Beam Divergence Research Articles

Guiding as a General Consequence of the Charged Particle Interaction with the Inner Surface of an Insulator Capillary—Guiding of 1 MeV Proton Microbeam through Polytetrafluoroethylene Macrocapillary

The transmission of energetic, 1 MeV proton microbeam through a single, cylindrical shaped, macrometer-sized polytetrafluoroethylene capillary was studied experimentally. The capillary axis was tilted with respect to the axis of the incident ion beam. The tilting, the aspect ratio of the capillary and the small beam divergence disabled the geometrical transmission of the beam through the target. The intensity, energy, deflection and charge state of the transmitted beam were investigated. We found that the pure guided transmission of a MeV/amu energy ion beam is observable. We clearly identified three completely different stages during the guiding process according to the measured energy distribution of transmitted particles. At the beginning the transmission intensity was low and only inelastic contributions with energy lower than 1 MeV were found in the spectrum. Later, in the second stage, the elastic peak appeared and became more and more significant. Finally, when the stable transmission evolved, only the elastic peak was present and the inelastic area was totally absent as a direct consequence of the ion guiding and as a result of the charged particle interaction with a charged inner surface of the insulator capillary.

A low-threshold single-mode higher-order topological photonic crystal nanowire array laser

A higher-order topological photonic crystal GaAs/AlGaAs/GaAs nanowire array laser based on 0D corner modes is proposed and analyzed. The structure is composed of a nanowire trivial photonic crystal sandwiched by two topological photonic crystals. Compared with the conventional deformed honeycomb lattice photonic crystal structure, an additional mode selection mechanism is introduced by adjusting the coupling strength of topological cavities formed at two topological interfaces. By optimizing the structure parameters, a large side-mode suppression ratio of 24 dB and a high-quality factor of 7935 are obtained. Due to the strong mode confinement, lasing is achievable at an ultrasmall diameter of 86 nm, with a high confinement factor of 7.96 and low threshold gain of 230 cm−1. Moreover, the laser exhibits a small beam divergence of 6.6°. This work may pave a way for the development of low-threshold single-mode miniaturized lasers.

Collinear acousto-optic filtering of divergent optical beams in the presence of optoelectronic feedback

The divergence of a diffracting light beam affects the characteristics of acousto-optic (AO) interaction. This is due to the side components of the optical beam spatial spectrum propagating in the crystal along the directions different from the beam axis. The divergence causes the AO diffraction efficiency decrease and the distortion of the AO device transmission function — its bandwidth increases and the side lobes are smoothed. We suggest the method to compensate the optical beam effect on the collinear AO diffraction transmission functions shape based on optoelectronic feedback circuit application connecting the AO cell optical output and its piezoelectric transducer. It is shown that such method makes it possible to narrow the transmission function passband and increase the spectral contrast of the system. It is found out that feedback does not affect the AO diffraction transfer functions. Thus, the introduction of feedback makes it possible to increase the spectral resolution of AO devices without compromising the AO device angular aperture.

Modeling the short wavelength infrared laser radiance reflection on the sea surface.

The sea surface is a complex dynamic structure dependent on atmospheric conditions, and for which physical and chemical properties change from water to foam. Its roughness determines how the surface reflects, absorbs, and emits radiance, and depends on multiple parameters such as wind speed and direction, and foam and turbulence induced from natural waves or from object displacement. In this paper, a model description is given for laser reflection on the sea surface in open water driven by the wind. The model allows calculation of the reflected laser radiance from the sea surface toward a receiver as a function of the incoming laser radiance with a known beam intensity profile. Each subarea of the sea surface seen by one pixel of the receiver is considered as an ensemble of facets, where each facet is defined by its x and y directional slopes. The wind speed and orientation determine the probability density function of the sea surface facet slope occurrence. In this paper, we have analytically expressed the reflected radiance on the sea surface as a function of the wind speed, receiver range, receiver heading, laser position, laser output aperture, and laser incoming radiance. Using the tolerance ellipse, the reflected radiance expression was approximated, and both direct and approximated results were compared. The richness in behavior of the reflected radiance and its dependence on the geometry of the problem were studied showing the impact of the receiver position, the laser position, heading, and beam divergence.

