Stability of Pulse Emission and Enhancement of Intracavity Second Harmonic Generation in Self-Mode-Locked Quantum Cascade Lasers

We report the observations of stable pulse emission and enhancement of intracavity second harmonic generation (SHG) in self-mode-locked Quantum Cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an rf signal allows the direct observation of the pulse laser emission in the time domain and reveals the stable train of pulses characteristic of mode-locked lasers. The onset of self-mode-locking in Quantum Cascade lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. From the measured increase of SHG a pulse duration of ~12 ps is estimated, which is in a good agreement with the pulse duration deduced from the optical spectral width.