High Beam Divergence Research Articles

Benchmarking Under- and Above-Canopy Laser Scanning Solutions for Deriving Stem Curve and Volume in Easy and Difficult Boreal Forest Conditions

The use of mobile laser scanning for mapping forests has scarcely been studied in difficult forest conditions. In this paper, we compare the accuracy of retrieving tree attributes, particularly diameter at breast height (DBH), stem curve, stem volume, and tree height, using six different laser scanning systems in a managed natural boreal forest. These compared systems operated both under the forest canopy on handheld and unmanned aerial vehicle (UAV) platforms and above the canopy from a helicopter. The complexity of the studied forest sites ranged from easy to difficult, and thus, this is the first study to compare the performance of several laser scanning systems for the direct measurement of stem curve in difficult forest conditions. To automatically detect tree stems and to calculate their attributes, we utilized our previously developed algorithm integrated with a novel bias compensation method to reduce the overestimation of stem diameter arising from finite laser beam divergence. The bias compensation method reduced the absolute value of the diameter bias by 55–99%. The most accurate laser scanning systems were equipped with a Velodyne VLP-16 sensor, which has a relatively low beam divergence, on a handheld or UAV platform. In easy plots, these systems found a root-mean-square error (RMSE) of below 10% for DBH and stem curve estimates and approximately 10% for stem volume. With the handheld system in difficult plots, the DBH and stem curve estimates had an RMSE under 10%, and the stem volume RMSE was below 20%. Even though bias compensation reduced the difference in bias and RMSE between laser scanners with high and low beam divergence, the RMSE remained higher for systems with a high beam divergence. The airborne laser scanner operating above the forest canopy provided tree attribute estimates close to the accuracy of the under-canopy laser scanners, but with a significantly lower completeness rate for stem detection, especially in difficult forest conditions.

Comparing mobile and terrestrial laser scanning for measuring and modelling tree stem taper

Abstract Measuring and modelling the shape of tree stems is a fundamental component of forest inventory systems for both commercial and biological purposes. The change in diameter of the stem along its length (a.k.a. 'taper') is one of the most important and widely used means of predicting tree stem volume. Until recently, the options for obtaining accurate estimates of stem taper and developing stem taper models have been limited to measurements of felled trees or the use of optical dendrometers on standing live trees. Here, we tested both a tripod-mounted terrestrial laser scanner (TLS; a Focus 3D 120 of FARO Technologies, Inc., Lake Mary, FL, USA), and a mobile laser scanner (MLS; the ZEB1 of the GeoSLAM Ltd, Nottingham, UK) to measure tree diameters at various heights along the stem of 20 destructively harvested broadleaf and needleleaf species using the outer hull modelling method, for the purpose of developing individual-tree and species-specific taper models. Laser scanner specifications were a major factor determining stem taper measurement accuracy. The longer-range, low beam divergence TLS could estimate stem diameter to an average of 15.7 m above ground (about 79 per cent of the canopy height), while the shorter-range high beam divergence MLS could estimate an average of 11.5 m above ground (about 45 per cent of the canopy height). Stem taper error increased with respect to height above ground, with the TLS providing more consistent and reliable diameter measurements (root mean square error (RMSE) = 1.93 cm; 9.57 per cent) compared with the MLS (RMSE = 2.59 cm; 12.84 per cent), but both methods were nearly unbiased. We attribute ~60 per cent of the uncertainty in stem measurements to laser beam diameter and point density, showing positive and negative correlations, respectively. MLS was unable to converge on the two tested taper models but was found to be an efficient means of easily sampling diameters at breast height (DBH) and reconstructing stem maps in simple forest stands with trees greater than ~10 cm DBH. TLS provided precision stem diameter measurements that allowed for the creation of similar taper models for three out of the four study species. Future work should focus on evaluating MLS systems with improved specifications (e.g. beam divergence and range), since these instruments will likely lead to dramatic improvements in reliable estimates of forest inventory parameters, in line with the current TLS technology.

