I. Introduction
Ultra-high resolution spectroscopy [1], THz coherent communications [2], THz metrology [3], or THz radio-astronomy [4], [5] are applications that need ultra-stable continuous-wave (cw) sources. While heterodyning two continuous optical waves in a photo-detector is a well-established approach to generate cw THz signals, stability usually relies on phase-locked loops (PLL) [6]–[9]. When the heterodyne beat is at THz frequencies, the major issue is to down-convert the THz beat to RF frequencies in order to implement standard PLLs. In this perspective, an important experimental step was made recently by using a nonlinear electro-optic modulator (EOM) to bridge the gap between widely spaced optical frequencies. The optoelectronic phase locked-loop principle (OEPLL) principle was successfully implemented to stabilize two-frequency laser beat signals at 100 GHz [10], [11], up to 250 GHz [12] in an improved set-up, and also to stabilize micro-combs at 140 GHz mode spacing [13]. In all cases, the frequency spacing is limited to the number of harmonics that the EOM can generate, typically 10.