Laser Beam Spot Research Articles

Determination of the laser intensity applied to a Ta witness plate from the measured x-ray signal using a pulsed micro-channel plate detector

The laser intensity distribution at the surface of a high-Z material, such as Ta, can be deduced from imaging the self-emission of the produced x-ray spot using suitable calibration data. This paper presents a calibration method which uses the measured x-ray emissions from laser spots of different intensities hitting a Ta witness plate. The x-ray emission is measured with a micro-channel plate (MCP) based x-ray framing camera plus filters. Data from different positions on one MCP strip or from different MCP assemblies are normalized to each other using a standard candle laser beam spot at ∼1 × 1014 W/cm2 intensity. The distribution of the resulting dataset agrees with results from a pseudo spectroscopic model for laser intensities between 4 and 15 × 1013 W/cm2 . The model is then used to determine the absolute scaling factor between the experimental results from assemblies using two different x-ray filters. The data and model method also allows unique calibration factors for each MCP system and each MCP gain to be compared. We also present simulation results investigating alternate witness plate materials (Ag, Eu and Au).

A heterodyne straightness and displacement measuring interferometer with laser beam drift compensation for long-travel linear stage metrology.

The laser beam drift seriously influences the accuracy of straightness or displacement measurement in laser interferometers, especially for the long travel measurement. To solve this problem, a heterodyne straightness and displacement measuring interferometer with laser beam drift compensation is proposed. In this interferometer, the simultaneous measurement of straightness error and displacement is realized by using heterodyne interferometry, and the laser beam drift is determined to compensate the measurement results of straightness error and displacement in real time. The optical configuration of the interferometer is designed. The principle of the simultaneous measurement of straightness, displacement, and laser beam drift is depicted and analyzed in detail. And the compensation of the laser beam drift for the straightness error and displacement is presented. Several experiments were performed to verify the feasibility of the interferometer and the effectiveness of the laser beam drift compensation. The experiments of laser beam stability show that the position stability of the laser beam spot can be improved by more than 50% after compensation. The measurement and compensation experiments of straightness error and displacement by testing a linear stage at different distances show that the straightness and displacement obtained from the interferometer are in agreement with those obtained from a compared interferometer and the measured stage. These demonstrate that the merits of this interferometer are not only eliminating the influence of laser beam drift on the measurement accuracy but also having the abilities of simultaneous measurement of straightness error and displacement as well as being suitable for long-travel linear stage metrology.

The Effect of Laser Beam Welding Parameters onto the Evolving Joints Geometry

In our research the effect of a new type of laser beam parameters during the laser welding have been investigated with 80 different welding parameters. The laser welding parameters such as the laser power, laser beam spot size on the surface and feed rate greatly affect the resulting weld geometry. S235 grade steel has been used. The operating equipment was a Trumpf 4001 4 kW disk laser with a diameter of 100 microns optical fiber. The effect of different welding parameters were evaluated from the metallographic cross-section of the welded joints. This article describes the effect of the different laser beam focusing and the welding feed rate.

Sub-Beam Size Temperature Measurement of Heavily Doped Silicon Heater Using Two-Wavelength Thermoreflectance Microscopy

This work describes a novel technique for simultaneously determining local temperature and thickness of a heavily doped Si heater having a submicron width by using two-wavelength thermoreflectance microscopy. The doped silicon line heater, whose thickness and width are, respectively, 480 nm and 900 nm, is fabricated by conventional microfabrication techniques on a fused silica wafer. The full width at half maximum (FWHM) of the focused laser beam is measured to be 2.00 μm and 2.28 μm for green (λ = 516 nm) and red (λ = 640 nm) lasers, respectively. Because the heater width is narrower than the focused laser beam size, the reflected beam contains background information (i.e., reflection from the fused silica substrate) in addition to the thermoreflectance signal from the doped silicon heater. With precise knowledge of the laser beam size, heater width, and exact location of the laser beam spot on the heater, one can quantitatively model the reflectance. In reality, however, due to the difficulty of aligning the laser beam with respect to the submicron-wide Si heater, precise determination of local temperature from thermoreflectance signal is not easily attained. In the present study, instead of aligning the laser beam to the center of the submicron silicon heater, the probe laser horizontally scans over a region of the heater. By taking into account the size of the focused laser beam and the width of the doped silicon heater, it is possible to determine the absolute temperature of a local region of the heater from the measured reflectance during the scanning, even though the width of the heater line is only 39% of the size of the laser beam.

