Spatial beam cleaning in mode-locked multimode fiber lasers

We propose and demonstrate the use of multimode fibers (MMF) inside a laser cavity as a new path to generate spatiotemporal modelocked pulses with high beam quality and high energy. Prior to our work, MMFs in optical cavities resulted in the generation of low-quality output beam profiles by spatiotemporal mode-locking. Here we present a versatile approach to reach high energy per pulse directly in the mode-locked MMF oscillator with a near single-mode output beam profile. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect inside the cavity achieved by controlling spatiotemporal pulse propagation with a dispersion-managed design. We demonstrate the versatility of our approach with Yb-doped and Er-doped multimode laser cavities which generate pulse energies of 24 nJ and 16 nJ, respectively. The high peak power reached in the MMF within the cavity induced a Kerr self-beam cleaning which produced a near Gaussian mode output (M2<1.13).