Harmonic Mode-Locking of 10-GHz Directly Modulated Weak-Resonant-Cavity Fabry–Perot Laser Diode in Self-Feedback Fiber Ring

Harmonic mode-locking of a directly modulated weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) at 10 GHz via the self-feedback of a fiber ring is demonstrated. The WRC-FPLD directly modulated at 10 GHz simultaneously functions as the gain medium and the active mode-locker in the self-feedback ring. The competition between the gain-switched lasing mechanism inside the WRC-FPLD and the harmonic mode-locking mechanism of the incorporated self-feedback fiber ring is monitored. The enhanced mode-locking force can be achieved at self-feedback ratio of 90% and modulation power of 28 dBm. The evolution of pulsating dynamics is dependent with modulation frequency, which enhances the mode-locking and shortens the pulsewidth at the longitudinal mode spacing. A mode-locking theory is used to simulate the coupled-cavity self-injected WRC-FPLD fiber ring. When the lasing mechanism transfers from intracavity gain-switching to intercavity harmonic mode-locking, the longitudinal mode of the WRC-FPLD is broadened with the mode extinction reduced from maximum to minimum. Due to the transformation from the gain-switching to harmonic mode-locking, the coupled-cavity self-feedback WRC-FPLD fiber ring delivers a pulsewidth of 22 ps, a timing jitter of 153 fs, a pulse extinction ratio of 13.65 dB, and a spectral linewidth of 7.6 nm, by setting the WRC-FPLD bias at three times above threshold (~60 mA) and the intracavity feedback ratio of 90%.