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Structured light fields have attracted much attention due to rich spatial degrees of freedom. The tailoring of an arbitrary structured light field on demand is the precondition for the application of structured light. Therefore, the computer holography method used to reconstruct a coherent light field wavefront has been naturally applied for generating structured light. In this work, we comprehensively demonstrate the principles and procedures of pure-phase computer-generated holography (PP-CGH) and binary-amplitude computer-generated holography (BA-CGH) methods for tailoring structured light, realized by two digitally programmable devices: liquid-crystal spatial light modulators ( Lc - SLM ) and digital micromirror devices (DMD), respectively. Moreover, we first compare the two approaches in detail and clarify the recipe to obtain a high tailoring accuracy and efficiency, which will help researchers to better understand and utilize the holographic tailoring of structured optical fields. [ABSTRACT FROM AUTHOR]

Since optical wireless power transmission (OWPT) transmits power by light, which has a narrow diffraction angle feature, it is a strong candidate for wireless power transmission systems supporting long ranges. To develop a realistic operational OWPT system, clarification of system level requirements is essential. In this study, to fill a gap between the concept/initial demonstration and an operational system, the required conditions were analyzed regarding the effects of beam alignment and shaping on the power generation ratio which is a system level efficiency factor with extension from the formerly reported one-dimensional analysis to three-dimensional to include errors in all degrees of freedom is presented. This extension is regarded as an indispensable methodology to evaluate the system level performance of general OWPT systems. Numerical requirements for beam alignment and shaping are derived for both non-cooperative and cooperative OWPT. In non-cooperative OWPT, the direction of the solar cell module is fixed, and the transmitter aligns its beam with the module. In cooperative OWPT, the module and transmitter mutually align in the same direction. Though the cooperative OWPT is more restrictive than the non-cooperative one, its advantages were clarified. [ABSTRACT FROM AUTHOR]

Direct Laser Interference Patterning (DLIP) using ultrashort pulsed laser sources is a single-step process to produce micro- and nanostructured surfaces by ablation. Spatial periods of a few micrometers are patterned using a laser scanner system with two interfering beams. In this study, the field of DLIP processing is expanded by handling spatial periods in the range of 1 µm and below. Precise periodic ablation is achieved with an optical setup tailored for a homogenous topography of spatial periods in the micro and sub-micrometer range. The lateral intensity distribution of the interference area used for ablation is shaped using beam shapers in order to achieve a homogenous intensity distribution and hence increase the homogeneity of the periodic texture in the ablated area. The shape of the interference area is formed into a square profile for the purpose of seamless stitching plenty of to be processed areas. This creates large area periodic textures with high homogeneity. Two beams with ultrashort laser pulses of 10 ps duration and a wavelength of 532 nm are used to structure line-like periodic surface textures with spatial periods of 650 nm. The beam shaping elements modify the lateral intensity distribution in the interference area and the affiliated profile. This enables precise patterning of tool steels with spatial periods in sub-micrometer range for applications in the field of life science surfaces. [ABSTRACT FROM AUTHOR]

Scaling the average power of ultrafast lasers into the kW range for high-throughput surface structuring and micromachining by the pulse energy requires an adapted processing strategy in order to minimize surface defects due to high fluences and maintain a high surface quality. One promising approach to distribute the pulse energy uniformly on the sample surface is spatial beam shaping using a spatial light modulator. The local phase was modulated by computer-generated holograms for the generation of uniform intensity distributions in the focal plane. The corresponding intensity distributions resulting from the different holograms were monitored with an on-axis camera to ensure accurate beam shapes during surface structuring and micromachining of the two materials stainless steel and cemented tungsten carbide. Uniform surface structures and flexible, dimensionally accurate cavities were machined with a shaped ultrafast laser beam and high pulse energies up to over 1 mJ on the sample surface. Various beam shapes with a cross-sectional area up to 0.42 mm² were generated on the sample surface for energy-efficient micromachining and homogenous surface structuring. The capability of high-energy pulse beam shaping was demonstrated by the fabrication of a large-area checkerboard pattern with small transition zones <10 µm between unprocessed and structured areas. [ABSTRACT FROM AUTHOR]

• Novel structures in the form of anomalous polygonal azimuthal-gap beams (APAB) with an extra new dimensionality, namely binarization threshold are investigated. This kind of anomalous polygonal azimuthal-gap beam utilizes the properties of free lens and azimuthal binary phase to control the number and the positions of the dark regions at the intensity profile. • The effectiveness of capture and move the particles are verified by utilizing the controllable anomalous polygonal azimuthal-gap beams in the experiment. • More than that, the polygonal array beams with multiple gaps are presented, where the number and position of the gaps, spatial position and size of the polygonal beam are controllable. Beams with controllable shapes and multiple zero intensity breaks hold particular significance in optical trapping, optical encryption, and optical communication. In this work, an anomalous polygonal azimuthal-gap beam (APAB) exhibiting controllable discontinuous intensity distribution is proposed and studied both theoretically and experimentally. The APAB features multiple zero intensity breaks within its polygonal intensity pattern by setting multiple the binarization thresholds and modulating parameters of the free lens phases. Additionally, the specific number and arrangement of azimuthal gaps by combining multiple APABs make up a beam array characterized by the various configurations of azimuthal-gaps and customized intensity profiles. The particle manipulation characteristics of the designed APABs are verified by optical tweezer experiments. It is demonstrated that the APABs utilize azimuthal-gap location as a new dimension to stably trap, locate, rotate, disperse particles, and guide particles, while the polygonal intensity guides particles along specific paths. Due to its unique properties, APAB will have many promising applications such as multi-particle trapping in 3D orientation, optical communication, lensless imaging, and optical multiplexing technology. [ABSTRACT FROM AUTHOR]

