Beam cleanup of a multimode Yb-doped fiber amplifier with an infrared sensitive Rh:BaTiO/sub 3/ crystal

Summary form only given. Single-mode, Er or Yb doped fiber lasers and amplifiers have made great progress in the last few years since output power exceeding 100 W have been demonstrated with a nearly diffraction limited beam quality. However, a further increase of the power is limited by nonlinear effects and by material damage. To limit the contribution of these mechanisms, it is proposed to increase the diameter of the fiber core. In such conditions the power density in the core is largely reduced but this is achieved to the detriment of the beam quality. Recently a significant effort has been devoted to the research of new techniques that allow to maintain a single-transverse mode propagation in large core fiber amplifiers. We propose and experimentally validate an original approach. The spatially multimode beam which is emitted by a large core Yb fiber amplifier is converted into single mode beam by means of the beam cleanup effect by photorefractive two-wave mixing (TWM).