Multimode optical fiber photonic doppler velocimetery

A new optical fiber head for Photonic Doppler Velocimetry (PDV) made from a combination of fiber types [multimode (MM) and single-mode (SM)] and lenses is described. The input laser beam is delivered by a SM fiber and imaged onto the target by simple optics, including an imaging lens centered inside a larger lens, whose role is to collect and image the back reflected light into the MM collection fiber. The large core of the MM fiber enhances the collection efficiency and also reduces its dependence on the target angle. Transmission through the MM fiber reduces the heterodyne fringe visibility considerably, but the Fourier analysis still enables very accurate resolution of the fringe frequency (and hence the velocity). The new PDV head with 20 GHz bandwidth was tested in a dynamical shock wave experiment to measure velocities of ∼3 km/s (∼3.9 GHz), and the results agreed very well with measurements by a standard SM PDV.