Nanofocused x-ray photon correlation spectroscopy

Here, we demonstrate an experimental proof of concept for nanofocused x-ray photon correlation spectroscopy, a technique sensitive to nanoscale fluctuations present in a broad range of systems. The experiment, performed at the NanoMAX beamline at MAX IV, uses a novel event-based x-ray detector to capture nanoparticle structural dynamics with microsecond resolution. By varying the nanobeam size from σ=88 nm to σ=2.5μm, we quantify the effect of the nanofocus on the small-angle scattering lineshape and on the diffusion coefficients obtained from nano-XPCS. We observe that the use of nanobeams leads to a multifold increase in speckle contrast, which greatly improves the experimental signal-to-noise ratio, quantified from the two-time intensity correlation functions. We conclude that it is possible to account for influence of the high beam divergence on the lineshape and measured dynamics by including a convolution with the nanobeam profile in the model.Received 17 December 2021Revised 19 May 2022Accepted 16 June 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.L032012Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsNanoparticlesSynchrotron radiation & free-electron lasersTechniquesCoherent X-ray scatteringX-ray photon correlation spectroscopyCondensed Matter, Materials & Applied PhysicsPolymers & Soft Matter

Mode engineering of semiconductor lasers with vertical periodic layered structures

High-power and low-divergence semiconductor lasers are useful in light detection and ranging, as well as material processing and so on. In this paper, an efficient approach is presented to explain the physics of mode selection in semiconductor lasers with vertical periodic layers for high power and low vertical beam divergence. The transmission matrix method is used to obtain the allowed band and forbidden band of the vertical periodic layered structure, and the band diagram of the layered structure of the semiconductor laser. The fundamental mode of the layered structure of the semiconductor laser lies in the forbidden band of the vertical periodic layered structure. Tapered lasers based on the designed layered structure are fabricated and can achieve an output power of 2.45 W. The measured vertical divergence angle of the tapered laser is below 10°, consistent well with the calculated result of 9.5°.

Liquid crystal-optical phased arrays (LC-OPA)-based optical beam steering with microradian resolution enabled by double gratings.

We demonstrate an optical beam steering setup based on a liquid crystal-optical phased array (LC-OPA) with high angular resolution and low beam divergence, due to the use of double-grating configuration to enhance the corresponding angular resolution of LC-OPA. The introduction of two nonparallel blazed gratings with a special included angle can achieve multiple diffractions for the incident light, leading to the realization of angular compression. Numerical simulation results show the angular compression ratio can be optimized by selecting the appropriate grating constant and the included angle of double-grating. Experimental results verify that the steered angle of the incident light can be compressed so that the angular resolution of the steered beam can correspondingly be improved more than six times. In addition, when the beam is multiply diffracted within the double-grating configuration, the divergence angle becomes smaller as the beam size is enlarged. Therefore, both the microradian steering resolution and the narrow beam divergence can be simultaneously obtained with our proposed setup. The efficiency of the steering where the beam diffracted four times within the double-grating configuration is 80%.

Status, scientific results and technical improvements of the NBH on TCV tokamak

The TCV tokamak contributes to physics understanding in fusion reactor research by a wide set of experimental tools, like flexible shaping and high power ECRH. A 1MW, 25 keV deuterium heating neutral beam (NB) has been installed in 2015 and it was operated from 2016 in SPC-TCV domestic and EUROfusion MST1 experimental campaigns (˜50/50%). The rate of failures of the beam is less than 5%.Ion temperatures up to 3.5 keV have been achieved in ELMy H-mode, with a good agreement with ASTRA predictive simulations. The NB enables TCV to access ITER-like βN values (1.8) and Te/Ti ˜1, allowing investigations of innovative plasma features in ITER relevant ELMy H-mode. The advanced Tokamak route was also pursued, with stationary, fully non-inductive discharges sustained by ECCD and NBCD reaching βN˜1.4–1.7.Real-time control of the NB power has been implemented in 2018 and presented together with the statistics of NB operation on the TCV. During commissioning, the NB showed unacceptable heating of the TCV beam duct, indicating a higher power deposition than expected on duct walls. A high beam divergence has been found by dedicated measurement of 3-D beam power density distribution with an expressly designed device (IR measurement on tungsten target).

