Intensity Distribution Of Beam Research Articles

Watt-level cladding-pumped bismuth-doped fiber laser operating near 1.31 [formula omitted]m

We report, to the best of our knowledge, the first CW bismuth-doped fiber laser operating at around 1.31 μm with an output power of >1 W utilizing a cladding-pumping configuration with multimode semiconductor laser diodes emitting at a wavelength of 793 nm. The Bi-doped fiber (BDF) serving as an active medium of the laser was based on a pedestal-index design, where P2O5−SiO2 glass core surrounded by P-doped cladding. The BDF construction offers advanced gain characteristics and an increased mode size (∼13 μm). The fiber preform was fabricated by the conventional modified chemical vapor deposition (MCVD) method combined with all gas-phase doping technique. The BDF with low-index polymer coating provided sufficient cladding peak absorption at 750 nm belonging to bismuth active centers associated with phosphorus atoms (BACs-P) that allowed to develop a continuous-wave laser with a linear cavity configuration and enable a maximum slope efficiency of ∼ 4% with respect to the absorbed pump power. In addition, near-field spatial intensity distribution of optical beam has good quality (M2<1.13) and close to Gaussian mode profile in x and y directions. The perspectives for optimization of BDF design and laser system configurations based on such fibers for further power scaling are discussed.

Talbot phenomenon in binary optical gratings under Gaussian illumination

Binary optical gratings are used in applications such as optical metrology. Due to the fact, that self-imaging phenomenon depends on the grating fill factor, in present work the effect of fill factor on Talbot image formation has been analyzed when the grating is illuminated with a Gaussian beam. A model based on Fresnel regime is used to determine the intensity distribution in the near field. Self-images are obtained under different initial beam conditions and it is found several regimes of the effect of Gaussian beam intensity distribution on the self-images are found: (I) decreasing intensity of grating slit at the beam border when Gaussian waist radius is 10 times greater than grating period, (II) transforming binary-like self-image to the cosine-like one in case when Gaussian waist radius ranges from 2 to 5 grating periods, and (III) disappearing regular grating structure when Gaussian waist radius is less than grating period. Additionally it is analyzed transform from Fresnel to Fraunhofer regime in case of Gaussian beam and effect of wavefront curvature radius value on the distance where different regimes appear.

Symmetric and asymmetric Tricomi-Gaussian beams in a gradient-index medium

In this paper, the propagation dynamics of the Tricomi-Gaussian Beam (TGB) in a gradient-index medium is studied. Influences of various beam parameters, the gradient-refractive index coefficient, and vortices on the propagation of TGB are investigated thoroughly. The research is mainly on the transmission characteristics of TGB with different modes, including symmetric and asymmetric patterns. Notably, the manipulations of beam modes, propagation trajectory, and intensity distribution of TGB by varying beam parameters α and β are explored. In addition, the autofocusing properties of TGB under off-axis conditions are investigated, which could find potential applications in optical trapping, laser energy focusing, and photonic waveguiding.

Twisted splitting and propagation factor of superimposed twisted Hermite-Gaussian Schell-model beams in turbulent atmosphere

We believe this to be a new superposition twisted Hermite-Gaussian Schell-model (STHGSM) beam hat is proposed. Analytic formulas for the intensity distribution and propagation factor of the STHGSM beam in non-Kolmogorov turbulence are derived by utilizing the generalized Huygens-Fresnel principle (HFP) and the Wigner function. The evolution characteristics of STHGSM beams propagating are numerically calculated and analyzed. Our findings indicate that the light intensity of the STHGSM beam gradually undergoes splitting and rotation around the axis during propagation through non-Kolmogorov turbulence, eventually evolving into a diagonal lobe shape at a certain distance of transmission. The anti-turbulence capability of the beam strengthens with higher beam order or twist factor values.

Simulation of the reflection of a high energy laser beam at the sea surface for hazard and risk analyses

The application of a high energy laser beam in a maritime scenario necessitates a laser safety concept to prevent injury to personnel or uninvolved third parties from uncontrolled reflections of laser light from the sea surface. Therefore, it is crucial to have knowledge of the amount and direction of reflected laser energy, which varies statistically and depends largely on the dynamics of the wavy sea surface. These dynamics are primarily influenced by wind speed, wind direction, and fetch. An analytical model is presented for calculating the time-averaged spatial intensity distribution of the laser beam reflected at the dynamic sea surface. The model also identifies the hazard areas inside which laser intensities exceed a fixed exposure limit. Furthermore, as far as we know, our model is unique in its ability to calculate the probabilities of potentially eye-damaging glints for arbitrary observer positions, taking into account the slope statistics of gravity waves. This is a critical first step toward an extensive risk analysis. The simulation results are presented on a hemisphere of observer positions with fixed radii from the laser spot center. The advantage of the analytical model over our numeric (dynamic) model is its fast computation time. A comparison of the results of our new analytical model with those of the previous numerical model is presented.

