Active-mode-locked terahertz quantum cascade lasers

In this work, we use a commercial mode-locked erbium-doped fs-laser emitting at 1.55 μm (193 THz), for the asynchronous sampling of a mode-locked QCL emitting at 2.5 THz. Mode-locking of the QCL is obtained by direct modulation of its bias current close to the laser roundtrip frequency. Asynchronous sampling is made possible by phase-locking the QCL source to the fs-laser, i.e. the pulse repetition rate of the QCL at ~ 13.3 GHz and its carrier frequency at 2.5 THz are simultaneously phase-locked to the repetition rate of the fs-laser-comb at 96.5 MHz. The advantage of this technique is twofold: (i) it enables coherent sampling of the THz QCL waveform over virtually unlimited time scales, allowing us to prove unambiguously that the device is operating in a regime of active mode-locking, and is generating ~10ps wide, transform limited pulses; (ii) it gives control of the QCL carrier-envelope phase shift, allowing THz frequency synthesis.