External cavity locking in self-mode-locked quantum dot lasers

Integrated optical frequency comb (OFC) sources with low radio-frequency (RF) linewidths are increasingly required for achieving stable mode-locking operation and low RF phase noise. This work centers on the demonstration of external cavity locking (ECL) in a self-mode-locked single-section quantum dot (QD) laser. It combines a detailed characterization of the self-mode-locking behavior with frequency stabilization of the generated OFC through ECL. Detailed analysis reveals that the feedback strength plays a dominant role in governing the ECL dynamics. Notably, the inherent chaos-free behavior of QD lasers under strong optical feedback enables straightforward locking, obviating the need for phase control or feedback attenuation, representing a significant advantage over conventional laser systems. Experimental results demonstrate that at a feedback strength of −6.3 dB, the 3-dB linewidth of the fundamental RF tone is reduced from 10.5 MHz to 249 kHz. Overall, by leveraging the self-mode-locking and feedback-resilient characteristics of QD lasers, this work paves the way for compact, integrated pulsed light sources with simplified architectures, offering promising prospects for applications in optical metrology, spectroscopy, and microwave photonics.