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We study a passively mode-locked semiconductor ring laser subject to optical feedback from an external mirror. Using a delay differential equation model for the mode-locked laser, we are able to systematically investigate the resonance effects of the inter-spike interval time of the laser and the roundtrip time of the light in the external cavity (delay time) for intermediate and long delay times. We observe synchronization plateaus following the ordering of the well-known Farey sequence. Our results show that in agreement with the experimental results a reduction of the timing jitter is possible if the delay time is chosen close to an integer multiple of the inter-spike interval time of the laser without external feedback. Outside the main resonant regimes the timing jitter is drastically increased by the feedback.

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Carrier scattering is known to crucially affect the dynamics of quantum dot (QD) laser devices. We show that the dynamic properties of a QD laser under optical injection are also affected by Coulomb scattering processes and can be optimized by band structure engineering. The nonlinear dynamics of optically injected QD lasers is numerically analyzed as a function of microscopically calculated scattering lifetimes. These lifetimes alter the turn-on damping of the solitary QD laser as well as the complex bifurcation scenarios of the laser under optical injection. Furthermore, we find a pump current sensitivity of the frequency-locking range, which is directly related to the nonlinearity of the carrier lifetimes.