Laser Beam Scanning Research Articles

Hermetic Package for MEMS Mirror

Applications for optical scanning and 3D sensing, mostly related to the consumer electronics and automotive markets, are becoming more and more demanding in terms of performance and power consumption. When using a MEMS based Laser Beam Scanning, enclosing the mirrors into a hermetic package helps to address these requirements. Two MEMS mirrors with different sizes and performance have been selected for this study. A unique package, suitable for both mirrors, has been properly designed. The package is based on a ceramic chip carrier with a soldering ring on the top surface. A metal cap with a sintered optical window is attached to the ceramic substrate. The optical window is tilted with respect to the mirror to avoid spurious reflections. The MEMS mirrors have been integrated onto the ceramic substrate by standard die attach and wirebonding technologies. Hermetic sealing has been developed under nitrogen atmosphere to guarantee the MEMS stable operation against humidity and contaminants. Flux-less SnAgCu solder preforms have been selected as sealing material. The solder reflow profile has been optimized in order to obtain a uniform solder fillet at the sealing region while minimizing the thermal stress to the mirrors. The hermeticity of the assemblies has been proved by fine and gross leak testing. The packages show a leak rate < 1.5 E-8 atm cc/s and pass the fine and gross leak test, defined accordingly to MIL-STD-883 TM 1014.18.

Wide-angle high-precision electronically controlled continuous beam scanning implementation based on MLAS cascaded AFOC

Microlens array scanner (MLAS), as a rapidly growing branch of the semisolid micro-mechanical beam scanning system, can achieve large-scale beam scanning through small-scale (μm) physical motion, which is an effective beam scanning scheme that balances aperture, field of view (FOV), inertia, and other vital indicators. However, the discrete addressing caused by the periodic structure of microlens array (MLA) will affect the scanning accuracy in applications. We design and develop a continuous laser beam scanning device with an effective aperture of 20 mm based on MLAS and adaptive fiber collimator (AFOC), and the scanning beam quality is close to the diffraction limit. In addition, the electrical control mechanism of the device is explained, which achieves a scan FOV of 20°, a scan angular velocity of 10,000°/s, and two-dimensional (2D) scan. This approach effectively realizes a beam scanning system with excellent comprehensive performance, including large aperture, large scan angle, high precision, high speed, tunable wavelength, and high power.

Magnetic response to microstructure and phase evolution in laser thermal treated FeSiB amorphous alloy

In the current work, laser thermal treatment of the FeSiB amorphous foil with a single linear laser track was carried out. The resultant microstructure and phase evolution were examined with the aid of x-ray diffraction, differential scanning calorimetry, and site-specific transmission electron microscopy. The laser power was kept constant at 100 W, whereas, laser beam scanning speeds were varied in the range of 500–235 mm/s, generating corresponding laser fluences of 0.42–0.91 J/mm2 on the sample surface. Laser fluences of up to 0.48 J/mm2 structurally relaxed the FeSiB foil, retaining the amorphous structure. Laser fluences of higher than 0.48 J/mm2 led to partial crystallization of FeSiB amorphous foils. The crystallite sizes were in the range of 11–31 nm (laser fluence of 0.49–0.91 J/mm2). α-FeSi formed as a major phase of partial crystallization while its quantity steadily increased from 3.6 to 46 vol. % with laser fluence (laser fluence 0.49–0.91 J/mm2). Fe2B formed in recognizable quantities (≥2%) for laser fluences ≥0.53 J/mm2. Laser fluences leading to structural relaxation and evolution of predominantly α-FeSi phase exerted minimal effects on ratios of intrinsic coercivities to saturation compared to the as-cast FeSiB amorphous foil. On the contrary, formation of Fe2B in significant quantities (≥2%) led to the steady increase in intrinsic coercivities and remanence to saturation ratios as a function of laser fluence indicating a loss in soft magnetic characteristics. Nonetheless, continuous increase in fractions of α-FeSi with laser fluence led to a steady improvement in saturation magnetostriction of the FeSiB foil.