Understanding Hybrid Additive Manufacturing of Functional Devices

The fabrication of electronic components in three-dimensional space is critical for building multifunctional devices. Traditional manufacturing approaches are limited to building planar device configurations. In order to address this limitation, our group has developed a novel hybrid additive manufacturing process that can deposit multi-material patterns on conformal substrates. This direct-write approach involves the combination of microextrusion, pico jet and laser systems. A multi-axis robot was employed to deposit slurries and colloids of conductive materials on different substrates. Fine deposition of the slurries was achieved by tuning the process parameters such as deposition line speed, extrusion pressure, curing temperature, Z-height, laser beam spot, material type and laser intensity. Precise deposition of the medium viscous colloidal liquids was achieved by pico jet unit. Sintering of the micro/nano particles was achieved using a fiber optic laser system and micro wave furnace. The conductance of the deposited electrical trace varied based on the material type and trace cross-section area. This research lays the foundation to build a three-dimensional hierarchy of electronic components. By embedding components at various depths and orientations complex electronic circuitry with versatile material compositions and design flexibility can be fabricated in situ.

Comparison of Titanium Metal Matrix Composite Surface Layers Produced During Laser Gas Nitriding of Ti6Al4V Alloy by Different Types of Lasers

Abstract The article presents the results of a comparative study of the nitriding process of titanium alloy substrate using two lasers with different characteristics of laser beams. One of the applied lasers was a high power diode laser emitting at a dominant wavelength of 808 nm, with a rectangular laser beam spot, and multimode energy distribution across the spot. The second laser was a solid state Yb:YAG disk laser emitting at a wavelength of 1.03 μm, with a circular beam spot, characterized by near Gaussian energy distribution across the spot. In a case of both lasers single stringer beads with a similar width and at similar energy input were produced. As a result of melting of the substrate with a laser beam in a pure gaseous nitrogen atmosphere composite surface layers with in situ precipitated titanium nitrides embedded in the metallic matrix of titanium alloy were produced, in both cases. However, the surface topography and structure is different for the surface layers produce by different lasers at the same processing parameters and width of laser beams.

Visualization of Defect-Induced Excitonic Properties of the Edges and Grain Boundaries in Synthesized Monolayer Molybdenum Disulfide

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive materials for next generation nanoscale optoelectronic applications. Understanding nanoscale optical behavior of the edges and grain boundaries of synthetically grown TMDCs is vital for optimizing their optoelectronic properties. Elucidating the nanoscale optical properties of 2D materials through far-field optical microscopy requires a diffraction-limited optical beam diameter sub-micron in size. Here we present our experimental work on spatial photoluminescence (PL) scanning of large size ( $\geq 50$ microns) monolayer MoS$_2$ grown by chemical vapor deposition (CVD) using a diffraction limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small as 200 nm allowing us to probe nanoscale excitonic phenomena which was not observed before. We have found several important features: (i) there exists a sub-micron width strip ($\sim 500$ nm) along the edges that fluoresces $\sim 1000 \%$ brighter than the region far inside; (ii) there is another brighter wide region consisting of parallel fluorescing lines ending at the corners of the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic peak, as large as $\sim 120$ meV, in the PL spectra from the edges. Using density functional theory calculations, we attribute this giant blue shift to the adsorption of oxygen dimers at the edges, which reduces the excitonic binding energy. Our results not only shed light on defect-induced excitonic properties, but also offer an attractive route to tailor optical properties at the TMDC edges through defect engineering.

