Coherent detection of an active mode-locked terahertz quantum cascade laser

We demonstrate coherent detection of the emission from a terahertz frequency quantum cascade laser which is actively mode-locked by modulating its bias-current with a microwave synthesizer. Coherent sampling of the terahertz pulse train is achieved by phase-locking the repetition rate and the carrier frequency of the terahertz quantum cascade laser to the repetition rate of the of a commercial erbium-doped mode-locked femtosecond fibre laser. We show that the terahertz pulses from the quantum cascade laser are transform limited, and reconstruct the full emitted waveform.