Damage and laser cleaning of optics on power laser systems

Inscription of Optical Waveguide in Silica or Saphirre Optical Fbers and Characterization at High Temperature

Fiber Bragg Gratings are structures, photo-inscribed with femtosecond laser, inside the fiber core and can be used as band pass optical filters (centered around the Bragg wavelength). Bragg wavelengths are easily multiplexed and they give to us the necessary information. Silica-based fiber Bragg gratings are point sensors and can measure temperature up to 1200°C. For higher temperature, sapphire based optical fiber are used, since they can withstand temperature up to 2000°C. However, the sapphire optical fiber is coreless, which leads to an extreme multimodal behavior. Consequently, the measure is less precise and the signal-to-noise ratio is low, compare to a classical silica-based grating. Moreover, each modification of the fiber surface, change the grating spectra.
The thesis objective is the creation of an optical waveguide inside the sapphire fiber, which will leads to less propagation modes inside the fiber, in order to obtain new perspectives for the monitoring in high temperature environments (airplane engines, nuclear reactors, …), which is one of the missions of the DRT/LIST-DIN. To obtain this result, the photo-inscription of a cladding is necessary: the cladding will be ring shape and the internal diameter – i.e. the core – will be few tenths of micrometers. Other techniques are also investigated, such as ion implantation, to create an amorphous sapphire cladding. Then these new structures will be characterized up to 2000°C and under high dynamic pressure (> 10 GPa).

High speed/High capacity distributed Fiber Bragg Grating sensing technique for Structural Health Monitoring (SHM) applications

Schedule-driven Non-Destructive Evaluations (NDE) are carried out during structure/equipment’ life to detect major degradations endangering safety and impairing service availability. In addition to NDE, Structural Health Monitoring (SHM) involves the use of in-situ Fiber Bragg Grating (FBG) sensing systems and algorithms to evaluate structure worthiness. FBGs are mostly used as strain/temperature sensors but are also used for acoustic sensing, as substitutes to piezoelectric actuators. The SHM of large structures or acoustic measurements for passive/active tomographic techniques simultaneously require a high capacity and readout rate. However, commercially available FBG readout units rely upon Wavelength-Division Multiplexing (WDM) or Optically Frequency-Domain Reflectometry (OFDR) techniques. WDM-based units are limited in capacity (several tens of sensors) but may reach high scan rate (MHz or beyond MHz). Conversely, OFDR-based units are limited in scan rate (typically several tens of Hz) but may accommodate large number of sensors (typically up to 2000). Tomography with acoustic techniques requires both high capacity and high scan rate with the aim to improve quality of image reconstruction. Optical Time-Stretch (OTS) is a time-domain technique that has potential to improve both capacity and scan rate and to open the way to efficient tomography reconstruction processes. The basics of OTS is to use a pulsed laser, a highly dispersive medium and a high bandpass photodetector in order to convert a Bragg wavelength shift into a time delay. The doctoral candidate will investigate several ways to implement OTS to SHM. Draw-Tower Gratings (DTG) and chirped gratings will be used for the measurement of strain profiles and acoustic field emission on metallic and carbon fiber-reinforced plastics (CFRP) composite structures. The candidate will first assess the performance of the OTS technique in laboratory (LSPM) with piezoelectric actuators and laser-ultrasonics (if available, with CNRS/PIMM). Then, the OTS device will be tested onto several demonstrators provided by partners within the MSCA USES 2 doctoral network: civil engineering structure (BAM, Berlin), hydrogen storage canister (Faber, Cividale del Friuli) and CEA DAM (Le Ripault) and finally onto a metallic pipeline for fluid transport (ENI, Milano). The doctoral candidate will move onto those test sites during three 2-month periods. He will implement the OTS technique and gather experimental feedback.

Quantum Cascade III-V/Si laser micro-sources

This thesis project focuses on the development of innovative micro-laser sources by combining III-V Quantum Cascade materials with Silicon Photonic Crystals. By integrating these advanced technologies, we aim to create hybrid lasers emitting in the middle infrared. This approach has significant advantages for medium-infrared spectrometry (MIR), a crucial technique for the chemical detection of gaseous, solid and liquid compounds.
The CEA-LETI Optical Sensor Laboratory offers a state-of-the-art research environment, where the candidate will have the opportunity to design, model, manufacture and characterize these devices. This thesis is part of a competitive but promising context, where technological advances could open new perspectives in areas such as "well-being and the environment". For Master 2 students who are passionate about photonics and emerging technologies, this research offers an opportunity to actively contribute to innovation in a growing field.

