Intensity Profile Of Beam Research Articles

Femtosecond laser polishing of additively manufactured parts at grazing incidence

Surface polishing is usually a requisite for making additively manufactured (AM) parts ready for practical applications. Due to its advantages of being flexible and noncontact, laser polishing has attracted increasing research interest. In this research, femtosecond (fs) laser polishing was established to post-process both the top surfaces and sidewalls of AM parts. The challenge to remove three levels of roughness (i.e., the initial surface roughness of the AM parts, the undulations newly introduced during fs laser polishing, and the micro-nanoscale surface features induced by fs laser irradiation) was identified and addressed. Mirror surfaces with Sa < 200 nm were achieved on stainless steel parts printed with an initial roughness >20 µm. Both parallel- and perpendicular-incidence were investigated for the polishing, with the former verified to be more effective in eliminating the initial roughness of the AM parts, due to the elongated focal intensity profile of a Gaussian beam irradiated on the AM part surfaces. The challenge of forming three-zone surfaces during the parallel-incidence was further addressed through a grazing-incidence polishing approach, and uniform smooth surfaces were realized. Fine-tuning the laser power enabled controlling the submicron surface features formed under fs laser irradiation, which determined the final achievable surface roughness.

On anomalous optical beam shifts at near-normal incidence

We develop the theory of optical beam shifts (both Goos–Hänchen and Imbert–Fedorov) for the case of near-normal incidence, when the incident angle becomes comparable with the angular beam divergence. The developed theory uncovers the unified origin of the anomalous beam shift enhancement via the geometric Berry phase singularity. Particularly, we predict the large Goos–Hänchen shift occurring for small incidence angles. We also propose a simple experimental scheme involving a quarter-wave q-plate that allows us to observe the giant transverse and longitudinal, spatial and angular beam shifts simultaneously. Finally, we analyze the relevant beam parameters, polarization structure, and intensity profiles of the shifted transmitted beam. Our results can find applications in spin–orbit photonics, polarization optics, sensing applications, and quantum weak measurements.

Theoretical study on the diffraction-based generation of a 2D orthogonal lattice of optical beams: physical bases and application for a vortex beam multiplication.

A comprehensive theoretical study on the generation of a 2D orthogonal lattice of optical beams based on the near-field diffraction and Talbot effect is presented. First we investigate the near-field diffraction of an optical beam with a finite lateral extension from an infinite 2D orthogonal grating. It is shown that the resulting diffraction patterns over the Talbot planes depend on the following parameters: the period and opening ratio (OR) of the grating, wavelength and spatial spectral bandwidth of the incident beam, and the propagation distance. In terms of these parameters, we find multiplication conditions: the certain conditions under which a 2D orthogonal lattice of the Fourier transform of the incident beam is generated on the Talbot planes. Therefore, if the incident beam is Fourier-invariant and all the established multiplication conditions are fulfilled, the intensity profile of each of the individual Talbot images resembles the intensity profile of the incident beam. We consider the Laguerre-Gaussian beams having zero radial index as an important class of the vortex beams. We explicitly show that these beams are Fourier-invariant and we calculate their spatial spectral bandwidth. As a result, in the illumination of a 2D orthogonal binary grating with this kind of vortex beam, a 2D orthogonal lattice of the incident optical vortex is generated at the Talbot planes. Considering the obtained multiplication conditions, for the first time, to our knowledge, we determine a multiplication interval. This interval covers the propagation distances at which the vortex beam multiplication occurs. Moreover, we obtain the maximum possible value of the grating's OR for the realizations of the vortex multiplication. It is shown that both the multiplication interval and the maximum value of the OR depend on the topological charge (TC) of the incident beam. With the aid of some practical examples and defining a multiplication quality factor, the mentioned results are verified quantitatively. In addition to the vortex beam multiplication effect, we consider another interesting phenomenon that results from the interference of the grating's first diffraction orders. We call this phenomenon the first diffraction orders interference (FDOI) effect. We show that both the multiplication and the FDOI effects occur simultaneously but at different propagation distances. It is also shown that the multiplication and FDOI intervals separate and distance from each other by increasing the TC of the incident beam.

Self-healing property of the self-rotating beam.

In this study, we demonstrate the self-healing of self-rotating beams with asymmetric intensity profiles. The proposed self-rotating beam exhibits an asymmetric intensity profile and self-healing properties in free-space propagation. In addition, the rotation direction and beam intensity profile of the self-rotating beam can be adjusted using the parameters a and b in the phase function. The effects of the position and size of the obstruction on the self-healing property of a self-rotating beam were studied both experimentally and numerically. The simulation and experimental results demonstrate that a self-rotating beam can overcome a block of obstacles and regenerate itself after a characteristic distance. Transverse energy flows were used to explain the self-healing properties. Moreover, the beam rotates during propagation, which can be used to capture and manipulate microscopic particles in a three-dimensional space. It is expected that these rotating beams with self-healing properties will be useful in penetrating obstacles for optical trapping, transportation, and optical therapy.

