Hybrid optical feedback architecture for improved performance of QDash comb laser

We propose and demonstrate a technique based on self-injection locking to significantly enhance the performance of a quantum-dash comb laser, implemented via a hybrid architecture that combines fiber-based and free-space elements. Packaged in a standard butterfly module, the laser was thoroughly examined in both free-running and hybrid optical feedback configurations. Resonant optical feedback resulted in a simultaneous and remarkable improvement in phase noise, optical linewidth, and long-term stability in a passive manner. Specifically, we successfully narrowed the RF linewidth of the comb modes down to 600 Hz, reduced the optical linewidth across all modes to between 430 kHz and 14 kHz, and effectively improved long-term operation by limiting the excursions in both frequency drift and RF linewidth over time by factors of 3 and 6, respectively. In contrast to previous works, which were constrained by methods that produced results separately, our novel architecture combines all performance goals simultaneously in one unified system. Our findings demonstrate that this hybrid feedback architecture could set a new benchmark for future developments in photonic integrated circuits, paving the way for these comb sources for diverse range of applications including precision spectroscopy, frequency metrology, optical clocks, LiDAR, and communications.