Figure 3: Beat-note detection.
From: Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers
(a) The upper sketch shows, in the frequency domain, the comb-like structure of the terahertz FCS beam, as well as the monochromatic QCL emission (νqcl). The red-coloured arrows indicate the beating between the QCL and the third-order neighbours teeth of the comb. The corresponding frequencies fall in the range between 2frep and 3frep (blue and green arrows, respectively), and are shown in the FFT spectrum below, together with the self-beatings among the comb teeth (as highlighted by the coloured labels). The beat-note frequency is given, apart from frep multiples, by its distance from the closest Nth and (N+1)th teeth of the comb. Therefore, once the order N is known, the QCL absolute frequency can be retrieved. (b) Zoomed view of the beat-note peak around 222 MHz (dashed box in a), acquired with a 20 kHz RBW. The 40 kHz FWHM gives the emission linewidth of the free-running QCL over 1 ms observation time.
