Mode-locked fiber lasers with switchable soliton states have attracted wide attention due to their flexible and controllable output pulse characteristics, which can simultaneously meet the requirements of multiple practical applications such as optical communication, optical sensing, precision machining, and biomedical imaging. Bright and dark solitons are two typical soliton types in mode-locked lasers. Interaction between different solitons can also generate bright-dark soliton pairs in which bright and dark solitons coexist. To obtain bright or dark soliton pulse outputs, a saturable absorption mechanism is introduced into the laser cavity to achieve mode-locking. Among these, the novel all-fiber saturable absorber based on nonlinear multimode interference is one of the ideal solutions for stable mode-locked pulse output due to its advantages including simple fabrication, low cost, and high damage threshold. Meanwhile, the inherent interference filtering effect enhances cross-coupling between pulses of different wavelengths, enabling the regulation of bright-dark soliton states. However, existing mode-locked fiber lasers based on nonlinear multimode interference typically output a single soliton state—either bright or dark solitons—while reports on lasers capable of flexibly switching among multiple mode-locked states, such as bright solitons, bright-dark soliton pairs, and dark solitons, remain scarce. Therefore, research on mode-locked fiber lasers with switchable soliton states holds significant theoretical importance and practical application value.

A saturable absorber with a single-mode-multimode-single-mode fiber structure was fabricated by splicing a gradient-index multimode fiber with two single-mode fiber segments. Using a 50 cm-long erbium-doped fiber as the gain medium, a ring-cavity passively mode-locked erbium-doped fiber laser was designed and constructed. When the pump power was increased beyond the mode-locking threshold, stable bright soliton mode-locked pulses were achieved by fine-tuning the intracavity polarization controller. Under the combined action of the pulse shaping mechanism and spectral filtering effect in the single-mode-multimode-single-mode fiber structure, flexible switching among multiple mode-locked states, including bright solitons, bright-dark soliton pairs, and dark solitons, was realized by further adjusting the polarization controller to modulate the intracavity pulse polarization state and interaction strength.

The results showed that the mode-locked laser exhibited a unique dual-wavelength structure (Fig.3), a repetition rate of 67.29 MHz (Fig.3), and a mode-locked pulse signal-to-noise ratio up to 70 dB (Fig.4), demonstrating excellent stability. The bright soliton pulse width was 3.55 ns (Fig.3), and the dark soliton pulse width was 2.96 ns (Fig.4). For the bright-dark soliton pair, the bright and dark soliton pulse widths were 1.66 ns and 3.09 ns, respectively (Fig.5). When the pump power was increased beyond the mode-locking threshold of 101 mW, flexible switching among the three mode-locking states—bright soliton, bright-dark soliton pair, and dark soliton—could be easily achieved by adjusting the polarization controller to modify the intracavity pulse polarization state and loss. At the maximum pump power of 302 mW, the output powers of dark solitons, bright solitons, and bright-dark soliton pairs were 11.49 mW, 11.32 mW, and 11.06 mW, respectively. Through linear fitting, the slope efficiencies of the mode-locked laser under different soliton states were calculated as 4.12%, 4.04%, and 3.97%, respectively (Fig.6).

The findings demonstrate the capability of the single mode fiber–graded index multimode fiber–single mode fiber structure to generate different solitons. The designed laser has a compact structure, low cost, high damage threshold, and flexibly controllable pulse states. It holds broad application prospects in fields such as intelligent optical communication, quantum physics, and precision machining, and provides an important reference for future research on multi-dimensional regulation of soliton lasers.