Mesoporous carbon nanospheres deposited onto D-shaped fibers for femtosecond pulse generation.

We demonstrate nonlinear optical modulation by combining mesoporous carbon nanospheres (MCNs) and D-shaped fibers (DFs). The MCNs are prepared by the silica-assisted strategy and the DFs are fabricated through a precision wheel polishing method. When the MCNs are deposited onto the DF as the saturable absorbers (SAs), the SAs possess broadband linear absorption and nonlinear saturable absorption properties. As the DF-MCNs SA is integrated into the laser cavities, ultrafast lasers at 1.56 and 2 μm were realized with minimal pulse duration down to a few hundreds of femtoseconds. Compared with the film and microfiber-based MCNs-SAs, the DF-MCNs SAs exhibit greater robustness and stronger evanescent field, and are more effective at generating femtosecond pulses. Our results verify that DF-MCNs as a kind of cost-effective and easily-prepared SA would be of great importance for stable and high-power femtosecond fiber lasers.