Collimated Research Articles

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Generation of Bloch surface beams with arbitrarily designed phases.

We proposed a new manipulation method for Bloch surface waves that can almost arbitrarily modulate the lateral phase through in-plane wave-vector matching. The Bloch surface beam is generated by a laser beam from a glass substrate incident on a carefully designed nanoarray structure, which can provide the missing momentum between the two beams and set the required initial phase of the Bloch surface beam. An internal mode was used as a channel between the incident and surface beams to improve the excitation efficiency. Using this method, we successfully realized and demonstrated the properties of various Bloch surface beams, including subwavelength-focused, self-accelerating Airy, and diffraction-free collimated beams. This manipulation method, along with the generated Bloch surface beams, will facilitate the development of two-dimensional optical systems and benefit potential applications of lab-on-chip photonic integrations.

Small sample measurements at the low energy muon facility of Paul Scherrer Institute

The low energy muon spin rotation spectroscopy (LE-μSR) is primarily used to investigate thin films, surfaces, and interfaces of materials, which has matured at the Paul Scherrer Institute (PSI) and is routinely employed by users of the low energy muon (LEM) facility. However, because of the large beam spot and low implanted muons rate, LE-μSR measurements on small samples are difficult, requiring an optimal sample size of 25×25mm2. Recently, we have boosted our ability to measure small samples, down to 5×5mm2 area, by beam collimation and tuning. This achievement is crucial for the measurements of many magnetic and superconducting materials. Furthermore, we have devised a method that allows us to measure five small area samples mounted together on the same sample plate. We expect this method to further improve the efficient use of beam time at LEM.

Investigating Slit-Collimator-Produced Carbon Ion Minibeams with High-Resolution CMOS Sensors

Particle minibeam therapy has demonstrated the potential for better healthy tissue sparing due to spatial fractionation of the delivered dose. Especially for heavy ions, the spatial fractionation could enhance the already favorable differential biological effectiveness at the target and the entrance region. Moreover, spatial fractionation could even be a viable option for bringing ions heavier than carbon back into patient application. To understand the effect of minibeam therapy, however, requires careful conduction of pre-clinical experiments, for which precise knowledge of the minibeam characteristics is crucial. This work introduces the use of high-spatial-resolution CMOS sensors to characterize collimator-produced carbon ion minibeams in terms of lateral fluence distribution, secondary fragments, track-averaged linear energy transfer distribution, and collimator alignment. Additional simulations were performed to further analyze the parameter space of the carbon ion minibeams in terms of beam characteristics, collimator positioning, and collimator manufacturing accuracy. Finally, a new concept for reducing the neutron dose to the patient by means of an additional neutron shield added to the collimator setup is proposed and validated in simulation. The carbon ion minibeam collimator characterized in this work is used in ongoing pre-clinical experiments on heavy ion minibeam therapy at the GSI.

METU-DBL: a cost effective proton irradiation facility

The Middle East Technical University Defocusing Beamline (METU-DBL) is designed to deliver protons with selectable kinetic energies between 15–30 MeV, and proton flux between 106–1010 protons/cm2/s, on a maximum 21.55 to 15.40 cm target region with a beam uniformity within ±6%, in accordance with the ESA ESCC No. 25100 specification for single event effects (SEEs) tests in the low energy range. The achieved high proton fluences, allow users to test space-grade materials; electronic circuits, ASICs, FPGAs, optical lenses, structural elements, and coating layers for LEO, GEO, and interplanetary missions.The total received dose on the Device-Under-Test (DUT) from secondary particles created during proton-material interactions at the first beam collimator and the beam dump never exceed 0.1% of the dose from primary protons. The METU-DBL is equiped with several measurement stations and services to the user teams. A secondary measurement station in a rotating drum that can hold multiple samples has been constructed next to the first collimator which provides neutrons for transmission experiments. At the target region, a robotic table is located, which provides mechanical and electrical mounting points to the samples and allows multiple samples to be tested in a row. A modular vacuum box can also be attached on the robotic table for any test that may require a vacuum environment. Power rails on the robotic table provide various outputs for the DUT. For the data acquisition, high-speed networking and a modular industrial PC are available at the target station. The design of the METU-DBL control software enables test users to integrate and optimize the data acquisition and controlling of the DUT.The beam properties at the target region are measured with the diamond, Timepix3, and fiber scintillator detectors mounted on the robotic table. With diamond and Timepix3 detectors, measurements are taken from the five different points (center and the four corners) of the test area to measure the proton flux and ensure that it is uniform across the full test area. Fiber scintillators on both axes (X and Y) scan the target area to cross-check the beam profile's uniformity. Secondary doses during the irradiation are measured by a Geiger-Müller tube sensitive to electrons and gammas above 0.1 MeV and by a neutron detector located at the entrance of the R&D room. The room cools down relatively fast after any irradiation (<1 hour).Accurate linear energy deposition rates and absorbed doses on the samples are calculated using MCNP6, FLUKA and Geant4 Monte Carlo simulations. Alanine dosimetry measurements that are calibrated against these simulations are also used to estimate the absorbed dose on the sample.