This study explores the uncharted territory of beam shaping through a novel deformable mirror system in directed energy deposition laser wire, an emerging area in Additive Manufacturing. While beam shaping has shown substantial benefits in laser processes like welding and powder bed fusion, its potential in this specific domain remains unexploited. The research investigates the influence of three near-elliptical Gaussian beam shapes on melt pool and bead geometries during deposition with stainless-steel wire. The study reveals three distinct processing modes achievable at the same total power through beam shaping, with significant modifications observed in melt pool and bead structures. Reduced bead geometry variation and enhanced process stability were achieved with the beam shape with major axis along the wire feeding direction, and with highest average power density and intermediate peak power density. The findings underscore the potential of beam shaping to enhance robustness and increase energy utilization and productivity in this process. • Beam shaping for laser directed energy deposition with feed-stock wire was studied. • A deformable mirror-based adaptive beam shaping system was utilized. • In-situ and post-process data were qualitatively and quantitatively evaluated. • Surface variation decreased by aligning elliptical beam with wire feed. • Effectiveness in energy utilization and productivity was revealed. [ABSTRACT FROM AUTHOR]

The laser powder bed fusion (LPBF) process utilizing a focused Gaussian-shaped beam faces challenges, including pore formation, melt pool fluctuation and liquid spattering. While beam shaping technology has been explored as a potential approach for defect mitigation, the beam-matter interaction dynamics during melting with shaped beams remain unclear. Here, we report the direct observation of ring-shaped beam-matter interaction dynamics, including pore formation, melt pool fluctuation and liquid spattering, and unveil defect mitigation mechanisms in ring-shaped beam laser powder bed fusion process. We find that, by spatially manipulating incident laser rays, the ring-shaped beam controls keyhole morphology, thereby managing the distribution of the reflected rays. This manipulation can effectively eliminate the formation of an unstable cavity at the keyhole tip, stabilizing the keyhole and mitigating keyhole pores. This enhanced keyhole stability effectively reduces the melt pool fluctuation, the formation of liquid breakup induced spatters and liquid droplet colliding induced large spatters in the laser powder bed fusion process. Additionally, the high-energy forefront of the ring-shaped beam effectively melts the powder bed, reducing agglomeration liquid spatter in the laser powder bed fusion process. The discovered defect mitigation mechanisms may guide the design of beam shaping strategies for simultaneously increasing the quality and productivity of metal additive manufacturing. [Display omitted] • The dynamics of ring-shaped beam induced mitigation of pore formation, melt pool fluctuation and liquid spattering in laser powder bed fusion process are characterized and quantified by in-situ high-speed X-ray imaging. • The keyhole pore mitigation is attributed to the stabilization of the unstable keyhole tip through spatial manipulation of incident and reflected laser rays. • The melt pool fluctuation mitigation is ascribed to the reduction of disturbances to the melt pool through improved keyhole stability. • The liquid spatter mitigation is attributed to the improved keyhole stability and powder capture enabled by high-energy forefront of the ring-shaped beam. • The condition and mechanism for simultaneous mitigation of pore formation, melt pool fluctuation and liquid spattering are identified. [ABSTRACT FROM AUTHOR]

Digital programmable coding metasurfaces (DPCMs) have recently attracted enormous attention and have been broadly applied, owing to their ability to manipulate electromagnetic (EM) wave behaviours and programmable multi-functionality. Recent DPCM works are divided into reflection and transmission types (R-DPCM and T-DPCM, respectively); however, there are only a few reported T-DPCM works in the millimetre-wave spectrum, owing to the difficulty of realising the large-phase controllable range while maintaining low transmission losses with electronic control components. Consequently, most millimetre-wave T-DPCMs are demonstrated only with limited functions in a single design. Additionally, all these designs use high-cost substrate materials that constrain practical applicability, owing to cost-ineffectiveness. Herein, we propose a 1-bit T-DPCM that simultaneously performs three dynamic beam-shaping functions with a single structure for millimetre-wave applications. The proposed structure is completely constructed using low-cost FR-4 materials, and operation of each meta-cell is manipulated using PIN-diodes, thus driving the achievement of multiple effective dynamic functionalities including dual-beam scanning, multi-beam shaping, and orbital-angular-momentum-mode generation. It should be noted that there are no reported millimetre-wave T-DPCMs demonstrating multi-function design, thus showing a gap in the recent literature of millimetre-wave T-DPCMs. Moreover, cost-effectiveness can be significantly enhanced, owing to the construction of the proposed T-DPCM using only low-cost material.