Ultrafast laser ablation of gold in liquids: Effect of laser pulse overlap-induced surface porosity on size distribution of formed nanoparticles

Pulsed Laser Ablation in Liquids (PLAL) manifested itself as a powerful tool for the synthesis of nanoparticles (NPs) from a variety of materials of high demand in biomedicine. The mechanisms and regimes of nanostructures formation, however, still require clarification in order to better control the final NPs characteristics. Here, we present a numerical study of femtosecond laser-ablative production of metal (gold) NPs in water ambient using an advanced atomistic continuum approach, combining the Molecular Dynamics (MD) and Two Temperature Model into the frames of a single MD-TTM computational method. The model describes non-equilibrium laser-induced phase transitions at atomic level and accounts for the effect of free carriers in continuum. With the MD-TTM model we investigated the effect of slight porosity arising due to a partial overlap of laser craters during the scanning of laser beam over the target surface under a high repetition rate of laser pulses. For that purpose, we perform a simulation of 270 fs laser pulse interaction with solid and porous gold targets at the incident fluence of 2.5 J/cm2. The obtained results revealed the manifestation of different regimes of ablation and different yield of the obtained NPs. The simulation results are compared with the corresponding experimental data. We found that depending on the scanning speed the ablation can follow thermal and spallation mechanisms of the material ejection, which are responsible for the appearance of fine and course populations of NPs correspondingly. In the case of clean target’s surface ablation, when at high scanning speed of 3840 mm/s each pulse hits a fresh area, the material ejection is governed by both mechanisms, which leads to the appearance of bimodal population of NPs. Alternatively, a moderate scanning speed of 840 mm/s results in a partial overlap of the sequential laser spots on the surface and generated slight porosity removes the accumulation of laser-induced stresses. The spallation mechanism of the material ejection is therefore suppressed, which results in generation of a monomodal fine population of NPs with a 20 times larger yield. The obtained data are of importance to predict and control size characteristics of laser-synthesized nanomaterials.

Incremental laser shock adhesive-clinching of Al/adhesive/Al/steel: Numerical simulation and experimental validation

This paper proposes the incremental laser shock adhesive-clinching technology (ILSAC) as a novel joining method for ultra-thin sheets. The ILSAC process enables the creation of linear-clinch-bonded hybrid joints with continuous and straight-line interlocking structures through laser beam scanning. No heat-affected zones exist and large-area connection can be easily achieved. Experiments and simulations are conducted to explore the ILSAC for Al/adhesive/Al and pre-slotted stainless steel sheets. The results show that the ILSAC can effectively produce hybrid joints exhibiting enhanced tensile strength and load-bearing capacity.

Lissajous MEMS laser beam scanner with uniform and high fill-factor projection for augmented reality display.

MEMS Laser beam scanning (LBS) has been identified as a key advancement for augmented reality (AR) displays due to its ability to create compact optical systems that generate bright, high-contrast images with minimal heat dissipation. This innovation can be attributed to the focus-free, efficient light-on-demand pixel projection mechanisms integral to LBS. The LBS, specifically in Lissajous-mode, outperforms the raster-mode in terms of larger scan angles and stability to external vibrations, by leveraging a MEMS mirror operating at bi-axial resonance. However, it tends to be hampered by small mirror aperture, low fill-factor, and inconsistent uniformity of image projection. In this research, a unique gimbal-less Lissajous MEMS scanner was proposed. It employs a bi-axial high frequency of 12,255 Hz and 7,182 Hz to achieve a resolution of 640 × 360 pixels and a video refresh rate of 57 Hz, all while maintaining a high image fill factor of 85.11%. The robust structure of the mirror is proven to sustain stable scanning under broad spectrum of external vibration disturbance up to 2,000 Hz. Furthermore, the large mirror diameter of 2 mm improves refined pixel projection and increased optical etendue for exit pupil. Mathematic model of Lissajous pixel-cells and image reconstruction simulation were established to validate the LBS's ability to generate a uniform and densely pixelated visual effect that fits for typical AR head-up display (AR-HUD). In a pioneering move, performance metric of figure-of-merit was defined to evaluate AR light-engines using varied picture-generation techniques, laying a foundation for guiding future AR system development.

