Intensity Distribution Of Beam Research Articles

A Simulation of Air Lasing Seeded by an External Wave in a Femtosecond Laser Filament.

Air lasers induced by femtosecond laser filaments play an important role in remote sensing applications. Few studies have been dedicated to the spatial distribution of external-seeded air laser radiation in the laser filament based on the numerical simulation method, which can pave the way to understanding the mechanism of the external-seeded air lasing process during filamentation. In this study, numerical simulations of the propagation of an air laser seeded by an external plane wave with a wavelength of 391 nm during femtosecond laser filamentation were performed. The results indicated that the air laser's beam intensity distribution varies from a ring pattern to a donut pattern when the filament length and nitrogen ion density are raised as a result of the defocusing and lasing effects of the filament plasma. Here, the ring pattern is formed by several thin rings, while the donut pattern refers to a notably thicker, ring-like structure. In addition, it has been demonstrated that the air laser's beam power would increase exponentially versus the filament length and the nitrogen ion density. The knowledge about the angular distribution of air lasing could be important for optimizing the detection geometry of the LIDAR setup, including the view angle and the size of the collecting optical component.

Capturing Vector Light Field on Azo Molecular Glass Submicron Pillar Array for Polarization Recording and Creating Optical Functional Surfaces

AbstractVector light fields possessing spatial‐variant states of polarization (SOPs) have attracted great research interest for various emerging applications. In this work, the responses of submicron pillar arrays of an azo molecular glass (IA‐Chol) to vector light fields are systematically investigated by using laser beams with different topological charges. The spatial‐variant SOPs in the illuminated areas are recorded by directional pillar deformations along the local electric field oscillation directions, and uncovered by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The degrees of pillar deformations are proven to be correlated to the intensity distributions of the vector beams in the transverse planes. The pillar deformations caused by the vector beam irradiations generate several optical effects related to the surface structures, as revealed by optical microscopy, polarizing optical microscopy, and diffraction measurements. Based on the understanding, structural colors covering a wide spectral range are created for the pillar arrays after irradiated with the vector beams. The complex images with various structural colors are produced by the irradiations with vector beams through photomasks. These investigations reveal the unique correlations between the pillar deformations and transverse polarization morphologies of vector beams and demonstrate a promising approach to fabricate optical functional surfaces.

Combined super-Gaussian pump intensity profile model and its application to a solid-state laser

The modeling of a combined super-Gaussian profile distribution for fiber-coupled laser diode beam intensity is investigated in this paper. By fitting the measured beam intensity distribution to the combined super-Gaussian model, the coefficient of determination is better than that of the Gaussian function and super-Gaussian function with order of 2. Applying this theoretical model to the fiber-coupled laser diode end-pumped Nd:GdVO4 laser at 1342 nm, the theoretical calculations are in good agreement with the experimental results.

Deep learning self-image update procedure in a wavefront sensor based on the Talbot phenomenon under Gaussian illumination

A feature-based image update procedure using machine learning is proposed to use in preprocessing of self-images in a Talbot wavefront sensor. A variant of the recurrent neural network with backpropagation, which is one of most widely applied machine learning tools, is utilized to stabilize intensity distribution in self-images in the case of an optical beam with a Gaussian profile. Once well trained, the neural network can decrease pit image shifts caused by beam intensity distribution in the case of a cosine-like grating. It is shown that based on the proposed recurrent neural network, it is possible to decrease the shift error caused by the Gaussian beam up to nine times depending on the aberration order and value. Despite the decreasing shift error, the value of the error of the restored aberration coefficient does not decrease significantly because of the feature-vector extraction method. It is shown additionally that due to the spatial spectrum wideness, the proposed self-image procedure is not applicable to binary gratings on the example of binary gratings with square pits. Adequate simulations are implemented to demonstrate the effectiveness and accuracy of the proposed approach.

Inverse design of optical correlation induced effects

A comprehensive theoretical framework for the inverse design of correlation induced effects with optical beams is introduced. Correlation induced effects are able to modify the intensity distribution of an optical beam drastically via effects such as correlation induced splitting, focusing, and shifting. The inverse design steps are given analytically, which allows the analysis of several related experiments. Finally, an algorithm for more complex numerical inverse design is overviewed and demonstrated.

