Longitudinal Intensity Distribution Research Articles

Cutting thick aluminum plates using laser fusion cutting enhanced by dynamic beam shaping

Cutting thick plates is affected not only by the laser power but also by the cut kerf width and the melt flow dynamics that determine the ejection of the molten material. Employing the same laser beam intensity distribution for various thicknesses is the limiting factor when cutting thicker plates. This paper investigates fiber laser fusion cutting of 25 mm thick aluminum with dynamic beam shaping (DBS). While both static and longitudinal dynamic intensity distributions fail to cut this thickness with a 4 kW laser power, a cut through is achieved using annular and elliptical intensity distributions. However, an improvement of 45% in cutting speed can be achieved using an elliptical intensity distribution compared to an annular one. In order to understand the effect of the beam shape, an infrared thermal camera is used to study lateral heat propagation when using different process parameters. Moreover, to analyze the melt flow when changing the DBS frequency, high-speed imaging is utilized to observe the molten material inside the cut kerf. Finally, the cut edge quality is investigated for different cutting conditions.

Experimental study of turbulence intensity on the wake characteristics of a horizontal-axis wind turbine

ABSTRACT A time-resolved particle image velocity (TR-PIV) system was used to test the wake flow of a small horizontal-axis wind turbine at tip speed ratios (TSRs) in the range of 4–6 under uniform inflow and turbulent inflow in order to study the impact of turbulence intensity on wind turbine wake characteristics. The impact of incoming turbulence on the wake structure of a wind turbine and its mechanism was examined by comparing and analyzing the turbulence properties of the wake flow, including velocity field, vortex structure, additional turbulence intensity, and energy distribution coefficient. It was found that tip vortex structure remained intact in the range of 9 R under uniform inflow, and the longitudinal distribution of turbulent intensity had double peaks, indicating that it was in the near wake zone. Under turbulent inflow, the three vortex systems could not be distinguished at the position of 5 R, and the longitudinal distribution of turbulent intensity did not have peaks, the wake vortex system was broken and the wake transitioned to the far wake zone. With the increase in the turbulence intensity and tip speed ratio, the contribution of the longitudinal component of turbulence intensity to the turbulent kinetic energy increases, which is also the reason for the rapid wake recovery. Additionally, the recovery of the wake velocity was sped up in the range of 3 R and gradually decreased after 3 R.

Longitudinal optical trapping and manipulating Rayleigh particles by spatial nonuniform coherence engineering

Focused structured light fields with spatially nonuniform correlation can exhibit multiple focal points in the longitudinal direction, which suggests that they are suitable for longitudinal optical trapping. Due to the controllable nature of the longitudinal intensity distribution, this class of structured light fields not only can trap two high-index particles simultaneously, but also can manipulate them in the longitudinal direction.

The formation of Bessel light beams at large distances from annular fields

In this work, the process of transformation of an annular beam in a Bessel-like field due to diffraction during propagation in a free space over long distances and due to the focusing effect is investigated. A number of models of annular fields are considered, including an analytical model in the form of a polynomial function in a bounded region of space, as well as an experimentally implemented model based on a scheme with two axicons. A comparison is made of the transverse and longitudinal intensity distributions for these models, and a high degree of stability of the structure of the longitudinal distribution of the axial intensity to a change in the model of the annular field is found. This distribution is characterized by the presence of an intense maximum with an asymmetric profile, the appearance of which is not connected with lens focusing. In the initial region of the pointed maximum, the process of formation of a Bessel beam from an annular beam arises, and a sharp increase in intensity takes place. It is also established that the focusing of an annular field at large distances essentially differs from focusing at short distances. In the case of large distances, the increase of the axial intensity does not take place in the vicinity of the focal plane, but much closer to the transmitter, and here the great increase of intensity caused by direct focusing is not identified. The transverse profile of a Bessel-like beam is calculated at large distances. It is shown that this profile is characterized by a small number of lateral rings, and the axial maximum and the first ring contain more than 90% of the light power. The problem of generation of a model annular field by a Fourier-type resonator with a special transparency mirror is considered.

