Development and Implementation of Capability Enhancements for Focused Laser Differential Interferometry

Enhancements to the capabilities of focused laser differential interferometry (FLDI) are developed and validated. These enhancements address shortcomings in existing FLDI systems to allow researchers greater flexibility in applying the technique and analyzing flows with complex features or geometries, like plasma and hypersonic flow. The primary shortcomings that are addressed regard the acquisition of FLDI measurements from large beam arrays, the creation of arrays of arbitrary shape and size, the limited use of FLDI in small-scale applications, and the integration of absorption spectroscopy methods with FLDI. First, a high-speed camera was shown to be a suitable alternative to photodiode arrays for data acquisition from a 2D array of FLDI beams in Mach 1 flow. The power spectral densities (PSD) of each beam were measured and compare favorably with values obtained from a photodiode for the same jet flow. Next, a spatial light modulator (SLM) was integrated into a 1-point FLDI to create a novel spatial light modulated FLDI (SLM-FLDI), and PSD measurements recorded for both an SLM-FLDI and a 1-point FLDI in the same region of a Mach 1.5 jet revealed strong agreement between the detected dominant frequencies. Then, small-scale designs of both 1-point and 2-point FLDI were developed. Each used an infrared laser to measure FLDI signals in stagnant, room temperature air for wavelengths from 1391.98 to 1392.95 nm. The resulting H₂O-induced absorbance was estimated via a standard tunable diode laser absorption spectroscopy (TDLAS) curve fitting approach. Electrical hygrometer and TDLAS measurements of molecular density agreed closely with the estimates. An up-scaled 1-point FLDI was used to record measurements of a small diameter jet using low humidity air and of the combustion products of a paraffin candle. Further measurements of the Mach 1.5 jet and a Hencken burner flame with equivalence ratios between 1-2 were recorded. These were used to develop a mathematical model incorporating aspects of FLDI and LAS. Comparison of the measurements and model indicates that the absorbing-FLDI (A-FLDI) signal effectively captures and encodes the local absorbance changes in the probe volume, which can then be accessed via the phase shift and individual beam absorbances.