2D imaging of laser wing effects and of soot sublimation in laser-induced incandescence measurements

The distribution of Laser-Induced Incandescence (LII) signal in sooting flames along the laser beam is imaged using two directions of observation: one counter to the propagation direction of the incident laser (backward LII) and one at right angles. It is shown that the effective probe volume, in which the LII signal is observed, is highly dependent on the laser irradiance profile. At high fluence, the LII from the central part of the beam decreases because of soot sublimation. This decrease can be compensated by an increase in the LII from the wings of the laser beam. This interaction is particularly important in the extraction of quantitative information in the backward LII case, which is the configuration best suited to the practical application of LII for in-situ particle concentration measurements in the exhaust of aero-engines.     

Laser-induced incandescence: excitation and detection conditions,material transformations and calibration

Successful implementation of laser-induced incandescence (LII) relies upon judicious choice of excitation and detection conditions. Excitation conditions encompass choice of excitation wavelength and laser fluence. Detection conditions include choice of detection wavelength, spectral band pass about the central wavelength, detection delay and duration relative to the excitation laser pulse usually corresponding to the peak of the signal intensity. Examples of applying these parameters to LII are illustrated by way of examples: soot/polycyclic aromatic hydrocarbon and metal aerosol systems. Tradeoffs must be recognized. Laser-induced chemical and structural changes of the aerosol must be considered, particularly in light of heterogeneous aerosols. Diagnostics of such changes are outlined as they will affect interpretation of the LII signal. Finally, calibration (for LII) must be chosen to be appropriate for aerosols from practical sources as they may be mixed organic and inorganic composition. A research paper in response to the call for papers from the Third International Discussion Meeting and Workshop on Laser-Induced Incandescence July 30th–August 1st, Ottawa, Canada (2008).     

DUV scattering measurements as a tool for characterization of UV-optical surfaces

A sensitive total-scattering measurement setup for the DUV spectral range is presented, which allows precise determination of both forward and backward scatter losses from optical components for excimer lasers with a sensitivity below 10 ppm for 248 nm and 50 ppm for 193 nm. Scattering from several different coated and uncoated DUV optical surfaces was monitored. For uncoated samples, the backward scatter losses are in good agreement with the predictions of scalar scattering theory, indicating that in this case scattering is mainly determined by surface effects. Although forward and backward scatter losses are of the same order of magnitude for uncoated samples, they differ by up to two orders of magnitude for high-reflection- and by one order of magnitude for anti-reflection-coated samples. The experimental data demonstrate that the anti-reflection coatings suffer from substantial losses due to forward scattering, whereas backward scattering is the predominant loss channel for high-reflection coatings. In addition, the strong influence of defects and impurities on the total scattering is demonstrated. Received: 5 June 2000 / Accepted: 6 June 2000 / Published online: 13 September 2000     

Soot volume fraction measurements in aero-engine exhausts using extinction-calibrated backward laser-induced incandescence

Control and reduction of soot particle emissions from aeronautic turbines requires a monitoring system suitable for quantification of these emissions. Currently, such emissions are estimated using the technique of smoke number. This is an extractive method, which is not sensitive enough for the low emission levels of modern gas turbines. Within a recent European project, AEROTEST, part of the project aimed at investigating an alternative soot monitoring technique, laser-induced incandescence (LII) as an in-situ optical diagnostic for quantification of soot emissions. For aero-engine applications, especially those involving large-scale turbines, it is necessary to perform the measurements at long distance from the turbine. The LII technique is favourable in this respect as it provides for non-intrusive measurements and, by detecting the isotropic LII signal along the same axis as the incoming laser beam (so called backward LII), both the laser and the detector can be built inside one system located several meters from the turbine. The concept was initiated in the previous European projects, AEROJET I and II. This paper describes the modified version of the system and the procedure developed to achieve reliable and quantitative soot volume fraction measurements in the exhausts of aero-engines. Application of the backward LII technique is demonstrated in the exhaust of a military turbojet engine for different engine speeds.     

Numerical investigation of the effect of signal trapping on soot measurements using LII in laminar coflow diffusion flames

Laser-induced incandescence has been rapidly developed into a powerful diagnostic technique for measurements of soot in many applications. The incandescence intensity generated by laser-heated soot particles at the measurement location suffers the signal trapping effect caused by absorption and scattering by soot particles present between the measurement location and the detector. The signal trapping effect was numerically investigated in soot measurements using both a 2D LII setup and the corresponding point LII setup at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh–Debye–Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The contribution of scattering to signal trapping was found to be negligible in atmospheric laminar diffusion flames. When uncorrected LII intensities are used to determine soot particle temperature and the soot volume fraction, the errors are smaller in 2D LII setup where soot particles are excited by a laser sheet. The simple Beer–Lambert exponential attenuation relationship holds in LII applications to axisymmetric flames as long as the effective extinction coefficient is adequately defined.     

