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.

We have developed an InAs/InP quantum dot (QD) gain material using a double cap growth procedure and GaP sublayer to tune QDs into the L-band. By using it, a passive L-band mode-locked laser with pulse duration of 445 fs at the repetition rate of 46 GHz was demonstrated. The 3-dB linewidth of the RF spectrum is less than 100 KHz. The lasing threshold injection current is 24 mA with an external differential quantum efficiency of 22% and an average output power of 27 mW. The relationship between pulse duration and 3-dB spectral bandwidth as a function of injection current was investigated.

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