Attenuation Of Laser Light Research Articles

Analysis of an Accurate Iterative Technique with Attenuation Compensation in Planar Laser Induced Fluorescence

Abstract Iterative techniques in Planar Laser Induced Fluorescence (PLIF) evaluations are theoretically analysed. Correction for laser light attenuation is also incorporated in the analysis. Conditions for the convergence of the results and the scalings for the errors are derived. The results are validated against experiments. It is shown that the optimal strategy for implementation of iterative PLIF evaluation with attenuation correction is on the ensemble mean of the fluorescence field. Compared to the implementation with instantaneous fluorescence images, this approach is not only far more efficient computationally, but also is assured to converge monotonically and extremely rapidly, with minimal errors. This is suggested to be a viable and useful technique for PLIF in general, but especially for cases where a reference cell is unfeasible for explicit attenuation correction.

Measurement of impinging and boiling droplet temperature by two-color/two-dye laser-induced fluorescence technique using continuous-wave laser

The 2-color/2-dye laser-induced fluorescence technique is a non-intrusive method for measuring temperature in millimeter-scale fluids. This study determines optimal conditions for fluorescein disodium (FL) and Sulforhodamine 640 (SRh640) with a low-cost continuous-wave laser to reduce dye saturation and morphology-dependent resonances. The signal images of both dyes, captured by two high-speed cameras, were clearly distinguishable from the background with high contrast under specific concentration conditions. As a result, three concentration combinations were selected based on preliminary experiments, and the fluorescence signal ratio was analyzed to establish a temperature relationship between 32 and 71 °C. The intensities of FL and SRh640 increases and decrease with the temperature, respectively. Errors resulting from laser light attenuation in single-signal images were reduced using the ratiometric method. Solution 1 (CFL = 1.55 × 10−4 / CSRh640 = 3.2 × 10−6 M) exhibited the highest sensitivity at 0.718 (%/ °C) and a strong linear relationship (r2 = 0.997), so it was determined to be suitable concentration for measuring the droplet temperature. The proposed method estimated the temporal temperature variations of ethanol droplets impinging on a heated surface at TW = 80, 150 and 200 °C. At TW = 80 °C, contact boiling initiated without a recoil process, and the heat transfer elevated the droplet's temperature near the surface, leading to bubble formation and growth. As surface temperature increases, the film boiling behavior can be observed. The temporal evolution of the average heat transfer rate was analyzed.

In situ monitoring of electrosprayed water microdroplets using laser and LED light attenuation technique: Comparison with ultra-high-speed camera imaging

An electrostatic spray (ES) of liquids is a simple way to generate microdroplets with a high surface-to-volume ratio. The ES generated by electrical discharges enables a fast transfer of reactive species from plasma into the liquid for an efficient generation of plasma-activated water. Here, we present a relatively simple, versatile, and cost-effective diagnostic technique for online monitoring of ES microdroplets which enables simultaneous and synchronized electrical and optical diagnostics of an electrical discharge. This technique is based on planar laser light attenuation monitored by a large area photo-detector covered by a slit. Two variants were tested and compared—one with two lasers and another with one laser and a broadband LED lamp. This technique enables estimations of the speed and size of microdroplets (down to ∼10 μm) and allows for monitoring the dripping frequency or studying fragmentation of microdroplets and water filaments. The ES characteristics obtained by this technique were successfully verified by ultra-high-speed camer:a imaging.

Quantification of particle separation using electrostatically charged spray mist in a dust chamber

In order to improve the efficiency of reducing fine dust emissions from diffuse sources, the commonly used water mist can be electrostatically charged. Thus, the attraction between particles and droplets increases due to electrostatic field forces. In the experimental investigations carried out, an electrostatic two-fluid nozzle was used to generate a charged spray mist according to the principle of charging by induction. In a chamber, dispersed dust was precipitated with the spray mist, while detecting the laser light attenuation. By means of comparative measurements it could be shown that faster sedimentation and separation takes place through electrostatic attraction. Parameter variations regarding the use of compressed air and water were investigated and the sedimentation time of the particle–droplet mixture in the chamber was shown. A size distribution was then calculated from the laser light attenuation, which shows that the proportion of small particles decreases with spray application. Furthermore, the influence of the polarity of droplets and solid particles on the reduction measure was determined.

