Laser Beam Path Research Articles

Velocity-map imaging with counter-propagating laser pulses.

Velocity-map imaging (VMI) is a key tool for studying outgoing electrons or ions following optical strong-field interactions of atoms and molecules and provides good momentum resolution even if the source volume of the fragments extends along a laser beam path. Here, we demonstrate within an enhancement cavity how, independently of the focal Rayleigh length, counter-propagating pulses longitudinally compress the ionization volume down to few tens of micrometers. We observe nonlinear above-threshold ionization (ATI) processes confined to the spatial overlap of femtosecond pulses, whereas the shortened ionization volume makes an electrostatic lens unnecessary for VMI.

Influence of laser power and beam path under nonuniform AC electric fields on 3D microvortex flow

This paper describes the effect of optical light on the generation and manipulation of microvortex flow named ‘twin opposing microvortex’ (TOMV) flow. This opto-electrohydrodynamic (OEHD) technique combines optical light, i.e. infrared (IR) laser (1064 nm), with non-uniform AC electric fields generated from a pair of indium tin oxide (ITO) electrodes. When the IR laser beam passes through the electric fields, a rapid and three-dimensional (3D) vortex flow is generated in a microchamber. When the laser beam passes through the electric fields, especially the exposed ITO electrode, the direction of the TOMV flow as well as its strength are controlled. With an AC signal of 107 kHz and various voltages below a peak-to-peak voltage of 10 V, laser power is varied up to 1.5 W and the path of a laser beam relative to the electrode (300 μm long and 16 μm wide) is manipulated. The maximum in-plane velocity outside the electrode region was obtained by micron-resolution particle image velocimetry (μPIV). When the laser beam passes through the left or right side of the lower electrode, the TOMV flow field rotates counterclockwise or clockwise, respectively. Applying optical light on an ITO electrode creates in situ and on-demand microvortex flow, which increases the feasibility of OEHD technique in various biological and chemical applications (e.g., mixing and delivering nanofluids in microfluidic devices).

Integrated-path multi-gas sensor using near-infrared diode lasers: An alternative to vehicle-driven point gas analyzer

Sensors for quantitative monitoring of greenhouse gases from high-emission facilities, such as wastewater treatment and industrial plants, are becoming essential to enforce regulatory compliance in climate action initiatives. A predominant sensing technique is the tracer-gas dispersion method (TDM) using closed-path gas analyzers typically installed in a vehicle to measure concentration as the gas plume is transected. Here, we validate for the first time the use of integrated-path gas sensors in TDM, measuring instantaneously the concentration along a predetermined laser beam path, thereby facilitating a stationary setup with orders of magnitude higher update rate and without the need for vehicles or road access. Our approach relies on a sensitive, integrated-path multi-gas sensor for real-time emission quantification using tunable diode laser absorption spectroscopy. An optical sensor head with spectral coverage from 1.52 μm to 1.65 μm is demonstrated, supporting multiplexed gas sensing in a modular fashion using low-power fiber-coupled diode lasers. The sensor measures real-time integrated-path concentrations of CO2 and CH4, and the tracer-gas C2H2 at 130 Hz with 0.5 ppm, 3 ppb, and 2 ppb sensitivity, respectively, at 1 s averaging. The integrated-path sensor is systematically benchmarked for absolute accuracy against a state-of-the-art point sensor, revealing an excellent match over 16 hours. Furthermore, the integrated-path sensor is deployed outdoors for emission quantification using TDM. A comparison to the gold-standard point sensor reveals identical results, demonstrating the integrated-path sensor as a potent substitute for vehicle-driven point sensors.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy via collimating fiber-array integration

This work reports on the implementation of a fiber-array with attached microlenses in the balanced-detection Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) scheme, enabling a highly compact probe laser beam path. The sample probe and the reference beam were coupled into a 1-mm spaced Fabry-Perot interferometer with a parallel beam separation of only 1.25 mm. A quantum cascade laser was used to induce photothermal waves, which were detected via a sample probe beam. A reference probe beam was used to realize balanced detection, allowing for significant noise reduction. The sensor’s metrological figures of merit were evaluated by detection of nitric oxide. For the targeted absorption band centered at 1900.08 cm−1 a 1 σ minimum detection limit of 64 ppbv was achieved with a 1-second integration time. This performance corresponds to a normalized noise equivalent absorption of 5.7 ×10−8 cm−1 W Hz−1/2.

