Laser Beam Size Research Articles

100 MW Peak Power Picosecond Laser Based on Hybrid End-Pumped Nd:YVO4 and Side-Pumped Nd:YAG Amplifiers

A mode-locked fiber laser with pulse duration of 4.8 ps and repetition rate of 30 kHz is amplified by end-pumped Nd:YVO4 amplifiers and side-pumped Nd:YAG amplifiers. The output power is amplified up to 31.8 W, achieving a pulse peak power of more than 100 MW and a gain of 59 dB. The beam quality is well preserved with ${M^2}$ factor of 1.26, based on the method of aberration self compensation. The laser beam size in amplifiers is optimized to prevent the unwanted damage due to the high pulse peak power density by using two different types of pumping scheme. It is demonstrated to be an ideal laser source for higher harmonic generation due to high peak power and good beam quality.

Experimental study of variable thermal lens in a two-stage chirped-pulse high-power Ti:sapphire amplifier at 1kHz

In general, misalignment, surface quality of optical components, thermal and nonlinear properties, doping level and inhomogeneity in the gain material can induce unexpected pulse propagation effects on intnese femtosecond laser pulses. Among them, the thermal lens in a thermally loaded material is more serious since it limits power scaling and degrades maximum achievable intensity at the focus by introducing distortion into the wavefront. Thereby, an understanding of thermally distortion wavefront is of great importance for optimum design of high power laser amplifiers and efficient serving femtosecond lasers in various specific applications. In this paper, we experimentally studied beam quality and wavefront distortion in a longitudinally pumped two-stage chirped pulse high power Ti:sapphire amplifier laser at 1 kHz operation in terms of pump power. The amplifier crystals are cooled thermoelectrically to below −10 °C. A simple formula for coupling the thermal lens in a two-stage amplifier consists of a regenerative amplifier followed by a single-pass amplifier pumped equally by the Nd:YLF laser at 527 nm with maximum average output power of 44 W and 130 ns pulse duration at 1 kHz is presented. The wavefront curvature of the compressed laser pulses with ∼40 fs pulse duration and center wavelength of 800 nm is evaluated directly by Shack-Hartmann Wavefront Sensor (SHWS) with help of the Zernike polynomials. While the beam radius and spatial beam quality M2 factor are estimated by knife-edge testing. Also, the optical wavefront distortion caused by an iris inserted into the beam path of a high power laser for quick spot size adjustment has been presented. When diameter of the iris is about 75% of the laser beam size (corresponding to 67% fraction transmission), laser beam distortion associated with the iris is negligible.

Skip the beat: minimizing aliasing error in LA-ICP-MS measurements.

Pulsed laser ablation sampling and sequential isotope detection can lead to signal beat in the registered signal intensities. In particular, if aerosol transport systems deliver ablated aerosol with temporal duration close to that of a single mass scan, such signal beat can become significant and lead to biased intensity ratios and concentrations. Averaging signal intensities based on the least common multiple of scan duration and laser pulse period can eliminate such a systematic bias and improve the accuracy of quantitative laser ablation experiments. The method was investigated for experiments using an ablation cell that provided aerosol washout times near 200ms and quadrupole-based ICP-MS acquisition using different dwell and settling times that were compared with and extended by numerical simulations. It was found that the systematic bias of acquired data could exceed the inherent noise of laser ablation inductively couple plasma mass spectrometry experiments and that the averaging method could successfully minimize the bias due to beating. However, simulations revealed that this was only the case for combinations of pulse frequency and scan duration in which the number of laser pulses within the averaged period was an integer multiple of the number of isotopes in the acquisition method. In element imaging applications, this averaging will necessarily increase the size of individual pixels and it depends not only on the laser beam size but also pulse repetition rate and the acquisition settings for a sequential mass spectrometer. Graphical abstract LCM averaging can prevent occurrence of a systematic bias in LA-ICPMS measurements.

