Chirp Manipulation of Harmonically Mode-Locked Weak-Resonant-Cavity Colorless Laser Diode With External Fiber Ring

The chirp manipulation of a colorless weak- resonant-cavity Fabry-Perot laser diode (WRC-FPLD) harmonically mode locking (HML) by 10-GHz direct modulation is demonstrated under the self-feedback control with an external single- or dual-fiber ring. With enlarging the self-feedback ratio through the fiber ring to 90%, the HML pulsewidth significantly shrinks to 14 ps for the colorless WRC-FPLD with a front-facet reflectance of 0.1%. Increasing the front-facet reflectance to 1% further shortens the pulsewidth to 10 ps. The interior modal linewidth of the colorless WRC-FPLD conversely broadens to 0.18 nm with enlarging the feedback ratio to 90%. The dynamic chirp of the WRC-FPLD fiber ring delivered HML pulse can be manipulated with controlling either the feedback ratio or the front-facet reflectance. The negative part of the chirp of the colorless WRC-FPLD with a front-facet reflectance of 0.1% can be eliminated to give the smallest absolute frequency chirp of 3 GHz within 28 ps by setting the self-feedback fiber ring at a self-feedback ratio of 10%. A theoretical model for the directly modulated and self-feedback colorless WRC-FPLD operated under HML scheme is established by modifying the injection-locking rate equations. The superiorities of the colorless WRC-FPLD, including longitudinal mode linewidth shrinkage and frequency chirping tenability, are successfully confirmed by experiments and simulations. The frequency chirp reduction, the modal linewidth narrowing, and the pulse shortening of the HML WRC-FPLD at relatively low front-facet reflectance can concurrently be achieved by incorporating the self-feedback single-fiber ring architecture. In comparison, such a chirp-magnitude manipulative WRC-FPLD can easily find more practical applications than a typical FPLD in fields of high-speed communications and ultrafast measurements.