Residual Stress Measurements on Rectangular Specimen SS316L using Numerical Computation Software and Selective Laser Melting

This research aimed to predict the residual stress of additively manufactured rectangular specimen using Selective Laser Melting (SLM) by means of non-linear numerical computation based on Thermo-mechanical method (TMM). The procedure starts with the geometrical and material modelling of rectangular specimen with regards to Austenitic Stainless Steel 316L (SS316L) in which the temperature dependent material data properties such as Young’s Modulus, Thermal Expansion Coefficient, Specific Heat Capacity and Thermal Conductivity are taken into account. The next phase consists of numerical computation procedure which is where the specimen is position 60° of inclination angle from the substrate plate. The support structure is to be generated within the lower surface of the specimen in order to avoid material to collapse during printing process. Laser heat source is to be modelled based on the laser beam width, power, efficiency and scanning speed in order for the numerical computation to accurately predict the thermal problem in SLM process. Furthermore, layer parameters used to fabricate the specimen such as hatch distance, hatch scan width and layer thickness are taken into TMM consideration. Similar set-up from numerical computation by means of laser and layer parameters to fabricate SS316L rectangular specimen is utilized in real fabrication process using SLM machine, Renishaw RenAM 500E. After fabrication of the specimen, electropolishing as for the sample preparation for X-Ray Diffraction (XRD) measurement are to be conducted by means of various depth on both sides of the specimen. For the validation procedure, residual stress on every depth is to be analysed and compared with the result from numerical computation. In conclusion, TMM simulation forecasting an acceptable residual stress on SLM product with relative error up to 14% and the computational time taken to predict the residual stress is only 56 minutes. This exploratory research using TMM simulation to predict residual stress on SLM product could benefit metal additive manufacturing (MAM) production as a whole by neglecting expensive trial and error approach.

Hybrid Multimaterial 3D Printing Using Photocuring-While-Dispensing.

Three-dimensional (3D) printing methods, such as vat photopolymerization (VPP) and direct-ink-writing (DIW) processes, are known for their high-resolution and multimaterial capabilities, respectively. Here a novel hybrid 3D printing technique that combines the strengths of VPP and DIW processes to achieve multimaterial and high-resolution printing of functional structures and devices, is presented. The method involves dispensing liquid-like materials via syringes into a photocurable matrix material and subsequently using a Galvano mirror-controlled laser beam to selectively photocure the dispensed material trace or the matrix material surrounding the trace. The laser beam scanning and syringe dispensing are synchronized with a set delay to control liquid diffusion and in situ fixture. The versatility of the method is demonstrated by fabricating intricate 3D ant and wheel prototypes using various materials available for VPP and DIW technologies. The proposed photocuring-while-dispensing strategy offers advantages over conventional multimaterial 3D printing methods, such as integrating materials regardless of photocurability and viscosity, and fabricating heterogeneous structures with complex geometries and high resolution. With its principle demonstrated, this multimaterial 3D printing process will open up a wide range of potential applications with diverse functionalities and materials.

Investigating the Effect of Interface Temperature on Molecular Interdiffusion during Laser Transmission Welding of 3D-Printed Composite Parts.

The present study investigated the influence of temperature on molecular interdiffusion at the interface during the laser transmission welding of 3D-printed continuous carbon-fiber-reinforced thermoplastic composites. In order to accurately measure the temperature at the weld interface, a series of thermocouples were embedded in the laser-absorbent composite part. Two different molecular interdiffusion models were implemented to calculate the degree of healing and to predict the effects of temperature on the welding process. The degree of healing and the weld line width were computed and compared with microscopy observations. The discrepancy between the two proposed numerical models was less than 6%. Both models showed good agreement with the experimental data, with an average error of 13.28% and 7.26%, respectively. The results revealed a significant correlation between the thermal history and molecular interdiffusion at the interface. Furthermore, the relationship between the welding parameters (laser beam scanning speed) and weld line width was established. The findings of this study provide a comprehensive understanding of the underlying mechanisms involved in the laser welding of 3D-printed composites and offer insights to optimize the welding process for enhanced weld quality and superior mechanical properties in the final product.

