Advancements in the Use of Artificial Saturable Absorbers for Modelocking of 2 µm Ultrafast Fiber Lasers

Over the past decade, the research on modelocked 2 $\mu m$ fiber lasers has increased rapidly. Conventionally, modelocking is achieved with the existing quantum well structures as well as 2D materials classified as Real Saturable Absorbers. As time progressed and keeping in mind the versatility, stability and robustness, demonstration of 2 $\mu m$ fiber lasers that are modelocked by Artificial Saturable Absorbers (ASA) gained importance. This class of Saturable Absorbers exhibits interesting properties, which can make them the best candidate to replace real material-based Saturable Absorbers. The progress of ASA based modelocking schemes in the 2 $\mu m$ regime is explored and discussed in detail, along with its underlying physics, citing the various uses and the increasing market for ultrafast fiber lasers. Toward the end of this review, a comparison is drawn between different ASAs based modelocking schemes to get the most favorable conditions for desired output parameters as per one's needs. These fiber lasers with an all-fiber ASA foundation have potential uses in the fields of medical, LIDAR, mid-IR generation, environmental sensing, industrial, and defence.