Research on high precision time-frequency transmission under low available bandwidth based on free space optical communication with special beam

As a powerful supplement to the wireless communication, free space optical (FSO) communication has many advantages, but the high-precision time-frequency transmission based on FSO communication is limited by complex equipment factors, especially under the condition of limited communication bandwidth. In this paper, a high-precision frequency transfer system based on a tunable optical field is established. Theoretical analysis yields the detection probability and intensity distribution of Bessel beams. In experiments, Bessel beams generated through optical field modulation exhibit enhanced laser data communication capabilities, achieving high-precision frequency transfer at 2e-12@1s, 3.3e-14@1000s,. This research introduces a novel approach for designing and optimizing frequency transfer methods between smaller platforms in the future.

Archimedes spiral beam: composite of a helical-axicon generated Bessel beam and a Gaussian beam

This paper introduces a structured beam with Archimedes spiral intensity distribution. The Archimedes spiral (AS) beam is the composite of a helical-axicon generated (HAG) Bessel beam and a Gaussian (GS) beam. We observed the spiral intensity patterns using computational holography, achieving the tuning over spiral arms number and spiral spacing. Analyzing the propagation dynamics of AS beams, we present that the spiral intensity will reverse beyond the maximum diffraction-free distance. Before and after the beam reverse, the spiral spacing remains constant, but the spiral direction is opposite. In addition, we obtain the Archimedes spiral equations to describe the spiral intensity patterns. Unlike the beams with Fermat and hyperbolic spiral patterns, the intensity distributions of AS beams are isometrically spiral. The isometric spiral intensity makes it possible to form particle isometric channels. AS beams have potential application prospects in particle manipulation, microscopic imaging, and laser processing.

Structurally Invariant Higher-Order Ince-Gaussian Beams and Their Expansions into Hermite-Gaussian or Laguerre-Gaussian Beams

Paraxial beam modes, which propagate in space and focus without changing their transverse intensity pattern, are of great value for multiplexing transmitted data in optical communications, both in waveguides and in free space. The best-known paraxial modes are the Hermite-Gaussian and Laguerre-Gaussian beams. Here, we derive explicit analytical expressions for Ince-Gaussian (IG) beams for several first values of the indices p = 3, 4, 5, and 6. In total, we obtain expressions for the amplitudes of 24 IG beams. These formulae are written as superpositions of the Laguerre-Gaussian (LG) or Hermite-Gaussian (HG) beams, with the superposition coefficients explicitly depending on the ellipticity parameter. Due to simultaneous representation of the IG modes via the LG and HG modes, it is easy to obtain the IG modes in the limiting cases wherein the ellipticity parameter is zero or approaches infinity. The explicit dependence of the obtained expressions for the IG modes on the ellipticity parameter makes it possible to change the intensity pattern at the beam cross-section by continuously varying the parameter values. For the first time, the intensity distributions of the IG beams are obtained for negative values of the ellipticity parameter. The obtained expressions could facilitate a theoretical analysis of properties of the IG modes and could find practical applications in the numerical simulation or generation of such beams with a liquid-crystal spatial light modulator.

Simulation of evaporation ablation dynamics of materials by nanosecond pulse laser of Gaussian beam and flat-top beam

Based on the established two-dimensional asymmetric model of the interaction between a nanosecond pulse laser and metallic aluminum, the effect of beam shaping on the evaporation ablation dynamics during the ablation of metallic aluminum by a nanosecond pulse laser is simulated. The results show that plasma shielding, which has a significant influence on the ablation properties of the target, occurs mainly in the middle phase and late phase of the pulse. Among the three laser profiles, the Gaussian beam has the strongest shielding effect. As the diameter of the reshaped flat-top beam increases, the shielding effect gradually weakens. The two-dimensional spatial distribution of target temperature is relatively different between ablation by a Gaussian beam and that by a flat-top beam. For the Gaussian beam, the center of the target is first heated, and then the temperature spreads in radial direction and axial direction. For the flat-top beam, due to the uniform energy distribution, the target is heated within a certain radial range simultaneously. Beam shaping has a great influence on the evaporation ablation dynamics of the target. For the Gaussian beam, the center of the target is first ablated, followed by the radial ablation. For the flat-top beam, the evaporation time of the target surface is delayed due to the lower energy density after the beam has been shaped. In addition, the target evaporates simultaneously in a certain radial range due to the more uniform distribution of laser energy. For each of the three laser profiles, the evaporation morphology of the target resembles the intensity distribution of the laser beam. The crater produced by the Gaussian beam is deep in the center and shallow on both sides, while it becomes relatively flat by the flat-top beam.