Modeling of nanoparticle-sensitized spatially localized dynamic photohyperthermia in a blood vessel under infrared laser irradiation with a scanning beam

Photoheating in a venous blood vessel under near-infrared laser irradiation by a scanning laser beam, which is synchronized with the blood flow velocity, is analyzed numerically by considering the light scattering, absorption, and heat transfer in the blood and surrounding tissues. The scanning regime of the laser irradiation with a wavelength of 1064 nm is found to increase strongly the photoheating of the blood while minimizing the overheating of the skin and the tissues surrounding the blood vessel. Simulations show that the photoheating effect can be additionally enhanced by light-absorbing nanoparticles in the blood. The proposed approach of dynamic laser photohyperthermia with the scanning laser beam is discussed for potential applications in laser phototherapy.

Influence of laser beam scanning pattern and polar angle on the fatigue behavior of additively manufactured specimens made of steel 316L

While Laser Powder Bed Fusion (LPBF) is a popular, rapidly advancing field of research, a comprehensive or deeper understanding of the mechanical, especially fatigue, properties of steels has not yet been fully achieved. In this work, a deeper investigation on the microstructural load capacity, critical defect size distribution and defect tolerance of fatigue performance of the commonly used 316L austenitic steel were performed. The 316L parts were generated via LPBF with three different scan patterns and polar angles. Resulting porosities were measured with Archimedes' method and cross section scanning with optical microscopy. To understand the relation of process parameters, microstructure and mechanical behaviors, the microstructure and mechanical properties were studied. Fatigue damage origins and fracture mechanisms were investigated. Results showed that apart from comparable hardness to the conventional steel, tensile properties (except Young's modulus) are slightly worse than the conventional values, owing to the fact that the presence of pores and inclusions promote earlier plastic deformations and ductile fracture. The fatigue strength is inferior compared to the conventional steel and most samples failed from lack of fusion. The surfaces of lack of fusion areas, triggering fatigue failure, are either smooth or show parallel lines. The different scan strategies lead to no big difference in the microstructures except the size of the largest lack of fusion area. The polar angles determine the orientations of lack of fusion to the loading axis and result in different sizes of fatigue triggering defects and deteriorate the matrix load capacity to different extents. Spherical pores within a certain range have limited impact. Parallel simulations confirm the defect tolerance determined by the morphology and critical size on the fatigue strength.

Solidification Crack Free Welding Strategy via Ultra High Precision Laser Scanning at a Single Grain of Directionally Solidified and Single Crystal Superalloys

In this study, possibility of solidification crack-free welding of directionally solidified CM247LC and single crystal CMSX-4 superalloys was metallurgically investigated using single-mode fiber laser scanning. The key metallurgical factors of solidification cracking have been identified as the formation of highly misoriented solidification grain boundaries (low level of epitaxial growth behavior) at the fusion zone that affects the repulsive force between two adjacent dendrites and dendrite coalescence undercooling at the terminal stage of weld solidification. In particular, under extremely low heat input and ultra-high-speed laser beam scan conditions (welding speed: 1,000 mm/s, heat input: 2 J/mm, and energy density: 65 J/mm<sup>2</sup>), an effective fusion zone could be achieved within a single directionally solidified grain of CM247LC alloy owing to the characteristics of single-mode fiber laser. The fusion zone successfully showed a 99.9% of epitaxial growth achievement ratio (that is, low fraction of highly misoriented solidification grain boundaries at the fusion zone) without solidification cracking. Based on these results on CM247LC alloy, solidification crack-free welds could be successfully achieved using quadruple-pass and square-type weld geometries.

Effect of interfacial thermal history on bonding mechanism of laser assisted joining of QP980-CFRTP with adjustable flat-top rectangular laser beam

Carbon fiber reinforced thermoplastic composite (CFRTP) - metal hybrid structures, which have been increasingly used in the industry, can significantly reduce structure weight and energy consumption. In this paper, an adjustable flat-top rectangular diode laser beam was used as a heat source to join the quenched and portioned QP980 steel and CFRTP in a lap joint configuration. Compared to conventional laser beam scanning process in the literatures, the rectangular laser beam provides a relatively uniform temperature distribution at interface. The relationship between interfacial thermal history and the joint performance was explored. Experimental results showed that thermal history had a significant impact on interfacial chemical compositions and the formation of joint defects via affecting the resin state, resulting in change of the joint fracture behaviors. Furthermore, higher interfacial temperature and sufficient heating/holding time contributed to the complete melting and diffusing of resin matrix on QP980 surface, filling the micro pores of the interface, and promoting the chemical bonding between QP980 and CFRTP. On this basis, a novel strategy to achieve excellent interfacial bonding via adjusting the interfacial thermal history was proposed to produce a high-quality joint with the peak load higher than 10 kN and the shear strength of 22 MPa.

