Time Integrated Detection and Applications of Femtosecond Laser Pulses Scattered by Small Particles.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2010)
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|Item Type:||Ph.D. Thesis|
|Title:||Time Integrated Detection and Applications of Femtosecond Laser Pulses Scattered by Small Particles|
In this thesis the time integrated detection of scattered femtosecond laser pulses, and illumination from a continuous wave semiconductor laser source with optical feedback and adaptable coherence length, is investigated for small particles. In this context the scattering function for small particles is numerically advanced. The angular distribution of local maxima of the scattering function is presented for small particles under ultrashort pulse illumination. The position of local maxima relating to scattering angle, particle size and refractive index is calculated with Fourier-Lorenz-Mie theory. Moreover, an estimate of the sensitivity of the scattering function to non-sphericity is presented, utilizing general Lorenz-Mie theory. For ultrashort pulse illumination the maxima of the supernumerary bows of the primary rainbow, which hold information on particle size and refractive index, are freed from disturbing interferences with reflection order and higher refractive order contributions. The presented results indicate the feasibility of precise in-situ measurements with Rainbow refractometry for highly spherical small particles with compact and cost-efficient sources of illumination and quantify the pulse duration or coherence length for the desired lower size range, which is essential information for future experimental studies. Moreover, a substantial reduction of morphology dependent resonances for microscopic water droplets is successfully demonstrated. For the first time, an electrodynamic Paul trap with novel geometry is applied to successfully observe the scattering of femtosecond laser pulses on small water droplets not only during a short period of milliseconds as in a droplet stream, but also over the temporal evolution of an evaporating droplet. Beyond Rainbow refractometry, the acquired results indicate the feasibility of new measurement techniques and the significant enhancement of existing methods. Under the aspect of the separation of scattering orders, interferometric particle imaging (IPI) is enhanceable by fitting the coherence length to the angle of detection. Illumination of small particles with femtosecond laser pulses is shown to facilitate direct size measurement from focused images of a thin spray by measurement of the intensity ratio of specific glare points on individual particles. Moreover, the smoothing of the diameter-intensity function for small particles allows for more precise determination of the Sauter mean diameter in the named size range. Due to the introduction of coherence length as a variable parameter for small particle characterization, the advantages of time integrated detection of femtosecond laser pulses scattered by small particles are underscored by the outlook of utilizing spatially compact and cost-efficient semiconductor laser devices with the important benefit of adjustable coherence length.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||16 Department of Mechanical Engineering > Fluid Mechanics and Aerodynamics (SLA)|
|Date Deposited:||08 Apr 2010 12:43|
|Last Modified:||07 Dec 2012 11:57|
|Referees:||Tropea, Prof. Dr.- Cameron and Damaschke, Prof. Dr.- Nils and Elsässer, Prof. Dr. Wolfgang|
|Refereed:||4 September 2009|