We experimentally investigate characteristics of saturable absorbers (SAs) based on nonlinear Kerr beam cleanup effect (NL-KBC). The SAs are formed by a long graded-index multimode fiber (GRIN MMF) with a short single-mode fiber served as a diaphragm. We studied the evolution of output spectrum and beam profiles from the GRIN MMF in order to investigate the mechanism of these SAs. We further performed saturable absorption measurements to evaluate their modulation depths and saturation intensities. We experimentally observed and first theoretically analyzed the “relaxation oscillation” behavior of the optical transmittance with increasing input intensity. We also studied their nonlinear polarization dynamics and observed the repolarized effect in NL-KBC regime. Our results confirm the optical properties of the SAs based on NL-KBC, and these SAs can find applications in Q-switched and mode-locked lasers.

中文翻译：

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基于非线性克尔束净化效应的可饱和吸收体研究

我们基于非线性Kerr束清除效应（NL-KBC），通过实验研究了可饱和吸收体（SAs）的特性。SA由长渐变折射率多模光纤（GRIN MMF）和短的单模光纤作为光阑形成。为了研究这些SA的机制，我们研究了GRIN MMF的输出光谱和光束轮廓的演变。我们进一步进行了饱和吸收测量，以评估其调制深度和饱和强度。我们通过实验观察并首先从理论上分析了随着输入强度的增加，光透射率的“松弛振荡”行为。我们还研究了它们的非线性极化动力学，并观察了NL-KBC体系中的复极化效应。我们的结果证实了基于NL-KBC的SA的光学特性，