Fluid flow vorticity measurement using laser beams with orbital angular   momentum
  We report on the direct measurements of fluid flow vorticity using a spatially shaped beam with a superposition of Laguerre-Gaussian modes that reports on the rotational Doppler shift from microparticles intersecting the beam focus. Experiments are carried out on fluid flows with well-characterized vorticity and the experimental results are found to be in excellent agreement with the expected values even for 200 ms integration times. This method allows for localized real-time determination of vorticity in a fluid flow. 
A. Ryabtsev, S. Pouya, A. Safaripour, M. Koochesfahani, and M. Dantus
physics.optics