Experimentally exploring the terahertz radiation in a long filament by a two-color laser field

We have experimentally investigated the propagation effect of terahertz (THz) radiation from a long filament by checking the evolution of THz temporal waveform through shifting the longitudinal position of a short filament. The results show that the relative delay and carrier-envelope phase of the THz waveform change linearly with the shifted distance of the filament. After measuring the longitudinal intensity distributions of laser fields in long filament, we reconstruct successfully the process of the polarization-controlled THz radiation in a long filament. Besides, we also discuss the efficiency and polarization of THz radiation from a long filament excited by a circularly polarized two-color laser field.

Sound field measurement and evaluation research for radiated acoustic fields in amplitude-variable sonochemical systems

The content of this study is based on the background of sonochemistry processes. First, the hydrophone is used to measure the sound pressure in the reactor (the experimental ultrasonic frequency range is 20 ± 1 kHz). The sound pressure signals are processed by threshold method, Kalman filter algorithm, and five-point three-time smoothing method. These methods eliminate singular items in the sound pressure signal, filter noise, smooth burrs, and so on. In addition, the sound intensity value is calculated by dual hydrophone method. In this paper, the average intensity of ultrasonic cavitation is studied. The radial distribution and longitudinal distribution of cavitation intensity of the same tool head are studied, and the effects of different power and different tool heads on cavitation intensity are also discussed in this paper. The experimental results show that the instrument designed in this paper can effectively measure the distribution of ultrasonic sound field and evaluate the performance of the tool head. We can infer from the experiments that the performance of the diamond tool head is better than that of the nine-section whip and the dumbbell tool head.

An Experimental Study of Turbulent Structures in a Flat-Crested Weir-Type Fishway

Fishways can assist fish species to overcome obstacles for performing spawning, feeding, and overwintering migrations. Flow structures in a flat-crested weir-type fishway were experimentally studied. Variations of time-averaged velocity, turbulence intensity, and Reynolds stress with longitudinal and vertical directions and flow rates were analyzed. Also, flow patterns in the longitudinal profile were given. The experiments were carried out in a large scale fishway model in the Hydraulics Laboratory at Zhejiang University of Technology. Two typical flow rates corresponding to detection and preference velocities of fish species were considered. Five different horizontal planes for each flow rate were taken. Eleven transverse lines were arranged for each horizontal plane. Ten measuring points were laid along each transverse line. Three-dimensional velocity at each measuring point was measured by acoustic Doppler velocimeter (ADV). Longitudinal and vertical time-averaged velocity distributions, longitudinal turbulence intensity distribution on the different horizontal planes, vertical turbulent intensity distribution along flow depth, and Reynolds stress distributions on the different horizontal planes and on the different cross-sections in the pool of fishway were analyzed. The experimental results showed that distribution of longitudinal velocity was characterized by topology, which constituted an apparent vertical vortex. Weir flow exhibited skimming flow in the fishway pool. Peak-value range of longitudinal turbulent intensity existed. The amplitude of variation in Reynolds stress near the surface layer reached the maximum, which provided a certain hydraulic condition for fish that favor jumping near the surface layer. This study uncovers three-dimensional flow structures, especially for turbulence characteristics, which can contribute to improving the design of crested weir fishway and to assisting fish species to pass smoothly through fishway, being of potential application value.

Distribution of erosion intensity in the Jingjiang reach influenced by the Three Gorges Dam

AbstractElucidation of the fluvial processes influenced by dams provides better understanding of river protection and basin management. However, less attention has been given to the erosion intensity distribution of riverbeds and its association with channel morphology and hydrological conditions. Based on hydrological and topographic data, the spatial and temporal distributions of erosion intensity (2002–2014) influenced by the Three Gorges Dam (TGD) were analyzed for the Jingjiang reach of the Yangtze River. The mechanisms underlying the distribution of erosion intensity in response to hydrological conditions were investigated. The results are as follows: (1) The erosion intensities of different discharges were not uniform, and moderate flow (10 000–27 000 m3/s) produced the largest erosion magnitude among all flow ranges. Owing to the hydrological changes caused by flood reduction and prolongation of moderate flow duration after the TGD began operating, up to 70% of the erosion amount was caused by moderate flows. (2) The lateral distribution of erosion intensity was extremely uneven, as the proportion of cumulative erosion of the low‐flow channel within the bankfull channel reached 88% in 2013. This caused the channel to become narrower and deeper. (3) The longitudinal distribution of erosion intensity was inhomogeneous. The erosion intensity in the wide reaches was greater than that in the narrow reaches, leading to smaller differences in channel morphology along the river. (4) Changes in hydrological conditions influenced by the TGD, significant reduction of sediment concentration along with flood abatement, and increased duration of moderate flow discharges were the main factors affecting erosion distribution in the post‐dam period. Our conclusions can be applied to the Yangtze River as a basis for riverbed change estimations, and river management strategies. Copyright © 2018 John Wiley & Sons, Ltd.

