Sub-100 fs Bound State Solitons and Period-Doubling Bifurcations in a Mode-Locked Fiber Laser

We investigate on bound-state mode-locking in the close-to-zero dispersion regime of an Er-fiber laser. A two-soliton bound state with 82 fs pulse duration and 0.58 ps pulse separation is achieved. Compound 2+1 type bound state solitons with ~100 fs pulse duration has also been observed. Both the double-pulse and the 2+1 type bound states could experience period-doubling bifurcations and exhibit the pumping hysteresis characteristics as the single pulse. To the best of our knowledge, this work provides the first observation of sub-100 fs bound solitons in a fiber laser. Such ultra-short bound state solitons show potential applications of multilevel coding unit for high-capacity telecommunication systems.