Few-mode fiber based saturable absorber for ultra-fast fiber laser at 2 μm

Abstract

The saturable absorption of the single mode-few-mode-single mode fiber (SMF-FMF-SMF) structure with respect to its passive mode-locking performance is still largely unexploited, although it is more conducive to the design and performance of the saturable absorber (SA) based on the nonlinear multimode interference effect. A series of FMF-based saturable absorbers with different FMF lengths are employed in passively mode-locked Tm fiber lasers for ultrafast pulse generations at 2 μm. The mode-locking performances using different FMF-based saturable absorbers were demonstrated. By comparing the saturable absorption characteristics and the respective mode-locking performances, we can conclude that it is easier to obtain the stable mode-locking output using saturable absorbers with the FMF lengths of 15.12 cm and 30.2 cm. The FMF saturable absorber with a 15.12 cm FMF has a large modulation depth of 40% and a saturation fluence of 108.23 μJ /cm2 which delivers ultrafast pulses with pulse width of 1.96 ps at 1878 nm. In addition, the harmonic mode-locking with stable and well-organized pulse trains could also be instantaneously achieved by increasing the pump power within the same cavity. Our results indicate that the SMF-FMF-SMF structure could operate as a SA while tuning its saturable absorption properties by changing the FMF length making it advantageous for mode locking fiber lasers.