Publications

Imaging using THz waves has been a promising option for penetrative measurements in environments that are opaque to visible wavelengths. However, available THz imaging systems have been limited to relatively low frame rates and cannot be applied to study fast dynamics. This work explores the use of upconversion imaging techniques based on nonlinear optics to enable wavelength-flexible high frame rate THz imaging. UpConversion Imaging (UCI) uses nonlinear conversion techniques to shift the THz wavelengths carrying a target image to shorter visible or near-IR wavelengths that can be detected by available high-speed cameras. This report describes the analysis methodology used to design a prototype high-rate THz UCI system and gives a detailed explanations of the design choices that were made. The design uses a high-rate pulse-burst laser system to pump both THz generation and THz upconversion detection, allowing for scaling to acquisition rates in excess of 10 kHz. The design of the prototype system described in this report has been completed and all necessary materials have been procured. Assembly and characterization testing is on-going at the submission of this report. This report proposes future directions for work on high-rate THz UCI and potential applications of future systems.

Wall impingement and fuel film deposition in gasoline direct injection engines under cold start conditions are major concerns for emissions reduction. However, it is challenging to study the dynamics of film deposition under realistic conditions because of the difficulty of measuring the thicknesses of these microscale films. Low-coherence interferometry provides a quantitative optical film thickness measurement technique that can be applied to study this problem. This work presents the first high-speed spectral low-coherence interferometry measurements of impinging gasoline direct injection sprays. The feasibility and practical concerns associated with high-speed low-coherence interferometry systems are explored. Two approaches to spectral low-coherence interferometry: Michelson interferometry and Fizeau interferometry, were implemented and are compared. The results show that Fizeau interferometry is the better option for measurements of impinging sprays in closed spray vessels. The high-speed low-coherence interferometry system was applied in the Fizeau configuration to measure time-resolved film thickness of impinging sprays under engine-relevant conditions to demonstrate its capabilities.

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