Laser Research Articles

Shock-Layer Measurements in T5 Shock Tunnel Hypersonic Flows Around a Cylinder Model

We report on near-body measurements of temperature and nitric oxide (NO) concentration in the hypersonic flows around a cylindrical test article in the Caltech T5 reflected shock tunnel. Flow measurements were made at 50 kHz using tunable diode laser absorption spectroscopy, deploying six lasers to probe an array of quantum state-specific transitions. Laser beams were positioned both in the freestream and behind the bow shock at specific locations deemed pertinent to computational fluid dynamics comparison and kinetic model evaluation. The fractions of laser beam pathlengths behind the shock in different spatial regions were also discerned, thus providing a measurement of shock location. This study consists of six total experiments (“shots”) across two Mach ∼5 conditions, characterized by total enthalpies of 8 and 16 MJ/kg and freestream velocities of 3.5 and 5 km/s, respectively. Freestream measurements generally concur with prior works in the T5, but with some non-trivial differences. Shock-layer measurements span from 2000 to 6000 K and feature noteworthy and expected variety among different zones within the post-shock region. Thermal equilibrium is generally held throughout the flowfield, but chemical nonequilibrium is commonly observed. NO is the primary spectroscopic target, but measurements of carbon monoxide, carbon dioxide, water, and atomic oxygen provide supplementary insights.

Spatial and temporal thermal management of a spintronic terahertz emitter

This work presents methods for addressing undesirable thermal effects induced by the pump beam of an oscillator laser to improve the efficiency of a terahertz (THz) spintronic emitter. We explore two approaches: spatial distribution of pump energy using a 2D lens array and temporal modulation of the pump duty cycle. Optimizing the spatial distribution approximately doubles the THz signal by increasing local heat dissipation, delaying the saturation limit. Similarly, temporal spreading of pump pulses by adjusting the duty cycle allows greater thermal relaxation within the heterostructure, enhancing the overall efficiency of THz wave generation.

Refractive channeling of heating wave driven by laser beam interaction with plasma of subcritical density

Refractive channeling of heating wave driven by laser beam interaction with plasma of subcritical density

Improved precision at high-spatial resolution of strontium isotope analysis in apatite and apatite inclusions by LA-MC-ICP-MS with 1013 Ω amplifiers

Recent advancements in in situ analytical techniques have generated new interest in measuring strontium (Sr) isotopes in rock-forming apatite and apatite inclusions in zircon, because they can bring new insights into the primary signatures of magmatic sources. This paper introduces a novel approach based on laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), which allows to achieve higher precision in situ analysis of Sr isotopes in small volumes of samples. The use of 1013 Ω current amplifiers for the acquisition of the signal on m/z 83, 83.5, 84, 85, 85.5, 86, 86.5, 87 and 88, along with a specific data reduction protocol, enhance both the internal precision and the external reproducibility of the 87Sr/86Sr ratio by a factor of four, presenting a significant improvement over the conventional use of 1011 Ω amplifiers. An internal precision better than 0.45‰ (i.e. 450 ppm; 2 s.e.) on the 87Sr/86Sr ratio can be achieved on the Durango apatite standard with a 13 μm square-shaped laser beam and a laser ablation pit of 60 μm depth. With a 5 μm beam, the precision is 2.2‰ on average with a 10 μm ablation pit depth for this standard. This analytical procedure was applied to investigate Sr isotopes in apatite and apatite inclusions within zircons from seven high BaSr magmatic rocks in the Northern Highland Terrane, Scotland. For these well-characterised, geochemically undisturbed samples, the data reveals similar Sr isotope compositions between apatite inclusions armoured in zircon, matrix apatites, and the bulk rock. This highlights the ability of apatite inclusions to faithfully record primary signatures of host magmas, elucidating their utility as reliable indicators in petrological studies of ancient samples.

Mode matching with a phase camera for gravitational-wave detectors

Mode mismatch causes a large fraction of optical losses in gravitational-wave detectors. This effect will need to be considerably reduced in next generation detectors in order to fully exploit advanced techniques such as squeezing. Phase cameras are wavefront sensors capable of imaging the amplitude and phase of a laser beam. We simulate a measurement scheme with the phase camera and show that the two resulting error signals can be used to act on the two mismatch degrees of freedom in a closed loop.

