High resolution flow measurement based on a bidirectionnal frequency shifting loop – MECHOUI

The consortium composed of Thales R&T (TRT), the Laboratoire Interdisciplinaire de Physique (LIPhy) end the Unité de Mécanique de l’ENSTA Paris introduces a novel architecture for optical fiber based distributed sensing with high spatial resolution and high bandwidth capability. This unique instrument in terms of performances will be tested in the frame of flow measurements and stall dynamics analysis.
The cornerstone is a bidirectional Frequency Shifting Loop (FSL), which has been studied for almost 10 years at LIPhy. This source generates two optical pulse trains covering a spectrum a few tens of GHz, with adjustable repetition rate and dispersion. Moreover, the bidirectional architecture performs the optical pulse compression operation in a fully analog way, with a tunable rate, through the recombination of both optical pulse trains emitted by the FSL on a photodetector.
Sending one of these pulse trains into an optical fiber acting as a sensor, the broad spectrum of the source allows a distributed measurement through light backscattering over a few meters, with a potential sub-cm spatial resolution. Besides, recombining the backscattered pulse train with the second pulse train emitted by the FSL on a photodiode provides pulse compression operation over the full spectral range of the frequency comb. This operation is performed at rate given by the repetition rate difference between the pulse trains emitted by the FSL. The sensing fiber can be analyzed at a kHz rate, requiring a low bandwidth acquisition system.
These parameters, along with the low invasiveness of the optical fiber, are particularly well adapted to the measurement of hydro and aerodynamic flows. We hope to achieve a map of the parietal pressure field on small scale objects tested in wind and water tunnels.
All the skills required to complete this goal are gathered in the MECHOUI consortium. First, the LIPhy will bring its expertise on the scaling, the modelling and the realization of a bidirectional FSL to provide a transportable demonstrator fulfilling the requirements set by the applications. This demonstrator will be paired with a highly sensitive distributed optical fiber sensor. The full expertise of TRT will be harnessed to optimize the signal backscattered by the fiber (by choosing an appropriate fiber and a proper processing of the optical signal) and the noise level at the detection stage. Moreover, the pulse compression signal will be numerically processed to realize an image of the full length of the distributed sensor with a refreshing rate faster than a ms. Finally, the sensor will be integrated to different models to check the performances of the bidirectional FSL in hydrodynamic, aerodynamic and aeroacoustic flow sensing thanks to the equipment and the expertise provided by ENSTA Paris.