Controllable spatial array of optical needles with independent axial intensity distributions for laser microprocessing

Due to their high length to width ratio nondiffracting beams are usually perceived as “optical needles”. We construct an “optical needle” with an arbitrary longitudinal intensity distribution and change the spatial position of it in the focal region of a lens. Next, we introduce a spatial array of independent “optical needles” and report on physical limitations due to mutual interference of individual beams. We employ a spatial light modulator as a toy model of an actual geometrical phase element and experimentally observe controllable spatial arrays with various numbers and spatial separations of individual beam. Lastly, we examine the distortions caused by propagation through planar air-dielectric interface and attempt to compensate it.

Size-controllable self-imaging of one-dimensional fractal grating

Fresnel diffraction of one-dimensional curved fractal grating is studied, and the reduced, identified and magnified Talbot images of fractal grating are obtained by the proposed optical system. Two kinds of fractal gratings formed by adding and multiplying periodic gratings are chosen as examples. The analytic expressions of the Fresnel diffraction for one-dimensional fractal gratings are deduced, and the size-controllable self-images are predicted to appear at the modified Talbot distance. The longitudinal and transverse diffraction intensity distributions of the curved fractal gratings are simulated. The uniformity of the longitudinal diffraction carpets can be controlled by changing the system parameters, and the transverse diffraction distributions take on the compressed, exact, and magnified images of fractal grating. These size-controllable Talbot imaging of fractal grating may extend the applications of fractal grating because of the additional flexibility.

Longitudinal intensity distribution near the focus produced by light through scattering media

Speckle pattern is formed when coherent light passes through scattering media. It has been demonstrated that after appropriately optimizing the phase of the incident light, a bright focal spot in the target point can be obtained for the case that the light passes through the scattering medium. However, until now the focused intensity distribution near the focus seems unclear. In this paper we experimentally investigate some factors influencing the longitudinal intensity distribution near the focus in details. It is shown that the desired longitudinal focused intensity distribution can be obtained by optimizing the incident light. The results may have potential applications in particle manipulation and laser processing, etc.

强聚焦混合偏振矢量光束在瑞利粒子上产生的辐射力

The radiation force of highly focused hybridly polarized beams on a Rayleigh particle was theoretically studied and the expression was derived.The influence of numerical apertures of an objective and radial coefficient on the radiation force distribution and the longitudinal intensity distribution on the focal plane were numerically simulated.The numerical results show that radiation forces by highly focused hybridly polarized vector beams can be used to trap three-dimensional Rayleigh particles,and the trapping property is affected by the numerical aperture and radial coefficient obviously.Copious experimental data indicate that it can be done only when the radial coefficient is greater than 3.

Talbot effect of curved grating

Fresnel diffraction of curved grating with the parabolic cylinder, the cylindrical surface or the elliptic cylinder profile, is studied theoretically and experimentally. The transmittance function of grating with the smaller curvature is expressed by Fourier series and the mathematic expression of the diffraction intensity in Fresnel region is deduced. The influence of the curvature of grating on the diffraction distribution is analysed theoretically. The simulation results show the longitudinal intensity distribution of curved grating takes on the divergence structure, and this structure gets clearer with increase of the curvature degree. The transverse distribution at Talbot distance deviates from the grating structure and the magnified image of grating appears at modified Talbot distance. One practical experiment is conducted and the experimental results verify these theoretic predictions. The work of this paper will be helpful for understanding the diffraction of curved grating and prompting the application of Talbot effect of grating in wave-front sense and optical encode.