Characterisation of a spark ignition system by planar laser-induced fluorescence of OH at high repetition rates and comparison with chemical kinetic calculations

This study reports the application of a novel, high speed laser-detector system for the time-resolved study of flame propagation in a well-controlled spark ignition system. The ignition system allowed full and reproducible control over the energy deposited during breakdown and the ensuing arc discharge of the spark plasma. Ignition was performed in a closed vessel which was filled with stoichiometric mixtures of methane and air. Four sequential snapshots of two-dimensional OH distributions were recorded during single ignition events by the use of planar laser-induced fluorescence (PLIF). From these OH distributions flame front velocities have been extracted with an accuracy of better than 2%. One-dimensional numerical simulations of the ignition event including detailed chemistry and transport processes have been performed. Experimental results and results from the simulations have been compared to each other with respect to flame front velocities as well as spatial concentration profiles of OH radicals. In general a good agreement was obtained. In this way the ignition system was carefully characterised. Received: 6 April 1999 / Published online: 27 October 1999     

A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere

We describe a near-infrared in situ tunable diode laser spectrometer developed for atmospheric measurements of CH₄ in the upper troposphere and lower stratosphere (UT/LS). The instrument is designed to provide fast-response (0.5–1 Hz) measurements and operate autonomously on the NASA WB-57F high-altitude aircraft. A single-mode InGaAsP distributed feedback laser diode operating at 1.6537 μm scans continuously over the R(3) rotation–vibration transition in the 2ν₃ band. We use a direct absorption technique incorporating a custom-designed long path length (252 m) low-volume (3.6 L) astigmatic Herriott cell. The present detection sensitivity is 5×10¹⁰ molecules cm^-3, corresponding to ∼20 ppbv in the UT/LS, with the main limit to instrument precision being background optical interference fringes. In-flight performance is demonstrated by presentation of recent data. Received: 25 January 2002 / Revised version: 5 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5373, E-mail: richard@al.noaa.gov     

<Emphasis Type="Italic">Z</Emphasis>-scan technique through beam radius measurements

A modification to the well-known z-scan technique for measuring optical non-linearities is introduced. It is based on directly measuring the beam radius in the far field instead of the transmittance of the irradiance through an aperture, as in the original version. It has the advantage of being insensitive to beam pointing instability and is almost insensitive to power fluctuations. Furthermore, the calculations required for the determination of the non-linear parameters are simplified. For demonstrating the advantages of the modified method, beam radius and transmittance measurements were simultaneously taken in the standard non-linear optical material CS₂. Separate fittings of these measurements gave almost the same values for the non-linearities, quite similar to those in the literature. A common fitting has been applied to both sets of measurements, enhancing the accuracy of the method. Received: 8 July 2002 / Revised version: 18 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +30-2610/997470, E-mail: gianetas@physics.upatras.gr     

The influence of wavelength in extinction measurements and beam steering in laser-induced incandescence measurements in sooting flames

The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C₂H₄/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm² at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.     

On the dependence of the laser-induced incandescence (LII) signal on soot volume fraction for variations in particle size

“The laser-induced incandescence (LII) signal is proportional to soot volume fraction” is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known. PACS 42.62.-b; 44.40.+a; 61.46.Df; 78.70.-g; 65.80.+n     

Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 μs for fluences below the vaporization threshold. Received: 20 October 1997/Revised version: 16 February 1998     

Laser-induced etching and deposition of W using a-SiO<Subscript>2</Subscript> microspheres

A regular lattice of a-SiO₂ microspheres on a quartz support is used as a microlens array for laser-induced surface patterning by etching and deposition of W in atmospheres of WF₆ and WF₆+H₂, respectively. Received: 22 July 2002 / Accepted: 30 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, Email: dieter.baeuerle@jku.at     

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.     