GENETIC ALGORITHM-BASED OPTIMIZATION OF THE LASER-BEAM PATH IN ADDITIVE MANUFACTURING

This study presents a methodology of genetic-algorithm-based optimization of the laser-beam path for improving laser-based additive manufacturing (AM). A simple thermal model was developed to simulate the effects of laser-induced heat input on the temperature distribution within the substrate during the fabrication of one layer. The optimization approach aims to find solutions with more homogeneous temperature properties that minimize the thermal gradient on the substrate caused by laser-based AM. The laser beam, i.e., the tool-path planning, is formulated as the search for the optimal sequence of cell depositions that minimize the fitness function, which is composed of two components, i.e., the thermal fitness and process fitness. The thermal fitness is expressed as the average thermal gradient, and the process fitness regulates the suitability of the proposed tool path for the implementation of the AM process. Various tool-path generators are proposed to initialize the initial population of tool-path solutions. Genetic-algorithm-based tool-path optimization is proposed, where custom initialization, crossover and mutation operators are developed for application in laser-based AM. Simulation studies demonstrate the effectiveness of the genetic-algorithm-based optimization in finding solutions that minimize the fitness function and therefore provide both thermally and, for the AM process implementation, more suitable laser-beam-path solutions.

Genetic Algorithm-Based Framework for Optimization of Laser Beam Path in Additive Manufacturing

In this study, a genetic algorithm-based laser beam (LB) path optimization method is presented to improve laser-based additive manufacturing (LBAM). To emulate the LBAM process, LB irradiation of a thin metal substrate is applied. The LB path generation is formulated as the search for the optimal sequence of LB irradiation into the cells on the substrate that minimizes the fitness function, which is composed of two components, i.e., thermal fitness and process fitness. The thermal fitness is expressed by the average thermal gradient, and a simple thermal model is developed to simulate the effects of laser-induced heat input on the temperature distribution in the substrate. The process fitness regulates the suitability of the proposed LB path for the implementation of the LBAM process. In addition to standardized tool paths (i.e., raster, spiral, etc.), novel LB path generators are proposed to define the initial population of LB path solutions. To implement a genetic algorithm-based LB path optimization, a framework is proposed, and custom initialization, crossover, and mutation operators are developed for application in LBAM. The effectiveness of the proposed approach is demonstrated through a simulation case study aiming to identify LB paths that minimize the fitness function and thus provide more suitable LB path solutions with respect to the defined fitness function. Compared with the traditional trial-and-error LB path formulations, the proposed approach provides an improved and automated method for an efficient laser beam path selection in LBAM.

Enhancement of optical levitation with hyperbolic metamaterials

The tightly focused laser beam in an optical trap has become a useful tool for many recent research areas. The momentum change in the photon-stream path of incident laser beam induces radiation force that enables trapping and manipulating mesoscopic micron-sized objects. In this study, we report the first analytical demonstration of optical trapping and levitation with radiation pressure on a transparent micron-sized spherical object made of hyperbolic metamaterial (HMM). The optical radial and axial forces acting on dielectric and HMM spherical particles are calculated using ray-optics approximation, assuming an optical levitation trapping setup. We compared the net force acting on the two objects, finding that the net radiation force exerted towards HMM particle is enhanced in the axial direction: The optical force enhancement in the HMM particle is more than ~ 8 times stronger compared to the induced force on the conventional dielectric particle with the corresponding material parameters. Besides, a better performance in the radial stabilization is observed for the HMM particle in comparison with the dielectric case, at which some oscillations and unstable saturation locations for the radial stabilization is monitored for TEM00 beam incidence. Furthermore, “zero-force” paths where radial stabilization of the HMM particle exists are also obtained for both TEM00 and TEM01∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$TEM_{01}^{*}$$\\end{document} laser beam incidences. Such phenomenon does not occur for particles of only dielectric and only metal material, which can be considered as another superiority of the proposed HMM particle.

