Optimal design of ultrabroadband self-similar pulse in fiber Mamyshev oscillators

The Mamyshev oscillator (MO) is a nonlinear system that achieves mode-locking through the spectral broadening caused by self-phase modulation and the spectral filtering effect. However, excess nonlinear accumulation in MO will lead to pulse splitting, which limits the generation of high-energy pulses and broadband spectra. Therefore, how to design an all-fiber MO so that it can not only achieve mode-locking, but also tolerate strong nonlinear to generate broad-spectrum optical pulses is important, but has not been reported. Here, combining the perfect saturable absorption effect provided by the MO and the self-similar pulse (SSP) that can tolerate strong nonlinearity without wave breaking, we construct a theoretical model to investigate the ultrabroadband SSP that breaks through the gain bandwidth limit in a 2-µm MO. Through systematically analyzing the effects of fiber length, filter bandwidth, filter separation, and gain saturation energy on the spectral width of the output pulse, using the optimal parameters, we obtained a 3-dB ultra-broad spectrum of 290 nm, which is the widest bandwidth achieved for single pulse obtained in thulium-doped fiber lasers. Benefiting from the linear chirp characteristics of the SSPs, a dechirped pulse of 37.54 fs ( \(\sim \) $\sim$ 6 cycles) has been achieved. This work provides theoretical guidance for experimentally obtaining ultrabroadband, few-cycle optical pulses in fiber lasers.