Fatigue crack detection using dual laser induced nonlinear ultrasonic modulation

In this study, a nonlinear ultrasonic modulation technique based on dual laser excitation is proposed for fatigue crack detection. Two pulse lasers are shot on the target specimen for ultrasonic generation. The corresponding ultrasonic responses are measured by a laser Doppler vibrometer (LDV) and analyzed to extract the crack induced nonlinear ultrasonic modulation. First, the effect of the pulse laser beam size on the frequency content of the generated ultrasonic waves is numerically and experimentally investigated. Then, this finding of the laser beam size effect is utilized to generate wideband (WB) and narrowband (NB) ultrasonic waves by adjusting the laser beam sizes of the two pulsed excitation lasers. Nonlinear ultrasonic modulation results from the interaction of WB and NB ultrasonic waves when a fatigue crack exists in the target specimen. The fatigue crack is then detected by comparing the spectral responses obtained under a single WB input and both WB and NB inputs. In the end, a fully noncontact dual laser ultrasonic system is developed and used to detect micro fatigue cracks in aluminum and steel plate specimens.

Exact solution of thermal response in a three-dimensional living bio-tissue subjected to a scanning laser beam

At present, laser plays an important role in biomedical treatments and surgical techniques. To guarantee patients’ safety, a deeply understanding of thermal responses of biological tissues during laser-tissue interaction is required. The existing analytical researches usually explore the bio-thermal process based on a one-dimensional heat conduction model because of the complication of the governing equations in three-dimensional case. The present study developed an analytical solution for the laser-tissue thermal interaction with a moving heat source based on a three-dimensional DPL thermal transfer model. The effects of the capillary vessel system and metabolism were under consideration. The expression of temperature in the cuboid bio-tissue was derived analytically by utilizing the Green’s function approach and its accuracy was verified through agreement with a numerical simulation. The influences of the laser moving speed, the spot size of laser beam and the two parameters of relaxation times were discussed.

The Effect of Collimating Lens Focusing on Laser Beam Shape in Matrix Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS).

Tissue imaging using matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a well-established technique that, in recent years, has seen wider adoption and novel application. Applications such imaging mass spectrometry (IMS) and biotyping are beginning to gain greater exposure and use; however, with limitations in optimization methods, producing the best result often relies on the ability to customize the physical characteristics of the instrumentation, a task that is challenging for most mass spectrometry laboratories. With this in mind, we have described the effect of making simple adjustments to the laser optics at the final collimating lens area, to adjust the laser beam size and shape in order to allow greater customization of the instrument for improving techniques such as IMS. We have therefore been able to demonstrate that improvements can be made without requiring the help of an electrical engineer or external funding in a way that only costs a small amount of time. Graphical Abstract ᅟ.

Thermo-Elastic Model of Epicenter Displacement by Laser Pulse Irradiated on Metallic Surfaces

In recent years, there is a much interest in developing of nondestructive testing (NDT) systems using the pulse-echo laser ultrasonics. The key idea is to combine a low-power and short-pulsewidth laser excitation with a continuous sensing laser; and use a scanning mechanism, such as five degrees-of-freedom (5DOF)-axis robot, laser mirror scanner, or motorized linear translation or rotation scanner stage, to scan the combined beam on the structure. In order to optimize the parameters of the excitation laser, a realistic theoretical model of the epicenter displacement in thermo-elastic regime is needed. This paper revisits and revises the study of Spicer and Hurley (1996, “Epicentral and Near Epicenter Surface Displacements on Pulsed Laser Irradiated Metallic Surfaces,” Appl. Phys. Lett., 68(25), pp. 3561–3563) on thermo-elastic model of epicenter displacement with two new contributions: first, we revised Spicer’s model to take into account the optical penetration effect, which was neglected in Spicer’s model; and second, the revised model was used to investigate the effect of laser rise time and beam size to the epicenter displacement. We showed that a pulse laser with short rise time generates an equivalent surface displacement with a pulse laser with long rise time, except a “spike” at the beginning of the epicenter waveform; also when the laser beam size increases, the epicenter displacement decreases. These two conclusions were then validated by experiments.

