Multimode optical fiber photonic doppler velocimetery

Abstract

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.

Technology and times scales in Photonic Doppler Velocimetry (PDV)

“…This under-appreciated feature means that interferometer behavior is not diminished by a rough target, if enough light can be collected from that surface. The same cannot be said for PDV, where rough surfaces and/or multimode fiber delivery degrades optical coherence of the return signal [38]. Some of that loss is offset by the scaling of signal amplitude with optical power ( √ P 1 ) in PDV, whereas VISAR signal amplitudes scale with P 1 .…”

Photonic Doppler velocimetry with 62.5 µm graded index multimode fiber at 1550 and 532 nm

“…Since PDV systems have been in use, the published studies on PDV systems with MM fibers are very limited. Horovitz et al investigated a focuser head containing a SM input fiber and a MM one for signal collection [9]. So, in their setup, only part of the system was MM.…”