Fe/TaC Coatings Produced on 145Cr6 Steel by Laser Alloying—Manufacturing Parameters and Material Characterization

This paper focuses on Fe/TaC composite coatings produced on 145Cr6 steel by laser alloying a TaC precoat in paste form. Fe/TaC coatings were produced in two consecutive steps. The first stage was the application of a precoat in paste form made from tantalum carbide and water glass on a steel substrate. Three TaC precoat thicknesses were produced: 30 µm, 60 µm and 90 µm. In the second step, the TaC precoat was remelted on a steel substrate using a 3 kW rated diode laser beam. A constant laser beam scanning speed of 3 m/min and three laser beam powers were used: 500 W, 800 W and 1100 W. In the study, microstructure, microhardness, chemical and phase composition and wear resistance were tested. The aim of the research was to check the possibility of producing composite coatings in which the reinforcing phase will be TaC, and the role of the matrix will be played by the material from the substrate. It was found that it is possible to produce the continuous composite coatings by remelting the TaC precoat with steel substrate. As microhardness increased, so did wear resistance. The coating microhardness obtained ranged from about 750 to 850 HV0.05 depending on the parameters used.

Fiber Laser Alloying of Additively Manufactured 18Ni-300 Maraging Steel Part Surface: Effect of Processing Parameters on the Formation of Alloyed Surface Layer and Its Properties.

The development of new efficient, economical, and safe methods for strengthening the working surfaces of parts is an important task in the field of improving the reliability and resourcefulness of critical equipment and structures. In the present paper, laser boronizing is investigated as an alternative method for improving the wear resistance of maraging steel parts manufactured by laser powder bed fusion (LPBF). After LPBF, the specimens' surface was covered with an amorphous boron paste (0.03-0.6 mm) and laser processed with a continuous-wave fiber laser in melting mode (λ-1070 nm; power-300 W; spot Ø-1.0 mm) at 500-1500 mm/min laser beam scanning speeds. Scanning electron microscopy, X-ray microanalysis, Knoop hardness, and dry sliding wear tests were applied to investigate the geometry, microstructure, hardness and its distribution, heat-affected zones, wear resistance, and wear mechanism of the alloyed layers. The boronized layers of thickness ~280-520 µm with microstructure from hypoeutectic to borides' mixture were obtained, whose hardness varied from ~490 to ~2200 HK0.2. With laser boronizing, the wear resistance was improved up to ~7.5 times as compared with aged LPBF samples. In further method development, the problem of thermal cracking and softening of the heat-affected zone should be solved.

Manufacturing Process, Microstructure and Physico-Mechanical Properties of W-Cr Coatings Reinforced by Cr3C2 Phase Produced on Tool Steel through Laser Processing

This paper presents study results of laser processing of W-Cr, WCr/Cr3C2 and Cr3C2 pre-coats applied on steel substrate in the form of paste. For this study, production parameters were selected to obtain the greatest possible durability of final coatings. Laser processing was carried out using a diode laser machine with a rated power of 3 kW. The laser beam scanning speed was constant at 3 m/min, but variable laser beam powers were used: 600 W, 900 W and 1200 W. Multiple laser tracks with 60% overlapping were used. After remelting the pre-coat with a steel substrate, new coatings were obtained. Following the experiment, microstructure, microhardness, wear, corrosion resistance and chemical composition were investigated. It was found that it is possible to produce W-Cr/Cr3C2 coatings through laser processing. These coatings do not have the characteristics of a composite coating; however, increasing the reinforcing phase in the pre-coat positively affects the wear resistance and microhardness. The addition of a reinforcing phase was found to lead to a microhardness of about 750–890 HV01 for 25% and 75% Cr3C2, respectively, in comparison to coating without Cr3C2. The wear resistance of coatings reinforced by chromium carbide improved more than twofold in reference to the W-Cr coating.

