Stable trains of ultrashort light pulses with large instantaneous intensities from mode-locked lasers are key elements for many important applications such as nonlinear frequency conversion [1-3], time-resolved measurements [4, 5], coherent control [6, 7], and frequency combs [8]. To date, the most common approach to generate short pulses in the mid-infrared (3.5-20 µm) molecular "fingerprint" region relies on the down-conversion of short-wavelength mode-locked lasers through nonlinear processes, such as optical parametric generation [9-11] and four-wave mixing [12]. These systems are usually bulky, expensive and typically require a complicated optical arrangement. Here we report the unequivocal demonstration of mid-infrared mode-locked pulses from a semiconductor laser.

The impact of upper state lifetime and spatial hole burning on pulse shape and stability in actively mode locked QCLs is investigated by numerical simulations. It is shown that an extended upper state lifetime is necessary to achieve stable isolated pulse formation per roundtrip. Spatial hole burning helps to reduce the pulse duration by supporting broadband multimode lasing, but introduces pulse instabilities which eventually lead to strongly structured pulse shapes that further degrade with increased pumping. At high pumping levels gain saturation and recovery between pulses leads to suppression of mode locking. In the absence of spatial hole burning the laser approaches single-mode lasing, while in the presence of spatial hole burning the mode locking becomes unstable and the laser dynamics does not reach a steady state anymore.

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A mode-locking mechanism by active gain modulation is studied numerically and experimentally. The parameter window for the emission of stable pulse trains was found. Pulses as short as 3ps (0.5pJ) were characterized by second-order autocorrelation.

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