Spatially resolved diagnostics for optimization of large ion beam sources

Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

Femtosecond multiple filamentation of an optical vortex in the mid-IR wavelength range in fused silica and fluorides

The results of experimental and theoretical study of the self-action of femtosecond optical vortices in the region of anomalous group velocity dispersion in fused silica and fluorides are presented. Multiple filamentation of an axially asymmetric annular beam with a phase dislocation of topological charge m = 1 at a wavelength of 1800 nm in a LiF crystal is investigated. It is found that for the experimentally recorded intensity profile of a vortex beam with two maxima on the diameter, the critical self-focusing power is approximately two times larger than the critical power of a unimodal Gaussian beam. In pulses with supercritical power in the vicinity of the intensity maxima, two coupled filaments, separated by a phase dislocation, are formed on the annular profile of the optical vortex, which prevents energy exchange during their formation. The length of vortex-beam plasma channels in a single pulse is found to be about 300 μm at a diameter of about 2 μm, which is close to the characteristics of plasma channels in a Gaussian beam.

Spatially modulated scene illumination for intensity-compensated two-dimensional array photon-counting LiDAR imaging

Photon-counting LiDAR using a two-dimensional (2D) array detector has the advantages of high lateral resolution and fast acquisition speed. The non-uniform intensity profile of the illumination beam and non-uniform quantum efficiency of the detectors in the 2D array deteriorate the imaging quality. Herein, we propose a photon-counting LiDAR system that uses a spatial light modulator to control the spatial intensity to compensate for both the non-uniform intensity profile of the illumination beam, and the variation in the quantum efficiency of the detectors in the 2D array. By using a 635 nm peak wavelength and 4 mW average power semiconductor laser, lab-based experiments at a 4.27 m stand-off distance are performed to verify the effectiveness of the proposed method. Compared with the unmodulated method, the standard deviation of the intensity image of the proposed method is reduced from 0.109 to 0.089 for a whiteboard target, with an average signal photon number of 0.006 per pixel.

Dynamic modulation of spatial intensity profile of a laser beam using a binary hologram

In this paper we first describe two existing schemes to construct binary amplitude or phase holograms for arbitrary complex amplitude modulation in the +1 order diffracted beam. We then propose another binarization scheme for complex amplitude modulation that uses a combination of fixed as well as statistically dependent threshold values for binarization. We perform a numerical investigation to compare the accuracy of the three binarization schemes to incorporate spatial intensity modulation in a laser beam and observe that the proposed scheme offers significant relative advantage in terms of aliasing. We present experimental results that supports the above observations.

Simulation of Heat Transfer in Single-Crystal Lithium Niobate in Interaction with Continuous-Wave Laser Radiation

The paper presents the simulation results of heat transfer in single-crystal lithium niobate (LiNbO3) in the form of cylinder of diameter mm and height mm in interaction with continuous-wave laser radiation with the output power of W and the wavelength of nm. The density of the LiNbO3 crystal is kg/m3; the thermal conductivity along the [001] direction is W/(m×K); the thermal conductivity in the (001) plane is W/(m×K); the specific heat at constant pressure is J/(kg×K); the absorption coefficient is %/cm @ 1064 nm. The laser beam propagates along the optical axis of the crystal. The laser beam intensity profile is represented as a Gaussian function, and the absorption of laser radiation of the single-crystal lithium niobate is described by Beer-Lambert’s law. The numerical solution of the non-stationary heat conduction problem is obtained by meshless scheme using anisotropic radial basis functions. The time interval of the non-stationary boundary-value problem is 2 h 30 min. The results of numerical calculations of the temperature distribution inside and on the surface of the single-crystal lithium niobate at times s are presented. The time required to achieve the steady-state heating mode of the LiNbO3 crystal, as well as its temperature range over the entire time interval, have been determined. The accuracy of the approximate solution of the boundary-value problem at the n-th iteration is estimated by the value of the norm of relative residual . The results of the numerical solution of the non-stationary heat conduction problem obtained by meshless method show its high efficiency even at a small number of interpolation nodes.

Simultaneous multiple laser beam intensity profile correction and its application to a vitiated bluff body combustor field.

Simultaneous high-speed stereo-particle image velocimetry, OH planar-laser-induced fluorescence (PLIF), and CH2O PLIF measurements in a vitiated bluff body combustor are considered. An ex situ, simultaneous, time-resolved laser sheet intensity profile correction procedure is introduced. This procedure is easily implemented experimentally and is capable of correcting multiple sheets at the same time. As a proof of concept, the procedure is applied to perform correction of the CH2O PLIF images in vitiated and unvitiated conditions. The challenges associated with CH2O PLIF under these combustor operating conditions are also discussed.