Excimer laser ablation of aluminum: influence of spot size on ablation rate

The dependence of ablation rate of an Al alloy on laser beam spot size (10–150 µm) was investigated using an ArF excimer laser operating at a wavelength of 193 nm and pulse width less than 4 ns. Ablation was conducted in air at a fluence of 11 J cm−2 and at a repetition rate of 20 Hz. Surface morphology and depth of craters produced by a variable number of laser pulses were characterized using optical and scanning electron microscopy. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used as an additional diagnostic technique to estimate the amount of material ablated from craters produced by a laser beam of different diameters. Laser beam spot size and number of laser pulses applied to the same spot were found to influence crater morphology, ablation rate, shape and amount of particles deposited at or around the crater rim. Ablation rate was found to be less dependent on spot size for craters greater than 85 µm. A four-fold increase in ablation rate was observed with decreasing crater size from 150 µm to 10 µm.

Charge-Trap Inactivation of Multi-Line-Beam Clc Poly-Si Tfts Using Channel Impurity Doping

Linearly-arranged long silicon crystal grains have been formed by using Multi-Line-Beam Continuous-wave laser Lateral Crystallization (MLB-CLC). Crystallinity of the laser-crystallized poly-Si thin film are biaxially oriented to (110)-(111)-(211) crystal orientation. The low-temperature poly-Si thin film transistors (LTPS-TFTs), however have a high-OFF leakage current and ON/OFF ratio become small. Charge traps on the long grain boundaries between source and drain would induce the high-OFF leakage current. In this work, we introduce a channel doping for inactivation of the traps, and high ON/OFF ratio is achieved. Poly-Si TFTs were fabricated as follows. Buffer SiO2 film (1 μm), a-Si (150 nm) and cap SiO2 (100 nm) were deposited on a quartz substrate. CW laser lateral crystallization with multi-line laser-beam spot was carried out on the a-Si films. The wavelength of the laser was 532 nm, and laser power and scan speed were 7.5 W and 0.5 cm/s, respectively. After the laser crystallization, the cap SiO2 was etched with buffered HF aqueous solution (BHF). In order to pattern the poly-Si active layer, lithography and dry etching (Cl2: 20 sccm, HBr: 20 sccm) were performed. Channel doping on channel region was carried out by boron (B) ions with 3 conditions (ion dose: 5×1012 cm-2, 7×1012 cm-2, 9×1012 cm-2, acceleration voltage: 19 keV). Gate SiO2 (42 nm) was deposited with ICP- CVD. Mo gate electrode (300 nm) was deposited by sputtering method. After resist patterning, the gate electrode was etched with an aqueous solution of H3PO4: HNO3: CH3COOH: H2O = 400: 25: 50: 25. A self-aligned S/D regions were formed by As ion doping (ion dose: 2×1015cm-2, acceleration voltage: 66 keV). Activation anneal was carried out at 550℃ in N2 ambient for 30 minutes. A sacrificial oxide layer in the S/D region was removed with BHF. After the removal, interlayer SiO2 film was deposited with APCVD. Contact holes were formed on the APCVD interlayer dielectric film with BHF. Mo metal pad was deposited by sputtering method, and finally the samples were sintered at 400°C in H2 ambient for 30 minutes. The IDS - VGS characteristic of the fabricated poly-Si TFTs are shown in Fig.1. As the boron impurity concentration increased, the off-leakage current was reduced. It is considered that boron impurity compensated the defect traps in the channel region. At the concentration of 1.3×1020 cm-3, the off leakage current at Vg=0 V was 3×10-11 A and ON/OFF ratio was 106. ON current was reduced by introducing the channel impurity. The electron field-effect mobilities for the boron impurities of 7.1×1019 cm-3, 1.0×1020 cm-3 and1.3×1020 cm-3 were 398, 333, and 261 cm2/Vs, respectively. By introducing boron impurity, the Coulomb scattering was occurred and the electron mobility was reduced. Figure 1

