Tunable condensed phase with a h-shaped envelop in a mode-locked fiber laser based on nonlinear multimodal interference technique

Abstract

We have demonstrated a partial mode-locked Er-doped fiber laser with nonlinear multimodal interference technique, which generates a tunable condensed phase with numerous unresolved aggregated solitons. The condensed phase has a h-shaped envelop with sharp leading edge and low amplitude trailing edge. The maximum span of a single envelop of condensed phase reaches 115 ns, and is beyond half of cavity roundtrip time (200 ns). Changing polarization states in cavity, the span varies from 115 ns to 24 ns, with the disappearance of condensed state of solitons, and the h-shape pulse grows. Benefiting from the quasi-degenerate modes existing in multimode fiber, the perturbation by mode noise in multimode fiber causes partial mode-locked operation in our experiments and results into the generation of condensed phase. The feature of saturated absorption and reverse saturated absorption of nonlinear modulator shapes the h-shaped envelop. The presented fiber laser is a promising tool to deep insight into complex nonlinear dynamics and laser physics in fiber lasers.