Direct laser deposition cladding of AlxCoCrFeNi high entropy alloys on a high-temperature stainless steel

AlxCoCrFeNi (x=0.3, 0.6 and 0.85) high entropy alloy (HEA) claddings were produced by coaxial direct laser deposition (DLD) on a 253MA austenitic steel substrate using a mixture of blended elemental powders. The effect of key processing variables on the formation of HEA claddings and the compositional mixing between the deposited layer and the substrate was investigated through a three-level parametric study on laser power, laser scanning speed, laser beam size, powder feeding rate and hatch distance. Using selected parameters, HEA claddings mostly free of defects were successfully manufactured with very minimal dilution. With an increase in the Al mole fraction from 0.3 to 0.6 and 0.85, the HEA claddings displayed an evolution of crystal structure from FCC, to FCC+BCC and BCC, accompanied by an increase in microhardness. The increased Al content also resulted in reduced microstructural stability of the coatings and hence higher level of thermal softening upon isothermal treatment at 1000°C.

Multiscale Stereolithography Using Shaped Beams

Current stereolithography (SL) can fabricate three-dimensional (3D) objects in a single-scale level, e.g., printing macroscale or microscale objects. However, it is difficult for the SL printers to fabricate a 3D macroscale object with microscale features. In the paper, a novel SL-based multiscale fabrication method is presented to address such a problem. The developed SL process can fabricate multiscale features by dynamically changing the shape and size of a laser beam. Different shaped beams are realized by switching apertures with different micropatterns. The laser beam without using micropatterns is used to fabricate macroscale features, while the shaped laser beams based on small apertures are used to fabricate micropatterned features. Accordingly, a tool path planning method for the multiscale fabrication process is presented to build macroscale and microscale features using different layer thicknesses, laser exposure time, and scanning paths. Compared with the conventional SL process using a fixed laser beam size, our process can manufacture multiscale features in a 3D object with fast fabrication speed and good surface quality.

Gaussian process-based surrogate modeling framework for process planning in laser powder-bed fusion additive manufacturing of 316L stainless steel

Laser Powder-Bed Fusion (L-PBF) metal-based additive manufacturing (AM) is complex and not fully understood. Successful processing for one material, might not necessarily apply to a different material. This paper describes a workflow process that aims at creating a material data sheet standard that describes regimes where the process can be expected to be robust. The procedure consists of building a Gaussian process-based surrogate model of the L-PBF process that predicts melt pool depth in single-track experiments given a laser power, scan speed, and laser beam size combination. The predictions are then mapped onto a power versus scan speed diagram delimiting the conduction from the keyhole melting controlled regimes. This statistical framework is shown to be robust even for cases where experimental training data might be suboptimal in quality, if appropriate physics-based filters are applied. Additionally, it is demonstrated that a high-fidelity simulation model of L-PBF can equally be successfully used for building a surrogate model, which is beneficial since simulations are getting more efficient and are more practical to study the response of different materials, than to re-tool an AM machine for new material powder.

Confinement – assisted shock-wave-induced thin-film delamination (SWIFD) of copper indium gallium diselenide (CIGS) on a flexible substrate

The laser structuring of CIGS (copper indium gallium (di)selenide) solar cell material without influence and damaging the functionality of the active layer is a challenge for laser methods The shock-wave-induced thin-film delamination (SWIFD) process allows structuring without thermal modifications due to a spatial separation of the laser absorption from the functional layer removal process. In the present study, SWIFD structuring of CIGS solar cell stacks was investigated. The rear side of the polyimide was irradiated with a KrF-Excimer laser. The laser-induced ablation process generates a traverse shock wave, and the interaction of the shock wave with the layer-substrate interface results in a delamination process. The effect of a water confinement on the SWIFD process was studied where the rear side of the substrate was covered with a ∼2mm thick water layer. The resultant surface morphology was analysed and discussed. At a sufficient number of laser pulses N and laser fluences Φ, the CIGS layer can be selectively removed from the Mo back contact. The water confinement, as well as the increasing laser beam size A0 and N, results in the reduction of the necessary minimal laser fluence Φth. Further, the delaminated CIGS area increased with increasing Φ, N, and A0.