Influence of laser shock peening pulse density and spot size on the surface integrity of X2NiCoMo18-9-5 maraging steel

Abstract Maraging alloys are a special class of steels with exceptional mechanical properties, suitable for critical and added value applications in aeronautical, aerospace engineering, and the tooling industry. Researchers have been studying several surface engineering techniques to enhance surface characteristics of maraging and tool steels with the aim of extending service life. The goal of the research presented in this paper was to investigate the influence of laser shock peening (LSP) pulse density and spot size on the surface integrity of X2NiCoMo18-9-5 maraging steel. Different pulse densities and spot sizes were chosen for this purpose. Surface integrity was analysed through roughness, residual stress, and microhardness measurements. The influence of each processing parameter was statistically evaluated using the analysis of variance (ANOVA) confirming a significant influence. LSP without the use of an absorbent coating generated high compressive residual stresses in the surface layer with relatively low surface roughness, a combination of characteristics which may have a beneficial influence on mechanical properties of 18% Ni maraging steel. According to the measurements taken in this research, maximal compressive residual stresses were not achieved using the smallest laser beam spot while maintaining constant pulse energy.

Optical alignment and tuning system for the HUST THz-FEL

A compact FEL oscillator with a radiation wavelength of 30–100μm is proposed by HUST and NSRL. The optical cavity is very sensitive to misalignment errors of the mirror, due to its near-concentric and symmetric structure. The magnetic axis of the undulator, the optical axis of the resonator, and the electron beam propagation axis must all be aligned with high precision for achieving saturated lasing. This paper introduces a high-precision, multi-degree-of-freedom controlled optical alignment system, which has the ability to align in the transverse and longitudinal directions. The alignment tolerances are given by theoretical analysis and numerical simulations with three-dimensional FEL code GENESIS and optical propagation code (OPC). To accomplish optical alignment, two auxiliary HeNe laser systems were introduced. By adjusting the HeNe laser beam spot on the wedge, the optical axis can be aligned to the magnetic axis, and the estimated errors meet the tolerances. Finally, the electron beam will be guided through the hole in the central wedge to complete the transverse alignment. The longitudinal alignment and tuning methods are also described.

Measurement and reliability issues in resonant mode cantilever for bio-sensing application in fluid medium

Cantilevers immersed in liquid experience viscous damping and hydrodynamic loading. We report on the use of such cantilevers, operating in the dynamic mode with, (i) frequency sweeping and (ii) phase locked loop methods. The solution to reliability issues such as random drift in the resonant peak values, and interference of spurious modes in the resonance frequency spectrum, are explained based on the actuation signal provided and laser spot size. The laser beam spot size and its position on the cantilever were found to have an important role, on the output signal and resonance frequency. We describe a method to distinguish the normal modes from the spurious modes for a cantilever. Uncertainties in the measurements define the lower limit of mass detection (mmin). The minimum detection limits of the two measurement methods are investigated by measuring salt adsorption from phosphate buffer solution, as an example, a mass of 14 pg was measured using the 14th transverse mode of a m × 100 μm × 1 μm silicon cantilever. The optimized measurement was used to study the interaction between antibody and antigen.

Direct Diode Laser Cladding of Inconel 625/WC Composite Coatings

Metal matrix composite coatings, composed of Inconel 625 alloy and tungsten carbide (WC), have been produced by laser cladding using a high power direct diode laser with a rectangular laser beam spot and a top-hat beam profile. The primary goal of the investigation was to understand the role of the shape of WC particles and the heat input level on the quality of the composite coating system used, especially on its erosion behaviour. The results indicated that angular WC particles are more susceptible to dissolution in the molten pool than spherical. However, the composite coatings containing angular WC particles exhibited significantly higher erosion resistance than those with spherical WC for both normal and oblique impacts. This is directly attributed to the excellent mechanical interlocking of the angular WC in the matrix. The WC/matrix interfacial decohesion has been observed in the coatings containing spherical WC at the oblique impact.

