RF linewidth of a monolithic quantum dot mode-locked laser under resonant feedback

The stability of a quantum dot (QD) mode-locked laser is experimentally shown to bifurcate under resonant optical feedback, which leads to either a reduction or an enhancement of the noise within the laser´s cavity. These two behaviours, which are theoretically known as the nearly exact resonant and stably resonant feedback conditions, are characterised using the RF linewidth of the QD device. Under the stably resonant case with relatively low feedback strength and constant temperature control, the RF linewidth narrows to a value as low as 170 Hz. Noise enhancement, which is a precursor to coherence collapse, is observed in the device under the nearly exact resonant case. Under proper conditions, the results presented show that the combination of external optical feedback and the relatively low threshold of QD mode-locked lasers make them attractive chip-scale sources for ultra-low noise photonic applications.