Generation of widely tunable single- and dual-wavelength in a figure-eight mode-locked fiber laser

•Dual-wavelength based on Sagnac filtering effect is implemented in a mode-locked all-fiber laser.•The dual-wavelength show high stability over 60 min.•Both single- and dual-wavelength mode-locking have widely tunable ranges. We report a single- and dual-wavelength tuning at the C + L band in a nonlinear amplifying loop mirror (NALM) mode-locked Er-doped all-fiber laser. The single-wavelength tuning range is 52 nm, and the dual-wavelength can vary from 1558 nm and 1581 nm to 1581 nm and 1606 nm, accompanied by a slight change in the spacing of the wavelengths. To our knowledge, this is the widest dual-wavelength tuning range achieved in an Er-doped mode-locked fiber laser. The optical spectrum of the dual wavelengths has no significant variation within 60 min, which indicates potential for applications such as dual-comb. Our work provides a compact and low-cost dual-wavelength all-fiber laser, which may facilitate the design and application of dual-wavelength sources.