High power multimode fiber amplifier with wavefront reshaping for high beam quality recovery

Abstract

The increase in fiber laser and amplifier output power is limited by nonlinear effects and material damage due to the high power density, especially in pulsed operation. Output powers of several hundreds watts have recently been achieved with large-mode-area quasi-singlemode fibers. To further increase the core diameter, an original alternative approach consists in correcting the beam profile after a multimode fiber amplifier by a nonlinear beam cleanup method. We report the conversion of a multimode and depolarized output beam of a highly multimode diode-pumped Yb-doped fiber amplifier to a diffraction limited, linearly polarized beam by two original beam cleanup methods: • the first converts the multimode beam to a singlemode beam using self-referencing two wave mixing process in an infrared sensitive photorefractive crystal (Rh:BaTiO 3). With this setup, up to 11.6 W singlemode output is achieved with a 78% multimode to singlemode photorefractive conversion efficiency, • the second converts a multimode beam into a singlemode beam by use of the Brillouin effect in a multimode gradient-index (GI) fiber with a self-aligned loop geometry. A preliminary conversion from a M 2 = 6.5 beam into a M 2 = 1.3 beam with 31% efficiency is reported. To cite this article: L. Lombard et al., C. R. Physique 7 (2006).