Collimation and energy enhancement in laser driven MeV protons

Enhancing the energy and controlling the divergence of proton beam have been crucial tasks since the development of laser-based ion accelerators. In this study, we employ 2D particle-in-cell EPOCH simulation to investigate a method to control the divergence and enhance the energy of MeV proton beam produced from the interaction of high intensity laser with a gold target. The energy and divergence of proton beam for the conventional laser-foil target configuration are compared with our proposed three laser-three target configuration. In the single laser-single target configuration, a high intensity laser irradiates a gold foil target resulting in proton acceleration from the plasma. In contrast, in the three laser-three target configuration, the generated proton beam is allowed to traverse through the configuration of two parallel Al plates irradiated with high intensity lasers. The new configuration provides the proton energy enhancements of 40% and more collimated beam compared to the single laser-foil target setup. This collimation and energy increase are due to the vertical and horizontal components of the electric field generated by the hot electron plasma of Al plates. The proton energy and divergence can also be tuned by varying the plasma electron density of the Al plasma. Also, the control of proton beam collimation is explored by varying target parameters as well, and it is observed that the separation between the Al plates and length of the Al plates plays major role in controlling the divergence of the beam. This study will have a significant impact in the ongoing laser fusion program, fast ignition, and laser ion therapy.

Rotated Feed-Combined Reconfigurable Transmit RIS With Disparate Deployment of 1-bit Hybrid Units for B5G/6G

This communication presents an initial disparate deployment of 1-bit hybrid units using p-i-n diodes in transmit reconfigurable intelligent surface (TRIS) combined with a rotated feed source to address the gain limitation of the existing 1-bit TRIS. Two types of hybrid units, with a radiation phase difference of 90° by the delay line, are proposed in this study. The TRIS aperture is divided into two sections based on two different radiation centers by rotated feed, and the hybrid units in each section are initially configured to implement optimized quasi 2-bit beam collimation in the 0° and 40° for enhanced quantization loss. It is found that the array envelope is higher even in radiation directions other than the optimized directions, despite the initial disparate deployment of each section. Beamforming for -theta direction in the H-plane is only considered for proof of concept with the smallest size of TRIS aperture. A 16×16 (7.6×7.6λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at 28 GHz) 1-bit hybrid TRIS offers gain enhancement of 0–5.6 dB in -70°–0° directions of the array envelope compared with the conventional 1-bit TRIS. The proposed 1-bit hybrid TRIS yields an improved aperture efficiency of 24.4%.

Suitability of Athena Monolithic Active Pixel Sensor for upstream monitoring of 6 MV, 6 FFF and 10 MV treatments

We are developing a thin, real-time radiotherapy verification sensor based on the Athena, a large-scale MAPS. The goal in radiotherapy verification is to measure the multileaf collimator positions and beam intensity to ensure the accuracy and safety of treatment delivery. Previously, results on this have been published. In this paper, we present results that clearly demonstrate that the Athena does not saturate, even at the highest beam intensities in a 6 FFF 10 × 10 cm2 field and thus is suitable for clinical deployment.

Folded Digital Meta-Lenses for on-Chip Spectrometer.

In-plane diffractive optical networks based on meta-surfaces are promising for on-chip application. The design constraints of regular antenna unit place ultimate limits on the functionalities of the meta-systems. This fundamental limitation has been reflected by the large footprints of cascaded meta-surfaces. Here, we propose a digital meta-lens with a large degree of design freedom, enabling significantly improved beam focusing, collimation, and deflection capabilities. A highly dispersive and compact diffractive optical system is constructed for spectrometer via five layers of meta-lenses in a folded configuration. The device only occupies a 100 μm × 100 μm chip area on a silicon photonic platform. Sparse and continuous spectra reconstruction is achieved over a 35 nm bandwidth. Fine spectral lines separated by 0.14 nm are resolved. In addition to such a compact and high-resolution on-chip spectrometer, it is also expected to be promising for imaging, optical computing, and other applications due to the great versatility of the digital lens design.