Optical Characteristics of Metallic Nanoparticles During Melting by Laser Radiation

The cross sections and efficiencies of absorption, scattering, and attenuation of laser light with λ = 532 nm and in the 475–625 nm range for two-layered spherical gold nanoparticles with radii r00 = 10, 25, 50, 75, and 100 nm and radii of the solid and liquid cores ranging from 0–r00 are calculated theoretically and studied. It is found that the variation in the optical parameters of a nanoparticle with changes in the aggregate state of the material (melting) has a decisive influence on the heating dynamics of the nanoparticle above the melting point and on the subsequent thermal processes.

Measurement of Particle Density During Explosive Particle Dispersal

AbstractA gauge based on laser light attenuation has been developed to determine the temporal history of particle number density during the explosive dispersal of inert particles. The optical scheme of the gauge employs narrow band pass and spatial optical filters that protect the optical sensors from ambient light and laser light scattered by particles. The gauge is used in field experiments to measure particle density at two locations in close proximity to spherical metalized explosive charges containing a packed bed of either iron, nickel, or glass particles saturated with nitromethane. The heavy iron particles penetrate the blast wave in the near field, whereas the lighter particles remain behind the blast wave. Comparisons with multiphase calculations indicate that the particle density field inferred from the light intensity signals is consistent with the computations until the point at which the combustion products containing soot arrives at the gauge, blocking the laser light.

Thermal Assessment of Ablation Limit of Subsurface Tumor During Focused Ultrasound and Laser Heating

Theoretical study on the thermal assessment of two types of tumor ablation techniques, viz., focused laser for ablating skin lesion and focused high-frequency ultrasound for ablating breast tumor has been presented in this article. Estimation of temperature rise and the induced thermal damage in the skin using laser heating have been done by integrating the bioheat transfer, the laser-light attenuation, and the thermal damage models. Further, ultrasound heating of deep seated tumor within the breast has been implemented to estimate the temperature rise and the induced thermal damage by combining the bioheat transfer, the vascularized, the pressure wave, and the thermal damage models. The theoretical models for skin, breast, and blood vessels have been constructed based on the anatomical details, thermophysical, optical, and acoustic properties available in the literature. The study indicates that the focused ultrasound heating can selectively raise the temperature of the tissue above the ablation limit sparing the surrounding healthy ones and imposes sufficient thermal damage to the entire tumor volume in a relatively short exposure time and longer cooling period. Whereas the laser-based heating would lead to collateral damage of the surrounding tissues and demands longer exposure time in order to achieve complete heating of the tumor volume. Heating of tumor at a uniform rate is a major issue in both the cases, and in the course of heating, the entire tumor volume in certain regions may experience irregular necrosis rate and char formation. Based on the comprehensive modeling efforts, the study further suggests two important thermal ablation criteria for complete and uniform heating of tumor volume at relatively short exposure time.

Improvements in filtered Rayleigh scattering measurements using Fabry–Perot etalons for spectral filtering of pulsed, 532-nm Nd:YAG output

In this manuscript, we investigate a new methodology for increasing the spectral purity of the second-harmonic output of an injection-seeded, frequency-doubled, Q-switched Nd:YAG laser operating near 532 nm. Specifically, tunable Fabry–Perot etalons (FPEs) are used as ultra-narrowband spectral filters, transmitting the desired single-mode output, while filtering out a significant portion of the broadband pedestal characteristic of injection-seeded lasers. A specific emphasis is placed on the design and optimization of the FPEs in the context of filtered Rayleigh scattering (FRS) measurements and how their utilization results in substantial increases in spectral purity, realizable attenuation of unwanted scattering, and applications in environments with high particulate levels. Experimental results show an increase in laser spectral purity of more than one order-of-magnitude (from 0.99997 to 0.999998) when using FPE filters, which led to a two-order-of-magnitude increase in achievable attenuation of laser light passing through a molecular iodine filter. The utility of the FPE-based spectral filtering of the pulsed Nd:YAG output for 2D FRS imaging was demonstrated in turbulent, isothermal gas-phase jets, seeded with varying levels of non-evaporating droplets with particle volume fractions (FVp) ranging from ~5 to >60 parts-per-million (ppm). After implementation of an optimized air-spaced FPE in the 532-nm output, no particle scattering was observed (based on visual and statistical analysis), even for the highest seed case (FVp ~ 60 ppm), and the gas-phase Rayleigh–Brillouin signals were collected without interference from the flowfield particulate. The current results suggest that the implementation of properly specified FPEs allows FRS to be applied in environments with high flowfield particulate levels; levels are well beyond what have been suitable for previous FRS measurements.