60‐1: Invited Paper: Emerging microdisplay technologies for AR/VR applications

LCD and OLED are presently the two dominating microdisplays for VR headsets. How to achieve high resolution density for wide field of view and eliminating the screen‐door effect, while keeping low power consumption remains a major challenge. For ultracompact and lightweight AR displays, micro‐LED, OLED‐on‐silicon, MEMS, laser beam scan, and front‐lit LCoS are strong contenders. To reduce the required display brightness for low power consumption, and to enable occlusion effect, a segmented smart dimmer is a viable approach. The pros and cons of each approach will be analyzed.

Interpretation of bimodal interference in and optimized operational modal analysis for long-range continuously scanning laser Doppler vibrometer measurements with a beam under white noise excitation

This paper explains the phenomenon of a dominant frequency in the spectrum of a raw response measured by a long-range continuously scanning laser Doppler vibrometer (LCSLDV) system being symmetrically shifted into two sharp peaks. This phenomenon is referred to as bimodal interference in this work, which gradually becomes obvious when the scanning length is greater than 0.5 m. Bimodal interference is significant because it makes natural frequencies unable to be estimated in LCSLDV measurements and this paper analyzes its nature. Natural frequencies and mode shapes of a structure under white noise excitation can be estimated by analysis of its responses measured by the LCSLDV system. The effect of bimodal interference has been ignored in previous LCSLDV and continuously scanning laser Doppler vibrometer (CLSDV) measurement studies, but its appearance makes the value of the dominant frequency not to be accurately identified. By deducing mathematical interpretation of bimodal interference with analysis of incidence and reflection of a continuously scanning laser beam in a tiny time interval and comparing experimental results with 21 groups of different conditions, it is concluded that bimodal interference is caused by signal coupling between the scanning frequency of the mirror and the vibration frequency of the measured structure. There is a positive correlation between the scanning frequency and the magnitude of the symmetric frequency shifts centered at the dominant frequency generated by bimodal interference. In experimental results of LCSLDV measurements with a distance of 60 m, the frequency shift ratio compared to the dominant frequency can be reduced to 0.06% when the scanning frequency is lower than 0.01 Hz. In this case, bimodal interference is not noticeable in the spectrum. The experiment was carried out on a cantilever beam machined based on a prescribed design. Modal parameters of the cantilever beam estimated by white noise excitation were successfully estimated by the LCSLDV system with a distance of 60 m. The maximum difference between the first four natural frequencies of the cantilever beam from the finite element model and the LCSLDV system was 2.97% and the minimum difference was 1.1%.

Microcantilever-integrated photonic circuits for broadband laser beam scanning

Laser beam scanning is central to many applications, including displays, microscopy, three-dimensional mapping, and quantum information. Reducing the scanners to microchip form factors has spurred the development of very-large-scale photonic integrated circuits of optical phased arrays and focal plane switched arrays. An outstanding challenge remains to simultaneously achieve a compact footprint, broad wavelength operation, and low power consumption. Here, we introduce a laser beam scanner that meets these requirements. Using microcantilevers embedded with silicon nitride nanophotonic circuitry, we demonstrate broadband, one- and two-dimensional steering of light with wavelengths from 410 nm to 700 nm. The microcantilevers have ultracompact ~0.1 mm2 areas, consume ~31 to 46 mW of power, are simple to control, and emit a single light beam. The microcantilevers are monolithically integrated in an active photonic platform on 200-mm silicon wafers. The microcantilever-integrated photonic circuits miniaturize and simplify light projectors to enable versatile, power-efficient, and broadband laser scanner microchips.

Research of Non-mechanical Beam Scanning Technology

Laser beam scanning technology has been widely used in various fields such as LIDAR, photoelectric detection and space optical communication, etc. Mechanical scanning has problems such as inflexibility and low scanning rate. This paper introduces the basic principles of acoustic-optical scanning, electro-optical scanning, optical phased array and MEMS scanning, and analyses the advantages and disadvantages of several scanning methods and the main application scenarios, and finds that the current optical scanning technology is developing towards large scanning angle, high accuracy, fast response and miniaturization, among which the liquid crystal optical phased array and MEMS scanning technology are relatively mature. The scanning angle of the LCD optical phased array can reach ±10°, and the scanning angle of the MEMS can reach 6.6°*4.4°, which has a broad development prospect.