• The need for the best direction of specular reflection is eliminated to the greatest extent. • Imaging quality and angle compatibility of terahertz fast reflective system are improved. • Design compact element to modulate propagation direction of wide-field illumination light. • A novel application of microlens array in terahertz imaging field. Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure. [ABSTRACT FROM AUTHOR]

The commonly used Gaussian intensity distribution during the laser-based processing of metals can negatively affect melt pool stability, which might lead to defects such as porosity, hot cracking, or poor surface quality. Hot cracking is a major factor in limiting production rates of high-strength aluminium alloys in laser-based processes such as welding or the powder bed fusion of metals (PBF-LB/M). Going away from a Gaussian intensity distribution to ring-shaped profiles allows for a more even heat distribution during processing, resulting in more stable melt pools and reduced defect formations. Therefore, the aim of this study is to investigate the influence of different laser beam profiles on the processing of high-strength aluminium alloys by using a multicore fiber laser, allowing for in-house beam shaping. Single weld tracks on the aluminium alloy EN AW-5083 are produced with varying laser powers and weld speeds, as well as different beam profiles, ranging from Gaussian intensity distribution to point/ring profiles. The molten cross sections are analyzed regarding their geometry and defects, and the surface roughness of the weld tracks is measured. By using point/ring beam profiles, the processing window can be significantly increased. Hot cracking is considerably reduced for weld speeds of up to 1000 mm/s compared to the Gaussian beam profile. Furthermore, the melt pool width and depth are more stable, with varying parameters for the point/ring profiles, while the Gaussian beam tends to keyhole formation at higher beam powers. Finally, a strong decrease in surface roughness for the point/ring profiles, accompanied by a significantly reduced humping effect, starting even at lower beam powers of 200 W, can be observed. Therefore, these results show the potential of beam shaping for further applications in laser-based processing of high-strength aluminium alloys.

Repetition-rate high-energy laser systems with high beam quality have been attracting growing attentions in various applications. However, the degradation of beam quality caused by thermal effects is nearly unavoidable in high-energy laser amplifiers operating at high repetition rates. One primary thermal effect is thermal-induced birefringence, which typically induces polarization changes at various spatial positions within the beam. This study explores the potential of leveraging the thermal-induced birefringence effect to manipulate the polarization distribution of the beam and, consequently, control the spatial intensity distribution of the beam by using an additional polarizer. This method demonstrates promise for beam pre-compensation, mitigating thermal-induced depolarization, and other laser applications in high-energy lasers with high repetition rates. [ABSTRACT FROM AUTHOR]

The purpose of this work is to design and manufacture low cost, efficient terahertz (THz) diffraction gratings using a 3D printing technique. We performed theoretical modeling and numerical simulations to determine the components' optimal parameters; then, we proceeded to 3D printing and final production of the desired component. Experimental characterization has shown that the components that we manufactured offer very satisfactory performances despite their insignificant cost, which is promising for providing THz components with a very high quality/price ratio. [ABSTRACT FROM AUTHOR]

• Effect of spatial beam shaping on the PBF-LB/M of Fe-2.9wt.%Si. • Experimental study based on the irradiance characteristics. • Effect of peak and ring intensities on densification and microstructure properties. • Adequate densification regardless of the beam profile. • Towards equiaxed grains by manipulating energy density and irradiance. • Grain size enlargement accompanied by higher texturization. Tuning the irradiance profile of the laser beam opens up new possibilities in terms of controlling the thermal field the material is subjected to during laser powder bed fusion (PBF-LB/M). This control can be exploited to manipulate the material's microstructure. In this work, a contemporary high power fibre laser with in-source beam shaping capability was used to test different irradiance profiles ranging from Gaussian to ring during the PBF-LB/M of Fe-2.9wt.%Si alloy. This soft magnetic alloy is used in electrical machines, and its magnetic properties are known to be correlated to the microstructure. Initially, the experimental work assessed the influence of peak and ring irradiance levels on densification and grain morphology. In a second experimental run, the influence of the beam profile on the conventionally employed energy density was verified. The results demonstrated that the use of combined Gaussian-ring profiles offers a method for manipulating grain size and shape from a columnar towards an equiaxed shape, without requiring a change in the scan strategy. [ABSTRACT FROM AUTHOR]

In recent decades, significant progress has been made in the field of laser beam shaping, driven by the growing demands in both scientific research and industrial applications. Notably, a class of beams known as nondiffracting beams, which exhibit such characteristics as self-healing and resistance to diffraction, has found practical utility in communication, imaging, and laser microprocessing. Although Bessel beams have become commonplace for various optical applications, further advances in beam engineering remain a need. One of the key requirements is the ability to control the elongation of the transverse profile of an optical needle, which is highly sought after for specific applications. In this study, we propose to achieve this control by utilizing nondiffracting Airy beams. The elongation of the optical needle's transverse profile is achieved through the implementation of a binary phase mask. We evaluate the performance of this method through both numerical simulations and experimental assessments, employing various metrics, such as the major-to-minor axis ratio, the length of the optical needle, and its stability. We present a specific case where a flat optical element is created using geometrical phase induced by nanogratings inscribed within the element's volume by means of a femtosecond laser. We validate the performance of this geometrical phase element through laser microprocessing of transparent glasses and report the results obtained from these experiments. • Elliptical Airy optical needle using binary phase masks is introduced. • Performance of the optical needle is benchmarked numerically and experimentally. • Binary phase mask is inscribed into a flat element for laser microfabrication. • Directional glass cracking along the elongation at the focus is reported. [ABSTRACT FROM AUTHOR]