A novel method for fully penetrated U-rib-to-deck joints: single-sided oscillating laser arc hybrid welding

U-rib-to-deck (RTD) joints represent one of the most critical and vulnerable welding details of orthotropic steel decks. Aiming for full penetration and high-level service performance, a novel method with single-sided oscillating laser-arc hybrid welding (OLAHW) technique was proposed for the RTD joint. The weld formation, microstructure and mechanical properties were extensively investigated. The results indicated that the porosity and lack of fusion (LoF) defects effectively suppressed with the beam oscillation during LAHW process. The OLAHW technique has good spatial accessibility with larger laser beam incidence angle of 20°. Beam oscillation led to an increase in acicular ferrite in the fusion zone (FZ) while simultaneously a reduction of martensite in the heat effected zone (HAZ). This was related to the cooling time, and the thermal simulation results revealed that the OLAHW sample demonstrated comparable cooling times in both the Laser-FZ and arc-FZ. Consequently, beam oscillation can generated more homogeneous microstructures and microhardness. Although the microhardness of the OLAHW sample in the FZ was increased compared to the conventional arc welding (CAW) and LAHW sample, the highest microhardness value remained within acceptable limits (380 HV). Moreover, the ultimate tensile strength of the OLAHW weld sample increased by 7% compared to the CAW weld sample. The application of the OLAHW technique demonstrates its feasibility for engineering applications in orthotropic steel decks manufacturing.

Glass capillary systems for micro-volume fluorometry

Capillary-based fluorometry can be considered a promising alternative to cuvette-based methods, enabling low-volume measurements while reducing analyte consumption and waste generation. In this work, we compared the performance of three types of glass container systems, i.e., anti-resonant hollow core fibers, capillaries, and standard cuvettes, using green fluorescent protein (GFP) as a reference analyte. Among these, when the volume of the analyte and the fluorescence intensity are assessed, a capillary accommodating 2.9 μL was found as an optimal choice. The capillary-based interrogation setup was further optimized to enhance sensitivity by adjusting the florescence-exciting laser beam position and optimizing the capillary’s shape. The obtained capillary-based setup achieved a noteworthy 85 % reduction in sample volume compared to previous studies utilizing capillary-based fluorometry, highlighting its significant contribution to material conservation. The system achieved a limit of detection as low as 26 μg/mL (1.01 nM) GFP concentration and a resolution of 3.5 μg/mL. With its low cost and potential for reusability, capillary-based fluorometry presents a compelling alternative to traditional cuvette-based fluorometry, offering a promising solution for micro-volume analysis.

Light Scattering Measurements of KCl Particles as an Exoplanet Cloud Analog

Salt clouds are predicted to be common on warm exoplanets, but their optical properties are uncertain. The Exoplanet Cloud Ensemble Scattering System (ExCESS), a new apparatus to measure the scattering intensity and degree of linear polarization for an ensemble of particles, is introduced here and used to study the light scattering properties of KCl cloud analogs. ExCESS illuminates particles with a polarized laser beam (532 nm) and uses a photomultiplier tube detector to sweep the plane of illumination. Scattering measurements for KCl particles were collected for three size distributions representative of modeled clouds for the warm exoplanet GJ 1214b. Our measurements show that Lorenz–Mie calculations, commonly used to estimate the light scattering properties of assumedly spherical cloud particles, offer an inaccurate depiction of cubic and cuboid KCl particles. All of our measurements indicate that Lorenz–Mie scattering overestimates the backscattering intensity of our cloud analogs and incorrectly predicts the scattering at mid-phase angles (∼90°) and the preferential polarization state of KCl scattered light. Our results align with the general scattering properties of nonspherical particles and underscore the importance of further understanding the effects that such particles will have on radiative transfer models of exoplanet atmospheres and reflected light observations of exoplanets by the upcoming Nancy Grace Roman Space Telescope and Habitable Worlds Observatory.

A new method to recycle Li-ion batteries with laser materials processing technology

