Intensity Distribution Of Beam Research Articles

Generation of dark hollow beams used in sub-diffraction-limit imaging in coherent anti-Stokes Raman scattering microscopy

Profile of a dark hollow beam in sub-diffraction -limit imaging is of crucial importance for its spatial resolution when using the coherent anti-Stokes Raman scattering microscopy, as far as the imaging quality is concerned. Therefore, the generation of dark hollow beams through a vortex phase plate will be theoretically analyzed based on the Fresnel diffraction theory. Influences of different incidence conditions on the intensity distribution of the generated dark hollow beams are also investigated. And it is shown that a perfect dark hollow beam could be produced when a Gaussian beam is vertically incident upon a first-order vortex phase plate, with the incident light wavelength equal to that of the phase plate. However, both the circular symmetry of the incident beam's intensity distribution and the alignment between the centers of Gaussian beam and phase plate may affect the intensity distribution of the dark hollow beam, which will almost be in circular symmetry though it may shift some distance from the image center when at a small incident angle. Furthermore, the dark hollow beam's intensity distribution will scarcely change when the central wavelength deviation is very small from the incidence light and the phase plate. These results may be of great value in generation of perfect dark hollow beams in sub-diffraction –limit imaging by coherent anti-Stokes Raman scattering microscopy.

Rotating elegant bessel-gaussian beams

We considered a new three-parameter family of rotating asymmetric Bessel-Gaussian beams (aBG-beams) with integer and fractional orbital angular momentum (OAM). Complex amplitude of aBG-beams is proportional to a Gaussian function and to n-th order Bessel function with complex argument. These beams have finite energy. The asymmetry ratio of the aBG-beam depends on the real parameter c ≥ 0. For c = 0, the aBG-beam coincides with the conventional radially symmetric Bessel-Gaussian beam, with increasing c intensity distribution of the aBG-beam takes the form of Crescent. At c >> 1, the beam is widening along the vertical axis while shifting along the horizontal axis. In initial plane, the intensity distribution of asymmetric Bessel-Gaussian beams has a countable number of isolated zeros located on the horizontal axis. Locations of these zeros correspond to optical vortices with unit topological charges and opposite signs on different sides of the origin. During the beam propagation, centers of these vortices rotate around the optical axis along with the entire beam at a non-uniform rate (for large c >> 1): they will rotate by 45 degrees at a distance of the Rayleigh range, and then by another 45 degrees during the rest distance. For different values of the parameter c zeros in the transverse intensity distribution of the beam change their location and change the OAM of the beam. Isolated intensity zero on the optical axis corresponds to an optical vortex with topological charge of n. Laser beam with the shape of rotating Crescent has been generated by using a spatial light modulator.

Laser Droplet Generation from a Metal Foil

Abstract Metal droplets are used in various droplet based innovative technologies, especially in joining of temperature sensitive components where the amount of applied energy and its spatial distribution are important. Laser light has already been proved to be a suitable energy source for metal droplet generation, since it enables high spatial and temporal control of the energy input. In this contribution, a novel process of laser droplet generation (LDG) from a metal foil is presented, which could provide increased flexibility of the generated droplet parameters compared to the existing LDG processes. In the novel LDGprocess, a horizontally placed foil is irradiated by a laser beam from above and the melted part of the foil is detached by shielding gas overpressure above the foil. The results of presented preliminary experimental study show that droplet formation from metal foil is possible by means of an annular laser beam. In addition to this, the space of process parameters is explored,showing influences of temporal intensity distributions of the annular laser beam, shielding gas overpressure, and foil thickness and material on the process outcome.

投影物镜热效应的快速求解算法

A new method is mentioned which can be used to simulate the thermal effect in the projection objective lens with fast speed. The method improves the solving period by separating the all solution to two steps: the first step is to obtain the transfer matrix between intensity distribution of the incident beam and thermal effect (including lens temperature and surface deformation). The second step is to express the intensity distribution with polynomial coefficients and multiply the fitting coefficients by the transfer matrix. The solving-time and the precision of the method are validated by analyzing the thermal effects on three different lenses with 0.2 s to get the node temperature and the Zernike coefficients of the surface deformation, while the general finite element analysis (FEA) method needs 600 s. Another advantage of the new method is that the solving-time isn't lengthened with more nodes. Temperature peak valley (PV) error is 0.002C, and fitting error of Zernike coefficient is 0.005 nm. The accuracy can match the requirement of calculating system aberration.