Direct Micromachining of Microfluidic Channels on Biodegradable Materials Using Laser Ablation.

Laser patterning on polymeric materials is considered a green and rapid manufacturing process with low material selection barrier and high adjustability. Unlike microelectromechanical systems (MEMS), it is a highly flexible processing method, especially useful for prototyping. This study focuses on the development of polymer surface modification method using a 193 nm excimer laser system for the design and fabrication of a microfluidic system similar to that of natural vasculatures. Besides from poly(dimethyl siloxane) (PDMS), laser ablation on biodegradable polymeric material, poly(glycerol sebacate) (PGS) and poly(1,3-diamino-2-hydroxypropane-co-polyol sebacate) (APS) are investigated. Parameters of laser ablation and fabrication techniques to create microchannels are discussed. The results show that nano/micro-sized fractures and cracks are generally observed across PDMS surface after laser ablation, but not on PGS and APS surfaces. The widths of channels are more precise on PGS and APS than those on PDMS. Laser beam size and channel depth are high correlation with a linear relationship. Repeated laser ablations on the same position of scaffolds reveal that the ablation efficiencies and edge quality on PGS and APS are higher than on PDMS, suggesting the high applicability of direct laser machining to PGS and APS. To ensure stable ablation efficiency, effects of defocus distance into polymer surfaces toward laser ablation stability are investigated. The depth of channel is related to the ratio of firing frequency and ablation progression speed. The hydrodynamic simulation of channels suggests that natural blood vessel is similar to the laser patterned U-shaped channels, and the resulting micro-patterns are highly applicable in the field of micro-fabrication and biomedical engineering.

Methodology for in situ synchrotron X-ray studies in the laser-heated diamond anvil cell

ABSTRACT A review of some important technical challenges related to in situ diamond anvil cell laser heating experimentation at synchrotron X-ray sources is presented. The problem of potential chemical reactions between the sample and the pressure medium or the carbon from the diamond anvils is illustrated in the case of elemental tantalum. Preliminary results of a comparison between reflective and refractive optics for high temperature measurements in the laser-heated diamond anvil cell are briefly discussed. Finally, the importance of the size and relative alignment of X-ray and laser beams for quantitative X-ray measurements is presented.

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.

Higher order mode laser beam intensity fluctuations in strong oceanic turbulence

Intensity fluctuations of the higher order mode laser beams are evaluated when these beams propagate in a medium exhibiting strong oceanic turbulence. Our formulation involves the modified Rytov solution that extends the Rytov solution to cover strong turbulence as well, and our recently reported expression that relates the atmospheric turbulence structure constant to the oceanic turbulence parameters and oceanic wireless optical communication link parameters. The variations of the intensity fluctuations are reported against the changes of the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of kinetic energy per unit mass of fluid, rate of dissipation of mean-squared temperature, viscosity and the source size of the higher order mode laser beam. Our results indicate that under any oceanic turbulence parameters, it is advantageous to employ higher order laser modes in reducing the scintillation noise in wireless optical communication links operating in a strongly turbulent ocean.

Analysis of Infra-Red Laser Power Process Using Response Surface Methodology

High-quality products and shorten manufacturing lead time are very important factors addressed by many companies. Typically, low-quality products are induced by inconsistent of manufacturing process performance. To get a constant laser power in Infrared (IR) Laser Marking process was a common problem facing by many engineers in the manufacturing industry. This consequences might effect on laser marking characteristics such as the possibility of burn marking or blur marking particularly on the products. Therefore, the objective of this paper is to study the interaction between Infrared (IR) Laser Marking input parameters such as current (amp), frequency (Hz), water flow (l/min) and laser beam size (mm) on laser power (watt) as the main response. Furthermore, the response of laser power from Infra-Red laser process can be predicted when all combinations of parameters are tested. The IR marking machine was deployed and experimental studies were conducted in order to meet this objective. The experiments were based on Design of Experiment (DOE) and Response Surface Methodology (RSM) because second order model or polynomial equation was needed to fit for the response. The relationship between input and output parameters can be visualized by 3D graphs which were developed from Mathematical modeling. Based on the experiment results, the parameters that have significant effects on the laser power are current, laser beam size and water flow. However, frequency does not have a significant factor in this study.

