Spatiotemporal mode-locking in a linear-cavity Er:ZBLAN fiber laser

Spatiotemporal mode-locking (STML) in fiber lasers has attracted significant attention due to its wide scientific and industrial applications, and achieved great progress in the near-infrared wavelength range. Here, we demonstrate both theoretically and experimentally a mid-infrared (MIR) STML laser based on a step-index multimode Er:ZBLAN fiber. The laser adopts a short linear cavity mode-locked by a large-relaxation-time saturable absorber, and generates stable 78.6 ps pulses at 2.79 μm with a repetition rate of 79.8 MHz. Through adjustment of the coupling condition, a wide variety of STML states are observed. Numerical simulations show that the modal dispersion is counteracted by the spatial filtering effect alone. This work provides a simple scheme for MIR fiber lasers to achieve STML, and helps to understand the underlying nonlinear dynamics.