Demonstration of a two-dimensional divertor Thomson scattering system on DIII-D

Recent upgrades providing two-dimensional divertor Thomson scattering (DTS-2D) measurements of Te and ne during a DIII-D plasma shot and a thorough description of system components and their functionality are presented. This system expands the capabilities of the existing single divertor Floor measurement location by introducing seven additional laser beam path options in the poloidal plane, spanning major radii from 1.062 to 1.335 m. The system redirects ∼1 J, 50 Hz Nd:YAG laser pulses to the new beam paths within 20 ms, stepping through each path on the divertor Floor every 200 ms during a plasma shot. The laser is redirected using an ex-vessel, fast-steering mirror to one of eight in-vessel beam paths oriented underneath the vessel tiles. Up to eleven measurement positions per beam path, from −1.35 to −1.13 m below the machine midplane, are available by dynamically refocusing the ex-vessel collection fiber array using a high-speed linear stage. Current measurement positions above the divertor Shelf are retained via a hole in the fixed, in-vessel mirror, allowing laser pass through.

Genetic Algorithm-based Optimization of Laser Beam Path in Additive Manufacturing

Genetic Algorithm-based Optimization of Laser Beam Path in Additive Manufacturing

High Power (~10 kW) Yb:YAG Ceramic Slab Laser Operating at 1030 nm

A high power quasi-continuous-wave (QCW) efficiently-cooled ceramic slab laser oscillator is reported. The maximum output power can reach 9.85 kW with the optical-to-optical efficiency of 60% with respect to the absorbed pump power as well as the slope efficiency up to 65.3%. To the best of our knowledge, this is the highest output power for any Yb:YAG ceramic laser oscillator. Beam aberration of the efficiently-cooled Yb:YAG ceramic slab laser is detected by a parallel probe beam along the optical path of laser beam. The peak-to-valley (PV) value of the wavefront distortion is 2.8 μm under full output power. The beam quality factor β is estimated to be about 3.7 times diffraction limit while without additional correction system. A numerical model of temperature field is constructed to analyze the wavefront distortion of the probe beam, and the calculated results are in agreement with the experimental data.

Revisiting time delay interferometry for unequal-arm LISA and TAIJI

Three spacecraft of LISA/TAIJI mission follow their respective geodesic trajectories, and their interferometric arms are unequal and time-varying due to orbital dynamics. Time-delay interferometry (TDI) is proposed to suppress the laser frequency noise caused by the unequal-arm. By employing the numerical orbit, we investigate the sensitivity of the first-generation TDI configurations and their corresponding optimal A, E, and T channels. The sensitivities of T channels from Michelson and Monitor/Beacon configurations diverge from the equal-arm case in frequencies lower than 10 mHz, and their performances vary with the inequality of the arm lengths. The mismatches of the laser beam paths are evaluated in a dynamic case, and the residual laser noise in the first-generation TDI could not satisfy the mission requirement.

Quantum microwave electric field measurement technology based on enhancement electric filed resonator

Rydberg atoms based quantum microwave measurement technology has significant advantages such as self-calibration, traceability, high sensitivity and stable uniformity of measurement. In this work, from the dimension of traditional electromagnetic theory, an electric field local enhancement technique for quantum microwave measurements is developed to improve the sensitivity of quantum microwave receiver. The theoretical basis of this method comes from the different mechanisms of realization of microwave reception in quantum microwave receivers and classical receiver. Classic receivers use antennas to collect microwave energy in space to signal reception; quantum microwave receivers measure the strength of the electric field in the path of a laser beam in an atomic gas chamber (the beam is about 100 µm in diameter) to realize the signal reception. Therefore, the sensitivity of quantum microwave receiver can be improved by increasing the electric field strength in the path of laser beam. The critical physical mechanism is the multi-beam interference at the open end and the short-circuited end of the structure. The results show that with the decrease of gap height of parallel plates, the enhancement factor of electric field strength increases rapidly and the power density compression capability is greatly improved. The |69D<sub>5/2</sub><inline-formula><tex-math id="Z-20230113213135">\begin{document}$\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221582_Z-20230113213135.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221582_Z-20230113213135.png"/></alternatives></inline-formula> experiments verify that the structure can achieve a 25 dB electric field enhancement at 2.1 GHz. This research is expected to be helpful in improving the sensitivity of measurement based on atomic measurement capabilities and in promoting the practical development of quantum microwave measurement technology.

