Tunable spectral modulation period and depth of loosely bound soliton pairs in an erbium-doped fiber laser

A loosely bound state of soliton pairs has been obtained in an erbium-doped fiber laser based on nonlinear polarization rotation mode-locking. The spectrum of the soliton bound state exhibits a tunable modulation period and depth by adjusting the polarization controller. The maximum modulation depth of the spectrum can reach 20.4 dB, which is a rare level for loosely bound solitons. This spectral tunability stems from the Lyot birefringence filtering effect of the cavity and mainly from the linear phase delay. The numerical simulation further confirms the experimental results.