NPR-NLMMI hybrid mode-locked fiber laser with an SMF-PCF-GIMF-SMF saturable absorber

This study demonstrates a hybrid mode-locked fiber laser by incorporating a single-mode fiber-photonic crystal fiber-graded-index multimode fiber-single-mode fiber (SMF-PCF-GIMF-SMF) structure as a saturable absorber (SA) into a nonlinear polarization rotation cavity. The SA exhibits a modulation depth of 5.8% and a saturation intensity of 59.8 μJ/cm2. Stable fundamental mode-locking is achieved at a center wavelength of 1555 nm with a 3 dB spectral width of 7.2 nm. Compared to the hybrid mode-locked laser employing a conventional SMF-GIMF-SMF SA, the proposed SA reduces the fundamental-frequency mode-locking threshold from 120 to 50 mW, improves the signal-to-noise ratio of the RF spectrum from 35 to 48 dB, and decreases the pulse width from 580 to 493 fs. Harmonic mode-locking up to the eighth order is achieved by adjusting the pump power and polarization controllers. The long-term stability of the laser is confirmed by the power fluctuation of the fundamental-frequency pulse train over an 8-h period. This compact, low-cost laser exhibits a high damage threshold and good stability, highlighting its significant potential for both fundamental ultrafast photonics research and industrial applications.