We have investigated the jitter, repetition frequency and spectra of a quantum-dot mode-locked laser operating at 40 GHz under continuous wave external light injection using an external cavity laser or under optical feedback. For the case of the continuous wave light injection we realized a shift of the optical emission of up to 15 nm. With optical feedback we have found a ninefold jitter reduction, down to a record low value of 1 ps over an integration range from 10 kHz to 1 GHz. Optical feedback fixes the repetition frequency while changing current and voltage applied to the diode. This is due to the fact that stable mode locking occurs at frequencies which are integers of the feedback loop frequency.