Two-point, parallel-beam focused laser differential interferometry with a Nomarski prism

A Nomarski polarizing prism has been used in conjunction with a focused laser differential interferometer to measure the phase velocity of a density disturbance at sampling frequencies >= 10 MHz. Use of this prism enables the simultaneous measurement of density disturbances at two closely spaced points that can be arbitrarily oriented about the instrument's optical axis. The orientation is prescribed by rotating the prism about this axis. Since all four beams (one beam pair at each measurement point) propagate parallel to one another within the test volume, any bias imparted by density fluctuations away from the measurement plane on the disturbance phase velocity is minimized. A laboratory measurement of a spark-generated shock wave and a wind tunnel measurement of a second-mode instability wave on a cone model in a Mach 6 flow are presented to demonstrate the performance of the instrument. High-speed schlieren imaging is used in both cases to verify the results obtained with the instrument.