Thesis defense of Qin LIU

Thesis defense of Qin LIU, PhD student in the Lasers group of the Laboratoire Charles Fabry, on 10 July 2024 at 2:00pm in the Auditorium of the Institut d'Optique Graduate School in Palaiseau, on the topic: " Coherent combination of semiconductor amplifiers: applications to atmospheric lidar and acousto-optic imaging ".

Abstract: " Diode lasers are compact, robust, and efficient which are in demand in the commercial market. However, the output power of individual diode devices are limited by the material damage threshold. Coherent beam combination, based on the constructive interference between coherent beams, is a power scaling technique that can improve the laser power while maintaining the beam quality of the individual combined emitters. This work targets the development of compact diode-laser-based setups based on coherent beam combination in QCW regime and the demonstration of their applications in atmospheric microlidar and acousto-optic imaging. The laser source is arranged in a master-oscillator power-amplifier configuration with a single-frequency distributed Bragg reflector laser at 828 nm injecting two commercial pulse-driven high-brightness tapered semiconductor optical amplifiers. The phase difference between the two beams is actively corrected to maintain long-term phase stabilization. Due to the delay of the project in collaboration with the Laboratoire des Sciences du Climat et de l'Environnement (LSCE), the demonstration of the laser source on lidar applications is conducted on an atmospheric microlidar designed by Cimel Electronique. A simplified version of the laser source based on the polarization beam combination of two tapered amplifiers (1 µs-10 kHz) is constructed to be compatible with the microlidar. The transmitted peak power reaches 6.1 W. The water vapor profile is successfully retrieved in good agreement with the radiosonde measurement. Another laser source based on coherent beam combination of two 100 µs-100 Hz tapered amplifiers with a maximal combined peak power of 8.9 W is applied to the Fourier Transform acousto-optic (FT-AOI) imaging setup in Institut Langevin. The first experimental acousto-optic images of two absorbing ink inclusions within a hydrogel are obtained with good signal-to-noise ratio. "