Beam Shaping Approach Research Articles

Design framework of a computer-generated hologram that performs volumetric beam shaping for advanced laser processing

Axial beam shaping is very effective for material laser processing, typically laser cutting, drilling, and grooving. We demonstrate a framework for designing a computer-generated hologram (CGH) that performs volumetric beam shaping. The procedure performs axial beam shaping with a continuous intensity distribution, unlike our previous research in which only discrete focal points were arranged three-dimensionally. This research is the more general approach for volumetric beam shaping. An important point in this research is finding an optimal interval in the optical axis direction and in calculating the CGH design. The design interval is half of the focusing length (the full width at half-maximum of the laser beam profile in the axial direction) given by the diffraction limit of the optical system. The optimal value is obtained using an axially shaped beam that is the reconstruction of the CGH calculated from Zernike polynomials. We also demonstrate that the optimal interval for evaluating the axially shaped beam is also half of the beam length. Following the CGH design procedure, we demonstrate CGHs that generate long-focus beams with an arbitrary axially shaped beam. We found a tradeoff relation between the focusing length and the intensity of the long-focus beam, suggesting that the use of a focused beam with an appropriate length according to the purpose will lead to improved processing efficiency.

HoloTile: Rapid and speckle-suppressed digital holography by matched sub-hologram tiling and point spread function shaping

In this work, a novel approach to rapid and speckle-reduced hologram generation is presented as HoloTile. HoloTile utilizes an analytic approach for Gaussian beam shaping to modify the Point Spread Function (PSF) of the holographic system into flat-top squares, such as to prevent interference between adjacent random phase values in the reconstruction plane. The PSF shape is determined by the inter-spectral spacing in the reconstruction plane that occurs due to tiling on the spatial light modulator. As such, the final hologram is the product of a once-calculated PSF shaping phase hologram and the tiled object holograms. HoloTile achieves speckle-reduced reconstructions and extremely fast hologram generation, due to the resolution of the object holograms.

Multi-variable compensated quantum yield measurements of upconverting nanoparticles with high dynamic range: a systematic approach.

Non-linear materials such as upconverting nanoparticles (UCNPs) are emerging technology with fast-growing applications in various fields. The power density dependence of the emission quantum yield (QY) of these non-linear materials makes them challenging to characterize using currently available commercial QY systems. We propose a multimodal system to measure QY over a wide dynamic range (1:104), which takes into account and compensates for various distorting parameters (scattering, beam profile, inner filter effect and bandwidth of emission lines). For this, a beam shaping approach enabling speckle free beam profiles of two different sizes (530 µm or 106 µm) was employed. This provides low noise high-resolution QY curves. In particular, at low power densities, a signal-to-noise ratio of >50 was found. A Tm-based core-shell UCNP with excitation at 976 nm and emission at 804 nm was investigated with the system.

Zero-order-free complex beam shaping

The unwanted zero-order diffraction is still an issue in beam shaping using pixelated spatial light modulators. In this paper, we report a new approach for zero-order free beam shaping by designing an asymmetric triangle reflector and introducing a digital blazed grating and a digital lens to the phase hologram addressed onto the spatial light modulator. By adding the digital lens phase to the previously reported complex-amplitude coding algorithms, we realized the generation of complex beams without the burden of zero-order diffraction. We comparatively investigated the produced complex light fields using a modified complex-amplitude coding algorithm to validate the proposed method.

Additively Manufactured Amplitude Tapered Slotted Waveguide Array Antenna With Horn Aperture for 77 GHz

In this work, a combined frontend of horn and slotted waveguide array antenna serving as feeding subsystem is presented. This setup allows to implement an amplitude taper in one plane while using the horn aperture to shape the far field radiation characteristics in the perpendicular plane independently and tailored to the individual demand of the application. Moreover, the feeding system is implemented as a loop resulting in a standing wave distribution for a wide frequency range in order to increase the antenna bandwidth. Two specimens with different horn apertures are additively manufactured in slotted waveguide technology from UV curable photopolymer resin using a digital light processing 3D printer and subsequently metal coated by electroless silver plating. A relatively large bandwidth of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{B}_{-10\text {dB}} = 7.2$ </tex-math></inline-formula> GHz and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{B}_{-15\,\text {dB}}$ </tex-math></inline-formula> of 4.9 GHz is achieved in measurements indicating a relative bandwidth of 9 % and 6 % respectively. Antenna Gain is characterized as 17.5 and 20.17 dBi in measurements while maintaining a sidelobe level of 20–22.5 dB at 77 GHz and exhibiting a radiation efficiency of 78.9 and 79.4 %. The proposed antenna architecture provides a flexible approach for beam shaping in applications where azimuth and elevation planes exhibit different or contrary requirements, e.g. broad vs. narrow field of view - especially suitable for automotive MIMO-radar sensors.

