In-Situ Observation of Beam Aggregation in a Ho-Doped Figure-9 Spatiotemporal Mode-Locked Laser

Spatiotemporal mode-locking has attracted widespread attention in recent years, because it provides an excellent platform to explore the intriguing transverse mode dynamics, an area that has not been extensively studied both theoretically or experimentally. Mode-locking induced beam aggregation is a most relevant phenomenon, as it may reveal how the spatial and temporal degrees of freedom interplay to meet the mode-locking conditions in a realistic setting. However, report on observation of this phenomenon has been scarce, and it is not yet possible to provide detailed analysis of such an effect. In this work, we investigate mode-locking induced beam aggregation in a Ho-doped Figure-9 spatiotemporal mode-locked laser. The laser is configured in a way so that the intra-cavity multimode beam profile can be characterized in an in-situ manner. We show unambiguously that it is essential to use the in-situ method, as multimode fiber output couplers may generate two beams with completely different transverse modes, therefore potentially leading to misleading interpretation of intra-cavity mode distribution. A number of beam aggregation snapshots are successfully obtained, each corresponding to a different intra-cavity mode excitation, as controlled by offset splicing. Also, our laser is the first demonstration of spatiotemporal mode-locked laser in the Ho-doped fiber band ( $\sim 2.1~\mu $ m). It is believed that the results reported here enrich the observations of mode-locking induced beam aggregation, and point to the importance of in-situ beam profile characterization in interpreting dynamics of spatiotemporal mode-locking.