Feedback-generated periodic pulse trains in quantum dot lasers

Quantum dot lasers have been shown to have greatly enhanced stability in the feedback configuration thanks to a high damping of the relaxation oscillations and they display different dynamics to those of conventional semiconductor lasers. For high feedback levels in conventional devices one obtains Low Frequency Fluctuations: sharp dropouts in intensity and subsequent gradual build-ups. Standard low frequency fluctuation-like traces are conspicuous by their absence in studies of feedback with quantum dot devices. We experimentally examine single mode quantum dot lasers at high feedback levels with a long delay and observe regular pulse-trains with a period equaling the external cavity round-trip time where each pulse features a distinctive broad trailing edge plateau. The distinctive pulse shape is very similar to the recently published strong pulse-asymmetry in two-section, passively mode-locked quantum dot lasers where this asymmetry was shown to result from the creation of different modal groups. We attribute the pulses in our experiment to the same phenomenon: each pulse corresponds to a simultaneous excitation of a number of the external cavity modes. We consider a model tailored specifically for quantum dot lasers with strong optical feedback and find it reproduces the experimentally observed trains extremely well.