Fast Laser Beam Research Articles

Fast spatial laser beam modulation for improved process control in Laser Powder Bed Fusion

We report on the implementation of a high frequency beam oscillation (wobbling) strategy for improving process control during laser powder bed fusion of single weld tracks on Inconel 625. Oscillation frequencies ranging from ~ 600 Hz to 7000 Hz, and different oscillation trajectories (circular, parallel or perpendicular to the direction of scanning) were explored. Highspeed imaging was used to elucidate the dynamics of the melt pool induced by the wobble beams, along with in situ absorptivity measurements to substantiate our hypothesis that the dynamic nature of wobble beams reduces absorptive losses due to laser-vapor interactions and results in improved coupling at the melt pool. Operando X-ray radiography was carried out to visualize sub-surface melt flow dynamics, correlate to spatter mechanisms and optimize the window for improving process stability. Our observations indicate that wobble beams increase the aspect ratio of the melt pool by up to 4×, depending on the oscillation frequency and energy input. Highspeed imaging and X-ray radiography reveal an optimized process parameter window for reducing spatter, improving absorptivity and creating a stable melt pool at high (several kHz) oscillation frequencies.

Broadband structured light using digital micro-mirror devices (DMDs): a tutorial

Laser beam shaping is a venerable topic that enjoyed an explosion in activity in the late 1990s with the advent of diffractive optics for arbitrary control of coherent fields. Today, the topic is experiencing a resurgence, fuelled in part by the emerging power of tailoring light in all its degrees of freedom, so-called structured light, and in part by the versatility of modern day implementation tools. One such example is that of digital micro-mirror devices (DMDs), for fast, cheap and dynamic laser beam shaping. In this tutorial we outline the basic theory related to shaping light with DMDs, give a practical guide on how to get started, and demonstrate the power of the approach with several case studies, from monochromatic to broadband light.

Diffraction Efficiency of MEMS Phase Light Modulator, TI-PLM, for Quasi-Continuous and Multi-Point Beam Steering.

The recent development of the Micro Electromechanical System (MEMS) Phase Light Modulator (PLM) enables fast laser beam steering for lidar applications by displaying a Computer-Generated Hologram (CGH) without employing an iterative CGH calculation algorithm. We discuss the application of MEMS PLM (Texas Instruments PLM) for quasi-continuous laser beam steering by deterministically calculated CGHs. The effect on the diffraction efficiency of PLM non-equally spaced phase levels was quantified. We also address the CGH calculation algorithm and an experimental demonstration that steered and scanned the beam into multiple regions of interest points, enabling beam steering for lidar without sequential raster scanning.

Fast and precise laser beam scanning by nonperiodic grating on a binary micromirror array

We report a precise and stable laser beam positioning method with a high scanning speed by a pixelated binary grating (PBG) realized by a compact on-off state digital micromirror device (DMD). The overall procedure for PBG pattern generation is proposed, analyzed, and experimentally verified. The resulting performances correspond to an angular resolution of 0.01 deg and angular repeatability of ± 2.54 × 10 − 4 deg over a beam steering range of ± 7.5 deg. The scanning speed of 40 deg/s is achieved in continuous scanning with a resolution of 0.01 deg. Arbitrary positions can be targeted in the scanning range with DMD’s frame rate of 4000 Hz. To verify its applicability in precision metrology, absolute distances to multiple off-axial targets were determined in parallel with a high speed. This method will open a wider application of DMD in precision LIDARs, three-dimensional calibration of precision machines, and high-resolution laser patterning.

