A femtosecond soliton erbium-doped fiber ring laser with the symmetrical saturable absorber (SA) of graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) is proposed and proved. Based on the nonlinear multimode interference (NL-MMI) effect of SA, the fiber laser achieved femtosecond soliton mode-locking operation and repetition rate tuning. The duration of soliton was 364 fs at 1562.5 nm and repetition rate was tunable from 10.29 MHz to 763.36 MHz. In the structure of SA, a short step index multimode fiber (SIMF) was inserted between two graded index multimode fibers (GIMFs) to generate more high-order modes, adjust the self-focusing length, and improve operation stability of mode-locked fiber laser. The SA also has the advantages of immunity to the external environment variation, high damage threshold and inoxidizability, which make the laser operate stably for a long time.

中文翻译：

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具有对称 GIMF-SIMF-GIMF 可饱和吸收器的飞秒孤子掺铒光纤激光器

提出并证明了一种具有渐变折射率多模光纤阶跃多模光纤渐变折射率多模光纤对称饱和吸收体（SA）的飞秒孤子掺铒光纤环形激光器（GIMF-SIMF-GIMF）。基于SA的非线性多模干涉（NL-MMI）效应，光纤激光器实现了飞秒孤子锁模操作和重复率调谐。孤子的持续时间为 364 fs，波长为 1562.5 nm，重复频率可在 10.29 MHz 至 763.36 MHz 之间调节。在SA结构中，在两根渐变折射率多模光纤（GIMF）之间插入短阶跃折射率多模光纤（SIMF），以产生更多的高阶模，调整自聚焦长度，提高锁模光纤的工作稳定性激光。SA还具有不受外界环境变化影响的优点，