Measuring photoluminescence spectra of self-assembly array nanowire of colloidal CdSe quantum dots using scanning near-field optics microscopy

A novel periodic array CdSe nanowire is prepared on a substrate of the porous titanium dioxide by using a self-assembly method of the colloidal CdSe quantum dots (QDs). The experimental results show that the colloidal CdSe QDs have renewedly assembled on its space scale and direction in process of losing background solvent and form the periodic array nanowire. The main peak wavelength of Photoluminescence (PL) spectra, which is measured by using a 100-nm aperture laser beam spot on a scanning near-field optics microscopy, has shifted 60 nm with compared to the colloidal CdSe QDs. Furthermore, we have measured smaller ordered nanometer structure in thin QDs area as well, a 343-nm periodic nanowire in thick QDs area and the colloidal QDs in edge of well-ordered nanowire.

Laser-induced infrared characteristic analysis for evaluating joint deviation during austenitic stainless steel laser welding

During high-power fiber laser welding, molten pool thermal radiation contains plenty of information which is related to the welding quality. In order to study the relationship between the molten pool characteristics and the welding stability during high-power (10 kW) fiber laser butt joint welding of type 304 austenitic stainless steel plates, the laser beam spot was controlled to move along the weld joint center, parallel to the weld joint with deviation, and cross the weld joint with two different slopes. Under these four different experimental conditions, the infrared radiation characteristics of a molten pool infrared image in laser welding process was investigated based on welding thermal radiation and heat conduction mechanism. An infrared-sensitive high-speed camera was used to capture the molten pool dynamic infrared images during laser welding. The image processing and pattern recognition algorithm were applied to analyze the molten pool characteristics. The molten pool width, keyhole area, keyhole swing, keyhole sharp degree, and position coordinates were defined as the characteristic parameters of a molten pool to analyze the welding stability. It was found that the joint deviation would affect the welding stability which could be reflected by the welding thermal balance, temperature distribution of a molten pool, and the shape of the corresponding infrared thermal image. Welding experimental results showed that the stability of high-power fiber laser welding process could be estimated by the proposed infrared characteristic parameters.

A repetitively pulsed xenon chloride excimer laser with all ferrite magnetic cores (AFMC) based all solid state exciter

Performance of repetitively pulsed xenon chloride excimer laser (λ~308nm) with solid state pulser consisting of magnetic pulse compression circuit (MPC) using all ferrite magnetic cores (AFMC) is reported. Laser system suitable for 100Hz operation with inbuilt pre-ionizer, compact gas circulation and cooling has been developed and presented. In this configuration, high voltage pulses of ~8µs duration are compressed to ~100ns by magnetic pulse compression circuit with overall compression factor of ~80. Pulse energy of ~18J stored in the primary capacitor is transferred to the laser head with an efficiency of ~85% compared to ~70% that is normally achieved in such configurations using annealed met-glass core. This is a significant improvement of about 21%. Maximum output laser pulse energy of ~100mJ was achieved at repetition rate of 100Hz with a typical pulse to pulse energy stability of ±5% and laser pulse energy of 150mJ was generated at low rep-rate of ~40Hz. This exciter uses a low current and low voltage solid state switch (SCR) that replaces high voltage and high current switch i. e, thyratron completely. The use of solid state exciter in turn reduces electromagnetic interference (EMI) effects particularly in excimer lasers where high EMI is present due to high di/dt. The laser is focused on a thin copper sheet for generation of micro-hole and the SEM image of the generated micro hole shows the energy stability of the laser at high repetition rate operation. Nearly homogeneous, regular and well developed xenon chloride (XeCl) laser beam spot was achieved using the laser.