Experimental study on beam characteristics of μHT-1 thruster under operating conditions adjustment

The micro Hall thruster μHT-1 is designed for space gravitational wave detection missions, and the beam characteristics of the thruster are studied by experiments. A Faraday probe mounted on a three-dimensional moving platform is designed for diagnosing to obtain the beam current density distribution, and the operating conditions of the discharge voltage and the mass flow rate are adjusted in the range of 700∼1200 V and 0.1–0.5 sccm, respectively. In addition, the study further analyze the variation trends of total beam current, discharge current, utilization rate, divergence angle and others. The test results indicate that the μHT-1 thruster can work stably in a wide range of operating conditions, and the beam current expresses a good axisymmetric distribution. Meanwhile, the beam current density gradually decreases along the axial direction and spreads along the radial direction. The current utilization efficiency fluctuates from 53.4 % to 67.7 % when the power is adjusted in the range of 1.45–5.76 W, and the divergence angle increases from 41.3° to 56.7°. The total beam current of the thruster shows a linear increase characteristics with the enhancing of the anode flow and the discharge voltage.

Propagation of hollow higher-order cosh-Gaussian beam in human upper dermis

Optical detection, measurement, and treatment methods are widely used in the medical industry nowadays. The evolution of radiated beams, received power and beam size play vital roles while developing devices. The propagation properties of hollow higher-order cosh-Gaussian beam (HHOCGB) while propagating in human upper dermis tissue are derived analytically and analyzed numerically. The impact of the hollowness parameter, beam order, operating wavelength, and Gaussian beam waists on the beam’s intensity profile is examined. Received power and beam size variations are analyzed considering operating wavelength and Gaussian waist width. According to the results, as the beam propagates, its profile rapidly evolves into a shape with a circular Gaussian peak in the center and petals at the corner. Dark hollow regions are observed among the petals. Furthermore, the received power by HHOCGBs with a higher Gaussian waist width is more than those received by beams with a lower Gaussian waist width. However, at far field, operating at a lower wavelength prevents the increase of the beam spread. Thus, the obtained results will be significant in the bio-optical disease detection and treatment technology development.

EVALUATION OF CONSUMER-GRADE AND SURVEY-GRADE UAV-LIDAR

Abstract. Driven by developments in the automotive industry, the availability of compact consumer-grade LiDAR (Light Detection and Ranging) sensors has increased significantly in recent years. Some of these sensors are also suitable for UAV-based surveying tasks. This paper first discusses the differences between consumer-grade and survey-grade LiDAR systems. Special attention will be paid to the scanning mechanisms used on the one hand and to different solutions for the transceiver units on the other hand. Based on the technical data of two concrete systems, the consumer-grade DJI Zenmuse L1 sensor and the survey-grade scanner RIEGL VUX-1UAV, the expected effects of the sensor parameters on the 3D point cloud are first discussed theoretically and then verified using an exemplary data set in Hessigheim (Baden-Württemberg, Germany). The analysis shows the possibilities and limitations of consumer-grade LiDAR. Compared to the low-cost sensor, the high-end scanner exhibits lower range measurement noise (5–10 mm) and better 3D point location accuracy. Furthermore, the higher laser beam quality of high-end devices (beam divergence, beam shape) enables more detailed object detection at the same point density. With moderate accuracy requirements of 5–10 cm, however, applications in the geodetic-cartographic context also arise for the considerably less expensive consumer-grade LiDAR systems.

