Dr. Gaël Le Blanc Profile

Dr. Gaël Le Blanc

at CEA-Gramat

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Publications (2)

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Proceedings Article | 8 March 2023 Presentation + Paper

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

Y. Barbarin, G. Lefrère, G. Bourcier, D. Plouhinec, T. Paccou, G. Le Blanc

Proceedings Volume 12428, 124280B (2023) https://doi.org/10.1117/12.2647537

KEYWORDS: Multimode fibers, Interferometers, Velocimetry, Doppler effect, High speed photonics

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Photonic Doppler Velocimetry (PDV) systems measure velocities in shock physics experiments on novel materials. In many experiments, diffusive materials limit the amount of Doppler signal collected, which degrades the measurement. Increasing the laser power comes with a cost when many channels are supplied at the same time and could damage some sensitive surfaces. The use of a multimode fiber with a 62.5 μm core diameter in the collimator or focuser brings more signal into the interferometer but, on the other hand, the multimode behavior of the fiber reduces interference contrast. PDV systems with 62.5 μm multimode fiber were tested at 1550 and 532 nm on a test-bench at low velocities. The interference amplitudes are discussed according to tilt angle and for different surface qualities. The trends differed with wavelength. A multimode system was also tested under real conditions during a ramp compression experiment with velocities greater than 100 m/s. Practically, the use of multimode fibers greatly facilitates probe alignment, especially at 532 nm. Furthermore, 532-nm singlemode fiber are rapidly limited in terms of their optical power. For the two wavelengths tested, the high-speed measurements show clear interference fringes with at least the same amplitude level as with singlemode PDV systems. The benefits of multimode PDV systems appear clearer for highly diffusive surfaces and when probe alignment cannot be guaranteed. At 532 nm, better results are expected using fibers with a core diameter in the range of 20 μm if the optical components become available.

Proceedings Article | 3 March 2022 Presentation + Paper

532-nm triature Photonic Doppler Velocimetry

G. Le Blanc, S. Bergey, J. Vich, T. d'Almeida, M. Roudot, J. Luc, Y. Barbarin

Proceedings Volume 12016, 120160X (2022) https://doi.org/10.1117/12.2616569

KEYWORDS: High speed photonics, Homodyne detection, Signal processing, Heterodyning, Doppler effect, Optical fibers, Velocimetry

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Photonic Doppler Velocimetry (PDV) systems are developed to measure velocities in physics experiments such as ramp compression experiment on novel materials. Better time resolution, in the fast acceleration phase, is required when the velocity reached is limited to few tens of m/s. This resolution being proportional to the wavelength, a green PDV system provides a factor 3 gain compared to a telecom based PDV. Furthermore, a Triature design allows to further increase the time resolution thanks to the phases signal processing. A 532-nm Triature PDV system was realized with singlemode fibers (4.5 µm core diameter) and compared to three standard PDV systems in reproducible ramp compression experiments. To minimize the optical losses, most of the fibers components were fused. The velocity profile measured has oscillations between 0 and 10 m/s. The first velocity peak is reached in 1.0 µs. The 1550-nm homodyne PDV system provides only 6 fringes and it clearly limits the time resolution. The 1550-nm heterodyne PDV system provides hundreds of fringes but variations of about 0.1 m/s are observed. The green homodyne PDV system provides almost the correct velocity profile by Short Time Fourier Transform. The 250-ns long acceleration phase is better resolved applying a signal processing with at least two phases.

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