One of the recent technological developments in the Laser Powder Bed Fusion (LPBF) process is the use of non-Gaussian beam profiles of power density distributions. Irrespective of the strategies used to change the beam profile, spatial beam shaping enhances process flexibility since it allows manipulation of the thermal field, the melt pool shape, and the microstructure. This work showcases the impact of novel ring and conventional Gaussian beams on density, melt pool characteristics, and mechanical properties of IN718 superalloy via LPBF. A novel laser source with beam shaping capabilities provided the beam profiles, ranging from Gaussian to ring beams. The results show clear differences between the crystalline patterns obtained with Gaussian and the other ring beams attempted. This study elucidates the impact of melting modes and solidification mechanisms on the texture index, arising from the interplay between laser beam shape modes and laser speed. It also shows how the crystalline configuration associated with each case studied influences the mechanical properties and how it is possible to modify the process parameters and the beam shape mode to extend control over the mechanical properties of the components manufactured using LPBF. This leads to a broader range of mechanical properties and grain size when tailoring the power density distribution towards ring beams. Additionally, the correlation between melt pool shape geometry, texture index, and mechanical properties is discussed, and the limitations and advantages of each mode are defined. [Display omitted] • The effect of Novel beam-shaping profiles on mechanical properties is unveiled. • The lamellar microstructure of the Gaussian mode remains when v(mm/s) changes. • The microstructure of the BS5 modes changes significantly with laser speed. • Exists a strong correlation between melt pool, texture index, and mechanical properties. • Laser beam shaping allowed high densification regardless of laser speed. [ABSTRACT FROM AUTHOR]

In this work, we present an introduction to the field of spatially partial coherent beams, while keeping in mind the transverse structure of an optical field. We look closely at the concept of spatial coherence and show some strategies to deal with it. We work stepâ€"byâ€"step with the reader and construct as an example, a partially coherent vortex beam. Finally, using numerical simulations, the richness in structure of a partially coherent field is revealed through its crossâ€"correlation function. [ABSTRACT FROM AUTHOR]

Optical conveyors of airborne particles were previously demonstrated using different techniques allowing particles to be conveyed along straight trajectories either down or upstream the laser radiation direction. Here, the operation of an optical conveyor which has the geometry of an Archimedes screw whose central axis is a predefined arbitrary trajectory in 3D space allowing to convey particles along any desired paraxial trajectory, is demonstrated. [ABSTRACT FROM AUTHOR]

This article describes a novel optical tweezer system which utilizes a magnetic fluid deformable mirror (MFDM) with a wavefront sensor (WFS) and controller to produce a curved Bessel beam for manipulating an optically trapped microparticle. The MFDM is proposed to control the wavefront phase of the laser beam since it offers the ability to produce a nearly perfect axicon in reflection. The working principle of the MFDM and its modeling and design processes are introduced. A decentralized control method is presented for the MFDM to generate the desired mirror surface shape, combing an axicon and an adjustable compensation phase profile that transforms the incident beam into a self-accelerating Bessel-like beam with curved trajectory. Using a prototype MFDM, an experimental optical tweezer system is set up to verify this optical micromanipulation method. The experimental results show the effectiveness of the proposed technique for the manipulation of optically trapped microparticles.

The anomalous vortex beam (AVB), whose paraxial local topological charge varies with propagation, has potential applications in quantum information, laser beam shaping, and other fields. However, there are currently no efficient optical devices to generate AVBs. In this paper, we propose an efficient pure-phase device called spiral axicons. We theoretically analyze the spiral axicon, and then experimentally verify its performance by implementing a spiral axicon on spatial light modulator. Our work provides an alternative method for generating AVB, which will facilitate its application in different fields.

A synthesis method for the design of large planar array antennas with phase-only control is here presented. The synthesis is based on the use of Zernike polynomials, as global basis function for the phase, to reduce the number of optimization variables with respect to the number of elements of the array. Invasive weed optimization (IWO) is applied to polynomial coefficients’ optimization to circumvent nonlinearity and local trapping issues typical of phase-only problems. The periodicity of the array factor is exploited to reduce the optimization to a Voronoi cell of the grating-lobes’ lattice and nonuniform meshing is introduced to best adapt the control stations to the beam shape requirements. The technique is applied to the design of shaped beams for continental coverage from geostationary satellites. [ABSTRACT FROM AUTHOR]

Within additive manufacturing, process stability is still an unsolved challenge. Process instabilities result from the complexity of laser deposition processes and the dependence of the quality of the workpiece on a variety of factors in the process. Because a stable process is dependent on many different factors, permanent precise inline monitoring is required. The suitability of the optical coherence tomography (OCT) measuring system integrated into a wire-based laser metal deposition (LMD-w) process for the task of process control results from its high resolution and high measuring speed, and from coaxial integration into the laser process, which allows for a spatially and temporally resolved representation of the weld bead topography during the process. To realize this, a spectral domain OCT (SD-OCT) system was developed and integrated into the beam path of the process laser. With the aid of suitable optics, circular scanning was realized, which allows for the 3D depth information to be displayed independently of the direction of movement of the processing head and the centrally running wire. OCT makes it possible to detect the process-typical topography deviations caused by process variations and thus paves the way for adaptive process control that could make additive laser processes more reproducible and precise in the future. [ABSTRACT FROM AUTHOR]

