Time-Resolved Mach-Zehnder Interferometry of Shock Waves

Introduction

The Mach-Zehnder interferometer (MZI), devised at the end of the 19th century [1], [2], is a complex yet highly useful apparatus to visualize and measure minute phase shift differences, which in gases correspond to density differences. Before the advent of other interferometric techniques made possible by the availability of high coherence length light sources such as lasers, this apparatus was arguably the only system that not only generated a pictorial record of a compressible flow with a reasonably large field of view but that also allowed one to obtain, with adequate accuracy, the density distribution within the flow. Designed for white light interference, the system has to be built in a way that optical path length differences between object beam and reference beam are of the order of a few micrometers. The associated considerable complexity of the apparatus and its stringent requirements for the quality of the components of the optical system have been detrimental to the popularity of this diagnostic tool. Consequently, once laser-based techniques such as holographic interferometry became available, which avoided most of the restrictions associated with the MZI, the importance of the MZI for flow visualization diminished. The MZI has, however, one important advantage inasmuch that it is suitable without modifications for time-resolved visualization if an appropriate combination of light source and camera is used, as was already illustrated several times in the past. Current camera technology allows one to obtain interferometric visualizations with frame rates beyond one million frames per second (fps). The main requirement for the light source is to provide sufficient illumination intensity and, if polychrome operation is desired, an adequate spectral range.