A synthetic phase-contrast imaging diagnostic with spatial filtering for gyrokinetic simulations

A Phase-contrast imaging (PCI) diagnostic provides measurements of line-integrated electron density fluctuations. Localisation along the laser beam path can be achieved with a spatial filter that selects the wave-vector directions of the fluctuations contributing to the PCI measurement and is a key feature of the PCI diagnostic installed on the TCV tokamak and also of a similar system planned for JT-60SA. We have developed a synthetic diagnostic that models measurements from PCI taking into account the effect of such a spatial filter. The synthetic tool is based on the principle of integrating over selected diagnostic volumes the electron density fluctuations generated by turbulence simulations, and applying an appropriate spatial filter in wave-vector space. We demonstrate the effect of the filter for a positive and a negative triangularity TCV discharge, and illustrate the potential of the synthetic diagnostic for better understanding the corresponding experimental results. We consider different types of filters and make first-principle estimates of the localisation of the measurement. Finally, using gyrokinetic simulations that include electromagnetic effects, collisions and four kinetic species, we make first predictions of the characteristics of the measurements using the planned set-up of PCI on JT-60SA.

PDA Laser Measurements of Droplet-Laden Flows in a Four-Stage Axial Compressor

Abstract Droplet-laden flows play a significant role in the development of turbomachinery. For example, fine water droplets are injected into the flow path of axial compressors to increase the power output of gas turbines by making use of evaporation cooling, often referred to as wet compression. Another example is the ingestion of rain droplets in aircraft engines. Therefore, thermodynamic and aerodynamic measurements of air and liquid are indispensable to better predict the following behavior of the droplets in the gas flow. This article presents a new PDA measurement setup for a droplet-laden four-stage axial compressor to analyze the droplet behavior in swirled flows. To reduce optical disturbances of the laser beam path, liquid films at the casing are suppressed by a boundary layer air injection. Supplementary investigations with this treatment show a negligible influence on the compressor behavior. To minimize refraction effects of the laser beams at the casing window, the laser device with the optical probes can be translated and rotated, thus having 4 degrees-of-freedom. Furthermore, the collected droplet data are compared with a mathematical description of velocity slip in superimposed air flows. For the first time, it can be shown experimentally that droplets smaller than 10 μm attain the velocity of air one stage after injection. For larger droplets, however, deviations from the air Mach number of up to 0.05 remain. In addition, detached liquid films behind the trailing edge of the stator blades with low velocities can be clearly detected.

Experimental investigation on the ductile machinability of fused silica during in-situ laser assisted diamond cutting

Fused silica finds widespread applications in optical equipment; nevertheless, the machining of fused silica to obtain high-quality surfaces was a common problem in the industry. In-situ laser assisted diamond cutting (LADC) is a promising process for machining hard and brittle materials. In this study, systematic numerical analysis and experiments were conducted to investigate the ductile machinability of fused silica in in-situ LADC. The laser beam path and power density passing through the diamond tool were determined through optical simulation. The corresponding laser heating experiment of fused silica was carried out using the finite element (FE) simulation, aiming to derive the temperature distribution under different laser power levels. The comparison between the temperature values obtained experimentally and those predicted by the FE simulation confirmed the high accuracy of the developed FE model. The influence of laser power on the ductile machinability of fused silica was investigated quantitatively through grooving and end face turning experiments. The experimental results have shown that the ductility and machinability of fused silica under in-situ LADC were improved. With the laser assistance, the critical depth of cut (DOC) was increased up to 294.9 %, from 82.06 nm to 324.03 nm. Compared to conventional single point diamond turning, a significant improvement in the surface quality was obtained, resulting in the formation of a smooth and homogenous surface with a Sa of 19.4 nm for a laser power of 11 W.

Laser Guide Star uplink beam: scattering and Raman emission measurements with the 10.4m Gran Telescopio CANARIAS

ABSTRACT Laser Guide Star Adaptive Optics (LGS-AO) is becoming routine in several astronomical observatories. The use of powerful lasers generates sensible Raman emissions on the uplink laser beam path, plus secondary Rayleigh scattering from atmospheric molecules and Mie scattering from aerosols. This paper reports the results of a campaign done with the 10.4m Gran Telescopio CANARIAS (GTC); this campaign was undertaken to assess the spectral and photometric contamination coming from a 589 nm laser uplink beam scattering and Raman emission induced on the GTC spectro-imager OSIRIS by laser launched ∼1 km off-axis. The photometric contamination is due to primary and secondary scattering of the uplink photons, as well by the Raman inelastic scattering. We have propagated the laser beam creating a mesospheric LGS, then pointed and focused the GTC telescope towards the uplink laser beam, at different heights and up to the LGS, taking into account the observing geometry. In our observations, the Raman emissions for O2 and N2 vibrational lines are visible at 20 km, weakening with altitude and becoming undetectable above 30 km. The scattering of the focused uplink beam is detectable at less than ±0.2 arcmin from the centre of the beam, while for the focused LGS the scattering is narrower, being detectable at less than ±0.1 arcmin around the plume. Recommendations for Laser Traffic Control Systems are given accordingly.

