Single-point and two-point focused laser differential interferometry (FLDI) are used to explore the structure of a turbulent boundary layer in a Mach 2.6 in-draft wind tunnel's test section. Single point FLDI is used to measure density fluctuation profiles above the wall. Comparison of these profiles to Schlieren images is used to establish criteria for inferring boundary layer thickness from the FLDI measurements. Two-point FLDI can measure velocities of disturbances which pass through two FLDI measurement volumes. Within the bounds of uncertainty, velocity profiles measured using FLDI compare well to earlier measurements made using Pitot probes, Particle Image Velocimetry (PIV), and Krypton Tagging Velocimetry (KTV). Bandpass filtering the FLDI signal allows measurements of disturbance velocities at different frequencies. Higher frequency bands yield greater disturbance velocities than lower frequency bands in the investigated region of the boundary layer.
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