Fig. 1. Diagram of MMI structure.

Fig. 2. Experimental setup of the nonlinear MMI mode-locked fiber laser. WDM, wavelength division multiplexer; PC, polarization controller; DCF, dispersion compensation fiber; OSA, optical spectrum analyzer.

Fig. 3. Optical spectra of mode-locked fiber lasers with different lengths of MMF in MMI structures.

Fig. 4. (a) Experimental setup for the absorption measurement of the MMI structure. ATT, attenuator. (b) Measured nonlinear absorption of MMI structure with L=48.8  mm.

Fig. 5. (a) Output optical spectra of fiber laser for CW without MMI and mode-locked with MMI for underlying values. (b) Mode-locked pulse shape (experimental data) with sech2 fit.

Fig. 6. (a) Typical laser output pulse train. (b) Radio frequency spectrum measured around the fundamental repetition rate. Inset is the radio frequency spectrum with high-order harmonic of the repetition rate.

Fig. 7. Optical spectra of mode-locked fiber lasers for adding different lengths of SMF or DCF.