Generation of three-dimensional optical bottle beams via focused non-diffracting Bessel beam using an axicon

In this paper, a new method is proposed to generate self-image three-dimensional (3-D) optical bottle beam (OBB) by focusing non-diffracting Bessel beam using an axicon. The theories of Hankel wave and interference are used to analyze the situation results of the beam after focusing Bessel beam by an axicon, the period of the self-imaged 3-D OBB is calculated. The longitudinal intensity distribution and the cross-sectional intensity distribution of the self-imaged 3-D OBB which contained a series of null intensity points along the propagation axis are numerically studied. Optical system is designed to prove the theoretical simulation. Furthermore, the self-reconstruction property of a self-imaged 3-D OBB is numerically studied and the minimum self-reconstruction distance is determined. The experimental results agree well with the theoretical predictions.

Angular spectrum analysis of near-field diffraction of a small aperture

The near-field diffraction of a nanometer aperture is studied in this paper according to the angular spectrum theory. The mathematic expressions of the near-field diffraction are presented, and the influences of the polarization of the electromagnetic field and the evanescent wave on the diffraction in near field are analyzed by means of vector and scalar diffraction theory, respectively. The transverse and longitudinal diffraction intensity distributions of a small aperture are numerically calculated, and the results show the near-field diffraction of a small aperture is influenced strongly by the vector property of the electromagnetic field and the evanescent component.

THE ORIGIN OF THE 6.4 keV LINE EMISSION AND H 2 IONIZATION IN THE DIFFUSE MOLECULAR GAS OF THE GALACTIC CENTER REGION

We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H_2ionization in the diffuse molecular gas of the Galactic Center (GC) region. We show that Fe atoms and H_2 molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A$* lasted at least several hundred years and released a mean 2-100 keV luminosity > 10^38} erg s^{-1}. The H_2 molecules of the diffuse gas can not be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.

Diffractive optical elements with radial four-level microrelief fabricated by two-photon polymerization

The two-photon polymerization technique is applied for the fabrication of diffractive optical elements (DOE) with a four-level microrelief. These DOEs form longitudinal intensity distribution (axial light segment) with dimensions required for ophthalmological applications.

Impressive laser intensity increase at the trailing stage of femtosecond laser filamentation in air

The longitudinal distribution of the laser peak intensity inside a half meter long femtosecond laser filament in air is studied by measuring the signal ratio of two nitrogen fluorescence lines, 391 nm and 337 nm. The experimental results reveal that laser peak intensity initially remains almost constant (~4.3 × 10(13) W/cm2) inside the filament. However, before the end of the filament, surprisingly the laser intensity undergoes dramatic increase. A maximum intensity as high as 2.8×10(14) W/cm2 could be reached. The experimental result is unexpected by the conventional intensity clamping scenario, according to which the laser peak intensity would feature low variation along a filament. The experimental result is then interpreted as being due to the generation of a short pulse at trailing stage of the filamentation with reduced diameter. This phenomenon might be of great interest owing to its potential application in high-order-harmonic generation and producing isolated single attosecond laser pulse through simple experimental approach.

Study on Refractive-Diffractive Logarithm Axicons in mm/submm Range

Large depth of field and uniform axial intensity distributions are important in many practical applications. Lots of optical elements have been proposed for this end, such as full-aperture logarithmic axicons (LAs), annular-aperture LAs and apodized annular-aperture LAs. In millimeter/submillimeter (mm/submm) range, the refractive-diffractive behaviors (including transverse and longitudinal intensity distributions) behind these axicons illuminated by a plane wave are calculated by rigorous electromagnetic analysis method in our paper. Many important features of these axicons are concluded by comparing their refractive-diffractive field distributions

Study on Refractive-Diffractive Logarithm Axicons in mm/submm Range

Large depth of field and uniform axial intensity distributions are important in many practical applications. Lots of optical elements have been proposed for this end, such as full-aperture logarithmic axicons (LAs), annular-aperture LAs and apodized annular-aperture LAs. In millimeter/submillimeter (mm/submm) range, the refractive-diffractive behaviors (including transverse and longitudinal intensity distributions) behind these axicons illuminated by a plane wave are calculated by rigorous electromagnetic analysis method in our paper. Many important features of these axicons are concluded by comparing their refractive-diffractive field distributions