Performance Analysis of a Low-Power Magnetically Shielded Hall Thruster: Computational Modeling

The applicability of a fully shielding magnetic field topology to a low-power xenon Hall thruster was demonstrated through testing of the MaSMi-60. Although the discharge channel lifetime was significantly increased, performance testing of the device revealed a peak anode efficiency of under 30%, which was lower than expected given the available data on high-power magnetically shielded Hall thrusters. Experimental measurements in a vacuum facility with operating pressures of suggest that the MaSMi-60’s current utilization, mass utilization, and beam divergence efficiencies were the key contributors to its moderate performance. To better understand the physics causing these performance deficiencies, a computational analysis including 2D plasma modeling of the MaSMi-60 was conducted. Results from the 2D numerical models confirmed that the MaSMi-60 achieved the parameters necessary for magnetic shielding. The physics governing the low mass utilization, current utilization, and beam divergence efficiencies were then identified and described using the computational model results. The low mass utilization is attributed to a long ionization mean free path in the discharge channel caused by the predominantly axial trajectory of the injected propellant. Insufficient magnetic field strength enabling excessive electron current to the anode was the primary cause for the poor current utilization. Lastly, the high beam divergence was due to an overly shielding magnetic field topology that promoted high-energy ions to be accelerated far off the thruster’s axis.

SIMULATION OF LASER TRANSMITTER BASED ON ND: YAG WITH PASSIVE COOLING FOR RANGEFINDING

Laser emitters used in rangefinders without cooling have several limitations: low energy, high beam divergence at the cavity exit that leads to large output telescope design that significantly increases rangefinder’s size, long time beam parameters instability due to laser crystal heating. In the article laser emitter for a transmitting unit of rangefinder based on cylindrical laser medium Nd:YAG with transverse diode pumping abled to solve all the abovementioned promblems is considered. Matrix optics and balance equations were used for system modeling. The emitter is based on plano-concave scheme cavity with internal concave lens aimed for matching the size of TEM00 mode with the size of laser medium and cavity tuning to stability threshold which allows to achieve divergence of the output pulse at diffraction level for 5 mm diameter medium and 300 mm cavity length. Cavity reconfiguration and mode size compliance is conducted by the longitudinal movement of lens relative to the rear mirror. Three-, twoand one-sided pump schemes are compared in terms of rod energy absorption uniformity and their effectiveness. The three-sided pump scheme model with a system of gold coated reflectors was designed that provides 54 % efficiency of pump when the pump energy is distributed uniformly along the cross-section of the laser medium. Based on the modeling in ANSYS the scheme of laser medium thermal control in vacuum conditions was suggested that implies introduction heat-conducting glue between the side edge of a laser medium and quantron body that allows to low the active medium heating in the 8 min cycle by 35 °С. Beam pulse energy under pump of 1.2 Joule equals to about 0.3 Joule with the 4 ns pulse time and 2 Hz in frequency.

Plume Control of a Cusped Field Thruster

The cusped field thruster uses several permanent ring magnets of alternating polarity to create a multistage cusp magnetic topology. It has demonstrated a long lifetime because of reduced plasma losses to the discharge channel surface. However, the high beam divergence problem has not been solved in this type of thruster. The purpose of this technical note is to describe a plume control method to reduce its plume angle. The results show that the magnetic field intensity in the plume region and the shape of the last separatrix have great influence on plume divergence.

Neutron Optics and Physics Beamline (J-PARC BL05)

A pulsed cold neutron beam for the study of neutron optics and fundamental physics (NOP) is under construction as the BL05 of the spallation neutron source of the J-PARC. The NOP beamline has three beam branches, which provide the highly-polarized beam, high intensity beam and low divergence beam for the neutron decay experiment, the neutron scattering experiment and the neutron interferometry experiment, respectively. We overview the physics to be studied at the NOP beamline at its beginning stage.

ION ACCELERATION USING CIRCULARLY POLARIZED PULSES: PHYSICS AND POSSIBLE APPLICATIONS

The acceleration of ions in the interaction of ultrashort, high intensity, circularly polarized laser pulses with overdense plasmas has been theoretically investigated. By using particle-in-cell (PIC) simulations it is found that high-density, short duration ion bunches moving into the plasma are promptly generated at the laser-plasma interaction surface. This regime is qualitatively different from ion acceleration regimes driven by fast electrons such as sheath acceleration at the rear side of the target. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch velocity and generation time as a function of pulse intensity and plasma density. The ion bunches have moderate energies (100 keV-1 MeV) but very high density and low beam divergence, and might be of interest for problems of ultrafast compression, acceleration or heating of high–density matter. In particular, we have studied their application to the development of compact sources of fusion neutrons. We analyzed two target schemes showing that intense neutron bursts with femtosecond duration are produced.

