23 August 2017 Spatially and temporally resolved diagnostics of dense sprays using gated, femtosecond, digital holography
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Abstract
This paper describes research that demonstrated gated, femtosecond, digital holography, enabling 3D microscopic viewing inside dense, almost opaque sprays, and providing a new and powerful diagnostics capability for viewing fuel atomization processes never seen before. The method works by exploiting the extremely short coherence and pulse length (approximately 30 micrometers in this implementation) provided by a femtosecond laser combined with digital holography to eliminate multiple and wide angle scattered light from particles surrounding the injection region, which normally obscures the image of interest. Photons that follow a path that differs in length by more than 30 micrometers from a straight path through the field to the sensor do not contribute to the holographic recording of photons that travel in a near straight path (ballistic and “snake” photons). To further enhance the method, off-axis digital holography was incorporated to enhance signal to noise ratio and image processing capability in reconstructed images by separating the conjugate images, which overlap and interfere in conventional in-line holography. This also enables digital holographic interferometry. Fundamental relationships and limitations were also examined. The project is a continuing collaboration between MetroLaser and the University of California, Irvine.
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James D. Trolinger, Andrei K. Dioumaev, Ali Ziaee, Marco Minniti, Derek Dunn-Rankin, "Spatially and temporally resolved diagnostics of dense sprays using gated, femtosecond, digital holography", Proc. SPIE 10373, Applied Optical Metrology II, 103730P (23 August 2017); doi: 10.1117/12.2274841; https://doi.org/10.1117/12.2274841
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