Theoretical and experimental researches for spectral beam combining based on a concave grating

A novel spectral beam combining (SBC) system based on a concave grating with a Rowland-circle mount was designed and built. Theoretically, the aberrations, as well as the normalized intensity distribution of a diffracted beam with the wavelength of 1550 nm were calculated, and the grating mount was optimized by using the recursion approach. The results show that the optimized mount is similar to paraboloid shape, under the optimized mount, the aberration is significantly reduced, and the intensity distribution of the diffraction beam is almost consistent with the intensity distribution of the output of a fiber. Further, the combining result of 5-element fiber lasers was simulated, which indicates that the beams of the fiber lasers can be made to spatially overlap. Experimentally, for the output beams of two fiber lasers with the wavelengths of 1536.4 nm and 1543.6 nm, the measured diffracted powers are 0.336 W and 0.368 W with the beam quality factors (Mx2) of 1.78 and 1.82. Further, the combined power and Mx2 were measured as well, which are 0.659 W and 2.52 with the combining efficiency of about 71%.

Two-Stage Link Loss Optimization of Divergent Gaussian Beams for Narrow Field-of-View Receivers in Line-of-Sight Indoor Downlink Optical Wireless Communication (Invited)

The predominant focus of research in high-speed optical wireless communication (OWC) lies in line-of-sight (LOS) links with narrow infrared beams. However, the implementation of precise tracking and steering necessitates delicate active devices, thereby presenting a formidable challenge in establishing a cost-effective wireless transmission. Other than using none-line-of-sight (NLOS) links with excessive link losses and multi-path distortions, the simplification of the tracking and steering process can be alternatively achieved through the utilization of divergent optical beams in LOS. This paper addresses the issue by relaxing the stringent link budget associated with divergent Gaussian-shaped optical beams and narrow field-of-view (FOV) receivers in LOS OWC through the independent optimization of geometrical path loss and fiber coupling loss. More importantly, the geometrical path loss is effectively mitigated by modifying the transverse intensity distribution of the optical beam using manipulations of multi-mode fibers (MMFs) in an all-fiber configuration. In addition, the sufficiently excited higher order modes (HOMs) of MMFs enable a homogenized distribution of received optical powers (ROPs) within the coverage area, which facilitates the mobility of end-users. Comparative analysis against back-to-back links without free-space transmission demonstrates the proposed scheme’s ability to achieve low power penalties. With the minimized link losses, experimental results demonstrate a 10 Gbps error-free (BER < 10−13) LOS OWC downlink transmission at 2.5 m over an angular range of 10° × 10° without using any optical pre-amplifications at a typical PIN receiver. The proposed scheme provides a simple and low-cost solution for high-speed and short-range indoor wireless applications.

Visualization of hydrogen jet using intensity projection of the laser beam

The main objective of this work is to propose a new hydrogen leakage visualization method to evaluate hydrogen jet pressure. A mathematical model is proposed to reveal the intensity distribution of the laser beam through the hydrogen jet at different outlet pressures. Our findings show that the hydrogen jet can be regarded as a gas lens with adjustable laser beam energy, and a change in jet pressure causes a change in the half-height width of the spectrum, which affects the intensity distribution of the laser beam after it passes through the hydrogen jet. The error between the light intensity distribution of the theory and the light intensity distribution obtained from the experimental results is 2.59%. The error is within the allowable range. Our technique shows that the variation of the hydrogen jet pressure can be estimated by analyzing the intensity data after the laser passes through the hydrogen jet.