Thermal Blooming Effect of Power-Exponent-Phase Vortex Beams Propagating through the Atmosphere

The thermal blooming effect of power-exponent-phase vortex (PEPV) beam propagating in the atmosphere is investigated by employing the multiple phase screen method. The influences of propagation distance, topological charge, power exponent, wind speed, and absorption coefficient on thermal blooming effect are analyzed in detail. The results show that (1) the thermal blooming effect exhibits a significant perturbation on the intensity and phase distribution of PEPV beams, with its influence becoming more pronounced as the propagation distance increases; (2) when the power exponent is fixed at 4, comparing the thermal blooming effect of PEPV beams with different topological charges indicates that a PEPV beam with topological charges of 3 is the most sensitive to thermal blooming; (3) when the topological charge is fixed at 3, the thermal blooming effect decreases with the increase in power exponent; and (4) an increase in wind speed or a decrease in absorption coefficient can reduce the thermal blooming effect. The research results obtained in this article have guiding significance for the application of research into high-energy PEPV beams in the atmosphere.

Generation of Vortex Beams via All-Dielectric Spin-Multiplexed Coding Metamaterials

The vector vortex beams are generated by the single-phase modulation of metasurfaces, which severely limits the freedom of design. We use the combination of the propagation phase and the geometric phase to construct a transmissive all-dielectric encoded metasurface. Under two orthogonal circularly polarized incidence conditions, vortex beams with different topological charge can be generated. We utilize the hybrid-order Poincaré sphere to characterize various polarization states of Poincaré vortex beams. Based on the near-field and far-field intensity distribution and phase distribution properties of the vortex beam, the design accuracy of the Poincaré vortex beam is confirmed. In order to improve the degree of freedom of control of vortex beams, we introduce the Fourier convolution principle in digital signal processing. The convolution operation of the encoding array and the phase gradient sequence is performed to construct a new encoding metasurface sequence, and the scattering angle of the vortex beam can be adjusted. Using the principle of coded convolution, functions with arbitrary deflection and beam splitting of vortex beams can be realized.

Frontal pyrometric snapshot measurements of the keyhole wall temperature in laser welding of pure aluminium

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

Cutting thick aluminum plates using laser fusion cutting enhanced by dynamic beam shaping

Cutting thick plates is affected not only by the laser power but also by the cut kerf width and the melt flow dynamics that determine the ejection of the molten material. Employing the same laser beam intensity distribution for various thicknesses is the limiting factor when cutting thicker plates. This paper investigates fiber laser fusion cutting of 25 mm thick aluminum with dynamic beam shaping (DBS). While both static and longitudinal dynamic intensity distributions fail to cut this thickness with a 4 kW laser power, a cut through is achieved using annular and elliptical intensity distributions. However, an improvement of 45% in cutting speed can be achieved using an elliptical intensity distribution compared to an annular one. In order to understand the effect of the beam shape, an infrared thermal camera is used to study lateral heat propagation when using different process parameters. Moreover, to analyze the melt flow when changing the DBS frequency, high-speed imaging is utilized to observe the molten material inside the cut kerf. Finally, the cut edge quality is investigated for different cutting conditions.

Fiber laser cutting of steel plate by twin spot beam setting in scanning direction

This study investigated the effects of laser beam intensity distribution on the reduction of dross height in fiber laser cutting of a steel plate with 3.2 mm thickness. A twin-spot beam was produced by splitting a single Gaussian beam into two beams using a special axicon lens, and these beams were set in the scanning direction for cutting experiments. The power ratio of two beams (R:F = Rear power:Front power) was varied to discuss the intensity balance for the effective reduction of dross. After cutting experiments, ray tracing analysis was conducted using an optical analysis to calculate the absorbed power density distributions in the kerf. A smaller dross height of 18 μm can be achieved at a power ratio of R:F = 8:2, and its value is lower than that by a single Gaussian beam. At a power ratio of R:F = 8:2, the front beam of lower power is irradiated at the upper part of the workpiece, and the rear beam of higher power is absorbed at the lower part of the workpiece. Thus, effective heat input to the lower part of the workpiece can contribute to a reduction of the dross height. Variation of power ratio in the rear and the front beams is effective in controlling the cutting front shape, and the uniformity of absorbed power in the thickness direction can be improved by setting the rear beam of about four times higher power to the front beam of lower power to obtain a smaller dross height in the case of a 3.2 mm steel plate.

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%.