Wavefront shaping for photobiomodulation - Application to neurodegenerative diseases

Photobiomodulation consists in using light, in the visible/near infrared range, to treat or slow down the evolution of a pathology. In the case of Parkinson’s disease, a neurodegenerative disease without any curative treatment so far, the brain region responsible for symptoms is located in the midbrain area, deep in the brain, which requires neurosurgery to position the optical implant. At Clinatec, 3 patients have been implanted with such an active implantable medical device so far, following years of preclinical research that showed the potential of the technique. At the moment the photon propagation in brain is mostly driven by scattering; therefore scattered photons reach regions of the brain that should not be stimulated and that also limits the yield. In this context, the PhD goal will be to develop wavefront shaping for a clinical use to restrain scattering to go towards a clinical use.
The technique has been published in 2007 by Vellekoop and consists in acting on the amplitude and phase of a coherent light source to compensate for scattering and therefore focus light in tissue. The final goal is to illuminate exclusively the desired brain region. The PhD work will include experimental and fundamental developments in optics, particularly for the feedback control (photoacoustics signal generated by the light pulse), but also in numerical simulation. The PhD will be located between Clinatec and Optics and Photonics lab of CEA LETI, and directed by LiPhy at UGA.

Study and quantification of the optical properties of nuclear fuels : towards a better understanding of the laser/mater interactions

Electricity production from nuclear sources has gained increased importance today in the fight against climate change and energy independence. Within this general context, this thesis aims to enhance the understanding of the properties and behavior of nuclear fuels under normal operating conditions as well as during hypothetical accident scenarios. To achieve this objective, innovative laser heating techniques are employed. Laser techniques not only enable precise characterization of the properties of irradiated (or fresh) nuclear fuels but also allow them to be subjected to controlled spatial and temporal gradients to study underlying fundamental mechanisms.
Accurate knowledge of the optical properties of materials, which govern the interaction of the laser beam with the considered substrate, is crucial in laser heating experiments. However, data on the optical properties of nuclear fuel are scarce, complex to measure, and often exhibit significantly different values. This thesis aims to make progress in this area, particularly targeting materials of interest. To do so, a combined approach of experimentation and modelling will be employed to quantify the optical properties of nuclear materials from ambient temperature to melting point.
The thesis will be conducted as part of the MATLASE CHAIR and in close collaboration between the LAMIR (Laboratoire d’Analyse de la MIgration des Radioéléments) within the Département d’Etude des Combustibles (IRESNE Institute, CEA-Cadarache)and the ILM team (Interaction Laser Matière) at the Institut Fresnel in Marseille, which will contribute its expertise in the field of high-power laser / material interactions and optical instrumentation for the development of the system and complex optical diagnostics.
This framework will enable the doctoral student to work in a stimulating scientific environment and allow them to disseminate their research both in France and abroad.
The candidate must have a master's degree or an engineering qualification in the field of optics, photonics, laser processes and/or materials.

Development of a lensless microfluidic instrument for in-situ measurement of facies-dependent dissolution kinetics

This thesis is part of an ambitious program designated as a priority research program. This project identifies the subsoil as a major reservoir of resources necessary for the energy transition.
One of the major issues is the dissolution of ores in the context of mining and extractive metallurgy. In particular, with the objective of process industrialization, the dissolution kinetics of ores must be compatible with the footprint of the installations, biocompatibility and the volume of reagents consumed.
The observation today is the very strong mismatch between the volume of experimental data produced and those necessary to model the chemical processes essential to demonstrate the viability of industrial processes.
This thesis proposes to develop a millifluidic prototype bench for mass kinetic data acquisition using lensless imaging techniques. This will make it possible to measure dissolution reaction kinetics using 3D reconstitution techniques, in-situ, under stable chemical conditions and with statistical representativeness allowing the original properties of the solid to be taken into account.
A large part of the research will be directed towards the development of the lensless optical technique in a millifluidic device and the mass production of chemical kinetic data for catalytic dissolution models.
The desired profile is that of a general physics and chemistry student, with a strong desire to learn in areas they are least familiar with, such as microfluidics or optics. At the end of this thesis, the student will acquire solid professional experience in applied research and will learn to evolve in a multithematic environment.

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