Advances in computer-generated holography for targeted neuronal modulation.

.Genetically encoded calcium indicators and optogenetics have revolutionized neuroscience by enabling the detection and modulation of neural activity with single-cell precision using light. To fully leverage the immense potential of these techniques, advanced optical instruments that can place a light on custom ensembles of neurons with a high level of spatial and temporal precision are required. Modern light sculpting techniques that have the capacity to shape a beam of light are preferred because they can precisely target multiple neurons simultaneously and modulate the activity of large ensembles of individual neurons at rates that match natural neuronal dynamics. The most versatile approach, computer-generated holography (CGH), relies on a computer-controlled light modulator placed in the path of a coherent laser beam to synthesize custom three-dimensional (3D) illumination patterns and illuminate neural ensembles on demand. Here, we review recent progress in the development and implementation of fast and spatiotemporally precise CGH techniques that sculpt light in 3D to optically interrogate neural circuit functions.

Improved stabilization scheme for extreme ultraviolet quantum interference experiments

Interferometric pump–probe experiments in the extreme ultraviolet (XUV) domain are experimentally very challenging due to the high phase stability required between the XUV pulses. Recently, an efficient phase stabilization scheme was introduced for seeded XUV free electron lasers (FELs) combining shot-to-shot phase modulation with lock-in detection Wituschek et al (2020 Nat. Commun. 11 883). This method stabilized the seed laser beampath on the fundamental ultraviolet wavelength to a high degree. Here, we extend this scheme including the stabilization of the XUV beampath, incorporating phase fluctuations from the FEL high gain harmonic generation process. Our analysis reveals a clear signal improvement with the new method compared to the previous stabilization scheme.

Proper Orthogonal Decomposition for a Turbulent Boundary Layer with Aero-Optic Distortion

The velocity field associated with the deflection of a laser beam passing through a heated incompressible boundary layer in the wall-normal direction was studied using variants of the classical proper orthogonal decomposition (POD) technique. Due to the heating of the boundary layer, temperature, density, and index of refraction fields were variable in the boundary layer and were instantaneously affected by turbulent coherent structures. The time-varying index of refraction field led to a time-varying beam path of a laser traversing the boundary layer; the final angle of the laser beam was measured simultaneously with particle image velocimetry. The relationship between coherent structures in the velocity field and the final beam angle was studied using conditional POD and space-time POD. Conditional POD was performed on the wall-normal velocity field using conditions of large upstream and downstream laser deflections. The modes from conditional POD were compared to modes from POD without conditions. Physical features within some conditional POD modes were found to be shifted in the streamwise direction compared to unconditioned modes, suggesting that the streamwise position of velocity features was correlated to laser deflection. Space-time POD resulted in the same modes as the conditional POD and showed the modes convecting in the streamwise direction without a significant change in the size of the structures. This suggested a hypothesis that the laser deflects when coherent structures reach particular positions relative to the laser position as the structures convect, aligning the largest local velocity and density gradients with the laser beam path.

On the laser beam absorption efficiency in laser welding of aluminium thin sheet with copper pipe

Welding is considered the most versatile joining method, with extensive applicability in every modern industrial field. Especially, laser welding technology is widely utilized due to its numerous advantages, and its capability to process a variety of different materials. Among the several process parameters that impact the welding results, the absorbed energy and the power density are of major importance. These parameters mainly depend on the utilized materials and their physical properties, as well as the laser beam inherent characteristics. Moreover, the welding of more complex shapes and geometries is even more challenging, since multiple scatterings are taking place as well. The current paper presents a simulation model of the laser beam path and its absorption in the case of welding an aluminium thin plate and a copper pipe. The Ray Tracing method has been employed, while, the model’s geometrical parameters, are dimensionless, reduced to the pipe's radius. The materials’ absorption coefficient is calculated as a function of the incident angle, while the multiple scatterings are taken into consideration. From the simulation, the total absorption, as well as the relevant power density are calculated for a series of different initial process control parameters, i.e., spot size, incident angle, relevant laser spot position.