Compensating deep focusing distortion for femtosecond laser inscription of low-loss optical waveguides.

Various beam shaping approaches were examined to counter the negative influence of surface aberration arising when inscribing optical waveguides deeply inside of glass with a femtosecond laser. Aberration correction was found unable to completely recover the low-loss waveguide properties, prompting a comprehensive examination of waveguides formed with focused Gaussian-Bessel beams. Diverging conical phase fronts are presented as a hybrid means of partial aberration correction to improve insertion loss and a new, to the best of our knowledge, means of asymmetric beam shaping. In this way, low-loss waveguides are presented over shallow to deep writing depth (2.8 mm) where morphological and modal properties could be further tuned with conical phase front.

Beam shaping for ultra-compact waveguide crossings on monolithic silicon photonics platform.

A beam shaping approach has been implemented to realize high-performance waveguide crossings based on cosine tapers. Devices with a compact footprint of 4.7µm×4.7µm were fabricated on the GLOBALFOUNDRIES 45 nm monolithic silicon photonics platform (45 CLO technology). Fabricated devices are found to be nearly wavelength independent (±0.035dB for 1260nm≤λ≤1360nm) with low insertion loss (∼0.2dB) and crosstalk (-35dB). The measured response of the devices is consistent with the three-dimensional finite-difference time-domain simulation results. The design stability is validated by measuring the device insertion loss on eight chips, which is found to be 0.197±0.017dB at the designed center wavelength of 1310 nm.

Double Interferometer Design for Independent Wavefront Manipulation in Spectral Domain Optical Coherence Tomography

Spectral domain optical coherence tomography (SD-OCT) is a highly versatile method which allows for three dimensional optical imaging in scattering media. A number of recent publications demonstrated the technique to benefit from structured illumination and beam shaping approaches, e.g. to enhance the signal-to-noise ratio or the penetration depth with samples such as biological tissue. We present a compact and easy to implement design for independent wavefront manipulation and beam shaping at the reference and sample arm of the interferometric OCT device. The design requires a single spatial light modulator and can be integrated to existing free space SD-OCT systems by modifying the source arm only. We provide analytical and numerical discussion of the presented design as well as experimental data confirming the theoretical analysis. The system is highly versatile and lends itself for applications where independent phase or wavefront control is required. We demonstrate the system to be used for wavefront sensorless adaptive optics as well as for iterative optical wavefront shaping for OCT signal enhancement in strongly scattering media.

Tuning Nanoparticle Electrodynamics by an Optical-Matter-Based Laser Beam Shaper.

Spatially modulated optical fields provide the perspective of tuning nanoparticle (NP) dynamics in a colloidal suspension. Here, it is shown that the lateral interferometric optical field created by a chain of optically bound Au NPs (i.e., optical matter) can tailor the electrodynamic interactions among more Au NPs. The free-standing NP chain, which is assembled and confined by an auxiliary optical line, shapes the main trapping beam and guides the self-organization of Au NPs under an optimized polarization direction. We find that the NP chain can largely enhance the anisotropic optical binding interaction of two nearby NPs but suppress the anisotropic interaction of multiple NPs, leading to isotropic self-organization. The dynamics and structural transitions of the NPs are well-reproduced in a simulation by using a coupled finite-difference time-domain (FDTD)-Langevin dynamics approach. Our work provides a new dual-beam optical trapping and in situ laser beam shaping approach to study and control interparticle electrodynamic interactions among colloidal NPs.

Hyperbolic phase function used in a spatial light modulator for flat top focus generation.

We report on a novel transversal beam shaping approach to convert a Gaussian beam to a flat top focus profile by employing a hyperbolic phase function in a liquid crystal on silicon (LCOS) spatial light modulator (SLM). The well-known Powell lens approach is compared to ours. The analytically obtained phase function for the flat top focus conversion problem is compared to our approach theoretically and experimentally. A strategy for the generation of squared and circular top hat foci is derived.

Generation of non-paraxial accelerating beams by active aberration compensation

We demonstrate the generation of non-paraxial accelerating beams by using an objective with an external spherical aberration compensation. A correction phase is applied at an illuminating plane before the objective, and such an aberration-corrected system enables a Fourier transform that translates a proper spatial spectral phase pattern into the desired non-paraxial accelerating beams. The feasibility of the scheme and the quality of the generated beams are further discussed for various conditions. Experimentally, we observe the accelerating beams as well as non-paraxial autofocusing beams following predesigned trajectories generated with such an aberration compensation method, providing an effective and alternative approach for non-paraxial beam shaping.