Evaluation of Local Voidage in Two Phase System by Optical Method

One of the important characteristics in predicting the flow of two phases (gas and liquid) in the industrial equipment is the gas percentage in the liquid, which is called voidage. This characteristic can be measured by different methods; the optical method is one of them. In the present work, a He-Ne laser light is employed to measure the voidage. The measurement depends on the principle of laser light attenuation during its passage through a mixture of fluids, which is governed by Beer-Lambert law. The attenuated light is measured using silicon type detector and an oscilloscope with a plotter. The gas phase was air while the liquid phase was contaminated water situated in a vertical column. The experiments were done with different air flow rates in water (1, 2, 3, 4,5and 6 liters per minute) with two spot sizes of laser (5mm dia & 10 mm dia). The results illustrated the possibility of measuring the voidage using laser beam and are compared with another one based on the liquid level variation (level ratio) in the column. Furthermore, the results provide that the 10mm dia spot size of laser beam gives better results.

Photophysical and DFT Characterization of Novel Pt(II)-Coupled 2,5-Diaryloxazoles for Nonlinear Optical Absorption

Several new bis-phosphine platinum(II) complexes with 2,5-diaryl-substituted oxazole-containing alkyne ligands have been synthesized and optically characterized in solution. Measurements of nonlinear absorption showed strong attenuation of laser light at 532 and 600 nm. The light absorption of the Pt complexes was shifted from the near-UV region for the ground state to the red region for the excited triplet state, and was associated with large extinction coefficients. The optical limiting effect can be explained by triplet-triplet excited state absorption in conjunction with fast excited singlet-to-triplet intersystem crossing and slow triplet-to-ground-state decay, in comparison with the pulse length of the laser. DFT calculations show good predictability of the S(0)-S(1) and S(0)-T(1) energy gaps and offer insight into the interaction strength between Pt and the alkyne ligands. The use of this type of ligand, with weak absorption for the Pt(II) complexes in the visual wavelength range as a key feature, enables the possibility to further improve these molecular systems for nonlinear absorption applications.

Attenuation and speckle modulation of laser light in dispersive dye-doped media: Competition of absorption and scattering processes

The effect of competition of laser light absorption and scattering in dispersive media (1μm polystyrene microspheres in ethylene glycol) with added dye (rhodamine 6G) was studied under the condition of near-resonant absorption of probe light in the host medium (solution of rhodamine 6G in ethylene glycol). The parameters of speckle modulation of transmitted light (the average speckle intensity and the oscillation index) were applied for characterization of light transport in the probed scattering systems as a function of dye concentration. The increase in dye concentration does not cause the expected decay in the transmittance of the examined dispersive systems for 532nm laser light, but results in the decrease of their turbidity. This effect is accompanied by the rise of the average intensity and the oscillation index of speckle-modulated light, and can be rationalized by partial matching of the real parts of refractive indices for the host medium and embedded scatterers. This interpretation was supported by statistical modeling of light transport through the examined scattering systems.

Pulsed IR laser ablation of organic polymers in air: shielding effects and plasma pipe formation

We report the effect of ‘plasma pipe’ formation on pulsed laser ablation of organic polymers in air under normal conditions. Ablation of polymers (PMMA, polyimide) is carried out in a wide range of CO2 laser fluences with special attention to plasma formation in the ablation products. Evolution of laser ablation plumes in air under different pressures is investigated with simultaneous registration of radiation spectra of the ablation products. An analysis based on thermo-chemical modelling is performed to elucidate the effects of laser light attenuation upon ablation, including plasma and chemical processes in a near-target space. The analysis has shown that the experimental observations of plume development in air can be explained by a combination of processes including formation of a pre-ionized channel along the laser beam propagation, laser-supported detonation wave and effective combustion of the polymer ablation products. A scenario of a streamer-like polymer plasma flow within an air plasma pipe created via laser-induced breakdown is proposed.

