HAL Portal IOGS (nstitut d'Optique Graduate School)
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Causality and Instability in Wave Propagation in Random Time-Varying Media
No full textInternational audienceWe develop a theoretical model to investigate wave propagation in media with random time-varying properties, where temporal fluctuations lead to complex scattering dynamics. Focusing on the ensemble-averaged field, we derive an exact expression for the average Green’s function in the presence of finite temporal disorder, and extend the analysis to the thermodynamic limit. In contrast to spatial disorder, causality prevents recurrent scattering, allowing us to achieve a nonperturbative solution. We introduce an effective medium description providing a simple analysis of the propagation regimes. Our findings offer new insights into wave dynamics in temporally disordered media, with potential applications in time-varying metamaterials, dynamic sensing, and imaging in turbulent or chaotic environments
Off-resonance intracavity up-conversion pumping architecture enabling multi-watt operation of a 2.3-µm Tm:YLF laser
No full textInternational audienceWe present an intra-cavity upconversion pumping scheme for Thulium lasers operating on the 3 H 4 → 3 H 5 transition. It is based on detuning the pump wavelength from the resonance of excited-state absorption, 3 F 4 → 3 F 2,3 , around 1 µm. This scheme is validated using a Tm:LiYF 4 -based laser. The pump source consists of a diode-pumped Nd:YVO 4 laser emitting at 1.064 µm. This architecture permits for power scaling of 2.3-µm Tm-lasers. An output power of 2.7 W is generated in the continuous-wave regime for 62.3 W of intracavity pump power at 1.064 µm corresponding to 21 W of primary laser-diode power, making this kind of pumping competitive in terms of power, laser gain and efficiency with direct diode pumping at 0.78 µm. We also describe the heat management in this double-cavity laser benefiting from the intra-cavity pumping architecture allowing to share the thermal load between two gain crystals. The low absorption caused by the non-resonant 3 F 4 → 3 F 2,3 Tm 3+ transition allows the Nd pump laser to reach higher intracavity power compensating for the low pump absorption efficiency in the Tm-crystal. The proposed off-resonance pumping scheme opens a new paradigm that holds great promise for high-power, high-gain 2.3 µm solid-state lasers based on thulium ions
Narrowline cooling of dysprosium atoms in an optical tweezer array
No full textInternational audienceWe perform narrowline cooling of single dysprosium atoms trapped in a 1D optical tweezers array, employing the narrow single-photon transition at 741 nm. At the trapping wavelength of 532 nm, the excited state is less trapped than the ground state. To obtain efficient cooling performances, we chirp the frequency of the cooling beam to subsequently address the red sidebands of different motional states. We demonstrate the effectiveness of the cooling protocol through Raman thermometry, which we characterize for our experimental conditions. We obtain an array of 75 atoms close to the motional ground state in the radial direction of the tweezers. Our results demonstrate the possibility to manipulate the motional degree of freedom of dysprosium in optical tweezers arrays, a key ingredient to exploit the potential of lanthanide-based tweezers platforms for quantum science
Douglas-Rachford Splitting for Hybrid Differentiable Models
No full textInternational audienceHybrid differentiable models that combine neural networks with numerical solvers have shown promise in scientific machine learning applications, particularly for modeling complex physical phenomena governed by partial differential equations (PDEs). However, many real systems exhibit incomplete or partially known physics. We address this setting by introducing a proximal optimization framework based on Douglas-Rachford splitting that cleanly separates the known physics from learned corrections. Our approach reformulates learning as a structured splitting of an incomplete-physics objective and a data-driven regularizer, yielding an unrolled architecture that alternates a numerical solver on the available physics and a neural network that compensates missing effects. Theoretically, we establish connections between our Douglas-Rachford iterations and gradient flows in PDE systems. Empirically, on the Allen-Cahn benchmark problem, our method improves accuracy over standard methods.</div
Accurate pulse-to-pulse stability measurement for femtosecond lasers