Dynamic optical beam shaping system to generate Gaussian and top-hat laser beams of various sizes with circular and square footprint for Additive Manufacturing applications

The speed of production in laser-based additive manufacturing processes is rather low compared to conventional manufacturing methods and could be improved by using a top-hat laser beam intensity profile, a variable spot size or a combination of both, enabling wider melt pools during processing. This while retaining or possibly even improving part quality. In this work, the design and implementation of a dynamic optical beam shaping system is presented to enable different shapes and sizes of top-hat beams at the build platform of a Laser Powder Bed Fusion machine which is equipped with a fiber laser emitting at 1070nm. The system has been demonstrated by assembling custom-made beam shapers with commercially available beam expanders and by measuring the intensity profiles. Initial results indicate the possibilities for tuning of the spot size at the build platform by a magnification factor ranging from 1x to 4x. Parameter optimization for the Gaussian beam with 4x magnification was performed and a first 316L demonstrator part is further discussed in this work to illustrate a gain in productivity of 44%, while retaining >99% density.

Understanding of adopting flat-top laser in laser powder bed fusion processed Inconel 718 alloy: simulation of single-track scanning and experiment

Laser beam intensity profile used in laser powder bed fusion (LPBF) process is generally Gaussian, which may not be the best to deliver site-specific microstructures. In this study, a flat-top laser with large-size laser spot and uniform intensity distribution is adopted in the LPBF process with Inconel 718 powder. Comparison investigations are conducted to the standard Gaussian laser. Firstly, the parameters for the flat-top laser are optimized by multi-physics simulation of single tracks. Then, the thermodynamic behaviors and morphologies of the molten pool are observed combined with experiments. A cubic sample is also fabricated utilizing double-laser system to reveal the different characteristics of the resultant microstructures. The results show that the flat-top laser can work at a relatively wider range of laser power without depression, which often occurs in the case of Gaussian laser. The molten pool induced by the flat-top laser has large width-depth ratio with a flat bottom boundary, and more moderate flow in the half bottom. These features combined with the temperature and flow fields of the flat-top laser provide advantageous conditions for directional epitaxial growth of columnar grains during multi-layer fabrication. The epitaxial growth is retained and can extends to 4 mm from the single-crystal baseplate. This demonstrates great potential of LPBF with the adopted flat-top laser to be applied in fabricating or repairing components with directional crystal structures or single-crystal structures.

Study of the output beam spatial characteristics of a cascade Raman laser with multimode diode pumping

We report a study of the spatial characteristics of the beam output from a multimode-pumped SRS cascade laser. The intensity profiles of transmitted pump radiation beams, as well as of the first and second orders Stokes SRS generation, is measured in two different laser configurations: in a classical laser with two pairs of reflectors and in a scheme with a half-open cavity for the second order Stokes component. The dynamics of changes in the intensity profiles of the radiation beams in the two considered resonators is shown to differ significantly. The data obtained for the beam profiles are analysed together with the power characteristics of the cascade Raman laser within the framework of a balance model of interaction between pump radiation and the Stokes components of the SRS.

Mirror-assisted backscattering interferometry to measure the first-order correlation function of the light emitted by quantum scatterers

We present a new method to obtain the first-order temporal correlation function, $g^{(1)} (\tau)$, of the light scattered by an assembly of point-like quantum scatterers, or equivalently its spectral power distribution. This new method is based on the mirror-assisted backscattering interferometric setup. The contrast of its angular fringes was already linked in the past to the convolution of $g^{(1)} (\tau)$ for different Rabi frequencies taking into account the incoming spatial intensity profile of the probe beam, but we show here that by simply adding a half waveplate to the interferometer in a specific configuration, the fringe contrast becomes $g^{(1)} (\tau)$ of the light scattered by atoms, which are now all subjected to the same laser intensity. This new method has direct application to obtain the saturated spectrum of quantum systems. We discuss some non-trivial aspects of this interferometric setup, and propose an analogy with a double Mach-Zehnder interferometer.

Dynamic aerosol and dynamic air-water interface curvature effects on a 2-Gbit/s free-space optical link using orbital-angular-momentum multiplexing