Continuous trench, pulsed laser ablation for micro-machining applications

The generation of controlled 3D micro-features by pulsed laser ablation in various materials requires an understanding of the material's temporal and energetic response to the laser beam. The key enabler of pulsed laser ablation for micro-machining is the prediction of the removal rate of the target material, thus allowing real-life machining to be simulated mathematically. Usually, the modelling of micro-machining by pulsed laser ablation is done using a pulse-by-pulse evaluation of the surface modification, which could lead to inaccuracies when pulses overlap.To address these issues, a novel continuous evaluation of the surface modification that use trenches as a basic feature is presented in this paper. The work investigates the accuracy of this innovative continuous modelling framework for micro-machining tasks on several materials. The model is calibrated using a very limited number of trenches produced for a range of powers and feed speeds; it is then able to predict the change in topography with a size comparable to the laser beam spot that arises from essentially arbitrary toolpaths. The validity of the model has been proven by being able to predict the surface obtained from single trenches with constant feed speed, single trenches with variable feed speed and overlapped trenches with constant feed speed for three different materials (graphite, polycrystalline diamond and a metal-matrix diamond CMX850) with low error. For the three materials tested, it is found that the average error in the model prediction for a single trench at constant feed speed is lower than 5 % and for overlapped trenches the error is always lower than 10 %. This innovative modelling framework opens avenues to: (i) generate in a repeatable and predictable manner any desired workpiece micro-topography; (ii) understand the pulsed laser ablation machining process, in respect of the geometry of the trench produced, therefore improving the geometry of the resulting parts; (iii) enable numerical optimisation for the beam path, thus supporting the development of accurate and flexible computer assisted machining software for pulsed laser ablation micro-machining applications.

Position alignment of laser beam spots in optical disc drives using cross-correlation

Optical pickup unit (OPU) is a data read/write head in optical disc drives, and precision orientation angle adjustment of a diffraction grating (GT) is an important process in OPU manufacturing for aligning laser beam spots on the same data track of an optical disc. This paper proposes a new method for detecting alignment of laser beam spots for utilization in automation of the GT adjustment process. While a conventional method uses complicated and expensive image processing techniques to an eye diagram of a data radio frequency (RF) signal, the proposed method measures the cross-correlation of two binary signals corresponding to two of the laser beam spots in consideration of the geometric relationship between the laser beam spots and data tracks. The proposed method is not only so simple as to be implemented using a digital logic circuit but also tolerant to poor signal condition due to radial off-track run-out and disc rotational speed error. A FPGA-based implementation of the proposed method is presented, and experimental results are provided to show its validity1.

On the limitation of dual-view triangulation in reducing the measurement error induced by the speckle noise in scanning operations

A triangulation-based laser displacement sensor using the diffuse reflection light has a strong practical advantage in its robustness against the tilting of the object surface to the sensor’s sensitive direction. On the other hand, it is often subject to higher-frequency noise-like measurement error in scanning operations. This error is caused by the movement of the speckle in the reflected laser beam spot with the scanning operation. This paper first discusses how the dual-view triangulation, using two image sensors aligned symmetrically about the laser beam axis, can reduce the scanning noise in principle. Then, its inherent limitation is discussed. Since different points on the target surface cast the specular reflection to each image sensor, the influence of the sensor’s lateral movement cannot be canceled by comparing the reflected spot’s displacement on each image sensor. This issue can be observed clearer when measuring a surface of a less random surface profile. It is experimentally validated.

Photon merging and splitting in electromagnetic field inhomogeneities

We investigate photon merging and splitting processes in inhomogeneous, slowly varying electromagnetic fields. Our study is based on the three-photon polarization tensor following from the Heisenberg-Euler effective action. We put special emphasis on deviations from the well-known constant field results, also revisiting the selection rules for these processes. In the context of high-intensity laser facilities, we analytically determine compact expressions for the number of merged/split photons as obtained in the focal spots of intense laser beams. For the parameter range of a typical petawatt class laser system as pump and a terawatt class laser as probe, we provide estimates for the numbers of signal photons attainable in an actual experiment. The combination of frequency upshifting, polarization dependence and scattering off the inhomogeneities renders photon merging an ideal signature for the experimental exploration of nonlinear quantum vacuum properties.