We report on experimental and theoretical studies of the stability regime of passive mode-locked quantum dot lasers, which is decisively larger than in quantum well lasers. A small range of Q-switched instability is observed at low gain currents. Transition to Q switching is inhibited due to fast damping of the relaxation oscillations. A double pulse mode-locking regime appears for longer cavities, and exhibits bistability and coupling to the fundamental mode-locking operation. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2822808͔Passive and hybrid mode-locked ͑MLd͒ quantum dot lasers ͑QDLs͒ are efficient sources of short pulses ideal for applications in high speed communication systems. 1,2 The mode-locking ͑ML͒ regime of quantum well lasers can exhibit peak power fluctuations induced by a Q-switching instability, which is detrimental for the laser performance. In most experimental situations, QDLs have only a small range of Q switching instability, a consequence of the strong damping of the relaxation oscillations induced by the fast carrier capture from the wetting layer to the dots.In this letter, we study the stability of the mode-locked regime in QDLs with pulse repetition rates of 20 and 40 GHz. We find that stable ML in QDLs occurs in a comparatively larger range of parameters than in quantum well lasers. We also demonstrate the bifurcation to a state of doubled pulse repetition frequency in the MLd regime in longer cavity devices and characterize its coupling to the fundamental ML regime.The devices used in this work are two-section QDLs with a total length of 1 mm integrated in a module and an unpackaged 2 mm device ͑saturable absorber length is always 1 / 10 of the total length͒. The active region was grown on a GaAs substrate, containing 15 self-organized InAs QD layers. The facet near the absorber was high refection coated, the front facet was as cleaved. Details of the growth and the processing are the same as described in Ref. 2.A number of different regimes have been observed experimentally: Q-switching modulated ML, pure ML and, finally, cw lasing. The stability domains of these regimes are displayed in Fig. 1 for the 1 mm QDL module. We measured the radio frequency ͑rf͒ spectra of the optical output in the range of 0 -50 GHz by means of a 55 GHz bandwidth photodetector and a 50 GHz electrical spectrum analyzer. The side band suppression ratio ͑SBSR͒ with respect to the fundamental ML frequency at 40 GHz was extracted for a large set of reverse bias voltages and gain currents and plotted in Fig. 1. The large area of proper ML with a SBSR better than 20 dB, i.e., with weak or absent bands besides the fundamental ML frequency, dominates the figure. The region of Q-switching modulated ML, i.e., the region with strong side bands in the range of a few gigahertz, is small compared to that observed with quantum well lasers. 3 Both regions are separated by a steplike change of the SBSR by more than 30 dB. In our experiments, we did not observe any dependence of the SBSR plot on the scanning dir...