Direct binary search for improved coherent beam shaping and optical differentiation wavefront sensing.

Spatially dithered distributions of binary amplitude pixels are optimized using a full direct binary search, taking into account the experimental configuration for amplitude modulation of coherent waves. This design process is shown to yield a significant reduction of the noise induced by binarization and pixelation over the region of interest. We demonstrate this approach for beam shaping and optical differentiation wavefront sensing, where the region of interest is in an image plane of the pixel distribution, and in the far field of the pixel distribution, respectively. The observed reduction in error compared to a standard error diffusion algorithm is significant for both applications because it improves performance without the tighter fabrication tolerance and cost associated with smaller pixels.

The voltage optimization of a four-element lens used on a hemispherical spectrograph with virtual entry for highest energy resolution

The methodology and results of a detailed four-element lens optimization analysis based on electron trajectory numerical simulations are presented for a hemispherical deflector analyzer (HDA), whose entry aperture size is determined by the injection lens itself and is therefore virtual. Trajectory calculations were performed using both the boundary-element method (BEM) and the finite-difference method (FDM) and results from these two different approaches were benchmarked against each other, to probe and confirm the accuracy of our results. Since the first and last electrode are held at fixed potentials, the two intermediate adjustable lens electrode voltages were varied over the entire available voltage space in a direct, systematic, brute-force approach, while minima in beam spot size on the 2-D position sensitive detector (PSD) at the exit of the HDA were investigated using a beam shaping approach. Lens voltages demonstrating improved energy resolution for the combined lens/HDA/PSD spectrograph system were sought with and without pre-retardation. The optimal voltages were then tested experimentally on the modeled HDA system using a hot-wire electron gun. The measured energy resolution was found to be in good overall agreement with our simulations, particularly at the highest resolution (∼0.05%) working conditions. These simulations also provide a detailed insight to the distinctive trajectory optics and positions of the first and second image planes, when the PSD has to be placed some distance away from the HDA exit plane, and is therefore not at the ideal optics conjugate image position. The substantial time savings afforded over usual trial-and-error experimentation should make this type of make-do simulation approach attractive to experimentalists.

Correction of linear-array lidar intensity data using an optimal beam shaping approach

The linear-array lidar has been recently developed and applied for its superiority of vertically non-scanning, large field of view, high sensitivity and high precision. The beam shaper is the key component for the linear-array detection. However, the traditional beam shaping approaches can hardly satisfy our requirement for obtaining unbiased and complete backscattered intensity data. The required beam distribution should roughly be oblate U-shaped rather than Gaussian or uniform. Thus, an optimal beam shaping approach is proposed in this paper. By employing a pair of conical lenses and a cylindrical lens behind the beam expander, the expanded Gaussian laser was shaped to a line-shaped beam whose intensity distribution is more consistent with the required distribution. To provide a better fit to the requirement, off-axis method is adopted. The design of the optimal beam shaping module is mathematically explained and the experimental verification of the module performance is also presented in this paper. The experimental results indicate that the optimal beam shaping approach can effectively correct the intensity image and provide ~30% gain of detection area over traditional approach, thus improving the imaging quality of linear-array lidar.

Improvement of Energy Deposition in Absorber-free Laser Welding through Quasi-simultaneous Irradiation

Laser transmission welding is usually known to put little thermal stress on the joining partners, indicated by a small heat affected zone (HAZ). However, this only applies when the joining partners have adapted optical properties. When it comes to welding of optically equal thermoplastics without absorbers, the main issue is the HAZ extending far from the interface. To enable welding without absorbers, lasers emitting within the polymer's intrinsic absorption bands are used. So far, different beam shaping approaches have already been investigated to achieve a selective energy deposition at the interface but, with little success to date. The approach presented in this paper is irradiating the welding path quasi-simultaneously to exploit the poor heat conductivity of polymers. Therefore, the influence of the irradiation regime on the seam formation is considered in detail. Another aspect investigated is the length of the irradiated contour which is a crucial factor in quasi-simultaneous welding. The results show that the energy deposition can be significantly improved when the welding contour length does not exceed a critical length determined by the capability of the welding system. However, by welding in segments the approach can also be applied to longer contours without any noticeable loss in welding time. The ideal irradiation regime obtained in the trials corresponds to an effective welding speed of 37mm/s and reduces the vertical extent of the HAZ by 30%.