Modality is a generic concept of wave-optics at the basis of optical information and communications. One of the challenges of photonics technologies based on optical orbital angular momentum consists in the production of a modal content for both the azimuthal and radial degrees of freedom. This basically requires shaping the complex amplitude of an incident light beam, which is usually made up from adaptive spatial light modulators or bespoke devices. Here, we report on the experimental attempt of a recent theoretical proposal [Opt. Lett.42, 1966 (2017)] toward the production of various optical vortex modes of the Laguerre–Gaussian type relying on the spin–orbit interaction of light. This is done in the visible domain from optical elements made out of silica glass. The idea consists in exploiting the combined effects of azimuthally-varying geometric phase with that of radially-varying propagation features. The proposed approach can be readily extended to any wavelength as well as to other families of optical modes, although some dynamic phase problems remain to be solved to make it a turnkey technology. [ABSTRACT FROM AUTHOR]

We present an approach for engineering the intensity trajectory and phase gradient of light beams with arbitrary shapes by estimating their parametric equations using Freeman chain code and by applying the fast Fourier transform. The analysis of the electric field distribution expected for a given curve allows the phase extraction over each local coordinate, generating a phase pattern to be displayed over a spatial light modulator. The intensity and phase gradient of eight different shapes is encoded during our experiments. The far field intensity profiles are captured and compared in shape to those designed, while the encoded phase is demonstrated by implementing a common path interference setup with a pair of beams from the spatial light modulator. The designed beams, initially drawn either by hand or generated with software, exhibit both the intensity and phase profiles encoded onto them.

Liquid crystal (LC) grating has drawn extensive attention due to its merits, including tunability, easy preparation process, cost effectiveness, and high diffraction efficiency. However, limited efforts have been made to explore LC grating for the generation of customized diffraction patterns, despite the fact that the patterning size of LC molecular director orientation has been tremendously reduced even to subwavelength scale via high-precision photoalignment techniques. Here, we propose a versatile method based on the binary particle swarm optimization algorithm to efficiently design aperiodic LC gratings for customized optical patterns generation, including needle beam, multi-foci, and hollow beam. The optical performance of the LC gratings was simulated using both the vectorial Rayleigh–Sommerfeld method (with discrete zero or π phase, in this article) and the actual director orientations. These results matched each other well, which proves the effectiveness of the optimization method based on discrete phase. This study paves the way for designing LC gratings with engineered functionalities, which may find applications in optical tweezers, laser delivery systems, and laser scanning imaging.

We theoretically propose and experimentally demonstrate a flexible approach to generate an arbitrary shape-invariant structured array beam during propagation with the prescribed intensity distribution. An algorithm for generating flexible desired phase is presented according the reverse engineering and Huygens–Fresnel theory. In this paper, we outline the theory for creating the shape-invariant array beams. The numerical simulation results show that the shape of generated array beams is invariant and its size increases gradually from the near field to the far field. The generation of shape-invariant structured array beams can be realized by the phase-only and amplitude-only elements encoded on the spatial light modulators. The generated arbitrary shape-invariant structured array beams are verified and are in excellent agreement with the theory.

Efficient beam supercollimation is proposed and shown experimentally by axisymmetric aperiodic photonic structures. The structure consists of equidistant layers of concentric refraction index rings, with the transverse period of the rings varying from layer to layer along the structure. Numerically designed and optimized axisymmetric photonic structures are inscribed in silica glass by femtosecond pulses. Strong enhancement (more than by one order of magnitude) of the axial beam component is demonstrated, resulting in the record performance of the beam supercollimation. [ABSTRACT FROM AUTHOR]

The design of a 1 × 0.33 m dual-polarized reflectarray antenna (RA) operating in Ku-Band (17.2 GHz) for a synthetic aperture radar (SAR) mission using a 12 U CubeSat is presented. The SAR RA has larger electrical size, larger aspect ratio, and smaller focal to aperture length ratio (F/L) compared to existing CubeSat RAs designed for communication payloads. Electrically small unit cells accounting for large incidence angle variations, improving phase resolution, and preventing grating lobes are designed. To illuminate the RA with the desired elliptical beam, a compact aperture-coupled microstrip patch array antenna was designed. Tests done on a CubeSat mockup integrating the dual-polarization feed source and a five-panel reflectarray showed good agreement between the simulated and the measured beamwidths, with low sidelobe and cross polarizations levels. [ABSTRACT FROM AUTHOR]

