Transverse Intensity Profile Research Articles

Method for exploring the topological charge and shape of an optical vortex generated by a spiral phase plate

A spiral phase plate (SPP) represents one of the most efficient and cost-effective optical devices used for vortex beam generation. In this work, we studied, both experimentally and theoretically, the effect of illumination of an SPP with a laser beam of a different wavelength than the one for which it was designed. We visualized the transversal intensity profile without any additional focusing element and the spatial phase of the beam was investigated using a Sagnac interferometer. SPPs with topological charges from 1 to 8 were used in these investigations. The results of the study revealed that the characteristic diffraction patterns which show rotational symmetry of order m, but no inversion symmetry, can be used to determine the absolute value m and the sign of the topological charge of the SPP. In addition, the same technique has the potential to be used for singularity manipulation and shaping the intensity distribution of vortex beams.

Rotating structured light based on perfect vortex

In this letter, we experimentally demonstrated a simplified rotating structured light consisting of two overlapping perfect vortices that are generated by focusing two coaxial Bessel beams with different longitudinal wavenumbers. Named perfect rotating wave (PRW), this beam inherits the characteristic of perfect vortex whose radius is controllable and frozen wave whose transverse intensity profile rotates along transmission direction. The no-sidelobes intensity distribution of PRW facilitates the orientation measurement. Because of the linear responds to the Bessel beams’ propagation distance, the orientation of PRW could be applied in remote sensing to measure optical path, temperature, refractive index and so on.

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61665006 and 61865011) and the Natural Science Foundation of Jiangxi Province of China (Grant Nos. 20171ACB21018, 20161BAB212041, and 20162BCB23012).

The nonlinear propagation of an intense Laguerre–Gaussian (LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation, catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian (FG) mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.

Study of a dark core beam generated by nonlinear thermo-optical effect

Thermal lensing with a CW pump in materials with large extinction coefficient is used to convert a probe beam with Gaussian transverse intensity profile into a ring-shaped beam with dark core. The radius of the dark core can be controlled by varying the pump power. A Mach-Zehnder interferometer was used to study the beam’s phase profile. A theoretical analysis of the phase profile yields estimates of the threshold pump power needed to generate a dark core and the maximum change of the refractive index achieved in the experiment. As an application of the dark core beam, a numerical simulation of the effect has been combined with the experimental data to get a quick in situ estimate of thermal conductivity by a non-contact method.

Centering a beam of light to the axis of rotation of a planar object.

We detail an experimentally simple approach for centering a beam of light to the axis of a rotating surface. This technique can be understood as a rotating analog to knife-edge profilometry, a common experimental technique wherein the intensity (or power) of various masked portions of a beam is used to ascertain the transverse intensity profile of the beam. Instead of collecting the light transmitted through a mask, we give the surface a variable reflectivity (such as with a strip of retro-reflective tape) and sample the light scattered from the surface as it rotates. We co-align the transverse position (not the tilt) of the axis of rotation and the beam centroid by minimizing the modulation amplitude of this scattered light. In a controlled experiment, we compare the centroid found using this approach to the centroid found using the canonical knife-edge approach in two directions. We find our results to be accurate to within the uncertainty of the benchmark measurement, ±0.03 mm (±2.9% of the beam waist). Using simulations that mimic the experiments, we estimate that the uncertainty of the technique is much smaller than that of the benchmark measurement, ±0.01 mm (±1% of the beam waist), limited here by the size of the components used in these experiments. We expect this centering technique to find applications in experimental and industrial fabrication and processing settings where alignment involving rotating surfaces is critical.

Multiband RadioAstron space VLBI imaging of the jet in quasar S5 0836+710

Context. Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. Aims. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. Methods. The RadioAstron observations at wavelengths of 18 cm, 6 cm, and 1.3 cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. Results. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02 mas to 200 mas. Brightness temperatures in excess of 1013 K are measured in the jet, requiring Doppler factors of ≥100 for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin–Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be Mj ≈ 12 and η ≈ 0.33. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.

