Two-laser System Research Articles

Ways to increase the productivity of L-PBF processes

One of the main limitations of additive manufacturing (AM) technologies consists in the relatively low build rate. Low productivity discourages companies from investing in AM machines, thus limiting the market of additive technologies. Machine manufacturers have introduced new solutions to their designs to increase the build rate, some of them are described in this paper. However, design improvements are not the only method to accelerate the process. The paper specifies factors that influence the build rate in the laser powder bed fusion process and provides an analytical assessment and comparison of the significance of how they affect its productivity. The influence that a change in selected parameters has on the process and the influence of multi-laser systems on its productivity are analysed in terms of the melted material quality. The processes from which the data for analysis were obtained were carried out on an SLM 280 machine with single- and dual-laser versions as well as on an SLM 500 with four lasers. Two types of samples, solid and thin-walled, both of the same volume, were tested. The data under analysis were the process times for both geometries, manufactured with different sets of parameters from the adopted range. Processing times were analysed to determine the main effects and interaction effects for extreme values of a given parameter. The height of the melted powder layer had the greatest influence on the build rate, which turned out to be greater even than the application of a two-laser system.

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.

Large dynamic range optical cavity based sensor using a low cost three-laser system.

An optical cavity based biosensor using a differential detection method has been proposed to provide a novel approach to point-of-care diagnostics. A two-laser based optical cavity based sensor system is designed and experimentally demonstrated. While the calculated differential value of the two-laser system shows larger responsivity compared to individual wavelengths, the dynamic range of the system is rather small. Because of that, it turned out its fabrication tolerance is also small based on the simulation. We employed a three-laser system and redesigned the optical cavity to increase the dynamic range and improve the fabrication tolerance. In this presentation, we report the measurement results of the three-laser system for the first time. This system increases the dynamic range threefold by a chaining action. Because of this large dynamic range, this three-laser system is anticipated to have a larger fabrication tolerance.

Preferential triplet over singlet emission of Zn in laser-induced plasmas

An experimental study is performed on the time-dependent intensity variations of Zn emission focusing on the triplet (Zn I 481.0 nm) and singlet (Zn I 636.2 nm) emission lines induced under three experimental conditions. A single nanosecond (ns) Nd:YAG laser in standard laser-induced breakdown spectroscopy (LIBS) setup is employed for the investigation of direct shock wave-induced emission characteristics with N2 ambient gas at 0.4 kPa and the different effects of He ambient gas at 2 kPa. An additional two-laser system consisting of ns and picosecond (ps) lasers in an orthogonal setup is used to study the exclusive role of a He-assisted excitation (HAE) process for the generation of those two Zn emission lines. The results of this study consistently exhibit the dominant triplet emission over the singlet emission marked by initial maximum intensity ratios of 8 and 12 obtained from the experiments using a single-laser setup in N2 and He ambient gases, respectively, indicating the significant contribution of the HAE mechanism to the enhanced and longer lasting Zn emission in He gas. The experiment using the special two-laser setup further demonstrates the exclusive role of the HAE process in the Zn emission featuring an even markedly higher triplet/singlet intensity ratio of 22. Thus, the results of this study suggest the possibly more general nature of dominant triplet emission phenomena previously found in laser-induced He and Ca emission spectra.

Low-cost optical cavity based sensor with a large dynamic range.

We have proposed an optical cavity based biosensor using a differential detection method for point-of-care diagnostics. For experimental demonstration of the proposed device using a two-laser system through refractive index measurements, an optical cavity structure is designed, fabricated, and measured. The differential value calculated with intensities of two different wavelengths has a larger responsivity with respect to the refractive index change as compared with the intesity of an individual wavelength. However, the repeatability test shows that the two-laser system has a tight fabrication tolerance due to its small dynamic range. In this paper, we introduce a three-laser system for an optical cavity sensor with a chained differential detection method and present the experimental measurement results showing a large dynamic range and large fabrication tolerance. The measured dynamic range of this system is more than three times greater than that of the two-laser system. The repeatability test using the three-laser system demonstrates a larger fabrication tolerance as a result of the increased dynamic range.

