Shifted Wave Interference Fourier Transform Spectroscopy of THz Quantum Cascade Laser Frequency Combs operating above 70 K

Quantum Cascade Lasers (QCLs) are compact and powerful sources for frequency comb generation in the mid-IR and THz regime [1] , [2] . The generation and locking of the frequency comb modes is based on four-wave mixing inside the active region and leads to FM modulated emission. Experimentally, the intensity profile in time resp. the phase relation between comb modes is measured by Shifted Wave Interference Fourier Transform (SWIFT) Spectroscopy [1] and leads to a linear frequency chirp of the instantaneous frequency in free running mid-IR QCLs [3] , [4] . In THz QCL the instantaneous frequency shows also different shapes despite the linear chirp [5] but the lack of extensive experimental SWIFT measurements in the THz regime does not give a conclusive picture.