Unspecified Louis Thamié Profile

Louis Thamié

at CEA-Gramat

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Proceedings Article | 7 June 2024 Presentation + Paper

Overpressure sensing through acousto-optics: a comparison between a self-mixing interferometer and an all-fiber Michelson interferometer

Sébastien Maqueda, Julien Perchoux, Clément Tronche, Louis Thamié, Alan Dufourmentel, Marc Genetier, Maylis Lavayssière, Yohan Barbarin

Proceedings Volume 13044, 130440B (2024) https://doi.org/10.1117/12.3013433

KEYWORDS: Sensors, Refractive index, Michelson interferometers, Interferometers, Optical sensors, Signal processing, Acoustooptics, Reflectors, Wave propagation

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Recent studies have shown that a compact self-mixing interferometer can be used for the characterization of shock waves. It measures dynamically (> 10MHz) the changes in the refractive index induced by the shock wave. Associated to an appropriate acousto-optic model, the pressure profile is computed with a 34mbar resolution. In the present work, we compare shock wave induced refractive index variations measurements by another method using a Michelson-type fiber-optic interferometer with phase analysis that has been developed for Photonic Doppler Velocimetry applications. The output signals of this system are processed in triature, which consists in analyzing the phase shift between the three interferometric signals. This bulkier system provides, in theory, a better resolution than the self-mixing interferometry sensing scheme. In the present paper, we compare these two optical methods to measure a shock wave pressure through experiments that were carried out with an open shock tube instrumented with commercial, bandwidth limited, pressure sensors. This configuration creates a spherical shock wave similar to those observed during on-field experiments with explosives. We describe the two measurement systems and the experimental setup design used for overpressure characterizations. Both sensing approaches have been carried out in the same experimental conditions and with shock wave pressure peak amplitudes of a few bars. We detail the two types of signal processing and we discuss the results obtained with the two optical methods, which are also compared to a piezoelectric reference sensor.

Proceedings Article | 7 June 2024 Presentation + Paper

Polymer chirped fiber Bragg grating for more sensitive shock velocity measurements

Y. Barbarin, L. Thamié, P. Hereil, S. Echaoui, A. Lefrançois

Proceedings Volume 13044, 130440C (2024) https://doi.org/10.1117/12.3013432

KEYWORDS: Polymers, Optical gratings, Fiber Bragg gratings, Sensors, Velocity measurements, Optics manufacturing, Reflectivity

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The performances of a single-mode polymer chirped fiber Bragg grating (CFBG) made in a microstructured PMMA fiber is investigated to measure shock and detonation velocities. The polymer material makes the fiber sensor more sensitive at lower pressure values than with silica. The optical characterization of the microstructured polymer CFBG is discussed. The single-mode fiber offers a stable reflected optical spectrum over time and the manufacturing process used provides a constant chirp rate along the grating. The maximum reflectivity of the microstructured polymer CFBG provides enough signal for the photoreceiver without significantly increasing the optical source power. The actual limitations include the global shape of the reflected spectrum, which comprises many dips, and the optical losses that reduce sensor sensitivity towards the end of the grating. Anyhow, a first detonation experiment intended to measure shock and detonation velocities in a wedge test was completed. The X-T diagram showing the shock and detonation wave positions as a function of time presents two slopes corresponding to a shock velocity of 4695m/s and a steady-state detonation velocity of 8392m/s. These values are very similar to the ones obtained with 48 piezoelectric pins, but the uncertainties remain high (>2%) for this first experiment. Nevertheless, the experience proves that the PMMA material is suitable for detonation physics experiments. Technical solutions were identified to improve sensor performance. First, optical losses could be reduced within the grating and a more constant reflectivity level could be obtained. Sensitivity would be similar along the full length of the grating. The second point of focus will be to prevent any dips in the reflected optical spectrum. With these improvements, we should achieve uncertainties of less than ±1% (at k=2) for shock and detonation velocity measurements.

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