Multiplexed Nondiffracting Nonlinear Metasurfaces

AbstractMetasurfaces and planar photonic nanostructures have drawn great interest from the optical scientific community due to their diverse abilities of manipulating electromagnetic waves and high integration. Most metasurfaces launch diffracting waves, and thus suffer from divergence, short working distance, and instability. Although much effort has been devoted to researching nondiffracting metasurfaces which can launch electromagnetic waves with constant transverse intensity profiles in free‐space propagation, the number of working channels is inherently limited as these meta‐devices are implemented in the linear optical regime. Here, the multiplexed nondiffracting nonlinear metasurfaces are theoretically proposed and experimentally realized, which can generate the representative nondiffracting Bessel beam and Airy beam. Three Bessel beams with different numerical apertures and topological charges and three Airy beams with different propagation curves and focal lengths can be generated by a combination of different spins and wavelengths. The complex properties of the nondiffracting beams can be designed and detected in a more comprehensive and concise way with Fourier analysis. This proof‐of‐concept represents a new strategy for realizing multiplexed nondiffracting metasurfaces with advantages of ultracompactness, high‐pixelation, and easy integration and paves the way for multi‐channel optical communication and manipulation.

Self-focusing and pulse sharpening of sub-picosecond laser pulse in n-InSb

The self-focusing and pulse sharpening of an intense short pulse laser in n-type indium antimonide, a semiconductor with non-parabolic energy band, is studied. The nonlinearity arises through the dependence of electron effective mass on quiver velocity and gives effective permittivity a saturating function of intensity. A laser beam with Gaussian transverse intensity profile undergoes periodic self-focusing. The faster self-focusing of the peak of the pulse causes significant sharpening of the pulse in course of propagation. For 10.6μm CO2 laser of picosecond pulse length and intensity ∼1 – 2 GW/cm2, significant self-focusing and pulse length shortening occurs over a length of 0.6 mm.

Effect of the Papaya Latex on the Visibility of Fingerprint

The effect of the papaya latex on the visibility of fingerprint was investigated. The fingerprints of the thumb without and with touching papaya latex for 5 and 10 min were prepared on the glass slides and captured by CMOS camera. The fingerprint images were reconstructed and the transverse intensity profiles were performed. The valley depth and the ridge width for these prepared fingerprints were determined. The results show that the visibility of the fingerprints increased with increasing touching time with papaya latex. It can be concluded that the finger surface needed to be scanned using any fingerprint sensors should be touched with papaya latex for about 10 min before image scanning to increase the image quality.

Energetic few-cycle pulse compression in gas-filled hollow core fiber with concentric phase mask**Project supported by the National Natural Science Foundation of China (Grant No. 61521093), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB1603), the International Science and Technology Cooperation Program of China (Grant No. 2016YFE0119300), and the Program of Shanghai Academic/Technology Research Leader, China

The compression of high-energy, linearly polarized pulses in a gas-filled hollow core fiber (HCF) by using a concentric phase mask is studied theoretically. Simulation results indicate that using a properly designed concentric phase mask, a 40-fs input pulse centered at 800 nm with energy up to 10.0 mJ can be compressed to a full width at half maximum (FWHM) of less than 5 fs after propagating through a neon-filled HCF with a length of 1 m and diameter of with a transmission efficiency of 67%, which is significantly higher than that without a concentric phase mask. Pulses with energy up to 20.0 mJ can also be efficiently compressed to less than 10 fs with the concentric phase mask. The higher efficiency due to the concentric phase mask can be attributed to the redistribution of the transverse intensity profile, which reduces the effect of ionization. The proposed method exhibits great potential for generating few-cycle laser pulse sources with high energy by the HCF compressor.

Coronal Loop Seismology Using Standing Kink Oscillations With a Lookup Table

The transverse structure of coronal loops plays a key role in the physics but the small transverse scales can be difficult to observe directly. For wider loops the density profile may be estimated by forward modelling of the transverse intensity profile. The transverse density profile may also be estimated seismologically using kink oscillations in coronal loops. The strong damping of kink oscillations is attributed to resonant absorption and the damping profile contains information about the transverse structure of the loop. However, the analytical descriptions for damping by resonant absorption presently only describe the behaviour for thin inhomogeneous layers. Previous numerical studies have demonstrated that this thin boundary approximation produces poor estimates of the damping behaviour in loops with wider inhomogeneous layers. Both the seismological and forward modelling approaches suggest loops have a range of layer widths and so there is a need for a description of the damping behaviour that accurately describes such loops. We perform a parametric study of the damping of standing kink oscillations by resonant absorption for a wide range of inhomogeneous layer widths and density contrast ratios, with a focus on the values most relevant to observational cases. We describe the damping profile produced by our numerical simulations without prior assumption of its shape and compile our results into a lookup table which may be used to produce accurate seismological estimates for kink oscillation observations.

