFe ratio in amphiboles and biotites using electron probe microanalysis (EPMA). The “flank method” applied in this study is based on the fact that FeL line spectra show different peak position and intensity for Fe2+ and Fe3+, which is associated with a resonant process involving both X-ray absorption and emission. The flank positions for Lα and Lβ, which correspond to sites with minimal Fe3+L3 absorption and maximal Fe2+L2 absorption respectively, are determined based on the L-line spectra difference of two Fe-rich ferric and ferrous garnet endmembers, i.e. andradite and almandine. Spectra intensities measured at the Lα and Lβ flank positions for selected amphibole and biotite references can be quantitatively correlated with their Fe2+ weight contents by a linear relation: Lβ/Lα = a + b*Fe2+, in which a and b are constants calibrated upon reference materials. It is emphasized that the constants a and b may vary significantly for different mineral groups and analytical conditions. Thus, a calibration is necessary for each analytical session. Our tests show that the potential beam damage during EPMA that may induce oxidation of the measured material can be minimized by using a large beam size (e.g., 20 μm). The Fe2+/ΣFe ratios determined using the EPMA flank method in collected natural calcic and sodic-calcic amphiboles (FeOT = 2.4–12.7 wt%, Fe2+/ΣFe = 0.45–75) and biotites (FeOT = 4.3-28.2 wt%, Fe2+/ΣFe = 0.67–0.87) are consistent within an error of ±0.1 with the data obtained from the colorimetric wet chemistry method. The EPMA flank method has the potential to be widely applied for accurate in-situ determination of Fe oxidation states in a wide range of minerals, provided that references are available.

Analysis of terahertz generation using tilted pulse fronts.

A spatio-temporal analysis of terahertz generation by optical rectification of tilted pulse fronts is presented. Closed-form expressions of terahertz transients and spectra in two spatial dimensions are furnished in the undepleted limit. Importantly, the analysis incorporates spatio-temporal distortions of the optical pump pulse such as angular dispersion, group velocity dispersion due to angular dispersion, spatial and temporal chirp, as well as beam curvature. The influence of the radius of curvature on the tilt angle is shown. Furthermore, the impact of group velocity dispersion due to angular dispersion on terahertz frequency, conversion efficiency and peak field is revealed. In particular, the deterioration of terahertz frequency, efficiency and field at large pump bandwidths and beam sizes by group velocity dispersion due to angular dispersion is expressed analytically.

A new tungsten wire calorimeter for the negative ion source testbed BATMAN Upgrade

Within the framework of the ion source development for the ITER and DEMO Neutral Beam Injection (NBI) systems, IPP Garching has recently upgraded the radiofrequency-driven negative ion source testbed BATMAN. One of the requirements for the ITER NBI system is to produce a beam power density homogeneity above 90% over its large beam size extracted from an ion source of 1 × 2 m2. The beam uniformity is going to be investigated, on a smaller scale, in BATMAN Upgrade with the new ITER-like beam optics. The testbed is equipped with several beam diagnostic tools to measure beam power, uniformity and divergence. A new tungsten wire calorimeter (TWC) has recently been developed to characterize the beam quantitatively and with an improved spatial and temporal resolution. The TWC consists of an array of thin tungsten wires with a diameter of 0.3 mm, placed in the beam path. The wires are heated by the beam up to 3000 °C and emit visible light, which is observed by an optical camera. While the existing TWC provides a qualitative impression of beam power characteristics and is placed at a distance of almost 2 m from the beam extraction system, the new TWC is positioned only 19 cm downstream from the extraction system and allows observation of the single beamlets for the first time in this testbed. In order to determine the correlation between the pixel intensity measured by the camera and the power density impinging on the wires, one wire is equipped with an ohmic heating system which allows heating up the wire with a known power, thus enabling calibration of the diagnostic tool.In this paper the design, including FEM simulations of the wire thermal behavior, manufacturing and installation of the new TWC inside the BATMAN Upgrade testbed are described and the first, very promising, observations of beamlet intensities are presented.

Steel Structures Research Update:Seismic Performance and Design of Steel Panel Dampers for Steel Moment Frames

Ongoing work on the seismic performance and design of steel panel dampers for steel moment frames is highlighted. Dr. Keh-Chyuan Tsai, professor in the Department of Civil Engineering at National Taiwan University, leads the team from National Taiwan University and the National Center for Research on Earthquake Engineering (NCREE) in Taipei. In 2018, At NCREE, one recent collaboration with the University of Washington included cyclic tests of a three-story chevron special concentrically braced frame (SCBF). Current seismic design provisions require large beam sizes to resist the unbalanced forces from the chevron braces after brace buckling. The research has explored options for alternative ductile mechanisms and reduced beam sizes. Steel research at NCREE has also included studies on steel beam-to-box-column moment connections and electro-slag-welded (ESW) joints in those connections. Meanwhile, research into premature fracture of ESW diaphragm-to-column joints in beam-to-box-column connections has revealed sensitivity to eccentricity in loading between the beam flange and the diaphragm. Further, a micromechanical-based stress-modified critical stress (SMCS) model has been developed and shown to be capable of predicting the crack initiation of the diaphragm-to-column ESW joint (Li et al., 2018).

