Observation of periodic optical spectra and soliton molecules in a novel passively mode-locked fiber laser

Due to the necessity of making a series of random adjustments after mode-locking in most experiments for preparing soliton molecules, the repeatability of the preparations remains a challenge. Here, we introduce a novel all-polarization-maintaining erbium-doped fiber laser that utilizes a nonlinear amplifying loop mirror for mode-locking and features a linear shape. This laser can stably output soliton molecules without any additional adjustment once the mode-locking self-starts. Moreover, it can achieve the transition from soliton molecule state to soliton state, and then to multi-pulse state by reducing the pumping power. The unconventional method of generating multi-pulses, combined with a wide pumping power range of 200--640 mW for maintaining mode-locking, allowed us to observe periodic optical spectra with two complete cycles for the first time. Based on the experimental facts, we develop a multistability model to explain this phenomenon. With its ability to switch between three stable states, this flexible laser can serve as a versatile toolbox for studying soliton dynamics.