Classical LDV techniques permit the recovering of the flow profile at the expense of the movement of projection and recovery optics. This operation limits the performances of the LDV and extends the measurement time. The aim of this communication is to present a novel monobeam distributed laser Doppler velocimeter (DLDV) which permits the continuous recovery of the flow velocity at any point belonging to the light beam, without moving the collimating optics. The DLDV applies the low-coherence multiplexing technique to a backscattering reference-beam LDV scheme. The DLDV uses a very-short coherence source (a superluminescent diode) which feeds an unbalanced interferometer. The sensing arm of the interferometer is launched in the flow while the remote reference arm permits the user to interrogate different flow volumes. The scheme has been implemented by all-fiber components. Preliminary measurements were given on a laboratory hydraulic circuit. The DLDV can be used as a conventional LDV, to measure with high accuracy a flow velocity component in a point of the duct, or in a distributed mode, to recover the flow velocity profile along a duct cross-section. Profiles of the flow were recovered at different average velocity and the first results confirm the exceptional capabilities of the novel instrument.
"Distributed laser Doppler velocimeter", Proc. SPIE 1797, Distributed and Multiplexed Fiber Optic Sensors II, (5 March 1993); doi: 10.1117/12.141299; https://doi.org/10.1117/12.141299