Time- and frequency-domain characterization of a mid-infrared difference-frequency-generated frequency comb via two-photon detection and heterodyne detection

Mid-infrared frequency combs are nowadays well-appreciated sources for spectroscopy and frequency metrology. Here, a comprehensive approach for characterizing a difference-frequency-generated mid-infrared frequency comb (DFG-comb) both in the time and in the frequency domain is presented. An autocorrelation scheme exploiting mid-infrared two-photon detection is used for characterizing the pulse width and to verify the optimal compression of the generated pulses reaching a pulse duration (FWHM) as low as 196 fs. A second scheme based on mid-infrared heterodyne detection employing two independent narrow-linewidth quantum cascade lasers (QCLs) is used for frequency-narrowing the modes of the DFG-comb down to 9.4 kHz on a 5-ms timescale.