Er-Doped Mode-Locked Fiber Laser With a Hybrid Structure of a Step-Index–Graded-Index Multimode Fiber as the Saturable Absorber

A mode-locked all-fiber laser based on the nonlinear multimode interference (NL-MMI) effect of the graded-index multimode fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step-index multimode fiber (SIMF) connected by a GIMF. Compared with the theoretically proposed SMF-GIMF-SMF structure, the introduction of the SIMF eliminates the restriction on the length of the GIMF and provides a more flexible method of making an SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulsewidth of ~446 fs, the bandwidth of 4.48 nm, and the fundamental repetition rate of 11.73 MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in the large-mode-area double-clad fiber laser make it highly attractive in ultrafast photonics.