Influence of high frequency wobbling parameters on the weld quality of aluminium LBW

Currently available commercial remote laser beam welding (LBW) scanners allow a fast laser beam manipulation up to 1 KHz. In this work, wobbling (fast laser beam oscillation) was applied to weld aluminium alloy (5754) sheets of 1.6 mm thickness that are subjected to stamping processes later on. Influence of process parameters (laser power and speed, incidence angle and wobbling frequency and amplitude) in weld quality of T joint and edge lap joint configurations was studied. In T joint configuration, the use of 4047 wire filler metal of 1.2 mm of diameter was beneficial to avoid excessive undercuts and provide good penetration, weld shape and dimensions. Metallographic characterisation confirmed the absence of other defects such as lack of fusion, pores or cracks. In addition, wobbling led to optimal results without removing the protective coating of Ti/Zr from the sheets. Finally, mechanical characterisation for post-welding stamping suitability assessment of the welds was performed using shear and bending tests.

A liquid crystal stackable phased array to achieve fast and precise nonmechanical laser beam deflection

In this study, a novel liquid crystal (LC) stackable optical phased array (SOPA) for fine tracking with fast and precise beam steering is proposed. The SOPA was composed of multiple layers of sliced phase shifters (SPSs). Each SPS had two parts for loading different voltages to form programmable phase steps. All LC materials were compatible with this architecture, including blue-phase, polymer network, and ferroelectric LCs. To verify and evaluate the properties of this SOPA scheme, a three-layered SOPA prototype was designed and fabricated according to the standard processing of LC devices. A couple of steering experiments, with steering angles ranging from −100 to 100 μrad in a step of 10 μrad, were performed. The steering efficiency recorded in all the experiments exceeded 90%. All the measured switching time of angle-to-angle were less than 10 ms. Herein, the fastest switching time reported was only 1.52 ms.

On the Grain Microstructure–Mechanical Properties Relationships in Aluminium Alloy Parts Fabricated by Laser Powder Bed Fusion

Recent years witnessed progressive broadening of the practical use of 3D-printed aluminium alloy parts, in particular for specific aerospace applications where weight saving is of great importance. Selective laser melting (SLM) is an intrinsically multi-parametric fabrication technology that offers multiple means of controlling mechanical properties (elastic moduli, yield strength, and ductility) through the control over grains size, shape, and orientation. Targeted control over mechanical properties is achieved through the tuning of 3D-printing parameters and may even obviate the need of heat treatment or mechanical post-processing. Systematic studies of grain structure for different printing orientations with the help of EBSD techniques in combination with mechanical testing at different dimensional levels are the necessary first steps to implement this agenda. Samples of 3D-printable Al-Mg-Si RS-333 alloy were fabricated in three orientations with respect to the principal build direction and the fast laser beam scanning direction. Sample structure and proper-ties were investigated using a number of techniques, including EBSD, in situ SEM tensile testing, roughness measurements, and nanoindentation. The as-printed samples were found to display strong variation in Young’s modulus values from nanoindentation (from 43 to 66 GPa) and tensile tests (from 54 to 75 GPa), yield stress and ultimate tensile strength (100–195 and 130–220 MPa) in different printing orientations, and almost constant hardness of about 0.8 GPa. A further preliminary study was conducted to assess the effect of surface finishing on the mechanical performance. Surface polishing was seen to reduce Young’s modulus and yield strength but improves ductility, whereas the influence of sandblasting was found to be more controversial. The experimental results are discussed in connection with the grain morphology and orientation.

In Situ SEM Study of the Micro-Mechanical Behaviour of 3D-Printed Aluminium Alloy

Currently, 3D-printed aluminium alloy fabrications made by selective laser melting (SLM) offer a promising route for the production of small series of custom-designed support brackets and heat exchangers with complex geometry and shape and miniature size. Alloy composition and printing parameters need to be optimised to mitigate fabrication defects (pores and microcracks) and enhance the parts’ performance. The deformation response needs to be studied with adequate characterisation techniques at relevant dimensional scale, capturing the peculiarities of micro-mechanical behaviour relevant to the particular article and specimen dimensions. Purposefully designed Al-Si-Mg 3D-printable RS-333 alloy was investigated with a number of microscopy techniques, including in situ mechanical testing with a Deben Microtest 1-kN stage integrated and synchronised with Tescan Vega3 SEM to acquire high-resolution image datasets for digital image correlation (DIC) analysis. Dog bone specimens were 3D-printed in different orientations of gauge zone cross-section with respect to the fast laser beam scanning and growth directions. This corresponded to the varying local conditions of metal solidification and cooling. Specimens showed variation in mechanical properties, namely Young’s modulus (65–78 GPa), yield stress (80–150 MPa), ultimate tensile strength (115–225 MPa) and elongation at break (0.75–1.4%). Furthermore, the failure localisation and character were altered with the change in gauge cross-section orientation. DIC analysis allowed correct strain evaluation that overcame the load frame compliance effect and helped to identify the unevenness of deformation distribution (plasticity waves), which ultimately resulted in exceptionally high strain localisation near the ultimate failure crack position.

