All-fiber saturable absorber based on nonlinear multimode interference with enhanced modulation depth.

We experimentally demonstrate a passively mode-locked fiber ring laser using a small-core fiber-graded index multimode fiber-single-mode fiber (SCF-GIMMF-SMF) structure as an effective saturable absorber based on the nonlinear multimode interference (NL-MMI) effect. A small-core fiber is used as an input single-mode fiber to improve the coupling of power into higher-order modes in a multimode fiber and to enhance the modulation depth of the structure. Stable fundamental mode-locked operation is obtained at a pump threshold of 166 mW and the output soliton pulses have a 647 fs duration at 1557.96 nm with a 5.2 nm spectral width at a repetition rate of 19.37 MHz. This NL-MMI SA can be applied to high-power mode-locked fiber lasers owing to its inherent high damage threshold, compact size, wavelength independent operation, and fabrication simplicity.