Structurally determinable optical soliton molecules enabled by a pre-designed microfiber resonator in a passively mode-locked fiber laser

The temporal structure of an optical soliton molecule (SM) produced in a passively mode-locked fiber laser relies directly on phase relations among the comprised solitons. However, such phase relations are typically determined by many global and local parameters of the fiber cavity. Thus, any parameter disturbance would exert complex distortion on the SM. This is always unpredictable and difficult to manipulate. One factor is the difficulty in decid-ing what parameter options have enabled the SM's formation. The second is the coupling effect among the related parameters. Consequently, the produced SMs usually exhibit considerable uncertainty and poor stability. How to obtain pre-defined SMs has long been a sought-after yet technically unsolved issue. Herein, as a preliminary investigation we demonstrate that employing a microfiber knot resonator (MKR) in a mode-locked fiber laser can enable the formation of artificially defined and structure-stabilized SMs. Specifically, the MKR enables the single soliton splitting into several ones through enhancing the local nonlinearity. But more importantly, it meanwhile functions as a notch filter that dominates and tailors the spectral evolution. The tailored spectrum is then mapped to the temporal domain, grouping the randomly split solitons into a structured and stabilized SM. Our results suggest an easy-to-access avenue in producing structure-determinable SMs in fiber lasers. (c) 2022 Optica Publishing Group