Intensity Distribution Of Beam Research Articles

Improved photoacoustic images via wavefront shaping modulation based on the scattering structure.

Multispectral optoacoustic tomography (MSOT) has become the dominant technical solution for photoacoustic imaging (PAI). However, the laser source of fiber output in the current MSOT method is typically a TEM00 Gaussian beam, which is prone to artifacts and incomplete due to the uneven distribution of the irradiated light intensity. Here, we propose a novel method to improve the quality of photoacoustic image reconstruction by modulating the wavefront shaping of the incident laser beam based on the designed scattering structure. In the experiment, we add the designed scattering structure to the current hemispherical photoacoustic transducer array device. Through experiments and simulations, we investigate and compare the effects of different scattering structures on laser intensity modulation. The results show that an ED1-C20 diffusion structure with a scattering angle of 20 degrees has the most effective modulation of the beam intensity distribution. And we choose gold nanoparticles of 50-100 nanometers (nm) diameters and index finger capillary vessels respectively as the medium of PAI. We obtain the highest ratio of PAI area increases of gold nanoparticles and index finger to devices compare without scattering structure is 29.69% and 634.94%, respectively. Experimental results demonstrate that our method is significantly higher quality than traditional methods, which has great potential for theoretical application in medical PAI.

Fiber laser cutting of steel materials with twin spot beam-twin spot setting in kerf width direction

In laser cutting, the temperature distribution would have significant influence on cutting characteristics, and the intensity distribution of a laser beam has a possibility to improve the cutting quality. In this study, a fiber laser beam of Gaussian distribution was divided into two beams by a roof axicon lens, and the cutting characteristics were investigated by using the twin spot Gaussian beam setting in the kerf directions. The cutting experiment of a cold-rolled steel plate with a thickness of 3.2 mm was carried out by a 3 kW fiber laser with a nitrogen assist gas, and the Gaussian mode of 114 μm spot and the twin Gaussian mode of two 110 μm spots were used with the variation of power ratio in twin spot processing. At the exit side of kerf by the twin spot process, the width of the cutting front in the low intensity side became wider than that in the high intensity side, and the dross could be reduced in the low intensity side due to sufficient ejection of the molten metal from the front wall rather than the side wall of kerf. The twin spot process could reduce the dross height below 18 μm in the low intensity side, which is smaller than that by the single Gaussian beam process.

Analysis of Azimuthal Doppler Shift of Anisotropically Absorbed Laguerre-Gaussian Beam Propagating in Transverse Flow

The particle flux onto a material is an important parameter in the study of the plasma-material interaction. With conventional Doppler spectroscopy, it is difficult to measure the flow velocity perpendicular to the material because only the velocity component projected on the wave number vector can be measured. To overcome the limitation, we are developing a transverse flow measurement method using the azimuthal Doppler shift of a Laguerre-Gaussian (LG) beam absorption spectroscopy. In this paper, the feasibility of this spectroscopy method has been examined by numerical analysis. The LG beam is anisotropically absorbed in the transverse flow due to the azimuthal Doppler shift. Since the anisotropic LG beam rotates with propagation, the spatial structure of resonance absorption in plasma and the intensity structure of the LG beam that has propagated through the plasma are inevitably different in the LG beam absorption spectroscopy. It was shown that the deviation from the original azimuthal Doppler shift is reduced to several percent at the position where the intensity distribution of the LG beam reaches its maximum value. Therefore, the transverse flow can be measured with sufficient accuracy by properly selecting the position on the beam cross-section used to evaluate the azimuthal Doppler shift.

Intensity uniformity improvement for a complex line beam

A complex line beam, whose intensity distribution is uniform along a line shape and phase distribution is linearly gradient, can be applied in automatic directional transportation and optical separation of microparticles. Generally, complex amplitude beam shaping methods are used to control both the amplitude and phase of an output beam with a single phase-only element. However, for the complex line beam, although the phase gradient can be controlled well, the intensity of the beam is unevenly distributed, i.e. most of the energy of the beam is distributed in the two ends of the lateral direction. In this paper, we propose an intensity-compensation method based on the complex amplitude beam shaping method, in which the target intensity distribution of the output beam is modulated with a Gaussian profile on purpose. The simulation and experimental results show that the intensity uniformity of the complex line beam has been increased by 80.6% while the phase of the beam has been controlled as linearly gradient.