Eye-Lens Dose Reduction using Region of Interest (ROI) Attenuators in Neuroimaging.

Lens dose can be high during neuro-interventional procedures, increasing the risk of cataractogenesis. Although beam collimation can be effective in reducing lens dose, it also restricts the FOV. ROI imaging with a reduced-dose peripheral field permits full-field information with reduced lens dose. This work investigates the magnitude of lens-dose reduction possible with ROI imaging. EGSnrc Monte-Carlo calculations of lens dose were made for the Zubal head phantom as a function of gantry angulation and head shift from isocenter for both large and small FOV's. The lens dose for ROI attenuators of varying transmission was simulated as the weighted sum of the lens dose from the small ROI FOV and that from the attenuated larger FOV. Image intensity and quantum mottle differences between ROI and periphery can be equalized by image processing. The lens dose varies considerably with beam angle, head shift, and field size. For both eyes, the lens-dose reduction with an ROI attenuator increases with LAO angulation, being highest for lateral projections and lowest for PA. For an attenuator with small ROI field (5 × 5 cm) and 20% transmission, the lens dose for lateral projections is reduced by about 75% compared to a full dose 10 ×10 cm FOV, while the reduction ranges between 30 and 40% for PA projections. Use of ROI attenuators can substantially reduce the dose to the lens of the eye for all gantry angles and head shifts, while allowing peripheral information to be seen in a larger FOV.

3D Printed Lenses for Vertical Beam Collimation of Optical Phased Arrays.

This article presents the design, fabrication, and characterization of edge-coupled 1D optical phased arrays (OPAs) combined with collimating lenses. Our concept was tested with two OPAs having different collimation ranges. Both OPA designs have 3-μm waveguide spacing and the maximum beam steering range is about 30° based on wavelength tuning around 1550 nm. The first generation had 37 channels with 108 μm of waveguide array width and the second generation had 512 channels with 1.5 mm array width. As the array outputs are edge coupled, suitable lenses are required to collimate the beam vertically. We report the comparison between a commercially available straight cylindrical lens and custom 3D printed curved cylindrical lenses. In the experiments, we demonstrate 1D beam steering of the light outcoupled from the waveguide facets and collimated by these lenses and analyzed parameters such as Rayleigh range and beam divergence. These parameters are estimated to be 9.9 mm and 7.0 mrad (0.4°), respectively, for the commercial lens, whereas 40.1 mm and 3.5 mrad (0.2°) for the dedicated 3D printed lens, showing a clear improvement.

Gain Enhancement of Millimeter Wave Antenna by Ultra-thin Radial Phase Gradient Metasurface for 5G Applications

In this article, gain augmentation of a microstrip antenna (MSA) operating at 28 GHz has been enunciated in a cost-effective manner using a phase-graded index lens. Different metamaterial unit cells with transmissive characteristics have been designed and placed in an array to create a radial phase graded metalens with locally varying refractive index and spatially varying surface impedance. Transformation of spherical wavefronts into planar wavefronts results in beam collimation which helps in obtaining a directive beam in the 5G frequency band. The ratio of focal length to the diameter (f/d) of the lens is 0.38. The gain of MSA is enhanced by 4.4 dB with radiation efficiency above 95% due to the focusing effect of the lens placed vertically above the antenna. The proposed planar lens is compact in size and easy to fabricate when compared with the conventional 3-D lens reported in existing works. The microstrip patch integrated lens prototype exhibits a gain of 12.4 dBi operating at 28 GHz. The performance of the lens has been analyzed theoretically and evaluated further by conducting experiments on the fabricated prototype. Measured results validate the full–wave simulation results carried out using finite element method.

Optimal dual-energy computed tomography scan parameters to detect small-sized urinary stones and their composition

To investigate the optimal scanning parameters of dual-energy computed tomography (DECT), which can accurately determine sensitivity (the detectability of urinary stones) and accuracy (the composition matching of urinary stones), and to apply them to clinical trials. Fifteen urinary stones were chemically analyzed, and their chemical compositions were considered a reference standard with which we compared the uric acid (UA) and non-UA compositions determined using DECT. The urinary stones were placed inside a bolus and scanned with a dual-source CT scanner under various selected dual-energy conditions (A to X) using various solid water phantom thicknesses. These datasets were analyzed using the Siemens syngo.via software tool (integrated into the CT system) for matching the sensitivity and accuracy assessments. This study showed that 80% of the highest sensitivity (detection of urinary stones) and 92% of the highest accuracy (composition matching of urinary stones) were achieved under condition A (a collimation beam width setting of 2 × 32× 0.6mm, an automatic exposure control setting of 80/sn140 peak kilovoltage, and a slice thickness of 0.5/0.5mm) (P < 0.05). Application of the DECT energy parameters presented in the study will help identify the sensitivity and accuracy of UA and non-UA stone analysis, even in patients with small-sized urinary stones and in conditions difficult for analysis.