The performance of various laser technologies, such as welding, laser powder bed fusion (LPBF), brazing, cladding, and sheet metal cutting, based on the use of high-power multimode fiber lasers, fiber-coupled solid-state, and diode lasers, can be improved using the patent pending beam-shaping optics providing optimal energy distributions by splitting the laser beam into several separate spots in the working plane and variable energy sharing between these spots. Various patterns, such as square, line, and rhombus, consisting of four or nine separate spots, are expected to eliminate or reduce spatter and to realize optimum temperature distributions in the melt pool and stabilizing the processes in the welding of tailored blanks, copper and aluminum parts in the production of batteries, zinc-coated steel, cladding, and LPBF. Because multimode lasers have a comparably low spatial coherence characterized by large beam parameter products or beam quality (M²) values, it is difficult to control the intensity distribution by methods other than imaging the fiber end with a collimator and a focusing objective. The proposed solution is a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis using special optical components and creating a working spot as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize common spot intensity distributions for particular applications. To ensure reliable operation with multi-kW lasers, the refractive optical components of the multispot devices are implemented from athermal optical materials characterized by insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The article presents descriptions of multispot optics and examples of intensity profile measurements and application results, while the reduction in spattering was observed using multispot laser welding. It is concluded that the melt pool flows homogenize when applying several laser spots compared to a single spot. The possibility of tailoring melt pool dimensions in LPBF was shown. [ABSTRACT FROM AUTHOR]

In this letter, we design, analyze, simulate, fabricate, and measure the performance of reflectarrays capable of forming cosecant-squared beam shape. Due to their unique pattern and abrupt change, achieving a cosecant-squared beam is challenging. In our effort, we evaluate the efficacy of a commonly used rectangular lattice and compare it, for the first time, to a triangular lattice. Two linearly polarized reflectarrays, each with 20 λ0 diameter in size, are fabricated and tested at 20 GHz. Through statistical methods, we quantify the gain agreement between the desired cosecant-squared mask and the measured beam patterns for both rectangular and triangular lattices. We have found that the rectangular lattice conforms better to the cosecant-squared mask at the design frequency of 20 GHz with a root-mean-square error of 6.27 dB. The triangular lattice conforms less, however, it does allow for an increase in gain bandwidth of 6.2% as compared to 4.5% for the rectangular lattice. Measured and simulated patterns are in a good agreement. [ABSTRACT FROM AUTHOR]

Femtosecond Laser Induced Surface Structures evolve from ripples to spikes upon the increase of the energy dose Φ. Nevertheless, for a fixed Φ, several process parameters (pulse energy, repetition rate, average power, overlapping, etc.) play an important role in determining the resulting morphology. Here we show that the beam intensity profile Î as well has an important bearing on the structures morphology evolution. In the case of a Gaussian beam, Î has been modified by varying the beam size d. We observed that for a stationary beam, and for a Φ value, passing from d = 13 µm to d = 200 µm makes possible the transition from ripples to spikes. This transition is made energetically advantageous by using a Top Hat profile. Compared to a 200 µm wide Gaussian beam, a 62 µm wide Top Hat reduces by 10 the Φ required to yield a similar surface morphology (coexistence of spikes and micro-groves). Confirming these results over a large surface could further validate the shaping of Î as a key approach to both fully exploit the output power of state of the art, kW class, femtosecond laser whilst increasing the surface texturing throughput.

We report on a partially coherent power-exponent-phase vortex beam (PC-PEPV), whose spatial coherence is controllable and the initial phase exhibits a periodic power exponential change. The PC-PEPV beam was generated experimentally with various spatial coherence widths, and its propagation properties were studied both numerically and experimentally. By modulating the topological charge (TC) and power order of the PC-PEPV beam, the structure of the vortex beam can be adjusted from circular to elliptic, triangular, quadrangle, and pentagon. When the power order is odd, the PC-PEPV beam with a negative TC can be generated, and the profiles of the PC-PEPV beam can be precisely controlled via adjusting the value of the power order. For the case of high spatial coherence width, the number of the dark cores in the polygonal intensity array of the PC-PEPV beam equals the magnitude of the TC. However, when decreasing the spatial coherence width, the dark cores vanish and the intensity gradually transforms into a polygonal light spot. Fortunately, from the modulus and phase distributions of the cross-spectral density (CSD), both the magnitude and sign of the TC can be determined. In the experiment, the modulus and phase distribution of the CSD are verified by the phase perturbation method. This study has potential applications in beam shaping, micro-particle trapping, and optical tweezers.

Nonlinear multiplexing holography emerges as a powerful tool to produce structured lights at new wavelengths. In this work, we propose and experimentally demonstrate an angle-multiplexing nonlinear holography in an angular noncritical phase-matching configuration. In experiment, various types of structured light beams, such as vortex beam, Airy beam and Airy vortex beam, are simultaneously output at second-harmonic waves along different paths. Because of the large angular acceptance bandwidth of noncritical phase-matching, one can achieve high conversion efficiency of angle-multiplexing nonlinear holography. Our method has potentially applications in high-capacity holographic storage and security encryption.