Dynamics of retinal photocoagulation and rupture

In laser retinal photocoagulation, short (<20 ms) pulses have been found to reduce thermal damage to the inner retina, decrease treatment time, and minimize pain. However, the safe therapeutic window (defined as the ratio of power for producing a rupture to that of mild coagulation) decreases with shorter exposures. To quantify the extent of retinal heating and maximize the therapeutic window, a computational model of millisecond retinal photocoagulation and rupture was developed. Optical attenuation of 532-nm laser light in ocular tissues was measured, including retinal pigment epithelial (RPE) pigmentation and cell-size variability. Threshold powers for vaporization and RPE damage were measured with pulse durations ranging from 1 to 200 ms. A finite element model of retinal heating inferred that vaporization (rupture) takes place at 180-190 degrees C. RPE damage was accurately described by the Arrhenius model with activation energy of 340 kJ/mol. Computed photocoagulation lesion width increased logarithmically with pulse duration, in agreement with histological findings. The model will allow for the optimization of beam parameters to increase the width of the therapeutic window for short exposures.

Antiinfectives and Low-Level Light: A New Chapter in Photomedicine

The purpose of this study was to identify synergistic effects in the interaction of light with biosystems in the presence of chemical agents. Their systematic analysis promises therapeutic strategies. Light intensities around 1000 Wm(2) potentially induce density variations in nanoscopic water layers adhering to surfaces in air or subaquatically. In permeable nanoscopic compartments in the interior of biosystems, this could result in powerful flow processes and bidirectional flows for repetitive applications of light. Consequently, external stimulation with light will force microorganisms and cells to incorporate a suitable antiinfective. Nanoscale biosystems, which respond to both light stimulation and antibiotics, are nanobacteria. Responses include growth, inhibition, and slime secretion. Slime secretion was provoked in vitro by gentamycin, an agent proposed for in vivo eradication, and blocked by light. Depending on the field of action, co-operative effects between light and an antiinfective can be exploited by considering two properties of the drug: transmission of light and resorption by the tissue. Antiinfectives can be administered in an active form or via drug delivery systems. In the latter case, a double action of the light could be exploited: stimulated release from the carrier and subsequent uptake by the targeted biosystem. The attenuation of laser light (670 nm) by antiinfectives was measured in films of different thickness of a vaginal suppository. The effect of 670-nm laser light - not absorbed by water - on nanoscopic water layers was examined by comparing the evaporation time of irradiated drops of water-based nanosuspensions with non-irradiated controls. The 6-microm-thick suppository films were virtually transparent to the laser light, and the 1-mm-thick films totally attenuated it. Nanosuspension drops irradiated with 670-nm light needed more time to evaporate than controls. Low-level light (LLL) therapy is compatible with antiinfectives, and even capable of boosting effects of superficially applied and/or absorbed antiinfectives. Temporal coordination between light treatment and drug administration maximizes drug effects and minimizes possible adverse effects. Irradiation should start when the drug concentration has reached its maximum in the desired field of action. Light-induced flow in nanoscale cavities could represent one mechanism of LLL therapy.

Phase-locked two-line OH planar laser-induced fluorescence thermometry in a pulsating gas turbine model combustor at atmospheric pressure

Two-line OH planar laser-induced fluorescence (PLIF) thermometry was applied to a swirling CH4/air flame in a gas turbine (GT) model combustor at atmospheric pressure, which exhibited self-excited combustion instability. The potential and limitations of the method are discussed with respect to applications in GT-like flames. A major drawback of using OH as a temperature indicator is that no temperature information can be obtained from regions where OH radicals are missing or present in insufficient concentration. The resulting bias in the average temperature is addressed and quantified for one operating condition by a comparison with results from laser Raman measurements applied in the same flame. Care was taken to minimize saturation effects by decreasing the spectral laser power density to a minimum while keeping an acceptable spatial resolution and signal-to-noise ratio. In order to correct for the influence of laser light attenuation, absorption measurements were performed on a single-shot basis and a correction procedure was applied. The accuracy was determined to 4%-7% depending on the location within the flame and on the temperature level. A GT model combustor with an optical combustion chamber is described, and phase-locked 2D temperature distributions from a pulsating flame are presented. The temperature variations during an oscillation cycle are specified, and the general flame behavior is described. Our main goals are the evaluation of the OH PLIF thermometry and the characterization of a pulsating GT-like flame.

