Sub-kHz RF linewidth of quantum-dash mode-locked laser by self-injection from symmetric dual-loop feedback and fiber delay

Sub-kHz RF-linewidth was demonstrated in two-section quantum-dash mode-locked lasers emitting near 1550 nm and 20 GHz repetition-rate using symmetric dual-loop and fiber delay. Compared to free-running lasing, RF-linewidth and timing-jitter reduced by ∼100× and ∼10×.