The formation of stable single solitons and various bound states based on a graded index multimode fiber (GIMF) is investigated experimentally for the first time. Taking advantage of the principle of nonlinear multimode interference, a 143.5 cm length of GIMF is bent and two polarization controllers are used to achieve stable passive mode-locking. The spectrum of single solitons centers on 1590.5 nm with a spectral bandwidth of 3.2 nm. 976 fs of pulse duration is obtained at the repetition rate of 14.31 MHz. The maximum output power of 14.572 mW with a corresponding single-pulse energy of 1 nJ is achieved. Meanwhile, all kinds of bound states, including soliton pairs, soliton triplets, soliton quartets, and multiple-pulse bound states with different pulse separations are realized. The bound soliton with two pulses obtained has a fixed pulse interval as the incident pump power is increased. Our results suggest that a GIMF might be employed as an effective saturable absorber for ultrafast photonics applications.

中文翻译：

---

基于渐变折射率多模光纤可饱和吸收体的各种束缚态观测

首次通过实验研究了基于渐变折射率多模光纤 (GIMF) 的稳定单孤子和各种束缚态的形成。利用非线性多模干涉原理，弯曲143.5 cm长的GIMF，并使用两个偏振控制器实现稳定的无源锁模。单个孤子的光谱中心在 1590.5 nm，光谱带宽为 3.2 nm。以 14.31 MHz 的重复频率获得 976 fs 的脉冲持续时间。实现了 14.572 mW 的最大输出功率，相应的单脉冲能量为 1 nJ。同时，实现了各种束缚态，包括孤子对、孤子三重态、孤子四重态和不同脉冲间隔的多脉冲束缚态。随着入射泵浦功率的增加，获得的具有两个脉冲的束缚孤子具有固定的脉冲间隔。我们的结果表明，GIMF 可用作超快光子学应用的有效饱和吸收体。