The preponderance of laser beam shapes cannot be ruled out during the implementation of an optical experiment nor during the formulation of its theoretical background. The present work elucidates the role of Gaussian and top-hat beam shapes in generating and analysing the photothermal beam deflection (PBD) signals. The complex geometrical optics models encompassing the perturbations in the phase and amplitude of the probe beam with one-dimensional (1D) and two-dimensional (2D) approaches is employed to curve fit the PBD signal and are compared. From the fitted curve, the thermal diffusivity and conductivity of the sample are calculated with the 1D and 2D models. A uniform intensity distribution over the sample, like a top-hat beam, is achieved using an optical lens system and verified using a beam profiler. When the phase and amplitude of the PBD signal are fitted at different positions of the lens, i.e., in focussed and defocussed conditions, it is observed that difference in the measured thermal characteristics is about 30% for the Gaussian pump beam profile, whereas it is only

In optical wireless power transmission (OWPT) systems, since beam size is finite, and relative position and attitude between transmitter and receiver is not always stationary, both beam alignment and shaping accuracies are important parameters. Analysis based on a power generation efficiency calculation model of general OWPT systems shows that their tolerances are quite demanding, especially for long range OWPT, and relaxation is inescapably necessary. This study introduces the fly eye lens as a candidate to relax these difficulties and, moreover, it features producing homogeneous irradiation onto the solar cell array. All of these are essential to OWPT systems. In this study, cooperative OWPT is discussed, in which solar cell array and power transmitter mutually align each other. Its efficiency calculation model is integrated with a fly eye module surrounded by reflective walls. System level requirements are analyzed regarding beam shaping and alignment in terms of power generation ratio, and it is clarified that this module largely relaxes requirements. In this module, beam power is confined within the module and will be eventually absorbed by the solar cell as the incident beam is within the acceptance angle. This feature avoids degradation of power generation ratio due to beam shape mismatch. These advantages bring progress towards building operational OWPT.

Submicron long focusing range beams are gaining attention due to their potential applications, such as in optical manipulation, high-resolution lithography and microscopy. Here, we report on the theoretical and experimental characterization of an elastomeric polydimethylsiloxane/single layer graphene (PDMS/SLG) axicon-like tunable device, able to generate diffraction-resistant submicrometric spots in a pump and probe configuration. The working principle is based on the phase change of an input Gaussian beam induced in the elastomer via the thermo-optical effect, while the heating power is produced by the optical absorption of the SLG. The phase-modified beam is transformed by an objective into a long focus with submicron diameter. Our foci reach an experimental full width at half maximum (FWHM) spot diameter of 0.59 μm at the wavelength of 405 nm, with the FWHM length of the focal line greater than 90 μm. Moreover, the length of the focal line and the diameter of the focus can be easily tuned by varying the pump power. The proposed thermo-optical device can thus be useful for the simple and cheap improvement of the spatial resolution on long focus lines.

This paper is an experimental characterization of a light-receiving module containing a fly-eye lens system with high tolerance to beam irradiation conditions. The fly-eye lens system, which is tolerant to fluctuations in beam shape, beam size, number of beams, beam incident position, and beam incident direction, was proposed, a light receiver module with a fly-eye lens system was constructed, and its characteristics were evaluated. The effect of the beam size on the fly-eye lens system was evaluated and the tolerance to misalignment of beam incident position was measured. When a GaAs solar cell was irradiated with a laser beam of 450 nm wavelength and 6 W light output through a 90 cm long water tank with tap water, a maximum output of 0.755 W was obtained as underwater OWPT. In addition, a fly-eye lens system with mirrors applied to four surfaces was proposed and fabricated as a light-receiving side module that can receive high incident angles from any direction of up, down, left, and right and its effectiveness was clarified through experiments.

An novel analytical method for a fiber laser beam shaper is investigated based on mode coupling theory and far field diffraction. A laser beam shaping device based on optical fiber taper is fabricated and experimentally verified. A Gaussian laser beam is shaped into a flat-top beam through a single abrupt tapered fiber. The amplitude and phase of core and cladding modes are calculated and applied in the prediction of beam shaping. The far field diffraction in calculation matched with the experimental result measured by laser beam profiler. [ABSTRACT FROM AUTHOR]

Optical dielectric metasurfaces composed of arrayed nanostructures are expected to enable arbitrary spatial control of incident wavefronts with subwavelength spatial resolution. For phase modulation, one often resorts to two physical effects to implement a 2π‐phase excursion. The first effect relies on guidance by tall nanoscale pillars and the second one exploits resonant confinement by nanoresonators with two degenerate Mie resonances. The first approach requires high aspect ratios, while the second one, known as Huygens' metasurfaces, is much flatter, and thus easier to manufacture. The two approaches are compared, more focusing on conceptual rather than technological issues, and fundamental limitations with the latter are identified. The origin of the limitations based on general arguments are explained, such as reciprocity, multimode/monomode operation, and symmetry breaking. [ABSTRACT FROM AUTHOR]

Detailed knowledge about the laser-material interaction, especially the distribution of laser power absorption, is a prerequisite for the simulation and optimization of laser material processing. In this work, an algorithm based on ray tracing is presented to calculate the propagation and the absorption of a laser beam inside a complex 3D cutting kerf. To model the laser beam precisely, a ray source based on high-power intensity measurements of the laser beam emitted from a highly multimode step-index fiber is set up. For the 3D reconstruction of the cutting kerf geometry, a semicircle model derived from three characteristic lines of so-called "frozen cuts" is applied. The presented approach enables a direct simulation of the laser absorption inside the cutting kerf considering light propagation properties like beam degeneration, shadowing effects, and multiple reflections. As a benchmark, it is finally applied to analyze cutting experiments in stainless steel with an axicon telescope. [ABSTRACT FROM AUTHOR]

