Interferometric velocity measurements through a fluctuating phase boundary using two Fresnel guide stars.

Laser optical techniques are widely used for flow measurements as they offer a high spatial and velocity resolution. However, undisturbed optical access to the measurement volume is desired. In order to measure through a fluctuating phase boundary, we present the use of adaptive optics. In an experiment, we prove that the Fresnel reflex of a phase boundary can be used as a proper guide star for adaptive velocity measurements with a single optical access. Interferometric flow measurements through a fluctuating phase boundary have been accomplished by a Mach-Zehnder interferometer.