Flexible tailoring of multifocal spot arrays of partially coherent radially polarized beam with periodical spatial coherence

In this work, a new kind of partially coherent vector beam, namely, partially coherent radially polarized multi-Gaussian Schell-model array (PCRP MGSMA) beam, is introduced and the focusing properties of such beam passing through a thin lens are characterized. One of the most remarkable features of such beam is its capability of producing tunable multifocal spot arrays in the focal plane, and each array lobe can be shaped into dark hollow, Gaussian-like and flat-topped profile by modulating the initial spatial coherence width More interesting, the modulation effect of multiple off-axis vortices can also be acted on each array lobe, thus the creation of triangle-like, square-like and pentagon-like hollow structure arrays can be realized for the high coherence case, but this remarkable characteristic will be vanished in the case of a very low coherence. In addition to the focal spot arrays tailoring, the focal intensity distribution of such beam can be flexibly shaped into diversified flat-topped profiles, such as linear, square and rectangular, by elaborately choosing the matching parameters of the source. These interesting results will be useful for optical multi-particle manipulation, high resolution imaging and material surface processing.

Terahertz Bessel beam generator.

The Bessel beam has broad application prospects in wireless energy transmission and high-speed communications. The traditional Bessel beam generation method has the problems of large volume, low efficiency, and complex manufacturing. To solve the above problems, we present a terahertz Bessel beam generator based on the reflective metasurface, which is composed of a metal pattern, dielectric layer, and bottom metal plate. Under the incidence of right circularly polarized (RCP) wave, the zero-order Bessel beam and zero-order symmetric double Bessel beam are generated. It can be found that the bottom angle of the axicon of the first-order Bessel beam is inversely proportional to the propagation distance of the Bessel beam. Comparing the electric field intensity distribution, phase distribution, and mode purity of the second-order Bessel beam and the second-order vortex beam in different observation planes, it can be seen that the energy of the higher-order Bessel beam is more concentrated and the field distribution is more stable than those of the ordinary vortex beam. The reflective terahertz Bessel beam generator has potential application value in terahertz wireless communications, measurement, radar detection, and imaging.

Theoretical and experimental research on a spatially modulated femtosecond bessel-like laser for microdrilling in silica glass

Beam shaping has been widely used in laser processing. Compared with Bessel beam, Bessel-like beam has unique advantages of fabricating high-aspect-ratio holes due to its tunable and longer non-diffraction length. However, the theoretical and experimental optimization of Bessel-like beams for laser manufacture has remained to be investigated. In this paper, we established a theoretical model of the intensity distribution and peak energy density of the Bessel-like beams based on a simplified phase of arc axicon. It was found that the longer propagation length and the lower peak energy density of the Bessel-like beams, contributed to the longer ablation length and higher ablation precision in the silica glass. The ablation length of the optimized Bessel-like beams was at least 1.6 times longer than that of the Bessel beam, which furtherly improved the fabrication efficiency of microdrilling.

Light field analysis for modeling and transmission characteristics of partially coherent light-emitting diodes

When analyzing the transmission characteristics of LEDs for long-distance lighting and communication applications, the light field is commonly assumed to be fully incoherent. However, in reality, the LED light source emits partially coherent light with a spatial coherence length on the order of microns. This paper is based on the generalized higher-order Lambert model of LEDs and aims to construct a Gaussian-Schell model for the LED beam (LED-GSM) on the near-field source plane, with a half-power angle of no more than 10o. Utilizing the cross-spectral density function transmission theory for partially coherent light, this paper provides the LED-GSM model’s spatial coherence length and beam radius at different distances and designs an experiment for measuring the spatial coherence length of LED beams. Experimental measurements of the spatial coherence length and beam spot size of LED beams at different distances are carried out using a Thorlabs LED528EHP light source. The experimental results match well with the theoretical simulations of the LED-GSM model, thus validating its effectiveness. Then, the proposed LED-GSM model is utilized to investigate the long-distance transmission characteristics of partially coherent narrow-beam LED light. Simulation results indicate that the spatial coherence length of the LED light field can reach tens to hundreds of millimeters over transmission distances of several kilometers. The beam radius is much smaller than that of the beam radius based on the fully incoherent model, and the beam intensity distribution also displays distinct differences.