Due to their subwavelength thickness, light weight and easy fabrication, metasurfaces show obvious advantages over 3D metamaterials. Among this, a new concept so-called digital coding metasurfaces which are described in a direct manner by quantized reflection or refraction phases was proposed. This paper proposes a scheme to achieve simultaneous and independent control of phase and amplitude responses via a reflective coding metasurface as it would be beneficial for full control of propagation of electromagnetic waves. The deflection of the reflected beam is dominated by designed coding patterns of digital unit cells while the amplitude adjustment depends on tuning values of loaded resistances on the unit cells. The simulation and experimental results confirm the function of the metasurface proposed above. The independent control of both phase and amplitude profiles of EM waves implies the potential application in beam shaping and high-quality holography. [ABSTRACT FROM AUTHOR]

In this paper, a fireworks algorithm based Gerchberg–Saxton (GS) algorithm for the design of diffractive optical element is developed, named FW-GS algorithm, in which, the explosion mechanism of the fireworks algorithm is introduced to efficiently find the initial phase, thereby ensuring a superior phase solution for the subsequent GS algorithm. By employing this approach to shape a Gaussian beam into a square beam, the results illustrated the notable improvement of 16.10 % in RMSE and 3.29 % in diffractive efficiency when compared to the traditional GS algorithm, while exhibiting stable optimization performance. Furthermore, the proposed algorithm demonstrated adaptability to various shaping targets, with RMSE improved by about 16 % and diffractive efficiency improved by more than 3 % for shaping to circular, annular, and rectangular beams. • A fireworks algorithm based Gerchberg–Saxton (GS) algorithm for the design of beam shaping diffractive optical element is developed, named FW-GS algorithm. • Compared with the traditional GS algorithm, the FW-GS algorithm optimized RMSE and diffractive efficiency when shaping Gaussian beam into square beam, while exhibiting stable optimization performance. • The FW-GS algorithm demonstrated adaptability to various shaping targets. [ABSTRACT FROM AUTHOR]

Bessel vortices are well-known for their long focal region and the unique property of carrying angular momentum. These characteristics are desired in applications such as laser micromachining, optical trapping, and imaging. Interference of several non-diffracting vortical beams results in light fields with complex intensity distribution structure — many single charged positive and negative vortices appear and experience complex dynamics under propagation. The trajectory of single-charged vortices creates braided structures as they move, annihilate, or are created during propagation. This influences the dynamics of the high intensity regions of the combined beam: high-contrast symmetrical structures are created in the intensity profile. We demonstrate an experimental generation of such complex fields by superimposing several higher-order vortical Bessel beams. We introduce different cone angles of the interfering beams and study modifications of the resulting beam profile. We analyze the vortical structure of the combined beam, track its evolution, and observe the creation of complex high-contrast structures. We implement a convenient technique for generating superimposed higher-order Bessel beams of different spatial frequencies and topologies using diffractive phase masks. We employ a spatial light modulator to experimentally realize these structured fields and to verify theoretical and numerical predictions. [ABSTRACT FROM AUTHOR]

In this study, a holistic evaluation strategy employing a complex test artifact, coupled with single line scan tracks, was employed to examine the influence of different laser beam energy profiles on various features of laser-based powder bed fusion of metals (PBF-LB/M). Using IN718, results revealed a significant influence of the laser beam on bead geometry. Specifically, the conventional Gaussian beam produced deeper, concave-shaped melt pools, while the ring beam resulted in wider and shallower beads. In terms of productivity, the artifacts (two each) fabricated using the Gaussian beam required 6 h and 6 min of build time, while those built using ring beam required only 3 h and 49 min (37% build time reduction) but a defocused Gaussian beam (not included in this study) can match the productivity of the ring beam. However, the holistic approach using the complex test artifact highlighted drawbacks associated with the ring beam, including increased lack of fusion porosity, compromised resolution for features below 0.5 mm, and less than 10% elongation to break in both as built and heat-treated conditions. While acknowledging the potential for improvement through further process optimization, the study concludes that the shallower melt pools produced by the ring beam may cause limitations as layer thicknesses increase. Microstructural analysis revealed distinct grain characteristics, with the Gaussian beam producing more equiaxed grains and the ring beam favouring columnar grains positioned along the build direction. The work primarily highlights the need for and benefits from a holistic analysis in PBF-LB/M technology, ensuring data driven outcomes. [ABSTRACT FROM AUTHOR]

Experimental studies of shaping a CO2 laser beam with a reflective diffractive optical element have been performed. To increase the aperture of the initial beam, we used a collimator containing a system of two plane-convex spherical ZnSe lenses. For the focal line formed with the diffractive optical element in combination with the collimator, in addition to a 1.3-fold increase in the length, a decrease in the maximum beam power density was found to occur in the laser spot. It was demonstrated that under the laser action it is possible to generate in a two-phase steel sample regions of full hardening, selective hardening, and annealing, alongside the initial structure. The formation of such structures is due to the distribution pattern of temperature fields and a difference in the cooling rate across the volume of the heat affected zone.

Fluorescence microscopy has long been a valuable tool for biological and medical imaging. Control of optical parameters such as the amplitude, phase, polarization, and propagation angle of light gives fluorescence imaging great capabilities ranging from super-resolution imaging to long-term real-time observation of living organisms. In this review, we discuss current fluorescence imaging techniques in terms of the use of tailored or structured light for the sample illumination and fluorescence detection, providing a clear overview of their working principles and capabilities.