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 $-0.154{\mathrm{ps}}^2$. The compression ratio can be also improved by strengthening the SPM at this stage.

Figure 1

(a) A photograph of the electrochemical exfoliation process and SEM images of graphene nanosheets obtained at dc voltages of $+3$ and $+6\mathrm{V}$. (b) The Raman scattering spectra. (c) Nonlinear transmittances. (d) Normalized absorbance of graphene nanosheets obtained at dc voltages of $+3$ and $+6\mathrm{V}$.

Figure 2

(a) The configuration of the passively-mode-locked EDFL with a graphene nanosheet. (b) The simulated pulse width and compression ratio of the passively-mode-locked EDFL versus cavity GDD with the given parameters. (SA represents saturable absorber.)

Figure 3

The autocorrelation traces and optical spectra of the passively-mode-locked EDFLs at different pumping powers with graphene nanosheets obtained by using the dc voltages of $+3$ and $+6\mathrm{V}$.

Figure 4

(a) The variations of pulse width and (b) spectral linewidth of the passively-mode-locked EDFLs at different pumping powers with the graphene nanosheets obtained by using the dc voltages of $+3$ and $+6\mathrm{V}$.

Figure 5

The shift of the Kelly-sideband peak in the pulse spectrum of the passively-mode-locked EDFL with a graphene nanosheet obtained by $+3\mathrm{V}$ under different pumping power.

Figure 6

(a) The optical spectra of the passively-mode-locked EDFLs with and without the graphene nanosheet in the wavelength domain. (b) The optical spectra of the passively-mode-locked EDFLs with and without the graphene nanosheet in the frequency domain.

Figure 7

(a) The experimental and simulated pulse-width variation versus different GDD by changing the SMF length. (b) The simulated compression ratio $\tau :{\tau }_0$ versus different GDD under the altering ${\delta }_n$ coefficients.