Intensity correlation scan (IC-scan) technique to characterize the optical nonlinearities of scattering media

Light scattering, whether caused by desired or spurious elements, is considered one of the main phenomena that present great challenges for the nonlinear (NL) optical characterization of turbid media. The most relevant disturbing factor is the random deformation suffered by the spatial intensity distribution of the laser beam due to multiple scattering. In this work, we report the intensity correlation scan (IC-scan) technique as a new tool to characterize the NL optical response of scattering media, by taking advantage of light scattering to generate speckle patterns sensitive to wavefront changes induced by the self-focusing and self-defocusing effects. Peak-to-valley transmittance curves, with a higher signal-to-noise ratio, are obtained by analyzing the spatial intensity correlation functions of the different speckle patterns, even in very turbid media where conventional NL spectroscopy techniques fail. To demonstrate the potential of the IC-scan technique, the NL characterization of colloids that contain a high concentration of silica nanospheres as scatterers, as well as gold nanorods, which act as NL particles and light scatterers, was performed. The results show that the IC-scan technique is more accurate, precise and robust to measure NL refractive indices in turbid media, overcoming limitations imposed by well-established Z-scan and D4σ techniques.

Properties of circular cosh-gaussian beams focused by a thin lens system under a turbulent atmosphere

The propagation of a Circular cosh-Gaussian beam (CiChGB) through a paraxial ABCD optical system in a turbulent atmosphere is investigated based on the extended Collins formula and Rytov theory. In particular, the analytical expression for the on-axis average intensity of a CiChGB focused by a thin lens in a turbulent atmosphere is derived in detail. From the obtained formula, the on-axis intensity distribution of the output beam is discussed numerically with a comparison to direct propagation in free space. The influences of the turbulence strength and initial beam parameters on the on-axis intensity are examined. It is found that the on-axis intensity near the focus region is less affected by atmospheric turbulence compared to the free space.

Single-pass cutting of frosted glass via change of laser incident medium.

We report a water medium-assisted composite laser cutting (WMACLC) technology for what is believed to be the first time to achieve single-pass separation of frosted glass (FG). The water medium was used to flatten the surface of FG to reduce the diffuse reflection and random refraction of the incident laser. The simulation results of picosecond pulsed laser Bessel beam (PPLBB) intensity distribution in FG showed that the peak intensity in the presence of water can reach about 24 times and 2.3 times that in the absence of water when the PPLBB is 0.08 mm and 0.3 mm below the upper surface of FG, respectively. A PPLBB with higher intensity can be formed along the thickness direction to realize the material modification. A coaxial CW laser provides the thermal tensile stress required for separation. Finally, high-quality separation of FG was achieved using the WMACLC technology with a speed of 50 mm/s. No deviation in the separation track and no edge collapse occurred. The roughness Sa of the separated sidewall is less than 0.3 µm.

Optical design and stray RF investigation of ITER EC H&CD equatorial launcher using mockup

The aim of this study is to assess power transmission characteristics of ITER EC H&CD equatorial launcher (EL) which will deliver 20 MW of millimeter wave power to the ITER tokamak. A precise optical mockup of the launcher is fabricated in order to experimentally verify the characteristics of 170 GHz beam transmission. To observe the intensity distribution of output beams, a mobile high-sensitivity power detection system is used for millimeter wave monitoring, which is in good agreement with quasi-optical calculations. Internal stray RF generation presents an additional concern regarding heat load inside the launcher especially in case of deteriorated input mode purity. Therefore, firstly, the stray RF power is quantified by observation after the beam duct (i.e., launcher exit) of the mockup. The results show that a significant portion of stray RF (>20%) is emitted and does not contribute to the heat load. For another approach to estimate the heat load, geometric optics is performed with various degrees of scattering. This approach indicates that nearly half of the stray RF power is emitted from the launcher exit. Thus, the internal heat load due to stray RF is calculated 44 kW/m2, which is an important result in relaxing the design conditions of the EL.