Efficient and cost-effective recycling of spent lithium-ion batteries is essential for the sustainable growth of the clean energy sector, conserves critical mineral resources, and contribute to environmental sustainability. The pyrometallurgy process, involving high-temperature smelting and solid-state reduction, plays a key role in the industrial-scale recycling of these batteries. Traditional smelting methods, however, face criticism for their substantial energy requirements and the loss of lithium in slag. In this study, an innovative laser-based in-situ pyrometallurgical process, hereinafter referred to as laser recycling, was developed to recycle Li-ion batterie materials without using slag, enabling the simultaneous recovery of Co, Ni, Mn, and Li. Lab-scale experiments were carried out to investigate the influences of laser power density and duration on the carbothermic reduction behavior of battery materials. The results showed that the maximum temperature reached approximately 1850 °C with a laser power between 1500 and 2000 W focused to an area of 20 mm in diameter within a few seconds. The laser recycling facilitates concurrent smelting and solid-state reduction, with carbothermic reduction completed in just 30 s due to rapid reaction kinetics, ultra-high temperatures, and the enhanced contact area resulting from surface tension-driven molten material flow under intense laser beam exposure. This laser recycling process reduced LiCoO2 and LiNi0.33Mn0.33Co0.33O2 to metallic Co or Co-Ni-Mn alloy, respectively, while Li was recovered as Li2CO3. The new process allowed for the near-total recovery of Co, Ni, and Mn in the alloy and virtually 100% Li recovery in the form of Li2CO3 by a vapor phase capture system. Additionally, continuous laser recycling in the battery material powder bed showed potentials to scale up for industry battery recycling. A mechanism for the laser recycling process was proposed. A preliminary discussion on the techno-economic implications of laser recycling was also provided.

Measurement of Gaussian laser beam radius using nanosecond-pulse laser etching of titanium film

A method for measuring the Gaussian laser beam radius based on nanosecond-pulse laser etching (NPLE) was proposed. The NPLE method has the advantages of simple operation, low cost and high accuracy. It can be used to directly measure the laser beam size in the range of 0.25 ∼ 6 W without attenuating the laser energy. In the experiments, 1064 nm pulsed laser beam was used to etch titanium film, the size and position of the laser beam waist were measured. The experimental results are consistent with the calibration values of the CCD method, it indicates that the NPLE method is feasible.

Design of an Optimized Terahertz Time-Domain Spectroscopy System Pumped by a 30 W Yb:KGW Source at a 100 kHz Repetition Rate with 245 fs Pulse Duration

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers.

Optimizing the Size of a Moving Annular Hollow Laser Heat Source

The physical phenomenon of the annular hollow laser surface treatment process is complex, and the internal mechanism involves multiple disciplines and fields. In addition to the general parameters of laser beams, such as laser power and scanning speed, an annular hollow laser beam exhibits unique physical characteristics, including hollow ratio and hollow area. The selection of the inner and outer annular radii of the laser plays a critical role in the study of metal surface heat treatment. From the point of view of heat transfer, the entransy dissipation theory is introduced in the metal surface treatment process with an annular hollow heat source. Firstly, using the principle of the extreme value of the entransy dissipation rate, under a constant heat flux boundary condition, the entransy dissipation rate is obtained through the temperature field distribution in the calculation area by numerical simulation. Secondly, the selection of the inner and outer ring radii of the annular laser is explored, and the average temperature difference of the heating surface is minimized to reduce the thermal stresses of the material. This paper seeks new insights into the geometric parameters of the inner and outer radii of the annular heat source.

Spatially Variable Ripple and Groove Formation on Gallium Arsenide Using Linear, Radial, and Azimuthal Polarizations of Laser Beam

We demonstrated the linear, radial, and annular ripple formation on the surface of GaAs. The formation of linear ripples was optimized by the number of shots and the fluence of 30 ps, 532 nm pulses. The radial and annular nanoripples were produced under the ablation using doughnut-like beams possessing azimuthal and radial polarizations, respectively. We compare the ripples and grooves formed by a linearly polarized Gaussian beam relative to an annular vector beam. The joint overlap of sub-wavelength grooves with ripples formed by azimuthally and radially polarized beams was reported. The conditions under which the shape of radial and ring-like nano- or micro-relief on the GaAs surface can be modified by modulating the polarization of laser pulse were determined. The resultant surface processing of GaAs using a laser beam with different polarization modes is useful for exploring valuable insights and benefits in different applications.

Multipass Faraday rotators and isolators

Faraday isolators are usually limited to Faraday materials with strong Verdet constants. We present a method to reach the 45° polarization rotation angle needed for optical isolators with materials exhibiting a weak Faraday effect. The Faraday effect is enhanced by passing the incident radiation multiple times through the Faraday medium while the rotation angle accumulates after each pass. Materials having excellent thermo-optical properties in the ultraviolet and mid-infrared range become available for optical isolators. Herriott-type multipass cells offer a simple and compact way to realize the desired propagation length in usual optical materials of standard sizes. A proof-of-principle experiment was carried out, demonstrating polarization rotation of a 532 nm laser beam by an angle of 45° in anti-reflection-coated fused silica surrounded by a standard neodymium ring magnet.

