Direct density measurements from intersectional FLDI

A widely utilized optical diagnostic, focused laser differential interferometry (FLDI), generates pseudo-point-like measurements with suppressed off-axis signals. However, prior studies reveal that off-axis disturbances with longer wavelengths remain integrated into the final FLDI measurement, making conversion of the measurement to true density units challenging. To overcome this limitation, intersectional FLDI (iFLDI) is proposed, utilizing two or more identical FLDIs that intersect at the region of interest. The coherence between the FLDIs is leveraged to identify and remove the contaminating off-axis noise. This novel, to the best of our knowledge, improvement is computationally tested with a high-fidelity simulation of a hypersonic turbulent flow with and without an inserted sinusoid. Two filtering methods are used to reconstruct the final signal. iFLDI is shown to approach an idealized point measurement in both spectral shape and amplitude relative to a single FLDI measurement. Crucially, the final, improved spectrum produces a quantitative measurement of the density change without the drawbacks of previously proposed calibration techniques.