Abstract When an orbital-angular-momentum (OAM) beam propagates through the dynamic air–water interface, the aerosol above the water and the water surface curvature could induce various degradations (e.g., wavefront distortion, beam wandering, scattering, and absorption). Such time-varying degradations could affect the received intensity and phase profiles of the OAM beams, resulting in dynamic modal power loss and modal power coupling. We experimentally investigate the degradation for a single OAM beam under dynamic aerosol, dynamic curvature, and their comprehensive effects. Our results show the following: (i) with the increase of the aerosol strength (characterized by the attenuation coefficient) from ∼0 to ∼0.7–1.3 dB/cm over ∼7 cm, the power coupling ratio from OAM −1 to +2 increases by 4 dB, which might be due to the amplitude and phase distortion caused by spatially dependent scattering and absorption. (ii) With the increase of the curvature strength (characterized by the variance of curvature slope over time) from ∼0 to ∼2 × 10−5 rad2, the power coupling ratio from OAM −1 to +2 increases by 11 dB. This could be caused by both the wavefront distortion and the beam wandering. (iii) Under the comprehensive effect of aerosol (∼0.1–0.6 dB/cm) and curvature (∼6 × 10−7 rad2), there is an up to 2 dB higher modal power loss as compared with the single-effect cases. (iv) The received power on OAM −1 fluctuates in a range of ∼6 dB within a 220 ms measurement time under aerosol (∼0.1–0.6 dB/cm) and curvature (∼6 × 10−7 rad2) effects due to the dynamic degradations. We also demonstrate an OAM −1 and +2 multiplexed 2-Gbit/s on–off-keying link under dynamic aerosol and curvature effects. The results show a power penalty of ∼3 dB for the bit-error-rate at the 7% forward-error-correction limit under the comprehensive effect of aerosol (∼0.1–0.6 dB/cm) and curvature (∼6 × 10−7 rad2), compared with the no-effect case.

Method for stabilization of the microwave modulation index in order to suppress the light shift of the coherent population trapping resonances

For the resonance of coherent population trapping (CPT), we show that in the case of a spatially inhomogeneous light shift (for example, due to the Gaussian transversal profile of the light beam intensity), the zero position of the error signal, formed by the use of phase-jump technique, depends on the integration time of the spectroscopic signal. Basing on this effect, we propose two-loop method to stabilize the microwave power at the point where the light shift vanishes.

Discrimination information for intensity distributions of a collimated wave beam

The results of a nonlinear thermodynamic analysis of the process of refractive distortions of a collimated wave beam profile at the optical path output are presented. The formalism of q-deformed entropy functionals is used, which makes it possible to describe both extensive additive systems and nonextensive systems for which the entropy superposition principle or internal energy is not implemented. Non-extensive thermodynamic systems should be considered as a class of nonlinear systems for which deviation from the Boltzmann–Shannon–Gibbs axiomatics is associated with the structure of the phase space, the presence of predetermined space-time correlations that determine the nonlocality of interactions admissible in the system. A method of discrimination information maps for various types of q-deformed entropies, a method of marginal projections of the first and second spatial moments for the video sequences series of the wave beam intensity profile are proposed. The values of the spatio-temporal correlations of the process, the stationarity time of the dispersion properties of the mappings are obtained. Similar properties are inherent in complex nonextensive systems and lead to nonlinear responses when creating multicomponent systems.

Optomechanical Self-Stability of Freestanding Photonic Metasurfaces

We develop an analytical framework for self-stabilizing optical manipulation of freestanding metasurfaces in three dimensions. Our framework reveals that the challenging problem of stabilization against translational and rotational perturbations in three dimensions is reduced to a simpler scattering analysis of the metasurface unit cell in two dimensions. We derive universal analytical stiffness coefficients applicable to arbitrary three-dimensional radial metasurfaces and radial beam-intensity profiles. The analytical nature of our framework facilitates highly efficient discovery of optimal optomechanical metasurfaces. Such use of metasurfaces for mechanical stabilization enables macroscale and long-range control in collimated, but otherwise unfocused light beams, and could open up avenues for manipulation beyond traditional optical tweezing and transport.

Two-dimensional matrix parabolic cylinder beams

We investigate the two-dimensional normalized linear Schrödinger equation describing the propagation of diffraction-free beams in free space using the traditional variable separation method. We discover that each beam component satisfies the standard Weber differential equation. From this fact, we construct exact solutions utilizing two parabolic cylinder functions, which are denoted as the two-dimensional (2D) parabolic cylinder beams with diffraction-free characteristics that can be described by two mode numbers. By using numerical simulation and choosing appropriate mode numbers, we exhibit intensity profiles of the 2D parabolic cylinder beams, which display a matrix structure. It is demonstrated that the beams can be completely controlled by the choice of two mode numbers.

Optimization of microwave emission from laser filamentation with a machine learning algorithm.

We demonstrate that is it possible to optimize the yield of microwave radiation from plasmas generated by laser filamentation in atmosphere through manipulation of the laser wavefront. A genetic algorithm controls a deformable mirror that reconfigures the wavefront using the microwave waveform amplitude as feedback. Optimization runs performed as a function of air pressure show that the genetic algorithm can double the microwave field strength relative to when the mirror surface is flat. An increase in the volume and brightness of the plasma fluorescence accompanies the increase in microwave radiation, implying an improvement in the laser beam intensity profile through the filamentation region due to the optimized wavefront.