Single-Chip Switchable Dual-Wavelength Vertical External-Cavity Surface-Emitting Laser

Dual-wavelength output devices have a wide range of applications in mid-infrared band difference frequency generation, anti-interference lidar, dual-wavelength holographic interferometry, and other applications. Vertical external cavity surface-emitting lasers (VECSELs) are a type of semiconductor laser that can achieve single-chip dual-wavelength output by designing the chip structure. In this paper, we present a single-chip VECSEL that can switch between dual-wavelength and single-wavelength output modes. The VECSEL can simultaneously emit coaxial laser beams at 967 nm and 1013 nm, with a wavelength spacing of about 45 nm. The degree of mismatch between the gain peaks of the two quantum wells in the gain chip and the corresponding cavity modes is different. By adjusting the pump power, the temperature of the active region can be changed, which alters the matching relationship between the gain peaks and the cavity modes and controls the output mode of the VECSEL. The dual-wavelength output mode maintains a stable wavelength spacing at different operating temperatures. The laser output mode can be switched between single-wavelength and dual-wavelength, and the beam divergence angle is less than 8°. The dual-wavelength output power can exceed 400 mW, and the long-wavelength output power can reach up to 700 mW.

Ultra‐Low Threshold Single‐Mode Upconversion Lasing in a Strong Coupled Microcavity via Four‐Photon Absorption

AbstractWith the development of ultra‐fast optical technology, the nonlinear interaction between light and matter has attracted great attention. Upconversion laser generation through multiphoton absorption (MPA) enables numerous critical applications such as quantum signal processing, biophotonics, and 3D microfabrication. Herein, an ultra‐low threshold single‐mode four‐photon absorption (4PA) upconversion lasing at room‐temperature (RT) by constructing a strong‐coupling microcavity is demonstrated. The planar microcavity significantly enhances the 4PA nonlinear interaction, which facilitates the obtainment of low‐threshold upconversion lasing with an excellent output divergence angle and beam quality. In addition, the temporal dynamic mechanism and polarization characteristics of single‐mode upconversion lasing are also comprehensively discussed. The work provides a feasible approach to fabricate a low‐threshold single‐mode upconversion laser through high‐order nonlinear MPA processes.

Design of a Compact Hologram System Capable of 3D Lesion Diagnosis in Clinic.

This paper proposes a small-sized hologram system for the 3D imaging of lesions in a clinical environment. In a general hologram system, the distance between the beam-generating device and the screen (400mm) and the size of the screen must be increased proportionally to obtain excellent image quality. However, in a clinical environment, the beam spread distance and screen size must be reduced. This paper proposes a method for reducing the beam divergence distance and screen size for clinical applications. To reduce the beam spread distance and screen size, a beam prism with a 45° refractive index is used to reduce the beam spread distance by 1/3. The direction of the bent light must be adjusted such that it can reach the screen accurately. However, because the reflected light may be refracted owing to the material properties of the mirror and cause loss, this problem can be solved by using a full reflection mirror. The beam spread distance of the designed hologram system is 200mm. The types of lesions obtained from the 3D images of the hologram include the lung, liver, and colon. The image resolution is 300 × 145. If the proposed method is used in a clinical environment, doctors can improve their understanding of the patient quickly and efficiently; thereby, shortening the treatment time. The proposed hologram system is expected to be useful in treatment rooms, operating rooms, and educational programs in medical schools.

Photonic eigenmodes and transmittance of finite-length 1D cholesteric liquid crystal resonators

Cholesteric liquid crystals exhibit a periodic helical structure that partially reflects light with wavelengths comparable to the period of the structure, thus performing as a one-dimensional photonic crystal. Here, we demonstrate a combined experimental and numerical study of light transmittance spectra of finite-length helical structure of cholesteric liquid crystals, as affected by the main system and material parameters, as well as the corresponding eigenmodes and frequency eigenspectra with their Q-factors. Specifically, we have measured and simulated transmittance spectra of samples with different thicknesses, birefringences and for various incident light polarisation configurations as well as quantified the role of refractive index dispersion and the divergence of the incident light beam on transmittance spectra. We identify the relation between transmittance spectra and the eigenfrequencies of the photonic eigenmodes. Furthermore, we present and visualize the geometry of these eigenmodes and corresponding Q-factors. More generally, this work systematically studies the properties of light propagation in a one-dimensional helical cholesteric liquid crystal birefringent profile, which is known to be of interest for the design of micro-lasers and other soft matter photonic devices.