Propagation properties of chirped cosh-Gaussian beams in strongly nonlocal nonlinear media

In this paper, we analytically investigated the evolution properties of chirped cosh-Gaussian beams in strongly nonlocal nonlinear media (SNNM). The expressions describing beam propagation, intensity distribution, and beam width are derived. The results show that in the absence of chirp, the evolution behavior of beam is similar to the solitons and breathers, which closely depends on the input power and initial parameter. In the presence of linear chirp, the optical beam periodically oscillates along a certain direction and exhibits the same structure as the unchirped beam. In the presence of quadratic chirp, the center symmetry of beam evolution is broken. Furthermore, we find that the evolution of beam width always changes periodically whatever the beam power is.

Transmission dynamics of circular–linear edge dislocation solitons in nonlocal nonlinear media

In this paper, the nonlinear propagation dynamics of circular–linear edge dislocation beams (CLED beams) in nonlocal media are studied theoretically. The transport formulae of CLED beams in nonlocal nonlinear media is derived. The evolution of the beam width, curvature, and intensity distribution are discussed in detail. Two different incident cases of CLED beams, namely beam waist and non-beam waist incidences, are discussed and compared. It is found that the off-axial distance can effectively adjust the position of linear edge dislocation and play a key role on beam propagation. The initial beam power and non-beam waist distance also play a key role in propagation. In particular, when specific parameters are selected, the CLED beam can form a transmission state with beam width and transverse pattern both invariant, i.e., CLED soliton forms. For general parameters, the transverse multi-peak patterns of the CLED beam evolutes periodically, i.e., a generalized higher-order soliton or breather forms.

Thermal lens effect with light's orbital angular momentum.

Thermal lens effect has been well developed and exploited for decades by using the Gaussian intensity distribution of a laser beam. In this paper, a new thermal lens effect by using a laser beam with Orbital Angular Momentum (OAM) is proposed. We find that the dynamic process for the formation of the OAM-thermal lens has reda rapid change towards the evolution direction at the beginning but then slowly approaches to a steady state for a while. This phenomenon is significantly different from the traditional Gaussian-thermal lens, thus it may be used to improve the sensitivity of the absorption spectrum for the chemical and biomedical analysis. Besides, theoretically and experimentally, the factors affecting the steady state of the OAM-thermal lens are also studied, hoping these may provide a useful reference for the research community. We also find a potential slow thermal-optical gate that can control of light passing through or blocking by changing the OAM of the heating beam. Our work opens the door which utilizes the structured light beam to study the thermal-optical effect, and more interesting phenomena remain to be explored.

The multi-consecutive optical bottles generated from the chirped elliptical Pearcey Gaussian vortex beams

The evolution of the chirped elliptical Pearcey Gaussian vortex (CEPGV) beams in free space is numerically investigated. The optical bottle and multi-consecutive optical bottles can be directly generated from an elliptical Pearcey Gaussian beam with central vortices and the second-order chirp. In particular, the focusing intensity of CEPGV beams will increase when the elliptical structure deflects larger or the ratio of the long axis to the short axis of the ellipse is larger. Furthermore, we can obtain multi-consecutive optical bottles by manipulating elliptical structure parameters and second-order chirp factors. In the meantime, the transverse intensity distributions at foci of CEPGV beams appear like a slit in a fixed direction. Our results show that the focusing intensity, the focal length and the transverse intensity distribution of CEPGV beams during the propagation can be controlled.

Polarization and coherence properties in self-healing propagation of a partially coherent radially polarized twisted beam.

With the help of generalized Huygens-Fresnel integral, an analytical expression for the self-healing of a partially coherent radially polarized twisted (PCRPT) beam is derived. The coherence and polarization properties of the PCRPT beam in self-healing propagation are studied in detail. It shows that the existence of the twist phase is a double-edged sword for the self-healing properties of the beam. With the increase of the twist factor, the self-healing ability of beam intensity distribution decreases. However, the anti-disturbance performance of beam polarization improves at the same time. Besides, the polarization and coherence distribution of the beam are proved that own a slight self-healing ability when the obstacle is small. Our results will be helpful to the fields of optical tweezers, microscopy, optical communication, and so on.

Diffractive deep neural network based adaptive optics scheme for vortex beam in oceanic turbulence.

Vortex beam carrying orbital angular momentum (OAM) is disturbed by oceanic turbulence (OT) when propagating in underwater wireless optical communication (UWOC) system. Adaptive optics (AO) is a powerful technique used to compensate for distortion and improve the performance of the UWOC system. In this work, we propose a diffractive deep neural network (DDNN) based AO scheme to compensate for the distortion caused by OT, where the DDNN is trained to obtain the mapping between the distortion intensity distribution of the vortex beam and its corresponding phase screen representing OT. In the experiment, the distorted vortex beam is input into the DDNN model where the diffractive layers are solidified and fabricated, and the intensity distribution of the modulated light field of the vortex beam can be recorded. The experiment results show that the proposed scheme can extract quickly the characteristics of the intensity pattern of the distorted vortex beam, and the predicted compensation phase screen can correct the distortion caused by OT in time. The mode purity of the compensated vortex beam is significantly improved, even with a strong OT. Our scheme may provide a new avenue for AO techniques, and is expected to promote the communication quality of UWOC system immediately.

