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Monday 2nd of september 2019, 2:00 pm, at ENSSAT

Antoine CONGAR defends his PhD thesis

« Spectral narrowing and intensity noise of coherent light sources based on gallium nitride laser diodes emitting in the blue range »

Single-mode InGaN/GaN laser diodes ; aperiodic grating ; fiber Bragg grating ; intensity noise


Many applications require coherent visible light sources, in particular in the blue range. Solutions commercially available are mostly expensive and not compact. In this thesis we study the possibility of creating blue single-mode light source based on InGaN/GaN laser diode which could be a low-cost device, easily integrated into photonic systems. The first work is the study of RIN of gallium nitride edge emitting laser diodes. The strong correlation between noise dynamics and spectral behavior impacted by mode clustering is highlighted. An analytical model is used to reproduce the experimental results. The second work is a study of blue/purple (400-450 nm) single- longitudinal-mode LD. Spectral filtering is achieved using high order aperiodic Bragg grating. A novel approach involving standard lithography technique with a resolution close to the micron is proposed unlike state of the art works using high resolution e-beam lithography. Release of the resolution constraint paves the way for mass production of low-cost single-mode devices. Aperiodic grating, mostly used in the telecom range, is adapted to 420 nm using numerical modelling. First single-mode structures are fabricated at EPFL. Measurements of optically pumped devices prove the feasibility of the technique. The last contribution of this thesis, concerns the spectral narrowing of InGaN LD by means of external cavity. Feedback is provided by a specially designed 400 nm Bragg grating optically written on a single-mode fiber. Using that setup, we obtain single-frequency emission with more than 40 dB side-mode suppression ratio, about 50 mW optical power and less than 2.4 MHz linewidth. Results are encouraging in both approaches, monolithic and external feedback, and offer various possibilities in terms of applications.


Committee members
Isabelle SAGNES Director of research, C2N, CNRSExaminator
Jean-Yves DUBOZ Director of research, CRHEA, CNRSExaminator
Dmitri BOÏKO Expert Senior, CSEM, NeuchâtelMember
Raphael BUTTÉ Lecturer & Researcher, EPFL, LausanneMember
Thierry GEORGES Engineer PhD, Oxxius, LannionMember
Monique THUAL Professor, Institut Foton, Université de Rennes Member
Stéphane TREBAOL Assistant Professor, Institut Foton, Université de Rennes PhD Manager
Pascal BESNARD Professor, Institut Foton, Université de RennesPhD Supervisor