Shaping propagation invariant laser beams

Propagation-invariant structured laser beams possess several unique properties and play an important role in various photonics applications. The majority of propagation invariant beams are produced in the form of laser modes emanating from stable laser cavities. Therefore, their spatial structure is limited by the intracavity mode formation. We show that several types of anamorphic optical systems (AOSs) can be effectively employed to shape laser beams into a variety of propagation invariant structured fields with different shapes and phase distributions. We present a propagation matrix approach for designing AOSs and defining mode-matching conditions required for preserving propagation invariance of the output shaped fields. The propagation matrix approach was selected, as it provides a more straightforward approach in designing AOSs for shaping propagation-invariant laser beams than the alternative technique based on the Gouy phase evolution, especially in the case of multielement AOSs. Several practical configurations of optical systems that are suitable for shaping input laser beams into a diverse variety of structured propagation invariant laser beams are also presented. The laser beam shaping approach was applied by modeling propagation characteristics of several input laser beam types, including Hermite–Gaussian, Laguerre–Gaussian, and Ince–Gaussian structured field distributions. The influence of the Ince–Gaussian beam semifocal separation parameter and the azimuthal orientation between the input laser beams and the AOSs onto the resulting shape of the propagation invariant laser beams is presented as well.

Beam shaping of edge-emitting diode lasers using a single double-axial hyperboloidal micro-lens

We report an innovative approach for beam shaping of edge-emitting semiconductor diode lasers using a single double-axial hyperboloidal micro-lens fabricated with femtosecond laser direct writing technology. The two hyperboloids of different axial lengths focus the light from fast and slow axes to an identical focal spot. The divergence angles were shaped from 60° and 9° to 6.9 and 32.3 mrad, respectively, and the single-end fiber coupling efficiency is measured higher than 80%. The device is simple in fabrication, robust in structure, and easy for operation, by which multiple reflections and absorptions at interfaces are reduced, and assembly errors are minimized.

Laser processing by using fluidic laser beam shaper

Laser beam shaping techniques are important to optimize a large number of laser material processing applications and laser-material interaction studies. The authors have developed a novel Fluidic Laser Beam Shaper (FLBS) with merits such as flexibility, versatility and low cost. This work presents a fundamentally new approach for laser beam shaping by using FLBS. A Gaussian beam profile is transformed to a flat top beam and an annular beam profile in the focal plane. The shaped laser beam is used for laser drilling to investigate the influence of the laser intensity profile in laser processing. The paper concludes with suggestions for future research and potential applications for further the work.

Experimental demonstration of Generalized Phase Contrast based Gaussian beam-shaper

We report the first experimental demonstration of Gaussian beam-shaping based on the Generalized Phase Contrast (GPC) approach. We show that, when using a dynamic spatial light modulator (SLM), this approach can rapidly generate arbitrarily shaped beams. Moreover, we demonstrate that low-cost binary-phase optics fabricated using photolithography and chemical etching techniques can replace the SLM in static and high power beam shaping applications. The design parameters for the binary-phase elements of the module are chosen according to the results of our previously conducted analysis and numerical demonstrations [Opt. Express 15, 11971 (2007)]. Beams with a variety of cross-sections such as circular, rectangular and square, with near flat-top intensity distributions are demonstrated. GPC-based beam shaping is inherently speckle-free and the shaped beams maintain a flat output phase. The non-absorbing components used in this beam-shaping approach have a high-damage-threshold and are thus ideally suited for high power applications.

Laser Processing by Using Diffractive Optical Laser Beam Shaping Technique

Laser beam shaping and homogenization techniques are substantial to optimize a large number of laser-material processing applications and laser-material interaction studies. Diffractive optical elements (DOE) play important role in provision of the process-adapted laser beam shaping. In this paper, we present an approach for laser beam shaping by using innovative DOE. The circular Gaussian beam was transformed to a square flat-top intensity profile in the focal plane of the objective lens by using novel DOE from TOPAG Lasertechnik GmbH. About 95% of the laser energy was coupled in the main beam with a nearly perfect rectangular shape. Experimental results have been achieved by applying the shaped beam for laser microfabrication. The picosecond and nanosecond lasers with moderate beam quality (M 2 =1.5) were used in the experiment. The shaped laser beam was applied for direct laser ablation of metal film on the glass substrate, drilling in a silicon wafer, scribing of thin-film solar cells and other technical materials. This technique allows creating a rectangular-shaped flat-top intensity profile in the focal plane that shows distinct advantages in laser material processing.