Switchable spectral sidebands based on modal interference effect in an Er-doped fiber laser

Recently, the nonlinear multimodal interference-based all fiber saturable absorber has been the focus of attention on ultrafast fiber lasers, owing to its intriguing properties of versatility, high damage threshold and instantaneous response time. Although, challenges present in the technology, such as complex perturbation induced by quasi-degenerate modes in multimode fiber, it is presented as an effective solution to control the output characterization and study the nonlinear dynamics in fiber lasers. In this work, we experimentally and numerically demonstrate the spectral sidebands in a passively Er-doped fiber laser based on multimodal interference technique. Kelly-type and triangular-type sidebands are achieved, and can be switchable by changing the polarization states of cavity, which are asymmetric distribution on both sides of the output spectrum. When the polarization states are varied, a wide sideband is obtained, which the width of sideband can be tuned from 0.13 nm to 2.3 nm. Coupled complex Ginzburg-Landau equation are provided to reveal the underlying principles of the tunable features in sidebands. The results of numerical simulation show the relevance between filtering induced by modal interference, high-order dispersion, polarization modal dispersion and experimental results. Our work lays a foundation for understanding of nonlinear dynamics in mode locking fiber lasers based on multimodal interference effect and provides a new way to generating versatile ultrafast source in engineering and scientific research.