No full textInternational audienceShort-term energy stability metrology of pulsed lasers is the source of widespread misleading comparisons. The most used value for this characterization in commercial products and laboratory applications is the rms pulse-to-pulse stability (P-to-P). For frequency-specific analysis, the relative intensity noise (RIN) power spectral density is measured and the integrated RIN over a certain bandwidth is given. In principle, integrated RIN and P-to-P correspond to the same value. In practice, the different experimental setups often cause results to differ significantly. In this paper, we detail the specifics of measurement setups for short-term stability characterization and focus on measurements with stability better than 1% rms. The sensitivity of the P-to-P approach with respect to the oscilloscope specifications is shown by measuring a commercial femtosecond amplifier using three different oscilloscopes. A simple modified setup, including a low-pass filter at the laser repetition rate to turn the photodetected pulse train into a quasi-sinusoidal response, is proposed. This approach allows to overcome the oscilloscope limitation in standard P-to-P setups, allowing to obtain reliable measurements lower than 1% rms with a resolution of the order of 0.1%
Optimisation de Paramètres Laser d’Inscription pour améliorer la diversité et la saturation de couleurs structurelle
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Laser-processable high chroma high angular tolerance color multilayer
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Optical fiber‐based sensors for real‐time monitoring for low‐energy proton beams
No full textInternational audienceThis work presents the first results from our investigation regarding the potential use of radioluminescent Ce ‐doped silica‐based optical fibers as sensors capable of monitoring the flux of low‐energy (≤ 15 MeV) protons. This technology is shown as a promising candidate for dosimetry at the Centro Nacional de Aceleradores (CNA), offering improved precision and real‐time radiation monitoring during radiation test campaigns on microelectronics or photonics components and devices. Two proton beam lines have been used to characterize the fiber dosimeters in two different CNA facilities, using a 18/9 Cyclotron and a Tandem 3 MV accelerators. The calibration of the sensor for monitoring the proton flux at 15 MeV is currently reliable within a 24% margin, due to the presence of significant beam fluctuations that prevent a unique correlation between radioluminescence levels and the associated flux values. However, this result can be improved through additional tests aimed at refining the calibration process. When using 3 MeV protons, the signal exhibits an enhanced radioluminescence, confirming the high sensitivity of optical fibers to low‐energy protons. Physically relevant insights are obtained, as the compact geometry of the optical fiber makes it sufficiently embedded to detect the 3 MeV proton Bragg peak within its sensitive core region. Nevertheless, further investigations are needed to fully understand the radioluminescence mechanism, especially under the high‐dose rate conditions involved
Visualization and quantification of coral reef soundscapes using CoralSoundExplorer software
No full textInternational audienceDespite hosting some of the highest concentrations of biodiversity and providing invaluable goods and services in the oceans, coral reefs are under threat from global change and other local human impacts. Changes in living ecosystems often induce changes in their acoustic characteristics, but despite recent efforts in passive acoustic monitoring of coral reefs, rapid measurement and identification of changes in their soundscapes remains a challenge. Here we present the new open-source software CoralSoundExplorer , which is designed to study and monitor coral reef soundscapes. CoralSoundExplorer uses machine learning approaches and is designed to eliminate the need to extract conventional acoustic indices. To demonstrate CoralSoundExplorer ’s functionalities, we use and analyze a set of recordings from three coral reef sites, each with different purposes (undisturbed site, tourist site and boat site), located on the island of Bora-Bora in French Polynesia. We explain the CoralSoundExplorer analysis workflow, from raw sounds to ecological results, detailing and justifying each processing step. We detail the software settings, the graphical representations used for visual exploration of soundscapes and their temporal dynamics, along with the analysis methods and metrics proposed. We demonstrate that CoralSoundExplorer is a powerful tool for identifying disturbances affecting coral reef soundscapes, combining visualizations of the spatio-temporal distribution of sound recordings with new quantification methods to characterize soundscapes at different temporal scales
