Carbon-nanotube-based passively mode-locked fiber lasers modulated with sub-loop

We have proposed a carbon-nanotube-based passively mode-locked fiber laser, which consists of two fiber ring resonators, for the first time to author's best knowledge. The proposed laser in the main cavity is coupled with a fiber sub-loop, which is connected with a 2 × 2 optical coupler. Without the sub-loop, uniform-spaced conventional solitons are achieved in the fiber ring laser. With the sub-loop, the fiber laser generates a train of pulses modulated with a periodic envelop. The modulation period and pulse-pulse separation are ∼160 and ∼4.3 ns, which are determined by the lengths of the main cavity and the sub-loop, respectively.