Formation and evolution of filtered solitons in figure-9 mode-locked fiber lasers

An erbium-doped filtered mode-locked fiber laser is demonstrated in this work. Mode-locked operation was achieved using the nonlinear amplifying loop mirror (NALM) technique incorporating a filtering effect. The proposed fiber laser employs a 1.8 m polarization-maintaining fiber (PMF) in the NALM loop, yielding a free spectral range (FSR) of 3.6 nm. The high birefringence of the PMF was utilized for optical filtering to construct the fiber laser. Stable filtered soliton pulses were obtained with a central wavelength of 1566 nm. Upon increasing the pump power to 304 mW and precisely adjusting the polarization controller, filter-induced noise-like solitons centered at 1562.6 nm were generated. This passively mode-locked fiber laser features a simple structure and stable performance, enabling switchable operation between two filtered soliton states, and thus holds significant application potential in fields such as all-optical communication.