Multiplexing of color images by theta modulation with laser beam in amplitude grating

Multiplexing of color images by theta modulation with laser beam in amplitude grating

Fast finite difference solver for optical microscopy in deep biological tissue

Optical scattering poses a significant challenge to high-resolution microscopy within deep tissue. To accurately predict the performance of various microscopy techniques in thick samples, we present a computational model that efficiently solves Maxwell’s equation in highly scattering media. This toolkit simulates the deterioration of the laser beam point spread function (PSF) without making a paraxial approximation, enabling accurate modeling of high-numerical-aperture (NA) objective lenses commonly employed in experiments. Moreover, this framework is applicable to a broad range of scanning microscopy techniques including confocal microscopy, stimulated emission depletion (STED) microscopy, and ground-state depletion microscopy. Notably, the proposed method requires only readily obtainable macroscopic tissue parameters. As a practical demonstration, we investigate the performance of Laguerre–Gaussian (LG) versus Hermite–Gaussian (HG) depletion beams in STED microscopy.

Theoretical investigation on the effective coupling optical system between an astigmatic tapered diode laser beam and single-mode optical fibers

In this study, we theoretically investigate the coupling efficiency of tapered diode lasers to single-mode fibers in four different coupling systems using an astigmatic beam propagation algorithm. The fast-axis lens and slow-axis lens have been used to mitigate astigmatism, which is a characteristic of the tapered diode laser and then the beam coupling into different single-mode fibers. Designers should carefully balance the pursuit of high coupling efficiency with the need to minimize astigmatism sensitivity in the design of coupling systems.

The effect of the external magnetic field on the power of the laser passing through the magnetized plasma and the calculation of the radius of convergence and the investigation of the condition of the beam's filamentation in the Medium

In this article, the power of a laser passing through a magnetized plasma medium which is affected by an external magnetic field is calculated.The change in the intensity of the magnetic field and its rotation around the axis of laser beam propagation in the plasma, on the power of the passing beam is investigated.This work is done based on the indirect effect of the magnetic field on the density distribution of electrons in the plasma, which we can consider the density changes as changes in the refractive index of the medium, so the laser power in this case is different from the case where there is no magnetic field. Also, the condition of beam filamentation has been researched as one of the third-order nonlinear phenomena, and the convergence radius has been calculated under these conditions.

An approach to modelling defocusing and keyhole reflectivity in keyhole laser processes

AbstractKeyhole laser-based processes have been adopted widely due to the high-quality welds and cuts that they produce and the accurately controlled dimensions of the end part. Fast-running simulation tools are required in order to estimate the dependence of process outputs, efficiency, and stability on the process variables. This work presents a fast-running simulation tool for keyhole laser-based processes with inert or no assisting gas that takes into account the effect of laser beam defocusing and the alteration of the effective reflectivity of the material due to the keyhole cavity. The model is built upgrading a successful model for the thermal phenomena based on finite differences and the enthalpy method. The presented model is compared to experimental results for the laser cutting process with both CO$$_2$$ 2 and fiber laser sources and inert assisting gas. The model accurately predicts cutting depth and kerf, heat-affected zone width, and the dependence of the cutting depth on the position of the focal plane. The model manages to achieve reduced computational time enabling its utilization in process digital twins.

Memory Response of Refined GN Models on a Rotating Fiber-Reinforced Magneto-Thermoelastic Medium Due to Pulsed Laser and Inclined Load

With the aid of the Caputo fractional derivative, this study constructs a novel mathematical model of magneto-thermoelasticity to investigate the transient phenomena for a fiber-reinforced rotating half-space under the influence of pulsed laser as heat source and inclined load in the context of refined three-phase-lag (TPL) Green–Naghdi (GN) models of generalized thermoelasticity, which is defined in an integral form of a common derivative on a slipping interval by incorporating the memory-dependent heat transfer. Together with the temperature gradient (Seebeck effect) and charge density influence, the generalized magneto-thermoelasticity model’s equations now include the modified Ohm’s law. An intense non-Gaussian laser beam heats the bounding plane’s surface. By using the normal mode analysis, the analytical expressions of the thermophysical quantities are produced in the physical domain. All physical quantities have been graphically depicted for different GN models (refined GN-II and GN-III models) to indicate the effect of memory-dependent derivative, Seebeck parameter, rotation and reinforcement of the medium under the influence of inclined load and pulsed laser.