Spatiotemporal mode-locking (STML) has become an emerging approach to realize organized wavepackets in high-dimensional nonlinear photonic systems. Mode-locking in one dimensional systems employs a saturable absorber to resist fluctuations in the temporal domain. Analogous suppression of fluctuations in the space-time domains to retain a consistent output should also exist for STML. However, experimental evidence of such a resistance remains elusive, to our knowledge. Here, we report experimental observation of such a spatiotemporal stabilizer in STML, by embedding a spatial light modulator (SLM) into a multi-mode fibre (MMF) laser. Mode decomposition reveals the mode content remains steady for an STML state when applying phase perturbations on the SLM. Conversely, the mode content changes significantly for a non-STML lasing state. Numerical simulations confirm our observation and show that spatial filtering and saturable absorber mainly contribute to the observed stability. The capability to resist the spatial phase fluctuations is observed to depend on the intracavity pulse energy as well as the modal pulse energy condensed in the low-order modes. Our work constitutes another building block for the concept of STML in multi-mode photonic systems.