Brain image quality according to beam collimation width and image reconstruction algorithm: A phantom study

PurposeTo compare quantitatively and qualitatively brain image quality acquired in helical and axial modes on two wide collimation CT systems according to the dose level and algorithm used. MethodsAcquisitions were performed on an image quality and an anthropomorphic phantoms at three dose levels (CTDIvol: 45/35/25 mGy) on two wide collimation CT systems (GE Healthcare and Canon Medical Systems) in axial and helical modes. Raw data were reconstructed using iterative reconstruction (IR) and deep-learning image reconstruction (DLR) algorithms. The noise power spectrum (NPS) was computed on both phantoms and the task-based transfer function (TTF) on the image quality phantom. The subjective quality of images from an anthropomorphic brain phantom was evaluated by two radiologists including overall image quality. ResultsFor the GE system, noise magnitude and noise texture (average NPS spatial frequency) were lower with DLR than with IR. For the Canon system, noise magnitude values were lower with DLR than with IR for similar noise texture but the opposite was true for spatial resolution. For both CT systems, noise magnitude was lower with the axial mode than with the helical mode for similar noise texture and spatial resolution. Radiologists rated the overall quality of all brain images as “satisfactory for clinical use”, whatever the dose level, algorithm or acquisition mode. ConclusionsUsing 16-cm axial acquisition reduces image noise without changing the spatial resolution and image texture compared to helical acquisitions. Axial acquisition can be used in clinical routine for brain CT examinations with an explored length of less than 16 cm.

Measurement of Telescope Transmission Using a Collimated Beam Projector

With the increasingly large number of Type Ia supernova being detected by current-generation survey telescopes, and even more expected with the upcoming Rubin Observatory Legacy Survey of Space and Time, the precision of cosmological measurements will become limited by systematic uncertainties in flux calibration rather than statistical noise. One major source of systematic error in determining SNe Ia color evolution (needed for distance estimation) is uncertainty in telescope transmission, both within and between surveys. We introduce here the Collimated Beam Projector (CBP), which is meant to measure a telescope transmission with collimated light. The collimated beam more closely mimics a stellar wave front as compared to flat-field-based instruments, allowing for more precise handling of systematic errors such as those from ghosting and filter angle-of-incidence dependence. As a proof of concept, we present CBP measurements of the StarDICE prototype telescope, achieving a standard (1σ) uncertainty of 3% on average over the full wavelength range measured with a single beam illumination.

All-fiber-device-coupled compact, transportable ultra-stable laser.

In response to the demand for operation in non-laboratory environments, there has been a trend toward the development of compact, transportable ultra-stable lasers. This paper reports on this sort of laser system assembled in a cabinet. The whole optical part utilizes fiber-coupled devices to simplify the integration. In addition, spatial beam collimation and alignment into the high-finesse cavity are realized by a five-axis positioner and a focus-adjustable fiber collimator, which significantly relax the alignment and adjustment. A theoretical analysis is performed on how the collimator adjusts the beam profile and coupling efficiency. The support structure of the system is specially designed as well so that it features robustness and transportation without performance degradation. The observed linewidth is 1.4Hz within a duration of 1s. After subtracting the linear drift of 70 mHz/s, the fractional frequency instability is better than 4 × 10-15, for the averaging time ranging from 1 to 100s, which is close to the thermal noise limit of the high-finesse cavity.

Impact of electronic portal image quality on Elekta AQUA® collimator isocenter.

The collimator radiation isocenter position determined in AQUA® v3.0 [Elekta AB, Stockholm, Sweden] software test "MLC Leaf and Jaw Position" was independently validated using an in-house MATLAB [Natick, MA: The MathWorks Inc.] script. The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated. Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X-axis and Y-axis. This corresponds to one image pixel and was found to be due to differences in software co-ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter. For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.

Role of Multidetector Computed Tomography in the Evaluation of Maxillofacial Trauma.