Folded Resonator for Glucose Solution Density Detection Based on Airy Beam Arrays

We presented an investigation on the evolution properties of the finite Airy beam array (FAiBA) in metamaterial’s slab system constituting of right-handed materials and double negative materials combining the generalized Huygens–Fresnel integral equation and the light transfer matrix. The beam intensity distribution on the receiver plane and its side transmission view on the (z, x) plane are demonstrated numerically with several array numbers. Moreover, we put forward a kind of folded resonator to detect glucose solution density based on the derived principle and conclusion; the influence of curvature radius on the emerging beam’s evolution characteristics is discussed, and the empirical formula between FAiBA’s received power and glucose solution density is derived by using the fit linear analysis. We hope that the present concept and the derived conclusions can be employed for monitoring in the environmental field, as well as the biomedical and food safety detection.

Enhance the delivery of light energy ultra-deep into turbid medium by controlling multiple scattering photons to travel in open channels

Multiple light scattering is considered as the major limitation for deep imaging and focusing in turbid media. In this paper, we present an innovative method to overcome this limitation and enhance the delivery of light energy ultra-deep into turbid media with significant improvement in focusing. Our method is based on a wide-field reflection matrix optical coherence tomography (RM-OCT). The time-reversal decomposition of the RM is calibrated with the Tikhonov regularization parameter in order to get more accurate reversal results deep inside the scattering sample. We propose a concept named model energy matrix, which provides a direct mapping of light energy distribution inside the scattering sample. To the best of our knowledge, it is the first time that a method to measure and quantify the distribution of beam intensity inside a scattering sample is demonstrated. By employing the inversion of RM to find the matched wavefront and shaping with a phase-only spatial light modulator, we succeeded in both focusing a beam deep (~9.6 times of scattering mean free path, SMFP) inside the sample and increasing the delivery of light energy by an order of magnitude at an ultra-deep (~14.4 SMFP) position. This technique provides a powerful tool to understand the propagation of photon in a scattering medium and opens a new way to focus light inside biological tissues.

Experimental investigation of LG beam propagating in actual atmospheric turbulence

The propagation properties of Laguerre-Gaussian (LG) beamare investigated experimentally in an actual turbulent atmosphere, including beam propagation, phase change, and singularity perturbation. The experimental results indicate that with the increase of propagation distance the beam gradually spread, the intensity distribution of LG beam is divergent and weakened, and the position of the singularity is unstable. In particular, we compare the propagation characteristics of the Gaussian beam and LG0,1 beam through the same atmospheric turbulence conditions. The experimental results show that the propagation stability of the LG beam is found to be better than that of the Gaussian beam.

Determination of the Bessel light beam cone parameter by Fourier analysis of the diametral distribution of the intensity

Power spectral density (PSD) of the intensity distribution of an ideal Bessel light beam (BLB) expressions are obtained. It is shown that the PSD of the intensity distribution of an ideal BLB is limited and the upper boundary of the PSD is equal to the doubled cone parameter (the product of the wave number and the sine of the half angle of the cone of wave vectors), and the number of zeros in the PSD of an ideal BLB located in the interval from zero frequency to the upper boundary of the spectrum, is equal to the order of the beam. A technique is proposed for estimating the cone parameter of BLBs by analysing the PSD estimates of diametral distributions – one-dimensional intensity distributions obtained in the transverse plane of the beam along a line passing through the beam axis from transverse two-dimensional BLB intensity distributions recorded in the form of digital computer images. The influence of clipping (the end of the distribution at the boundary of the sensitive sensor), Gaussian and cosine modulations of the diametral distribution on the shape of the spectrum and, in case of clipping, the error in the cone parameter estimation using the proposed method is investigated. The technique is tested on the example of the analysis of experimental BLBs of different orders (BLBm, m = 0, 1, 2). The proposed technique can be used to assess the quality parameters of the BLB. The results of the analysis of the influence of cosine modulation on the shape of the PSD can be used to assess the magnitude of the rounding of the apex of the axicon forming the BLB for control the quality of the axicon manufacturing.

