Thesis of Quentin Demazeux

Defense of thesis Quentin Demazeux - Laboratory PhLAM

Abstract :

The routine operation and investigation of modern accelerator-based light sources, such as Free-Electron Lasers (FELs), demand high-fidelity, single-shot diagnostics capable of operating at MHz repetition rates. Accurate, non-destructive characterization of the longitudinal electron bunch shape or the resulting terahertz (THz) radiation is essential for optimizing these complex facilities. While recent advancements in Electro-Optic Spectral Decoding (EOSD) have successfully achieved breakthroughs in temporal reso-
lution with the Diversity Electro-Optic Sampling (DEOS) method and MHz acquisition rates with Photonic Time-Stretch (PTS), key challenges remain, particularly regarding signal reconstruction fidelity and readouts bandwidth limitations. This thesis addresses these shortcomings by overcoming the fidelity loss due to non-linear dispersion on chirped probe laser pulse for broadband THz, as well as the readout bandwidth constraints for the characterization of high-frequency narrowband THz.
For the characterization of broadband THz pulses, the fidelity limitation caused by the non-linear dispersion of the chirped probe laser is overcome by a novel Self-Adaptive Dispersion (SAD) algorithm developed in this thesis. The SAD algorithm accurately reconstructs the full THz signal while simultaneously characterizing the complex amplitude (amplitude, phase, and precise dispersion) of the chirped probe laser. In doing so, it enables a faithful reconstruction of the broadband THz signal over long temporal windows. The robustness and the high-fidelity performance of the DEOS-SAD system are demonstrated through routine, single-shot, MHz-rate diagnostics of the electron bunch shape at the European XFEL and FLASH facilities. For the characterization of high-frequency narrowband THz pulses, the acquisition bandwidth constraint of conventional electro-optic detection systems is overcome by introducing a novel heterodyne electro-optic detection technique combined with Photonic Time-Stretch. This method reduces the necessary readout bandwidth while preserving signal integrity. Its feasibility was demonstrated at the FELBE facility, where it enabled the single-shot diagnostics of the Carrier-Envelope Phase (CEP) of the FEL output using a low-bandwidth acquisition system.

Keywords : Accelerator, Photonics, Ultrafast, Free-Electron Laser, Terahertz, single-shot