| 1. |
- Badiei, Shahriar, 1969, et al.
(författare)
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The Rydberg matter laser: excitation, delays and mode effects in the laser cavity medium
- 2005
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Ingår i: Applied Physics B-Lasers and Optics. - : Springer Science and Business Media LLC. ; 81:4, s. 549-559
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Tidskriftsartikel (refereegranskat)abstract
- Temporal and temperature effects are studied in Rydberg matter (RM) formed from K atoms and N-2 molecules as the active medium in a cavity. The function of this setup as a laser was recently described. Temperature-variation studies show that the photons re-exciting the RM clusters usually have a longer wavelength than the photons emitted in the stimulated emission process in the cavity. The deficit is probably covered by background photons. Very long time constants observed after emitter temperature changes indicate that long-wavelength photon energy is accumulated in the RM clusters. Long-wavelength modes are located farther from the RM emitter. The modal structure can be TEM01 or TEM00, as observed clearly by the spatial structure in rapid pulsing experiments. The in-cavity chopped beam signal is delayed by approximately 50 mu s. The initial growth rate of the signal during chopping is temperature dependent. Tailing is also observed by chopping, but rapid pulsing of the beam with a spinning mirror does not show any delay of the start of the lasing. The conclusion is that delays exist in the stimulated emission process. The broad intense band appearing at 11 000 nm is shown to be formed partly by light in the range 3500-5000 nm, probably by standing wave interaction at the grating surface (grating bands).
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| 2. |
- Bladh, Henrik, et al.
(författare)
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Experimental and theoretical comparison of spatially resolved laser-induced incandescence (LII) signals of soot in backward and right-angle configuration
- 2006
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 83:3, s. 423-433
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Tidskriftsartikel (refereegranskat)abstract
- In-situ measurements of soot volume fraction in the exhausts of jet engines can be carried out using the laser-induced incandescence (LII) technique in backward configuration, in which the signal is detected in the opposite direction of the laser beam propagation. In order to improve backward LII for quantitative measurements, we have in this work made a detailed experimental and theoretical investigation in which backward LII has been compared with the more commonly used right-angle LII technique. Both configurations were used in simultaneous visualization experiments at various pulse energies and gate timings in a stabilized methane diffusion flame. The spatial near-Gaussian laser energy distribution was monitored on-line as well as the time-resolved LII signal. A heat and mass transfer model for soot particles exposed to laser radiation was used to theoretically predict both the temporal and spatial LII signals. Comparison between experimental and theoretical LII signals indicates similar general behaviour, for example the broadening of the spatial LII distribution and the hole-burning effect at centre of the beam due to sublimation for increasing laser pulse energies. However, our comparison also indicates that the current heat and mass transfer model overpredicts signal intensities at higher fluence, and possible reasons for this behaviour are discussed.
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| 3. |
- Bladh, Henrik, et al.
(författare)
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Influence of spatial laser energy distribution on evaluated soot particle sizes using two-colour laser-induced incandescence in a flat premixed ethylene/air flame
- 2009
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 96:4, s. 645-656
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Bladh, Henrik, et al.
(författare)
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On the dependence of the laser-induced incandescence (LII) signal on soot volume fraction for variations in particle size
- 2008
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Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. ; 90:1, s. 109-125
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Tidskriftsartikel (refereegranskat)abstract
- “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.
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| 5. |
- Bruebach, J., et al.
