Generation of conventional solitons and bound state solitons in a passively mode-locked fiber laser based on GIMF–NCF–GIMF structured saturable absorber

Abstract A new all-fiber structured saturable absorber, consisting of graded index multimode fiber-no-core fiber-graded index multimode fiber (GIMF–NCF–GIMF), in a passively mode-locked erbium-doped fiber laser is proposed and demonstrated. The conventional solitons with a center wavelength of 1575.5 nm, a pulse width of 707 fs, and a signal-to-noise ratio of 66 dB are generated at the repetition frequency of 18.919 MHz. The different lengths of each part of the saturable absorber (SA) can usually allow the fiber laser system to reach mode-locking, which shows that the length limits on this SA can not be quite as serious. Meanwhile, by only adjusting the polarization controller, tightly bound solitons and loosely bound solitons can be readily generated. The pulse trains of tightly bound solitons exhibit a separation of 1.52 ps, a pulse width of ∼850 fs, and centered at 1573 nm, whereas the pulse separation of loose bound solitons are from 23 ps to 58 ps, which shows wide variations of the time dispersion. More importantly, it is found that tightly bound solitons and loosely bound solitons can exist simultaneously for higher-order solitons, for the first time to the best of our knowledge. Such an SA is easy to fabricate, highly stable and of low cost, and has high potential in ultrafast photonics.