PAL-XFEL laser heater commissioning

X-ray free electron lasers (XFELs) require electron beams with a peak current of several kA and a transverse emittance below 1mm mrad. The uncorrelated energy spread of the beam should be as small as 0.01% of the beam energy. A photoinjector generates an electron beam satisfying the requirements, however the uncorrelated energy spread of the beam is too small, about one keV. A beam with such a low uncorrelated energy spread may suffer from the longitudinal microbunching instability when the beam is accelerated and compressed through a linear accelerator. A laser heater system in the injector minimizes the microbunching instability growth by controlling the uncorrelated energy spread of the beam. In this paper, we introduce the PAL-XFEL laser heater system and the commissioning result. The laser heating effects depending on the undulator gap, infrared laser pulse energy and laser beam size are studied. Experimental studies on the laser heating depending on the relative spatial position between the infrared laser and electron beam are discussed in detail.

Self-focusing of a laser beam in the rippled density magnetoplasma

In the present paper, we have investigated the self-focusing of intense laser beam in rippled density magnetoplasma. The differential equation for the beam width parameter is obtained by following WKB and paraxial approximations and solved it numerically. The behaviour of beam width parameter with dimensionless distance of propagation is extensively investigated for different values of laser and plasma parameters and is strongly influenced by the magnetic field. The results are discussed and presented graphically. It is seen that the density ripple of suitable wave number increases the self-focusing property of laser beam in magnetoplasma. Meanwhile, an increase in the values of magnetic field increases the strength of the self-focusing, especially in shorter distance of propagation. Furthermore, increase in plasma frequency (plasma density) causes more reduction in the spot size of laser beam and consequently increases the rate of self-focusing of laser beam in magnetoplasma.

The investigation of laser shock peening effects on corrosion and hardness properties of ANSI 316L stainless steel

Laser shock peening (LSP) is known as a post processing surface treatment which can improve the mechanical properties of some materials. Shock waves are generated by confining the laser-induced plasma to cause a large pressure shock wave over a significant surface area. In the present study, effects of LSP on the electrochemical corrosion and micro hardness properties of 316L stainless steel alloy were investigated by changing the laser parameters such as the laser spot size, the average number of impacts, and the laser intensity. Since laser parameters do not cover the desired region of LSP, we have to use the proper design of experiment method, in which the D-optimal design of MATLAB was selected. Results revealed that by increase in irradiance, number of impacts and spot size of laser beam, improvement in the surface micro hardness, and corrosion resistance is achieved. Also, due to unexpected drop into the outcome of our experiments, it was found that the contamination of the transparent overlay and reduction of the absorption coefficient of the absorbent layer play a key role to reduce the efficiency of the mechanical impacts. So, by changing the experimental conditions, even better results are expected.

Nonlinear propagation of Gaussian laser beam in an inhomogeneous plasma under plasma density ramp

Nonlinear propagation of Gaussian laser beam in an inhomogeneous plasma under plasma density ramp is studied. The differential equation for beam width parameter is derived by parabolic wave equation approach under paraxial approximation. For different values of plasma density, laser intensity and initial beam width, the behavior of beam width parameter with the normalized propagation distance is examined. It is found that the beam width parameter decreases with a higher rate. The frequency of the oscillations increases while amplitude decreases and the laser beam focuses up to long distance. Further, increase in relative plasma density causes reduction in the amplitude of spot size of laser beam near the propagation axis.