Maxillofacial fractures are among the commonest injuries occurring in trauma patients. Multislice computed tomography (CT) is a widely used radiological investigation that accurately reveals the number, location, and extent of the fractures as well as concomitant soft tissue injuries and has been found to be superior in the diagnosis of maxillofacial fractures owing to high sensitivity and specificity. This study was performed to assess the efficacy of axial, coronal, sagittal, and three-dimensional (3D) reformatted images in the detection of fractures in maxillofacial trauma. This was a cross-sectional descriptive study conducted on 49 adult patients with maxillofacial injuries undergoing multislice CT using a multidetector SiemensSOMATOM Emotion eco 16 slice CT scanner (Siemens AG, Munich, Germany). CT protocol consisted of non-contrast axial 16-slice helical series beam collimation ~ 3 mm, pitch ~ 0.8 - 1, tube current ~ 270 mAs, voltage ~ 130 kV, Total exposure time ~ 18 seconds, total radiation ~ 200 mGy. Along with the axial, coronal and sagittal images were reconstructed with 0.5 mm increment. 3D volume-rendering images were also obtained. 3D images were compared with axial images, coronal and sagittal plane images. The maximum number of cases was in the age group of 21-30 years with the male: female ratio being 5.12:1. The most common cause of injury was road traffic accidents (RTA). Mandible fractures were found to be the most common (20 patients, 40.8%) followed by fractures of nasal bone (18 patients, 36.7%). The incidence of frontal bone fractures was found to be the least(six patients, 12.24%). Our study found that 3D images are superior to axial in assessing the extent and degree of displacement of maxillofacial fractures in general. The maxillary sinus was found to be the most commonly fractured sinus (19 patients, 38.7%). Sphenoid sinus fractures were the least common (seen in two patients, 4.08%). CT findings correlated with the operative findings in most types of fractures. Multidetector CT with multiplanar and 3D reformation is highly accurate in the identification of fractures and assessing the extent and degree of displacement of fractures; hence, it is the imaging modality of choice in maxillofacial trauma.3D images are much better for the detection of maxillofacial fracturescompared to axial, coronal, or sagittal views, especially in maxilla and mandibular bone fractures. It is also found to be betterat providing information on thepatterns of the fracture lines and the displacement of the fracture fragments. Another added advantage of multidetector CT is that it is a non-invasive technique with good accuracy and a short scan time.

Intense nonlinear laser field longitudinal vector potential breaks motion symmetry of stimulated electron and produces highly collimated radiation

The effects of longitudinal fields on electron dynamics, power, and radiation spectrum in tightly-focused circularly polarized laser pulses are investigated in detail. When the longitudinal field vector potential is not considered at the intense relativistic laser field, the longitudinal acceleration of the electron subjected to the ponderomotive force is oscillatory, and the electron will be pushed out of the laser field. The peak radiation power, along with peak angle, varies exponentially with the laser peak amplitude. The longitudinal field leads to the appearance of spatial radiation vortex states when the laser peak is strong, increasing the collimation of the power and spectrum of spatial radiation as well as the focus.

Free-Space Diffused Light Collimation and Concentration

Collimating and concentrating broad-band diffused light can increase the yield, decrease the cost, and open new opportunities for solar-generated electricity. Adherence to the second law of thermodynamics requires that collimation, and therefore the reduction of étendue or entropy, of diffused sunlight, i.e., light scattered by clouds or the atmosphere, can only occur if the photons lose energy during the process. This principle has been demonstrated in luminescent solar concentrators; solar photons are energetically down-shifted by a luminophore and the emitted photons are trapped within a transparent matrix and guided toward an edge lining solar cell. However, this process suffers from low efficiency as the photons are trapped within the waveguide for a long time, encountering many instances of accumulating loss mechanisms. Here, we theoretically describe and experimentally demonstrate the first free-space diffused light collimation system which overcomes these efficiency losses. The high photon energy solar spectrum is allowed to enter the system from all angles, whereas the re-emitted luminescent photons can only escape under a desired emission cone. We achieved this through doping a polymethylmetacrylate waveguide with Lumogen Red dye, which we cover on one side with a Lambertian reflector for photon recycling and induced randomization and on the top face with a complex multilayer dielectric nanophotonic coating stack. We experimentally found an angular concentration of 118% within the designed escape cone, where isotropic emission corresponds to 100%, thereby verifying the reduction of étendue in free space experimentally. Such free-space collimation systems will enable efficient redirection of sunlight toward solar panels, thereby increasing yield, decreasing heating through the emission of low energy photons, and expanding the range of available surfaces from which sunlight can be harvested.