Conventional and bound-state soliton mode-locked by an artificial absorber based on SMF–GIMF–SMF structure

We report on the experimental observation of various soliton dynamics within an erbium-doped fiber laser (EDFL) configured with a single-mode fiber–graded-index multimode fiber–single-mode fiber structure, referred to as the SMF–GIMF–SMF structure, which acts as an artificial saturable absorber (SA). Our observations include conventional solitons, as well as both tightly and loosely bound solitons, and loosely bound solitons with fixed time separations. The conventional soliton, centered around 1573[Formula: see text]nm, exhibits an exceptionally low self-start mode-locking threshold pump power of 29[Formula: see text]mW. It has a pulse width of 650[Formula: see text]fs, a signal-to-noise ratio (SNR) of 68[Formula: see text]dB, and a repetition rate of 21.98[Formula: see text]MHz. The tightly and loosely bound solitons observed are centered around 1570[Formula: see text]nm, with a pulse width of 740[Formula: see text]fs. These solitons are characterized by time separations of 12.8[Formula: see text]ps and 1.8[Formula: see text]ps, respectively. Additionally, we observed several loosely bound solitons with a fixed time separation of 20.4[Formula: see text]ps but varying intensity ratios among the bound solitons. These findings contribute to a deeper understanding of the SMF–GIMF–SMF structure’s role as an SA in EDFLs and provide insights into the diverse soliton behaviors it can support.