Use of photoacoustic excitation and laser vibrometry to remotely detect trace explosives.

In this paper, we examine a laser-based approach to remotely initiate, measure, and differentiate acoustic and vibrational emissions from trace quantities of explosive materials against their environment. Using a pulsed ultraviolet laser (266nm), we induce a significant (>100 Pa) photoacoustic response from small quantities of military-grade explosives. The photoacoustic signal, with frequencies predominantly between 100 and 500kHz, is detected remotely via a wideband laser Doppler vibrometer. This two-laser system can be used to rapidly detect and discriminate explosives from ordinary background materials, which have significantly weaker photoacoustic response. A 100 ng/cm2 limit of detection is estimated. Photoablation is proposed as the dominant mechanism for the large photoacoustic signals generated by explosives.

Resonance photoionization of a diarylethene derivative in the presence of cyclodextrins using multi-color multi-laser irradiation

Abstract Photochromism is isomerization of a molecule induced by light, and is attractive for applications like actuators and optical memory. However, the photodynamics of photochromic materials under excitation by three lasers at three different wavelengths are poorly understood. In this article, we investigate the properties of the well-known photochromic material 2,3-bis(2,4,5-trimethyl-3-thienyl) maleic anhydride (1) upon excitation by one, two and three lasers. In these experiments, α-, β- and γ-cyclodextrin (CD) are added to 1 to form inclusion complexes to improve the solubility of 1 in water/acetonitrile. First, the formation of 1/CD inclusion complexes is evaluated by changes in the 1H NMR spectrum of 1 and calculations. We find that the area of each molecule of 1 in the 1/CD inclusion complexes exposed to the solvent depends on the cavity size of the CD, and that the CD also affects the ratio of the antiparallel and parallel conformations of the open form (OP) of 1. The transient absorption of hydrated electrons in the 1/CD systems is then observed at 720 nm by flash photolysis using one laser (266 nm), two lasers (266 and 532 nm) and three lasers (266, 532 and 355 nm). The ionization quantum yields of 1 depend on both the type of CD present, which is rationalized by the ability of 1 to isomerize in the different-sized cavities of α-, β- and γ-CD, and on the laser pulse configuration. The ionization efficiency obtained using the two-laser system is higher than that measured with the one-laser system. This confirms that two-photon ionization of 1 proceeds through two-step two-photon excitation with S0 → S1 and S1 → Sn singlet-state transitions. The photoionization efficiency of 1 in the presence of CDs determined using three laser pulses increases upon opening of the closed-ring isomer (CL) of 1 by 532 nm irradiation (the second laser pulse). This effect is particularly obvious in the presence of γ-CD. This investigation of the photodynamics of 1 in inclusion complexes using multiple lasers contributes to our understanding of how we can influence photochromism using host species and different laser wavelengths.

Generation of cnoidal waves by a laser system with a controllable saturable absorber

We demonstrate the cnoidal wave formation in a two-laser system with a saturable absorber in the cavity of one of the lasers. Another laser is used to activate the saturable absorber in order to control the pulse shape, width, intensity and frequency. Using the three-level laser model based on the Statz--De Mars equations, we show that for any value of the saturable absorber parameter there exists a certain modulation frequency for which the pulse shape is very close to a soliton shape with less than 5% error at the pulse base. Such a device may be prominent for optical communication and laser engineering applications.

