Coherent Broadening and Tuning of QCL Frequency Combs via RF-Injection

Quantum cascade laser (QCL) optical frequency combs [1] , [2] are emerging as promising sources in mid-infrared spectroscopy due to their ability to act as coherent broadband sources at room temperature with equidistant phase-locked modes. Their short sub-picosecond upper-state lifetime enables high modulation speeds of the device in the order of tens of gigahertz [3] . Since the devices are electrically driven, the intermode beating, which is in the RF-domain, can be read out electronically as current-modulations. Through external RF-modulation of the driving current, the comb-modes can be injection-locked for stabilization as well as enhancement of the four-wave-mixing process which is one of the main driving mechanisms of comb formation in QCLs [5] .