Artifacts in femtosecond transient absorption spectroscopy

The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed. Received: 29 May 2001 / Final version: 15 October 2001 / Published online: 29 November 2001     

Determination of the temporal response function in femtosecond pump-probe systems

A method of determination of the true temporal response function for pump-probe type of experiments with femtosecond time resolution is presented. An analytical formula allowing calculation of group velocity dispersion (GVD) modified and sample thickness-sensitive pump-probe cross correlation function is provided. The reliability of the formula is further experimentally verified with measurements of ultrafast stimulated Raman scattering and transient absorption signals. Received: 31 October 2000 / Revised version: 6 February 2001 / Published online: 27 April 2001     

Mid-infrared polarization spectroscopy of carbon dioxide

The use of mid-infrared polarization spectroscopy (PS) for the detection of CO₂ has been demonstrated. The P(13) and P(14) resonances of the (0 0⁰ 0)→(1 0⁰ 1) transition of CO₂ were probed using a single-mode optical parametric generator system to produce a high-intensity laser beam at approximately 2.7 μm. The experiments were performed in an atmospheric pressure CO₂ jet and also in a sub-atmospheric pressure gas cell. The experimental results were compared with the results of the time-dependent density-matrix equations using direct numerical integration. The Zeeman-state structure of the upper and lower energy levels was included in the multi-state formulation of the density-matrix equations. Fifty-eight Zeeman states and two bath levels were included in the numerical analysis of the P(14) transition. The measured and calculated PS line shapes were in good agreement, and the absolute experimental signal level agreed with the theoretical calculation to within a factor of five. Received: 20 March 2002 / Revised version: 16 August 2002 / Published online: 11 December 2002 RID="*" ID="*"Present address: Innovative Scientific Solutions Inc., 2766 Indian Ripple Road, Dayton, OH 45 440, USA RID="**" ID="**"Corresponding author. Fax: +1-765/494-0539, E-mail: Lucht@purdue.edu RID="***" ID="***"Present address: School of Mechanical Engineering, Purdue University, W. Lafayette, IN 47807-2040 USA     

Influence of spatial laser energy distribution on evaluated soot particle sizes using two-colour laser-induced incandescence in a flat premixed ethylene/air flame

Time-resolved laser-induced incandescence (LII) has been developed rapidly during the last decade as a useful non-intrusive technique for particle size determination. Still several parameters should be investigated in order to improve the accuracy of LII for particle sizing and the spatial distribution of the laser energy is one of these. Generally a top-hat profile is recommended, as this ensures a uniform heating of all particles in the measurement volume. As it is generally not straightforward to create a uniform beam profile, it is of interest to establish the influence of various profiles on the evaluated particle sizes. In this work we present both an experimental and a theoretical investigation of the influence of the spatial profile on evaluated sizes. All experiments were carried out using a newly developed setup for two-colour LII (2C-LII) which provides online monitoring of both the spatial and temporal profile as well as the laser pulse energy. The LII measurements were performed in a one-dimensional premixed sooting ethylene/air flame, and evaluated particle sizes from LII were compared with thermophoretically sampled soot particles analysed using transmission electron microscopy (TEM). The results show that although there is some influence of the spatial laser energy distribution on the evaluated particle sizes both in modelling and experiments, this effect is substantially smaller than the influence of the uncertainties in gas temperature and the thermal accommodation coefficient.     

激光诱导炽光法与消光法诊断烧蚀羽烟消光性能

为了研究流场中碳纤维增强环氧树脂复合材料在激光辐照时产生的烧蚀羽烟对入射激光的屏蔽效应,通过对朗伯-比尔定律进行分析,得到了评价羽烟消光性能的平均质量消光系数的表达式,其与羽烟场浓度和激光透过率相关。采用激光诱导炽光法（LII）和激光消光法,搭建了羽烟消光性能联合诊断实验平台,使待测激光落于LII的激发光平面上,通过同步采集待测激光的透过率和LII信号,获得激发光平面上羽烟浓度场和激光消光比,得到羽烟在不同气流速度下的平均质量消光系数。实验得到气流速度为7,10,20 m/s时羽烟对1064 nm激光的归一化质量消光系数分别为2.51,1.08,1.00。实验发现,质量消光系数受到气流速度影响,当气流速度较低时质量消光系数曲线波动幅度大,且曲线均值较大;当气流速度较高时质量消光系数趋于稳定且均值较小。     

Laser-induced nano-patterning by means of interference subpatterns generated by microspheres

Interference patterns generated by a regular lattice of SiO₂ microspheres on a transparent support are used for the surface patterning of polyimide (PI) foils. Using 248 nm excimer-laser radiation, thousands to millions of holes with a full width at half maximum (FWHM) of 160±40 nm can be generated with a single laser pulse. Received: 1 March 2002 / Accepted: 4 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at