Abstract

Single-mode 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 (V. Dominic et al., Electron. Lett. 35, 1158, 1999). However, a further increase of the power is limited by nonlinear effects and by material damage arising from the huge power density carried along the core of the fiber. 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 since the propagation is now multimode. In the present paper, we propose to convert the spatially multimode beam which is emitted by a large core Yb fiber amplifier into a single mode beam by using a photorefractive two-wave mixing process in a IR sensitive Rh :BaTiO₃ crystal. In order to demonstrate the interest of the technique, we will present a theoretical model that permits us to calculate the efficiency of the laser by taking into account the photorefractive gain, the amplification of the fiber and the gain saturation. Influence of the main parameters such as the input power of the master oscillator, the core diameter and the beam ratios will be emphasized. We will show that output powers from 100 W to 1 kW into a diffraction- limited beam are theoretically achievable. This technique is also experimentally demonstrated in the low-power regime with an Yb-doped 20pm core diameter multimode fiber amplifier. Qualitative results of multimode to single-mode conversion are presented. This approach may open new perspectives for compact fiber laser sources delivering high power with beam quality close to the diffraction limit for industrial applications.

© 2001 Optical Society of America