Laser fluorescence velocimeter

A new laser velocimeter was proposed which utilizes the laser induced fluorescence from atomic species imbedded in a flow, rather than scattering from particles, to determine the flow velocity. Simple theoretical models indicated that the phase shift from the space and time modulated fluorescence signal produced within a probe volume created by crossed laser beams of two different frequencies could be used to measure the fluid velocity. Experiments using static cells containing iodine verified that adequate modulation could be detected using phase-locked amplifiers. An underexpanded jet of nitrogen seeded with sodium was constructed to provide a flow-field in which a proof-of-principle experiment could be performed. This experiment was unsuccessful. A more detailed analysis of optical absorption by individual atoms in a bichromatic optical field showed that there is only a narrow range of velocities for which a single atom can absorb radiation at either of the optical frequencies. This effect greatly reduces the visibility of the fluorescence fringes within the probe volume and further limits the range of applicability of the technique.