Generation of cavity-birefringence-dependent multi-wavelength bright-dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright-dark pulse pair in a mode-locked thulium-doped fiber laser (TDFL). The nonlinear polarization rotation (NPR) and nonlinear optical loop mirror (NOLM) were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation. By incorporating different lengths of high birefringence polarization-maintaining fiber (PMF), the fiber laser could operate stably in a multi-wavelength emission state. Compared with the absence of the PMF, the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright-dark pulse pair.