Transformation of the Beam Intensity Distribution and Formation of a Uniform Ion Beam by Means of Nonlinear Focusing

Transformation of the Beam Intensity Distribution and Formation of a Uniform Ion Beam by Means of Nonlinear Focusing

Optimization of Beam Mode for High Efficiency Laser Thermal Forming within Metallurgical Constraints

In the laser forming (LF) process, laser induced temperature distribution within the work-piece is of paramount importance. Through control of process parameters and depending on work-piece geometry, the temperature distribution can be altered to achieve either localized plastic compressive strains or elastic-plastic buckling. Conventionally, three process parameters are manipulated in order to control the temperature distribution within the work-piece; traverse speed, average power and spot size. Additionally, the intensity distribution and geometrical shape of the beam incident on the work-piece surface can be manipulated. The latter has the potential to be useful in maintaining bend angle per pass whilst working within strict metallurgical constraints. In this paper, the effect of beam intensity distribution and geometrical shape on the LF of automotive grade high strength DP 1000 steel sheet is investigated numerically and experimentally, with particular emphasis on optimization for minimal micro-structural transformation.

Propagation characteristics of a high-power broadband laser beam passing through a nonlinear optical medium with defects

AbstractThe intensity distributions of a high-power broadband laser beam passing through a nonlinear optical medium with defects and then propagating in free space are investigated based on the general nonlinear Schrödinger equation and the split-step Fourier numerical method. The influences of the bandwidth of the laser beam, the thickness of the medium, and the defects on the light intensity distribution are revealed. We find that the nonlinear optical effect can be suppressed and that the uniformity of the beam can be improved for a high-power broadband laser beam with appropriate wide bandwidth. It is also found that, under the same incident light intensity, a thicker medium will lead to a stronger self-focusing intensity, and that the influence of defects in the optical elements on the intensity is stronger for a narrowband beam than for a broadband beam.

Speckle effects in target-in-the-loop laser beam projection systems

Abstract In target-in-the-loop laser beam projection scenarios typical of remote sensing, directed energy, and adaptive optics applications, a transmitted laser beam propagates through an optically inhomogeneous medium toward a target, scatters off the target’s rough surface, and returns back to the transceiver plane. Coherent beam scattering off the randomly rough surface results in strong speckle modulation in the transceiver plane. This speckle modulation has been a long-standing challenge that limits performance of remote sensing, active imaging, and adaptive optics techniques. Using physics-based models of laser beam scattering off a randomly rough surface, we show that received speckle-field spatial and temporal characteristics can be used to evaluate the intensity distribution of the beam projected onto the target. We derive analytical expressions that directly couple the measured target-return wave statistical characteristics, or ‘speckle metrics’, with characteristics of the laser beam intensity distribution on the target surface. We also show how measured speckle metrics can be utilized for evaluation of laser beam quality at the target surface and for adaptive compensation of atmospheric turbulence-induced phase aberrations.

Modal analysis of a seeded free-electron laser

It has been shown that the direct seeding can enhance the performance of a free-electron laser (FEL) in terms of its spectral, temporal, and coherence properties and reduces fluctuations in FEL output energy and arrival-time jitter. The properties of the used seed photon pulse are of high importance. In this paper, we describe the influence of the ${M}^{2}$ onto the achievable power contrast between the direct seeded and the unseeded FEL radiation. The results of these studies are compared with the data from the high harmonic generation direct seeding experiment ``sFLASH'' in Hamburg, Germany. A method to measure ${M}^{2}$ from a single transverse intensity distribution of the high harmonics beam at waist is discussed.