(författare)
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Determination of surface normal temperature gradients using thermographic phosphors and filtered Rayleigh scattering
- 2006
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 84:3, s. 537-541
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Tidskriftsartikel (refereegranskat)abstract
- Wall temperature as well as the temperature distribution within or close-by the boundary layer of an electrically heated axisymmetric jet impinging on a flat plate were monitored to deduce wall-normal temperature gradients. The radial surface temperature profile of the plate was determined by coating it with thermographic phosphors (TPs), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm) and the temporal decay of the phosphorescence intensity was measured zero-dimensionally by a photomultiplier tube. In this case the 659-nm emission line of Mg3F2GeO4:Mn was monitored. The non-intrusive measurement of gas temperatures near the surface was performed two-dimensionally by filtered Rayleigh scattering (FRS). A tunable frequency-tripled single-longitudinal-mode alexandrite laser beam at 254 nm was formed into a light sheet pointing parallel to the surface. The scattered light was imaged through a very narrow linewidth atomic mercury filter onto an intensified charged coupled device (ICCD). The elastic stray light from surfaces was strongly suppressed, whereas Doppler-broadened light was detected. Thermographic phosphors proved to be reliable for the measurement of surface temperatures. Dependent on the specific experimental conditions, problems appeared with signals interfering with the FRS radiation close-by the surface. Results and challenges of this approach are discussed.
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| 6. |
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| 7. |
- Cai, Y., et al.
(författare)
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Propagation of laser array beams in a turbulent atmosphere
- 2007
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Ingår i: Applied physics. B, Lasers and optics (Print). - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 88:3, s. 467-475
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Tidskriftsartikel (refereegranskat)abstract
- The propagation of phase-locked and non-phase-locked laser array beams of radial and rectangular symmetries in a turbulent atmosphere are investigated based on the extended Huygens-Fresnel integral. The beamlet used in our paper for constructing the laser array beams is of elliptical Gaussian mode. Analytical formulae for the average irradiance of phase-locked and non-phase-locked radial and rectangular laser array beams are derived through vector integration and tensor operation. The irradiance properties of these laser array beams in a turbulent atmosphere are studied numerically. It is found that both phase-locked and non-phase-locked radial and rectangular laser array beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The propagation properties are closely related to the parameters of laser array beams and the structure constant of the turbulent atmosphere.
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| 8. |
- Delhay, J., et al.
(författare)
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Soot volume fraction measurements in aero-engine exhausts using extinction-calibrated backward laser-induced incandescence
- 2009
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 95:4, s. 825-838
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Foltynowicz, Aleksandra, 1981-, et al.
(författare)
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Noise-immune cavity-enhanced optical heterodyne molecular spectrometry : Current status and future potential
- 2008
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Ingår i: Applied physics. B, Lasers and optics (Print). - Berlin / Heidelberg : Springer Berlin/Heidelberg. - 0946-2171 .- 1432-0649. ; 92:3, s. 313-326
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Tidskriftsartikel (refereegranskat)abstract
- As a result of a combination of an external cavity and modulation techniques, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is one of the most sensitive absorption techniques, capable of reaching close-to-shot-noise sensitivities, down to 5×10-13 fractional absorption at 1 s averaging. Due to its ability to provide sub-Doppler signals from weak molecular overtone transitions, the technique was first developed for frequency standard applications. It has since then also found use in fields of molecular spectroscopy of weak overtone transitions and trace gas detection. This paper describes the principles and the unique properties of NICE-OHMS. The historical background, the contributions of various groups, as well as the performance and present status of the technique are reviewed. Recent progress is highlighted and the future potential of the technique for trace species detection is discussed.
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| 10. |
- Franke, Axel, et al.
(författare)
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Application of advanced laser diagnostics for the investigation of the ionization sensor signal in a combustion bomb
- 2005
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 81:8, s. 1135-1142
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Tidskriftsartikel (refereegranskat)abstract
- The ionization sensor is an electrical probe for diagnostics in internal combustion engines. Laser-induced fluorescence (LIF) imaging of fuel, hydroxyl (OH), and nitric oxide (NO) distributions has been employed to extend our knowledge about the governing processes leading to its signal. By monitoring the flame propagation in quiescent and turbulent mixtures, the cycle-to-cycle variations in the early sensor signal was attributed to the stochastic contact between flame front and electrodes. An analysis of the relationship between gas temperature and sensor current in the post-flame gas suggests a dominant role of alkali traces in the ionization process at the conditions under study. Significant cooling of the burned gas in the vicinity of the electrodes was observed in quiescent mixtures. Imaging of the post-flame gas in turbulent combustion revealed moving structures with varying NO and OH concentrations, which were identified as sources of variation in the sensor current.
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