Propagation of vortex optical beams through artificial convective turbulence

The propagation of vortex optical beams through artificial convective turbulence is studied experimentally. It is found that as the refractive turbulence intensifies, instantaneous intensity distributions of vortex beams become speckled, and the initial ring structure of a beam distorts and disappears. In the averaged intensity distributions, the ring structure is gradually blurring with strengthening the refractive turbulence and becomes close to the Gaussian one with the intensity maximum at the center of the beam. The random wandering of vortex beams, whose initial transverse size increases with an increase of the topological charge, and beams, whose initial transverse size remains unchanged with a change of the topological charge, is compared. It is shown that the amplitude of random displacements of a vortex beam energy centroid is independent of the topological charge.

Periodic oscillation behavior of chirped cosh-Gaussian beam in parabolic potential

In this paper, based on the normalized linear Schrödinger equation, we analytically and numerically investigate the propagation dynamics of cosh-Gaussian beams in a medium with parabolic potential. The obtained results show that cosh-Gaussian beams perform a periodic oscillation behavior and the parabolic potential determines the focusing ability, including the focal distance as well as peak intensity at the focus. Especially, the intensity distributions of cosh-Gaussian beam are related to the initial parameter. We also demonstrate the effect of chirp factors on the beam and find the periodic oscillating behavior caused by linear chirp can be used to manipulate the propagation trajectory. The influence of quadratic chirp on the evolution of beam is also discussed and we find that it plays a significant role in the modulation of the energy localization.

Long-term repeatability improvement using beam intensity distribution for laser-induced breakdown spectroscopy

The relatively low measurement repeatability has long been considered as a major obstacle to the widespread use and commercialization of laser-induced breakdown spectroscopy (LIBS). Although many efforts have been made to improve the signal repeatability in the short term, how to improve the long-term signal repeatability is critical in practical applications and has rarely been studied. Moreover, the mechanisms behind the degradation of long-term repeatability are not fully revealed. This study proposes a new method to improve the long-term repeatability of LIBS measurement, which modifies the spectral intensity based on laser beam intensity distribution. It first pre-processes the beam intensity distribution profiles and spectral intensity. Then the relationship between the relative deviations of beam and spectral intensities is modelled using Partial Least Squares Regression (PLSR). The proposed method was tested on copper and silicon samples, and the spectra and laser beam intensity distribution were recorded for more than thirty days. Day-to-day variations in beam intensity distribution were observed. Such variations can lead to changes in spectral intensity, resulting in degraded signal repeatability. By modifying the spectral intensity, the long-term signal repeatability was improved. Specifically, in terms of day-mean spectral intensity, the valid correction rates were above 70% for both of copper silicon sample in most cases. Long-term RSD decreased from ∼13.5% to ∼4% for copper and decreased from ∼10.7% to 6.5% for silicon sample. These results indicate that the proposed method provides a viable method for improving the long-term repeatability of LIBS measurement.

Sensitivity Analysis of a 6 MeV Photon Beam Monte Carlo Model.

This study aimed to optimize efficiency in Monte Carlo (MC) simulation using sensitivity analysis of a beam model. The BEAMnrc-based model of 6 MV beam of a Siemens Primus linac was developed. For sensitivity analysis, the effect of the electron source, treatment head, and virtual phantom specifications on calculated percent depth dose (PDD) and lateral dose profiles was evaluated. The optimum mean energy (E) and the full width at half maximum (FWHM) of the intensity distribution of the electron beam were calculated as 6.7 MeV and 3 mm, respectively. Increasing E from 6.1 to 6.7 MeV, increased the PDD in the fall-off region by 4.70% and decreased the lateral profile by 8.76%. Changing the FWHM had a significant effect on the buildup region of PDD and the horns and out-of-field regions of the lateral profile. Increasing the collimators opening by 0.5 mm, PDD increased by 2.13% and the central and penumbra regions of profiles decreased by 1.98% and 11.40% respectively. Collimator properties such as thickness and density were effective in changing the penumbra (11.32% for 0.25 cm increment) and the out-of-field (22.82% for 3 g/cm3) regions of the lateral profiles. Analysis of a 6 MV model showed that PDD profiles were more sensitive to changes in energy than to FWHM of the electron source. The lateral profiles were sensitive to E, FWHM, and collimator opening. The density of the collimator affected only the out-of-field region of lateral profiles. The findings of this study may be used to make benchmarking of an MC beam model more efficient.