Absorption focused laser differential interferometry for near-simultaneous steady and fluctuating flow property measurements

A focused laser differential interferometer (FLDI) system using a single tunable diode laser has been assembled and tested in the lab, in order to make near-simultaneous and colinear measurements of high-frequency flow density fluctuations and other steady flow properties. The tunable diode, emitting near 760 nm, probes the A-band absorption spectrum of molecular oxygen, a gas species ubiquitous in many wind tunnel facilities, avoiding the need for flow seeding for these measurements. Non-focused LDI and focused versions of the absorption interferometer system were tested using a pressure-controlled test cell with 100% <i>O</i>₂, where both systems accurately measured the pressure over the tested range from 0.01 atm to 0.3 atm, with higher uncertainties in pressure estimates using the FLDI system. The expected operation of the interferometer aspect of LDI and FLDI systems was also observed, where the LDI system demonstrated constant sensitivity to a compressed air jet along the beam path, whereas the FLDI system demonstrated the ability to filter the signal away from the plane of focus. Further testing of the system at NASA Langley Research Center's Probe Calibration Tunnel was used to evaluate various options for laser modulation and their impact on the interferometer and absorption portions of the measurement. Both direct absorption and wavelength modulation techniques are demonstrated for this system, as well as a discussion of the benefits and drawbacks of each approach.