Threshold behaviour of optical frequency comb self-generation in an InAs/InGaAs quantum dot laser

We report on a significant reduction of both the radiofrequency beat note line width at 40.7 GHz and the integrated relative intensity noise of a 1 mm long edge-emitting monolithic Fabry–Perot InAs/InGaAs quantum dot semiconductor laser emitting from the ground state at 1250 nm by injection current control. For increasing injection currents, first an unlocked multi-mode behavior is observed and then, at a certain current above lasing threshold, self-locking of the longitudinal modes due to the internal non-linear effects occurs yielding a beat line width of 20 kHz (−3 dB) in contrast to tens of megahertz for lower injection currents. These results are confirmed by simulations.