Fraunhofer diffraction of a Gaussian beam by a four-sector binary grating with a half period fringes shift between adjacent sectors

The study of Fraunhofer diffraction of a Gaussian laser beam by a computer-generated, binary four-sector grating (FSG) is presented. In the four equal angular sectors of the FSG, parts of a binary rectilinear grating are embedded in such a way that, two neighboring parts are shifted by a half spatial rectilinear grating period. Analytical expressions, describing the diffracted wave field amplitude and intensity distributions in the focal plane of a converging lens, are derived. The zeroth-diffraction-order beam is a Gaussian beam whose transverse cross section has a reduced Gaussian radius. The higher-diffraction-order beam transverse intensity profiles have four bright spots divided by dark crossed lines and a central dark, non-vortex core. They can be described as cosine-Laguerre–Gaussian Kummer beams. The numerically calculated diffraction patterns, as well as the interference patterns of the diffracted beam with a slightly inclined plane wave, are supported by experimentally obtained ones. In both investigations (theoretical and experimental) the interferograms confirm the non-vortex nature of the diffraction spots.

A novel method for laser radar cross section calculation of complex laser targets with partial and Gaussian beam irradiation

A directional-diffuse light (DDL) algorithm is proposed to calculate the laser radar cross section of complex targets with partial irradiation by changing the size of the viewing volume based on graphical electromagnetic computing and OpenGL. To assess the performance of the DDL algorithm, we theoretically and experimentally compare the DDL algorithm with the spotlight (SL) algorithm: another algorithm used in partial irradiation cases. We find that there is a higher degree of consistency between the theoretical or experimental and simulated data calculated by the DDL algorithm compared with the SL algorithm. Moreover, we propose a directional-diffuse Gaussian light (DDGL) algorithm by including an attenuation factor for characterizing the transverse intensity profile of the Gaussian laser beam based on the DDL algorithm. We theoretically and experimentally analyze the effects of the DDGL algorithm and the DDL algorithm and show that the DDGL algorithm efficiently improves the accuracy of numerical simulation relative to the DDL algorithm. Finally, we compare the experimental data with the simulated data calculated by the DDGL algorithm at different in-plane bistatic angles. The experimental results demonstrate that the DDGL algorithm can accurately illustrate laser scattering properties of targets with partial and Gaussian beam irradiation.

Evolution of the Transverse Density Structure of Oscillating Coronal Loops Inferred by Forward Modeling of EUV Intensity

Abstract Recent developments in the observation and modeling of kink oscillations of coronal loops have led to heightened interest over the last few years. The modification of the Transverse Density Profile (TDP) of oscillating coronal loops by nonlinear effects, particularly the Kelvin–Helmholtz Instability (KHI), is investigated. How this evolution may be detected is established, in particular, when the KHI vortices may not be observed directly. A model for the loop’s TDP is used that includes a finite inhomogeneous layer and homogeneous core, with a linear transition between them. The evolution of the loop’s transverse intensity profile from numerical simulations of kink oscillations is analyzed. Bayesian inference and forward modeling techniques are applied to infer the evolution of the TDP from the intensity profiles, in a manner that may be applied to observations. The strongest observational evidence for the development of the KHI is found to be a widening of the loop’s inhomogeneous layer, which may be inferred for sufficiently well resolved loops, i.e., >15 data points across the loop. The main signatures when observing the core of the loop (for this specific loop model) during the oscillation are a widening inhomogeneous layer, decreasing intensity, an unchanged radius, and visible fine transverse structuring when the resolution is sufficient. The appearance of these signatures are delayed for loops with wider inhomogeneous layers, and quicker for loops oscillating at higher amplitudes. These cases should also result in stronger observational signatures, with visible transverse structuring appearing for wide loops observed at the resolution of current instruments.

Spatiotemporal Analysis of Coronal Loops Using Seismology of Damped Kink Oscillations and Forward Modeling of EUV Intensity Profiles

Abstract The shape of the damping profile of kink oscillations in coronal loops has recently allowed the transverse density profile of the loop to be estimated. This requires accurate measurement of the damping profile that can distinguish the Gaussian and exponential damping regimes, otherwise there are more unknowns than observables. Forward modeling of the transverse intensity profile may also be used to estimate the width of the inhomogeneous layer of a loop, providing an independent estimate of one of these unknowns. We analyze an oscillating loop for which the seismological determination of the transverse structure is inconclusive except when supplemented by additional spatial information from the transverse intensity profile. Our temporal analysis describes the motion of a coronal loop as a kink oscillation damped by resonant absorption, and our spatial analysis is based on forward modeling the transverse EUV intensity profile of the loop under the isothermal and optically thin approximations. We use Bayesian analysis and Markov chain Monte Carlo sampling to apply our spatial and temporal models both individually and simultaneously to our data and compare the results with numerical simulations. Combining the two methods allows both the inhomogeneous layer width and density contrast to be calculated, which is not possible for the same data when each method is applied individually. We demonstrate that the assumption of an exponential damping profile leads to a significantly larger error in the inferred density contrast ratio compared with a Gaussian damping profile.

