Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection

In this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio-frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm$^{-1}$ to 60cm$^{-1}$ can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red-shifted compared to the free-running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum-bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red- or blue-shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free-running beatnote, we can generate two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz. In the time-domain we show with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi-continuous output to pulsed ($\tau_p \approx 55$ ps) output by ramping up the injection power to 35 dBm.