A compact tunable narrow-band LD based on emission through the substrate and an external abnormal-reflection mirror

Due to the high beam divergence of standard laser diodes (LDs), these are not suitable for wavelength-selective feedback without extra optical elements. This letter reports on first prototypes of compact tunable LDs with wavelength-selective feedback via a so-called abnormal reflecting mirror of a low divergent beam coming from a special LD, emitting through the substrate. With this setup, tunable narrow-band (<0.7 nm) lasing is observed showing good temperature stability. Tuning over a range of 5 nm at a maximum power of 0.2 W was demonstrated.

Simulation of heat generation for transcatheter Nd-YAG laser photocoagulation of myocardium

Nd-YAG laser photocoagulation is a successful technique for the intraoperative treatment of arrhythmia. Experimental endocardial transcatheter laser application, however, occasionally causes tissue vaporization and perforation. Intramural heat generation was simulated in a Monte Carlo model as a function of laser beam profile and divergence versus individually varying tissue properties. Heat generation at the tissue surface was dominated both by the beam profile and the individual tissue properties. High beam divergence may help to reduce heat accumulation in subsurface regions. Heat distribution in depth as well as backscattering are not substantially influenced by beam parameters but are mainly determined by the optical tissue properties. Surface colling can help to avoid overheating. A catheter, concept for safe endocardial photocoagulation is presented including a highly divergent beam of maximized, spot size, an effective catheter flushing, and shielding of blood.

Scientific opportunities at the proposed 6–7 GeV advanced photon source

The availability of synchrotron radiation from bending magnet sources has contributed in a major way to new research in physics, chemistry, biology, materials science, and technologies. The newly proposed 6–7 GeV Advanced Photon Source based on insertion device sources (undulators and wigglers) will enhance the research capability in all these areas many-fold and provide new opportunites in even wider ranges of scientific and technological research. The characteristics of the synchrotron radiation from the insertion devices on a 6–7 GeV Advanced Photon Source will be discussed, highlighting their high brilliance and low beam divergence. Some of the unique scientific opportunities provided by such an insertion-device-based X-ray source will be presented.

Twin channel laser with high cw power and low beam divergence

We report on a new GaAlAs/GaAs semiconductor laser, the Twin Channel Laser, consisting of two closely spaced channel substrate planer (CSP)-type lasers having high cw power (50–80 mW), high differential efficiency (45–55%), low beam divergence (4–6° by 25°), almost single longitudinal mode operation at high output power (50 mW), and high T0 (180°&amp;lt; T0 &amp;lt; 200°). The favorable output characteristics are related to the strong coupling between the individual emitters which is a consequence of the large optical gain between the emitters.

Ion beam extraction from a plasma with aberration reduction by method of mutual exclusion

Using an algorithm for solution of the Poisson-Vlasov equations for ions extracted from a plasma, an electrode shape is considered which has high optimum perveance and low beam divergence.

Influence de la temperature sur les proprietes optiques du verre fondu contenant des oxydes de metaux de transition

An original method is proposed for the determination of the influence of temperature on optical properties of molten glass containing transition metal oxides. Two high-power continuously tunable lasers are the light sources, and a new cell system is used which allows measurements through molten glass with a windowless system. The laser's high spectral radiant intensity and low beam divergence provide very high resolution good accuracy, and a simpler optical arrangement. Temperature and atmosphere are controlled in two automatically powered furnaces by thermocouples and radiation pyrometry. Transmissitivity and reflectivity measurements, made almost at the same time, are used to compute absorptivity and specific extinction factors of colouring agents. Results are reported for some industrial soda-lime-silica glasses containing iron in various concentrations.

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The availability of synchrotron radiation from bending magnet sources has contributed in a major way to new research in physics, chemistry, biology, materials science, and technologies. The newly proposed 6–7 GeV Advanced Photon Source based on insertion device sources (undulators and wigglers) will enhance the research capability in all these areas many-fold and provide new opportunites in even wider ranges of scientific and technological research. The characteristics of the synchrotron radiation from the insertion devices on a 6–7 GeV Advanced Photon Source will be discussed, highlighting their high brilliance and low beam divergence. Some of the unique scientific opportunities provided by such an insertion-device-based X-ray source will be presented.