Entrainment of inclusions from the dross in stirred reactors for melt treatment

The dross appearing at the surface of an aluminium melt consists, in addition to aluminium, of mainly oxide films but also other non-metallic inclusions. The inclusion particles are often poorly wetted by the metal and accumulate at the metal surface. However, in impeller stirred reactors for melt refining, stirring may be exceedingly strong, and inclusions accumulated at the metal surface may be entrained into the bulk metal. Concerning inclusion removal, such effects may deteriorate the effectiveness of melt refining. The surface entrainment of buoyant particles in a water model of an impeller stirred refining reactor is investigated. Under varying operating conditions the particle concentration in the bulk liquid is measured by laser light attenuation. The observed entrainment of particles is explained theoretically by a three-dimensional flow analysis. It is found that the entrainment is to a large extent explained by the turbulent dispersion and the mean velocity field. However, more refined modelling of the surface flow and the layer of buoyant particles floating on top of a liquid, is needed in order to predict the conditions for the beginning re-entrainment. This is crucial for a more quantitative prediction of the experimental data. Some metallurgical consequences of the findings are discussed.

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. III. Comparison of A-X excitation schemes.

Laser-induced fluorescence (LIF) has proven a reliable technique for nitric oxide (NO) diagnostics in practical combustion systems. However, a wide variety of different excitation and detection strategies are proposed in the literature without giving clear guidelines of which strategies to use for a particular diagnostic situation. We give a brief review of the high-pressure NO LIF diagnostics literature and compare strategies for exciting selected transitions in the A-X(0, 0), (0, 1), and (0, 2) bands using a different detection bandpass. The strategies are compared in terms of NO LIF signal strength, attenuation of laser and signal light in the hot combustion gases, signal selectivity against LIF interference from O2 and CO2, and temperature and pressure sensitivity of the LIF signal. The discussion is based on spectroscopic measurements in laminar premixed methane-air flames at pressures between 1 and 60 bars and on NO and O2 LIF spectral simulations.

Measurements of laser light attenuation following cryogen spray cooling spurt termination.

Cryogen spray cooling (CSC) is used to minimize the risk of epidermal damage during laser dermatological surgery. However, while CSC can protect the epidermis from non-specific thermal damage, the cryogen film on the skin surface may pose a potential problem of laser light attenuation due to optical scattering. This study is focused on measuring the light transmittance changes that occur following cryogen spurt termination. The wavelengths studied were chosen for their clinical relevance to treatment of hypervascular skin lesions (594 nm) and laser-assisted hair removal (785 nm). Following delivery of cryogen spurts to the surface of an epoxy skin phantom, continuous records of light transmittance for 594 and 785 nm were obtained using an integrating sphere-based light collection apparatus. Shortly after spurt termination, there was negligible light attenuation by the cryogen film at the two wavelengths studied. For the typical clinical use of a 30 milliseconds spurt duration and 30 milliseconds delay between spurt termination and delivery of the laser pulse, a minimum average transmittance value of approximately 97% was measured.

Influence of the working atmosphere on the excimer laser ablation of Al 2O 3–TiC ceramics

In this paper, the influence of the atmosphere on the ablation rate of Al 2O 3–TiC ceramics using nanosecond duration pulses of 248 nm laser radiation and He, Ne, N 2, air, Ar and Kr atmospheres, at pressures between 10 −4 and 1 bar, was investigated. The effects of plume confinement due to ambient gas and the mechanisms of laser light attenuation are discussed. Analytical and numerical models were developed and used to explain the observed reduction in the ablation rate with increasing atomic/molecular weight of the ambient gas and its pressure.

Quantitative NO-LIF imaging in high-pressure flames

Planar laser-induced fluorescence (PLIF) images of NO concentration are reported in premixed laminar flames from 1–60 bar exciting the A-X(0,0) band. The influence of O2 interference and gas composition, the variation with local temperature, and the effect of laser and signal attenuation by UV light absorption are investigated. Despite choosing a NO excitation and detection scheme with minimum O2-LIF contribution, this interference produces errors of up to 25% in a slightly lean 60 bar flame. The overall dependence of the inferred NO number density with temperature in the relevant (1200–2500 K) range is low (<±15%) because different effects cancel. The attenuation of laser and signal light by combustion products CO2 and H2O is frequently neglected, yet such absorption yields errors of up to 40% in our experiment despite the small scale (8 mm flame diameter). Understanding the dynamic range for each of these corrections provides guidance to minimize errors in single shot imaging experiments at high pressure.