Passively synchronized dual-color soliton fiber laser based on single-walled carbon nanotubes

•The dual cavity structure of the laser can generate two types of optical solitons.•The two cavities of the laser can operate independently as two sub-lasers.•Passively synchronized mode-locked state can be achieved by nonlinear interaction.•The laser can generate dual-color soliton molecules in synchronization. In this work, we propose a uniquely designed fiber laser to generate dual-color soliton molecules, which are stable bound states made up of solitons with different carrier frequencies, pulse widths and soliton energies. This fiber laser consists of two resonators, which are cascaded by a pair of wavelength division multiplexers (WDMs), and a single-walled carbon nanotube saturable absorber (SWCNT-SA) is placed in the common branch shared by two sub-cavities to achieve optical soliton mode-locking. The optical solitons generated by this laser have central wavelengths of 1530.43 nm and 1547.62 nm with pulse widths of 0.832 ps and 1.198 ps, respectively. The synchronous mode-locked operation of the two sub-cavities can be easily achieved by adjusting the optical delay line (DL) in one of the resonant cavities. When the synchronized state is reached, two solitons with different central wavelengths operate synchronously and stably at a consistent repetition frequency of ∼8.1505 MHz and dual-color soliton molecules are generated. The fiber laser has the advantages of simple structure and controllable generation of soliton molecules, which provide a new idea for the design of soliton molecule ultrafast fiber laser.