Research on the measurement method for a large laser beam profile based on CCD diffuse transmission imaging

The intensity distribution of a laser beam (laser beam profile) is one of its most important characteristics because it yields information on the non-uniformity, centroid, centroid jitter and beam quality. A novel measurement method for obtaining a large laser beam intensity distribution based on CCD diffuse transmission imaging is proposed. The measurement principle is presented. The measurement components of the system have been designed. The layout of the system and the design of a diffuse transmission sampling target are emphasized. The methods of correcting geometric and intensity distortion of the spot are described in detail. The method to calibrate the power coefficient is also given. After the laser spot correction, we can obtain the precise laser beam intensity profile as well as the laser power. The presented test results validate the method. In the measurement field of large laser beams, the method can measure the profile with a non-uniformity of less than 1%, and laser power within 2% error compared with the calibration power meter. The measurement system can be used as a standard measurement instrument after being calibrated when manufactured.

Dependence of residual amplitude noise in electro-optic phase modulators on the intensity distribution of the incident field

Our results demonstrate that photorefractive residual amplitude modulation (RAM) noise in electro-optic modulators (EOMs) can be reduced by modifying the incident beam intensity distribution. Here we report an order of magnitude reduction in RAM when beams with uniform intensity (flat-top) profiles, generated with an LCOS-SLM, are used instead of the usual fundamental Gaussian mode (TEM00). RAM arises from the photorefractive amplified scatter noise off the defects and impurities within the crystal. A reduction in RAM is observed with increasing intensity uniformity (flatness), which is attributed to a reduction in space charge field on the beam axis. The level of RAM reduction that can be achieved is physically limited by clipping at EOM apertures, with the observed results agreeing well with a simple model. These results are particularly important in applications where the reduction of residual amplitude modulation to 10−6 is essential.

Optical induction of 3D rotational symmetry refractive lattices by combined interferometric-mask method in doped LiNbO3 crystals

A novel combined interferometric–mask method for the formation of micro- and nanometric scale three-dimensional (3D) rotational symmetry quasi-crystalline refractive lattice structures in photorefractive materials is demonstrated experimentally. The method is based on micrometric scale spatial modulation of the light by amplitude mask in the radial directions and along the azimuthal angle and the use of counter-propagating beam geometry building up Gaussian standing wave, which defines the light modulation in the axial direction with half-wavelength periodicity. 3D intensity pattern can be represented as numerous mask-generated 2D quasi-periodic structures located in each anti-node of the standing wave. The formed 3D intensity distributions of the optical beams can be imparted into the photorefractive medium thus creating the micro- and sub-micrometric scale 3D refractive index volume lattices. The used optical scheme allows also the formation of 2D lattices by removing the back-reflecting mirror. 2D and 3D refractive lattices were recorded with the use of 532 nm laser beam and rotational symmetry mask in doped lithium niobate crystals and were tested by the probe beam far-field diffraction pattern imaging and direct observation by phase microscope. The formed rotational symmetry 3D refractive structures have the periods of 20–60 μm in the radial directions, 60 μm along the azimuthal angle and half-wavelength 266 nm in the axial direction.

Coherent combination of mutual injection phase-locked fiber lasers with a corner cube reflector

We present a new method of the mutual injection phase-locked coherent combination of fiber lasers based on corner cube reflector (CCR). The theoretic calculation of phase difference and far-field intensity distribution of coherent beams obtained by the CCR has been carried out on this work. The relation between the far-field optical intensity distribution and the CCR rotation angles is analyzed in the case of mutual injection. The coherent combination experiments of mutual injection phase-locked with two and four fiber lasers are performed. The experimental results reveal that the far-field intensity distribution in the two free running fiber lasers is good agreement with the theoretical calculation. In the two internal mutual injection structures, the coherent combination output power goes up as high as 54W, and power combined efficiency of more than 90% and fringe contrast about 90% are obtained. The far-field intensity distribution is the same as the typical coherent combination pattern of two-dimensional fiber laser arrays, the experimental results also show that the coherent combination output with more laser beams and higher power with a good beam quality could be realized by the current scheme.

Modeling electro-optical characteristics of broad area semiconductor lasers based on a quasi-stationary multimode analysis

To increase the brightness of broad area laser diodes, it is necessary to tailor the optical properties of their waveguide region. For this purpose, there is the need for simulation tools which can predict the optical properties of the complete device and thus of the outcoupled light. In the present publication, we show a numerical method to calculate typical intensity distributions of the multimode beam inside a high-power semiconductor laser. The model considers effects of mode competition and the influence of the gain medium on the optical field. Simulation results show a good agreement with near and far field measurements of the analyzed broad area laser diodes.