Fast laser beam steering into multiple diffraction orders with a single digital micromirror device for time-of-flight lidar

The sampling rate and angular resolution of diffraction-based beam steering employing a digital micromirror device (DMD) can be simultaneously enhanced by at least an order of magnitude by synchronizing multiple nanosecond laser sources and pulses during each DMD actuation. A time-of-flight single-chip DMD lidar with three sources measures a range at a 3.34 kHz sampling rate and a 3.4° angular resolution across a 48° field of view. Employing multiple diffraction orders of the DMD improves the sampling rate at least by a factor of 2 and up to the number of diffraction orders supported by the DMD. An improved sampling rate of 6.68 kHz with a 9.6° angular resolution is experimentally demonstrated by illuminating micromirrors multiple times within a single transition period of the micromirrors.

Acousto-optical deflector for non-mechanical manipulating using optical tweezers

Optical tweezers are widely used in various fields of science and technology, such as biophysics, cytology and solid-state physics. Most of the existing optical tweezers use mirror or mirror-lens systems to manipulate the position of the trap. Such systems require precise alignment and do not allow the trap to be moved quickly from one arbitrary point to another due to the inertia of the mirror and lenses. We discuss acousto-optic scanning characterized by high precision and repetition rate for manipulating micro-objects using optical tweezers. Bragg diffraction of light via ultrasonic waves allows creating robust solid-state devices for precise and fast laser beam deflection. We describe a scheme of the optical tweezers with PC-driven two-dimensional scanning implemented by two sequential acousto-optical cells.

Fast Laser Beam Characterization

Fast Laser Beam Characterization

Fast Laser Beam Characterization

AbstractThe power‐to‐weight ratio plays a key role in automotive construction: the lighter the powertrain components of the car or truck, the more efficient the vehicle. Moreover, it significantly cuts emissions and fuel consumption. And engineers are always striving to eliminate parts that are at higher risk of wear and tear. With these goals in mind, automotive experts at Daimler have successfully optimized the design and assembly of their differential gears, substituting bolted joints with laser welds (Fig.1). Not only has this reduced the mass of each part by over 15 percent, the new procedure requires no rework of the parts, and it has also brought down losses due to splashing of nuts and bolts through the oil reservoir of the differential gear during operation. In order to meet the stringent standards that Daimler is committed to, the company implemented a digital process chain to guarantee and document all relevant quality parameters. One key feature thereof is the non‐contact, automated characterization of process laser beams using Ophir's BeamWatch Integrated.

A fast dual wavelength laser beam fluid-less optical CT scanner for radiotherapy 3D gel dosimetry I: design and development

Three dimensional dosimetry by optical CT readout of radiosensitive gels or solids has previously been indicated as a solution for measurement of radiotherapy 3D dose distributions. The clinical uptake of these dosimetry methods has been limited, partly due to impracticalities of the optical readout such as the expertise and labour required for refractive index fluid matching. In this work a fast laser beam optical CT scanner is described, featuring fluid-less and dual wavelength operation. A second laser with a different wavelength is used to provide an alternative reference scan to the commonly used pre-irradiation scan. Transmission data for both wavelengths is effectively acquired simultaneously, giving a single scan process. Together with the elimination of refractive index fluid matching issues, scanning practicality is substantially improved. Image quality and quantitative accuracy were assessed for both dual and single wavelength methods. The dual wavelength scan technique gave improvements in uniformity of reconstructed optical attenuation coefficients in the sample 3D volume. This was due to a reduction of artefacts caused by scan to scan changes. Optical attenuation measurement accuracy was similar for both dual and single wavelength modes of operation. These results established the basis for further work on dosimetric performance.