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method. Based on the plane-wave angular spectrum expansion and Fresnel approximation, the analytical expressions of the reflected field, internal field as well as transmitted field in each region are obtained. Through numerical simulations, the intensity distributions of the incident beam, reflected beam, internal beam as well as transmitted beam are presented at oblique incidence. Besides, we also compare the intensity distributions of the geometrical-optics beam field, the first order beam mode field and the actual beam field, which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab. Meanwhile, the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated. Finally, the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail. The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.

Utilizing a Diffractive Focus Beam Shaper to Enhance Pattern Uniformity and Process Throughput during Direct Laser Interference Patterning.

Uniform periodic microstructure formation over large areas is generally challenging in Direct Laser Interference Patterning (DLIP) due to the Gaussian laser beam intensity distribution inherent to most commercial laser sources. In this work, a diffractive fundamental beam-mode shaper (FBS) element is implemented in a four-beam DLIP optical setup to generate a square-shaped top-hat intensity distribution in the interference volume. The interference patterns produced by a standard configuration and the developed setup are measured and compared. In particular, the impact of both laser intensity distributions on process throughput as well as fill-factor is investigated by measuring the resulting microstructure height with height error over the structured surface. It is demonstrated that by utilizing top-hat-shaped interference patterns, it is possible to produce on average 44.8% deeper structures with up to 60% higher homogeneity at the same throughput. Moreover, the presented approach allows the production of microstructures with comparable height and homogeneity compared to the Gaussian intensity distribution with increased throughput of 53%.

Transversal and axial modulation of axicon-generated Bessel beams using amplitude and phase masks for glass processing applications.

The control of laser-induced microcracks in the volume of transparent materials is essential for scribing processes. In this paper, we investigate the effect of various amplitude and single-level phase masks on both transverse and axial intensity distribution of the conventional axicon-generated Bessel beams. Furthermore, we demonstrate the volumetric crack control induced by an asymmetrical central core with an appropriately selected intensity level to avoid the influence of peripheral intensity maxima. Proper alignment of cracks and intra-distance between the modifications results in the reduced separation stress of the scribed samples. Furthermore, the additional amplitude modulation of the incident Gaussian beam is introduced to flatten the axial intensity distribution of the axicon-generated Bessel beam.

A method for spatio-temporal characterization of pulse beams with an all-optical image sensor

A method for analyzing the spatio-temporal intensity distribution of pulse beams, such as photons, electrons and other particles, with an all-optical image sensor has been developed. The image sensor measures the photon or particle intensity by probing the induced refractive index change due to their excitation in the semiconductor. In order to prove this scheme, the interaction of the electron beam with the image sensor has been simulated, indicating that the transient phase grating can be created in the semiconductor. The transient phase grating can be probed with a laser to evaluate the spatio-temporal intensity distribution of the incident electrons. In addition, an InP-based image sensor has been established for analyzing pulse laser beams. A pump-probe technique has been utilized to record images with varying delays between the analyzed and probe lasers and a differential method has been developed to improve the temporal resolution to be less than the duration time of probe lasers. Sequential images, representing the spatio-temporal distribution of the analyzed laser beams with several hundreds of picoseconds temporal resolution, have been obtained, indicating the ability of this scheme for analyzing the spatio-temporal distribution of pulse laser beams and paving the way for promising applications in characterization of pulse particle beams.

Period of droplet quasi-Bessel beam generated with the round-tip axicon

We demonstrate the experimental study of the axial intensity distribution of a quasi-Bessel beam with a droplet structure of the central core, formed by an axicon with the round-tip, and the results of the theoretical calculations. We show that the period of droplet quasi-Bessel beam is determined by the shape of the surface rounding and the angle at the top of the axicon lens and depends on the distance to it. The analysis of this dependence makes it possible to restore the shape of the round-tip of the axicon without 3D scanning. Keywords: bessel beams, axicon, droplet beams.

Influence of dynamic beam shaping on the geometry of the keyhole during laser beam welding

In deep penetration laser welding the shape of the keyhole is one of the determining variables for the energy input for each material and therefore has a significant influence on the resulting weld seam. A variation of the intensity distribution on the sample's surface changes the shape of the keyhole as well as the distribution of the locally absorbed irradiance within the keyhole. Coherent beam combining through optical phased arrays enables an almost arbitrary variation of the local intensity distribution of the laser beam as well as switching between different intensity distributions at high frequencies. The impact of dynamic beam shaping on the longitudinal and transversal extension of the keyhole was analyzed by means of streak images. Subsequently the frequency of changes in the extent were determined by the application of a fast Fourier transform on the grayscale values of the streak images. The results indicate a high correlation between the temporal variation of the intensity and the geometry of the keyhole.