Self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process

This paper presents an investigation of self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process. The pump beam interacts with a pre-excited electron plasma wave thereby generating a back-scattered wave. On account of Gaussian intensity distribution of laser beam, the time independent component of the ponderomotive force along a direction perpendicular to the beam propagation becomes finite, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density affects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and SRS-reflectivity by taking full non-linear part of the dielectric constant of collisionless plasma with the help of moment theory approach. It is observed from the analysis that focusing of waves greatly enhances the SRS reflectivity.

Two complementary ways of thin-metal-film patterning using laser beam interference and direct ablation

Interference of several identical laser beams allows for the fabrication of periodical structures over large areas. We present two complementary techniques for the patterning of thin films using the laser beam interference and direct ablation of material. The first one is based on precise control of the laser fluence relative to the ablation threshold of the film. Additional means for pattern management are provided by separate control of the intensity and phase of the interfering laser beams. Interference of four or six laser beams permits fabrication of periodical arrays of circular holes. Flexibility and sensitivity to the process parameters of the method beyond of simple hole arrays are shown for the thin Cr film ablation. Another versatile means for pattern generation in the films is the scanning of the periodic laser beam distribution over a workpiece in a sub-period area. By translation of the sample in small steps between laser shots, the structures with their shape independent of the interfering beam intensity distribution can be fabricated by overlapping the ablated holes. Slit- and cross-shaped periodical structures were fabricated using direct structuring with interfering nanosecond and femtosecond pulses. The thin-film structures fabricated by both techniques can be used as spectrally-selective elements for terahertz and infrared radiation.

Multiple ionization of neon by soft x-rays at ultrahigh intensity

At the free-electron laser FLASH, multiple ionization of neon atoms was quantitatively investigated at photon energies of 93.0 and 90.5 eV. For ion charge states up to 6+, we compare the respective absolute photoionization yields with results from a minimal model and an elaborate description including standard sequential and direct photoionization channels. Both approaches are based on rate equations and take into account a Gaussian spatial intensity distribution of the laser beam. From the comparison we conclude that photoionization up to a charge of 5+ can be described by the minimal model which we interpret as sequential photoionization assisted by electron shake-up processes. For higher charges, the experimental ionization yields systematically exceed the elaborate rate-based prediction.

Reliability analysis of an auto-tracked FSO link under adverse weather condition

In this paper, the effects of auto-tracking subsystems on the Bit-Error-Rate (BER) and reliability of free-space optical (FSO) communication links are investigated. Considering environmental influences on the laser beam intensity distribution in receiver, we study the propagation properties of Gaussian laser beams through a complete optical path. Then it is tried to derive an analytical formula for intensity distribution along the complete optical path as well as on the receiver's detector plane based on the Collins integral. Based upon the intensity formula, we calculate the values of signal to noise ratio (SNR), BER, and power while considering the effects of atmospheric losses due to absorption, scattering and turbulence on them. Using mentioned values, the role of auto-tracking subsystems on link reliability improvement in adverse weather condition is described. The related results are illustrated by graphs obtained by calculations and simulations.

Study on ion irradiation response of Gafchromic films for the intensity distribution measurement of a large-area beam

It is required to precisely evaluate the large-area transverse intensity distribution of various ion beams applied to materials and biological research at an azimuthally-varying-field cyclotron in Japan Atomic Energy Agency. The response of radiochromic films, Gafchromic HD-810 and EBT2, was, therefore, investigated using proton and several heavy-ion beams for developing a measurement technique of the transverse beam intensity distribution. The optical density of the films increased linearly with the particle fluence of the beam in a low-fluence region. The linear-response range of the fluence was strongly dependent on ion species. By analyzing the film response to absorbed dose, it was found that the sensitivity of the film decreased for heavy-ion beams. The practical fluence range of the Gafchromic film HD-810 for an arbitrary ion beam was determined from the measurement result. The present technique with the use of the two types of Gafchromic films enables us to measure the intensity distribution of a large-area ion beam easily in a wide fluence range. Using the technique, we measured the relative intensity distribution of a large-area ion beam focused by multipole magnets and evaluated its size and uniformity.

An optimized photon pair source for quantum circuits

We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%.