Power Density Distributions in Electron Beams

Measuring and controlling the power density distribution of electron beams used for welding is critical for producing repeatable welds and for transferring welding parameters between different machines. On any given machine, the power density distribution is controlled by defocusing the beam relative to its sharpest focused condition. However, measuring the power density distribution can be difficult due to the intense nature of welding beams and is further complicated by imperfect electron optics that can distort the beam, making it difficult to quantify. The enhanced modified Faraday cup (EMFC) diagnostic method was used here for beam analysis that utilizes computed tomography to reconstruct the beam’s power density distribution. These results were compared to the International Standards Organization (ISO) method for characterizing laser beams using a second-moment D4σ calculation. For ideal Gaussian-shaped beams, both methods would give the same result. However, for imperfect beams, the calculated D4σ diameter was shown to be about 25% larger relative to the FWe2 diameter measured by the EMFC due to the heavier weighting of data in the tails of the beam by D4σ. Although both methods produce repeatable welds, it is important to understand the differences in the reported beam diameters, divergence angles, and beam parameter products when transferring parameters between machines.

Low-frequency acoustic collimated beam

In recent years, we have developed a variety of unique acoustic sources that can generate a highly collimated acoustic beam (very low beam spread) of low frequency (10-250 kHz) without any side lobes. An acoustic beam with the above characteristics is extremely important and leads to deeper penetration, due to low frequency, and increased lateral resolution, due to collimation. These novel acoustic sources are based on frequency mixing in nonlinear medium, linear array superposition, generation of Bessel-like acoustic beams through radial modes of piezo disks, etc. These sources were used extensively for wellbore integrity monitoring and fracture detection on simulated boreholes in the laboratory. A few different approaches of generating a Low-Frequency Collimated Beam, along with potential applications will be discussed.

Towards Clinical Proton Minibeam Radiation Therapy (pMBRT): Development of Clinical pMBRT System Prototype and pMBRT-Specific Treatment Planning Method

Proton minibeam radiation therapy (pMBRT) is an emerging spatially fractionated RT (SFRT) modality that can provide very high therapeutic index compared to conventional radiotherapy methods and clinically-available SFRT methods (GRID and LATTICE). The biological data collected thus far encourage the preparation of clinical trials in pMBRT. This work is to facilitate the clinical translation of pMBRT by developing (1) the first clinical pMBRT system prototype worldwide, readily available for both small-animal biology studies and large-animal pMBRT trials; (2) pMBRT-specific treatment planning method with peak-valley dose ratio (PVDR) optimization capability for large animal and patient pMBRT trials, which is currently unavailable. The pMBRT system is based on clinically-used pencil-beam-scanning proton machine equipped with large-field clinical-size pMBRT collimator, pMBRT-dedicated treatment planning system, and KV/CBCT imaging guidance. The multi-slit brass collimator has 10 × 10 cm field size, 0.4mm width per slit and 4 mm center-to-center distance. The divergence of slits is tailed to the divergence of the proton beam. A unique universal collimator design is implemented, so that we can keep the outer fitting to the snout and conveniently inter-change collimators as needed. The pMBRT-specific treatment planning method jointly optimizes PVDR and dose objectives, to meet a minimal PVDR threshold and maximize PVDR, to avoid the situations where meeting dose objectives can compromise PVDR when PVDR were not optimized. In addition, the survival fraction for organs at risk is also optimized. The dose calculation engine is based on the Monte Carlo method using TOPAS. The optimization algorithm utilizes total variation and L1 sparsity regularization to maximize PVDR and iterative convex relaxation method to solve the optimization problem. Monte Carlo simulations via TOPAS were performed to design this large-field multi-slit collimator, using the beam structure and the beam data specific to our proton system, with mean dose rate of 8 Gy/min under clinical condition with the collimator in place. The feasibility of using this pMBRT system for small-animal studies has been demonstrated, with customized 3D printed holder for immobilizing small animals, on-board KV imaging system for accurate small-animal positioning, and the GAFchromic film for verifying radiation dose and PVDR. On the other hand, the efficacy of pMBRT-specific treatment planning method (with PVDR optimization capability) to improve PVDR has been demonstrated using retrospective patient planning studies in comparison with standard proton treatment planning method (without PVDR optimization capability). The initial development of a clinical pMBRT system prototype and pMBRT-specific treatment planning method of PVDR optimization capability has been completed with ongoing efforts to make this system ready for large-animal pMBRT studies.