Alternate analytic formulation of optical force on a dielectric sphere in the ray optics limit

In the past, the optical force on a micrometer-sized dielectric sphere in a single-beam gradient laser trap was formulated by considering 2D distribution of rays for a plane-wave excitation. However, laser beams usually have a Gaussian transverse intensity profile, which, upon tight focusing, leads to a 3D optical trap. Here, we systematically formulate a generalized ray/geometric optics formalism for estimating force (and potential) for both flat-top and Gaussian laser beams using 2D distribution of light rays as well as 3D distribution of light cones. We also compare our method with the exact Mie theory. In addition, we present a detailed discussion on the nature of force (and potential) considering the optical Kerr effect under high-repetition-rate ultrafast pulsed excitation.

Vectorial diffraction- and dispersionless pulsed “optical knife” beams with femtosecond duration

Mathieu beams are nondiffracting solutions of the wave equation in elliptical coordinate system and can be perceived as “Optical Knives” due to unique asymmetric transverse intensity profile. Width of these “Optical Knives” can be controlled due to extra degree of freedom – eccentricity parameter. Femtosecond pulsed beams are of significant interest due to their importance in material processing and growing applicability elsewhere. Here we derive vectorial Mathieu beams and superpose them to create femtosecond pulsed beams, where diffraction spreading and dispersive broadening is compensated by a given angular dispersion. Intensity profiles, durations and polarization states of different “optical knife” pulsed beams are discussed in detail.

Higher-order mode suppression in twisted single-ring hollow-core photonic crystal fibers.

A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP01-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP11-like core mode phase-matches to the LP01-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D=0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP11-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.

Experimental study of wavefront distortion in the dark hollow beam generated using axicon

Axicons are the optical components widely used for the generation of dark hollow beam in a controlled manner. In certain circumstances, distortion in the transverse intensity profile (i.e. wavefront distortion) appears in DHB produced by axicon-lens assembly. In this paper, we performed the detailed experimental study for exploring the reasons for the appearance of wavefront distortion including its control and removal in partially coherent dark hollow beam produced from axicon-lens assembly. This experimental study find its crucial importance in the applications, where axicon is used for producing dark hollow beams, e.g. in eye surgery, atom trapping, laser machining and laser resonator. We also observed that the effect of chromatic aberration near the region of induced wavefront distortion in the depth of focus changes with propagation and can be controlled by varying spatial coherence of the beam. This control on the effect of chromatic aberration will be helpful in attaining achromatic dark hollow beam from axicon.

Coronal loop density profile estimated by forward modelling of EUV intensity

Aims. The transverse density structuring of coronal loops was recently calculated for the first time using the general damping profile for kink oscillations. This seismological method assumes a density profile with a linear transition region. We consider to what extent this density profile accounts for the observed intensity profile of the loop, and how the transverse intensity profile may be used to complement the seismological technique.Methods. We use isothermal and optically transparent approximations for which the intensity of extreme ultraviolet (EUV) emission is proportional to the square of the plasma density integrated along the line of sight. We consider four different models for the transverse density profile; the generalised Epstein profile, the step function, the linear transition region profile, and a Gaussian profile. The effect of the point spread function is included and Bayesian analysis is used for comparison of the models.Results. The two profiles with finite transitions are found to be preferable to the step function profile, which supports the interpretation of kink mode damping as being due to mode coupling. The estimate of the transition layer width using forward modelling is consistent with the seismological estimate.Conclusions. For wide loops, that is those observed with sufficiently high spatial resolution, this method can provide an independent estimate of density profile parameters for comparison with seismological estimates. In the ill-posed case of only one of the Gaussian or exponential damping regimes being observed, it may provide additional information to allow a seismological inversion to be performed. Alternatively, it may be used to obtain structuring information for loops that do not oscillate.