A way to improve the uniformity of nanometer-thickness graphite film synthesized on polycrystalline Ni substrate: From large grain to small grain

A pellicle is an important part of mask protection in extreme ultraviolet (EUV) lithography, and nanometer-thickness graphite film (NGF) is an ideal material for pellicle fabrication. In our previous work, we synthesized large-scale NGF on polycrystalline Ni foil by a “two-stage” chemical vapor deposition process with an EUV transmittance of 79 ± 2%. The nonuniformity of EUV transmittance is mainly caused by the thicker NGF formed on the Ni grain boundaries (GBs). Here, we present a simple and cost-effective method to weaken the effects of the Ni GBs by sputtering thin Ni films onto Ni foils. The Ni film provides smaller grains with high GB density so that the non-uniformity of NGFs caused by Ni GBs was averaged out by the large EUV beam size. We investigated the grain growth of Ni films with different thicknesses, and 1-μm Ni film/Ni foil had the minimum average grain size of ∼6 μm. The NGF synthesized on it showed a higher EUV transmittance with better uniformity of 86.3 ± 0.9%. Fullerenic CC and sp3 CC bonding were detected by the X-ray photoelectron spectroscopy, and the NGF on the Ni GBs showed a higher defect density.

Femtosecond laser eyewear protection: Measurements and precautions for amplified high power applications

Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 KHz repetition rate. The typical maximum peak laser irrandiance employed was ca. 3 TW/cm2 (800 nm wavelength, 450 uJ/pulse with 80 fs FWHM pulse duration) or less when damage occurred, depending on the sample. While many samples maintained their integrity under these test conditions, many plastic samples showed signs of failure which reduced their OD, in some cases transmitting 4 to 5 orders of magnitude higher than expected. In general, glass filters performed significantly better than plastic filters, exhibiting less physical damage to the substrate and less absorber degradation.

Transverse beam profile measurement system for the Duke storage ring

The transverse beam emittance is a crucial parameter determining the brightness of an electron storage ring based synchrotron radiation source. The beam emittance is typically determined using the transverse electron beam size and the beta function at a particular location. For a low energy storage ring, the direct imaging method using visible/UV light has many advantages, including being simple, straightforward, and cost-effective. The resolution of such a system can be quite adequate for measuring electron beams with a reasonably large transverse beam size. In this work, we present the development of a new transverse beam profile measurement system for the Duke storage ring. This new system has been characterized to allow absolute measurements of the electron beam size while achieving better system resolution than previously thought possible for the direct imaging technique. The preliminary measurement results show that this system is capable of measuring the horizontal beam size over a wide range of the electron beam energies and currents. The system has also been demonstrated as a useful tool to study the intra-beam scattering related emittance increase in the storage ring.

Multi-dressing and optical pumping effects in nonlinear gain from parametric-amplification four-wave mixing

A higher degree of entanglement of entangled sources in quantum communications is always desired and can be achieved with high nonlinear gains. The dressing of atomic coherence and optical pumping can be considered as effective parameters to obtain such entangled sources. In this regard, we have experimentally studied doubly dressed effects in a small range of frequency, and optical pumping and de-pumping effects over a wide frequency range in multi-level atomic system in order to enhance the nonlinear gain. The multi-mode phenomenon that results from the multi-peak of Autler–Townes splitting can be observed at the large beam size of the pinhole of the pump field. Theoretical calculations fit well with the experimentally measured results. The dressing of atomic coherence and optical pumping lead to the enhancement of nonlinear gain up to 8.2 when the original nonlinear gain is 4.7.

A Serendipitous Pulsar Discovery in a Search for a Companion to a Low-mass White Dwarf

We report the discovery of a previously unidentified pulsar as part of a radio campaign to identify neutron star companions to low-mass white dwarfs (LMWDs) using the Robert C. Byrd Green Bank Telescope (GBT). PSR J0802-0955, which is coincident with the position of a WD with a mass of 0.2 solar masses, has a pulse period of 571 ms. Because of its relatively long pulse period, the lack of radial velocity (RV) variations in the radio data, and GBT's large beam size at the observing frequency of 340 MHz, we conclude that PSR J0802-0955 is unassociated with the LMWD at roughly the same position and distance.

Geometric Field Errors of Short Models for MQXF, the Nb3Sn Low-β Quadrupole for the High Luminosity LHC

In the framework of the high-luminosity upgrade of the large hadron collider, the U.S. LARP collaboration and CERN are jointly developing a 150-mm aperture Nb3Sn quadrupole for the Large Hadron Collider (LHC) interaction regions. Due to the large beam size and orbit displacement in the final focusing triplet, MQXF has challenging targets for field quality at nominal operation conditions. Three short model magnets have been tested and around 30 coils have been built, allowing a first analysis of the reproducibility of the coil size and turns positioning. The impact of the coil shimming on field quality is evaluated, with special emphasis on the warm magnetic measurements and the correlation to field measurements at cold and nominal field. The variability of the field harmonics along the magnet axis is studied by means of a Monte-Carlo analysis and the effects of the corrective actions implemented to suppress the low-order unallowed multipoles are discussed.

Tunable angular shearing interferometer based on wedged liquid crystal cells.

The concept of a liquid crystal wedge as a tunable angular shearing interferometer is introduced and demonstrated to combine both high stability and high tunability. Different wedges are fabricated from planar aligned nematic liquid crystal cells with thickness gradients. These wedges are shown to produce stable interferograms from the polarization interference between the ordinary and extraordinary waves propagating in different directions at the output of the cell. The fringe periods, ranging from 70μm to 1.25mm, can be precisely controlled by a low voltage. Despite the wedge-shaped structure, no inhomogeneity has been detected when the wedge is driven adiabatically and the interferograms are uniform over regions as large as 5×5 mm. Moreover, dynamical measurements show that the wedges behave as a succession of multiple cells with different thickness, giving rise to a moving front of stabilizing fringes when driven dynamically. All the observations show that the device is suitable for large beam size and tunable shearing interferometry, with attractive features for applications such as phase sensing, photoalignment or photolithography.