Robust mode-locking in all-fiber ultrafast laser by nanocavity of two-dimensional heterostructure

The fiber-based saturable absorber (SA) that enables mode-locking within a ring cavity serves as the core component of the ultrafast all-fiber lasers. However, the integration of SAs onto fibers with high compactness suffers from imbalanced saturable absorption properties and unstable mode-locking performance. Here, we present a robust mode-locking SA by integrating a nanocavity composed of a two-dimensional graphene heterostructure on the fiber end facet. We demonstrate a significant reduction in the saturation intensity (~65%) and improved soliton dynamic processes through precise modulation of the optical field within the heterostructure. The designed heterostructure facilitates the formation of a stable single-soliton state for robust mode-locking. A high tolerance to intracavity polarization variations is achieved in the heterostructure-SA (~85% compared to 20% for bare graphene). Our designed heterostructure-SA represents an important advancement in the development of ultracompact mode-locked all-fiber lasers, offering enhanced integrability and stability.