Wavelength and interval tunable second-order bound solitons in mode-locked fiber laser based on Lyot filter

The wavelength and interval tunable second-order bound solitons are generated in a nonlinear polarization rotation mode-locked fiber laser based on a Lyot filter in this paper. The Lyot filter structure is composed of two polarization controllers, two in-line polarizers and polarization maintaining fiber. The transmission curves of Lyot filter have the characteristic of periodic modulation, so it has spectral filtering effect. The change of two polarization controllers of Lyot filter can cause a shift of the transmission curve and the variation in the modulation depth. The wavelength tunable second-order bound solitons can be obtained due to the shift of transmission curve. A second-order bound solitons with a maximum wavelength tuning range of 18.56 nm can be obtained in the cavity. In addition, the second-order bound solitons with different pulse intervals can be obtained due to the variation in the modulation depth. But the second-order bound solitons will be turned into complex higher-order bound solitons through increasing the pump power. It can be recovered to second-order bound solitons by adjusting the polarization controllers. The numerical simulation further verifies the phenomenon on second-order bound solitons, which is consistent with experimental results.