A voice-coil actuator based motorized optical mount for high-performance laser beam routing

We present a motorized optical mount based on a voice-coil actuator from an obsolete computer hard drive. Instead of using complex servo motors and motion sensors as the commercial motorized mounts, our homemade mount adopts a buffering circuit for motion control. The mount flips a 1 inch fused silica mirror in 600 ms, and the flip-to-flip spatial repeatability is better than 85 µrad. This performance is comparable to the commercial ones, while the cost is only one-tenth of the commercial price. Our device provides a convenient and low-cost solution for fast and precise laser beam routing that can be applied for various applications in optical experiments.

Fast targeted gene transfection and optogenetic modification of single neurons using femtosecond laser irradiation

A prevailing problem in neuroscience is the fast and targeted delivery of DNA into selected neurons. The development of an appropriate methodology would enable the transfection of multiple genes into the same cell or different genes into different neighboring cells as well as rapid cell selective functionalization of neurons. Here, we show that optimized femtosecond optical transfection fulfills these requirements. We also demonstrate successful optical transfection of channelrhodopsin-2 in single selected neurons. We extend the functionality of this technique for wider uptake by neuroscientists by using fast three-dimensional laser beam steering enabling an image-guided “point-and-transfect” user-friendly transfection of selected cells. A sub-second transfection timescale per cell makes this method more rapid by at least two orders of magnitude when compared to alternative single-cell transfection techniques. This novel technology provides the ability to carry out large-scale cell selective genetic studies on neuronal ensembles and perform rapid genetic programming of neural circuits.

Fast and precise Laguerre–Gaussian beam steering with acousto-optic deflectors

Novel fluorescence microscopy techniques and two-color laser direct imaging photolithography methods that enable resolution an order of magnitude beyond the diffraction limit require Laguerre-Gaussian beams and a fast and precise laser beam steering device to obtain images and produce microstructures. An acousto-optic deflector (AOD) is a suitable choice and provides high-speed random access beam positioning with subnanometer precision as well as beam intensity control in a single element. In high-resolution applications, the impact of an AOD on beam quality plays a major role. We study the transfer function of an AOD for a fundamental Gaussian and a doughnut-shaped Laguerre-Gaussian beam by measuring the beam quality as a function of the diffraction angle after passing through the device. It is demonstrated that an AOD introduces negligible distortion and degradation to the beam profile and is therefore highly suitable for use in super-resolution imaging and photolithography techniques where manipulation of Laguerre-Gaussian doughnut-shaped beams is required.

基于CMOS传感器的高速高精度激光光束自动准直系统

基于CMOS传感器的高速高精度激光光束自动准直系统

High Speed Laser Micro Processing Using High Brilliance Continuous Wave Laser Radiation

Laser micro processing using a high power single-mode continuous wave fibre laser in combina-tion with a fast galvanometer scanner as well as an ultra fast polygon scan systems was investigated. As a key technology in high rate laser ablation a maximum laser power up to 3 kW and scan speeds up to 18,000 m/minwere applied. With the ultra fast laser beam deflection and laser a smallfocal spot diameter of 21 µm laser dwell times less than 100 nanoseconds were achieved. As a result laser intensities comparable to the q-switched lasers in the range of

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.

Development of a fast position-sensitive laser beam detector

We report the development of a fast position-sensitive laser beam detector. The detector uses a fiber-optic bundle that spatially splits the incident beam, followed by a fast balanced photodetector. The detector is applied to the study of Brownian motion of particles on fast time scales with 1 A spatial resolution. Future applications include the study of molecule motors, protein folding, as well as cellular processes.