Julien Lumeau

We present a synchronously pumped diamond Raman laser that yielded a maximum second-order Stokes-shifted average output power of 106 W at a wavelength of 1420 nm. At this power, the laser generated 8.4 ps long pulses at a repetition rate of 50 MHz, while the beam propagation factor was M ² =4.85. Low-power replicas of the output pulses, with an average power of up to 5 W, were then compressed to a pulse duration of about 560 fs.

Thermo-induced wavefront deformation is a major limitation for thin-film optical components operating under high-power continuous-wave laser irradiation. In this work, we investigate several mitigation strategies for reducing these distortions in reflective and transmissive configurations. The study combines wavefront and thermal measurements with a previously validated finite-element model. A comparison of substrate materials shows that Zerodur is particularly effective in reflection because of its near-zero thermal expansion, while sapphire is more favorable in transmission due to its high thermal conductivity and reduced refractive-index gradients. A rear-side Ta ₂ O ₅ coating is also shown to partially compensate thermo-mechanical stresses, leading to a reduction of at least 25% in reflected wavefront deformation for both fused silica and Zerodur substrates. Finally, active edge heating is investigated as a means of reducing radial thermal gradients and further mitigating deformation. These results demonstrate that thermo-induced wavefront distortions can be efficiently reduced through an appropriate combination of substrate selection, component design, and thermal management strategies.

Accurate optical characterization is essential for uranium dioxide (UO ₂ ) for understanding the laser-matter interaction phenomena involved in the material characterization techniques used in nuclear fission research. In this study, we investigate the optical properties of mono- and polycrystalline UO ₂ using a custom spectral ellipsometer, complemented by transmission measurements. The instrument performances are assessed on Si and Ge wafers combined with silica thin film characterization. Our results reveal a refractive index of 2.3 and exhibit a band gap at 2 eV with strong absorption below the band gap. While transmission measurements confirm the band gap position, they indicate absorption levels 4 to 40 times lower than those derived from ellipsometry. This disparity, already reported in the literature, is observed on the same type of sample, implying a thermodynamically non-stable surface.

Precise optical monitoring of the layer thickness is crucial for the fabrication of complex error-sensitive thin-film filters. In this study, we extend a previously developed pseudo-polychromatic monitoring algorithm by integrating automatically placed time-controlled layers. While time-based monitoring is often applied empirically for convenience, we highlight its positive impact on the performance of the pseudo-polychromatic approach. Concerning automated selection of time-control, several configurations were tested on three multi-bandpass filter designs. Results show that hybrid strategies can reduce equivalent random thickness errors by more than 0.1–0.2 nm, compared to purely optical approaches. Although results are specific to our monitoring scheme, this work opens the door for broader analytical studies on the influence of time-controlled layers across a range of commonly used optical monitoring techniques.

Precise optical monitoring of the layer thickness is crucial for the fabrication of complex error-sensitive thin-film filters. In this study, we extend a previously developed pseudo-polychromatic monitoring algorithm by integrating automatically placed timecontrolled layers. While time-based monitoring is often applied empirically for convenience, we highlight its positive impact on the performance of the pseudo-polychromatic approach. Concerning automated selection of time-control, several configurations were tested on three multi-bandpass filter designs. Results show that hybrid strategies can reduce equivalent random thickness errors by more than 0.1-0.2 nm, compared to purely optical approaches Although results are specific to our monitoring scheme, this work opens the door for broader analytical studies on the influence of time-controlled layers across a range of commonly used optical monitoring techniques.

High-power continuous-wave lasers are widely employed in industrial, defense, and research applications. Yet their interaction with optical thin-film components introduces challenges due to photo-induced thermal effects. This study investigates wavefront deformations arising from thermal gradients in thin-film optical filters under 1 µm high-power laser illumination. We implement an experimental platform that combines a high-resolution wavefront sensor in an imaging configuration with an infrared camera. The setup enables simultaneous mapping of optical path difference (OPD) and surface temperature on the same component during irradiation. Reflected and transmitted wavefront deformations are quantified, and a finite-element model reproduces the coupled thermo-elastic and thermo-optic response. The phase metrology coupled to a frequency filtering method demonstrates a sensitivity better than 2 nm in OPD for a 1 K temperature rise. Finite-element predictions agree with experiment over a 25 mm clear aperture, validating the methodology. By bridging synchronous measurements and modeling, the approach delivers a practical workflow for qualifying and optimizing thin-film components in high-power laser systems.

This study demonstrates that self-injection locking (SIL) of a distributed feedback (DFB) laser to a high-Q fiber Fabry Perot (FFP) resonator, fabricated with highly nonlinear fiber, allows optical frequency combs (OFC) generation with a laser power as low as 100 mW. More precisely, cavity soliton (CS) regime has been observed in this configuration, along with other types of combs. The laser stabilization using SIL is described. Then the system’s behavior is analyzed through modeling the laser’s dynamics and comparing the model results to experimental tuning curve measurements. Our findings highlight the critical role of the initial phase of the fiber link between laser and FFP in determining the stability and effectiveness of the locking process. We explore the dynamics of the nonlinear SIL process while varying the laser current, revealing the transition from modulation instability to chaotic comb states, and eventually to soliton formation as the system moves from an effective blue-detuned to an effective red-detuned regime. Notably, the inclusion of self-phase modulation (SPM) in the SIL model predicts accessibility of the narrow soliton existence range. These results highlight the potential of SIL in FFP resonators for low-power, stable OFC generation, offering a promising path forward for practical applications.

Fiber Fabry–Perot (FFP) resonators of a few centimeters are optimized as a function of the reflectivity of the mirrors and the dimensions of the intra-cavity waveguide. Loaded quality factor in excess of 10⁹, with an optimum of ~4 × 10⁹, together with an intrinsic quality factor larger than 10¹⁰ and intrinsic finesse in the range of 10⁵ have been measured. An application to the stabilization of laser frequency fluctuations is presented.

The saturable absorption of 2D Bi₂Te₃ layers is studied by using the Z-scan technique employing infrared 400 fs laser pulses. Optimization of the nonlinearities has been carried out by measuring the third-order nonlinear susceptibilities as a function of the film thickness. A thorough optimization of the thin film annealing conditions has been performed and is presented. For each thickness, the annealing parameters have been separately investigated. Scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometry studies have also been performed on the as-deposited and crystallized 2D layers.