A new Multi Leaf Collimator for the fission neutron therapy facility at FRM II

As a part of the general upgrade program of the fission neutron therapy facility MEDAPP at the research reactor FRM II, a newly designed, electronically operated Multi Leaf Collimator (MLC) was introduced. The design of the new collimator was optimized in order to increase dose output and to allow patient positioning closer to the collimator. The new collimator allows divergent and convergent beam shaping in order to maximize the effective area of the fission source used for irradiation. In comparison to the old MLC, the radiation dose for the staff is reduced by the motorized operation of the new MLC. The advanced beam shaping abilities of the new collimator are investigated by a comparison with depth dose curves and beam profiles of the old MLC. For the separation of neutron and gamma dose deposition in water, the two chamber dosimetry method recommended for mixed field dosimetry was applied. A re-calibration of the set of ionization chambers used for dosimetry in mixed neutron-gamma radiation fields at FRM II was performed. The calibration was done in a reference neutron field from 3H(p,n)3He-reactions in a low scattering environment. An overall increased dose output as intended by the design of the new MLC was shown. Also, improved dosimetric characteristics of the treatment field due to the optimized beam shaping abilities of the new MLC were demonstrated. In the calibration measurements of the magnesium ionization chamber used in the two chamber method, an unexpected increase of the neutron sensitivity by a factor of 8 was measured. The new MLC will enable more conformal dose delivery to the target volume in patients and therefore help to reduce healthy tissue damage by the application of neutron radiation.

Stimulated Raman scattering by circularly polarized quadruple Gaussian laser beam and co-propagating electron beam in plasma

This study investigates the interplay between a co-propagating relativistic electron beam and a quadruple Gaussian laser beam in plasma, focusing on the suppression of stimulated Raman scattering (SRS) growth. The presence of the laser beam induces the excitation of a pair of plasma waves and side-scattered electromagnetic waves. As the side-scattered wave and pump wave couple together, they exert a ponderomotive force on the electron beam and plasma electrons, resulting in an enhancement of the plasma wave amplitude. Nonlinear coupling between the density perturbation in the plasma, induced by the plasma wave, and the pump wave leads to the excitation of a nonlinear current responsible for the growth of the side-scattered electromagnetic wave associated with SRS. Furthermore, the growth rate of SRS is shown to be highly sensitive to the phase matching between the relativistic electron beam and the plasma wave. In cases of phase mismatch, the growth rate experiences a significant reduction. Additionally, the effectiveness of the electron beam in driving the stimulated Raman process is greatly affected by the energy spread of the electron beam. A substantial reduction in effectiveness is observed due to this energy spread.

