Two-photon fluorescence laser sheet imaging for high contrast visualization of atomizing sprays

Two-photon excitation laser induced fluorescence (2p-LIF) is used here for imaging an optically dense atomizing spray. The main advantage of the approach is that very little fluorescence interference originating from multiple light scattering is generated. This leads to high image contrast and a faithful description of the imaged fluid structures. While point measurement 2p-LIF imaging is a well-known approach used in life science microscopy, it has, to the best of the authors' knowledge, never been tested for analyzing liquid structures in spray systems. We take advantage of this process, here, at a macroscopic scale (similar to 5 x 5 mm field of view) by imaging the central part of a light sheet of 10 mm height. To generate enough 2p-LIF signal at such a scale and with single-shot detection, ultra-short laser pulses of 25 fs, centered at 800 nm wavelength and having 2.5 mJ pulse energy, have been used. The technique is demonstrated by imaging a single spray plume from a 6 hole commercial Gasoline Direct Injection (GDI) system running at 200 bar injection pressure. The proposed approach is very promising for detailed analysis of liquid breakups in optically dense sprays and can be used for other fluid mechanics related applications. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement