Towards Quantitative Diagnostics using Short-Pulse Laser Techniques
Sammanfattning: Laser based diagnostic tools have had an exploding impact on fundamental as well as applied research in a number of disciplines, such as biomedicine, physical chemistry etc over the last decades. Whereas fundamental research often investigate phenomena in extreme conditions, applied research aims at performing in-situ measurements, in order to understand the many aspects and entangled phenomena that affects the research of interest. Moreover, laser measurements performed in applied research often aims at presenting images, since variations, fluctuations and the complexity of dynamical events are better viewed and understood in its context, which is provided in an image. Therefore, a number of diagnostic techniques have been developed and demonstrated in this work to facilitate laser-based studies in problematic environments. Issues that often makes optical measurements problematic could be scattered light and interfering photo-induced signals. Picosecond lasers and short gated ICCD cameras has been used to suppress interfering signals in Raman as well as fluorescence studies. Moreover, a model so simulate the detection has been developed that should be used to evaluate the potential of performing temporal filtering. By using this evaluation tool, a specific experimental setup can be evaluated in Raman as well as fluorescence measurements, if the system is characterized and the temporal shape of the signals are known. Temporal filtering should be considered as a complementary to other filtering techniques, such as spectral or polarization filters. The simulating detection model was further developed to determine temporal shapes of signals in an image. We call this technique DIME, Dual Imaging with Modeling Evaluation, since two acquired images are combined and evaluated by modeling the detection of these two images. The most common application of such a scheme is fluorescence lifetime imaging (FLI), which is a widely used optical tool in biomedicine. In comparison to traditional FLI techniques, the DIME concept allows higher signal-to-noise ratios. Furthermore, a rapid lifetime determination algorithm is presented, called RGP-LD (Ramped Gain Profile-Lifetime Determination), that shows promising potential for fluorescence lifetime imaging, especially in combination with DIME. The DIME concept has been utilized to achieve quenching corrected fluorescence images of formaldehyde in a flame, as well as quantitative oxygen concentration measurements in toluene seeded N2/O2 flows. DIME was also evaluated in combination with an optical measurement technique called SLIPI (Structured Laser Illumination Planar Imaging).The combination of techniques was able to provide quantitative fluorescence lifetime data even though the signal was collected through multiple scattering media. Picosecond lasers and fast detection systems, such as streak cameras, MCP-PMT, where used to demonstrate and develop picosecond LIDAR (LIght Detection And Ranging). Such a scheme allows single ended measurements in case of limited optical access, which most often is the case in practical applications.
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