Mirror Vibration Tolerance Studies in X-ray Free-Electron Laser Oscillator

The X-ray free-electron laser oscillator (XFELO) has received significant attention due to its ability to produce fully coherent, high-brightness, and highly stable X-ray beams. Despite these advantages, the operation of the XFELO can be impeded by the surrounding environment. Specifically, vibrations of the optical components within the cavity can lead to poor alignment, which can diminish the interaction between the light and electrons in the undulator. Consequently, the quality of the output X-rays may be compromised. This study aims to investigate the impact of mirror vibrations on the output laser at various vibration frequencies. Firstly, we develop three single-frequency vibration models at 10 Hz, 0.01 MHz, and 1.1 MHz to investigate the changes in energy, spectral width, beam size, and beam divergence angle of the output laser. Secondly, we build a more complex multi-frequency vibration model based on the single-frequency one to simulate the realistic vibration of the mirror. Finally, we utilize the multi-frequency vibration model to investigate the tolerance limits of the output laser to vibration amplitude at different vibration frequencies of the mirror. The results show that the tolerance of the amplitude near the low and middle frequencies has less effect on the output power, which is approximately 250 nrad or more. However, in certain particular instances, particularly in the vicinity of the resonant frequency, there will be deviations from the tolerance limit. These deviations can result in values that are excessively high or excessively low. The study could prove useful in the future installation of XFELOs.

Propagation Characteristics of a Partially Coherent Gaussian Schell‐model Array Vortex Beam in the Joint Turbulence Effect of a Jet Engine and Atmosphere

AbstractThis work investigates the joint effects of jet engine exhaust‐induced turbulence and atmospheric turbulence on the propagation of a partially coherent Gaussian Schell‐model Array (GSMA) vortex beam. Using the two‐process propagation method, analytical formulae are derived for the cross‐spectral density, spectral density, degree of coherence, and beam width of the considered beam. The results show that the considered beam takes different shapes; when the spatial coherence is large, the spectral density of the GSMA vortex beam takes an elliptical shape, whereas when the spatial coherence is smaller, the spectral density remains a Gaussian shape. The evolution profile of the degree of coherence weakens gradually when the propagation distance, topological charge, and turbulence strength increase. Moreover, the profile of the degree of coherence takes the Gaussian profile when the propagation distance is longer or turbulence atmospheric is stronger. Furthermore, the results reveal that the corresponding beam spreads faster with a larger propagation distance, lower spatial coherence, and high‐strength turbulence. This study also concludes from the results that the beam is affected more when its propagation is near the jet engine exhaust, which means that this latter has a significant impact.

Designing a sky-scanning Fabry–Perot interferometer system with a large size integrating sphere

Integrating spheres can be manufactured at sizes ranging from 1 mm to 3 m in diameter. It has been agreed widely by manufacturers that fitting a large-sized integrating sphere as per the customer requirement is a complex process. Thus, these spheres due to its large beam divergence from the exit port, have never been deployed for the calibration of aeronomy applications. In this article a optical system is described to integrate a large-sized integrating sphere with the Fabry–Perot etalon and thus devise a set-up to generate the concentric ring pattern and record it on the detector. The experimental set-up was verified by optical simulation using ZEMAX (OpticStudio) so as to validate the system’s performance in the laboratory. The important parameter finesse is calculated and its accuracy is found to be 80%.

Unusual dynamics and nonlinear thermal self‐focusing of initially focused magnetoacoustic beams in a plasma

AbstractUnusual thermal self‐focusing of two‐dimensional beams in plasma which axis is parallel to the equilibrium straight magnetic field is considered. The equilibrium parameters of plasma determine scenario of a beam divergence (usual or unusual) which is stronger as compared with a flow without magnetic field. Nonlinear thermal self‐action of a magnetosonic beam behaves differently in the ordinary and unusual cases. Damping of wave perturbations and normal defocusing in gases leads to reduction of the magnitude of initially planar perturbations at the axis of a beam. Additional thermal self‐focusing nonspecific for gases occurs in plasma under some condition which counteracts this reduction. The theory and numerical examples concern thermal self‐action of initially focused (defocused) magnetosonic beam. Dynamics of perturbations in a beam is determined by dimensionless parameters responsible for diffraction, damping of the wave perturbations, initial radius of a beam's front curvature, and the ratio of viscous to thermal damping coefficients.