Abstract : The recent development of quasi-singlemode large mode area fibers has led to a quick rise in fiber laser power. Indeed, the large core prevents nonlinear effects and increases damage threshold. In order to further increase the core diameter, we propose an alternative approach using a very large core multimode fiber amplifier followed by a nonlinear beam cleanup interaction to restore a diffraction limited beam profile. Two concepts have been studied:
- in the first one the beam quality is restored by two-wave mixing in a Rh:BaTiO3 photorefractive crystal. An original scheme has been proposed and experimentally validated. This mode converter can change any depolarized, aberrated and coherent laser beam into a diffraction-limited beam,
- the second one is a result of a detailed theoretical and experimental study of stimulated Brillouin backscattering (SBS) in multimode fibers. It is well known that both phase conjugation and beam cleanup behavior are observed by SBS in multimode fibers, but the reasons why each effect occur are not clear. The study defines the conditions in which phase conjugation (short step index fibers) or beam cleanup (gradient index fibers) are more likely to occur. An original geometry has been proposed and experimentally demonstrated. We obtain efficient and highly stable beam cleanup in a multimode gradient index fiber by using a self-aligned loop geometry. Beam cleanup is achieved by efficient reflection of the multimode beam to the fundamental gradient index fiber mode.