Nonlinear beam self-cleaning in a coupled cavity composite laser based on multimode fiber

We study a coupled cavity laser configuration where a passively Q-switched
Nd:YAG microchip laser is combined with an extended cavity, including a doped multimode
fiber. For appropriate coupling levels with the extended cavity, we observed that beam selfcleaning
was induced in the multimode fiber thanks to nonlinear modal coupling, leading to a
quasi-single mode laser output. In the regime of beam self-cleaning, laser pulse duration was
reduced from 525 to 225 ps. We also observed a Q-switched mode-locked operation, where
spatial self-cleaning was accompanied by far-detuned nonlinear frequency conversion in the
active multimode fiber.