Induced photon statistics in three-level lasers

The statistical properties of light emitted by a three-level laser are theoretically investigated. In the model considered the three-level medium is coherently excited by another laser having arbitrary photon statistics. It is shown that under certain conditions the photon statistics of the three-level laser is determined by that of the exciting laser. This feature is referred to as induced photon statistics. It is proposed to use this phenomenon to analyze the statistics of a laser involved in a feedback process. Arranging an experiment in a way such that the laser of interest serves as a coherent pump for the probe three-level laser one is able to directly observe the statistics of the feedback-controlled laser by simply detecting the induced photon statistics in the three-level probe system. The analysis of the operation of such two-laser system presented demonstrates, in particular, that the feedback itself is unable to produce a nonclassical state of light which is in complete agreement with the conclusion of Wiseman and Milburn [Phys. Rev. A 49, 1350 (1994)].

Experimental study of laser Doppler vibrometer and ambient vibration for vibration-based damage detection

This paper presents a study on the application of a laser Doppler vibrometer (LDV) for structural health monitoring using ambient vibration. The work covers three important issues namely, data acquisition, system identification, and structural damage detection. A two-laser system is employed for data acquisition using ambient response of the structure. Modal parameters are estimated by the eigensystem realization algorithm, after first deriving the impulse response functions from both laser responses using the Natural Excitation Technique. In damage detection, a new matrix-updating-based method is proposed. The essential feature of this method is the non-iterative solving technique of inverse problem, which allows damage to be located and quantified by employing the modal parameters obtained before and after damage. Numerical simulation and laboratory-scaled experiments using bolted lap joint plate demonstrate that the proposed technique can detect locations and magnitude of damage with incomplete modal information.

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaOx thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaOx films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.

Experimental Testing of a Curvilinear Gas Shroud Nozzle for Improved Plasma Spraying

The performance of a new gas shroud nozzle attachment for plasma spraying was tested experimentally using particle diagnostics, flow visualization, and coating characterization techniques. A nozzle attachment with curvilinear inner walls was tested and compared with a commercially available conical nozzle. Particle temperatures were measured with a high-speed ratio pyrometer and particle velocities were measured with an intensified camera and a two-laser illumination system. Flow visualization was performed by seeding the surrounding air with smoke. Particle temperatures at the spraying distance were 300 K higher with the curvilinear insert. The plasma jet was narrower but the particle velocity distribution at the spraying distance was unchanged. Higher temperatures and improved particle melting with the curvilinear insert resulted in a reduction in coating porosity (from 7.0 to 7.2 to 4.5–5.1%) and an increase in coating adhesion strength (from 27.2 to 42 MPa). Shrouding as injected through a circular slot around the nozzle exit was also seen to provide better protection than gas injected with the standard sixteen-port configuration.

TO‐PRO‐3 iodide: A novel HeNe laser‐excitable DNA stain as an alternative for propidium iodide in multiparameter flow cytometry

A new red emitting fluorophore, TO-PRO-3 iodide (TP3), which is best excited by an HeNe laser (633 nm), has been compared with propidium iodide (PI) for measuring relative DNA content. TP3, which has a peak absorbance at 642 nm and emission at 661 nm, has been tested on peripheral blood lymphocytes (PBL) and keratinocytes in a two-laser system. As an example, we present a three-color flow cytometric application utilizing TP3 in combination with fluorescein-isothiocyanate (FITC) and phycoerythrin (PE) conjugated to monoclonal antibodies in this paper. A subequilibrium concentration of 1 microM TP3, most preferably used in combination with RNase treatment, proved to be a powerful alternative for DNA amount determination. In human- and mouse-Balb/MK-keratinocyte populations with different S-phase fractions, PI and TP3 showed a good correlation. Finally, in the triple labelling experiment we clearly demonstrated that TP3 is readily applied to the analysis of binding of two antibodies and relative DNA content simultaneously.

Laser-induced dissociation of hydrogen fluoride

Laser-induced dissociation of hydrogen fluoride has been studied using a quantum-mechanical approach in which the fast Fourier-transform technique has been applied. The dissociation probability has been calculated as a function of the frequency of the laser and the vibrational quantum number and the results are compared with those of classical trajectory calculations. The dissociation probability from the ground state has also been calculated for a two-laser system as a function of the frequency of the two lasers.

