Optimizing the Self-Amplitude Modulation of Different 2-D Saturable Absorbers for Ultrafast Mode-Locked Fiber Lasers

Abstract

Passive mode locking of erbium-doped fiber lasers (EDFLs) started by different two-dimensional (2-D) saturable absorbers (SAs) is demonstrated and compared. The self-amplitude modulation (SAM) coefficients of four 2-D SAs, including graphene, MoS2, and n-/p-type Bi2Te3, are also estimated to compare their mode-locking performance at first stage. By analyzing the nonlinear absorption of those 2-D SAs, graphene possesses the largest SAM coefficient $(\gamma _{{\rm{Graphene}}})$ of 8.7x10–4 better than other 2-D SAs ( $\gamma _{{\rm MoS}2}= 5.5\times 10^{\hbox{--} 4}$ , $\gamma _{{\rm n-Bi2Te}3}= 7.2\times 10^{-4}$ , and $\gamma _{{\rm p-Bi2Te}3}= 7.9\times 10^{\hbox{--} 4}$ ). When operating the EDFL under low-gain condition, the mode-locked EDFL pulsewidth is mainly correlated with the SAM coefficient of SA. All of these 2-D SAs can generate the sub-700 fs EDFL pulsewidth at first mode-locking stage. In particular, graphene can produce the narrowest EDFL pulse with 615-fs pulsewidth because of its largest SAM coefficient to start up the mode locking. When inducing the soliton mode locking by mutually compensating the group-delay dispersion and self-phase modulation, the pulsewidth can be further compressed to sub-400 fs with enlarging pumping power. The graphene mode-locked EDFL effectively compresses its soliton pulsewidth to 343 fs with the widest spectral linewidth of 7.86 nm among all cases. All the obtained EDFL pulses exhibit similar compression ratio of ∼0.55.