Dark Rectangular Pulses From a Dumbbell-Shaped Mode-Locked Double-Clad Er:Yb Laser

The formation of dark rectangular pulses from a dumbbell-shaped mode-locked double-clad erbium-ytterbium fiber laser is experimentally demonstrated. The laser operates under a large net-anomalous cavity dispersion regime, with a passive mode locking mechanism that relies on a nonlinear amplifying loop mirror (NALM) along with a nonlinear optical loop mirror (NOLM). This design facilitates the control of nonlinear effects within the cavity and enables the generation of dark pulses as the dominant mode locking regime. The variations of dark-rectangular pulse emission and its spectral evolution are experimentally investigated as a function of the pump power, revealing a dependence analogous to bright dissipative soliton resonance (DSR) pulses, where the dark pulse duration increases with increasing pump power, a behavior that is opposite to previously reported dark rectangular pulses. At the maximum pump power of 9.5 W, the mode-locked fiber laser generates dark rectangular-shaped pulses with duration of 571 ns, average output power of 600 mW, fundamental repetition rate of 320 kHz.