Characteristics of ultrafast passively mode-locking soliton fiber laser utilizing higher-order mode fibers

For a large range of enthralling applications in telecommunications, spectroscopy and material processing, soliton fiber lasers have been intensively explored as compact light sources of generating ultrashort pulses with stable peak power, owing to the balance between optical Kerr effect and cavity dispersion. Meanwhile, to fulfill the ever-increasing capacity demand for high-bit-rate fiber-optic communication systems, space division multiplexing technique has attracted much attention using few-mode fibers (FMF). Nonetheless, to achieve high output energy in present ultrafast fiber lasers, key challenges include low repetition rate and possible pulse breaking due to excessive nonlinear phase accumulation. To resolve aforementioned issues, multiple-pulsing or bound solitons with discrete fixed pulse separations have been purposed in passively mode-locked fiber lasers, whereas random phase and inter-pulse interference would impair the laser performance. In this work, we systematically investigate the formation and dynamics of synchronized soliton pulses in multiple spatial modes operating in large intermodal dispersion by solving the complex cubic–quintic Ginzburg–Landau equation. For a mode-locked laser cavity design at 1560 nm with maximum output power of 150 mW utilizing a FMF, whose large mode area can reduce nonlinearities caused by high-peak power intensities, the loop length of FMF laser cavity is 5-meter-long with 2000-ppm erbium-doped concentration. The FMF has a triple cladding index profile supporting more than two linearly polarized modes. Our results show that due to soliton fission by intermodal nonlinear coupling, energy is transferred from higher order modes to fundamental mode to stabilize mode-locking and realize high-repetition-rate fiber laser.