Pulse-Width Saturation and Kelly-Sideband Shift in a Graphene-Nanosheet Mode-Locked Fiber Laser with Weak Negative Dispersion

The optimized soliton mode locking of the erbium-doped fiber laser (EDFL) and its pulse-shortening dynamic with the graphene nanosheet is demonstrated by precisely detuning the weakly negative group-delay dispersion (GDD) and maintaining strong self-phase-modulation (SPM), to obtain the shortest pulse width of 449 fs with a spectral linewidth of 6.02 nm. The pulse evolution with the mode-locking mechanism changing from the self-amplitude-modulation of the saturable absorber, to the soliton compression caused by the GDD and SPM is experimentally and numerically investigated in detail. Under high pumping powers, the enlarged up-chirp inside the EDFL cavity can induce a significant Kelly-sideband shift of up to 0.5 nm. The passively-mode-locked EDFL pulse width is controllable by detuning the GDD and SPM parameters, so that the pulse width can be compressed from 642 to 449 fs while reducing the negative GDD from -0.354 to -0.154 ps(2). The compression ratio can be also improved by strengthening the SPM at this stage.