Two‐ and three‐color immunofluorescence using aminocoumarin, fluorescein, and phycoerythrin‐labelled antibodies and single laser flow cytometry

Antibodies coupled to 7-aminocoumarin (AMCA) emit a bright blue fluorescence under ultraviolet (UV) excitation and are therefore ideal for three-color immunofluorescence (IF) with fluorescein (FITC) and phycoerythrin (PE) labeled reagents; however, due to the different absorption spectra, the use of these fluorophores for multicolor flow-cytometric analysis requires a double light excitation source (e.g., two-laser system). We report a strategy which uses a single argon-ion laser to simultaneously excite AMCA, FITC, and PE, thus allowing the flow cytometric analysis of three immunological parameters. When the UV-visible argon-ion laser is fitted with an appropriate set of mirrors, the 35.1-363.8 nm (UV) and 488 nm wavelengths (accounting for 80 mW and 520 mW, respectively) are simultaneously generated; these lines can then be exactly focused on the same observation point by an achromatic cylindrical lens. A number of comparative analysis were performed with this instrumental set up to verify the sensitivity of AMCA IF and its possible application for multicolor immunophenotypic evaluation of blood cell subsets. When AMCA- and FITC-labeled antimouse Ig antibodies were assessed for their ability to detect limiting amounts of mouse monoclonal antibody bound to cells, the former was less sensitive than the latter. A number of factors, including differences in excitation energy (80 mW for AMCA and 520 mw for FITC) and extinction coefficients (1.9 x 10(4) for AMCA and 6 x 10(4) for FITC) could explain this result.(ABSTRACT TRUNCATED AT 250 WORDS)

Band structure of stationary emission spectra from a three-level molecule pumped by strong radiation fields

A theory of the emission spectrum from a multilevel molecular system pumped by strong resonant radiation fields based on the double space theory of the density matrix is developed. We are especially concerned with a three-level, one-laser system, and a three-level, two-laser system. It is shown that the band structure cannot always be estimated by the energy level structure of the dressed states. In fact, in both model systems, triplet structures of the emission spectra can be observed in the case where the magnitude of the molecule-field interaction is larger than that of the detuning. The characteristic features of the band structure are interpreted in terms of the dressed state picture based on analytically solvable models. Several numerical calculations are performed to show how the multiplet band structure in the emission spectrum is influenced by the detuning and the power of the pump field(s).

LASER IGNITION OF OIL SPILLS

ABSTRACT A major oil spill in the Arctic, whether from a tanker or an oil rig, could result in large concentrations of oil among broken ice and/or thousands of oiled melt pools. The remoteness of the area and the inadequacy of other countermeasures make in-situ burning the only possible response. Helicopter-deployable igniters have been developed to ignite the oil, but studies have shown that use of these devices has severe logistical constraints. The use of a helicopter-borne laser system as an alternative to, or in conjunction with, the igniters has been pursued from conceptual development to completion of the engineering design phase. The concept was examined by theoretical analysis combined with laboratory studies. This work indicated that a dual-laser system would ignite both fresh and weathered crude oils at temperatures representative of an arctic spring or fall day. A series of outdoor experiments was carried out in Kanata, Ontario, Canada, in March 1985. These experiments, under climatic conditions very similar to those in the Canadian Arctic in June, demonstrated that a two-laser system would ignite both fresh and weathered crudes. An engineering feasibility and design study was undertaken. The study confirmed that a system having the required performance can be assembled from existing, proven hardware and operated effectively from a helicopter. The development of the laser ignition of oil spills from concept through engineering design is described.

Conservative Hamiltonian for the theory of multiple-photon absorption

A conservative Hamiltonian is developed for the classical dynamical theory of the multiple-photon absorption process and is the analogue of the time-dependent Hamiltonian. Applications to semiclassical theory are discussed for the interaction of a diatomic molecule with one and two lasers. It is demonstrated that the two-laser system can significantly lower the threshold to chaotic motion. 3 figures, 1 table.