FM to AM Transition of RF Driven THz QCL Comb States

Quantum Cascade Lasers are powerful frequency comb sources in the THz and their repetition rate can be stabilized by means of RF injection locking. By exceeding certain RF injection powers, the spectral shape can be changed significantly with often unidentified effect on the temporal intensity profile. Here we perform temporal intensity profile measurements by means of Shifted Wave Interference Fourier Transform Spectroscopy by employing a fast, antenna coupled superconducting NbN Hot Electron Bolometer. We investigate the intensity profile of strong RF injected heterogeneous THz QCLs showing a strongly AM emission. Additionally, we measure fundamental as well as harmonic comb states under reduced RF injection which exhibit a FM emission.